An orifice plate is a device used for measuring flow rate, reducing pressure or restricting flow (in the latter two cases it is often called a restriction plate).
Description
An orifice plate is a thin plate with a hole in it, which is usually placed in a pipe. When a fluid (whether liquid or gaseous) passes through the orifice, its pressure builds up slightly upstream of the orifice but as the fluid is forced to converge to pass through the hole, the velocity increases and the fluid pressure decreases. A little downstream of the orifice the flow reaches its point of maximum convergence, the vena contracta (see drawing to the right) where the velocity reaches its maximum and the pressure reaches its minimum. Beyond that, the flow expands, the velocity falls and the pressure increases. By measuring the difference in fluid pressure across tappings upstream and downstream of the plate, the flow rate can be obtained from Bernoulli's equation using coefficients established from extensive research. In general, the mass flow rate q m {\displaystyle q_{m}} measured in kg/s across an orifice can be described as
q m = C d 1 − β 4 ϵ π 4 d 2 2 ρ 1 Δ p , {\displaystyle q_{m}={\frac {C_{d}}{\sqrt {1-\beta ^{4}}}}\epsilon {\frac {\pi }{4}}d^{2}{\sqrt {2\rho _{1}\Delta p}},}
where
The volume flow rate q v {\displaystyle q_{v}} measured in m3/s is
q v = q m ρ 1 . {\displaystyle q_{v}={\frac {q_{m}}{\rho _{1}}}.}
The overall pressure loss in the pipe due to an orifice plate is lower than the measured differential pressure, typically by a factor of 1 − β 1.9 {\displaystyle 1-\beta ^{1.9}} .
Application Orifice plates are most commonly used to measure flow rates in pipes, when the fluid is single-phase (rather than being a mixture of gases and liquids, or of liquids and solids) and well-mixed, the flow is continuous rather than pulsating, the fluid occupies the entire pipe (precluding silt or trapped gas), the flow profile is even and well-developed and the fluid and flow rate meet certain other conditions. Under these circumstances and when the orifice plate is constructed and installed according to appropriate standards, the flow rate can easily be determined using published formulae based on substantial research and published in industry, national and international standards. An orifice plate is called a calibrated orifice if it has been calibrated with an appropriate fluid flow and a traceable flow measurement device. Plates are commonly made with sharp-edged circular orifices and installed concentric with the pipe and with pressure tappings at one of three standard pairs of distances upstream and downstream of the plate; these types are covered by ISO 5167 and other major standards. There are many other possibilities. The edges may be rounded or conical, the plate may have an orifice the same size as the pipe except for a segment at top or bottom which is obstructed, the orifice may be installed eccentric to the pipe, and the pressure tappings may be at other positions. Variations on these possibilities are covered in various standards and handbooks. Each combination gives rise to different coefficients of discharge which can be predicted so long as various conditions are met, conditions which differ from one type to another. Once the orifice plate is designed and installed, the flow rate can often be indicated with an acceptably low uncertainty simply by taking the square root of the differential pressure across the orifice's pressure tappings and applying an appropriate constant. Orifice plates are also used to reduce pressure or restrict flow, in which case they are often called restriction plates.
Pressure tappings There are three standard positions for pressure tappings (also called taps), commonly named as follows:
Corner taps placed immediately upstream and downstream of the plate; convenient when the plate is provided with an orifice carrier incorporating tappings D and D/2 taps or radius taps placed one pipe diameter upstream and half a pipe diameter downstream of the plate; these can be installed by welding bosses to the pipe Flange taps placed 25.4 mm (1 inch) upstream and downstream of the plate, normally within specialised pipe flanges. These types are covered by ISO 5167 and other major standards. Other types include
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