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# Depressurization Fundametal

## Depressurization Fundametal

(OP)
dear all,

i am a new junior engineer. I really want to understand about depressurization, i have read API RP 521 1997 edition and the FEED document of the depressurization of some facilities here, but to be honest, i still dont understand.

could you please give/recommend me an article, books or other references. so i can understand about the fundamental concept of depressurization more easily ? since form the FEED document, it is calculated by Hysys dynamic depressurization, i can't understand the basic concept of the calculation.

I want to understand the philosophy of why comparing fire, adiabatic, and isochoric case so eventually i can calculate the maximum blow down flow rate and minimum metal mass temperature . and how to determine the isolatable section so i can determine the number of SDV and BDV.

i hope you can understand my confusion o understand depressurization concept. thanks in advance :)

### RE: Depressurization Fundametal

If I'm understanding your question, it's just a mass balance.

Take the very simple example of a compressed tank of nitrogen at constant temperature. You can calculate the mass of nitrogen initially in the vessel. If you have to reduce the pressure to some value, you can calculate how much nitrogen is remaining in the tank. The difference is the amount of nitrogen that has to be removed or blown down.

The blowdown systems I've seen limit the flow of fluid either due to the Cv of the depressuring valve or by using an orifice. The mass flow through the valve/orifice is not constant because as the pressure in the vessel decreases, the flow through the valve/orifice decreases even in this simple case. So you have to pick a valve Cv or orifice size that over the required period of time, the total mass that flows through the valve/orifice equals what you need to remove (see the first paragraph).

In the real world, it's much more complicated. You may have vaporizing liquids as the system pressure falls during the depressurization. You may have thermal capacities from pipes, vessels, catalyst, etc to include. Maybe you have you have additional incoming streams or outgoing streams or energy streanms. But ultimately, it's a matter of removing enough mass from the system to reach the desired final pressure in the specified time period.

### RE: Depressurization Fundametal

Pls note that many companies in the engineering business / oil and gas operating companies are moving on to the AspenTech - Blowdown depressuring routine which they have recently purchased from Imperial College, London.

The major bug in the simpler simulator models is the requirement to manually input a value for isentropic efficency, which is most likely in this Hysys Dynamics routine also.

Am not sure if the Dynsim routine from Simsci avoids the requirement for this input

There are a a few other more elaborate models, but they are proprietory to the companies that develop and maintain the code for these programs.

You may want to bring this up to the attention of the plant owner and see what they want done.

### RE: Depressurization Fundametal

most simulators solve a blowdown mainly from a thermodynamic point of view,
meaning at n steps (P initial->P final) a mass & heat balance is solved in the way suggested by T2k,
to understand how it works you can solve for example a series of H-V flash or some equivalent operations,
being Volume constant you have to remove n kg of fluid = V*(Dens(Tin,Pin)-Dens(Tfin,Pfin)),
for a pure fluid (constant composition in vapor and liquid phases) perhaps, for the purpose of learning, you may try to solve with the help of a Mollier chart,
for a mixture (variable composition in vapor and liquid phases) you would need a software cause compositions change at each step,
there is a old paper (Chemputers 1994) discussing how to solve this with prode calculator, I have coded the procedure in VBA with Excel and Prode, results are not too different from std. procedures,
note that, to keep in account the complex mechanisms of heat transfer between phases and metal, one may adopt some generic parameter as efficiency.
A more accurate procedure should attempt to estimate heat transfer and non-homogeneous phase equilibria, see the thesis of Angela Wong (it's available from University College London) discussing BlowSim,
this may give more accurate results (of course depending from the availability of parameters required by procedure)

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