There is a loss of pressure across the plate. The principle consideration is selecting an orifice plate is the ratio of its bore diameter (d) to the internal diameter of the pipe (D). This is often called the “beta ratio”. If the d/D ratio is too small, the loss of pressure becomes too great. If the ratio is too great the loss of pressure become too small to detect and too unstable. Ratio from 0.2 to 0.6 generally provides best accuracy.
Several procedures have been developed for calculating the correct size of an orifice to make it suitable for measuring a particular rate of flow.
The orifice plate, flow nozzle and venturi tube operate on the same principle, and the same equation is used for the three. Concentric sharp edged orifice plate should be used for all normal applications. Eccentric or segmental orifice plate should be used for liquid containing solids. Beveled or rounded – edged orifice plate should be used for viscous fluids.
Plate material for industrial fluids normally should be stainless steel or such superior material demanded by the process conditions.
Orifice plates are not generally recommended for applications where:-
1. Wide variations in flow rate occur.
2. Tolerance less than 3% is required.
3. Highly viscous fluids and slurries are to be measured.
4. Piping lay outs do not permit adequate straight length to be used.
5. System allowable pressure drop is very small.
There must be a long continuous run of straight pipe leading up to any of these primary elements. Considerable information is available concerning the length of straight pipe required between such devices as elbows and valves and the primary elements. When sufficient pipe is not possible, the disturbances can be reduced or eliminated by the installation of straightening vanes.