Noise, insufficient flow and increased discharge temperature of crack gas compressor

[Mohammad Ka]

Hi everyone
During start up of an ethylene plant, discharge temperature of the crack gas compressor (fifth stage) was drastically increased, meanwhile very high amount noises like fast opening and closing of a valve was observed. Moreover the compressor faced with insufficient flow. In this case, opening of the antisurge valve was 100%.
Other trends could be listed as below:
1- Outlet temperature of crack gas from quench tower was increased about 4-5 deg.C.
2-Level of discharge and suction drums of the crack gas compressor (stage 4) were increased during starup.
3-Discharge temperature of 5th stage of the CG compressor was increased to 130 C.
4- Fluctuation of flow was observed in all five stages of the compressor.
5-Alongside the flow fluctuation some small fluctuinations of pressure drop was observed in caustic tower.
6- No vibration in CG compressor was detected by DCS.

The following activities to remedy have been done:
* Antisurge valve was opened and fully inspected . No failure was detected.
* All suction and discharge drums, check valves, orifices, cooling water exchangers and caustic tower in the line were inspected and no chocking or unusual thing was seen.
* About 1000kg mixed yellow oily water was observed in cooling water exchanger of 5th stage of the crack gas compressor. Antisurge valve is branched from outlet of this exchanger. The water was completely drained.
*Stage 5 of the compressor was inspected by a borescope camera and no failure was detected.

After these remediations and start the plant for the next three times, the noise, insufficient flow and increased temperature were again repeated.
I was wondering if anyone could propose a solution for this problem.

 

[georgeverghese]

What about the FE on the CGC stage 1 suction which is part of antisurge loop - was it checked and found to be clean? Are there any low point piping pockets in this compressor suction line upstream of the FE ? Check the impulse tubing going to the FE dp cell also - it should not be pocketed, since it is in high dewpoint temp cracked gas service. A lot of this noise is probably from the operation of the antisurge valve - is the valve specified to be low noise?

 

[Mohammad Ka]

Thank you Georgeverghese
There is no FE on antisurge loop Of CGC stage 1 and no piping pockets in the suction. However, a pumping trap is installed in upstream of suction line CGC stage 1. Antisurge valve with a difuser is specified to be a low noise. A part of Pid related to the suction line is attached.

 

[georgeverghese]

It is indeed suprising that this suction line to the CGC is pocketed, first time for me to see such an arrangement for this critical piece of kit. The diagram shows a 40inch line acting as the low point drip leg feeding this pumping trap. The diagram doesnt show the line going to stage 1 of this compressor after the take off to this drip leg.
a)If there is no FE on this suction line, then how does the antisurge control loop work?
b)The take off to the antisurge valve is shown to be taken from stage 4 discharge - why is it not taken from stage 5 discharge?
c)How does this pumping trap work? Your first post does not say anything about checking the operating integrity of this unit.

 

[Mohammad Ka]

a) antisurge control the loop via PT and TT on the suction line of compressor and also FE on discharge of CGC stage 4 and stage 5.
b) because a caustic tower is located between stage 4 and 5.
c) the pumping trap is simply worked by steam and the liquid level is constantly checked by field operators.