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Discharge Design Pressure For Centrifugal Compressors


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#1 SawsanAli311

SawsanAli311

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Posted 11 March 2020 - 01:26 PM

Dear All,

based on your experience, what is the optimum criteria for setting the discharge design pressure and hence the PSV set point at compressor outlet, I recommend the following:

take the suction upstream system PSHH trip with the consideration that under a blocked outlet scenario, the upstream system will pack with no credit for any flaring at the upstream via any pressure control valve to flare.
Then, consider compressor speed up to the maximum continuous speed of 105% of the rated speed since under the blocked outlet scenario, the anti-surge valve will be triggered by its high discharge pressure limit loop dictating as well the performance controller to speed up along with it by something called loop decoupling. Approximating the increase in the speed into a compression head will be fan laws where 1.05^2 correspond to 1.1 increase from the rated head or i..e compression ratio.

then finally considering PSHH suction * 1.1 would give the discharge design pressure downstream the compressor.

 

Or do you propose to consider suction PAHH (alarm) with a multiplier of 1.1 of the normal compression ratio . Overall margin of 5% can be applied to the above term.

In case a PSV is to be eliminated, then a higher margin needs to be considered to mimic the maximum surge differential pressure i.e. maximum suction PSHH + 1.3* normal differential head.

Appreciate your views.

 



#2 Pilesar

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Posted 13 March 2020 - 06:31 AM

It sounds like you are thinking through to get reasonable results which is certainly better than guessing. I don't have a better formula to replace yours. I would also consider actual values for pressure. Whether your final calculated answer were 4 atmospheres or 40 atmospheres makes a difference on how I would adjust such a calculated result. A PSV is to protect equipment and piping and is not necessarily based on a theoretical design pressure calculation. If there is existing equipment, I would generally use maximum allowable working pressures and ignore design pressures when setting PSVs. Downstream equipment to be protected must also be considered which may not be rated up to your calculated pressure. Even in setting compressor case design pressure for new equipment, I might round your calc up to the pipe flange rating if it were not much higher. 



#3 SawsanAli311

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Posted 16 March 2020 - 11:43 AM

Thanks Pilesar, 

 

When it comes to pipeline design pressure , setting the discharge PSV set point becomes an important aspect. At the same time, the selected setting shall cater for future de-bottlenecking and capacity increase as well as compressor's speed up under blocked outlet or previous compressor's stage recycle operation. 






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