8 replies to this topic

[benabed]

Dear freinds.
During the commissioning of a new oil treatment plant where I am working the contractor failed to maintain an interrupted opération of the gas lift compressors(motor driven). The gas lift compression section consists of two parallel centrifugal compressors with two stages for each one(single shaft).

The compressors trips in most cases was du to low suction pressure and as a justification, the contractor attributed the problem to the available gas flow rate coming from the wells which is less than the minimum required without any indication about how much this minimum is?!

The frequent trips of the compressors during the commissioning resulted in the failure of the dry seal of one compressor. As a result the owner is doing a modification to import gas from another location for the start up of the compressors.

From my understanding of the compressor opération this problem has no thing to do with the gas flow rate, but is du to the compressor control which is not able to cope with the fluctuating nature of the feed gas from the wells.

I need your comments and assistance in this matter, because this problem is causing a loss of production opportunity du to the shut down of the gas lift compressors.


Regards.
L.Benabed

[Steve McGahey]

Sounds to me like you need to start on full recycle and a low speed until sufficient inventory has built up.

[djack77494]

This doesn't sound like a dificult problem to me. Control forward flow to maintain your minimum needed suction pressure. Also, confirm with your compressor and/or controls vendor that the anti-surge controls are working and tuned properly. Anti-surge controls are not so ordinary, and you should have the experts have a look at them. These controls will recycle whatever quantity of gas in necessary. You shouldn't be experiencing frequent compressor trips.

[astro]

benabed

Have you tried sourcing the compressor's performance curve (polytropic head vs flow) and mapping the operating point as the machine approaches its trip point?

You're always better off understanding a machine's operation by relating it to the performance curve and assessing the system resistance and how the 2 marry up. You could possibly estimate driver power from motor amps or similar to provide you assurance of your operating point assessment.

You could then cross check this data against any trip points that may have been configured, to at least rule out a problem there.


I'd agree with Steve McGahey that this could be simply resolved by starting up under full recycle. This relates to the performance curve. If you're dealing with an open discharge (i.e. low polytropic head) situation the power draw will be high because you're effectively trying to run at high flow. Get the curve and work through the logic. The answers should fall out by themselves.

Dear friend

Every compressor has a minimum operation requirement in order to start up. Called permission to start. Now you need to go back and see what was the compressor design data. i.e ( P, F, T and gas composition ) etc..

otherwise you need to do a compressor modifications according to your present condition.

[benabed]

Dear Hussein.

We had started up the compressor in the past and at each time it trip by low suction pressure in most cases, so we didn't have a problem of permission to start. I think it is a control problem. As i know there is no minimum required flow for the start up because the control can recycle the flow needed as djack said. Also the minimum suction pressure is overrided at every start up. Thank you for your input.

[A.Subburayan]

Dear benabed,

Do you have the compressor controls controller?If you say yes,please check the ranges of the control and trip transmitters both in the field and DCS/TMR.If thers is any deviation from the design/specification,please correct them.The following are suggested as the actual control scheme was not informed.

Since compressor is getting frequently tripped on suction pressure low,control scheme can be:

1.Give suction PT to compressor inlet valve through suction pressure controller
2.Add program to throttle the inlet valve fast enough to bring back the suction pressure.
3.Link suction pressure indication to antisurge controller so that incase of sudden suction pressure drop below the required set point, anti surge valve(ASV) can open and cope up the situation(in this condition ASV will open along with suction valve throtlling action when the pressure drops below the set point).
4.It is better to maintain the suction pressure bit more to avoid sudden pressure drop and tripping of the compressor due to low suction pressure.
5.Until identifying the exact root cause, try to run the compressor with ASV and related controls on manual. Be careful: NEED VERY CLOSE MONITORING AND CONTOL WHEN CONTROLLERS ARE ON MAUAL MODE. ONCE IDENTIFIED THE PROBLEM TRY TO FIX IT. OTHERWISE, CALL VENDOR TO TUNE THE CONTROLLERS AND FIX FOR YOU.

A.Subburayan,Qatar

[iyer]

Tune the antisurge! I assume you did do a surge test (or the vendor did)

[demank]

Dear Benabed,
Mr. A.Subburayan give a good explanation, you can follow it.

Btw, The gas lift compression section consists of two parallel centrifugal compressors with two stages for each one(single shaft). So there are two stage rite?
Then as preiously posting, it need recycled stream when the discharge pressure from the first stage is too low (below surge point).
And, how if the inlet pressure of the first stage is also too low (which may cause surging in first compressore)?
I think you need install another smaller compressor to overcome this.

NB: Do you install Anti-Surge Vessel before entering compressor?
How About surging controll on existing plant?