I have a rupture disc on a process condensate line (mainly water with slight vapours) at 3.5 kg/cm2a and 123C. Rupture disc setting is at 6.6 kg/cm2. The outlet line of rupture disc is connected to a blow down vessel which is at atmospheric pressure. I want to calculate exactly the back pressure due to rupture disc downstream piping up to the blow down vessel, how to do that?

Chem01,

Please refer to API Std 521 2007, section 7.3.1.3.5.

Joe,

I have API RP 521 4th ed only, where can i found API 521 2007 edition....

Check out here... http://www.api.org/Publications/

The conservative, simple approach is to ASSume an infinite supply of condensate at the rupture disk at 7.26 kg/cm^2 (10% over pressure) flowing to the blow down vessel at atmospheric pressure. I would recommend the homogeneous equilibrium model for this two-phase flow problem. The easiest way is to solve it numerically by breaking the line into say 10 equal length pieces, converge to an answer, increase the number of segments, say 20 pieces, converge to a solution, compare that to the last solution to see if the difference is significant or not, and repeat the process until the difference is not significant.