4 replies to this topic

[Miguel]

Hello all,

 

I would like to calculate the time it takes to depressurize a vessel. 

 

I have a pressure vessel under 30bara with propane and would like to depressurize to 1 bar using a control valve. As i remember this is an adiabatic expansion ? 

 

i would like to build this in a spreadsheet  could anyone give me some guidance on how to calculate or some theory behind this.  My first law thermodynamics is a bit rusty.

 

Thank you 

 

regards,

Edited by Miguel, 17 September 2015 - 08:19 AM.

[proinwv]

I suggest that you obtain a copy of Flow of Fluids TP-410.  This is inexpensive and an excellent reference.

 

The problem would also involve some calculus as the pressure continually changes, or you could set it up in a spread sheet and divide it into many separate sections. The more, the more accurate. 

[serra]

multiple steps (direct / numerical integration) from P Initial -> P Final is a good option,

if there is no change of phase (all gas) and limited heat transfer you may get reasonable estimates from ideal gas laws,

in other cases you may wish to consider properties such as compressibility, cp, vapor pressure etc.

for propane these are available from tables, Mollier charts etc. or (in Excel) from a library such as Prode,

 

the topic has been discussed at cheresources, you may find old threads.

[Miguel]

Hi, 

i have looked it up in cheresouces the information is quiet scattered and not quiet clear

@Proinwv. Crane  is a good source for fluid dynamics. But im still lacking some good examples and step by step description of the things to consider when calculating.

 

I have been researching  i have found out that off course,  this is a compressible flow problem, and we are dealing with an isentropic process.  depending on the critical ratio sonic, super sonic or choked flow need to be considered.

I would need a good fluid dynamics book where compressible flow with examples is described. specially flow through a nozzle. i could take it from there. 

 

any recommendations ? 

 

Thank you !

[serra]

if the pressure drop is about that of control valve you may adopt the ISA formulation to estimate the flow at specified conditions,

you may keep in account piping etc. if required, the procedure is simple,

given a initial amount of gas at t, p you can calculate the flow through the control valve (and piping) as Kg/h or equivalent unit,

multiply  that value for time step (say 1 minute)  to obtain the gas discharged in first time step,

estimate the new pressure in the vessel (for a pure fluid  a Mollier chart can be useful, as alternative you may estimate values with ideal gas law or solving a flash operation with a library such as Prode or a process simulator).

At the end of first step continue with a second time step and so on....

as suggested by proinwv you may adopt time steps about 1/10 or less of time estimated to depressurize the vessel.

Edited by serra, 18 September 2015 - 06:25 AM.