if it's for a bursting disc on critical flow condition
you may consider to use methods similar to PSV sizing,
for a two-phase flow HEM usually gives reliable values,
for two phase critical flow I simulate the discs with HEM
model or (for a different case) with
constant energy flash operation,
these procedures are complex to solve by hand
so I use a library (PRODE PROPERTIES)
which works directly in Excel,
for the frictional losses of the relief piping and all piping components
similar methods should be applied,
see for example "Easily Size Relief Devices and Piping for Two-Phase Flow"
by Leung on CEP 96
Edited by PaoloPemi, 11 November 2013 - 03:40 PM.