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dear all,
it happened sometimes to have read the following requirements for Blowdown Valve and Emergency Shutdown valves:
"Upon instrument air supply failure, the system shall maintain sufficient pressure in the buffer vessel to allow for at least three valve strokes"
My question: is there a rationale in having such an air buffer when the shutdown/blowdown valves are normally design for fail-safe action?
thanks
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Air Buffers
Started by SafetyUser, Apr 24 2009 07:18 AM
2 replies to this topic
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#2
djack77494
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Posted 24 April 2009 - 09:40 AM
Safety,
You may prefer (for various reasons) to have the valves in some position other than the "fail safe" position during various failures. For example, if you have a compression system with large volumes of high pressure gases, you might not want to blowdown all the trapped volume in the event of an instrument air failure. Perhaps you need to scrub the gas or you just need to disperse any generated fumes and your problem is reduced if you can limit the flowrate of gas to be vented. Then why fail all BDV's open at one instant. Why not control the venting of trapped gas or just let them vent more slowly over a longer period of time. Also, there may be situations where a valve failing open is generally safer than failing closed but that there may be times when you'd rather not have it fail open. A "fail locked" option may be better. If you have an air actuator on the valve, you can maintain a degree of control by having "an air bottle" available for stroking the valve without the need for an external air supply. How could that be bad?
You may prefer (for various reasons) to have the valves in some position other than the "fail safe" position during various failures. For example, if you have a compression system with large volumes of high pressure gases, you might not want to blowdown all the trapped volume in the event of an instrument air failure. Perhaps you need to scrub the gas or you just need to disperse any generated fumes and your problem is reduced if you can limit the flowrate of gas to be vented. Then why fail all BDV's open at one instant. Why not control the venting of trapped gas or just let them vent more slowly over a longer period of time. Also, there may be situations where a valve failing open is generally safer than failing closed but that there may be times when you'd rather not have it fail open. A "fail locked" option may be better. If you have an air actuator on the valve, you can maintain a degree of control by having "an air bottle" available for stroking the valve without the need for an external air supply. How could that be bad?
#3
Whit123
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Posted 12 May 2009 - 09:18 PM
QUOTE (djack77494 @ Apr 24 2009, 10:40 AM) <{POST_SNAPBACK}>
Safety,
You may prefer (for various reasons) to have the valves in some position other than the "fail safe" position during various failures. For example, if you have a compression system with large volumes of high pressure gases, you might not want to blowdown all the trapped volume in the event of an instrument air failure. Perhaps you need to scrub the gas or you just need to disperse any generated fumes and your problem is reduced if you can limit the flowrate of gas to be vented. Then why fail all BDV's open at one instant. Why not control the venting of trapped gas or just let them vent more slowly over a longer period of time. Also, there may be situations where a valve failing open is generally safer than failing closed but that there may be times when you'd rather not have it fail open. A "fail locked" option may be better. If you have an air actuator on the valve, you can maintain a degree of control by having "an air bottle" available for stroking the valve without the need for an external air supply. How could that be bad?
You may prefer (for various reasons) to have the valves in some position other than the "fail safe" position during various failures. For example, if you have a compression system with large volumes of high pressure gases, you might not want to blowdown all the trapped volume in the event of an instrument air failure. Perhaps you need to scrub the gas or you just need to disperse any generated fumes and your problem is reduced if you can limit the flowrate of gas to be vented. Then why fail all BDV's open at one instant. Why not control the venting of trapped gas or just let them vent more slowly over a longer period of time. Also, there may be situations where a valve failing open is generally safer than failing closed but that there may be times when you'd rather not have it fail open. A "fail locked" option may be better. If you have an air actuator on the valve, you can maintain a degree of control by having "an air bottle" available for stroking the valve without the need for an external air supply. How could that be bad?
Agree, it is an excellent idea. I have seen plants with a storage buffer tank for Instrument air, sufficient for at least 30 minutes operation for ALL valves, so that the valves can be operated even in case of emergency/power failure. The fail safe action that you mention is generally for that specific valve, in case of a diaphragm rupture or failure of the postioner/SOV/etc. If the air supply fails for ALL valves simultaneously, then it is better to have a buffer tank to maintain the instrument air pressure so that you can take a more "planned" shutdown of the entire system.
