ISO 9001: 2015 OrganizationS
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TAC Guidelines on Water Spray Systems:
Definitions and terminology relating to the components of the water spray systems are as
follows:
(a) Water Spray System: A special fixed pipe system connected to a reliable source of
fire protection water supply and equipped with water spray nozzles for specific water
discharge and distribution over the surface or area to be protected. The piping system is
connected to the water supply through an automatically actuated Deluge Valve which
initiates flow of water. Automatic actuation is achieved by operation of automatic detecting
equipment installed along with water spray nozzles. There are two types of systems
namely High Velocity and Medium Velocity systems. The former is useful for liquids with
flash point above 65 °C and the latter for flash point below 65 °C.
(b) Spray Nozzle and Valves: A normally open water discharging device which, when
supplied with water under pressure will distribute the water in a special directional pattern
peculiar to the 'particular device. Nozzles used for High Velocity Water Spray systems are
called "Projectors" and nozzles used for Medium Velocity Water Spray systems are called
"Sprayers". Both these nozzles are made in a range of orifice sizes with varying discharge
angles so that discharge can be controlled for optimum protection.
Types of Valves Used with Fire Water Piping System /Water Hydrants
(c) Deluge Valve: A quick opening valve which admits water automatically to a system
of projectors or sprayers and is operated by a system of detectors and/ or sprinklers
installed in the same areas as nozzles.
(d) Control of Burning: Application of water spray to equipment or areas where a fire
may occur to control the rate of burning and thereby limit the heat release from a fire until
the fuel can be eliminated or extinguishment effected.
(e)Exposure Protection: Application of water spray to structures or equipment to limit
absorption of heat to a level which will minimize damage and prevent failure whether
source of heat is external or internal.
(f) Impingement: The striking of a protected surface by water droplets issuing directly
from projectors and/or sprayers.
(g) Run Down: The downward travel of water along a surface caused by the momentum
of the water or by gravity.
(h) Slippage: The horizontal component of the travel of water along the surface beyond
the point of contact caused by the momentum of water.
(i) Insulated Equipment: Equipment, structure vessels provided with insulation which
for the expected duration of exposure, will protect steel from exceeding a temperature of
454 "C (850 °F) for structural members and 343 °C (650 °F) for vessels.
(j) Density: The unit rate of water application to an area or surface expressed in
litres/min/ m
(k) Automatic Detection Equipment: Equipment which will automatically detect one or
more components directly related to combustion such a heat. Smoke, flame and other
phenomenon and automatic actuation of alarm and protection equipment.
(l) Fire Barrier: It is a continuous wall or floor that is designed and constructed to limit
the spread of fire.
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(m) Range Pipes: Pipes on which sprinklers are attached either directly or through short
arm pipes which do not exceed 30 cm in length.
(n) Distribution Pipes: Pipes which directly feed the range pipes.
(4)Foam System: It uses fixed foam apparatus either automatic or manual. It may
consist of one or more portable foam extinguishers suspended in such a way that flame or
heat releases a cord or fusible link to operate the extinguisher automatically. Discharge
rate may vary from 15 to 4000 gpm. Foams are of two types - chemical and mechanical.
Chemical foam is produced by a chemical reaction of CCX, bubbles and a foaming agent.
Mechanical foam is created when air and water are mechanically agitated with a foam
solution.
Firefighting foam (gas-filled bubble solution) is lighter than most flammable liquids.
Therefore, it forms a floating blanket on burning liquid, cuts off oxygen supply and also
cools the fuel.
Foam system is generally used to protect fuel tanks, oil and paint storage rooms,
asphalt coating etc. It can be injected on the liquid surface in a tank to provide blanketing
effect and to cut off flames and vapours.
Foam is of two types - Low expansion and High expansion foam. Low expansion
foam is of four types Chemical foam, Mechanical -or air-generated foam, Protein foam and
Synthetic (fluorinated surface-active agent) foam. Foam generators of different types are
available. Foam-water sprinkler and spray systems use mechanical foam equipment with
a deluge sprinkler system. High-expansion foam is best suited for class A and B fires in
confined spaces such as sewers, basement. It is made by mixing a small amount (@ 1.5%)
of foam liquid into a foam generator where water and large quantities of air are mixed.
Accumulated foam can act as an insulating barrier for the surface not involved in fire.
Thus, it prevents fire spread. Ventilation is necessary to vent the displaced air and gases
when foam is being applied.
Fire Detection and Alarm Systems: Various types of detectors are available operating
on principles of thermal expansion, thermoelectric sensitivity, thermo conductivity or
photosensitivity to detect presence of smoke, increase in temperature, light intensity or
total radiation. Their types are: Thermal expansion detectors. Radiant energy detectors.
Light interference detectors and ionization detectors. They should be properly located
depending upon their range. They simply give alarm and cannot extinguish fire. They make
us alert for fire-fighting.
Fire detectors (A & B) and LPG detector (C)
Though fire detection and alarm systems are separate systems but the latter has to
operate just after the former operates. Therefore, they are considered together. IS 2175
and 2189 also deal with them together.
Two main functions of any fire detection system are
1. To give alarm to start up extinguishing procedure, and
2. To give early warning to area occupants to escape.
It is wrong to speak 'fire detectors'. Actually, they detect sensible heat, smoke density or
flame radiation to operate before actual fire follows. Their 'sensor' detects measurable
quantity of these parameters. A decision-making device coupled with the sensor, compares
the measured quantity with a predetermined ' value, and when it is different, an alarm is
sounded. A detector both detects and signals.
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Human being is a good detector as he can act m a flexible way i.e. run away, put
out the fire or call the fire department. No other detector can work in such selective
manner.
Selection of the type of detector is important. For example, low risk areas need
thermal detectors, a ware house may have infrared and ionization detectors and a
computer area requires ionization or combination detectors.
Location and spacing should be determined to obtain the earliest possible warning.
Sensitivity, reliability, maintainability and stability are important factors for selection.
Fire process has four stages - incipient stage, smoldering stage, flame stage and
heat stage. Many types of fire detectors are available for various situations and useful at
different stages of a fire. Thermal detectors are of fixed temperature detectors, rate-
compensated thermal detectors, rate of rise thermal detectors, line thermal detectors and
the bulb detection system.
Smoke detectors are of photoelectric type and are of two classes - The beam
photoelectric or reflected beam photoelectric detectors.
Flame detectors are of infrared (IR) or ultraviolet (UV) type.
Ionization (combustion products) detectors are the single chamber or dual chamber
ionization detector and the low-voltage ionization detector.
Fire Alarm system may be separate to run manually or connected with fire detectors
and operable automatically. All workers must be made aware of the sound pattern and its
meaning. Fire alarm sound should be distinguishable from other sound m that area. It
should be clearly audible to all facility personnel. Sound for beginning of fire and end of
fire should be kept different.
10.5 BLEVE (Boiling Liquid Expanding Vapour Explosion):
Boiling Liquid Expanding Vapour Explosion (BLEVE), also referred as a fireball, is a
combination of fire and explosion with an intense radiant heat emission within a relatively
short time interval.
When a tank or pressure vessel containing liquid or liquefied gas above its boiling
point (so heated) fails or ruptures the contents release as a turbulent mixture of liquid and
gas, expanding rapidly and dispersing in air as a cloud. When this cloud is ignited, a fireball
occurs causing enormous heat radiation intensity within a few seconds. This heat is
sufficient to cause severe skin burns and deaths within a few hundred meters depending
on the mass of the gas involved. A BLEVE involving a 50- tone propane tank can cause
'"third-degree burn at @ 200 mt and blisters at @ 400 mt
Road/rail accident to a tank car/wagon or due to weakening of structure by fire or
physical impact on an overstressed vessel/tank can cause a BLEVE.
Types of Explosion
Dust Explosion: It is possible due to flammable dusts of wood, coal, food(starch, flour,
sugar, cocoa, feed stuffs), chemicals, plastics (urea formaldehyde, resin, polyethylene,
polystyrene), metals(aluminum, magnesium) etc.
It results from rapid combustion of fine solid particles like iron, aluminum, wood,
starch etc. Many solid particles when reduced to fine powder becomes very flammable and
explosive.
At a starch/corn plant at Ceder Rapids, Iowa in 1919, 43 people were killed and at
Peking, Illinois in 1924, 42 people were killed due to dust explosion.
At a starch plant at Ahmedabad, 29 workers injured and out of them 20 died due
to starch dust explosion on 19-12-1991.
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Explosion characteristics of dust suspension are as under:
1. Explosibility classification.
2. Minimum explosible concentration.
3. Minimum ignition temperature.
4. Minimum ignition energy.
5. Maximum permissible oxygen concentration to prevent ignition.
6. Explosion pressure characteristics.
(a) Maximum Explosion Pressure.
(b) Maximum Rate of Pressure Rise.
(c) Average Rate of Pressure Rise.
Sources of Ignition for Dust Explosions:
(1) Flames, heat or hot surfaces
(2) Welding and cutting
(3) Mechanical sparks
(4) Self-heating
(5) Static electricity and
(6)Electrical equipment.
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Preventive Methods for Dust Explosion:
(1) Avoidance of dust suspensions
(2) Wet process
(3) Elimination of source of ignition and
(4) Inserting.
Methods of Protection Against Dust Explosion:
(1) Isolation
(2) Containment
(3) Explosion suppression and
(4) Explosion venting.
Dust fires can occur in dust deposits and are of two types - flaming and smoldering fires.
Halon Gas Extinguisher (Halon Alternatives):
Halon 1011, 1211 or 1301 a liquid gas is filled in extinguishers. It is usedin place of CO,
extinguishers but is lighter in comparison. 1.5,3 and 6 kg cylinders and bigger sizes are
available in wheel mounted model. By pressing a knob in cap-assembly it can be started.
Nose should be covered to avoid direct inhalation.
It is suitable for class B and C fires. See IS 11108 for Halon 1211.
Halon is a fast extinguishing agent. It is ideal for intense and rapid fires. It is non-conductive
and leaves no traces when applied. Therefore, it is also suitable for electrical fires, computer
rooms etc.
Halon interrupts the chain reaction at the flame zone of fire. It is two times as effective as
CO2 on a weight basis and five times as effective as CO2 on volume basis.
Halon is stored under pressure in a cylinder. A squeeze grip type nozzle is provided on top of
the cylinder valve depending upon capacity. It is available in 2,4,5,25 and 50 kg capacities.
Mostly two types of Halons (halogenated agents) are used as they are less toxic - (1) Halon
1211-Bromochloro difluoromethane i.e. CF2BrCI and (2) Halon 1301 - Bromo-trifluromethane
CF2Br.
Portable Fire Extinguishers:
In addition to the fixed fire installations stated in next part, portable (first-aid) fire
extinguishers are always desirable for quick manual use on small fires and fort the period till
automatic equipment or outside fire fighters work. All such extinguishers should be (1) of
reliable make, standard (IS) and properly identified (2) of right type depending upon the class
of fire (3) sufficient in number (4) properly located where they are necessary and readily
accessible (5) recharged periodically, inspected and maintained in good working condition and
(6) known by the operators who are trained to use them.
Their types are: (1) Water type (2) Soda acid type (3) Carbon dioxide type (4) Foam
type (5) Dry chemical powder type and (6) Vaporizing liquid type. IS:2190 is most useful for
selection, installation and maintenance of portable first aid fire extinguishers. Details of these
six types are also given in IS:940, 6234, 934, 2878, 933 and 2171. Tables of their suitability
according to class of fire and scale i.e. their range or area coverage arc also given therein.
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Based on them, number of extinguishers can be determined. Methods of their testing and test
form are also prescribed. Refer them for further details.
For small fires mostly portable fire extinguishers are used. They are explained below
in brief:
(1) Soda Acid (Water Type) Extinguisher: This extinguisher is useful for class A fire
(wood, paper, fabrics, rubbish etc.). It should not be used on fires of electricity, oil, chemical
or metal. It is available in both the shapes cylindrical and conical. Its normal capacity is 9
Litres (weight 14 Kg) and to be used in a range of 6 to 8 mt. It consumes within I to 1.5
minute. It should be checked every 3 months. It is held vertically up (not inverted). By
standing 4 to 5 mt. away from the fire, after opening the plunger, it is struck on the hard
surface. A small H„SC) (Sulphuric acid) bottle breaks and due to its mixture with soda
bicarbonate solution, C0„ (Carbon dioxide) is generated. Pressure of CO, throws water at a
distance. Its handle and bottom are held by two hands and water is sprayed on fire to
extinguish it.
(2) Foam Extinguisher: It is used on class B small fires. It: should not be used on electrical
or metal fire. It is available in 9-litre cylinder and used in 4 to 6 mt range. It consumes within
1.5 minute. It is available in wheel mounted trolley of 18 Litre and 150 Litre capacity for
longer use. It should be checked every 3 months.
By standing 3 to 4 mt away from the fire, the plunger is. pulled up and turned right
up to a slot. It is shaken by turning 180" twice. Then it is held inverted. By chemical reaction
CO is generated which throws foam outside. The foam is not thrown directly in fire but it is
thrown on nearer hard surface so that because of striking further foam is generated and
spread on burning surface. It stops oxygen availability for burning and controls the fire.
Foam is effective up to 120 °C temperature only.
(3) CO2 (Compressed gas) Extinguisher: It is useful on class E i.e. electrical fire because
CO2 is nonconductive gas. It can be used on class B and C fire also, as it diminishes oxygen
to control fire. It is not advisable to use it in a closed room as more CO2 may be inhaled.
Therefore, open doors and windows before using it in a room. It should not be used on fires
of metal, sodium, potassium and metal hydrides.
It is available in 2 kg,4 kg, 6.8 kg and 22.5 kg capacities. Small cylinders have handles
and big cylinders have wheels. Its range is 1 to 1.5 mt. CO2 pressure is at 64 to 70 bar. It
should be checked every three months.
(4) Dry Chemical Powder (DCP) Extinguisher: This can be used on any class of fire.
Therefore, it is known as 'universal type extinguisher'. It is generally used on fire of flammable
liquid. It is not effective on fire of benzene, ether, EO and CS2. For metal fire, special powder
extinguishers are available. 1,2,5 and 10 kg extinguishers in cylinders and 68 kg in wheel
models are available.
A 10 kg cylinder is consumed within 12 to 15 seconds and its range is 3 to 6 mt. A 68
kg cylinder is consumed within I to 1.5 minute and its range is 6 to 8 mt. Both should be
checked at 3 months interval.
By standing 6 to 8 mt near the fire, the cylinder is shacked twice by turning 180°, a
safety clip is removed and plunger is pressed or struck so that CO, bottle breaks and it throws
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dry chemicals out. The dry powder blankets the burning surface, stops 0, contact and CO,
coming out also diminishes 0 proportion. Therefore, fire is controlled by double action. Its
long nozzle should be turned in wind direction like a broom.
(5) Halon Gas Extinguisher (Halon Alternatives): Halon 1011, 1211 or 1301 a liquid gas
is filled in extinguishers. It is used in place of CO, extinguishers but is lighter in comparison.
1.5 ,3 and 6 kg cylinders and bigger sizes are available in wheel mounted model. By pressing
a knob in cap-assembly it can be started. Nose should be covered to avoid direct inhalation.
Halon is a fast extinguishing agent. It is ideal for intense and rapid fires. It is non-
conductive and leaves no traces when applied. Therefore, it is also suitable for electrical fires,
computer rooms etc.
Halon interrupts the chain reaction at the flame zone of fire. It is two times as effective
as CO2 on a weight basis and five times as effective as CO2 on volume basis.
Halon is stored under pressure in a cylinder. A squeeze grip type nozzle is provided on
top of the cylinder valve depending upon capacity. It is available in 2,4,5,25 and 50 kg
capacities. Mostly two types of Halons (halogenated agents) are used as they are less toxic -
(1) Halon 1211-Bromochloro difluoromethane i.e. CF2BrCI and (2) Halon 1301 -
Bromotrifluromethane CF2Br
Fixed Fire Installations: Fixed automatic fire installations are desirable from the design
stage, as they can be used for longer time and are more effective than the portable type.
Deflagration: It is an explosion with a resulting shock wave moving at a speed less than the
speed of sound in unreacted medium.
Deflagration is very rapid auto combustion of particles of explosive as a surface
phenomenon. It may be initiated by contact of a flame or spark but may be caused by impact
or friction. It is a characteristic of low explosives.
Deflagration or detonation is a form of explosion, the former is due to low burning
velocity (flame speed as 1 m/s) while the latter is due to high burning velocity (flame speed
as 2000-3000 m/s). A detonation generates high pressure and is more destructive than a
deflagration. The peak pressure caused by a deflagration in a closed vessel can reach up to
70-80 kPa (8 bar), whereas in case of detonation it easily reaches up to 200 kPa (20 bar).
A deflagration can turn into a detonation while travelling through a long pipe. In that
case deflagration velocity exceeds that mentioned above.
Detonation: It is an explosion with a resulting shock wave' moving at a speed more than the
speed of sound in unreacted medium.
Detonation is extremely rapid, self-propagating decomposition of an explosive accompanied
by a high pressure-temperature wave that moves at from 10009000 m/sec. It may be initiated
by mechanical impact, friction or heat. It is a characteristic of high explosives which varies
considerably in their sensitivity to shock, nitroglycerine being one of the most dangerous in
this regard. Whether a deflagration or detonation takes place depends on the material
involved and the conditions under which it occurs. A vapour phase explosion requires some
degree of confinement for a detonation to take place.
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Detonation of a gas-air mixture is possible directly by a powerful ignition source or by
transition from deflagration. Such transition requires a strong acceleration of the flame front.
It is possible in pipelines but rarely possible in vessels.
A number of substances are listed which can produce detonation in gas-air mixture.
Some commonly known substances are:
Acetone Ethylene
Acetylene Hydrogen
Benzene Methane
Chloroform Methanol
Cyclohexane Naphthalene
Diethyl ether Trichloro ethylene
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Session - 11
SALIENT FEATURES OF FIRE EXPLOSION AND TOXICITY INDEX
The most widely used relative ranking hazard index is Dow's Fire Explosion Index (F&EI) and
Mond's Toxicity Index (TI) together is called as Fire Explosion and Toxicity Index (FETI). It
involves objective evaluation of the realistic fire, explosion, toxicity and reactivity potential of
process or storage units.
Dow’s Fire and Explosion Index
The Dow’s Fire and Explosion Index is one the most common Preliminary Hazard Analysis
(PHA) adopted into the chemical and process engineering field.
As many other risk assessment analyses, the DOW FEI is based on historical data, gathered
from past report about industrial near-misses and accidents.
The primary goals of the analysis are:
QUANTIFY the harm (potential and realistic)
Identify the main hazardous source
COMMUNICATE the risk
FEI as part of the QRA, should be developed as part of a Risk Assessment procedure, after
the Hazard Identification Analysis (e.g. HAZOP).
The Analysis is applied to reach the quantification of the “realistic” maximum loss that can
occur in the plant. The Realistic Maximum Loss should not to be confused with the worst or
most dangerous scenario. In fact, great features of the analysis, is that the introduction, in
the calculation process, of several different parameters about the substance involve and
process safety parameters either, grants to develop a realistic picture of the process safety,
without the underestimation of the risk.
As part of the Quantified Risk Analysis school, the FEI arise doubt and skepticism about its
reliability, especially because is based on statistic model, and past experience. The FEI results
should be considered as an additional useful tool in the decision process.
The most relevant aspect to take into account during its application, is the common sense
and good critical judgment, especially for the results interpretation.
To develop the Dow FEI study, the following documents are required:
An Accurate Plot Plan
Data-sheets of the main Process Units
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A Fire & Explosion Index Hazard Classification Guide
A Complete Fire & Explosion Index Form with the additional annex
Replacement Cost Data for the installed process equipment under study
An excellent knowledge and experience about the process, or the plant is necessary required
by the operator. If the operator is a consultant, or generally, he did not work on the process
design, the analysis should be developed by a small team.
Preface:
The DOW’s FEI can be developed by a single operator or by a small team, composed by up to
three members. Greater is the number of the members, greater should be the complexity of
the plant or units analyzed and, consequently, the time required for the analysis.
Commonly, the best way to develop the analysis, is with a single operator with a deep
knowledge of the plant and FEI procedure either.
The first step of the study, is the IDENTIFICATION of the critical process unit. A process, or a
plant, could be composed by thousand items, and assess the risk related to each of them
could be time expensive and generally, useless. Only those items, or units, with a critical role
inside the process should be studied, and to simplify the identification of those units, the
process items are commonly divided into the following categories:
Raw Material Storage Tank/Vessels/Area
Process Stream Storage Tank/Vessels
Reactor Feed Pumps
Reactor(s)
Strippers
Recovery Vessels
Flash Drums
K.O. Drums
Other
Once that the plant has been divided into the unit categories, for each of them the critical
item(s) should be identified. The critical items are defined as “process units that could have
a relevant impact from process prevention standpoint “. Those items are commonly known as
“Appropriate” or “Pertinent” Process Units (APU or PPU).
General Criteria to choose those items are:
Substance(s) processed and its physical conditions
Quantity stored by the unit
Process Conditions (Normal or operating conditions)
Design Conditions
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Capital Density (Pound/ Square Foot)
Past Cases or Reports
Corrosion Data
Other
Primary Considerations:
The DOW FEI Analysis should be applied only on those items affected by the following points:
5000 lb or about 600 Gal of Flammable, Toxic, Explosive or reactive material are
restrained into the item.
Equipment arranged in series, not effectively isolated from each other’sneeds careful
consideration on the amount of material to be considered. On the other hand, rarely
more than 3 or 4 items, on the same circuit should be evaluated. Address risk to each
of them is a common error, overestimating the risk.
Pick and exact time which the study is applied to. For example, the start-up phase
could be the most dangerous one, but its conditions are limited for a brief session of
time. Commonly the normal process conditions should be applied, or abnormal
conditions identified by a previous hazard identification technique.
Calculation of The Material Factor:
The MF or Material Factor, it is the first parameter of the FEI calculation. The MF is the measure
of the intrinsic potential energy released by the combustion, explosion or chemical reaction
of the substances restrained in the equipment under study. The MF is calculated from the Nf
and Nr. Those parameters are NFPA rating expressing the flammability and reactivity of the
substance respectively.
Those factors are present into the NFPA 325M or NFPA 49 and are defined at ambient
temperature. The flammability, or in generally, the reactivity of substances rises with the
temperature. If the process condition is different from the ambient temperature, a corrective
factor must be adopted defined as “Temperature Adjustment of Material Factor”.
Mixtures and unlisted substances have a proper procedure to identify their material factor.
General Process Hazards Factor (F1):
The general Process Hazard Factors are six different elements, playing a primary role in the
hazardous scenario like explosions of fires. Those parameters are quite common, and are
applicable to most process situation. Although it may be not necessary to take all the penalties
they represent.
Once again, those parameters should be chosen on the normal or operational conditions or,
in generally, during the plant time phase chosen for the study.
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The six General process hazards factor are:
Exothermic Chemical Reaction(S)
Material Handling and Transfer
Enclosed or Indoor Process Units
Access to the area
Drainage and Spill Control
For each of this parameter, a different grade of penalty must be chosen. The severity of the
penalty is as greater as the parameter is more hazardous. To know the value of the penalty,
look at the DOW FEI guide.
Special Process Hazards Factor (F2):
The special process hazard are factors that contribute primarily to the probability of a loss
incident. There are twelve factors, describing the major cause and effect of the potential
incident. The factors are:
Toxic Material(S)
Sub Atmospheric Pressure (Under 500 mmHg)
Operation IN or NEAR Flammable Range
Dust Explosion
Relief Pressure
Low Temperature
Quantity of Flammable/Unstable Material
Corrosion and Erosion
Leakage – Joint and Packing
Use of Fire Equipment
Hot Oil Heat Exchanger System
Rotating Equipment
Each factor has contributed to a penalty range. The choice of the correct value is related to
the type and condition of the substance, process design, and grade of maintenance.
Process Unit Hazard Factor (F3):
The process Unit Hazard Factor is the product of the General Process Hazard Factor (F1) and
the Special Process Hazard Factor (F2). The Hazard Factors are multiplied, instead that
summed, because generally there are a compounding effect between them.
The Process Unit Hazard Factor is commonly present in the range 1 up to 8. If the final
calculation gives a Process Unit Factor greater than 8, use a maximum of 8.
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Determination of Fire and Explosion Index (FEI):
Once that the Material Factor and The Process Unit Hazard Factor have been calculated, the
Fire and Explosion Index can be estimated.
Fire & Explosion Index = (Process Unit Factor) x (Material Factor)
The Fire and Explosion Index is used for estimating the damage would probably result from
an incident in a process plant. The FEI estimates the harm caused by the potential loss of
control of the process. The direct and indirect effect of a fire/Explosion increase with the
degree of hazard related to the FEI.
Boiler: A closed vessel in which water is heated by combustion of fuels or heat from other
sources.
Unfired Pressure Vessels: air tanks, steam-jacketed kettles, digesters, vulcanizers, and other
such vessels
High-temperature Water: Water kept in closed systems under a high pressure so that it
remains a liquid rather than turning into steam.
Pressure Parts: Any component of a vessel, boiler, or water heater that retains steam, hot
water, or other fluids under pressure.
ASME: American Society of Mechanical Engineers
NBIC: National Board of Boiler and Pressure VesselInspector’s Code
Boilers and unfired pressure vessels found in workplaces—offices, hospitals, manufacturing
plants, hotels, garages, warehouses
Contents within vessels—gases, vapors, liquids, solids toxic and benign substances. Pressures
ranging from almost full vacuum to thousands of psi temperatures ranging from hundreds of
degrees below zero to well over 1000°F.
Safety Hazards
Fires
Burns
Explosions
Asbestos
BLEVE: boiling liquid expanding vapor explosion This is a type of explosion that can occur
when a vessel containing a pressurized liquid is ruptured. Such explosions can be extremely
hazardous.
A BLEVE results from the rupture of a vessel containing a liquid substantially above its
atmospheric boiling point. The substance is stored partly in liquid form, with a gaseous vapor
above the liquid filling the remainder of the container.
Boilers and high-temperature water heaters
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Four elements for compliance with ASME
Design
Fabrication
Testing
Installation
ASME Boiler and Pressure Vessel Code National Board of Boiler and Pressure Vessel Inspectors
Code governs inspection, repair, and alteration of boilers and pressure vessels after placed in
service Boilers and pressure vessel operations are strictly regulated within jurisdiction of
states, municipalities, or Canadian provinces.
Synopses of boiler and pressure vessel laws, rules, and regulations are available from Uniform
Boiler and Pressure Vessel Laws Society.
National Fire Protection Associations Boiler-Furnace Standards (NFPA 85A–85E) Compliance
with ASME is authorized by inspectors commissioned by the National Board of Boiler &
Pressure Vessel Inspectors—licensed by state or provincial governmental authority charged
with enforcement.
I Rules for Construction of Power Boilers
II Material Specifications
III Nuclear Power Facility Components
IV Rules for Construction of Heating Boilers
V Nondestructive Examination
VI Recommended Rules for Care and Operation of Heating Boilers
VII Recommended Rules for Care of Power Boilers
VIII Rules of Construction for Pressure Vessels
Division 1
Division 2—Alternate rules
Division 3—Alternate rules
IX Welding and Brazing Qualifications
X Fiberglass-Reinforced Plastic Pressure Vessels
XI Rules for In-service Inspection of Nuclear Power Plant Components
ASME Code—covers design, fabrication, and inspection requirements only during construction
of boilers and pressure vessels
NBIC Code—standard governs after initial installation of boiler or pressure vessels
NBIC guidelines for inspection, repair, alteration, rating, and rerating for the remainder of
boiler or pressure vessel’s service life Manufacturer’s instructions—installation and
maintenance
Train Operating Personnel:
to operate equipment properly
to make routine safety checks
to call qualified maintenance personnel if necessary or appropriate National Board
Certificates of authorization to perform specific tasks NB stamps
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Common Causes of Explosions in Pressure Vessels
Errors in design, construction, and installation, improper installation, human failure, and
inadequate training of operators, Corrosion/erosion of construction materials; Failure or
intentional defeat of safety devices; failure or override of automatic control devices; Failure
to inspect and test thoroughly, properly, and frequently; Improper application of equipment;
over-firing Lack of planned preventive maintenance; Establish testing and servicing program.
Always test safety and relief valves under pressure. Have repairs made immediately.
Check and service the boiler in and out of heating season. Keep a boiler log.
Unfired Pressure Vessels (UPV)
Vessels designed to contain fluids under internal pressure or vacuum and not heated directly
through the combustion of fuels or other external heat sources. Found in commercial and
industrial facilities. Heat can be generated from chemical reactions within vessel or by
applying a heating medium within the vessel or circulating it around the vessel (jacket).
Examples: compressed air tanks, propane tanks, deaerators and condensate tanks, steam-
jacketed kettles, pulp mill digesters, rubber vulcanizers
UPV Design
ASME Code UPV exceptions: vessels subject to federal regulations vessels with a nominal
capacity of 120 gal (450 l) or less of water under pressure, in which any trapped air serves
only as a cushion vessel having an internal or external operating pressure not exceeding 15
psi (103 kPa), with no limitation on size; vessels with an inside diameter not exceeding 6 in.
(15 cm), and no limitation on pressure hot-water storage tanks heated by steam or other
indirect means heat input of 200,000 Btu (59,000 J/s) or less, water temperature of 200ºF
(93ºC) or less, and nominal capacity of 120 gal (450 l) or less.
UPV—Code Divisions
Design: ASME Code, Section VIII
Division I—normally covers vessels with ratings of 3,000 psi or less.
Division II—normally covers vessels used at pressures exceeding 3,000 psi.
Other Codes—American Petroleum Institute’s code or state and local codes may be enforced.
Codes may impose size or service limits more restrictive than ASME Code. Secondhand vessels
Prior to purchase, written report that equipment meets requirements of jurisdiction where it
is to be installed. Have the equipment inspected by a NB licensed inspector.
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Session - 12
ASSESSMENT OF RELIABILITY OF VESSELS, TEST CHECKS
Methods of Pressure Vessel Testing
Pressure vessels need to be structurally sound to maintain their internal pressure and not to
allow whatever material is contained inside to leak out. Testing is intended to ensure that
pressure vessels don’t contain any flaws like punctures, cracks or loose connections that could
compromise their efficacy.
Two primary types of tests that are performed on pressure vessels include hydrostatic and
pneumatic tests. The key difference between these two types is that hydrostatic testing uses
water as the test medium, and pneumatic testing uses a non-flammable, non-toxic gas like
air or nitrogen.
A concern with pneumatic testing is that, if a fracture occurs during testing for some reason,
it could lead to an explosion. This makes hydrostatic testing a safer option since the volume
of water does not rapidly increase when it is suddenly depressurized. However, there are
situations where pneumatic testing is a viable option.
Hydrostatic testing involves filling a vessel entirely with water, pressurizing it up to one and
a half times its design pressure limit and then watching for any leakage. Adding a tracer or
a fluorescent dye to the water inside can make it even easier to see where there may be
leaks. Hydrostatic testing could cause damage to a pressure vessel if the water is pressurized
too much or if the pressure causes a small fracture to spread rapidly.
Beyond these basic types of testing, OSHA identifies five non-destructive testing (NDT), also
called non-destructive examination (NDE), methods that are widely used on pressure vessels:
1. Visual Test (VT)
One type of testing is a visual inspection, which can give a good overview of a vessel’s general
condition. First making sure the surface of a vessel is clean and well-lit, pressure vessel
inspectors will examine any part of the vessel they can observe. They will look at things like
any welded seams, such as those around appendages or along the length of the vessel’s shell.
They may see that the vessel appears to be in good working condition, or they may observe
issues like cracking, corrosion, erosion or hydrogen blistering. While a visual inspection can
reveal some problems, it can only take you so far. Some other non-destructive testing
methods can further reveal whether a pressure vessel’s construction and function are sound.
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2. Liquid Penetrant Test (PT)
A liquid penetrant test is a means of checking for flaws on a pressure vessel that are open to
the surface. First, an inspector flows a very thin liquid, known as a penetrant, into the possible
flaw. Typically, the penetrant is sprayed on and allowed time to soak in. A tester can add a
fluorescent chemical to the penetrant so that it will be even more visible under ultraviolet
light.
Two primary types of tests that are performed on pressure vessels include hydrostatic and
pneumatic tests. The key difference between these two types is that hydrostatic testing uses
water as the test medium, and pneumatic testing uses a non-flammable, non-toxic gas like
air or nitrogen.
A concern with pneumatic testing is that, if a fracture occurs during testing for some reason,
it could lead to an explosion. This makes hydrostatic testing a safer option since the volume
of water does not rapidly increase when it is suddenly depressurized. However, there are
situations where pneumatic testing is a viable option.
Hydrostatic testing involves filling a vessel entirely with water, pressurizing it up to one and
a half times its design pressure limit and then watching for any leakage. Adding a tracer or
a fluorescent dye to the water inside can make it even easier to see where there may be
leaks. Hydrostatic testing could cause damage to a pressure vessel if the water is pressurized
too much or if the pressure causes a small fracture to spread rapidly.
After letting the penetrant dry, the inspector then wipes off the penetrant left on the surface
and uses a developer to draw out any penetrant that has seeped into cracks. As the penetrant
comes up to the surface, it reveals the magnitude of the problem. This method of testing is
most often used on welded seams, but it can also be used on bars, plates, pipes and more.
3. Magnetic Particle Test (MT)
In a magnetic particle test, the inspector runs a magnetic current through the pressure vessel,
typically using the prod method, where an electric current flow between contact probes. If
there are any defects in the shell’s material, a “flux leakage field” will appear. In other words,
these defects will interrupt the flow of the magnetic current, causing magnetism to spread
out from them.
The flux leakage fields become visible when the inspector spreads ferromagnetic particles on
the vessel. In a wet magnetic particle test, these particles consist of a wet suspension in a
liquid, and in a dry magnetic particle test, they consist of a dry powder. As with the liquid
penetrant test, the particles can be treated, so they fluoresce under black light. As the metal
particles are attracted to the magnetic current, they reveal the approximate dimensions of
any flaws that have created flux leakage fields.
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4. Radiographic Test (RT)
Unlike the previous three methods, radiography can detect more than flaws that are near or
on the surface — it is a volumetric method, meaning it can detect issues inside the vessel.
Radiography uses gamma or X-rays to produce a picture of the vessel. Just as with medical
radiography, holes, discontinuities and other differences in density decrease the attenuation
of the X-ray, leading to greater exposure on the film. On the negative film, these more
exposed areas appear darker. Open voids will appear pretty obvious in a radiographic image,
but more minor cracks can be elusive. Ionizing radiation can be dangerous, so these tests
should only be conducted by an experienced professional. An inspector must also be
experienced to accurately interpret the image that is produced and correctly identify if and
where there are any defects in the pressure vessel.
5. Ultrasonic Testing (UT)
Ultrasonic testing is another volumetric method. It uses sound waves to measure a material’s
thickness or detect any defects. An electronic system produces high-voltage electrical pulses,
and in return, a transducer creates high-frequency ultrasonic energy. As the ultrasonic sound
waves move through the material, if they encounter a discontinuity, the discontinuity will
reflect back some of the energy. The transducer converts this reflected wave into an electrical
signal, which is then shown on a display.
Generally speaking, ultrasonic testing must be read in real-time since it doesn’t produce a
lasting record like radiography does. However, some modern UT equipment is designed with
a means of recording the signals.
Pressure Vessel Testing Benefits
Pressure vessel testing, as we’ve seen, is required at certain stages, but it’s also something
all manufacturers and end users should want to prioritize since it’s so critical to maintaining
their operations and people’s safety. If a pressure vessel holds a poisonous gas, a rupture
could allow for a dangerous gas leak. Even if the material inside isn’t poisonous, a ruptured
vessel could lead to an explosion or a serious fire.
An event like this could quickly bring your operations to a halt. Consider what operations in
your business are, in some way, dependent on a pressure vessel. Now imagine those
operations ceasing until the pressure vessel is replaced. Unplanned downtime can result in a
great financial loss.
An accident from a pressure vessel could also severely damage equipment within the vicinity
of the pressure vessel. It could cost hundreds, thousands or even millions of dollars to replace
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the damaged equipment. And, of course, any equipment out of commission will add to the
issue of unplanned downtime.
An even graver consequence than financial loss is if any workers are poisoned or caught up
in an explosion or fire. Employers are responsible for maintaining a safe working environment
for their workers, and an injury or death due to a faulty pressure vessel can seriously
compromise this environment.
None of this will seem important if you assume your pressure vessels are in good shape. While
this is hopefully the case, you cannot know for sure unless your vessels are tested. The results
of a pressure vessel inspect may surprise you. According to OSHA, recent pressure vessel
inspections have revealed the fact that many pressure vessels in workplaces are cracked or
damaged.
Regular inspections can make all the difference in preventing a dangerous failure. Pressure
vessel inspection frequency depends on a variety of factors, but a general rule of thumb is
that you should have your pressure vessel inspected every five years. These inspections
should be thorough and involve a visual inspection, a hydrostatic pressure test, a thickness
evaluation, a stress analysis and an inspection of any pressure release valves.
Pressure Vessel Inspection Services from NTS
Whether you’re a manufacturer of pressure vessels or an end user who depends on storage
tanks, heat exchangers or process vessels for your operations, consider partnering with NTS
for pressure vessel inspection services. NTS is a private test, inspection and certification
company with a longstanding reputation for excellence. For more than 50 years, we have
acquired the expertise and physical capabilities to offer an impressively wide range of
engineering services. We have partnered with the defense, aerospace and automotive
industries, just to name a few.
We have an ever-expanding network of laboratories and certification houses across the U.S.
We work with your engineering team to make sure we meet your needs and provide any
necessary solutions. If you’re looking to get a pressure vessel to market and want to avoid
delays, our team can help you make sure your product meets all standards for safety and
quality promptly.
For example, we can conduct pneumatics testing safely and efficiently in a controlled
environment. Our NTS Santa Clarita location, which is accredited like all of our testing
facilities, is home to our largest pneumatics test capabilities. Here, we can test factors like air
flow, management subsystems for pressure and temperature, as well as compressed air
components. We can also test countless other aspects of your products, including factors like
structural stress, external thermal control and vibration.
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Session - 13
INSPECTION TECHNIQUES
5 Most Popular Inspection methods
The purpose of this article is to outline 5 popular methods: visual inspection, ultrasonic
techniques, radiography, thermography and acoustic emissions. Each of these methods is
explained, followed by a qualitative discussion of its implementation.
Method 1: Visual Inspection
Visual inspection is an inexpensive method for detecting equipment flaws and defects. A
trained technician is likely to detect improper structural installations, certain types of
impending structural failure, welding flaws, corrosion development, and cracks.
Method 2: Ultrasonic techniques
Ultrasonic testing utilizes sound waves whose frequencies (50 kHz - 50 MHz) are above the
audible range for the human ear.
The piezo-electric effect of the ultrasonic transducer makes it possible to transmit and receive
from within the equipment. The instrument makes it possible to inspect the internal structure
of the equipment, and to detect thickness changes, welds, cracks, voids, delamination and
other types of material or structural defects.
The limitation of this method is that data acquisition and evaluation depend on the expertise
of the technician. This makes it difficult to arrive at non-subjective readings and precision.
Method 3: Radiography
Radiographic methods utilize X-ray or gamma rays (electromagnetic radiation) to examine
the internal structure and integrity of the equipment. Because these waves have short wave
lengths, they can penetrate and travel through structural materials such as steel and metallic
alloys.
In the oil and gas industry, this NDT method is useful for inspecting welds on pipelines and
pressure vessels. It is also useful for inspecting non-metallic materials such as concrete and
ceramics. Operating this type of NDT requires conformance to safety regulations.
Method 4: Thermography
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Thermographic inspection measures the difference between the temperature of a pipeline and
the surrounding environment. The measurement helps to detect (a) defects in pipeline
insulation, and (b) leakage of oil or gas.
The marriage of this NDT technique and drone services will make this method of inspection
more efficient and cost effective.
Method 5: Acoustic Emissions
This method detects the presence of rarefaction waves produced by leaks in pipelines. When
a fluid leak occurs, negative pressure waves propagate in both directions within the pipeline.
Detection of these acoustic waves helps identify leakage in pipelines.
Conclusion
This article presents a summary of 5 popular inspection methods, their implementation, and
their limitations. It is not possible within the scope of the article to present an exhaustive
discussion on all NDT inspection methods. However, the article highlights the importance of
NDT methods for the oil and gas industry.
This session discusses the prominent types of erosion and what are the causal factors behind
them. Learners will also learn about methods to prevent corrosion/erosion.
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Session - 14
CORROSION, EROSION, CAUSES, INSPECTION AND PREVENTION
Reasons of Pressure Vessel Failure are many. Wrong selection of material of construction,
mechanical failure due to overpressure, overheating, external loading (e.g. platform, stairs,
ladders, supports, brackets etc.),excessive stress (uneven or over tightening), brittle fracture,
creep (due to fire or maloperation), mechanical fatigue and shock (due to pressure or flow
variations, vibrations, expansion effects), thermal fatigue and shock (due to temperature
difference and rate of change of temperature), hydrogen attack (blistering or embrittlement)
and corrosion failure are some of the reasons.
Corrosion is an electrochemical reaction between a metal and its environment. It
results in a loss of metal or weakening of it Corrosion reaches deeply, creates maintenance
problems and incurs cost of loss in lacs of rupees over the years.
Corrosion failure has also many reasons to occur. General, local and external corrosion,
galvanic, crevice, knife-line, intergranular and stress-related corrosion, scaling, exfoliation,
corrosion pitting and erosion are some common types of corrosion in process plants including
pressure vessels.
Corrosion due to oxidation at high temperature is called scaling, e.g. steam boilers.
Exfoliation is a type of scaling caused by oxidation in steam atmosphere e.g. feed water
heaters. General corrosion takes place due to a corrosive chemical or impurity over the
exposed surface.
Inter granular corrosion occurs in stainless steels heated upto 500-800 °C and then
exposed to corrosive conditions.
Galvanic corrosion happens due to current flowing between two dissimilar metals which
form a galvanic cell. It occurs when two such metals are joined together at a weld. A typical
pair is iron and copper. Corrosion pitting results from electrochemical potential set up by
differences of oxygen concentration inside and outside the pit. The oxygen-lean part acts as
anode and the metal surface as cathode.
Knife-line corrosion takes place between parent and weld metals, e.g. austenitic stainless
steels. Crevice or contact corrosion occurs at the point of contact of a metal and non-metallic
material, e.g. threaded joints. Erosion is a type of corrosion and is caused by flow restriction
or change of direction, e.g. elbows, tees, baffles, nozzles and valves and point opposite to
inlet nozzle. It is increased if the flow contains solid particles or by bubbles in liquids and by
two phase flow. Wet steam flow, air jet flashing flow and pump cavitation can cause severe
erosion. External corrosion occurs by material of insulation. Leaching of chloride salts from
insulation can corrode pipe work.
Underground piping can be corroded by soil due to electrochemical action and cathodic
protection is used to control it.
Stress corrosion cracking is the result of corrosion and static tensile stresses. Corrosion
fatigue is caused by corrosion and by alternating fatigue stresses. Chlorides are a common
cause of stress corrosion cracking. Stress may be internal or external. Stress corrosion
cracking caused by an alkaline solution is known as caustic embrittlement, which has been a
frequent cause of failure in boilers. Therefore, treatment to boiler feed water (removal of
caustic and chloride content) is necessary. Control measures are elimination of corrodents,
reduction of residual stresses and vibrations etc.
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In plants handling nitric acid and nitrates, "nitrate stress corrosion cracking" of mild
steel is possible. This was the reason of crack in the reactor at Flixborough resulting in removal
of the reactor and temporary installation of the 20" pipe which gave way and the disaster
took place.
At high stresses and temperatures, traces of other metals like zinc can cause rapid
and severe zinc embrittlement of some types of steels. Wetting of the steel by molten zinc is
a favorable condition to zinc embrittlement. This may cause local fire and catastrophic failure.
To avoid this, zinc-coated items should not be placed in direct contact with stainless steel or
in positions where they can drip molten zinc on it. For example, galvanized wire netting used
in insulation should not be in direct contact with stainless steel pipe. During welding and
fabrication, zinc contamination of stainless steel should be prevented. Special metallurgical
examination will reveal zinc embrittlement.
Corrosion Prevention is of high importance as it prevents accidents and reduces cost
of corroded materials. Substitution of non-corrosive or less corrosive material (e.g. SS instead
of MS) tolerated by the process technology and economy and selection of such material from
the design and erection stage avoids most of the corrosion problems. Then selection of powder
coated metal parts (sheets, structural members, machine parts, guards, covers etc.) instead
of painted, give long life. Mild steel parts of tanks structures piping, machines and vessels
must be regularly painted by anti-corrosive paints. Protection from rain and plant water,
dripping and leaking of corrosive chemicals, oxidation and contact of zinc and copper is
necessary. Rapid cleaning of spillage, good housekeeping, cathodic protection, control of flow,
fluctuations and vibrations, water softening and removal of salts, checking of scale formation
on plates and tubes, thickness measurement and defect monitoring by NDT methods stated
in foregoing Part 9.5.2 and latest instruments and equipment, scanning by computer
methods, descaling, de-choking, scrapping, timely repairing and preventive maintenance are
also useful to avoid corrosion and erosion.
More Methods to Stop Corrosion and Erosion:
1. Two compatible metal prevent or slow down the rate of corrosion.
2. A strategically placed gasket i.e.to provide insulating material between the two metals.
3. Cathodic protection and conversion coating.
4. Crevice corrosion can be avoided by choosing materials having corrosion resistance.
Stainless steels are prone to crevice corrosion and not recommended for such use.
5. Dezincification (removal of zinc from brass) can be prevented by using alloys of brass
containing Sn, As, P or Sb.
6. Use of non-metallic material like plastic.
7. Applying monomolecular film(inhibitor)of grease, paint, synthetic organic coating or a
plastic sheet (liner) over the surface.
8. Use of oxygen scavengers (e.g. Sodium sulphite and hydrazine) to add into boiler water to
remove oxygen.
9. Inhibitors like phosphonates are used in cooling water for corrosion control.
10. Use of acid pickling as corrosion inhibitors.
11. Use of heavy oils or greases, waxes dissolved in solvents or sulphonate salts dissolved in
petroleum as a barrier between die environment and the metal surface.
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However, it should be verified that chemicals being selected as inhibitors should not be
carcinogenic as they can cause cancer. For example, p-t-butyl benzoic acid, sodium nitrite,
nitrosamines, thiourea etc. are carcinogenic and should not be used.
12. Non-metallic materials like plastic, rubber and synthetic elastomers can also be
attached by corrosion or cracking due to solvent, environmental stress or thermal effect.
Corrosion process in plastic takes place because of swelling, softening or loss of physical
properties. Polyurethane, polyethylene, polystyrene, ABS, acetal homopolymers and
polyethersulfone are the plastics having good resistance against corrosion. Rubber lining (e.g.
chloroprene, nitrile and butyl rubber) on steel tank prohibits attack of strong acids.
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Session - 15
PLANT COMMISSIONING
Pressure Vessel:
Horizontal Pressure Vessel in Steel
A pressure vessel is a container designed to hold gases or liquids at a pressure substantially
different from the ambient pressure.
Pressure vessels can be dangerous, and fatal accidents have occurred in the history of their
development and operation. Consequently, pressure vessel design, manufacture, and
operation are regulated by engineering authorities backed by legislation. For these reasons,
the definition of a pressure vessel varies from country to country.
Design involves parameters such as maximum safe operating pressure and temperature,
safety factor, corrosion allowance and minimum design temperature (for brittle fracture).
Construction is tested using nondestructive testing, such as ultrasonic testing, radiography,
and pressure tests. Hydrostatic tests use water, but pneumatic tests use air or another gas.
Hydrostatic testing is preferred, because it is a safer method, as much less energy is released
if a fracture occurs during the test (water does not rapidly increase its volume when rapid
depressurization occurs, unlike gases like air, which fail explosively).
In most countries, vessels over a certain size and pressure must be built to a formal code. In
the United States that code is the ASME Boiler and Pressure Vessel Code (BPVC). These
vessels also require an authorized inspector to sign off on every new vessel constructed and
each vessel has a nameplate with pertinent information about the vessel, such as maximum
allowable working pressure, maximum temperature, minimum design metal temperature,
what company manufactured it, the date, its registration number (through the National
Board), and ASME's official stamp for pressure vessels (U-stamp). The nameplate makes the
vessel traceable and officially an ASME Code vessel.
The earliest documented design of pressure vessels is described in the book Codex Madrid I,
by Leonardo da Vinci, in 1495, where containers of pressurized air were theorized to lift
heavy weights underwater, however vessels resembling what are used today did not come
about until the 1800s where steam was generated in boilers helping to spur the industrial
revolution. However, with poor material quality and manufacturing techniques along with
improper knowledge of design, operation and maintenance there was a large number of
damaging and often fatal explosions associated with these boilers and pressure vessels,
with a death occurring on a nearly daily basis in the United States. Local providences and
states in the US began enacting rules for constructing these vessels after some particularly
devastating vessel failures occurred killing dozens of people at a time, which made it
difficult for manufacturers to keep up with the varied rules from one location to another and
the first pressure vessel code was developed starting in 1911 and released in 1914, starting
the ASME Boiler and Pressure Vessel Code (BPVC).[1] In an early effort to design a tank
capable of withstanding pressures up to 10,000 psi (69 MPa), a 6-inch (150 mm) diameter
tank was developed in 1919 that was spirally-wound with two layers of high tensile strength
steel wire to prevent sidewall rupture, and the end caps longitudinally reinforced with
lengthwise high-tensile rods. The need for high pressure and temperature vessels for
petroleum refineries and chemical plants gave rise to vessels joined with welding instead of
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rivets (which were unsuitable for the pressures and temperatures required) and in 1920s
and 1930s the BPVC included welding as an acceptable means of construction, and welding
is the main means of joining metal vessels today.
There have been many advancements in the field of pressure vessel engineering such as
advanced non-destructive examination, phased array ultrasonic testing and radiography,
new material grades with increased corrosion resistance and stronger materials, and new
ways to join materials such as explosion welding (to attach one metal sheet to another,
usually a thin corrosion resistant metal like stainless steel to a stronger metal like carbon
steel), friction stir welding (which attaches the metals together without melting the metal),
advanced theories and means of more accurately assessing the stresses encountered in
vessels such as with the use of Finite Element Analysis, allowing the vessels to be built safer
and more efficiently. Today vessels in the USA require BPVC stamping but the BPVC is not
just a domestic code, many other countries have adopted the BPVC as their official code.
There are, however, other official codes in some countries (some of which rely on portions
of and reference the BPVC), Japan, Australia, Canada, Britain, and Europe have their own
codes. Regardless of the country nearly all recognize the inherent potential hazards of
pressure vessels and the need for standards and codes regulating their design and
construction.
Pressure Vessel Features
Shape of a Pressure Vessel
Pressure vessels can theoretically be almost any shape, but shapes made of sections of
spheres, cylinders, and cones are usually employed. A common design is a cylinder with end
caps called heads. Head shapes are frequently either hemispherical or dished (tori spherical).
More complicated shapes have historically been much harder to analyze for safe operation
and are usually far more difficult to construct.
Spherical gas container
Cylindrical pressure vessel.
Picture of the bottom of an aerosol spray can.
Fire Extinguisher with rounded rectangle pressure vessel
Theoretically, a spherical pressure vessel has approximately twice the strength of a cylindrical
pressure vessel with the same wall thickness,and is the ideal shape to hold internal pressure.
However, a spherical shape is difficult to manufacture, and therefore more expensive, so most
pressure vessels are cylindrical with 2:1 semi-elliptical heads or end caps on each end.
Smaller pressure vessels are assembled from a pipe and two covers. For cylindrical vessels
with a diameter up to 600 mm (NPS of 24 in), it is possible to use seamless pipe for the shell,
thus avoiding many inspection and testing issues, mainly the nondestructive examination of
radiography for the long seam if required. A disadvantage of these vessels is that greater
diameters are more expensive, so that for example the most economic shape of a 1,000 litres
(35 cu ft), 250 bars (3,600 psi) pressure vessel might be a diameter of 91.44 centimeters
(36 in) and a length of 1.7018 meters (67 in) including the 2:1 semi-elliptical domed end
caps.
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Construction Materials
Composite overwrapped pressure vessel with titanium liner
Many pressure vessels are made of steel. To manufacture a cylindrical or spherical pressure
vessel, rolled and possibly forged parts would have to be welded together. Some mechanical
properties of steel, achieved by rolling or forging, could be adversely affected by welding,
unless special precautions are taken. In addition to adequate mechanical strength, current
standards dictate the use of steel with a high impact resistance, especially for vessels used in
low temperatures. In applications where carbon steel would suffer corrosion, special corrosion
resistant material should also be used.
Some pressure vessels are made of composite materials, such as filament wound composite
using carbon fiber held in place with a polymer. Due to the very high tensile strength of carbon
fibre these vessels can be very light, but are much more difficult to manufacture. The
composite material may be wound around a metal liner, forming a composite overwrapped
pressure vessel.
Other very common materials include polymers such as PET in carbonated beverage
containers and copper in plumbing.
Pressure vessels may be lined with various metals, ceramics, or polymers to prevent leaking
and protect the structure of the vessel from the contained medium. This liner may also carry
a significant portion of the pressure load.
Pressure Vessels may also be constructed from concrete (PCV) or other materials which are
weak in tension. Cabling, wrapped around the vessel or within the wall or the vessel itself,
provides the necessary tension to resist the internal pressure. A "leakproof steel thin
membrane" lines the internal wall of the vessel. Such vessels can be assembled from
modular pieces and so have "no inherent size limitations".There is also a high order of
redundancy thanks to the large number of individual cables resisting the internal pressure.
Safety Features
Leak before burst
Leak before burst describes a pressure vessel designed such that a crack in the vessel will
grow through the wall, allowing the contained fluid to escape and reducing the pressure,
prior to growing so large as to cause fracture at the operating pressure.
Many pressure vessel standards, including the ASME Boiler and Pressure Vessel
Code[citation needed] and the AIAA metallic pressure vessel standard, either require
pressure vessel designs to be leak before burst, or require pressure vessels to meet more
stringent requirements for fatigue and fracture if they are not shown to be leak before
burst.
Safety Valves
Example of a valve used for gas cylinders.
As the pressure vessel is designed to a pressure, there is typically a safety valve or relief
valve to ensure that this pressure is not exceeded in operation.
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Maintenance Features
Pressure vessel closures
Pressure vessel closures are pressure retaining structures designed to provide quick access
to pipelines, pressure vessels, pig traps, filters and filtration systems. Typically pressure
vessel closures allow maintenance personnel.
Uses
An LNG carrier ship with four pressure vessels for liquefied natural gas. Pressure vessels are
used in a variety of applications in both industry and the private sector. They appear in
these sectors as industrial compressed air receivers and domestic hot water storage tanks.
Other examples of pressure vessels are diving cylinders, recompression chambers,
distillation towers, pressure reactors, autoclaves, and many other vessels in mining
operations, oil refineries and petrochemical plants, nuclear reactor vessels, submarine and
space ship habitats, pneumatic reservoirs, hydraulic reservoirs under pressure, rail vehicle
airbrake reservoirs, road vehicle airbrake reservoirs, and storage vessels for liquefied gases
such as ammonia, chlorine, and LPG (propane, butane). A unique application of a pressure
vessel is the passenger cabin of an airliner: the outer skin carries both the aircraft
maneuvering loads and the cabin pressurization loads.
Safety and Health Codes Board to Formulate Rules& Regulations
The Board is authorized to formulate definitions, rules, regulations and standards which
shall be designed for the protection of human life and property from the unsafe or
dangerous construction, installation, inspection, operation, maintenance and repair of
boilers and pressure vessels in this Commonwealth.
In promulgating such rules, regulations and standards, the Board shall consider any or all of
the following:
1. Standards, formulae and practices generally accepted by recognized engineering and safety
authorities and bodies.
2. Previous experiences based upon inspections, performance, maintenance and operation.
3. Location of the boiler or pressure vessel relative to persons.
4. Provisions for operational controls and safety devices.
5. Interrelation between other operations outside the scope of this chapter and those covered
by this chapter.
6. Level of competency required of persons installing, constructing, maintaining or operating
any equipment covered under this chapter or auxiliary equipment.
7. Federal laws, rules, regulations and standards.
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Installations, Repairs and Alterations Conforming to Rules and Regulations
(a) No boiler or pressure vessel which does not conform to the rules and regulations of the
Board governing new construction and installation and which has been certified by the Board
shall be installed or operated in this Commonwealth after twelve months from July 1, 1973.
Prior to such date no boiler or pressure vessel shall be installed and operated unless it is in
conformity with the rules and regulations established pursuant to this chapter which were in
existence on July 1, 1972.
(b) This chapter shall not be construed as in any way preventing the use, sale or reinstallation
of a boiler or pressure vessel constructed prior to July 1, 1972, provided it has been made to
conform to the rules and regulations of the Board governing existing installations prior to its
reinstallation or operation.
(c) Repairs and alterations shall conform to the rules and regulations set forth by the Board.
Exemptions
The provisions of this article shall not apply to any of the following:
1. Boilers or unfired pressure vessels owned or operated by the federal government or any
agency thereof;
2. Boilers or fired or unfired pressure vessels used in or on the property of private residences
or apartment houses of less than four apartments;
3. Boilers of railroad companies maintained on rail borne vehicles or those used to propel
waterborne vessels;
4. Hobby or model boilers as defined in
5. Hot water supply boilers, water heaters, and unfired pressure vessels used as hot water
supply storage tanks heated by steam or any other indirect means when the following
limitations are not exceeded:
a. A heat input of 200,000 British thermal units per hour;
b. A water temperature of 210 degrees Fahrenheit;
c. A water-containing capacity of 120 gallons;
6. Unfired pressure vessels containing air only which are located on vehicles or vessels
designed and used primarily for transporting passengers or freight;
7. Unfired pressure vessels containing air only, installed on the right-of-way of railroads and
used directly in the operation of trains;
8. Unfired pressure vessels used for containing water under pressure when either of the
following are not exceeded:
a. A design pressure of 300 psi; or
b. A design temperature of 210 degrees Fahrenheit;
9. Unfired pressure vessels containing water in combination with air pressure, the
compression of which serves only as a cushion, that do not exceed:
a. A design pressure of 300 psi;
b. A design temperature of 210 degrees Fahrenheit; or
c. A water-containing capacity of 120 gallons;
10. Unfired pressure vessels containing air only, providing the volume does not exceed eight
cubic feet nor the operating pressure is not greater than 175 pounds;
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11. Unfired pressure vessels having an operating pressure not exceeding fifteen pounds with
no limitation on size;
12. Pressure vessels that do not exceed:
a. Five cubic feet in volume and 250 pounds per square inch gauge pressure;
b. One and one-half cubic feet in volume and 600 pounds per square inch gauge
pressure; and
c. An inside diameter of six inches with no limitations on gauge pressure;
13. Pressure vessels used for transportation or storage of compressed gases when
constructed in compliance with the specifications of the United States Department of
Transportation and when charged with gas marked, maintained, and periodically requalified
for use, as required by appropriate regulations of the United States Department of
Transportation;
14. Stationary American Society of Mechanical Engineers (ASME) LP-Gas containers used
exclusively in propane service with a capacity that does not exceed 2,000 gallons if the owner
of the container or the owner's servicing agent:
a. Conducts an inspection of the container not less frequently than every five years, in which
all visible parts of the container, including insulation or coating, structural attachments, and
vessel connections, are inspected for corrosion, distortion, cracking, evidence of leakage, fire
damage, or other condition indicating impairment;
b. Maintains a record of the most recent inspection of the container conducted in accordance
with subdivision a; and
c. Makes the records required to be maintained in accordance with subdivision b available for
inspection by the Commissioner;
15. Unfired pressure vessels used in and as a part of electric substations owned or operated
by an electric utility, provided such electric substation is enclosed, locked, and inaccessible to
the public; or
16. Coil type hot water boilers without any steam space where water flashes into steam when
released through a manually operated nozzle, unless steam is generated within the coil or
unless one of the following limitations is exceeded:
a. Three-fourths inch diameter tubing or pipe size with no drums or headers attached;
b. Nominal water containing capacity not exceeding six gallons; and
c. Water temperature not exceeding 350 degrees Fahrenheit.
Employment and Appointment of Inspectors and Other Personnel
The Commissioner is authorized to employ persons to enforce the provisions of this chapter
and the regulations of the Board. He shall be authorized to require examinations or other
information which he deems necessary to aid him in determining the fitness, competency,
and professional or technical expertise of any applicant to perform the duties and tasks to be
assigned.
The Commissioner is authorized to appoint a Chief Inspector and to certify special inspectors
who shall meet all qualifications set forth by the Commissioner and the Board. Special
inspectors shall be authorized to inspect specified premises and without cost or expense to
the Commonwealth. Reports of all violations of the regulations or of this chapter shall be
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immediately made to the Commissioner. Other reports shall be made as required by the
Commissioner.
Examination of Inspectors; Certificate of Competency Required
A. All applicants for the position of inspector authorized by § 40.1-51.9 shall be required to
have successfully completed an examination monitored by the Examining Board and to have
received a certificate of competency from the Commissioner prior to commencing their duties.
A fee as set under subsection A of shall be charged each applicant taking the inspector's
examination.
B. Each inspector holding a valid certificate of competency and who conducts inspections, as
provided by this chapter, shall be required to obtain an identification card biennially, not later
than June 30 of the year in which the identification card is required. Application for the
identification card shall be made on forms furnished by the Department upon request. Each
application shall be submitted to the Department, accompanied by a post-office money order
or check drawn to the order of the Treasurer of Virginia in the amount as set under subsection
A
Financial Responsibility Requirements for Contract Fee Inspectors
A. Contract fee inspectors inspecting or certifying regulated boilers or pressure vessels in the
Commonwealth shall maintain evidence of their financial responsibility, including
compensation to third parties, for bodily injury and property damage resulting from, or
directly relating to, an inspector's negligent inspection or recommendation for certification of
a boiler or pressure vessel.
B. Documentation of financial responsibility, including documentation of insurance or bond,
shall be provided to the Chief Inspector within thirty days after certification of the inspector.
The Chief Inspector may revoke an inspector's certification for failure to provide
documentation of financial responsibility in a timely fashion.
C. The Safety and Health Codes Board is authorized to promulgate regulations requiring
contract fee inspectors, as a condition of their doing business in the Commonwealth, to
demonstrate financial responsibility sufficient to comply with the requirements of this chapter.
Regulations governing the amount of any financial responsibility required by the contract fee
inspector shall take into consideration the type, capacity and number of boilers or pressure
vessels inspected or certified.
D. Financial responsibility may be demonstrated by self-insurance, insurance, guaranty or
surety, or any other method approved by the Board, or any combination thereof, under the
terms the Board may prescribe. A contract fee inspector whose financial responsibility is
accepted by the Board under this subsection shall notify the Chief Inspector at least thirty
days before the effective date of the change, expiration, or cancellation of any instrument of
insurance, guaranty or surety.
E. Acceptance of proof of financial responsibility shall expire on the effective date of any
change in the inspector's instrument of insurance, guaranty or surety, or the expiration date
of the inspector's certification. Application for renewal of acceptance of proof of financial
responsibility shall be filed thirty days before the date of expiration.
F. The Chief Inspector, after notice and opportunity for hearing, may revoke his acceptance
of evidence of financial responsibility if he determines that acceptance has been procured by
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fraud or misrepresentation, or a change in circumstances has occurred that would warrant
denial of acceptance of evidence of financial responsibility under this section or the
requirements established by the Board pursuant to this section.
G. It is not a defense to any action brought for failure to comply with the requirement to
provide acceptable evidence of financial responsibility that the person charged believed in
good faith that the owner or operator of an inspected boiler or pressure vessel possessed
evidence of financial responsibility accepted by the Chief Inspector or the Board.
Right of Access to Premises; Certification and Recertification; Inspection
Requirements
A. The Commissioner, his agents or special inspectors shall have free access, during
reasonable hours to any premises in the Commonwealth where a boiler or pressure vessel is
being constructed, operated or maintained, or is being installed to conduct a variance review,
an owner-user inspection agency audit, an emergency repair review, an accident
investigation, a violation follow-up, and a secondhand or used boiler review for the purpose
of ascertaining whether such boiler or pressure vessel is being constructed, operated or
maintained in accordance with this chapter.
B. On and after January 1, 1973, no boiler or pressure vessel used or proposed to be used
within this Commonwealth, except boilers or pressure vessels exempted by this chapter, shall
be installed, operated or maintained unless it has been inspected by the Commissioner, his
agents or special inspectors as to construction, installation and condition and shall be certified.
A fee as set under subsection A of § 40.1-51.15 shall be charged for each inspection certificate
issued. In lieu of such fees both for certification and recertification, an authorized owner-user
inspection agency shall be charged annual filing fees as set under subsection A of § 40.1-
51.15.
C. Recertification shall be required as follows:
1. Power boilers and high pressure, high temperature water boilers shall receive a
certificate inspection annually and shall also be externally inspected annually while
under pressure if possible;
2. Heating boilers shall receive a certificate inspection biennially;
3. Pressure vessels subject to internal corrosion shall receive a certificate inspection
biennially;
4. Pressure vessels not subject to internal corrosion shall receive a certificate inspection
at intervals set by the Board, but internal inspection shall not be required of pressure
vessels, the content of which are known to be noncorrosive to the material of which
the shell, heads or fittings are constructed, either from the chemical composition of
the contents or from evidence that the contents are adequately treated with a
corrosion inhibitor, provided that such vessels are constructed in accordance with the
rules and regulations of the Board;
5. Nuclear vessels within the scope of this chapter shall be inspected and reported in such
form and with such appropriate information as the Board shall designate;
6. A grace period of two months beyond the periods specified in subdivisions 1, 2, 3 and
4 of these subsections may elapse between certificate inspections. The Chief Inspector
may extend a certificate for up to three additional months beyond such grace period
subject to a satisfactory external inspection of the object and receipt of a fee as set
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under subsection A of § 40.1-51.15 for each month of inspection beyond the grace
period.
D. Inspection requirements for operating equipment shall be in accordance with generally
accepted practice and compatible with the actual service conditions and shall include but not
be limited to the following criteria:
1. Previous experience, based on records of inspection, performance and maintenance;
2. Location, with respect to personnel hazard;
3. Qualifications and competency of inspection and operating personnel;
4. Provision for related safe operation controls; and
5. Interrelation with other operations outside of the scope of this chapter.
E. Based upon documentation of such actual service conditions by the owner or user of the
operating equipment, the Board may, in its discretion, permit variations in the inspection
requirements as provided in this section.
F. If, at the discretion of the Commissioner, a hydrostatic test shall be deemed necessary, it
shall be made by the owner or user of the boiler or pressure vessel.
G. All boilers, other than cast iron sectional boilers, and pressure vessels to be installed in
this Commonwealth after the six-month period from the date upon which the rules and
regulations of the Board shall become effective shall be inspected during construction as
required by the applicable rules and regulations of the Board.
H. Ninety-one days after expiration of a certificate for any boiler or pressure vessel subject to
this section, the Commissioner may assign an agent or special inspector to inspect such boiler
or pressure vessel, and its owner or operator shall be assessed a fee for such inspection. The
fee shall be established in accordance with subsection A of § 40.1-51.15.
Issuance of Certificates; Charges:
The Commissioner may designate special inspectors and contract fee inspectors to issue
inspection certificates for boilers and pressure vessels they have inspected. If no defects are
found or when the boiler or pressure vessel has been corrected in accordance with regulations,
the designated special inspector or contract fee inspector shall issue a certificate on forms
furnished by the Department. The designated special inspector or contract fee inspector shall
collect the inspection certificate fee required under § 40.1-51.10 at the time of the issuance
of the certificate and forward the fee and a duplicate of the certificate to the chief inspector
immediately.
Each designated special inspector or contract fee inspector may charge a fee as set under
subsection A of § 40.1-51.15 for each certificate issued, but the charge shall not be
mandatory. No charge shall be made unless the inspector has previously contracted therefore.
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Suspension of Inspection Certificate; Injunctive Relief
A. The Commissioner or his authorized representative may at any time suspend an inspection
certificate when, in his opinion, the boiler or pressure vessel for which it was issued, cannot
be operated without menace to the public safety, or when the boiler or pressure vessel is
found not to comply with the rules and regulations herein provided. Each suspension of an
inspection certificate shall continue in effect until such boiler or pressure vessel shall have
been made to conform to the rules and regulations of the Board, and until such inspection
certificate shall have been reinstated. No boiler or pressure vessel shall be operated during
the period of suspension.
B. Notwithstanding any other provision of this chapter to the contrary, in the event of violation
of any provision of this chapter or the regulations promulgated thereunder, the Board or the
Commissioner may petition any appropriate court of record for relief by injunction, without
being compelled to allege or prove that an adequate remedy at law does not exist.
Owner-User Inspection Agencies
Any person, firm, partnership or corporation operating pressure vessels in this Commonwealth
may seek approval and registration as an owner-user inspection agency by filing an
application with the chief inspector on forms prescribed and available from the Department,
and request approval by the Board. Each application shall be accompanied by a fee as set
under subsection A of § 40.1-51.15 and a bond in the penal sum of $5,000 which shall
continue to be valid during the time the approval and registration of the company as an
owner-user inspection agency is in effect. Applicants meeting the requirements of the rules
and regulations for approval as owner-user inspection agencies will be approved and
registered by the Board. The Board shall withdraw the approval and registration as an owner-
user inspection agency of any person, firm, partnership or corporation which fails to comply
with all rules and regulations applicable to owner-user inspection agencies. Each owner-user
inspection agency shall file an annual statement as required by the rules and regulations,
accompanied by a filing fee as set under subsection A of § 40.1-51.15.
Violation for Operating Boiler or Pressure Vessel Without Inspection Certificate;
Civil Penalty
A. After twelve months following July 1, 1972, it shall be unlawful for any person, firm,
partnership or corporation to operate in this Commonwealth a boiler or pressure vessel
without a valid inspection certificate. Any owner, user, operator or agent of any such person
who actually operates or is responsible for operating such boiler or pressure vessel thereof
who operates a boiler or pressure vessel without such inspection certificate, or at a pressure
exceeding that specified in such inspection certificate shall be in violation of this section and
subject to a civil penalty not to exceed $100. Each day of such violation shall be deemed a
separate offense.
B. All procedural rights guaranteed to employers pursuant to § 40.1-49.4 shall apply to
penalties under this section.
C. Investigation and enforcement for violations of this section shall be carried out by the
Department of Labor and Industry. Civil penalties imposed for violations of this section shall
be paid into the general fund.
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Posting of Certificate
Certificates shall be posted in the room containing the boiler or pressure vessel inspected. If
the boiler or pressure vessel is not located within the building the certificate shall be posted
in a location convenient to the boiler or pressure vessel inspected, or in any place where it
will be accessible to interested parties.
For Invalid Inspection Certificate for Insured Boiler or Pressure Vessel
No inspection certificate issued for an insured boiler or pressure vessel based upon a report
of a special inspector shall be valid after the boiler or pressure vessel for which it was issued
shall cease to be insured by a company duly authorized to issue policies of insurance in this
Commonwealth.
Operations and Maintenance
A. A hobby or model boiler must be attended by a person reasonably competent to operate
such boiler when in operation. For the purposes of this section, a hobby or model boiler may
be considered as not being in operation when all of the following conditions exist:
1. The water level is at least one-third of the water gauge glass;
2. The fire is banked and the draft doors closed or the fire is extinguished; and
3. The boiler pressure is at least twenty pounds per square inch below the lowest safety
valve set pressure.
B. All welding performed on hobby or model boilers shall be done by an "R" stamp holder in
accordance with the inspection code of the National Board of Boiler and Pressure Vessel
Inspectors.
C. Repairs to longitudinal riveted joints are prohibited.
Variances: Upon application pursuant to the provisions of subdivision 9 of § 40.1-6, the
Commissioner may allow variances from a specific statutory requirement of this article
provided the applicant proves by clear and convincing evidence his hobby or model boiler
meets substantially equivalent construction and operating criteria and standards.
Civil Penalty:
A. It shall be unlawful for any person, firm, partnership or corporation to operate in the
Commonwealth a hobby or model boiler without a valid certificate. Any such person shall be
subject to a civil penalty as provided by § 40.1-51.12.
B. Any owner or user who leaves or causes to leave a hobby or model boiler unattended while
in operation at an event to which members of the general public are invited shall be in violation
of this article and subject to a civil penalty not to exceed $5,000. Each instance of such
violation shall be deemed a separate offense.The chapters of the acts of assembly referenced
in the historical citation at the end of these sections may not constitute a comprehensive list
of such chapters and may exclude chapters whose provisions have expired.
The Virginia General Assembly is offering access to the Code of Virginia on the Internet as a
service to the public. We are unable to neither assist users of this service with legal questions
nor respond to requests for legal advice or the application of the law to specific facts.
Therefore, to understand and protect your legal rights, you should consult an attorney.
The Code of Virginia online database excludes material copyrighted by the publisher, Michie,
a division of Matthew Bender. Copyrighted material includes annotations and revisors' notes,
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which may be found in the print version of the Code of Virginia. Annotated print copies of the
Code of Virginia are available in most Virginia public library systems, from LexisNexis (1-800-
446-3410), and from West, a Thomson-Reuters business (1-800-344-5008).
Maintenance and Use in Accordance with Manufacturers’ Instructions
Manufacturers and suppliers are required to specify servicing requirements to purchasers of
machinery. This would include service periods for conditions of “normal use”. To ensure safe
use, all machinery used in the work place, including pressure vessels must be serviced and
maintained in accordance with the manufacturer’s recommendations.
The Department owning the vessel is responsible for establishing the equipment maintenance
contract and meeting the cost of maintenance.
The Local Estates Representative should be contacted when a maintenance contract is being
established or renewed – they may know of alternative suppliers offering more favorable
terms or if a College-wide contract is in place. In any case, the local Estates representative
will need to be made aware of service visits, as this can then be coordinated with insurance
inspections when necessary, usually at time of thorough examination.
Equipment may require more frequent and more stringent servicing as well as additional
control measures depending on its location and use. Advice should be sought from the
manufacturer in this instance. See 8. Information required by the Insurance Inspector
Requirement to Inspect and Examine
If a pressure vessel or part of a pressurized system fails, the consequences could be severe
(explosion, scalding etc.). Therefore, the User is responsible for ensuring that College-owned
(including grant-funded) pressure systems are maintained, operated safely, and precautions
taken to prevent over-pressurization.
In addition, the User must ensure that a competent person (the College Insurance Inspector)
makes a thorough annual inspection and regular examination of any College-owned pressure
systems containing steam or having a pressure x volume equal to 250 bar litres or more.
The scope and frequency of the examination is defined in a Written Scheme of Examination,
drawn up by the College Insurance Inspector. Usually a written scheme is produced as part
of the commissioning process, but in any case, will be provided by the Insurance Inspector
on the first examination. The Decision Tree in Appendix 1 helps to determine the duty holders.
Written Scheme of Examination (WSE)
If the supplier has not provided documentary evidence on commissioning and testing, advice
must be obtained from the College Insurance Inspector(s) on whether a WSE should be drawn
up before first use of any relevant pressure equipment. The scope of the examination depends
on the complexity of the equipment and the harm resulting in the event of failure.
The User must determine the scope of the WSE – if for example the equipment is likely to be
weakened due to chemical or environmental conditions then pipe work should be included or
if sudden failure of pipe work would give rise to danger. In practice, the College Insurance
Inspector would do this on the User’s behalf provided he is supplied with certain information
(see below).
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The College Insurance Inspector sends the original WSE to the Estates Helpdesk, who in turn
will send copies to the local Estates representative. The Insurance Inspector will notify any
subsequent modifications to the WSE (after repair for example) to the Estates Helpdesk, who
will notify to the local Estates Contact by means of an amended hard copy and an electronic
version, which is then circulated as an update. Appendix 2 – Local Estates Contacts
Information Required by the Insurance Inspector
To determine the depth and frequency of an examination, the Inspector will need to be
informed of the use and environmental conditions that the equipment is subjected to, for
example:
i)If environmental conditions are extreme, if corrosive chemicals such as acids or salts are
used within the vessel or are present in the external atmosphere, then physical degradation
of the equipment components is expected and the service frequency may need to be
increased. Similar consideration must be given to extremes of temperature, moisture, dust,
etc.
ii)If domestic-type equipment is being used for research purposes (as this would not be
considered to be normal use by the manufacturer or supplier). Note that domestic pressure
cookers must not be used for laboratory-type work within the College.
The User should note under such circumstances, maintenance and servicing periods may need
to be more frequent then those specified by the manufacturer. See 5. Maintenance and use
in accordance with manufacturers’ instructions
Arranging an Insurance Inspection
This should coincide with annual maintenance, as the machine may need to be stripped down
for the Inspector to access its workings. To ensure that maintenance coincides with inspection,
two actions are necessary.
i)Your vessel(s) will need to be registered with the Estates Help Desk and the Dept/Divisional
Safety Officer.
ii)Your maintenance contractor’s details will need to be registered with the local Estates
Contact who will try to co-ordinate the visit of the Inspector with that of the service engineer.
The College currently meets the cost of the Insurance inspection.
Once the Inspector has visited, he will issue a certificate. This will be issued as hard and
electronic copy to the Estates Helpdesk.The Estates Helpdesk will send hard copy to the local
Estates representative and to the Department Safety Officer.
If the Insurance Inspector identifies a safety problem, he will inform the user and the local
Estates representative, who will be required to isolate and remove from service the piece of
equipment pending its repair by the department. See 15. Failing an inspection - actions to be
taken
Procedures for Registration with Estates
You must register existing, newly purchased and second-hand equipment by email to
[email protected]. You will need to provide all the information required on the
Estates form. See Appendices Pressure Vessel Registration Form.
In addition, the supplier will provide commissioning and testing data. Pressure vessels may
continue to be used in departments for many years, and may even be moved between
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buildings, campuses etc. It is important that original commissioning and testing data is not
lost, so Users should send a copy of the commissioning and testing data to the local estates
contact, and give the original to the Department Safety Coordinator.
On receipt of the completed form, the Estates Helpdesk will arrange for the Insurance
Inspector to visit the Department to make a WSE and inform the local Estates contact,
maintenance provider, User, Department Safety Coordinator and Divisional Safety Advisor
that the Insurance Inspector has been notified. Estates will add the item(s) to the College
Register for subsequent annual inspections.
Newly purchased equipment must be registered in the same way, but the insurance inspection
will only be necessary 12 months after installation.
If registering newly installed second-hand pressure systems, unless the supplier has provided
commissioning and testing data, you will also need to ask the Helpdesk whether or not a WSE
needs to be drawn up before first use. Estates will contact the College Insurance Inspector
to determine if this is necessary and to arrange a visit.
The maintenance and inspection of fixed installations (those that form part of the building
pressure system) is organized by the Local Estates contact on some campuses. It remains
the responsibility of the department to check that maintenance and inspection is being done.
If vessels are already registered, they may have been marked with a unique identifier – such
as a consecutive number and a Building/campus code. The unique identifier will identify the
equipment regardless of where it is located within the College. The unique identifier will
appear on the Inspector’s report, and on the College and Divisional database of pressure
vessels. The serial number will act as the unique identifier where this is not in evidence.
Entry of Non-authorized Personnel into Laboratories
Insurance Inspectors, service engineers, Estates personnel etc. may not enter biological
Containment Laboratories, plant rooms or other high-risk workplaces unless permit-to-work
or equipment decontamination procedures are followed accordingly (see College Guidance
Note).
Rented and Second-hand Vessels
Mobile vessels are often on long-term hire. The owner of any pressure vessel is required to
have it maintained and tested (and examined where relevant). This applies to nitrogen
pressure vessels owned by nitrogen supply companies such as BOC. BOC are responsible for
carrying out the annual maintenance and testing on all of their vessels and would normally
fix a label to the equipment showing when this was next due. They usually provide a copy of
the test certificate to the hirer (on Hammersmith Campus these are kept by Stores), or a
copy may be obtained directly from the BOC. The hirer is responsible for checking that tests
have been carried out, and for ensuring that vessels failing the test are removed from service
and either repaired or replaced by the supply company.
The purchase of second-hand pressure systems should be avoided to minimize risks. They
may be in poor condition and the maintenance history may be unknown. Before use of such
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equipment it is essential that a full-service visit (combined with an Insurance Inspection if
over 250 bar litre) is arranged. See 10. Procedures for registration with Estates
Laboratory Equipment Which is Integral with the Building
Some equipment is linked to a building steam generating plant, for example non-self-
generating autoclaves. This type of machine will be examined in accordance with a WSE, as
it forms part of a larger, unseen steam system. Estates will arrange for the maintenance and
testing of plant supplying the service to the autoclave. However, departments are responsible
for ensuring that maintenance and examinations of any attached autoclaves are carried out
(as in 6-10). Other items such as gas generators and some compressed gas supply lines may
also be part of building-wide pressure system. The Estates Department currently recharges
user departments for the cost of maintaining this type of system. Pressure vessels operating
via a standard 13 Amp plug are unlikely to fall into this category. If you are uncertain, contact
the Estates Helpdesk for advice.
Procedures for Inspection and Re-inspection after Repair or Modification
On occasion, equipment will need to be repaired or modified. The Estates Helpdesk and the
local Estates contact must be informed immediately of any planned repairs or modifications
so that these may be notified to the Inspector, who may need to make an extra inspection,
perhaps whilst the machine is still stripped down by the service engineer.
Self-modifications made for example, as part of an experimental procedure must be subject
to a full risk assessment.
Failing an Inspection, Actions to be Taken
The Insurance Inspector will take the following actions if he considers there to be
“imminent danger” from a pressure vessel:
Inform local laboratory staff, and advise that the machine is removed from use
immediately.
Issue a Site Defect Notice to the User or other responsible person (who must sign it),
stating what requirements are needed to make the machine safe (either repair or
scrapping), and a time scale in which to do it.
Telephone the Estates Helpdesk and the local Estates Contact, who will complete a
Dangerous Occurrence Form.
Inform the Health and Safety Executive (HSE).
The local Estates contact, once informed by the Insurance Inspector, will email the User,
copying the Department Safety Coordinator, their named deputy, the Head of
Section/Department and the Divisional Safety Advisor, advising that action is required.
Users are responsible for carrying out required actions (repair or disposal), but must not
return a repaired item to service until authorized to do so by Estates.
The DSC or deputy should check that the required action has been carried out, and send
written confirmation of this to the local Estates contact, to the Estates Helpdesk (who will
inform the Inspector), the Divisional Safety Advisor and the Head of Section/Department.
HSE may write or check personally whether the appropriate action has been taken; if action
is not taken by the specified deadline, HSE may serve an Improvement Notice or a Prohibition
Notice depending on the danger involved.
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Pressure System Inspection Frequency
Equipment Inspection Type Inspection Frequency
3 years
Heating boiler External 2 years
3 years
Pressure vessel, corrosive External and internal 5 years
service External
Pressure vessel, non-
corrosive service
Vacuum vessel External
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Session - 16
PLANT INSPECTION
The phrases "Industrial plant inspection activities" refers to the inspection or integristy
department in oil, gas, chemical or other industrial plants, which is responsible for carrying
out the inspection in an operating unit. The inspections are done according to the following
timelines: Periodic external inspections.
A safety inspection is an on-site walk through to identify potential hazards to occupants and
personnel and options for remedial action. Safety inspections are also important for property
insurance issues. These actions prevent future incidents, injury/illness, or
property/equipment damage.
Prepare checklist and guidelines for Inspection and share with your faculty.
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Session - 17
PRESSURE SYSTEM HAZARDS AND CONTROL
General Safety Requirements for Compressed Air:
1. All pipes, hoses, and fittings must have a rating of the maximum pressure of the
compressor. Compressed air pipelines should be identified (psi) as to maximum
working pressure.
2. Air supply shutoff valves should be located (as near as possible) at the point-of-
operation.
3. Air hoses should be kept free of grease and oil to reduce the possibility of
deterioration.
4. Hoses should not be strung across floors or aisles where they are liable to cause
personnel to trip and fall. When possible, air supply hoses should be suspended
overhead, or otherwise located to afford efficient access and protection against
damage.
5. Hose ends must be secured to prevent whipping if an accidental cut or break
occurs.
6. Pneumatic impact tools, such as riveting guns, should never be pointed at a person.
7. Before a pneumatic tool is disconnected (unless it has quick disconnected plugs),
the air supply must be turned off at the control valve and the tool bled.
8. Compressed air must not be used under any circumstances to clean dirt and dust
from clothing or off a person’s skin. Shop air used for cleaning should be regulated
to 15 psi unless equipped with diffuser nozzles to provide lessor pressure.
9. Goggles, face shields or other eye protection must be worn by personnel using
compressed air for cleaning equipment.
10. Static electricity can be generated through the use of pneumatic tools. This type
of equipment must be grounded or bonded if it is used where fuel, flammable
vapors or explosive atmospheres are present.
Safety Requirements for Operating Compressed Air Machinery:
All components of compressed air systems should be inspected regularly by qualified and
trained employees. Maintenance superintendents should check with state and/or insurance
companies to determine if they require their own inspection of this equipment.
Operators need to be aware of the following:
Air Receivers:
The maximum allowable working pressures of air receivers should never be exceeded except
when being tested. Only hydrostatically tested and approved tanks shall be used as air
receivers.
1. Air tanks and receivers should be equipped with inspection openings, and tanks
over 36 inches in diameter should have a manhole. Pipelug openings should be
provided on tanks with volumes of less than five cubic feet.
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2. The intake and exhaust pipes of small tanks, similar to those used in garages,
should be made removable for interior inspections.
3. No tank or receiver should be altered or modified by unauthorized persons.
4. Air receivers should be fitted with a drain cock that is located at the bottom Of the
receiver.
5. Receivers should be drained frequently to prevent accumulation of liquid inside the
unit. Receivers having automatic drain systems are exempt from this Requirement.
6. Air tanks should be located so that the entire outside surfaces can be easily
inspected. Air tanks should not be buried or placed where they cannot be seen for
frequent inspection.
7. Each air receiver shall be equipped with at least one pressure gauge and an ASME
safety valve of the proper design.
8. A safety (spring loaded) release valve shall be installed to prevent the receiver
from exceeding the maximum allowable working pressure.
9. Only qualified personnel should be permitted to repair air tanks, and all work must
be done according to established safety standards.
Air Distribution Lines:
1. Air lines should be made of high-quality materials, fitted with secure connections.
2. Only standard fittings should be used on air lines.
3. Operators should avoid bending or kinking air hoses.
4. Air hoses should not be placed where they will create tripping hazards.
5. Hoses should be checked to make sure they are properly connected to pipe outlets
before use.
6. Air lines should be inspected frequently for defects, and any defective equipment
repaired or replaced immediately.
7. Compressed air lines should be identified as to maximum working pressures (psi),
by tagging or marking pipeline outlets.
1. Only qualified personnel should be allowed to repair or adjust pressure regulating
equipment.
2. Valves, gauges and other regulating devices should be installed on compressor
equipment in such a way that cannot be made inoperative.
3. Air tank safety valves should be set no less than 15 psi or 10 percent (whichever
is greater) above the operating pressure of the compressor but never higher than
the maximum allowable working pressure of the air receiver.
4. Air lines between the compressor and receiver should usually not be equipped with
stop valves. Where stop valves are necessary and authorized, ASME safety valves
should be installed between the stop valves and the compressor.
5. The Safety valves should be set to blow at pressures slightly above those necessary
to pop the receiver safety valves.
6. Blowoff valves should be located on the equipment and shielded so sudden blowoffs
will not cause personnel injuries or equipment damage.
7. Case iron seat or disk safety valves should be ASME approved and stamped for
intended service application.
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8. If the design of a safety or a relief valve is such that liquid can collect on the
discharge side of the disk, the valve should be equipped with a drain at the lowest
point where liquid can collect.
9. Safety valves exposed to freezing temperatures should be located so water cannot
collect in the valves. Frozen valves must be thawed and drained before operating
the compressor.
Air Compressor Operation:
1. Air compressor equipment should be operated only by authorized and trained
personnel.
2. The air intake should be from a clean, outside, fresh air source. Screens or filters
can be used to clean the air.
3. Air compressors should Never be operated at speeds faster than the manufacturers
recommendation.
4. Equipment should not become overheated.
5. Moving parts, such as compressor flywheels, pulleys, and belts that could be
hazardous should be effectively guarded.
Compressed Air Equipment Maintenance:
1. Only authorized and trained personnel should service and maintain air compressor
equipment.
2. Exposed, Noncurrent-carrying, metal parts of compressor should be effectively
grounded.
3. Low flash point lubricants should not be used on compressors because of its high
operating temperatures that could cause a fire or explosion.
4. Equipment should not be over lubricated.
5. Gasoline or diesel fuel powered compressors shall not be used indoors.
6. Equipment placed outside but near buildings should have the exhausts directed
away from doors, windows and fresh air intakes.
7. Soapy water of lye solutions can be used to clean compressor parts of carbon
deposits, but kerosene or other flammable substances should not be used.
Frequent cleaning is necessary to keep compressors in good working condition.
8. The air systems should be completely purged after each cleaning.
9. During maintenance work, the switches of electrically operated compressors should
be locked open and tagged to prevent accidental starting.
10. Portable electric compressors should be disconnected from the power supply before
performing maintenance.
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Session –19
PLANT MODIFICATION
Pressure Measurement: Principles
How Is Pressure Measured?
As with most measure and pressure measurement methods have varying suitability for different
applications. Measurement engineers need to be familiar with several techniques in order to
select the one that is most appropriate for their specific requirements.
Deadweight Tester:
The most fundamental pressure measurement technique, and favored as well for primary
calibration of pressure sensors, is the deadweight tester, or piston gauge. This device uses
calibrated weights (masses) that exert pressure on a fluid (usually a liquid) through a piston.
Deadweight testers can be used as primary standards because the factors influencing accuracy
are traceable to standards of mass, length, and time. The piston gauge is simple to operate;
pressure is generated by turning a jackscrew that reduces the fluid volume inside the tester,
resulting in increased pressure. When the pressure generated by the reduced volume is slightly
higher than that generated by the weights on the piston, the piston will rise until it reaches a
point of equilibrium where the pressures at the gauge and at the bottom of the piston are
exactly equal.
Various pressure ranges can be achieved by varying the area of the piston and the size of the
weights. For extremely accurate and precise pressure calibrations, many corrections must be
made, exact areas and weights must be known, and great care must be taken in the procedure.
Obviously, this is not a practicable method for day-to-day pressure measurements.
Fluid Head—Manometers:
The height of a column of liquid, or the difference between the heights of two liquid columns,
is used to measure pressure head in devices called U-tube manometers. If a fluid is installed in
an open U-shaped tube, the fluid level in each side will be the same. When pressure is applied
to one side, that level will go down and the level on the other side will rise until the difference
between the heights is equal to the pressure head. The height difference is proportional to the
pressure and to the density of the fluid. The U-tube manometer is a primary standard for
pressure measurement.
Although many manometers are simply a piece of glass tubing formed into a U shape with a
reference scale for measuring heights, there are many variations in terms of size, shape, and
material. If the left side is connected to the measurement point, and the right is left open to
atmosphere, the manometer will indicate gauge pressure, positive or negative (vacuum).
Differential pressure can bemeasured by connecting each of the legs to one of the measurement
points. Absolute pressure can be measured by evacuating the reference side. A mercury
barometer is such an absolute pressure measuring manometer indicating atmospheric
pressure.In some versions, the two legs of the U are of different diameters. Some types
incorporate a large-diameter "well" on one side. In others, one tube is inclined in order to
provide better resolution of the reading. But they all operate on the same principle. Because of
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the many constraints on geometry of installation and observation, and their limited range,
manometers are not practical or effective for most pressure measurements.
Force-Summing Devices:
Mechanical pressure gauges and electromechanical pressure sensors incorporate an elastic
element called a force-summing device that changes shape when pressure is applied to it.
The shape change is then converted to a displacement. Of the wide variety of force-summing
devices, the most common are Bourdon tubes and diaphragms. Bourdon tubes provide fairly
large displacement motion that is useful in mechanical pressure gauges; the lesser motion of
diaphragms is better in electromechanical sensors.The motion of the force-summing device can
be linked to a linear variable differential transformer, which acts as the electromechanical
transduction element. Alternatively, it can be linked, usually through a motion amplifying
mechanism, to the wiper of a potentiometer. To reduce acceleration error, a balancing mass
may be provided.
Mechanical Pressure Gauges:
In mechanical gauges, the motion generated by the force-summing device is converted by
mechanical linkage into dial or pointer movement. The better gauges provide adjustments for
zero, span, linearity, and (sometimes) temperature compensation for mechanical calibration.
High-accuracy mechanical gauges take advantage of special materials, balanced movements,
compensation techniques, mirror scales, knife-edge pointers, and expanded scales to improve
the precision and accuracy of readings. The most accurate mechanical gauges, test gauges, are
used as transfer standards for pressure calibration, but for applications requiring remote
sensing, monitoring, or recording they are impractical. Their mechanical linkages also limit their
frequency response for dynamic pressure measurements.
Electromechanical Pressure Sensors:
Electromechanical pressure sensors, or pressure transducers, convert motion generated by a
force-summing device into an electrical signal. These sensors are much more useful and
adaptable than mechanical gauges, especially when applied in data acquisition and control
systems. In well-designed transducers, the electrical output is directly proportional to the
applied pressure over a wide pressure range. For rapidly changing—dynamic—pressure
measurement, frequency characteristics of the transducer are an important consideration.
Types of Pressure Sensors
Pressure sensors are available with a variety of reference pressure options: gauge (psig),
absolute (psia), differential (psid), and sealed (psis). All use a force-summing device to convert
the pressure to a displacement, but that displacement is then converted to an electrical output
by any of several transduction methods. The most common are strain gauges, variable
capacitance, and piezoelectric.
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Strain Gauge Transducers:
Strain gauge transducers are based on metal or silicon semiconductor strain gauges. The
gauges can be discrete units attached to the surface of the strained element or unbonded
gauges. The gauge material can be sputtered onto a diaphragm or diffused into a silicon
diaphragm structure. The most common force-summing device for strain gauge transducers is
the diaphragm, which may be flat or sculptured. Strain gauges are also used on Bourdon tubes
and bellows assemblies.
Strain gauges are made of materials that exhibit significant resistance change when strained.
This change is the sum of three effects. First, when the length of a conductor is changed, it
undergoes a resistance change approximately proportional to change in length. Second, in
accordance with the Poisson effect a change in the length of a conductor causes a change in its
cross-sectional area and a resistance change that is approximately proportional to change in
area. Third, the piezoresistive effect, a characteristic of the material, is a change in the bulk
resistivity of a material when it is strained. All strain gauge materials exhibit these three
properties, but the piezoresistive effect varies widely for different materials. Metal strain gauges
are networks of wire or patterns of thin metal foil fabricated onto or into a backing material and
covered with a protective film. Their design permits the use of a large active length (= large R)
in a small area. They are made of specially formulated alloys with relatively large piezoresistive
effects. Silicon strain gauges are doped to resistivity levels that produce the optimum
combination of piezoresistive and thermo-resistive effects. Strain gauge materials are
characterized by their strain sensitivity, but when fabricated into strain gauges they are
characterized by their "gauge factor," defined as relative resistance change divided by strain.
Bonded Strain Gauges:
Discrete metal or silicon strain gauges are usually bonded (glued) to the surface where strain
is to be measured, and provide an output proportional to the average strain in their active area.
The typical gauge factor is around 2; a strain of 1 µin./in. would produce a resistance change
of 2 µ/. Unstrained resistance ranges from 120 to several hundred ohms. Because a significant
length of wire or foil is necessary to provide high unstrained resistance, metal strain gauges
cannot be made extremely small.
Unbonded Strain Gauges:
Unbonded strain gauge transducers use relatively long strands of strain gauge wire stretched
around posts attached to a linkage mechanism. The linkage is designed such that when pressure
increases, half of the wire isfarther stretched and the other half is less so. The primary
advantage of unbonded over bonded is a higher gauge factor, on the order of 3. Because no
adhesives are required, they can also be designed and fabricated for use at higher
temperatures. Unbonded strain gauge transducers tend to be large.
Sputtered Strain Gauges: Strain gauge material may be sputtered onto a nonconductive
diaphragm to create the strain gauges. Location and orientation are controlled by masking, and
the molecular bond created by the sputtering process eliminates any problems with adhesive
bonding. Gauge factors are similar to those of unbonded gauges. Surface preparation and other
process controls are quite critical. The fabrication process offers some of the advantages of a
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silicon diaphragm, such as good linearity and highnatural frequency, as well as the good
temperature characteristics of metal gauges.
Semiconductor Strain Gauges:
These devices are made of semiconducting silicon. Their gauge factor is dependent on the
doping level—more lightly doped, higher resistivity material has a higher gauge factor.
However, it also has greater thermal sensitivity, causing both resistance and gauge factor to
change significantly with temperature. Most silicon gauges are doped to provide a gauge factor
of 100?200, which gives acceptable temperature characteristics. Discrete silicon strain gauges
are used just as are metal gauges, glued to the strained surface in the desired orientation to
provide maximum sensitivity for pressure measurement. In addition to their higher gauge factor
(which provides higher sensitivity), they are also smaller, allowing more miniaturization.
Bonded Discrete Silicon Strain Gauges:
Early silicon strain gauge transducers used discrete silicon strain gauges bonded with adhesives
to the surface of a strained element. These devices were similar to bonded metal strain gauges,
except that the silicon typesprovided much higher output and had greater temperature errors.
Furthermore, the silicon gauges were smaller than metal gauges, so the sensors could be made
smaller.
Diffused Diaphragm Sensors:
Discrete strain gauges, metal or silicon, require tedious micro assembly for installation, but
diffused diaphragm sensors can be fabricated using semiconductor masking and processing
techniques. This approach provides precision location and orientation of the gauges for optimum
linearity and sensitivity, allows extreme miniaturization, and reduces assembly costs. It also
removes the variability of the adhesive and its application.
Sculptured-Diaphragm Sensors:
Early diffused silicon diaphragm pressure transducers used a simple, flat silicon diaphragm of
uniform thickness. Silicon micro fabrication techniques (MEMS) allow great flexibility in the
mechanical design of the diaphragm.
Anisotropic etching provides precise control of etching directions in the silicon crystal. Extremely
small yet complex shapes can be fabricated, permitting the diaphragms to be shaped for
optimum combinations of linearity, sensitivity, and frequency response characteristics.
Variable Capacitance Transducers:
When one plate of a capacitor is displaced relative to the other, the capacitance between the
two plates changes. If one of the plates is the diaphragm of a pressure sensor, the capacitance
can be correlated to the pressure applied to it. This change of capacitance is either used to vary
the frequency of an oscillator or is detected by a bridge circuit. If the dielectric material is
maintained constant, this mechanism provides a very repeatable transducer. The primary
advantages are low hysteresis; good linearity, stability, and repeatability; static pressure
measurement capability; and a quasi-digital output. However, complicated electronics are
required.
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Piezoelectric Transducers:
Piezoelectric (PE) pressure transducers use stacks of piezoelectric crystal or ceramic elements
to convert the motion of the force-summing device to an electrical output. Quartz, tourmaline,
and several other naturally occurring crystals generate an electrical charge when strained.
Specially formulated ceramics can be artificially polarized to be piezoelectric, and they have
higher sensitivities than natural crystals. Unlike strain gauge transducers, PE devices require
no external excitation. Because their output is very high impedance and their signal levels low,
they require special signal conditioning such as charge amplifiers and noise-treated coaxial
cable.
Some designs of PE transducers (ICP or voltage mode) therefore include an integral preamplifier
within the transducer's case. The output can then be an amplified (mill volt level) low output
impedance signal, greatly reducing cabling problems and simplifying signal conditioning. The
integral amplifier requires external power from a constant-current supply, using the same two
conductors as the signal circuit. The signal conditioner has a blocking capacitor to block the DC
power supply voltage and to transmit an AC signal.Because the PE transducers are self-
generating, dependent on changes of strain to generate electrical charge, they are not usable
with DC or steady-state conditions. They have an inherent low-frequency roll-off that is
dependent on the signal conditioning's low-frequency time constant.Their primary advantage is
their ruggedness, and, without integral electronics, their usefulness at high temperatures. If
not properly compensated, though, they are sensitive to shock and vibration and may exhibit
large changes of sensitivity with temperature variations.
Other Electromechanical Sensors:
Virtually every technique for converting motion to an electrical signal—variable reluctance,
variable inductance, force balance, vibrating wires, vibrating columns and tubes, piezoelectric
film, and Hall effect—has been tried in pressure transducer design. Several varieties of fiber-
optic sensors have also recently become available. These make use of variable reflectance,
phase coherence, and microband effects to convert the sensed pressure into light variations
that can be excited and caused to transmit signals via optical fibers. These sensors may be
advantageous in environments of high-amplitude electromagnetic fields or pulses. Some
"hybrid" systems use conventional transducers, then convert the electrical outputs to optical
signals for fiber-optic transmissions.
Scanners:
Multichannel scanning pressure measurement systems are sometimes the best selection when
many measurement points are required. Two types are available: mechanical and electronic.
Mechanical scanners use only one sensor and mechanically route the pressure sequentially from
each measurement point to the sensor. Electronic scanners use many sensors in a common
body, and electrically time-multiplex the signals to data acquisition equipment. In both types,
tubing transmits the pressure from measurement points to one sensor.
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ISO 9001: 2015 Organization
Session – Safety in Chemical Industry
Pressure-Scanning Valves:
A pressure-scanning valve is a pneumatic switch capable of sequentially multiplexing a number
of pressures to a single transducer. The most common design is based on a matched pair of
lapped surfaces with one rotating relative to the other. The transducer is typically flush mounted
very close to the valve in order to minimize the volume of gas subject to the changes in
pressure. The valve rotor is driven by a stepper motor, and the valve position is indicated by a
rotary encoder. A periodic recalibration can be incorporated into the system by supplying
accurately known pressures to one or more ports. The maximum scanning rate is dependent
on the accuracy required. If the dwell time at each measurement position is long enough for
the pressure equilibrium to be achieved, the accuracy is that of the transducer. Equilibrium time
is a function of the traveling volume and the magnitude of the pressure change. The typical
scanning rate for aerodynamic or jet engine pressure measurements is 5?10 measurements/s.
Multiple scanners can be time sequenced to provide faster effective scanning rates.
Electronic Pressure Scanners:
Combining miniature semiconductor strain gauge transducers and solid-state electronic
multiplexing into an integrated measurement system provides much higher rates than possible
with mechanical scanners. A multiple transducer array, a low-level multiplexer, and an
instrumentation amplifier in a shared housing make up the typical system. Some systems also
include a pneumatic valve that can be automatically switched to subject each sensor to a
calibrated pressure at any time. The calibrated pressure is supplied by a calibration manifold.
Because there is no mechanical switching of pressurized passages, there is no need to delay
measurements while a traveling volume is stabilized. Each transducer is always measuring, and
its output is periodically sampled by the electronic multiplexer scanning. Scanning speeds can
be 10,000 to 20,000 sps. Of course, the connecting tubing between the measurement point
and the sensor will still impose a physical low-pass filter.
A steam explosion is an explosion caused by violent boiling or flashing of water into steam,
occurring when water is either superheated, rapidly heated by fine hot debris produced within
it, or heated by the interaction of molten metal’s (as in a fuel–coolant interaction, or FCI, of
molten nuclear-reactor fuel rods with water in a nuclear reactor core following a core-
meltdown). Pressure vessels, such as pressurized water (nuclear) reactors, that operate above
atmospheric pressure can also provide the conditions for a steam explosion. The water changes
from a liquid to a gas with extreme speed, increasing dramatically in volume.
Steam explosions are not normally chemical explosions, although a number of substances react
chemically with steam (for example, zirconium and superheated graphite react with steam and
air respectively to give off hydrogen, which burns violently in air) so that chemical explosions
and fires may follow. But many large-scale events, including foundry accidents, show evidence
of an energy-release front propagating through the material (see description of FCI below),
where the forces create fragments and mix the hot phase into the cold volatile one; and the
rapid heat transfer at the front sustains the propagation.
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ADIS 7 Safety in Chemical Industry - Overall (1)
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