## Volumetric flow rate from tank

## Volumetric flow rate from tank

(OP)

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?

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?

## RE: Volumetric flow rate from tank

Is the concern that the flow element is measuring less than what you calculated?

Is your experiment being done at standard temperature?

Has your flow meter been calibrated?

Patricia Lougheed

******

Please see FAQ731-376: Eng-Tips.com Forum Policies: Eng-Tips.com Forum Policies for tips on how to make the best use of the Eng-Tips Forums.

## RE: Volumetric flow rate from tank

## RE: Volumetric flow rate from tank

The concern is that we are testing a method for our systems as a way to check flow, but our experimental results are not matching up.

The temperature is 68 F, which is close to the standard temperature of 70 F used in our instrumentation.

Our flow meter has been calibrated and we have used several different flow meters with similar results. We also have several pressure gauges measuring the tank pressure with identical results.

## RE: Volumetric flow rate from tank

## RE: Volumetric flow rate from tank

It doesn't seem likely that a tank that was sold as a 400 gallon tank would be in actuality 250 gallons.

I measured the straight side length of the tank to be 73" with a circumference of 114". The wall thickness is 0.244" from U1A form. The heads are elliptical.

If I plug this information into http:

## RE: Volumetric flow rate from tank

## RE: Volumetric flow rate from tank

## RE: Volumetric flow rate from tank

dvd - The flowmeter is a Dwyer variable area flowmeter with the exhaust open to atmosphere

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We are using the RMC type with a scale reading from 40 to 400 SCFH. We have no piping attached to the exhaust of the meter, so this should be at atmospheric pressure.

"Also, for pressure-fed flowmeters on air or gas service, the discharge piping should be as short and open as possible. This will allow operation of the flow tube at near atmospheric pressure and insure the accuracy of the device." - From Dwyers manual

## RE: Volumetric flow rate from tank

you have constant volume, and a pressure drop (presumably at constant temperature), so you can calc the mass that's left the tank, no?

## RE: Volumetric flow rate from tank

## RE: Volumetric flow rate from tank

Patricia Lougheed

******

Please see FAQ731-376: Eng-Tips.com Forum Policies: Eng-Tips.com Forum Policies for tips on how to make the best use of the Eng-Tips Forums.

## RE: Volumetric flow rate from tank

## RE: Volumetric flow rate from tank

dvd - I've confirmed constant temperature at the flowmeter. Would a slight temperature difference cause a 48% difference in results?

vpl - I'm not sure what you mean by full. Are you saying that the nitrogen gas is not occupying the full volume of the tank?

btrueblood - I can actually measure the pressure to 1 decimal place (it should be 62.0 to 61.0 psig). There are two inspection ports on the tank that I will check to see if I might have actually found the location of Jimmy Hoffa.

## RE: Volumetric flow rate from tank

I mean your measurement is a change in pressure, ie a delta-P, which has (at least) 10% error associated with it (0.1 psi resolution in a measured change of 1 psi). Similar problem with time measurement. Finally, what is the change in tank temperature (you could assume adiabatic conditions, but ought to take it into account, and not just assume isothermal without checking to see if it makes any difference).

## RE: Volumetric flow rate from tank

## RE: Volumetric flow rate from tank

## RE: Volumetric flow rate from tank

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## RE: Volumetric flow rate from tank

## RE: Volumetric flow rate from tank

## RE: Volumetric flow rate from tank

## RE: Volumetric flow rate from tank

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

http://

I also made a spreadsheet in Excel with the calculations:

http://

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.

## RE: Volumetric flow rate from tank

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.