What is a Orifice Plate

 Orifice Plate 

The orifice plate is the most common form of restriction that is used in flow measurement. An orifice plate is basically a thin metal plate with a hole bored in the center. It has a tab on one side where the specification of the plate is stamped. The upstream side of the orifice plate usually has a sharp, edge. below Figure shows a representative orifice plate.

When an orifice plate is installed in a flow line (usually clamped between a pair of flanges), increase of fluid flow velocity through the reduced area at the orifice develops a differential pressure across the orifice. This pressure is a function of flow rate. 

With an orifice plate in the pipe work, static pressure increases slightly upstream of the orifice (due to back pressure effect) and then decreases sharply as the flow passes through the orifice, reaching a minimum at a point called the vena contracta where the velocity of the flow is at a maximum. Beyond this point, static pressure starts to recover as the flow slows down. However, with an orifice plate, static pressure downstream is always considerably lower than the upstream pressure. In addition some pressure energy is converted to sound and heat due to friction and turbulence at the orifice plate. below Figure shows the pressure profile of an orifice plate  installation.

On observing above Figure, one can see that the measured differential pressure developed by an orifice plate also depends on the location of the pressure sensing points or pressure taps.

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