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# Circulating Gas through centrifugal compressor

## Circulating Gas through centrifugal compressor

(OP)
I am interested in input on how I might evaluate the attached system. Gas is being circulated through a two stage compressor. Normal suction pressure at the tank to be protected is 100 psig, normal discharge pressure is 250 psig. I am interested in an approach to dealing with letdown of the FCV. In addition to the flow through the control valve, the gas is fed through a process loop.

A conservative case would be to assume that the RV would need to handle the letdown rate through the control valve assuming the max Cv of the valve, an upstream pressure of 300 psig, and a downstream pressure of 220 (200 x 1.1 ovp). However, since the protected system serves as the suction side of the compressor, so this appears to be overkill.

Another approach I am considering is to assume that the RV must deal with the energy from the compressor that would be converted into thermal energy - essentially an external heat input case.

Thoughts?

### RE: Circulating Gas through centrifugal compressor

This is a closed system?

To do anything you need to work the scenarios that could lead to overpressure. From what I imagine is a basic sketch, I'm struggling to come up with anything that isn't thermal related.

Unless the compressor can receive new gas from somewhere else that can then over pressure the vessel assuming the outlet valve is closed then any flow into the tank will be matched by mass flow out of the tank, give or take the odd molecule.

Much will also go on the relative sizes of the vessel and the compressor / flow rates involved. A large tank and small compressor will be a lot different from the opposite.

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