Measuring Pressure Across the Orifice
2019-06-03
In order to read only static pressure in a pipe the tapping hole must not be exposed to the velocity component of the flow. Usually a pressure gauge is mounted in a hole drilled through the wall, square to the wall. At the inside wall of a pipe the velocity is zero and only static pressure is present.
An orifice plate flow meter uses static tapping points located at either side of the orifice. By convention the upstream static tapping is one pipe diameter up from the orifice and the downstream tapping is half a pipe diameter from the orifice at the location of the ‘vena contracta’ where the flow lines are most narrow.
The pressure difference between the two points is proportional to the flow and once the pressure difference is known it is put into an equation to find the velocity. From the flow velocity and cross-sectional areas of pipe and orifice, the volume of product flowing can be calculated.