Terms of Use

On Flow Measurement F-0007

Compensation Calculation For Variable Area Flowmeters


Variable area flowmeters (Rotameters) theoretically involve indication errors when the operating condition deviate from the design condition. However, these errors can be compensated by calculations. The calculation procedure by fluid types are shown below.


Compensation(Liquid)e.xls
Compensation(Gas)e.xls
Compensation(Sat.Steam)e.xls

 

IN CASE OF LIQUID MEASUREMENT

 

The liquid density change from the design condition produces indication error.

Actual Flow rate = Indicated flow rate × Correction coefficient

Density correction coefficient
Design density (g/cm3)
to be referred to Approval drawing
Density of the measuring liquid (g/cm3)
the density of actual liquid
Density of float (g/cm3)
differs by material, Refer to the following table.

Float material Density of float *
Stainless steel 7.7 g/cm3
PVC 2.7 g/cm3
* Figures may have deviations due to final design.
Compensation Calculation Example
Original design condition of flowmeter
  Fluid : Water
Design density : 1.0 g/cm3
Actual process condition
  Fluid : Alcohol
Actual density : 0.8 g/cm3
Indication of flowmeter : 10m3/h

Actual flow rate of Alcohol = 10 × 1.134 = 11.34 m3/h

 

IN CASE OF GAS FLOW MEASUREMENT

In Gas Flow Measurement,
  Density of the gas to be measured (under Normal condition, 0°C,1atm)
Operating pressure
Operating temperature
as the factors of compensation.
Actual Flow rate = Indicated flow rate × Correction coefficient

· Density compensation
Density correction coefficient
Design density [kg/m3(nor)]
to be referred to Approval drawing
Density of the gas to be measured [kg/m3(nor)]

· Pressure compensation
Pressure correction coefficient
Design pressure (MPa)
in Gauge pressure,to be referred to Approval drawing
Actual operating pressure (MPa)
in Gauge pressure

· Temperature correction
Temperature correction coefficient
Design temperature(°C)
to be referred to Approval drawing
Actual operating temperature(°C)

Compensation Calculation Example

Original design condition of flowmeter
  Fluid : Air
Design density : 1.293 kg/m3(nor)
Design pressure : 0.2 MPa
Design temperature : 20°C

Actual process condition
  Fluid : CO2 Gas
Design density : 1.977 kg/m3(nor)
Actual operating pressure : 0.4 MPa
Actual operating temperature : 50°C
Indication of flowmeter : 5m3/h(nor)


Actual flow rate of CO2 gas = 5 × 0.994 = 4.97 m3/h(nor)

 

IN CASE OF (SATURATED) STEAM FLOW MEASUREMENT

 

Correction calculation is done by the density of the steam to be measured

Actual Flow rate = Indicated flow rate × Correction coefficient

Density correction coefficient
Density of actual steam (kg/m3)
Design density (kg/m3)
Compensation Calculation Example
Original design condition of flowmeter
  Fluid : Steam
Design density : 5.163 kg/m3
Design pressure : 0.902 MPa
Design temperature : 180°C

Actual process condition
  Fluid : Steam
Design density : 2.549 kg/m3
Actual operating pressure : 0.3479 MPa
Actual operating temperature : 150°C
Indication of flowmeter : 10 t/h

The steam density in operating condition is to be referred to Steam Table.

Actual Flow rate at 150°C = 10 t/h × 0.703 = 7.03 t/h

 

 

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