Pressurization of Compression Train

[chemicalengr]

I'm working for a client who would like to increase the startup times of their compression trains. Currently, the trains are pressurized in stepwise manner from lowest to highest pressure, through restriction orifices installed in 2" bypass lines.

The client would like to increase the size of the restriction orifices in order to enable the pressurization to occur at a faster rate.

What are the limitations that should be imposed in a case such as this? I've calculated velocites, Mach #'s, rhov^2 and am beginning to look at Acoustic Vibration (Carucci and Mueller).

In certain cases, the results of these calculations are contradicting each other - for example the Mach # falls within the recommended range (< 0.7), but the maximum recommended velocity is exceeded (> 30 m/s). Which parameter is considered most important in designing a system of this type and is there a "Rule-of-Thumb" for how long the maximum recommended values may be exceeded?

Keep in mind that as the downstream system is pressurized, the velocity and Mach # begin to decrease.

 

[dcasto]

max velocity is only a concern on errosion, its a clean gas in a compressor, I'd ignore that. Mach .9 or less is fine for the short time period.

This is assuming the lines are purged and you are not mixing or compressing air or something in the lines. also keep an eye on pressure rate of change. I'd limit it to say 100 psi every 30 seconds.

 

[chemicalengr]

dcasto, Where do your limits of Mach .9 or less and rate of pressure change come from? - literature, experience, ???

 

[dcasto]

From experience at pipeline companies. In the old days they had internal documents with these engineering parameters.

The rate of change is the toughest one. That could be sped up quite a bit after some experience. I use this safe value until I've actually seen the system pressure up. We would put an RO on the purge system to limit flow of depressure or repressure. If the system was all single phase and the piping was well anchored, we'd increase the size of the RO. for example we'd repressure the compressor to 800 psig in about 10 minutes. But we'd go from 800 to 2400 in less than 30 seconds.