5 replies to this topic

[kpa]

I am working on a reformer section. Currently P&ID shows attached arrangement of compressor circuit.

Compressor is been provided with two auto isolation valves on suction and discharge respectively for emergency situations which trips the compressor.

The minimum flow recycle line (surge line) is been circulated from air cooler as there is considerable temperature rise across the compressor.

I have following querries:

1. During surge condition of the compressor, there will not be any problem because of the isolation valves and spectacle blinds near the auto isolation valves.

2. The auto isolation valves are provided as FL (fail lock) positions, why not FC (fail close)?

Please advice.

Thank you in advance

kpa

Attached Files

•  Compressor__Antisuge_Control_valve_Ckt.xls   25KB   170 downloads

[djack77494]

I looked over your schematic and am not sure what valves will and will not have automated actuators. Though I am certainly NOT a compressor expert, I have seen installations where the anti-surge valve is inside the suction and discharge isolation valves. In you case, for example, you might have a fail closed inlet isolation valve, then tie in your anti-surge (spill-back) line, then go through your air cooler, your trim cooler and into your compressor. On the discharge side, you'd come out of the compressor, split off for the spillback line, and then have your fail closed discharge isolation valve. The anti surge valve would fail open.

Though I would prefer an arrangement as described above, it does have the disadvantage of increasing the gas volume trapped between your isolation valves. Only by examining your actual piping layout or design can you tell how significant that aspect of the design compares to the positive aspects.

Good luck in your design,
Doug

[rxnarang]

I looked over your schematic and am not sure what valves will and will not have automated actuators. Though I am certainly NOT a compressor expert, I have seen installations where the anti-surge valve is inside the suction and discharge isolation valves. In you case, for example, you might have a fail closed inlet isolation valve, then tie in your anti-surge (spill-back) line, then go through your air cooler, your trim cooler and into your compressor. On the discharge side, you'd come out of the compressor, split off for the spillback line, and then have your fail closed discharge isolation valve. The anti surge valve would fail open.

Though I would prefer an arrangement as described above, it does have the disadvantage of increasing the gas volume trapped between your isolation valves. Only by examining your actual piping layout or design can you tell how significant that aspect of the design compares to the positive aspects.

Good luck in your design,
Doug

I would do this. As Doug says the voulme trapped between auto shutdown valve increases, so you have to evaluate if the compressor will not surge when the spill back lines opens. I believe the maximum time for spillback loop to react is of the order of 500ms, which is a function of opening time of the anti-surge valve, and the volume trapeed between valves.

Regards

Attached Files

•  Compressor_cheresources1..xls   25.5KB   116 downloads

[pawan]

Not clear about Auto Or Manual Actuation. (Got it they r Remote actuated).

But, During surge,

1. Blinds are not going to do anything.
2. control depends on Antisurge response.
3. Both Isolations can't fail simultaneously.
4. U r not dependant on one valve, its a combination 3 valves.
5. U cant consider antisurge without considering cooler in the loop.

6. The arrangement is the best combination followed, in this case u cant have FC Isolation it has to be FL.

The other arrangement requires,

1. Create another loop in the main cycle.
2. Piping rearrangement for gas lines which are bigger usually.
3. No significant advantage.

I am not able to understand the Objective behind Ur queries.
I dont see any significant gain if U do modification for Anti Surge within the valves.

[djack77494]

Only the anti-surge valve and associated instrumentation protect the equipment against surging. The other valves and the blinds serve other (non-related) purposes.

To elaborate a bit further, the isolation valves are important for safety, maintenance, and perhaps other purposes. Compressors have seals and they can develop leaks. For an air compressor, this is a loss of efficiency and not much more. For other compressors, it is more serious. I tend to think in terms of natural gas or other light hydrocarbon compressors if "compressor" is not otherwise defined. If you detect a leak near such a compressor, you want to immediately isolate the source, and you hope the trapped gas volume that will have to be vented is minimal and will be vented safely. This is best achieved by having remotely actuated isolation valves as close to the compressor as possible. Depending on the circumstances, you may also want an actuated vent valve to rapidly depressure the trapped volume.

Develop valve failure positions with common modes of failure in mind. For example, if your instrument air and/or electric power were to fail, all the valves could move to their failure positions. Set them with this in mind. (BTW, I have a very low opinion of the so called "fail locked" option. To me, that means that if it were best for the valve to be in the open position, it will probably be closed at the time of failure. Also, it is more difficult to implement.)

Doug

[kpa]

Thank you all for your views, I got to know more detail things related to auto isolation valve.

Actually, I was more worried about the manual block valve upstream to the auto-isolation valve in the compressor suction.

If this valve is closed by mistake then there will not be any flow to the suction during anti-surge condition. So, to avoid such circumstances to occur, I was planning to make anti-surge line inside the suction and discharge isolation valves.

But, for doing so I am planning to make following arrangement.

Discharge side, I can easily shift the taping before isolation vale, but for suction side, I have to consider the separate line (loop) from downstream of cooler to the downstream of auto isolation valve. Has anybody seen this type of arrangement anywhere?

Secondly, I fully agree with Mr. Doug for not giving Fail Locked positing of the Auto isolation valve. As these are more difficult to implement and need special arrangement than Fail Close positioning.

I do not see any special advantage of putting Fail Lock position of the valves in my case because in any case valve is failed it will automatically trip the compressor.

So can I change two auto isolation valves to Fail Close position instead of Fail Lock position?

Thank you inadvance
kpa