Usually, when setting up a differential pressure transmitter, we need to decide what variable the device will measure, such as flow, level, volume, mass, and others. Then we have to calculate the flow range of the orifice plate.
The working principle of orifice plate flow meters
When an orifice plate is placed in a pipe carrying the fluid whose rate of flow is to be measured, the orifice plate causes a pressure drop which varies with the flow rate. This pressure drop is measured using a differential pressure transmitter and can be used to calculate the flow rate.
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Variables in orifice plate flow measurement
- The detail of the fluid movement inside the pipe and orifice plate has to be understood.
- The fluid having a uniform cross-section of flow converges into the orifice plate’s opening in its upstream. When the fluid comes out of the orifice plate’s opening, it’s cross-section is minimum and uniform for a particular distance and then the cross-section of the fluid starts diverging in the downstream.
- At the upstream of the orifice, before the converging of the fluid takes place, the pressure of the fluid (P1) is maximum. As the fluid starts converging, to enter the orifice opening its pressure drops. When the fluid comes out of the orifice opening, its pressure is minimum (p2) and this minimum pressure remains constant in the minimum cross-section area of fluid flow at the downstream.
- This minimum cross-sectional area of the fluid obtained at downstream from the orifice edge is called VENA-CONTRACTA.
- The differential pressure sensor attached between points 1 and 2 records the pressure difference (P1 – P2) between these two points which becomes an indication of the flow rate of the fluid through the pipe when calibrated.
Orifice flow plate for liquid, gases and steam
Orifice plate types
Like most instrumentation, the ideal type of orifice plate is dependent on the application. Orifice plate flow meters are built in different forms depending on the application-specific requirements. The shape, size, and location of holes on the orifice plate define the following orifice plate types:
- Concentric Orifice Plate
- Eccentric Orifice Plate
- Segment Orifice Plate
- Quadrant Edge Orifice Plate
To know more about flow meters, you can read our article on flow meter types
Concentric Orifice Plate
It is made up of SS and its thickness varies from 3.175 to 12.70 mm. The plate thickness at the orifice edge should not be exceeded by any of the following parameters:
- 1 – D/50 where, D = The pipe inside diameter
- 2 – d/8 where, d = orifice bore diameter
- 3 – (D-d)/8
*Beta Ratio(β): It is the ratio of orifice bore diameter (d) to the pipe inside diameter (D).
Eccentric Orifice Plate
It is similar to Concentric Orifice plate other than the offset hole which is bored tangential to a circle, concentric with the pipe and of a diameter equal to 98% of that of the pipe. It is generally employed for measuring fluids containing
- Media having Solid particles
- Oils containing water
- Wet steam
Segment Orifice Plate
It has a hole which is a semi-circle or a segment of a circle. The diameter is customarily 98% of the diameter of the pipe.
Quadrant Edge Orifice Plate
This type of orifice plate is used for flow such as crude oil, high viscosity syrups or slurries, etc. It is conceivably used when the line Reynolds Numbers range from 100,000 or above or in between to 3,000 to 5,000 with an accuracy coefficient of roughly 0.5%
Equations for orifice plate flow rate calculation
Maximum flow: Max flow = K x √ΔPmax
Constant: K = max flow/√ΔPmax
Differential pressure: ΔP = (flow/K)²
Output (4-20 mA): Output mA = [(%ΔP) x 16 mA] + 4 mA
Differential pressure transmitter with metal sensor for measuring pressure differences
Advantages of orifice plate flow meters
- It is a very cheap and easy method to measure flow rate.
- It has predictable characteristics and occupies less space.
- Can be used to measure flow rates in large pipes.
To know more about coriolis flow meters, you can read our article on Coriolis flow meters
Limitations of orifice plate flow meters
- The vena-contracta length depends on the roughness of the inner wall of the pipe and the sharpness of the orifice plate. In certain cases, it becomes difficult to tap the minimum pressure (P2) due to the above factor.
- Pressure recovery at downstream is poor, that is, overall loss varies from 40% to 90% of the differential pressure.
- In the upstream straightening vanes are a must to obtain laminar flow conditions.
- Gets clogged when the suspended fluids flow.
- The orifice plate gets corroded and due to this after some time, inaccuracy occurs. Moreover, the orifice plate has low physical strength.
- The coefficient of discharge is low.