|
[Instrumentation Table of Contents]
[Sensors and Transducers Suppiers Menu]
The orifice plate Flow Detector is the
simplest of the flow-path 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 (as shown in Figure F1).
Figure F1
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.
|
