Volumetric gas flow correction for density

[tc112]

I am trying to verify the actual volumetric flow rate in a test rig we have setup. The unit is a waste gas combustion device used to burn vapors from oil storage tank batteries. The test rig is flowing C3H6 (Propylene) at 10 oz/sq in, approx. 60 deg F, and at a rate that is supposed to be around 90 CFM. I originally purchased an expensive PD rotary gas flow meter that fit the application perfectly, except that I didn't yet understand that the definition of a standard cubic foot varies based on the gas composition. My meter was only setup for a range of SG from .55 to .87 (C3H6 SG=1.453) and is reading much lower that I had expected. I also tried to verify the flow rate using a thermal mass flow meter (hot wire anemometer), but come to find out it had been calibrated in air and the correction factor the manufacturer gave me adjusted the flow rate up by approx. 55%, this doesn't seem right to me. I have tried to correct the numbers I have for both the meter and the anemometer using the Ideal Gas Law without success. Should the molar flow rate be the same regardless of gas composition for the same P, V, and T? I have been using the universal gas constant for my molar flow rate calcs and corrected density calcs but should I be using the Individual gas constant instead?? From what I have done so far it looks like for a given temp and pressure, the cu ft/lbmol is the same for all gases??

 

[zdas04]

Whenever I find myself working in moles, I am pretty certain that I am going to mess it up. I always do. I don't like moles in my garden and I really don't like them in my work. I try to leave them for the ChemE guys because I have proven myself too inept to use this perfectly reasonable statement of mass.

I never purchase any measurement technology that does not allow input of gas composition (especially) or compressibility, that choice by an instrument company seems to point to other unpleasant shortcuts in their code that they don't tell you about. But you have one and want to convert crap answers to data. The first thing I would do is make sure that everyone is OK with +/- 30% numbers because you are not going to be much better than that. Then you need to go back to first principles:

[ρ]=P*SG/R(air)/T/Z

Vol(condition 1) * [ρ]1 = Vol(condition 2) * [ρ]2

So if you have a meter that is assuming 0.7 SG, then to convert the to your actual SG you need to calculate compressibility (yes, even at your low pressure, your gas is very much non-ideal) of the 0.7 SG gas (you have to make a of assumptions, you basically have to construct a mix of gases that gets you to the midpoint SG and calculate a compressibility for it) and your actual gas.

Finally Vol(condition 1) * [ρ]1 / [&rho]2 is the corrected flow rate. Where Vol(condition 1) is what you are getting from your meter.

David Simpson, PE
MuleShoe Engineering

"Belief" is the acceptance of an hypotheses in the absence of data.
"Prejudice" is having an opinion not supported by the preponderance of the data.
"Knowledge" is only found through the accumulation and analysis of data.

 

[tc112]

David,
Thanks for your reply. In your first equation did you mean MW instead of SG? I know that SG is just a ratio of MW's but is this a valid substitution? I have the P-H Diagram for Propylene from the GPSA data book so I can get the density for a given T&P from it but I dont know if that accounts for Z?? If so I just need to calculate the Z for my 0.6 SG mixture using Z=(MW*P)/(R*T*rho) but how can I calculate Z without knowing rho first and vice versa?

 

[zdas04]

R(universal) = R(air) * MW(air) = R(gas) * MW (gas) ==> R(gas) = R(air) * MW(air) / MW(gas)

SG(gas) = MW(gas) / MW(air)

Therefore

R(gas) = R(air)/SG(gas)

A very useful relationship since SG seems to be far more available than MW.

There are several correlations for compressibility (I use Hall Yarbourough), but mostly they work with SG below 1.0. For anything heavier than that I use REFPROP.EXE from NIST.

David Simpson, PE
MuleShoe Engineering

"Belief" is the acceptance of an hypotheses in the absence of data.
"Prejudice" is having an opinion not supported by the preponderance of the data.
"Knowledge" is only found through the accumulation and analysis of data.

 

[hacksaw]

sre you wanting to verify your thermal mass flow corretion?