The orifice plate is a metal disk with a concentric hole as shown in the figure below. It is the simplest device used in almost all industrial application because of cheapness and availability in wide range of sizes.
There are certain limitations of the orifice plate. For very high flow rates, the permanent pressure losses are very high. Over a period of time, the sharp edge of the hole wear out and the particles in the flowing fluid build up behind the hole reducing diameter. Hence discharge coefficient gradually changes.
This problem can be eliminated by using eccentric hole near the bottom of the pipe which sweeps out built-up particles behind the plate. Sometimes bubbles of vapour or gas tend to built up behind plate and obstruct the flow. This can be avoided by mounting the orifice plate in vertical run of the pipe.
An orifice plate is a device used for measuring the volumetric flow rate. It uses the same principle as a Venturi nozzle, namely Bernoulli’s principle, which states that there is a relationship between the pressure of the fluid and the velocity of thefluid. When the velocity increases, the pressure decreases and vice versa.
Orifice plates are most commonly used for continuous measurement of fluid flow in pipes. They are also used in some small river systems to measure flow rates at locations where the river passes through a culvert or drain. Only a small number of rivers are appropriate for the use of the technology since the plate must remain completely immersed; i.e., the approach pipe must be full, and the river must be substantially free of debris.
A restrictive flow orifice, a type of orifice plate, is a safety device to control maximum flow from a compressed gas cylinder. In the natural environment, large orifice plates are used to control onward flow in flood relief dams. In these structures, a low dam is placed across a river. In normal operation, the water flows through the orifice plate unimpeded as the orifice is substantially larger than the normal flow cross section.
However, during floods, the flow rate rises and floods out the orifice plate, which can only pass a flow determined by the physical dimensions of the orifice. Flow is then held back behind the low dam in a temporary reservoir, which is slowly discharged through the orifice when the flood subsides.