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.

Flange Taps

Flange taps are the most widely used pressure tapping location for orifices. They are holes bored through the flanges, located one inch upstream and one inch downstream from the respective faces of the orifice plate. A typical flange tap installation is shown in below Figure.

The upstream and downstream sides of the orifice plate are connected to the high pressure and low-pressure sides of a DP transmitter. A pressure transmitter, when installed to measure flow, can be called a flow transmitter. As in the case of level measurement, the static pressure in the pipe-work could be many times higher than the differential pressure created by the orifice plate. In order to use a capsule that is sensitive to low differential pressure, a three valve manifoldhas to be used to protect the DP capsule from being over ranged. The three valve manifold is discussed in more detail in the level measurement.

Corner Taps

Corner taps are located right at upstream and downstream faces of the orifice plates (see below Figure).

Vena Contracta Taps

Vena contracta taps are located one pipe inner diameter upstream and at the point of minimum pressure, usually one half pipe inner diameter downstream (below Figure).

Pipe Taps

Pipe taps are located two and a half pipe inner diameters upstream and eight pipe inner diameters downstream. When an orifice plate is used with one of the standardized pressure tap locations, an on-location calibration of the flow transmitter is not necessary. Once the ratio and the kind of pressure tap to be used are decided, there are empirically derived charts and tables available to facilitate calibration. Advantages and Disadvantages of Orifice Plates

Advantages of orifice plates include:

High differential pressure generated
Exhaustive data available
Low purchase price and installation cost
Easy replacement

Disadvantages include:

High permanent pressure loss implies higher pumping cost.
Cannot be used on dirty fluids, slurries or wet steam as erosion will alter the differential pressure generated by the orifice plate.

What is a Orifice Plate and explain