Problem in crack gas compressor start up

[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%.It should be noted that 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
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? 

 

[Mohammad Ka]

rotw,
- The maps are predicted. Can you please tell me how this solve the conflict?
- I am using a third party simulator , AspenHysys.
For sure, I do like to access the dropbox.

 

[rotw]

- Predicted map reflect vendor prediction model of performance. When machine is installed in the field, the performance deviate from the predicted model. The magnitude of the deviation depends case by case. Up to ~5 to 10% deviation have been reported. Thus it is recommended to base the analysis upon 'as tested' maps. But I understand, this information is not available.

- You could give me your email and I will add you as member in the dropbox. But this would only be useful to you, if you require me to analyse a particular scenario. I could then share for example the results in dropbox if this is more suitable.

- By the way, if there is any chance to receive the trends in a more readable state, it would be also better.



Life is not about waiting for the storm to pass. It's about learning dance in the rain.

 

[Mohammad Ka]

georgeverghese,
Talking with instrumentation engineers of the plant, it is revealed ASV on stage 5 could be activated by discharge pressure or FIC or MV by DCS operator or anti-surge algorithm internal controller as soon as reaching the min. governor speed. On the other hand, the activating factors on ASV stage 4 are suction pressure / FIC / MV by DCS operator / internal controller. Each PV is compared with relevant SP and the maximum difference would be selected to activate the ASV after min. speed.
So, ASV on stage 5 is activated by high pressure on 5D and NOT low pressure on 5S while high pressure on 1S my activate ASV on stage 4 without waiting for the algorithm.
I suppose 17barg is stage 5D pressure and not 5S. If we assume high pressure activation on 5D (based on these data)and the characteristic of ASV is Cv=463gpm, the valve will show about 84 t/hr ASV2 recycle capacity at 17barg on 5D and about 10barg on 5S. The result is same when discharge and suction of stage 5 are 15.37barg and 9.71barg, respectively (This is the successful run). Do you believe this recycle capacity is sufficient?
In all failed starts, the discharge flow of stage 5 is less than 139t/hr while in the normal one is higher than 150t/hr. Is this suspicious or not?

 

[georgeverghese]

In theory, ASV2 should be operated by low set PIC on 5S also.
So what is the min governor speed which allows these controllers to take over? We see most of these trends indicate speed at about 4600rpm during early part of startup when surging occured.
Yes, you are right, 17barg is the 5D pressure I was referring to, not 5S.
The high set discharge PIC which operates ASV2 would be set at >35barg or so, so it cannot be the one which operated ASV2 in all these startups.
The full Cv of ASV2 is stated to be >600, so we should use say 90% of the wide open Cv of ASV2 for the rating check of ASV2 for these failed startups.
In any case, it is obvious that good operating procedure would be to
a)Ensure that neither ASV1 or ASV2 should be operated say more than 90% open during startup / pressure ramp up.
b)If ASV2 is approaching 90% open, then more fresh feed from the furnaces must be enabled. Running the compressor with ASV2 at 100% open during pressure ramp up is bad operation. c)Speed also should be sufficiently high so that compressor stage 5D discharge pressure can be developed.