2 replies to this topic

[sunny143]

Good day folks,

 

Attached is the sketch of the scenario taken for your review;

 

My Question is

 

"If surge valve FCV fails open then i assume that the forward flow will be decreased and suction KO drum pressure will be increased which ultimately will required high compressor horsepower and shall trip the compressor." 

 

If the same scenario lies for 2nd stage of compression then opening of 2nd stage spill back line will cause low forward flow and hence will cause high suction pressure and stop the flow from 1st stage of compression and ultimately opening 1st stage spill back line.

 

Please share your experience for the scenarios i have mentioned.

Attached Files

•  ASV Fail Open Case.jpg   98.62KB   33 downloads

[cg89]

@sunny143

 

Anti surge valve is designed to fail open and in that case, your 1st stage suction KOD pressure will rise. Now you must have pressure protection (PSV) of compressor till 1st stage (by calculating settle out pressure). Generally settle out will always be less than the pressure your 1st stage KOD will see in case of anti surge valve fail open. So in case of fail open of anti surge valve, PSV will pop open and compressor will trip due to low low pressure in suction (PAL trip should be there in compressor suction).

 

Even i would like to post a question in this thread.

 

1) When we provide more than one anti surge line (individual anti surge for individual stage)?

Any response will be highly appreciated.

[Babu Prasad]

"If surge valve FCV fails open then i assume that the forward flow will be decreased and suction KO drum pressure will be increased which ultimately will required high compressor horsepower and shall trip the compressor."

Any sudden opening of anti surge will increase load to compressor and trips the driver on overload which inevitable to save the compressor from surging. By default anti surge valve must be fail to open in case of any instrument air fails to prevent compressor from surging.

If the same scenario lies for 2nd stage of compression then opening of 2nd stage spill back line will cause low forward flow and hence will cause high suction pressure and stop the flow from 1st stage of compression and ultimately opening 1st stage spill back line.

If second stages fails open fully there will be no forward flow and same time first stage also will force open due to back pressure. Generally all suction drums will be equipped with suction pressure control valve to flare/atmosphere to release any excess amount gas which cannot pushed forward due to under design flow rate.

Conventional surge controller which depends only real suction flow will have slow response over any process change and require operator intervention to load the machine.  Nowadays advance surge controller will get varies input from suction pressure/temperature ,diff pressure across each stage and will initiate quick action based on inputs.  So machine will trip only if any surge valve fails open fully due to malfunction otherwise advance controller configured such way that machine loads and unloads smoothly without operator intervention and trips open the anti surge valve according to  configuration of surge limit/count.

In positive displacement compressor suction lines are provided with PSV to avoid over pressurization of suction lines in case of recycle valve gets wide opened. But centrifugal compressor suction lines does not require PSV as pressure will not buildup beyond designed discharge pressure compressor.  

Edited by Babu Prasad, 06 March 2015 - 09:07 AM.