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Depressurisation time

Depressurisation time

Depressurisation time

Hi all,

I would like to ask for advice. could anyone please advise me how to determine the time required for depressuring a vessel (low temperature vessel)?

I want to determine the time when depressure the vessel to atmospheric pressure. Below the brief description of the problem:

The design temperature of the vessel is -162 oC and the design pressure is 24 barg. the problem is when the blowdown valve (valve to dry flare) fails to open it will depressure the vessel. I want to calculate the time required to depressure the vessel to atmospheric pressure.
I am hoping to getting some advices from you.

RE: Depressurisation time

The process is to consider the initial mass in the closed volume and then do an iterative calculation:

1) Determine flow through depressuring valve (a suited formula) for a given time step assuming upstream condition constant
2) Calculate new mass in vessel (old-"lost")
3) Determine new pressure and temperature by assuming known density (from step 2 and the fixed volume) and then solving for a isentropic process (thats true for whats happening inside the vessel) then repeat from 1) with new P and T

- If you have a fire case you will add the heat as a step 4
- Software such a HYSYS has this algorithm build into it
- Remember to check you timestep so that the P/T is not significantly lower at step 3) that what it was at the beginning at the timestep.
- If condensation occurs you must compensate for this
- Its possible to do it as an excel exercise but setting up the therdynamics is a big job. If you have software that can collaborate with excel that makes it easier - but then this software will usually do the whole thing

Best regards, Morten

RE: Depressurisation time

Oh year, and if you need a specific blow down time (say 15 minutes) then add a new loop where you adjust your "valve" until you get the correct blow down time

RE: Depressurisation time

On some depressurization questions, I reference Milton Beychok's website, http://www.air-dispersion.com/ , but it was "not found" just now. I hope this is a temporary issue. Lots of good information on his website!

Good luck,

To a ChE, the glass is always full - 1/2 air and 1/2 water.

RE: Depressurisation time

Morten - Has Aspen incorporated the newly aquired Blowdown software into Hysis? Is that what you were referring to when you mentioned Hysis? I think Aspen plans to add Blowdown to AspenOne, but I don't have that in the version being used by my company.

Orlando - If you have access to this Blowdown software, I think that's the idea tool for this task you mention. This depressurization must be done at a controlled rate to prevent embrittlement failure. Refer to the API 521 (6th ed) for depressurization guidance, which was significantly expanded from the previous edition.

RE: Depressurisation time

@Don, its been there for at long as i can remember - even when they called it HYSIM (before Aspen). They changed the "motor" some revisions back. It used to be a special "package" that was called from HYSYS. Now it looks like HYSYS dynamics - and i think it is but with limited options.

RE: Depressurisation time

Hi everyone,

Thank you for all your comments and inputs.

Just to let you know that, I have done the hysys simulation for the absorber column and it was ok and even the valve fully open the flare can handle the problem of de-pressuring the vessel (correct me if I am wrong).

One of my concerns is to do a manual calculation to determine the time. I have not got the idea how to calculate it. Could you please provide or direct me to find the way to do the calculation?

I am sorry, I am a young engineer. I would be appreciated if you can advise me

RE: Depressurisation time

I told you how to do it manually?

RE: Depressurisation time

nowadays most simulator include procedures to simulate a depressuring cycle,
note that these can have limits, for example (in general) they don't consider non-equilibrium conditions as discussed in API (last revision),
for these cases (where std. procedures don't work well) a software like Excel (or a similar tool) in union with a thermo engine (I have Prode Properties but you can use a simulator as well as suggested by Morten) allows to define specific procedures...

Probably in your case there is most gas and the solution should be easy,
as suggested by Latexman there are several methods which allow quick estimates,
see for example


as alternative you can adopt a direct integration procedure,
this topic has been discussed many times,
for a recent discussion see


RE: Depressurisation time

More info. to my post above, I just found out this morning that mbeychok passed away earlier this year. We have also lost his website. It's a sad day when someone with so much knowledge and experience is gone. It's even sadder when we lose one of their main knowledge archives.

Good luck,

To a ChE, the glass is always full - 1/2 air and 1/2 water.

RE: Depressurisation time

Thank you for all your inputs... I will work it out and I will get back if I have any further concerns.

Thank you all, Orlando

RE: Depressurisation time


What is the vessel volume?
What is the gas type in the vessel?
What is the outlet orifice diameter of the valve?

RE: Depressurisation time

What kind of hydrocarbon (Butane, Propane, Methane) each has different chemical properties such as Cp/Cv, Molecular weight.

RE: Depressurisation time

The gases that come out from the vessel to flare mostly nitrogen and methane

RE: Depressurisation time

To my calculations:
Gas = Nitrogen
Orifice discharge coefficient = 0.8
Gas temperature of 113 Kelvin (-160C)
I get the following:
Time to reach vessel pressure of 1 atm is ~66 minutes for Adiabatic discharge.
Time to reach vessel pressure of 1 atm is ~53 minutes for Isothermal discharge.

I used closed form inhouse propriety formulas for vessel discharge.

RE: Depressurisation time

Sorry, Correction, I used incorrect orifice diameter.
Time to reach vessel pressure of 1 atm is ~99 minutes for Adiabatic discharge.
Time to reach vessel pressure of 1 atm is ~76 minutes for Isothermal discharge.

RE: Depressurisation time

Hi Israelkk,

I am sorry... If you don't mind, can you please provide me your spread sheet? I really need some help in doing hand calculation.

Thanks a lot for you guidance so far

RE: Depressurisation time

This is not a spreadsheet. It is custom made program. I can not give it.

RE: Depressurisation time

I see...

Thank you israelkk

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