If a constriction is placed in a pipe as shown in figure 1 below, the flow must be higher through the restriction to maintain mass flow at all points.
Since the mass flow must be equal at all points, the flow velocity must increase in the region of A2. As there is no net gain or loss of energy, the pressure must therefore decrease at A2.
The commonest differential pressure flow meter is the orifice plate, illustrated in figure 2 below:
The orifice plate is inserted into the pipe with upstream tapping point at D and downstream tapping point at D/2 where D is the pipe diameter. The plate should be drilled with a small hole to release bubbles (liquid) or drain condensate (gases). An identity tag should be fitted showing the scaling and plant identification.
The D-D/2 tapping is the commonest but other tappings illustrated in figure 3 may be employed where it is not feasible to drill the pipe.
(a) D-D/2 is most likely the commonest
(b) Flange taps used on large pipes with substantial flanges
(c) Corner taps drilled through flange
(d) Plate taps, tappings built into the orifice plate
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(e) Orifice carrier, can be factory made and require no drilling on site
(f) Nozzle, gives smaller head loss
Orifice plates suffer from a loss of pressure on the downstream side (also termed to as the head loss). This can be as high as 50%. Other differential pressure flow sensors like Venturi tube and Dall tube have lower losses of around 5% but they are bulky and more expensive. Another low loss device is the Pitot tube. Conversion of the pressure to an electrical signal requires a differential pressure transmitter and a linearizing square root unit. This square root extraction is a major limit on the turndown as zeroing errors are magnified. A typical turndown is 4:1.
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Also read: How to Connect a DP (Differential Pressure) Flow Sensor to a DP Transmitter
