Actual Gas flow measurement in compressor 2

[MarioAlc]

Hi!

I am supervising the FAT of a Compressor Control System (CCS). The CCS vendor gave the flow measurement in deltaP (inchH2O) because they just need this to generate the H vs Q curves.
We required to show the measurement in actual and standard units using the actual pressure and temperature.

In order to do so, they will use some formulas to convert deltaP to actual flow. I don't know the details of the formulas but I assume they need MW, Z, Cp/Cv and gas viscosity. Those values are nowhere on their screens and are accessible for the operator to modify. However, the gas properties will change over time and from summer to winter.

My question is if you know how this is normally done for flow calculations? Do you think they have to show the gas properties and give an easy way to be modified by the operator?
Is it ok if the formula is just adjusted with actual pressure and temperature or the operator/engineer should change all the other gas properties?

I am struggling with this actual and standard measurement because I want a reliable flow shown in the screen but they only get deltaP and P/T downstream the orifice.

Thank you very much

 

[rotw]

It depends on your control system supplier methodology. Is this for centrifugal?
In which case in general, supplier defines a characterizer for the controller based on gas properties you give to them.
They define calibration ranges for the flowmeter and other instruments accordingly.
The mass flow and standard flows are calculated using a set of constants that the control system supplier will predefine (edit: so called flow scaling coefficients which are typically function of flow element differential pressure, and pressure and temperature at flow element).
If you operate with a different gas MW, supplier would normally do verifications though the calculations are supposed to use invariant system of variables. Since due to this certain design parameters may be exceeded (calibration ranges, controller characterizer, control valve capabilities, etc.).
Normally changes of MW or gas properties should be accounted for and included as part of your design specification and input to control system supplier. If these changes of gas properties were not expected, then you need to get in touch with control system vendor for reconfirmation or upgrade. They may propose upgrade to increase their sales.
For other type of compressor like recip, I cannot say much but I suspect there are also constants defined to arrive at standard and volume flow and probably similar issues on calibration ranges of instruments that could be exceeded when gas properties change have not been accounted for in the original design specifications.

 

[georgeverghese]

Re arranging eqn 6-111 in Perry Chem Eng Handbook 7th ed, we have

Q. sqrt(rho/dp) = K, where Q = m3/sec actual, rho = kg/m3, dp = Pa.
Compute the value of K for the design case operation, input values for which can be obtained from the orifice plate design data. Let this be = Kd. This value remains practically constant for all other flow estimations

Correction factors are applied when actual rho varies from the design case rho.

For any other flow reading, we have a dp reading, a P and T reading. Compute gas density for this reading by

rho = MW. P / Z.R.T using consistent units, where z is gas compressibility for the current operating gas at operating P and T.

Then compute Q with Q = Kd. sqrt(dp/rho).
To get Qs in std units, we have Qs / Q = (P/Ps).(Ts/T).(Zs/Z)