Compressor casing Design pressure

[vjr0512]

As per API 617 the compressor casing design pressure shall be minimum the relief valve set pressure for the discharge side as per API 617 7th Ed Chapter-2 Para. 2.3.1 and in case this relief valve set pressure is not available it shall be 1.25 times of the max discharge pressure" according to Ch-2 Para. 2.3.1.2. What is the basis for arriving at the set pressure for this relief valve at the compressor discharge side as we have the rated speed and the minimum continuous speeds (105% of rated speed) for the compressor which will give different discharge pressures.
Another question is how we can arrive at the maximum discharge pressure as the compressors are normally variable speed machines. Further if we consider only the rated speed of the compressor the surge limit pressure will be higher than the 110% of the rated discharge pressure. As the surge control is an inherent protection for the compressor operation do we not have to design the compressor casing for the surge limit discharge pressure for the rated speed of the compressor.

As the compressor will have a minimum continuous speed (MCS) and the compressor has to be designed for operation at this MCS do we not have to design the casing for the discharge pressure at the MCS.

Thanks to clarify

 

[georgeverghese]

If there were no blocked discharge relief valve, the compressor casing and discharge system would have to be specified for a design pressure which is in excess of that developed when all of the below are coincident
a) Compressor is at high speed alarm or trip ( usually 105% of rated speed)
b) Compressor suction is at high pressure trip
c) Gas is high mol wt and suction temp is low
with no credit taken for surge trip

If there is a relief valve, then the RV capacity at 110% of RV pressure setpoint should account for the compressor capacity developed at the corresponding superimposed backpressure created at compressor discharge for all of the same conditions as above

Sometimes you may find that when running these calcs for RV capacity, the power demand exceeds that available from the driver - then we would have to drop speed to get to max available power. For max power available, exclude fouling derates for gas turbines and reduce other derates as appropriate.

PS MCS = MAX (?) cont speed = 105% of rated speed

 

[rotw]

For me it is pretty simple what API 617 says:

1/ If there is no PSV, the design pressure of the casing is 1.25 x max discharge pressure of any of the operating cases specified in the data sheet. In principle API criteria (1.25 factor) results in having the pressure at MCS / surge covered, means within the design pressure. When it is not the case, to my knowledge this is outside API 617 provisions. In short applying API criteria does not require to look at what is happening at MCS, trip speed whatsoever and surge, as choking as it may seem, this is the way it is done. So I dont understand the previous discussion, at least it is very confusing.

2/ If there is a PSV, different story....the design pressure of the casing is set at the PSV pressure.

In some situations, it may be that there will be too little margin between max compressor discharge pressure and design pressure when the later is set to the PSV pressure, engineering review is required and eventually the design pressure could be set slightly higher than the PSV set pressure.

To the question, what is the criteria for sizing the PSV ? At least it is clear that this issue (sizing criteria) is not covered by provisions of API617. So referring to API617 would not help much.

"If you want to acquire a knowledge or skill, read a book and practice the skill".

 

[georgeverghese]

Obviously, if the API 617 1.25x rule results in a higher design pressure than what the process engineer would have computed using the scenario setting described ealier, then the API rule would need to be included also for the casing.

The scenario described in my previous post is based on real operating conditions that could occur at some time in the life of the compressor, and would usually be the minumum basis for setting design pressure or for determining the blocked discharge relief load for the machine and the systems further downstream.

In some cases on multistage compressors, interstage design pressures may need to be even higher when compressor shutdown - settleout scenarios are evaluated.

The margin between the PZHH - trip setting (or max shutdown - settleout pressure, whichever is higher) on the compressor discharge and the setting of the PSV should match the type of PSV used:
At least 10% margin for conventional and bellows operated RVs'
At least 5% for pilot operated RVs

This is to prevent the PSV from simmer during actual operations and / or to allow for the RV to reseat after relief ( which depends on blowdown setting for the RV).