I'm now working with centrifugal compressor and trying to reduce start up time. During start up compressor is being purged with process gas by sequence control. Purging timer is set so that three volume changes in compressor and corresponding lines is being acheived. Can somebody help me with one question. Do anybody knows any procedure or practise where i can found how many volume changes should be applied during start up. Compressor is handling CH4 gas.

If you are purging to atmosphere, calculate the sonic velocity through your purge valve (calculated at upstream pressure*Forged Steel Valves) divided by the area of the purge valve.  That gives you the volume flow rate of the purge.  Then purge 3 * Volume * (8 bar / 1 bar) or 24 times the compressor volume at that volume flow rate.

The rate of flow into the compressor is somewhat irrelevant as long as the inlet piping is bigger than the exhaust piping (otherwise the pressure will tend to decrease with time and the calculation becomes impossibly complex).  

Introducing gas above 0.6 Mach into an air filled system is a very good way to blow the system up.  At some value around 0.6 Mach, density increases rapidly and mixing stops.  The consequence of these two factoids is that you can create a "pseudo piston" that will compress the gas in front of it until the inlet gas does enough work to slow below 0.6 Mach.  About a dozen people die each year re-discovering this fact.  Every single compressor explosion that I've ever reviewed has been caused by this phenomenon (most of the time the investigator refers to it as "dieseling", or using the heat of compression to raise the temperature of an explosive mixture above the auto-ignition temperature).

The more I learn about your process the worse it sounds, but yes, you can use volumetric flow.

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