Paper ID : 20100101 ACHIEVING REDUCED FLARING THROUGH ...

Petrotech-2010
31 October-3 November 2010, New Delhi, India

Paper ID : 20100101

ACHIEVING REDUCED FLARING THROUGH CONTROL VALVES

R. M. SEHGAL, S. K. PATHAK

ONGC, Hazira Plant – India
Email : [email protected]

Abstract

Flaring is a very important safety measure at natural gas facilities as it safely disposes of gas during
emergencies, power failures, equipment failures or other "upsets" in the processing.
Flaring is both a concern to the public and a government priority because of the potential health and
environmental risks associated with the activity and also because it wastes a valuable non-renewable
resource Flaring is an environmental concern with regards to global warming and acid deposition.
Emissions of carbon dioxide and unburned natural gas from flares contribute to the greenhouse gas
effect and global warming.

Hazira plant handles approximately half of the total gas presently available for sales in the country.
For controlling flare in each individual process unit trains like Gas Sweetening Unit, Gas Dehydrating
unit, a flare valve is provided. However if during normal operations the flare control valve is not
properly closed then it is possible that some gas may pass through it. This state of valve is called
passing. Under such a situation we are losing unprocessed gas to atmosphere and also causing
atmospheric pollution. That is why it is important to rectify such leaks to enhance plant efficiency.

With the increased focus on the effects of climate change on the environment, paper highlights efforts
of ONGC Hazira Plant for reducing the carbon footprint and abating greenhouse gases brought on by
plant upsets and how flare control valve 37-PV-1220 of unit train 37 pertaining to Gas Sweetening
Unit was modified to achieve zero flaring. The modification was done within the parameters of defined
ANSI standards related to control valves.

Introduction

ONGC Hazira Plant is situated on the western shore of India and spread over an area of 6.25 square
kilometers. It processes around 46MMSCMD of sour gas and 5500 Mf of sour condensate everyday.
Hazira plant handles approximately half of the total gas presently available for sales in the country.

The plant is equipped with an integrated flare system having a network which connects all the flare
loads from various process trains etc to the elevated flare stack (capacity 730 Tones/ hour). Flaring is
a very important safety measure at natural gas facilities as it safely disposes of gas during
emergencies, power failures, equipment failures or other "upsets" in the processing.

Flaring is both a concern to the public and a government priority because of the potential health risks
and environmental concerns associated with the activity and also because it wastes a valuable non-
renewable resource.

For controlling flare in each individual process unit trains like Gas Sweetening Unit, Gas Dehydrating
unit, a flare valve is provided. However if during normal operations the flare control valve is not
properly closed then it is possible that some gas may pass through it. This state of valve is called
passing. Under such a situation we are losing unprocessed gas to atmosphere and also causing
atmospheric pollution. That is why it is important to rectify such leaks to enhance plant efficiency. The
case study discusses how the problem of flare control valve 37-PV-1220 of unit train 37 pertaining to
Gas Sweetening Unit was rectified to achieve zero flaring. The modification was done within the
parameters of defined ANSI standards related to control valves.

The control valve is a critical link in the process-control chain in any plant. They can be used for
regulating flow or for regulating and relieving pressure.

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Petrotech-2010
31 October-3 November 2010, New Delhi, India
The flare valve PV-1220 is a 14 inches, 600 Class Globe Type valve having equal percentage
characteristics.
Before we move further it is pertinent we learn about few important terms related to Control Valves
like Capacity, Flow Coefficient and Flow characteristics.
Capacity of Control Valve is The rate of flow through valve under stated conditions.
Flow coefficient Cv: is a constant used to establish the capacity of valve. It is the number of US
gallons per minute of water at 60 deg. F that will flow through the valve with 1 PSI pressure drop a
stated pressure and % of rated travel TRIM are the internal components of valve that modulate the
flow of controlled fluid.
Flow Characteristic The relationship between Valve capacity and valve travel is known as the flow
characteristic of the valve. Valves can have quick opening, linear, equal percentage flow
characteristics.
A quick opening flow characteristic produces a large flow rate change with minimal valve travel until
the valve plug nears a wide open position. At that point, the flow rate change is minimal with valve
travel.
A linear flow characteristic is one that has a flow rate directly proportional to valve travel.

An equal percentage flow characteristic is one in which a flow rate change is proportional to the flow
rate just prior to the change in valve position. Equal increments of valve travel result in equal
percentage changes to the existing flow rate.
The change in flow rate observed regarding travel will be relatively small when the valve plug is near
its seat and relatively high when the valve plug is nearly wide open. Therefore, a valve with an
inherent equal-percentage flow characteristic provides precise throttling control through the lower
portion of the travel range and rapidly increasing capacity as the valve plug nears the wide-open
position.
Valves with equal-percentage flow characteristics are used on pressure control applications, on
applications where a large percentage of the pressure drop is normally absorbed by the system itself
with only a relatively percent f' of maximum flow rate.

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Petrotech-2010
31 October-3 November 2010, New Delhi, India

The purpose of characterizing control valves is to allow for relatively uniform control stability over the
expected operating range.

The selection of valve body material and trim material is typically based on pressure, temperature,
and corrosive and erosive properties of the liquid. Typical materials that one can cast into a valve are
carbon and stainless steel, Monel, Hastelloy, iron, and bronze. Valve seats are an integral part of a
valve. Seat material must be compatible with the process fluid. Valve seats can be either metallic or
nonmetallic.

In addition, the amount of valve leakage is important, and the process and design requirements
dictate how much leakage is allowable. Control valve seats are classified as to and in accordance
with ANSIFCI 70-2-1991 for leakage (Refer Table)

Valve seat leakage classifications

Leakage class designation Maximum allowable leakage

I-

II 0.5% of rated capacity

III 0.1% of rated capacity

IV 0.01% of rated capacity

V 5x10-12m3/s of water per mm of seat
diameter per bar differential (0.0005 ml/min
per inch of seat diameter per psi differential)

VI Not to exceed certain specified and stringent
amounts

DETECTING PROBLEM OF PASSING, INVESTIGATION AND SOLUTION

There are no direct methods to pinpoint that flare valve is passing. However the following methods
are used to arrive at a conclusion:

1. Increase in flare of the flare header which is also indicated the flare header meter
2. Sweating on pipeline on which valve is installed.
3. Once the valve is identified, the valve is closed with hand-wheel and it is confirmed whether

any reduction in reading of flare meter has occurred.

Once it is established that valve is passing, attempts of rectification of problem starts.

A thorough investigation of the problem and its causes was undertaken. In this application, the control
valve is required to provide tight shut-off capability. There are actually six different seat leakage
classifications as defined by ANSI/FCI 70-2-1976.

Most control valves application provide valve seat leakage classification as per CLASS IV.
CLASS IV is also known as METAL TO METAL. Certain kind of leakage rate is always expected from
a valve with a metal plug and metal seat.

CLASS VI is known as a SOFT SEA T classification. SOFT SEAT VALVES are those where either the
plug or seat or both are made from some kind of composition material such as Teflon.

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Petrotech-2010
31 October-3 November 2010, New Delhi, India
Therefore to provide tight shut off it was decided to provide a soft seat instead of metallic one. This
would confirm to Class VI of valve leakage classification Table and would be improvement over the
original valve leakage Class V.
To carry out the modification, shutdown of process unit train was taken. Prior to the unit train GSU
shutdown, tripod with chain pulley arrangement was made for lifting of the big control valve. Once
shutdown of GSU was taken, upstream and downstream isolation valves were completely closed. For
safety reasons, all fluids were drained.
First actuator was disengaged and removed and then valve was dropped from bonnet. The old stem
and plug assembly was taken out. The orientation of parts, such as gaskets, seat rings, plugs and
actuators was marked for proper re-assembling.

INTERNALS OF VALVE

Sowhat maygowrongwithacontrolvalveandreduceoverallperformance?

• Actuator

BrokenSpring
Air leakage(diaphragm, tubing)

• Positioner

Linkage
Pneumatics(I/P,Relay)
Calibration/ Performance
Lossof air supply

• TrimWear (plug/ seat)

Seatleakage
Bodydamage

• PackingWear

FugitiveEmissions
StemWear/ Corrosion

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Petrotech-2010
31 October-3 November 2010, New Delhi, India
A new stem and plug assembly with Teflon coated seat was installed. The valve was reassembled.
The Line gaskets were also changed. Stroke check was also done.

VIEW OF STEM AND PLUG ASSEMBLY WITH SOFT RING
RESULTS AND CONCLUSIONS
After the valve was taken into service, no passing was observed from the flare valve. Thus the
modification of having a soft seat had worked successfully. Based upon above experience all flare
valves have been fitted with soft seats.

Modified Flare valve in Line
Valve in Line after modification

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Petrotech-2010
31 October-3 November 2010, New Delhi, India
About the authors
1. Sh. R. M. SEHGAL---CE(INSTRUMENTATION)-I/C Process Instrumentation
Joined ONGC in 1986 and has worked in both upstream and downstream streams. Presently he is
posted at ONGC Hazira Plant. He is responsible for process maintenance, continuous plant
operations and process optimization troubleshooting. Principal Experience encompasses various
fields of Engineering like Design, Erection and Commissioning, Maintenance, Turnaround
Management, Contract Management & Safety. Well experienced in Gas Processing Plants and has
carried out numbers of modifications for efficient process control and has been instrumental in
introducing new instrument and control technologies like Guided Wave Radar for Level Measurement,
SIL rated Fire &Gas Systems, Safety PLC for rotating equipment protection , SMART Positioners etc
in Hazira Plant. He is a senior member of ISA.
2. Sh. S.K. PATHAK—GM(P)-Head Operations Hazira Plant
He has around thirty years of varied experience in Oil & Gas sector including Asset Management,
Failure Analysis, and Data Management. He is having hands on experience on all process units of
gas processing complex like Gas Sweetening Unit, CFU, and Sulphur Recovery Units. He has carried
out number of modifications for improvements in operations and increasing production. He is a strong
advocate of adoption of new technologies for minimizing human intervention in process operations.

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