6 replies to this topic

[DUmesh]

How is Pressure,Temperature and Molecular Weight (density ) compensated flow can be calculated.

Regards
Umesh

[sri]

Umesh

It's not clear. Pl re-phrase it in the form given data
and required data.

[latexman]

You can start with the ideal gas law and a few definitions and derive it. You should come up with something like this:

rho1/rho2 = P1/P2 x MW1/MW2 x T2/T1

[sri]

Umesh,

Why have u given density in brackets for molecular weight? Density and Molecular weight are two different characteristics of gas.

Molecular weight depends only on the composition of gas, whereas Density depends on composition, pressure and temperature.

For a given gas, relationships exists between pressure, specific volume/ density and temperature. Z factors have to be applied for real gases.

Furthermore, gas laws between two conditions (say 1 & 2) can only be applied if there is no phase transformation between state 1 & 2. For example, if you are compressing a rich gas at low pressure (state 1) to high pressure and cooling it to low temperatures, condensation of gas will occur and separated gas (state2) will become leaner. Now, we cannot apply gas law directly between condition 1 & 2, as composition/characteristics of gas (molecular weight) changes between state 1 & 2.

However, as given by latexman, using gas laws for ideal gases, you can get the corelation between pressure, temperature, density/specific volume and molecular weight. However, this may not be accurate for real gases.

Hope other members also endorse my views.

[sri]

Umesh,

if you are looking for dynamic corrections in the measured flow due to change in gas quality, online pressure, temperature transmitters and online gas chromotagraphs are installed to compensate the same in flow computers.

Flow is directly proportional to the sq root of static pressure and inversely proportional to the sq root of specific gravity. However, it will be largely inaccurate if we do the corrections manually.

[DUmesh]

Hello Sri,

Thanks for your reply. I have asked this question as I have completed the Mass balace after completion I have found error in total closure. On assumptions, I have considered some flowmeters are wrong. But instrumentation people disagree on this as there is difference in design and actual mole wt, resulting in error in flowmmeters.

So I am confused . I have used the same formula for corrections, so I do not understand how I can convince them. For assumption, I have considered Custody meter is right because we are
paying on that basis.

Reply awaited

Umesh

[pawan]

Dear Umesh
You can also go through this article on my blog - Flow Measurement - Errors & Corrections.

There you can learn the basics of correction applied to flow measurement.