I haven't actually used it yet, but I'm familiar with the Aspen program named "Blowdown" which is used for designing high pressure blowdown systems, mainly for hydrocarbon applications. This is a highly regarded software program that Aspen acquired several years back, and incorporated it into their program suite.

Any 2phase service pipeline to be used for blowdown purposes should be pigged clear of all liquid before use in this new service. This then (a) prevents mechanical damage to the line from high momentum liquid slugs that would surge through the line during blowdown (b) reduces residual backpressure on the blowdown sources a the end of the blowdown. If the line is low point pocketed and has been in blowdown service for some time, it should be pigged clear again after some time when residual backpressure is seen to be high at the end of a blowdown event.