Control valve at the suction of centrifugal compressor 1

[Jack Nicholson]

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.

 

[bimr]

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.

 

[hacksaw]

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,

 

[LittleInch]

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.

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[Jack Nicholson]

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

 

[LittleInch]

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

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[Jack Nicholson]

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

 

[bimr]

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.

 

[rotw]

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.

 

[LittleInch]

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.[tt][/tt]

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[Jack Nicholson]

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.

 

[LittleInch]

Ok. Edited


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[LittleInch]

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

Do you not have the control narrative available?

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[rotw]

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.

 

[Jack Nicholson]

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

 

[georgeverghese]

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.

 

[rotw]

/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 control, load sharing and anti-surge.pdf

 

[georgeverghese]

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.

 

[bimr]

LittleInch (Petroleum)

Attached an article on centrifugal air compressor basics so that you can see that you are misinformed on the procedures for controlling the inlet flow .

 

[LittleInch]

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.

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Also: If you get a response it's polite to respond to it.