Suction throttling valve: is a mechanical stop always required? 1

[JustSomeRoark]

Gents,

I have seen a few gas compressors driven by a fixed speed electric motor controlled by suction throttling where the inlet throttling valve has a mechanical stop, or some other means to prevent the valve from fully closing. I've enquired a couple senior process engineers at my company about this requirement and their advise is that it is required to prevent reaching sub atmospheric pressure at the suction of the compressor.

However, if the normal operating suction pressure is sufficiently high I cannot envisage any scenario which may cause said vacuum.

Could anyone share their expertise on the topic? Why is this mechanical stop required? For what scenarios shall it be considered? Do you have any experience with constant speed drive, centrifugal compressors controlled by suction throttling where the inlet throttling valve is allowed to close fully?

Many thanks in advance!

Regards.

 

[georgeverghese]

This is most likely to address the case where an overly sensitive suction side PIC over reacts to a reduction in compressor throughput ( due to some operational upset at consumers downstream of compressor) and slams down on its PCV in order to prevent compressor suction pressure from going beyond setpoint. This may in turn cause the compressor to go into surge if the antisurge controller is less sensitive than this feed line PIC, and is too slow to compensate for the over reacting PIC.

While a mechanical min stop at the field PCV is one solution for an over reactive suction line PIC, a "soft" min stop at the PCV at DCS will do just as well. But there may be cases where even at this min stop position, suction line pressure to the compressor continues to escalate, this being due to a drastic reduction in downstream take off. For this case, an excess pressure blowoff PIC / PCV ( leading to flare for example) may be required at the compressor discharge or suction, if you want to avoid a compressor trip. Other control solutions to address this may also be possible.

 

[rotw]

First thing is throttling valve inside or outside antisurge loop, if inside mechanical stop is recommended in addition to min. stop by control system. A closed valve on failure or whatsoever could otherwise lead to surge of the machine despite antisurge control action.
Second excessive throttling on low pressure application with explosive mixture could lead to hazard if there is vaccum pressure and external gas is ingested thereby rising the risk of mixture attaining explosive level.

 

[MortenA]

Is the valve perhaps flow to close? If so it may be to avoid excessive torgue to open the valve

 

[JustSomeRoark]

Many thanks for your replies, @georgeverghese and @rotw.

I understand your scenario, but is it realistic to consider that, even assuming that the antisurge controller fails to react (control loop failure), the full closure of the suction PCV may lead to vacuum conditions at the compressor suction? I think that this may be the case if the suction pressure is slightly above atmospheric (i.e. a higher DP in the valve may drop the pressure below 14.7 psia), but I fail to see how this could occur in cases with suction pressures of, say, 200 psia.

Think for example about a compressor with a suction pressure of 200 psia (refer to the sketch attached:

https://files.engineering.com/getfi...8-9e9d-4d83-bb21-103b12ea1ffe&file=sketch.png

). The PCV normally drops 10 psi, so the pressure upstream of the PCV is 210 psia. The compressor performance curve shows a pressure ratio Rc = 3 at normal operating conditions and Rc = 4 at surge. Hence in normal operating conditions the discharge pressure is 600 psia. Assuming the most stringent scenario of the PCV suddenly closing and the ASC loop failing to react (i.e. the ASV stays closed), in order to reach a pressure below 14.7 psia at the suction, the compressor should develop a pressure ratio of 600/14.7 = 40.8! That's way too much increase above the normal Rc, isn't it? Would you agree this is realistic?

On the other hand, if the suction pressure of the same compressor was 16.7 psia (2 psig), the discharge pressure would be 50.1 psia (Rc = 3). In this case, if the inlet flow reduced suddenly and the Rc increased to the max reasonable value (Rc = 4), for a constant discharge pressure the suction pressure would be 50.1 psia / 4 = 12.525 psia = -2.175 psig. In this case, for the same compressor, the fact that the suction pressure is nearly atmospheric could cause indeed a sub-atmospheric pressure.

In summary, yes, suction pressure below atmospheric may be an issue should the PCV close too much but only if the normal suction pressure is around 0 psig. Would you agree with this train of thought?

Regards

 

[rotw]

quote
Assuming the most stringent scenario of the PCV suddenly closing and the ASC loop failing to react (i.e. the ASV stays closed),
unquote

1/ - The AS-Valve is fail open, on actuator failure or if signal is de-energized, valve goes fully open anyway. By the way, your point involves double failure if by 'sudden closure of throttling valve' you mean a failure. Double failure is typically outside any Hazop scenario. In any case, if AS valve stays closed for any reason and there is no forward flow to compressor, it surges in matters of seconds.
2/ - Throttling valve is outside ASV loop so it will not create obstruction in case of throttling valve failure. In some cases, throttling valve is inside the loop to gain advantadge to decrease load on motor when it is started direct on line. This is not the case here, so this is fine.
3/ - Nothing unsual with the sketch, standard flow diagram.
4/ - Suction pressure is well above atmospheric pressure, speaking of vaccum pressure is almost irrelevant.

I think mechanical stop is in this case optional, adding a layer of protection.

quote
Assuming the most stringent scenario of the PCV suddenly closing and the ASC loop failing to react (i.e. the ASV stays closed), in order to reach a pressure below 14.7 psia at the suction, the compressor should develop a pressure ratio of 600/14.7 = 40.8! That's way too much increase above the normal Rc, isn't it? Would you agree this is realistic?
unquote

Not this is not realistic. The compressor would have surged long long time before that fictitious scenario. Also this is a dynamic behavior which requires a special simulation to gain insight of what is really going on.

 

[georgeverghese]

In the case when the suction PCV is fully closed, the compressor goes into full recycle by action of the antisurge controller. The discharge side check valve ( d/s of cooler) would have fully closed, and there wont be any net flow out to the consumers, since there is no fresh feed to the compressor. Doubt if this will cause the compressor suction pressure to drop below atmospheric, even if normal operating suction pressure is close to atm.

However, it is possible that during normal operations, fluctuations in the suction side PIC operation can cause suction side pressure to momentarily drop below atm for the case when normal suction pressure is 2psig, even with the mechanical min stop on the suction PCV. Think the main concern with momentary subatmospheric dips may not be with air ingress through suction piping, but air ingress at the shaft seals - especially with the simpler configurations where there is no inert gas barrier. You may have to change out seal type if this is the case and may well be justified on process safety grounds - check with your mechanical / rotating machinery engineers.