4 replies to this topic

[coolxz]

I have a 2 stage compression train and they are centrifugal compressors driven by a fixed speed motor on a common shaft. There is a PSV at the discharge of each of the compressor. The sizing case for the PSV would be blocked discharge. Can I size the PSV based on the rated flow of the compressor? Compressor curves are available, but I don't quite know how I can make use of it to determine the relief load? (I am quite new to compressors)

Any help on this will be very much appreciated.

[kkala]

Search the Cheresources forums, you may trace useful info like http://www.cheresour...cked-discharge/.

Edited by kkala, 18 June 2011 - 03:33 PM.

[kkala]

Having searched the matter further, http://www.cheresour...rge-prv-design/ may be more useful in your case. Read the attached file in Ankur's post there, where PSV at discharge seems to serve the need of antisurge protection.
Look also at http://www.cheresour...gal-compressor/.

Edited by kkala, 19 June 2011 - 05:43 AM.

[paulhorth]

Coolxz,

I copy below a comment I made on another recent post.

It is vital to understand that the sizing case for the relief valve is ALWAYS MORE than the normal mass flow through the machine as shown on the material balance. The sizing case should take into account the flow at maximum suction pressure and maximum speed or max driver power, with no credit for the antisurge valve. On a recent gas plant project that I was involved in, this was not understood by the designer. When the operator discovered the error, the startup of the plant was delayed for six months while temporary fixes were put in place including a high suction pressure trip and a speed limiter. In the long term, larger relief valves are being fitted.
This maximum mass flow at the relieving pressure has to be derived from the performance curves, usually with assistance of the compressor supplier.

In your case, the "Rated Flow" ( which I find can mean different things to different people) may correspond to the maximum flow as determined above, in which case that's fine. It helps that your machine is fixed speed.

Paul

[chemsac2]

coolxz,

It is conservative to consider normal rated capacity of compressor. Actual relief rate would be lower than normal capacity for a fixed speed machine. API 521, section 5.5 has following on this:

"The quantity of material to be relieved should be determined at conditions that correspond to relieving conditions instead of at normal operating conditions. The required relieving rate is often reduced appreciably when this difference in conditions is considered. The effect of frictional-pressure drop in the connecting line between the source of overpressure and the system being protected should also be considered in determining the required relieving rate."


As suggested by Paul, relief rate should be calculated at maximum suction pressure, but also RELIEVING pressure of PSV on compressor discharge. To exactly calculate relief rate for such a case, follow steps below:
- Get compressor head versus capacity curve for maximum speed
- Calculate maximum compressor suction pressure
- Calculate maximum compressor discharge pressure going backward from PSV (at relieving pressure) to compressor discharge flange
- Calculate compressor differential head from suction and discharge pressures
- from compressor head - capacity curve, read capacity corresponding to above calculated which is relief rate for your PSV

As long as maximum suction pressure does not change, gas composition does not change and motor is fixed speed, relief rate would be lower than normal operating flowrate. Else relief rate can go higher than normal rated flowrate.

Regards,

Sachin