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An orifice plate is a device that measures the flow rate of fluid in a pipe. Orifice plates typically are round metal plates with a hole in the middle. The plate itself is usually sandwiched between the ends of two flanged pipes on the system in which flow is to be measured.
Orifice plates can be made of several different materials, and the material of construction will depend on the fluid being measured. Carbon steel is commonly used for hydrocarbons. Stainless steel is used mainly for more corrosive fluids. Chemical compatibility is important because corrosion can negatively impact performance.
The hole, or bore, in an orifice plate usually has a sharp side and a beveled side. The sharp side of the plate should be on the inlet side of the pipe. The beveled side is on the outlet side of the pipe. The orifice plate must be installed in the proper direction; the side geometry plays a large role in how the fluid passes through the orifice.
An orifice plate measures flow by relating the pressures and velocities of the fluid at the inlet and outlet sides of the plate. The fluid on the inlet side is forced to pass through the orifice bore. Shortly after passing through the orifice, the fluid reaches a point of maximum convergence, known as the vena contracta. At this point, the velocity and the pressure of the fluid are different from what they were in the inlet side of the pipe. The differences in the pressure and velocity are used to calculate the flow rate through Bernoulli's principle.
To measure the differential pressure, small pipes — sometimes known as "legs" — are installed on the inlet and outlet sides of the plate and are connected to a differential pressure meter. These pipes must be free of debris or heavy fluid buildup. Material other than the fluid being measured in the pipe legs will hurt the accuracy of the differential pressure measurement.
Orifice plates are commonly used for fluid flow measurements because they are relatively cheap, easy to install, easy to maintain and well understood in the chemical processing industry. Orifice plates do not have moving parts to maintain. The typical accuracy of an orifice meter can be expected to range from 90-95 percent. One of the downsides of orifice meters is that they can introduce large permanent pressure drops in a piping system.
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