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Propane pumps

Propane pumps

(OP)
I want to calculate the loss of delivery caused by natural thermodynamic phenomena of liquefied gases.
Is a vane pump.
How can I know the amount of propane evaporated at the inlet pipe pump.
I mean that :
The amount of propane evaporated can be determined by the ideal gas equation , knowing the total tank volume and initial liquid volume.

Vapor space volume= Total tank volume - volume of liquid

PV= ZNRT
V =vapor volume
N = amount of moles in the vapor space .
Multiply the moles by the molecular weight to get the weight.
Then use the ecuation again at higher temperature .
The difference in moles or weight is the amount evaporated.
ITs right?

are there any program in order to calculate the amount of propane,or butane evaporated when there is a drop pressure
From storage tank to inlet pump.
Could you help me ? Thanks

RE: Propane pumps

i'd use a p-H (pressure-enthalpy) chart.

secondly, one can only hope the design of such system does not allow for any of the propane to vaporize before entering the pump.

i know firsthand of a situation in which the propane pumps were operated dry.  a catastrophic failure, not only in terms of repair, but legal.
-pmover

RE: Propane pumps

pmover is right

You dont have much use for the ideal gas equation since you have a single component (propane) in liquid state - and the ideal gas equation is for - gasses!

If you are vorried that propane might evaoprate due to pressuredrop and e.g. heat influx then you should find an equation that determine vapour pressure - e.g. Antoine if you dont want anything fancy or look at the suggested diagram since the evaporation will cause the liquid to cool and thus lower the vapor pressure.

This will most likely be a lot easier if you have a process simulation program (allthough i know some might think its overkill - and they may even be right )

Best regards

Morten

RE: Propane pumps

gorkus,

This type of calculation can be done without simulation software, but you have to several broad assumptions.

The equation you need to use is the energy rate balance equation from the 1st law of thermo.  Assuming that the pumping process is adiabatic with respect to the propane storage tank and assuming that fluid velocity is the same at the drain valve of the tank and the pump inlet and assuming that the change in height between the bottom of the tank to the pump inlet is negligible, you can simplify the inlet piping to the pump as a simple throttling process, where the enthaply of the fluid at the bottom of the tank is equal to the enthalpy at the pump inlet.

Now, given a specific pressure drop from the drain valve of the tank to the inlet side of the pump, along with the assumption that the propane is pure liquid at the bottom of the tank, you can determine the quality of the propane at the inlet of the pump due to the vapor formation from the pressure drop in the inlet line.

Once you have obtained the quality it should be downhill from there.  You should be able to determine the quantity of vapor by volume at the inlet based on the quality of the fluid at the inlet and the line size.

The part where this analysis significantly deviates from reality is the assumption that the propane is 100% liquid phase at the bottom of the tank.  If the tank you are using does not have a vapor equalization line, then this assumption is true only at time<=0.001 seconds, because of the internal boiling that will occur the instant you remove liquid volume from the tank, thus increasing the vapor space, thus decreasing the pressure.

I have used this analysis before and approached it from the standpoint that the results obtained will be the most optimistic performance.  Such that, the true vapor content at the inlet to the pump will be higher than the values obtained in the theoretical analysis.

Good luck.

Steve

RE: Propane pumps

(OP)
Q= dU+W
dU= -W
W = -1/2 m V2
The value of enthalpy of the fluid at the bottom of the tank , is a data? (Liquid Enthalpy).
this value of enthalpy = -1/2mV2 its ok?
But I don´t Know How can I get the quality of the propane at the inlet of the pump due to the vapor formation from the pressure drop at a gine specific pressure drop.

Could you help me?

RE: Propane pumps

I think I've seen elsewhere Montemayor's response to the last query. Am I right ?

RE: Propane pumps

(OP)
but
hL ( flashed liquid enthalpy), What is this?
hL could be calculate as follow:
(PV)^CP/CV=cte=K
Then I can Know final volume of liquid at the inlet of the pump.

w= K[ Vf^(1-cp/cv)-Vi^(1-cp/cv)]
K= (PV)^cp/cv

dU=-w
H= U+PV
this is the value of Flashed liquid enthalpy

RE: Propane pumps

Why are you getting a phase change at the pump suction?  In the LPG systems I am familiar with, LPG (C3 or C4) in storage facilities is maintained at or slightly above bubble point.  Usually in these type of systems the pressure in the storage vessel is maintained at bubble point using a makeup gas system or a C3/C4 vapor-return system with a compressor.  This prevents liquid flashing and phase change at the pump suction and in the suction piping.

Anyway... I would use use a process simulator like HYSYS or WinSim to do this.  If you don't have that you only need a propane property table or a p-h diagram.  Assume an adiabatic or isenthalpic throttling process and you will not be far off.  From this you can determine the quality at the pump suction and that will give you the approximate vapor volume.

Thanks!
Pete

RE: Propane pumps

(OP)
but If there is not a vapor line return?
Could you tell me any steps for determine the quality at the pump suction?
Thank you

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