5 replies to this topic

[andiez]

Hi all,

I am actually in the process of designing a blowdown valve in a system full of vapour. As per dep,

'For designing the depressuring system capacity it shall be assumed that the initial pressure
in the equipment under consideration is the relief valve set pressure or the pressure which
will be reached after 15 minutes of fire exposure, whichever is the lower'

Based on this, is there any method of calculting the final pressure after 15 mins using HYSYS as the DEP doenst giv much explanation regarding this...

Also my system consists of several equiments .. during, blowdown should i take the operating pressure at the upstream or the downstream? upstream has higher pressure.

Thanks in advance.

Regards,
Andy Lum

[mohammad reza]

Andy


according to API-521 , if your vessel thickness less than 25 mm you have to depressurize your system up to 8 bar , if it more than 25 mm , it depressurized up to 50% percent of design pressure.

BY use of HYSYS software, it is very easy to calculate both your blowdown rate and CV.

But about system consists of several equipments, if your means is compressor station , you have to first reach the settle out pressure , then depressurize it.

Mohamamd reza

[joerd]

is there any method of calculting the final pressure after 15 mins using HYSYS

Short answer is yes, you can model your case using the dynamic depressuring utility. Specify system volume etc, initial conditions (you probably want to assume the highest pressure), and a fire heat input. Then run the utility, you should see the pressure go up until the relief valve set pressure is reached. Read the user manual first, it helps you to set up the problem.

[andiez]

Thank for the reply,

yes i hav use the HYSYS utility specifying no flow at both the vapor and liquid outlet... n selecting the operating mode of Fire API 521. however i found out that there is no increase in the pressure after 15 min. so i was wondering was it the correct way?... my system is a saturated gas... and dry gas...

Is the any manual or book explaining the method of calculating the settle out pressure?

Thanks.

Regards,
Andy Lum

[mohammad reza]

for caculation of settle out pressure you can use hand calculation (By Excell or by use of HYSYS soft ware.

[andiez]

sorry but how do we calculate settle put pressure using HYSYS? im not familiar with it. For calculation by hand do u hav any reference o literature describing the method...

Regards,
Andy Lum