Depressurizing: Vessel and Overhead Line (24")

[barbara]

Dear All

I'm simulating with Hysys the depressurization of a vessel, I should like to know if I have to considered also the gas in overhead line (d=24" and length=40000mm).
The line is positioned on the pipe rack (H=9000 mm).

- I have to increased the volume of vessel to take into account the volume of line?
- I have to simulate the depressurization of the line only for the cold case (which cv valve and final pressure????)?

thk for your help!

Barbara

[sphere2004]

You should add the volume of piping that is part of system being depressurized.
System depressurization final pressure is normally 50% of design pressure or 6.9 barg whichever is lower to prevent catastophic failure of vessels during fire (pool fire) within 15 minutes as per API 521. This bascially sets the valve Cv.
I am not sure what you mean by cold case. Is the system you are designing for emergency depressurization(high rate depressurization) or for opertional/maint depressurization/blowdown? Pl. check with your seniors.

[barbara]

Thks for your reply!
"The cold case" is referred to "Adiabatic Depressurization".
With the "fire case (P_final= 50% P_Design and t=15 min)" we can find:
- cv of valve
- the peak flow

and with the "adiabatic case (P_final= P_atm)" we can find the minimum temperature.

I have an other doubt: I use Hysys to simulate the depressurization, in section Heat flux -> Heat loss Parameter -> Detailed I have to define for the "convection" the:
- Fixed U
- Updated U
Someone can explaine me the difference between this Fixed U and Updated U? When I have to use the first and when the second?

thks

Barbara