There are so many parameters and each case is so different.
Which type of compressor ? This is not indicated.
In a centrifugal compressor and in absence of back up seal gas in case of trip, well you should blow down fast enough to not damage the shaft seals for instance, while if you need to start up in partial pressure condition to help your driver, well this is a different story.
In case you have a loss of containment (failure of sealing or whatsoever), you may need to depressurize but what about the gas: is it flammable, toxic etc ?
Do you blow off the gas to atmosphere, do you route the gas to flare, do you vent to safe location ?
Do you expect to receive a definitive answer to this question, really ?
Thank you for the additional questions. I was hoping that there is a general industry "rule of thumb".
For the additional information,
the station has six recip gas compressors in very lean gas service. After an ESD, EBD triggers on each unit (while the station inlet and out is closed automatically) blows down to a header then routed to a safe location (silencer). I do have equipment limits, which would be a minimum, thus dictating the higher limit on vent header sizing. But I would like to it be smaller (cheaper) if I could take longer time to vent and make vent piping (& associated equipment) smaller.
Maybe it is wise to discuss with recip compressor manufacturer if they have minimum depressurization rate requirement, so that you can keep the gas "longer" in the loop while under pressure without damaging the equipment. The maximum I guess you know already. In general for some equipment it is the O'rings that does not tolerate a too fast depressurization, so this could set your maximum (example: O'rings or sealing elements with elastomer materials).
Might be also that what drives this minimum rate of blow down is another component in your loop, e.g. any valve sealing element or whatsoever (just speculating here)...
Presume you have done the vent header capacity calcs by using compressible flow equations? Are there PSVs connected to this vent header?
Have you acounted for all the credible simultaneous venting scenarios (say localised blowdown on one compressor plus safety relief on another), for which there are limits on built up backpressures on PSVs'?
If there are PSV's connected to this vent header, is there an emergency rupture pin bypass ( or RD or similar) across the silencer? All these will affect the vent header size.
API 520 has a guideline for blowdown estimation to depressurize in 15 minutes to 100 psig or 50% of the system MAWP. I've seen this used in a few places.
If you have an emergency, you want to reduce the pressure in the station if the decision is made to blow down the unit. I'd at least start with the above guideline and see what sort of flows you come up with, do those seem reasonable and then start looking at 'does it work with your compressors?' and are there any other reasons you may want to blowdown faster or slower.
There are many types of packing designs for recip compressors in the API standard for recip compressors - which one is used for these machines? - the simpler ones pose a greater risk for external leaks than others, and may therefore require a faster blowdown than 100psig in 15minutes.
