Depressuring systems
Depressuring systems
(OP)
Hello, I have a gas depressuring system in wich a pressure vessel has to be depressurized from about 29 bar to environmental pressure, using a pressure relief valve.
This is the question and I wonder if you could help me: do you know a mathematical model that is able to tell me what is the minimum temperature value of the pressure vessel wall? I mean: if the gas reaches a minimum temperature value of -50°C at the end of the depressuring process, may I know what is the minimum wall temperature value?
This point is very important for me, because the wall material choice in the vessel depends on this minimum temperature value.
Thank you
This is the question and I wonder if you could help me: do you know a mathematical model that is able to tell me what is the minimum temperature value of the pressure vessel wall? I mean: if the gas reaches a minimum temperature value of -50°C at the end of the depressuring process, may I know what is the minimum wall temperature value?
This point is very important for me, because the wall material choice in the vessel depends on this minimum temperature value.
Thank you





RE: Depressuring systems
RE: Depressuring systems
What I did I assumed the vessel temp is maintained with the fluid temp as it depressurizes. Depending on the vessel sizes and the wall thickness of steel, heat content in the vessel wall is quite significantly more as compare with the fluid. As such the final temp may not be as low as you assumed only the fluid depressurizes.
Just courious, how can you depressurize the vessel with a relief valve to atm pressure?
RE: Depressuring systems
1) The vessel mass
2) The vessel wall thickness
3) The thermal conductivity and the specific heat of the wall material
3) The depressuring time from the beginning to the final pressure value (and so, the time needed to reach the final temperature value!)
If I have to consider the process as it is, this is a not stationary heat transfer process and it is necessary to write the equations concerning this case. I only tried to find a semplified way.
RE: Depressuring systems
I am very sure you can depressurise a system in Hysys provided you know the time you want to depressurise it in.
If you go into Hysys/utlities, you can obtain the depressurisation utility.
Provided you know your metal mass, lqd inventory, timeyou want to depresursie it, inventory mass and inventory vessel heat capacity , initial pressre and temperature and what pressure you want to depressurise it, you can do it in hysys.
hope this helps
svv
RE: Depressuring systems
P.S.: Please, forgive me for my terrible english!
RE: Depressuring systems
You guessed right about the important parameters in this problem. All of these can (and should) be input into the HYSYS utility somewhere.
HYSYS will perform a sequential flash in a series of time steps to achieve a "pseudo-dynamic" result. Heat transfer from wall to gas is calculated based on the temperature difference (flux = heat transfer coefficient * temp. difference), the wall temperature drops according to heat loss = mass * Cp * (Tinitial - Tfinal), the gas temperature drops due to depressuring/flashing.
Take care if you use HYSYS 2.4 or lower, I found a bug in unit conversion, when you input the heat transfer coefficient to the wall (or the heat flux, I don't remember) it doesn't convert to SI properly, so you should check your results.
HYSYS 3.0 has a better depressuring utility, based on a real dynamic approach.
You can check support.aspentech.com for more details.
Regards,
Joerd
RE: Depressuring systems
Emergency depressurization is usually applied in case of an external fire, to prevent the metal temperature to reach a level at which stress rupture could occur. Note that a safety relief valve will not prevent this from happening! This is indeed described in API RP 521, section 3.19. Probably this is also what Hysys is referring to.
With respect to the material selection, note that the low temperatures (e.g. -50 °C) only occur in combination with low pressures. Assumption here is that the vessel will not be repressurized before it has heated up again (operating manual!). Because of this fact, it may still be possible to use carbon steel instead of stainless, without having the risk of brittle fractures. Please ask a material specialist for details.
RE: Depressuring systems
When designing a blowdown system, the key issue is to blowdown duration. API has indicated for a 1" wall thickness vessel, a 15 min blowdown duration. Many designers has taken this as the design blowdown time duration. If you look at Fig 1 in API you will see that for a 1" wall thickness plate it take about 15 min to reach 1200 deg F which is about critical temp when the material start to deform. For a 1/2" wall thickness vessel, looking at Fig 1 again, it only take 7 minute to heat up to 1200 deg F. So if you design the blowdown system to depressurize within 15 min for all cases, you will probably have BLEEVE before the system complete it blowing down.
So, set the blowdown duration depending on the wall thickness of the vessel. For a system blowdown watch out for the piping wall thickness. Piping wall thickness is usually much less than that of vessel wall. I have not seem many piping system more than 1" wall thickness unless for very high pressure system.
Also the blowdown system should include the delay in activation of the blowdown, if any. Some companies set a time delay of about 5 min to allow the operators to confirm a fire situation before starting to blowdown.
RE: Depressuring systems
RE: Depressuring systems
Am I missing something?
Jack M. Kleinfeld, P.E. Kleinfeld Technical Services, Inc.
Infrared Thermography, Finite Element Analysis, Process Engineering
www.KleinfeldTechnical.com
RE: Depressuring systems
A inexpensive simulation tool is design II/winsim:
www.winsim.com
Best Regards
Morten