An orifice plate is the most common type of restriction that is used in flow measurement.
An orifice plate is primarily a thin metal plate with a hole bored in the center. It has a tab on one side where the specifications such as Tag number, orifice bore diameter (in inches), the material of orifice plate (ex: SS316) are stamped. The stamping is made plate upstream side of the flow direction. The upstream side of the orifice plate is usually has a sharp edge.
The Orifice Plate is provided with a drain hole or vent hole as per the gas and liquid services.
Orifice plate drain hole:
It is a small hole at the bottom of the orifice plate, which is required in gas flow service where liquid or condensate entrain may take place. The drain hole is not recommended in orifice plates for the services of viscous fluids, slurry, dirt liquid flow measurements because of the chance that the hole may get plugged.
Orifice plate Vent hole:
It is provided at the upper portion of the orifice plate, which is required for liquid service to allow gas bubbles to escape.
The thickness of the orifice plate depends on pipeline size and differential pressure.
The disadvantage with the vent hole and the drain hole is that its size can affect the accuracy of flow measurement.
Materials: Stainless steel 316, Stainless steel 304, Hastelloy C, Monel, Carbon steel, Titanium, etc.,
Types of orifice plates:
It is the most widely used, has a circular hole in the middle. It is installed in the pipeline with the hole concentric to the pipe. The concentric orifice plate is not recommended for slurries and dirty liquids, where the solid particles accumulate near the orifice plate.
Concentric orifice plates are preferable to use in a vertical meter tube if at all possible.
Concentric orifice plates can be used in clean liquids, vapors, or gases.
The beta ratios of concentric orifice plates range from 0.25 to 0.75.
Eccentrically bored orifice plates are plates with these orifice plates are used liquids containing sediments of solids or flush liquid condensate deposits.
Segmental Orifice: the bore of this plate is a semicircle. These are used for measurements where gas or liquid flow containing solids in the stream. A segmental opening is placed either top or bottom of the pipe.
The use of Segmental and eccentric orifices are recommended where horizontal meter runs are required.
The inlet bore of this orifice plate is rounded to a quarter circle. This orifice plate is principally used to measure flow rates of Reynolds number from 10,000 or above. 3,000 to 5000 depending on the beta ratio.
This type of orifice plate is used for flows such as heavy crudes, slurries, syrups, and viscous flows.
Advantages of orifice plates:
High differential pressure.
Available in many materials. SS 316, SS 316L, SS 304, Monel, Hastelloy c.
Possibility of change of their characteristics due to erosion and corrosion.
Pressure loss will be very high compared with any other primary flow sensor.
Flow characteristics are square root.
They are holes bored through the flanges located 1” upstream side and 1” downstream side from respective faces of the orifice plate.
Flange taps should not be used in pipe sizes less than 2 inches.
Corner taps are located right at the upstream side and downstream side faces of the orifice plates. As shown in the figure below, the pressure is measured at the corner between the orifice plate and the pipe wall.
Corner taps for Pipe size less than or equal to 1 inch.
taps are located 2.5 pipe diameters (PD) upstream and 8 pipe diameters (PD) downstream. With the help of pipe taps, the smallest pressure difference because of the tap distance from the orifice.
Taps are located 1 pipe diameter upstream and 1 pipe diameter downstream. For pipe sizes greater than 6 inches chooses radius taps.
Vena contracta taps and VENA CONTRACTA:
with an orifice installed in the piping work, as the flow passes the static pressure increases the upstream of the orifice and then decreases sharply the downstream of the orifice reaching a minimum pressure is called vena contracta.
Vena contracta taps are located one pipe diameter upstream and downstream at the point of minimum pressure. Usually half pipe inner diameter downstream.