DEP_80.45.10.11_FEB-11

ECCN EAR99 DEP 80.45.10.11-Gen.
February 2011
Page 51

5.4.4 Positive displacement compressors – General
5.4.5
1. Relief protection is required for positive displacement compressors for the blocked outlet
scenario.

NOTE: The API specification for the compressor may limit the set pressure and the relieving pressure in
order to protect the compressor against overpressure and or other electro/mechanical limits (e.g., to
prevent the compressor motor from being overloaded and to manage compressor rod loading).

2. Process conditions for the blocked outlet case shall be used.

NOTE: Use of design machine capacity might not be conservative if the suction density or pressure
increases as a result of the blocked-in condition.

3. Approval by the Principal is required on the following design criteria:

a) Suction pressure

b) Molecular weight

c) Nitrogen circulation during unit commissioning

Reciprocating compressors – Additional considerations

1. The design SHALL [PS] assume a blocked outlet of each stage. This pertains to each
stage of a reciprocating compressor.

NOTE: Where there are no block valves between stages, an interstage relief valve might be required to
prevent overpressure in the event of loss of interstage cooling. A Remote Contingency, however,
would be internal failure of the downstream stage. Loss of downstream stage pumping would
essentially shift all of the gas compression work to the first stage. A relief valve sized for the blocked
outlet flow conditions at the interstage MAWP is required to prevent overpressure.

2. Reciprocating compressors shall be protected from rod packing failures in the distance
piece by an adequately sized vent line or pressure relief device as follows.

NOTE: Reciprocating compressor distance pieces often have low pressure ratings, e.g. 140 kPa (ga)
[20 psig].

a) Pressure relief device or vent line shall be sized using one of the following
methods.

Method 1: Assume an annular gap of 0.8 mm (1/32 in) between the compressor
rod and the packing gland. Calculate flow using an equivalent square
edge orifice area.

Method 2: Provide a pressure relief device with inlet size equal to the nominal pipe
size of the vent piping. That is, if the vent piping is DN 40 (NPS 1-1/2),
use a pressure relief device with a DN 40 (NPS 1-1/2) inlet flange.

- Specify the largest orifice area available for the vent line nominal
pipe size.

- Specify a rupture disk or a balanced PRV

- No need to perform inlet or outlet hydraulics since sizing is arbitrary.

- An alarm shall be provided to indicate to operations that there is
abnormally high packing leak-off rate (e.g., a high temperature alarm
or a high pressure alarm).

b) Regardless of which method is used, the vent or pressure relief device shall be
installed so that there is no block valve downstream that if accidentally closed,
would simultaneously block the normal vent path and the pressure relief path.

c) In both methods any valves in the relief path shall be locked or car sealed open.

An acceptable design would be to have the PRV piped around the vent block valve
provided that there is NOT another block valve further downstream in the flare
lateral. If there is another valve further downstream in the lateral then closure of that
valve could potentially cause the vent flow to be blocked and the relief valve
discharge to be blocked simultaneously which may result in equipment overpressure.
If this is the case the PRV discharge shall be routed independently of the vent line.

ECCN EAR99 DEP 80.45.10.11-Gen.
February 2011
Page 52

5.4.6 3. Safeguards to prevent upstream equipment overpressure due to backflow through
reciprocating compressors shall be evaluated on a case-by-case basis. If check valves
5.5 are required for reciprocating compressors, proper check valve selection and orientation
is critical to ensure reliable valve operation.
5.6
5.6.1 Steam turbines
5.6.2
5.7 1. For new turbines, the relief load, the set pressure, and the allowable accumulation shall
5.7.1 be specified by the turbine vendor.

2. New condensing and non-condensing steam turbines SHALL [PS] have pressure relief
devices located on the low pressure turbine discharge side.

3. In units where administrative controls (in lieu of appropriately sized relief devices) are
used to prevent blocked outlet conditions on existing steam turbines, these
administrative controls may also be used for new non-condensing turbines provided that
the turbine case has been hydrotested at a pressure at least equal to the steam supply
pressure, and provided that the Principal so approves.

LUBE-OIL SYSTEMS

If pump deadhead causes overpressure of machinery lubricating oil equipment (pumps,
piping, filters, or coolers) and if the lubricating oil filters and coolers each have capacities
less than 500 L, then the relief protection may be provided by an internal or an external oil
pressure limiting valve that complies with the performance requirements of
DEP 31.29.60.32-Gen.

It is the practice to take credit for an oil pressure limiting valve to provide both overpressure
protection and pressure control for lubricating oil set applications. In view of the product
involved, the equipment size, the requirement for stable operation, the degree of
instrumentation, typical operating procedures, and the satisfactory performance of these
valves, the use of such valves in lieu of internal or external PRVs is acceptable.

EJECTORS

Overpressure of equipment upstream of ejectors (e.g., steam ejectors or eductors used to
create vacuum) shall be evaluated for the blocked outlet case. A blocked outlet may result
in backflow of the motive fluid to the upstream equipment

If the source of overpressure is downstream of the eductor nozzle, credit can normally be
taken for the flow backwards through the process inlet line. The flow resistance of an
eductor flowing “backwards” shall be assumed to be equivalent to the resistance of a
piping tee branch connection with the same size as the process inlet line.

NOTE: Often the relief device is located upstream of the eductor (e.g., on the vacuum column).

INTERFACES WITH PIPELINES

For gas pipeline systems, ASME B31.8 permits the use of either pressure relief devices or
series pressure reducing control valves or regulators. The use of series pressure reducing
control valves as the sole means of overpressure protection may not be adequate for any
downstream ASME B31.3 piping or ASME equipment. If pipeline overpressure protection is
provided by series pressure reducing control valves or regulators, the potential
overpressure of downstream ASME B31.3 piping and ASME equipment shall be evaluated
as follows:

a) Blocked-in condition with no control valve responding (size for normal flow).

b) Single control valve/regulator driven wide open (Code allowable overpressure).

c) Both control valves/regulators driven wide open. Valves/regulators shall be sized as
a Remote Contingency, in accordance with (4), with credit taken for normal flow-out
if viable.

NOTE: There can be high pressure trips on gas users that can reduce or eliminate normal flow-out and

can result in an overpressure scenario.

ECCN EAR99 DEP 80.45.10.11-Gen.
February 2011
Page 53

5.8 DOUBLE-SEATED VALVES
5.8.1
5.8.2 If double-seated valves are used then overpressure protection of the valve body
SHALL [PS] be evaluated.
5.9
Double-seated valves in liquid service can experience thermal expansion overpressure in
the valve body cavity when the valve is closed. Body cavity over-pressure can also occur
in condensing vapour services (e.g. valves in steam/condensate systems when
condensate is trapped). Double-seated valves that are subjected to temperature changes
and consequent expansion of the liquid medium SHALL [PS] be specified with a valve
design that prevents overpressure of the body cavity; see 3.12.6 in DEP 31.38.01.11-Gen.
for details. For double-seated valves that are provided with internal body cavity relief to just
one side (e.g., a drilled hole), notations shall be made on the PEFS to ensure that the relief
path is specified on the proper side of the valve and that the valve is installed in the proper
direction.

INSTRUMENTATION

Pressure relief protection is required on the body of coriolis motion flow meters if the
tube-side operating pressure is greater than the hydrotest pressure of the casing. If this is
the case, a DN 15 (NPS ½) or DN 20 (NPS ¾) rupture disk shall be provided on the purge
connection. The instrument data sheet shall adequately specify the rupture disks so that
future replacements can be made. Such rupture disks shall be sized on the basis of a
pinhole leak. Full-bore tube rupture is not a design consideration in view of experience with
these devices.

ECCN EAR99 DEP 80.45.10.11-Gen.
February 2011
Page 54

6. STORAGE TANKS
6.1
GENERAL
6.2
Pressure/vacuum relief for storage tanks SHALL [PS] be in accordance with ISO 28300.
6.3
6.3.1 NORMAL VENTING

6.3.2 For new tanks the normal inbreathing/outbreathing loads shall be calculated using the
6.3.3 method in Section 4.3.2 of ISO 28300. That is, the alternative method described in Annex
A of that standard should not be used for new tanks.
6.3.4
FIRE CASE

Fire case overpressure protection is not required for plastic tanks since loss of containment
is highly likely and the tank wall above the liquid level will probably melt before
overpressuring.

Fire case overpressure protection is required for fiberglass tanks.

Fire case overpressure protection is intrinsically provided by floating roof tanks designed in
accordance with API Std 650 or DEP 34.51.01.31-Gen., in that their ventilation ports are
adequate for the fire relief loads. Fire case relief calculations are not required.

Emergency venting, including the fire case, can be provided by a frangible roof joint.

a) API Std 650 or DEP 34.51.01.31-Gen., SHALL [PS] be used to determine roof
frangibility.

NOTE: Tanks with a diameter less than 15 m (50 ft) designed to earlier API Std 650 editions might not
meet the frangibility criteria specified in the current API Std 650.

b) Tanks that do not have a frangible roof SHALL [PS] have emergency vents or
emergency manway hatches provided for the fire case and any other emergency
venting scenarios.

c) The Principal shall specify whether a frangible roof or a pressure relief device shall
be used for emergency venting.

ECCN EAR99 DEP 80.45.10.11-Gen.
February 2011
Page 55

6.4 RELIEF CRITERIA
6.4.1
Set pressures and allowable accumulation and underpressures for venting storage tanks
under normal and emergency conditions (including fire case) shall be as follows:

Set pressure and allowable accumulation
Note 5

Design Storage Non-fire case Non-fire case Fire case vent Fire case
pressure tank/ typical vent allowable set pressure allowable
vessel set pressure equipment
equipment relieving relieving
pressure pressure

103 kPa (ga) ASME VIII = design 1.1*design up to 1.21 × design
[15 psig] or Vessel 1.1 × design

higher

Between Tank < design 1.1*design up to 1.2 × design
17.2 kPa (ga) API 620 1.1 × design
and 103 kPa (ga)
[2.5 psig and

15 psig]

up to Tank << design design up to 1.2 × design
17.2 kPa (ga) API 650 1.1 × design
Appendix F
[2.5 psig]

Full range of Tank << design < design < design design
design pressures API 650

up to weight of
roof

Atmospheric Tank ~ 0.21 kPa 0.42 kPa (ga) 0.65 kPa (ga) 0.75 kPa (ga)
(no design (existing (ga) [1.7 in H2O] [2.6 in H2O] [3 in H2O]
specified)
only) [0.85 in H2O] Note 1 Note 2
API 650

Allowable underpressures

Design Vacuum Storage tank/vessel Typical Vacuum Vent Allowable Equipment

Set Pressure Relieving Underpressure

103 kPa (ga) Vessel < design Design
[15 psig] or higher ASME Section VIII

Between 17.2 kPa (ga) Tank < design Design
and 103 kPa (ga) API 620

[2.5 psig and 15 psig]

up to 17.3 kPa (ga) Tank < design Design
[2.5 psig] API 650 Appendix F

Atmospheric (no design Tank (atmospheric) ~ –0.21 kPa –0.442 kPa
specified) API 650 [–0.85 in H2O] [–1.7 in H2O]

NOTES: 1. The 0.65 kPa (ga) set point is based on mid-range of set points of typical emergency manway vents. It is set
higher than tank design to minimize normal fugitive emissions.

2. The 0.75 kPa (ga) relieving pressure is established as an upper limit. This upper limit is considered acceptable
since this is well below the stress level that would occur if the tank is overfilled.

3. API Std 650 storage tanks that have a frangible roof typically have a pressure rating of only 0.37 kPa (ga)
(governed by the weight of the roof plates) and a vacuum rating of –0.43 kPa (–1 oz/in2).

4. API Std 650 storage tanks have lower allowable overpressure/underpressure because they have lower design
safety factors and because they have potential roof weld fatigue.

5. These requirements are applicable to API tanks. For tanks designed in accordance with BS 2654 or
EN 14015, DEP 34.51.01.31-Gen. shall apply.

6.4.2 Since a weight-loaded pressure vacuum vent typically requires a pressure that is 80 % to
100 % higher than the set pressure to achieve full rated capacity, the set points of such
devices shall be lowered well below the tank design pressure and vacuum rating in order
to comply with the allowable accumulation and under pressures. Consult with the tank vent
manufacturer on tank vent flow performance.

ECCN EAR99 DEP 80.45.10.11-Gen.
February 2011
Page 56

7. DOCUMENTATION

The engineering assumptions and calculations for relief/flare loads shall be documented
and included with the other calculations and documentation requirements specified in
DEP 80.45.10.10-Gen. and DEP 80.36.00.30-Gen.
The required documentation shall be provided to the location.

ECCN EAR99 DEP 80.45.10.11-Gen.
February 2011
Page 57

8. REFERENCES
In this DEP, reference is made to the following publications:

NOTES: 1. Unless specifically designated by date, the latest edition of each publication shall be used,
together with any amendments/supplements/revisions thereto.

2. The DEPs and most referenced external standards are available to Shell staff on the SWW (Shell
Wide Web) at http://sww.shell.com/standards/.

SHELL STANDARDS

Definition of temperature, pressure and toxicity DEP 01.00.01.30-Gen.
levels

Preparation of safeguarding memoranda and DEP 01.00.02.12-Gen.
process safeguarding flow schemes

Pressurised bulk storage installations for LPG DEP 30.06.10.12-Gen.

Lubrication, shaft-sealing and control oil systems and DEP 31.29.60.32-Gen.
auxiliaries (amendments/supplements to API 614)

Piping – General requirements DEP 31.38.01.11-Gen.

Pipeline overpressure protection DEP 31.40.10.14-Gen.

Instrumentation of depressuring systems DEP 32.45.10.10-Gen.

Instrumented protective functions DEP 32.80.10.10-Gen.

Vertical carbon steel storage tanks – Selection and design DEP 34.51.01.31-Gen.
(based on EN 14015)

Relief devices – Selection, sizing and specification DEP 80.36.00.30-Gen.

Design of pressure relief, flare and vent systems DEP 80.45.10.10-Gen.

Emergency depressuring DEP 80.45.10.12-Gen.

AMERICAN STANDARDS

Sizing, selection, and installation of API RP 520 Part I
pressure-relieving devices in refineries – Part I:
Sizing and selection

Pressure-relieving and depressuring systems API Std 521

Design and construction of large, welded, low-pressure API Std 620
storage tanks

Welded steel tanks for oil storage API Std 650

Venting atmospheric and low pressure storage tanks API Std 2000
(non-refrigerated and refrigerated)

Issued by:
American Petroleum Institute
Publications and Distribution Section
1220 L Street Northwest
Washington DC. 20005
USA.

ASME boiler and pressure vessel code

Section I – Power boilers ASME I

Section VIII – Pressure vessels ASME VIII

Process piping ASME B31.3

Gas transmission and distribution piping systems ASME B31.8

ECCN EAR99 DEP 80.45.10.11-Gen.
February 2011
Issued by: Page 58
American Society of Mechanical Engineers
Three Park Avenue NFPA 58
New York NY 10016-5990
USA EN 13445
EN 14015
Standard for the storage and handling of liquefied
petroleum gases ISO 23251
ISO 28300
Issued by:
National Fire Protection Association
470 Atlantic Avenue, Boston
Massachusetts, 02210, USA.

EUROPEAN STANDARDS

Unfired pressure vessels

Specification for the design and manufacture of site
built, vertical, cylindrical, flat-bottomed, above
ground, welded, steel tank for the storage of liquids
at ambient temperature and above

Issued by:
CEN
Rue de Stassart 36
B-1050 Brussels, Belgium

Copies can also be obtained from national standards organizations

INTERNATIONAL STANDARDS

Petroleum, petrochemical and natural gas industries
– Pressure-relieving and depressuring systems
Petroleum, petrochemical and natural gas
industries — Venting of atmospheric and low-
pressure storage tanks

Issued by:

ISO Central Secretariat
1, ch. de la Voie-Creuse
Case postale 56
CH-1211 Genève 20
Switzerland

Copies can also be obtained from national standards organizations.

ECCN EAR99 DEP 80.45.10.11-Gen.
February 2011
Page 59

9. BIBLIOGRAPHY

NOTE: The following document is for information only and does not form an integral part of this DEP.

Flashing through Orifices and Pipes, B. Fletcher, AIChE Loss Prevention
Symposium in Denver 1983.