Volumetric flow rate from tank

[aterwill]

I have an experiment set up where I am measuring the flow rate from a 375 gallon tank (50.13 ft3). I am trying to calculate the flow rate from the tank, but my results are not matching up. The flow meter I have on the tank is measuring 360 SCFH (6 SCFM).

The tank is being filled with nitrogen gas at intervals and once a valve closes it stops being filled. During this time I record the start pressure and the final pressure before the valve opens and resumes filling the tank. The start pressure (when the valve closes) is 62 psig and final pressure is 61 psig and this occurs in 23 seconds.

P1 - 76.7 psia
P2 - 75.7 psia
T - 528 R
V - 50.13 ft3

(76.7 psia - 75.7 psia) / 14.7 psia * 50.13 ft3 = 3.41 ft3
3.41 ft3 / 23 s = 0.148 ft3/s = 533 ft3/hr

Am I doing something wrong or missing something in the calculation?

 

[TheTick]

Does the tank bulge?

 

[aterwill]

btrueblood - the flow meter is open constantly at 360 SCFH. The pressure is regulated to it at 50 psig. The tank swings 1 psi up when the valve opens and 1 psi down when the valve closes. The nitrogen gas from the process delivers more than the tank is flowing on its output.

I've attached a link of how the process is setup here:

http://dl.dropbox.com/u/41855723/VOLUME FLOW RATE.pdf

I also made a spreadsheet in Excel with the calculations:

http://dl.dropbox.com/u/41855723/volume flow rate.xls

I calculate the mass of N2 at each state and then determine the mass difference between the states. Then using the density of N2 at standard conditions of 70F and 14.69 psia the flow rate is calculated. If I left the process as adiabatic and didn't change the temperature between states the flow rate would be 536 SCFH. If I change the temperature by 2F to 66F the resulting flow rate is closer to the meter reading.

Next step will be to experimentally measure the temperature of the gas in the tank.

 

[btrueblood]

aterwill,

Could not open the first link, and don't like opening excel files from unknown sources, sorry. But, rereading your first post it starts to make a bit more sense. Can you measure the temperature of the N2 just before the meter - you are throttling from 50 psig to atmospheric, so there should be substantial Joule-Thompson cooling occurring there, affecting your reading if not accounted for properly (unless I'm still missing something).

Similar effects are happening in the tank - the throttling thru the inlet valve is injecting cold N2 into the tank, which then warms to some extent due to heat flow through the tank walls. Knowing the temp's. at both ends will/should help you get closer agreement between the two measurements.