Flow Measurement
There are various methods used to measure the flow rate of stream, water, lubricants, air, etc., in a nuclear generating station. The common used of flow measurement tool is DP cell type flow detector.
Flow Detectors:
To measure the rate of flow by the differential pressure method, some form of restriction is placed in the pipeline to create a pressure drop. Since flow in the pipe must pass through a reduced area, the pressure before the restriction is higher that after or downstream. Such a reduction in pressure will cause an increase in the fluid velocity because the same amount of flow must take place before the restriction as after it. Velocity will vary directly with the flow and as the flow increases a greater pressure differential will occur across the restriction. So by measuring the differential pressure across a restriction, one can measure the rate of flow.
Orifice Plate:
The orifice plate is the most common form of restriction that is used in flow measurement. An orifice plate is basically a thin plate with a hole bored in the center. It has a tab or one side where the specification of the plate is stamped. The upstream side of the orifice plate usually has a sharp, edge.
When orifice plate is installed in a flow line (usually clamped between a pair of flanges), increase of fluid velocity through the reduced area at the orifice developed a different pressure across the orifice. This pressure is a function of flow rate.
With an orifice plate in the pipe work, static pressure increase slightly upstream of the orifice (due to back pressure effect) and then decreases sharply as the flow passes through the orifice, reaching a minimum at a point called the vena contracta where the velocity of the flow is at a maximum. Beyond this point, static pressure starts to recover as the flow slow down. However, with an orifice plate, static pressure downstream is always considerably lower than the upstream pressure. In addition some pressure energy is converted to sound and heat to friction and turbulence at the orifice plate.