mass flow rate through a pipe 4

[jmckennon]

I have a 6000 psig nitrogen cylinder. I'm not sure the setting of the regulator outside of the cylinder. The nitrogen flows to a nitrogen accumulator tank, and fills it so that the pressure in the accumulator is the same as the pressure in the cylinder. On the outside of the accumulator is a fast acting valve. We use this valve to pulse nitrogen into our system. The pipe coming from the fast acting valve is a 3/4' schedule 80 pipe. I need to figure out how to find the flow rate of nitrogen if the fast acting valve goes from closed to 100% open and then closes again in .75 seconds. Any help would be appreciated!!

 

[TD2K]

Can you block in the accumulator and measure the change in pressure?

With the accumulator's volume you can work out the mass change and divide by the number of valve cycles and the time each cycle the valve is open.

 

[Ktysai]

jmckennon

There are a lot of unknown features down there. I've gave you the stripped formula. I'm not such a bright student in pneumatics, although I deal with this field a lot, but only when I make the control the rest I leave it to the best

TD2K is right, at least pressure gauge is needed in both chambers. Maybe the best way is to get a flow meter, but you have pretty big pressure involved there.

The flow exactly cannot be found easily, perhaps a simulation in Solidworks / Cosmos flow analysis can help with that.

Cheers!



It's nice to be important, but it's more important to be nice!

 

[zekeman]

Well, if you assume the accumulator holds at 6000psig, which reasonable, and the pressure into the receiving end remains less than about 3000psig, you have a choked flow situation where the flow is controlled by the tank pressure and the cross sectonal flow ares of the pipe. That flow is
Ap[sqrt[(k/RT)*{(k+1)/2}^-{(k+1)/2/(k-1)}]
K=1.4
R=55
T= absolute temp accumulator

 

[zdas04]

Man this problem is a mess. First, you didn't say what the valve technology was (it matters, a gate valve gets to full flow in about 15% of travel, a ball valve gets to full flow around 25%). Second, you don't say what portion of the 0.75 seconds is spent in the valve's throttle range. Third, you started off by saying you had a regulator and ended up with equalizing the accumulator and supply pressures--which is it, is the pressure at the end of the 0.75 seconds regulator output or supply tank pressure?

All that aside, if you want to know the mass flow rate (which I'm assuming from the title of the thread), then you have to know starting and ending pressure in the accumulator, volume of the accumulator, and possibly pipe volume if it is material. With that, you can determine the mass of nitrogen that transferred in the 0.75 seconds and divide mass by time to get mass flow rate.

David

 

[racookpe1978]

Keep it simple.

First, answer the above questions. There are too many unknowns to calculate the answer to enough detail.

Let the tank pressure steady at a known temperature.
Put an ACCURATE calibrated gage on the tank, and record temperature and tank pressure.

Then, cycle the valve once into an empty accumulator exactly as you would do for a normal cycle. Vent the accumulator, reset your piping and system. Record tank pressure.

Repeat the valve cycle, vent the accumulator, reset the system. Repeat 30 times.

Compare start pressure to end pressure.

 

[rb1957]

personally, 1st i'd install a regulator that i knew the setting of.

do you want the mass flow rate as a function of time, or just the mass flow for an opening event ? the previous post is a good way to experimentally derive the latter. i think, with more information, you could mathematically derived the steady state mass flow rate, but some varibles (coefficient of discharge of the valve, viscosity effects of your tubing) are difficult to accurately quantify.

 

[chicopee]

Get hold of the valve manufacturer and find out if they have test results for you.

 

[jmckennon]

THanks for all the help everyone! I solved the problem by finding the total volume of N2 the accumulator can hold, using the ideal gas law, and finding the difference in the number of moles of N2 that were still in the cylinder after the valve opens as opposed to how many were in there previous to the valve opening. The answer I obtained was correct.

Thanks again for all the help!