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Natural gas compressor Cold Gas Bypass valve sizing question

Natural gas compressor Cold Gas Bypass valve sizing question

Natural gas compressor Cold Gas Bypass valve sizing question

(OP)
Dear colleagues,

I am currently working on a natural gas compression plant, and the dynamic simulation study carried out by a third party contractor concluded that most of the compressors shall be furnished with a Cold Gas Bypass loop (i.e. an additional on/off valve bypassing the antisurge valve providing additional recycle capacity to the ASV) in order to ensure the stringent surge prevention criteria established by the Client.

We have been provided a Cv of these CGBVs which has been calculated assuming an xT factor of 0.7. If we look at tables provided by several Vendors, these valves calculated by the Dyn Simulation contractor are very, VERY small (i.e. 2'' valve for a inlet/outlet lines sized at 10/18'' respectively based on 150 ft/s, which is the normal max gas velocity used for gas in intermittent service).

Something we are considering is that the xT of full bore ball valves is usually much lower (refer to Figure 5 in https://www.valin.com/resources/blog/gas-flow-cont... ) than globe valves, so taking this parameter into account perhaps the Cv calculated by the DynSim provider may be larger, allowing us to provide larger on/off valves.

Have any of you experience when specifying valves for this kind of service? Are full bore, ball valves OK for it? It seems that in this case valves of the capacity dictated by the Dynamic Simulation provided are too small for the inlet/outlet pipes. Can anyone think of any reason why?

Many thanks. Should you need any further info to tackle this issue please do let me know.

RE: Natural gas compressor Cold Gas Bypass valve sizing question

In most cases I've come across, this quick response recycle capacity function is assigned to a hot gas by pass. Anyway, if the dyn sim contractor believes a cold gas bypass is adequate for this service, that would be for you to verify.
This quick open bypass valve would be triggered into action when there is an unplanned emergency shutdown sequence initiated trip of the compressor or if the tracked rate of approach to the surge line exceeds a predetermined value. It only needs to deal with say 30-50% of the recycle flow seen at max speed a short distance away from the surge line to make up for any response lag anticipated on the ASV. Cannnot comment on the xT value for these quick open valves. Preferably, this CGB fail open valve should be fitted with a quick exhaust pneumatic relay at the actuator for obvious reasons. Globe valves would be preferable to handle the high dp seen at the time of recycle; trim should be quick open type.

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