An orifice plate is a device used to measure fluid flow rate in a pipe. It consists of a round metal plate with a hole in the middle, and is installed between two flanged pipes. Orifices can be made of various materials depending on the fluid being measured. The plate has a sharp and blunt side, and must be installed in the correct direction. The flow rate is calculated by measuring the pressure and velocity differences of the fluid at the inlet and outlet sides of the plate using Bernoulli’s principle. Orifices are inexpensive, easy to install and maintain, and have no moving parts. However, they can introduce large permanent pressure drops into a piping system.
A flat orifice is a device that measures the flow rate of fluid in a pipe. Orifice plates are typically 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 where the flow is to be measured.
Orifices can be made of a variety of materials and the material of construction will depend on the fluid being measured. Carbon steel is commonly used for hydrocarbons. Stainless steel is mainly used for more corrosive fluids. Chemical compatibility is important because corrosion can negatively impact performance.
The hole, or bore, in an orifice usually has a sharp side and a blunt side. The cutting edge of the plate should be on the inlet side of the pipe. The beveled side is on the outlet side of the tube. The orifice plate must be installed in the correct direction; lateral geometry plays a large role in how 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 hole. 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 pressure of the fluid are different from what they were at the inlet side of the pipe. The pressure and velocity differences are used to calculate the flow rate using Bernoulli’s principle.
To measure the differential pressure, small tubes, also called “legs”, are installed on the inlet and outlet sides of the plate, connected to a differential pressure gauge. These pipes must be free of debris or heavy fluid buildup. Material other than the measured fluid in the tube legs will affect the accuracy of the differential pressure measurement.
Orifices are commonly used for fluid flow measurements because they are relatively inexpensive, easy to install, easy to maintain, and well understood in the chemical processing industry. Orifices have no moving parts to maintain. The typical accuracy of an orifice gauge can be expected to be between 90 and 95 percent. One of the disadvantages of orifice meters is that they can introduce large permanent pressure drops into a piping system.
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