HEAD FLOW METERS
Figure 2 Orifice Plates
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.
between a pair of flanges and are
installed in a straight run of
smooth pipe to avoid disturbance
of flow patterns from fittings and
Three kinds of orifice plates are used: concentric, eccentric, and segmental (as shown in Figure
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.