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With an orifice plate
flowmeter, the restriction is in the form of a plate which
has a hole concentric with the pipeline. This is referred to
as the primary element.
To measure the differential
pressure when the fluid is flowing, connections are made
from the upstream and downstream pressure tappings, to a
secondary device known as a DP (Differential Pressure) cell.
From the DP cell, the information may be fed to a simple
flow indicator, or to a flow computer along with temperature
and/or pressure data, which enables the system to compensate
for changes in fluid density.
In horizontal lines carrying
vapors, water (or condensate) can build up against the
upstream face of the orifice. To prevent this, a drain hole
may be drilled in the plate at the bottom of the pipe.
Clearly, the effect of this must be taken into account when
the orifice plate dimensions are determined.
Correct sizing and
installation of orifice plates is absolutely essential, and
is well documented in the International Standard ISO 5167. |
