Flashing transient in closed vessel

[barqueiroenmarte]

Dear all,

I am looking for any method to estimate the pressure variation inside a closed pressure vessel which is initillay filled with, for example, propane, at atmospheric temperature, when the liquid is taken out from it by means of a pump.
As far as I know, at a first stage, the propane inside the vessel will be at its vapor pressure (liquid+vapor phase). When the pump starts working, the liquid level will decrease, so the pressure in the vessel will decrease as well and some flashing vaporization will start in order to try the recover the steady state, this is, the vapor pressure. I suppose that the variables wich define the process are: the vessel volume, the volume of liquid and vapor at the beginning of the process, the fluid characteristics (i.e. vapor pressure), the pump flowrate, the surroundings temperature, etc but I am not able to know how the process exactly works in order to calculate the pressure in the vessel in each moment.

I would be really very grateful if anybody could make any suggestion or any reference to a book, technical paper, document, etc where this process is studied.

Best regards

 

[dcasto]

I have not studied it mathmatically, just from actual operations. I can't find the thread where I put up the results earlier.

Mathmatically you could use API rp520 for assumed radiation heat gain (about 300 BTU/hr/ft^2) as the enrgy into the vessel. This energy would then vaporize propane to replace the volume leaving by way of the pump. so if propane needs 150 BTU/lb to vaporize, then ..... 300/150 2 lb/hr/ft^2. Then you can do a heat transfer from the outside air. There are papers on that, but I believe that ranges from 1 to 2.0 BTU/hr/ft^2. Wind and shadows and all effects this.

Here is the bottom line. The heat transfer into a 30,000 gallon tank where we removed 250 GPM could not keep up and the propane did not come to equilirium (steady state). This cause the propane to flash in the line going to the pumps. The pumps responded by passing a liquid with some bubbbles in it and they would collasp on the way to the meter. The volume the pumpscould move would drop from 275 down to 250 gpm over about a 1 hour time period. In the winter time, the pump would cavitate completely and shutdown (or the loading meter would on low flow). If we pumped from 2 30,000 gallon tanks we would do OK. Actually this only happened if we started with the tank less than 50% full. The combination of more NPSH and heat in the tank due to a larger amount of liquid comes into play.