On Flow Measurement

F-0004 How To Read The Flowrate

Different types and designs of float are used for glass tube variable area flowmeters.
The reading positions by float are specified by Standard and in Japan, JIS B7551 "Float type Variable Area Flowmeters" governs in which the following description is shown;

JIS B7551

In our actual products, the following types of float are used and their reading points are as follows
(position of Arrow)

F-0005 Description of Accuracy of Flowmeters

Accuracy is the key factor of flowmeters. Two different ways of indication of the flowmeter accuracy are often used as follows;

[1] ± OO% of Full Scale ( OO% F.S.)

This is to indicate the accuracy of flowmeter against full scale range.
For example, when the subject flowmeter has 100m3/h full scale range with 1.5% F.S. accuracy, the absolute error which might be involved is ;

100m3/h × 0.015 = 1.5m3/h

This estimated error is applicable through the whole scale range. Even when the actual flow rate is 50m3/h (50% of Full Scale), still the possible error is 1.5 m3/h.
Thus, practical accuracy at 50% of full scale is;

1.5m3/h × 50m3/h = ± 3.0 %

The actual / practical accuracy curve of the Full Scale Accuracy is expressed by the following Graph;
(Typical accuracy graph of Rotameters)

 

 

[2] ± OO% of Reading (Indicated value) ( OO% R.D.)

This means that the accuracy is against the measurement value. ± 1.0% R.D. means 1.0% accuracy is guaranteed for through the scale range. However, flowmeters may often have minimum possible measuring point with accuracy limit. Lower than this point, the accuracy will not be guaranteed or different/separate description of accuracy is applicable.
The following is the typical indication curve of "± 0.5% R.D" from our Magnetic Inductive Flowmeters;

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.

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

F-0010 Recommended position of flow control needle valves for purgemeters

Generally, a needle valve is provided for small sized glass tube rotameters (Purgemeters) for flow control purpose. This is to explain the recommended location (Inlet / Outlet) of valve by application

 

For Liquid measurement application

The location of valve may not affect the accuracy and any location(Inlet / Outlet) is acceptable.

For Gas measurement application

For gas measurement, the location of control valve may differ depending on the application.

In case of Atmospheric discharging ( 0MPa pressure outlet)
P-820 Valve at Inlet (Lower) is recommended.
In this case, the flowmeter is calibrated according to 0MPa (1atm).
In case of Atmospheric discharging ( 0MPa pressure outlet)
P-100 Valve at Outlet (Upper) is recommended.
In this case, the flowmeter is calibrated according to customer specified pressure.
In case of Vacuum application (Vacuum in outlet)
P-710 Valve at Outlet (Upper) is recommended.
If the valve is located Inlet (Lower), the pressure in tapered tube becomes vacuum and the movement of float will be unstable.

The valve provided for purgemeters is for flow control purpose, not for process shut down, Excessive rotation for closing may cause breakage of the valve.
Install separate closing valve upstream when complete shut down is needed.

F-0011 Change of Indicator direction of AM-1□□□

There could be a requirement of change of indicator direction than originally designed. In this case, the direction of indicator can be altered even during the operation. Follow the following procedure;

1. Standard assembling of AM-1□□□ type metal tube variable area flowmeters

Unless otherwise specified, AM-1□□□ type metal tube variable area flowmeters are assembled as per the following table.

Model When the indicator observed from front Changeable inlet and outlet direction
inlet direction outlet direction
AM-1□□2 Bottom Top right Bottom to
Top left
Bottom to
Top left
Bottom to
Top left
AM-1□□3 Bottom right Top right Bottom left
to Top left
Bottom rear
to Top rear
Bottom front
to Top front
AM-1□□6 Left Right Right to Left Rear to Front Front to Rear
AM-1□□7 Right Left Left to Right Rear to Front Front to Rear

These directions of indicator may be rotated to change by 90°angle.
Follow the following for changing of indicator direction.

2. Procedure


  1. Requited tools : Hex Spanner for M10 size, + headed driver
  2. Remove Side cover of indicator (1) and Cap (2).
  3. Remove Indicator fixing nut (3) (4 X M10).
  4. Pull up the indicator vertically observing the inside of indicator. Do not damage the lead pipe during this operation.If connecting cable obstructive for free rotation of indicator, disconnect and remove such cable. first and rotate the indicator. Then, connect again the cable.Turn off the power supply in hazardous area.
  5. Rotate the indicator to the required angle and fix onto 4 stud bolts. Confirm that the following magnet of main lever does not touch the lead pipe and moves freely.
  6. Tighten the nuts equally. Before closing the side cover, confirm again that the following magnet of main lever does not touch the lead pipe and moves freely.
  7. The indicator is not the pressure resistant part. Thus, the direction change of indicator can be conducted even during the operation. However, do not touch upper flange bolts since they are pressure resistant part.

Caution

Change of indicator direction may not be conducted for special designed products.

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