Centrifugal VSD compressor discharge design pressure criteria

[Sawsan311]

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.

In case a PSV is to be eliminated, then a higher margin needs to be considered to mimic the maximum surge differential pressure.

Appreciate your views.

 

[georgeverghese]

I would use the following, if you wish to avoid having to cater for a discharge side PSV rated for blocked in flow conditions:
a) Suction at PSHH
b) Compressor at 105% of max normal speed
c) Highest expected gas mol wt

To get the max polytropic head at the surge point, use ( in the absence of vendor H vs Q curves), the generic curves in the GPSA.
In many cases, overall CAPEX may be lower if you were to use a lower design pressure and install a PSV catering to blocked in conditions, since CAPEX for all downstream equipment may then be lower.

 

[Sawsan311]

Thank you Mr. georgeverghese for your reply,

I think what you are proposing is for the case when a PSV is required since it one of the scenarios dictate the machine to recycle reaching the maximum operating speed of 105% even if the suction PSHH is reached in case the upstream pressure control loop to flare has failed or an upstream compressor stages has been speeding up with a consequent trip under discharge PSHH.

If a PSV is to be avoided, then we need to address the maximum surge differential pressure with a multiplier of 1.3 * normal differential pressure added to maximum suction pressure as PSHH.

Regards,