## Depressurisation time

## Depressurisation time

(OP)

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.

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

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

## RE: Depressurisation time

David Simpson, PE

MuleShoe Engineering

In questions of science, the authority of a thousand is not worth the humble reasoning of a single individual.Galileo Galilei, Italian Physicist## RE: Depressurisation time

Good luck,

Latexman

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

## RE: Depressurisation time

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

## RE: Depressurisation time

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

## 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

"http://www.ijee.ie/articles/Vol13-2/ijee924.pdf"

as alternative you can adopt a direct integration procedure,

this topic has been discussed many times,

for a recent discussion see

"http://www.cheresources.com/invision/topic/23595-v..."

## RE: Depressurisation time

Good luck,

Latexman

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

## RE: Depressurisation time

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

1. V = 24 m3

2. Gas type = Hydrocarbon

3. Orifice diameter = 1.26 cm

Thanks all

## RE: Depressurisation time

## RE: Depressurisation time

## RE: Depressurisation time

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

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

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

## RE: Depressurisation time

Thank you israelkk