The orifice plate is the simplest of the flowpath restrictions used in flow detection, as well as the most
economical. Orifice plates are flat plates 1/16 to 1/4 inch thick. They are normally mounted between a pair of
flanges and are installed in a straight run of smooth pipe to avoid disturbance of flow patterns from fittings
and valves. Three kinds of orifice plates are used: concentric, eccentric, and segmental. The concentric orifice
plate is the most common of the three types. As shown, the orifice is equidistant (concentric) to the inside
diameter of the pipe. Flow through a sharp-edged orifice plate is characterized by a change in velocity. As the
fluid passes through the orifice, the fluid converges, and the velocity of the fluid increases to a maximum
value. At this point, the pressure is at a minimum value. As the fluid diverges to fill the entire pipe area, the
velocity decreases back to the original value. The pressure increases to about 60% to 80% of the original input
value. The pressure loss is irrecoverable; therefore, the output pressure will always be less than the input pressure.
The pressures on both sides of the orifice are measured, resulting in a differential pressure which is proportional to
the flow rate. Segmental and eccentric orifice plates are functionally identical to the concentric orifice. The circular
section of the segmental orifice is concentric with the pipe. The segmental portion of the orifice eliminates
damming of foreign materials on the upstream side of the orifice when mounted in a horizontal pipe.
Depending on the type of fluid, the segmental section is placed on either the top or bottom of the horizontal pipe to
increase the accuracy of the measurement. Eccentric orifice plates shift the edge of the orifice to the inside of the
pipe wall. This design also prevents upstream damming and is used in the same way as the segmental orifice
plate. Orifice plates have two distinct disadvantages; they cause a high permanent pressure drop (outlet pressure
will be 60% to 80% of inlet pressure), and they are subject to erosion, which will eventually cause
inaccuracies in the measured differential pressure.