Compressor, and discharge PSV capacity

[22082002]

I have to size the PSV on discharge of recip-compressor for blocked outlet case.
I believe, it should be sized for 110% of comp. rated capacity, as the segment has to be protected with margin, when compressor is running at it's rated capacity.
Others feel it should be at rated capacity only.
I feel the compressor is definitely capable of reaching it's rated capacity and can deliver little bit more...

Thank you, in advance for spending time on this and I look forward to your advice..

Regards,
SAA

PS:
Please comment on the rated capacity and whether compressor can deliver little bit more..

 

[quark]

If you are not changing the speed of the compressor, there is no chance of increase in capacity. This seems to be superfluous to me. Just check the density variation of the working fluid. If it is an air compressor, then don't worry. Africa can never reach Antarctica at any point on the globe

Regards,

 

[22082002]

Well, I agree that africa will not reach antartica.. (appreciate your way of putting this, definitely a good mood changer for a boring monday morning start...)

The compressor is a recycle gas compressor, containing ~76% hydrogen. Mole. wt. can be varying...

I feel, it can go up by 1-2% above rated compressor, as compressor vendor will, no matter how small, will have his safety margin with regards to rated capacity.

What do you feel?

Regards,
SAA

 

[aikmeam]

I think it's not such a bad idea after all to have a 10% margin. It may well cushion future upgrade, if any.

Check the rated capacity of the PSV you select, it may also already have the 10% you required.

 

[TD2K]

I would initially size the PSV for the compressor rated discharge (rated capacity being the vendor confirmed capacity and not necessarily what is on the process data sheet) and see what your calculated area is versus the next standard PSV area. That will typically give you a certain margin and PSVs, if they are ASME stamped, are derated by another 10%.

Yes, I would want some margin to account for changes in MW or other process conditions but I'm not sure I would arbitraily use 10% on top of the compressor rated capacity.

 

[EGT01]

22082202,

You didn't mention to where the relief valve would discharge.
Just so there is no misunderstanding, it's not back to suction is it?

 

[22082002]

No,it will go to flare.

 

[quark]

These type of things are mostly subjective, in my view. If I am not sure I would go in a democratic way(just count up yes and nos). As suggested by Aikmeam the PSV may already have the required redundancy.

As far as compressor rating is concerned, actually it is always less at site than what is being suggested by the manufacturer.

If you still feel insecure, the required redundancy in capacity should be the percentage change in density.

I vote for TD2K anyhow.

Regards,

 

[hornfan]

For capacity, I would use the manufactures rated capacity (should be stated on the as-built data sheet or the data sheet returned with the bid) with no margin if it is different than the rated capacity that went out on the data sheet for inquiry. As stated before, the actual orifice area in the relief valve will almost always be greater than the calculated required orifice area of the valve. There are problems with oversizing a relief valve.

I don't know what your system looks like, but are you protecting equipment or just piping? The allowable overpressure is different if you are just protecting piping, typically 10% for equipment and 33% for piping. The cylinders themselves are usually rated for much more that anything downstream.

 

[EGT01]

Theoretically, you only need to size your discharge PSV based on the rated capacity of your compressor at relief conditions. But it may be more prudent to let your compressor manufacturer/vendor handle sizing to that criteria. In fact, it might be a good idea to talk to the compressor manufacturer about this anyway.

Assuming that there are no differences in the suction conditions for the manufacturer's rated capacity at design conditions and at relief conditions, wouldn't the capacity of the compressor at relief conditions (higher discharge pressure) actually be lower than at design conditions? It would then seem appropriate to just use the rated capacity at design conditions without a percentage increase.

 

[22082002]

Thank you gentlemen, for sharing your thoughts on this subject. I have now kept it for as rated capacity.

Best Regards,
Satish