5 replies to this topic

Hi,
Im trying to do a PSV on a centrifugal compressor for blocked discharge case. The company normal practice seems to be to use just the rated flow through the compressor increased for the maximum compressor speed.

the compressor in question is gas turbine driven & the turbine has lot of "fat" it can deliver about 50% more power.

Im not sure if the approach stated above represents the worst case relief load possible from the machine. If I account for suction pressure increase, then mass flux from the machine works out higher than the rated mass flow.

My system is a pipeline into an inlet separator & from here to a 2 stage centrifugal compressor system. the inlet separator has its own XVs & is not designed for settle out.

im assuming relief pressure as the discharge pressure & suction pressure as the max suction pressure (dump valve to flare set pressure on the inlet separator), using this head Ive checked the flowrate from the curves at 105% speed & used this mass flowrate as relief load.

however Im ending up with about 20% more mass flowrate than the rated flow.

Please help me out, Im new with compressors. Sorry for the long post...again its my first post.........

awaiting an early reply.

[JoeWong]

I would think the company practice is just a first estimation when the compressor curve is not available.

Nevertheless above figure shall be checked and validated with the compressor curve and more define pressure setting for compressor blocked discharge case during EPC phase.

If your findings shown you have 20% more the "rated flow" as claimed, your relief valve and flare system shall be designed to take it.

When you mentioned "rated flow", i guess your flow is related to normal operating condition. During blocked discharge, the condition may have been changed and it affect all parameters. Sometime, eventhough you have more mass flow at elevated condition, your PSV which has been designed for rated flow at operating condition may still be sufficient to handle it. nevertheless, you flare system may need to be reviewed. Again, higher mass flow in flare system does not mean that you have to change out the flare system...as higher mass flow, higher pressrue drop, higher backpressure, it might still within the ulleage of the flare system...

[ponnusj]

I am designing a PSV on a compressor discharge, similar to the one mentioned above. I calculated the relief rate as the flow at surge point @ 105% speed of the compressor, which works out to be lower than the rated flow rate. Any comments ???

[ponnusj]

Seems that there is not much information sharing about relief rate estimation. I have been searching high and low for answers/comments !!!

[Art Montemayor]

ponnusJ:

My comments on your query are:

1) Please start an independent thread on your query. By interjecting your request into this thread you are, in effect, high-jacking some else’s thread. Answering the OP’s and your requests now become a confusion.
2) The reason you haven’t received a response during an entire day is probably because no one is expecting you to interject your request in someone else’s thread.
3) It seems that YOU are not sharing much information about your relief rate estimation. Why haven’t you submitted your calculations or reasons for selecting the surge point as your relief point?
4) Please furnish complete basic data regarding the application in order to facilitate a member being able to understand what the problem/application is.
5) Start a new thread with this topic in this Forum.

Thank you.

[ponnusj]

Art,

The first post was very similar to my situation. That's why I posted into it. My comment was that there is not much information available in chemical engineering forums (in general) about relief rate estimation. Will start a new one.