Orifice Plate
Orifice Plate
The orifice plate is a metal disk with a
hole in it which is inserted into the pipe carrying a flowing fluid.
This hole is normally concentric with the disk. Over 50% of the
instruments used in industry for measuring volume flow rate are of
this type. The use of the orifice plate is so widespread because of
its simplicity, cheapness and availability in a wide range of sizes.
However, the best accuracy obtainable with this type of obstruction
device is only ±2% and the permanent pressure loss caused in the flow
is very high, being between 50% and 90% of the pressure difference in
magnitude.
Other problems with the orifice plate
are a gradual change in the discharge coefficient over a period of
time as the sharp edges of the hole wear away, and a tendency for any
particles in the flowing fluid to stick behind the hole and gradually
build up and reduce its diameter. The latter problem can be minimized
by using an orifice plate with an eccentric hole. If this hole is
close to the bottom of the pipe, solids in the flowing fluid tend to
be swept through, and the build-up of particles behind the plate is
minimal.
A very similar problem arises if there are any bubbles of vapor or gas
in the flowing fluid when liquid flow is involved. These also tend to
build up behind an orifice plate and distort the pattern of flow. This
difficulty can be avoided by mounting the orifice plate in a vertical
run of pipe.

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