3 replies to this topic

[Asalahi]

Dear friends

 

I have saturated liquid propane as refrigerant at  at 929kpaa at pressure vessel and try to work out the blowdown pipe size and valve. I have only VMG as software but I couldn't find any dynamic tools. I' m thinking of developing spread sheet for my calculation. Could anyone help out how to work out the flow rate to decrease the pressure by 50% in 15 minutes. I don't know how to account for vaporisation of liquid as pressure drops.

any thought? total initial liquid mass and vessel volume in known

 

Thanks 

[Art Montemayor]

If you are proposing to “blowdown” the liquid contents of a propane process vessel, I don’t believe that it is normal or recommended to do so.  The term “blowdown” is applied to a gaseous or vapor flow rate that is vented from a process and usually is directed to a flare (in the case of hydrocarbon vents).  What, exactly, are you proposing to carry out?  Please submit a detailed sketch and all related basic data so this topic can be discussed and commented upon accurately and in detail by our members.  Are you analyzing the Pool Fire Case for safety reasons?

 

Please explain in detail the procedure and reasons behind your proposal.  For example, your explanation does not make good sense.  You state you need to vaporize the liquid contents of a tank in order to blow it down to “50% in 15 minutes”.  This doesn’t make sense.  As you vaporize some of the liquid, the vapor pressure remains constant.  You either have failed to accurately describe what you propose or you don’t understand the principle of a saturated, pressurized liquid (propane, in this case).

[fallah]

Asalahi,

 

Art did explain outstanding points regarding blowdown in general and LPG blowdown in your special case. Actually, as he mentioned blowdown for depressurizing wouldn't apply to only liquid LPG; and for the stored LPG in which, let say, liquid propane is in equilibrium with its vapor inside a pressure vessel, if blowdown for depressurizing would be done due to external pool fire the blowdown time would be almost 8 minutes or less to avoid the risk of BLEVE...

[Nicolau Goula i Masllorens]

Asalahi,

 

VMGSim has a dynamics engine able to model the system that you suggest. Furthermore, you will find the Blowdown Assistant to be a very useful tool for this particular case.

 

If your intention is to carry out a Minimum Design Metal Temperature (MDMT), VMGSim even offers the option to track the wall temperature separately for each phase in the vessel, as well as continuous updates of the HTC. VMGSim also keeps track of the kinetic head in all the unit operations, so you may be able to find lower temperatures in nozzles and probably after relief valves compared to other simulators.

 

If you want to evaluate a fire scenario, VMGSim has various standard fire models and also offers the user the option to input custom equations for this purpose.

 

However, before attempting anything in VMGSim, I strongly suggest to consider Art and Naser's comments regarding the proper specification of the system. Simulation software is a very valuable tool for the Engineer, but it should never replace a sound reasoning and understanding of the system. Results must be taken sceptically until proved otherwise.

 

Of course, if you need any help on using VMGSim, you can always contact the Technical Support team in your region.

 

Nicolau Goula