Blowdown calculation

[Techwus]

Anyone know where to look for some equations pertaining to blowing down a vessel. What I am looking for is what happens to the LNG in the vessel as the pressure is released and the fluid is sent to a flare.
All I could find was in API 521 where it talked about depressurization, but the calculation had heat included, which I would not
Anyone have any ideas

[Yengineer]

I was asked to calculate ***hing simmilar - the time to depressurise a gas pipeline.
Now my situation its simpler (I have only gas - no phase changes)
My steps were as follow:
Calculate standart system volume at initial conditions (Pi; ti - std m3)
Calculate standart system volume at final conditions (Pfn; tfn - std m3)
Set the number of steps (n) which system will follow during depressurising Pi to Pi-n..... to Pf
Calculate compressibility factor for each of new sets of conditions which system will follow during depressurising
Calculate the volume difference in system between each step of condition Vi - Pi-n (V diff - std m3)
Given the system set of conditions (P, t) and the valve or oriffice parameters - calculate the flow time

Cumulative time is your total depressurising time. Calculations will look similar as table below:

P	Z	Volume	Volume (std)	Vdiff	t	cumm time
100	0.7	1000	5000
800	0.75	800	4000	1000	20	20
600	0.8	600	3000	1000	18	38