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Control valve at the suction of centrifugal compressor

Control valve at the suction of centrifugal compressor

(OP)
Hi. I have a question. In our plant, sales gas at the top of column compress and throttle and goes back to column as a recycle stream. At the suction of compressor we have a control valve named PV-149. Is there anyone explain to me how this valve can be used?
P.S.: Compressor is centrifugal with fixed rotation speed.

RE: Control valve at the suction of centrifugal compressor

The inlet valve may be used to throttle the blower back. The amount that the blower may be throttled back depends on the surge limit of the compressor. Throttling the blower is a method to save energy.

RE: Control valve at the suction of centrifugal compressor

from your process flow diagram, it appears that the valve is set up for back pressure control, to avoid pulling air into the flare system or other flare streams into the compressure suction.

You also have a condensate receiver upstream, you may also have to maintain a certain amount of pressure there,

RE: Control valve at the suction of centrifugal compressor

Very difficult to say without reading the control narrative or actually knowing what that valve is controlling on.

I doubt it's back pressure as you could then starve the compressor of flow.

Given it's a fixed speed compressor, it might be there to limit the inlet pressure to the compressor so that the outlet pressure doesn't exceed a certain value.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

RE: Control valve at the suction of centrifugal compressor

(OP)
a PIC which gets pressure at the discharge of compressor would command this valve

RE: Control valve at the suction of centrifugal compressor

Then you've answered your own question.

It is there to limit/control the outlet pressure of the compressor by controlling the inlet pressure/flow

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

RE: Control valve at the suction of centrifugal compressor

(OP)
But I think it's weird control strategy. Why a control valve at suction of compressor? How this valve control outlet pressure?

RE: Control valve at the suction of centrifugal compressor

It is controlling the inlet flow, not the pressure. The pressure will vary slightly as the flow changes. As the flow decreases, the pressure will change as shown on the performance curve.

The PIC is probably there to prevent the blower from going into a surge condition.

RE: Control valve at the suction of centrifugal compressor

I think one approach of control of throttle valve is controlling the pressure at the suction drum so it stays at a specified value. The valve opens or close thereby creating a back pressure which keeps the suction drum pressure at specified value. In this case, the discharge pressure of the compressor is resulting, or so to say floating. The only way to control the discharge pressure, if needed, would be to use flow recycle as additional control method by opening FV-139.

In this example it seems different philosophy applies since it was indicated the suction throttle valve is controlled based on compressor discharge pressure (62 bar). So for instance if the sales gas mass flow decreases, the discharge pressure would increase because the compressor speed is constant. In this case, the controller would gradually close the throttle valve as a result the suction pressure of the compressor reduces. Correspondingly, the actual flow increases which would then reduce discharge pressure by some amount (because the compressor point move to the right of the curve) in addition to the reduction due to decrease of suction pressure ; these are not conflicting objectives.

One point here:
When the suction throttle valve is actuated towards the closed position, it creates a back pressure on the feeding line upstream, which would reduce the mass flow ; this effect however is conflicting. Here I am not sure if because this would be of secondary order it would supposedly help to make the control loop stable. Maybe someone could help and clarify.



RE: Control valve at the suction of centrifugal compressor

Bimr. With all due respect I believe you are incorrect. A control valve creates a pressure difference across it as a result of flow passing through it. Both are affected as a result of control valve movement.

Whoever designed the systemclearly didn't mind if the inlet pressure goes up or down and maybe the inlet pressure is quite variable, but the compressor, not a "blower", might like to operate in a fairly small inner pressure band

It's more usual to control on outlet pressure as the risk is that the control valve closes to much and starves the compressor, but not unheard of.

Rotw. We can't see the entire system so it's not clear, but I agree with your second para.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

RE: Control valve at the suction of centrifugal compressor

(OP)
Dear LittleInch.
Don't have an attitude, but rubbish is a little bit rude.
The controller impose action such that it couldn't reduce below 50% opening in order to avoid surge.

RE: Control valve at the suction of centrifugal compressor

Ok. Edited

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

RE: Control valve at the suction of centrifugal compressor

Hosein. You seem to know more about this system than you've been letting on.

Do you not have the control narrative available?

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

RE: Control valve at the suction of centrifugal compressor

When you control flow, this means you measure the flow value and manipulate a control element (throttle valve, speed, recycle valve, IGV) until the measured flow matches the target flow value. This is not the case here; LittleInch is correct.

In addition to normal operation controls, suction throttle valve can also be specified because of compressor start up requirement (transients). For instance, if you have a restart up at pressurized condition, your motor might not be capable to cope with the start up torque requirement. Throttle at the suction is an option to avoid oversizing the motor or blow down and also can be a provision to mitigate any dynamic effects, which usually are hard to predict/understand. But this method has limitation and would be ineffective if the departing restart up suction pressure is relatively low or is close to atmospheric, in fact the effect seen on power reduction would be limited also going sub atmospheric poses other risks and may incur safety consequences.


RE: Control valve at the suction of centrifugal compressor

(OP)
LittleInch, Nope! I don't know more than I letting on. Cause this is a little bit confusing arrangements.

RE: Control valve at the suction of centrifugal compressor

We dont have the full picture here - get the impression that PCV 149 maintains constant suction pressure to the recycle gas compressor, because there are operating cases where the suction header pressure downstream of 310-E-1 ( going to the methanol plant) can vary and go higher than normal operating suction pressure.

Without this PCV, at higher suction pressure, the shaft gas seals on the compressor may not work as intended. Flow through the compressor is controlled at FV 111 at the inlet to 310-T-1. Antisurge min flow is enabled at FV 139.



RE: Control valve at the suction of centrifugal compressor

/quote/
PCV 149 maintains constant suction pressure to the recycle gas compressor
/unquote/

Like I already stated, generally if you do need to control suction throttle valve to maintain a suction pressure at a target value, it is done on sensing the suction pressure upstream the throttle valve, NOT downstream. Maintaining the suction pressure to compressor constant or at a target value is not the proper way. I am not aware of set up done for maintaining suction pressure to compressor constant due to reasons related to seal gas system . You may want to look at this resource for insight on what can go wrong:

http://www.chemwork.org/PDF/discussion/Compressor%...

RE: Control valve at the suction of centrifugal compressor

Error in my previous reponse : PCV 149 maintains a constant compressor discharge pressure ( by throttling in on compressor suction press) so that temp fluctuations on inlet temp at 310-T-1 are minimised. Am presuming that FV 111 is part of a TC on FC cascade loop, with temp control of the mix feed at 310-T-1.

RE: Control valve at the suction of centrifugal compressor

bimr, on the contrary.

One - inlet guide vanes work at a very low pressure as the inlet pressure into the compressor is 1 bara therefore it isn't perhaps as easy to see the effect as other control valves like the one we're talking about. Inlet guide vanes work in a different fashion to normal control valves as they affect swirl and create inefficiencies in the inlet to reduce flow, but at some level they reduce flow by creating a pressure drop across them.

however the text is quite clear - "In the throttle range of these devices, these devices act to change the pressure the first stage impellor sees"

It is not physically possible to separate flow from pressure in a control valve. There is no magic way to affect changes in flow without changes in pressure.

The control logic being used in the example above is maybe a little odd, but it controls the outlet pressure of the compressor by throttling or controlling the inlet pressure, assuming that the inlet pressure is always a bit higher than it needs to be. FLow and pressure then balance out in the compressor so that with asteady flow, the control valve finds it correct position to meet the process requirement.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

RE: Control valve at the suction of centrifugal compressor

That is what I stated above and you disagreed with:

"It is controlling the inlet flow, not the pressure. The pressure will vary slightly as the flow changes. As the flow decreases, the pressure will change as shown on the performance curve."

The reason to do this on the inlet side is that it is possible to save energy. You will not save the energy if you do it on the discharge side.

RE: Control valve at the suction of centrifugal compressor

Lets reverse the problem as follows;

Suppose there is no throttle valve. The machine has fixed speed.

Process is working at its nominal capacity. Compressor is running normally.

Then Operator wants to increase methanol production capacity by 20%. What happens?

The gas is circulated into a big loop. If you increase the flow, pressure drop increases through the process loop.
Machine has constant speed. So with flow increase, discharge pressure reduces along constant speed curve. So this will not work.

What can therefore be envisaged, is to throttle the suction valve PV-149 at normal operation (and live with the wasted energy costs); say you throttle by 10% on normal case. Now comes the increase of capacity case, say the 20% extra methanol capacity. In this case, the valve is gradually opened, and so compressor generates higher discharge pressure ratio. In this way, discharge pressure corresponds to a known flow capacity level by knowing in advance system resistance and behavior. By manipulating the discharge pressure set point, the plant control can increase capacity to desired level as a bonus suction throttle valve is less closed so more energy efficient. We have to be clear PID is pressure control although indirectly the ultimate goal is flow capacity change.

Its also possible to develop similar thinking when sales gas composition is off spec etc. There would be a min and max case and throttle valve offers some flexibility. flexibility is limited, however and is at energy cost. The most flexible solution and energy efficient would be variable speed machine but that would be at higher capital cost.






RE: Control valve at the suction of centrifugal compressor

Addition, there is no risk of vacuum whatsoever (sealing system, explosive mixture etc.) as the suction pressure is at 30 bar.
Pressurized re-start up when applicable may also take advantage of the same throttle valve to relax starting torque requirement which I already mentionned.

RE: Control valve at the suction of centrifugal compressor

Hi Hosein,
My idea is that two control valves of PV-149 and FV-139 works together to control the compressor. They may work on override concept, the normal loop can be through PV-149 (which is more economical control option) and the override loop is handled by FV-139 when there is not enough flow back to the tower.

RE: Control valve at the suction of centrifugal compressor

(OP)
Mr Shahryar Thanks. Great explanation ...

RE: Control valve at the suction of centrifugal compressor

(OP)
Dear ROTW.
Quoted: /The gas is circulated into a big loop. If you increase the flow, pressure drop increases through the process loop / Unquoted.
When we increase methanol flow, the recycle flow through compressor decreases.
And in constant speed, if flow decrease, discharge pressure will increase.

RE: Control valve at the suction of centrifugal compressor

We can see the outlet, but where is the inlet of gas??

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

RE: Control valve at the suction of centrifugal compressor

hosein elbaghli, no disrespect but I think you know more than letting know.

RE: Control valve at the suction of centrifugal compressor

For FV 139 to act as (a) low suction pressure recycle (as mentioned by @shahyar), which is required when there is low net fresh feed appearing at 310-T-1, and (b) compressor min flow recycle.

On closer inspection, this arrangement seems a little odd :
a) the location of FV 139 discharge seems to be wrong; it should be tied in downstream of PV 149, and not upstream, to allow for times when FV139 is acting as min flow recycle (and also when running on low pressure recycle mode).
b)though I'm more used to seeing the excess suction pressure blowoff PCV (in this case PV 135) upstream of the station inlet block valve (in this case UV300/346), I dont see anything wrong with the way it is set up here.

RE: Control valve at the suction of centrifugal compressor

(OP)
Dear Mr. ROTW.
Frankly no, I'm really curios about this and I'm learning right here too.
I would not put you out, if I was fluent on this subject.
Really appreciate dear all. I think I get what I want. Really thanks for your coporate.

RE: Control valve at the suction of centrifugal compressor

Every time I look at this drawing I find different / odd things with it.

1) Is it me or how does 244 ton/hr of gas get heated from minus 93 to + 30, apparently only by 75 tons/hr of the same gas going from 40C to minus 88???
2) It's pretty clear that the re-cycle line has a compressor in order to go from 30 bar to 32 bar, plus pressure losses through 4 HXs. Why the designer thinks it takes up to 30 bar to do this I don't know, but that's what they seem to be aiming for.
3) What's the flow coming into presumably 301-T-1 or does the gas just appear out of thin air?

To do this with a fixed speed compressor and to ensure that the inlet into the compressor is always a certain amount, the control valve PV 149 drops the pressure to some unknown value which is then compressed to 60+ barg. The valve then opens and closes in response to outlet pressure which will vary with flow requirement for the re-cycle loop in general - not clear what sets flow in this system - but flow is massaged by opening the compressor re-cycle line. Keeping the compressor re-cycle line at a reasonable temperature, there are 2 Hxs, one air, one water for some reason - ambient temp issues?? who knows.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

RE: Control valve at the suction of centrifugal compressor

Hosein Eghbali,

What is sketch coming from, FEED, concept study phase?
existing machine? Don't you have any other accompanying document? emails? Ok not emails.

Thanks

georgeoverghese,

a) the location of FV 139 discharge seems to be wrong; it should be tied in downstream of PV 149, and not upstream, to allow for times when FV139 is acting as min flow recycle (and also when running on low pressure recycle mode).

Guess if you bring gas back to compressor suction you need to pass through the knock out drum to avoid risk of condensate / water carry over penetrating the machine. Anti-surge/recycle loop is a fast circuit and the gas here is taken downstream after cooler.

RE: Control valve at the suction of centrifugal compressor

@rotw, Agreed, the PCV 149 and the return recycle stream from FV139 should both be upstream of the suction scrubber in standard design practice.
Does seem like this recycle gas compressor is part of the column top reflux - compression - refrigeration loop for the natural gas de ethaniser column 310-T-1. Net flow of 160tons/hr of de ethanised gas (mainly methane) flows out to the natural gas reformer at the front end of the methanol plant for syngas production.

RE: Control valve at the suction of centrifugal compressor

This suction side PCV149 would be throttling for all part load refrigeration duties, which is a waste of the energy in the feedstream to the compressor. Better operating flexibility woud be obtained with a VSD on this compressor. The PCV149 would then be operating only when required refrigeration flow is below the turndown limit of the VSD.
More OPEX / CAPEX savings can be obtained if we reduced the cold end temp approach on the last HX 310-E-4 from the current 5degC (93 - 88 degC) to say 2-3degC, which should be possible for compact plate type exchangers. This will help to decrease the discharge pressure on the refrigeration compressor.

RE: Control valve at the suction of centrifugal compressor

@LittleInch,

Agreed, the enthalphy balance across the 3 exchangers doesnt seem right, given that the Cp value for 2 streams would be about the same, and there is probably no phase change for the stream being cooled. Suspect the error here is on the column top exit temp of -93degC, which could actually be higher for a lower C2 recovery ratio at the column.

The 30bar drop across FV111 would possibly be what is required to get the exit stream to a temp adequate to enable the desired column top temp / create enough reflux and to meet the column spec on C2 content in the column over head vapor exit stream.

Another energy saving option that would reduce the compressor discharge pressure is to enable an isentropic drop with an expander rather than an isenthalpic drop with the JT valve FV111. A mechanically coupled booster recompressor on the suction or discharge of 310-C-1 is also included as a speed brake for the expander typically. This increases mechanical complexity however.

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