## flow compensation from flow meters

## flow compensation from flow meters

(OP)

hi

What is the flow compensation calculation for a flow meter reading specifically a gas phase.

What is the flow compensation calculation for a flow meter reading specifically a gas phase.

## RE: flow compensation from flow meters

## RE: flow compensation from flow meters

V = volume, P= absolute pressure, Z = supercompressibility factor, T = absolute Temperature, std = standard conditions or compensated for flowing conditions, and flow= the flowing conditions.

There are other compensations made to the meter itself that adjust readings, such as reynolds number, expansion of the steel, compensating for average density or density changes, coefficient of discharge adjustments, and on. All of this is covered in API MPMS standards.

## RE: flow compensation from flow meters

If the measurement is based on differential pressure (such as with orifice plate, venturi or rotameter type) the correction is approximately:

V

_{TRUE,O}= V_{I,O}*SquareRoot(Rho_{CAL}/Rho_{O})Where:

V

_{TRUE,O}= actual operating gas flow in m3/h ft3/h or whateverV

_{I,O}= flow rate indicated by flow meter in actual m3/h, ft3/h, etc.Rho

_{CAL}= density of the gas at which the flow meter shows the flow correctlyRho

_{O}= density of the gas at the current operating conditionsThe gas densities are calculated from:

Rho = MolWt*P*T/z/R/T

With

MolWt = Molecular weight of the gas

P = Absolute pressure

T = Absolute temperature

z= Compressibility factor of gas at T and P

R = Ideal gas constant (If P is in bar abs, T in Kelvin, Rho in kg/m3 then R = 8314.472)

If you just have T and P and the conditions aren't too different from the calibration conditions, assume z doesn't change much and use:

V

_{TRUE,O}= V_{I,O}*SquareRoot(P_{CAL}/P_{O}*T_{O}/T_{CAL})with the O subscript being for the current operating P and T conditions and CAL the P and T conditions at which the flow meter reads correctly.

There are standards covering some kinds of meters (e.g. ISO 5167 for orifice plates and venturi tubes) and manufacturers, e.g. of rotameters-like devices, may have their own more sophisticated corrections.

## RE: flow compensation from flow meters

You can easily derive the correct equation from the basic orifice equation. For a gas flow reading in standard ft3/h, the correction is:

Qstd = Qstd,DCS * Sqrt[(Mw.T/P)DCS * (P/Mw.T)actual]

where "actual" refer to current operation and "DCS" is the numbers used for sizing/calibrating the flow meter on the DCS. I assumed Z is constant.

## RE: flow compensation from flow meters

For other meters there is some sort of "K" factor or "meter factor" that is usually provided by the manufacturer to try to convert the measured parameter (e.g. angular velocity of a turbine meter) into a flow rate more accurately. I've often thought of these factors as "multiply by zero and add the right answer", but that is just my prejudice--many meters with meter factors result is very accurate (i.e., low uncertainty, high repeatability) measurement.

The technique and magnitude of any meter factors is very much dependant on both the technology and manufacturer of the device, but there is no meter factor for square-edged orifice measurement.

David

## RE: flow compensation from flow meters

API does allow a meter factor for an orifice, you can take the meter to a lab (or bring the lab to the meter) and flow calibrate it against a traceable standard. Then you can apply the calibration meter factor in real time along with the other compensation factors.

## RE: flow compensation from flow meters

David

## RE: flow compensation from flow meters

A good source of data is to visit Emerson Mobrey's web site and request a copy of their manual for the gas flow computers. Lots of good stuff there.

JMW

www.ViscoAnalyser.com

## RE: flow compensation from flow meters

By compensation I mean:

- The orifice was sized based on the physical properties estimated by the process engineer.

- Based on these properties, the dP cell ranges and meter max flow are determined and put into the DCS.

- Thus, the flow displayed on the DCS (or whatever) is still based on the estimated fluid physical properties.

- If the actual fluid has a different composition and at different temp+pres, then the flow displayed must be "compensated" or "corrected" to yield the real flow.

- It has nothing to do with the C factor. In fact, we typically do not adjust the C factor when we do this "compensation".

## RE: flow compensation from flow meters

David

## RE: flow compensation from flow meters

Method 1.

Example: Assume dPcell range = 100 inH2O for 4-20 mA.

dP of 50 inH2O equals 12 mA equals 0.5 signal equals 0.707 after square root. The DCS multiply it with the meter max of say 100 BPSD to give you 70.7 BPSD.

If your composition and conditions are different from that used to calc the 100 BPSD, then you have to adjust the flow.

Method 2.

What you are saying is that you take the 50 inH2O and stick it into the orifice calc and get the right answer directly.

Both methods will give the same answer, it is just that in my limited experience method 1 is implemented in refineries.