Another method by which flow rate can be determined involves the use of an orifice plate-type flowmeter in which an orifice is installed in the pipeline as shown in Figure 2.14.
The figure also shows the presence of two pressure gages, one each on either side of the orifice. This arrangement enables us to determine the pressure drop (AP) across the orifice when the fluid flows through the pipe and given by AP = Pi- P2- The higher the flow rate, greater will be the pressure drop.
The actual flow rate can be determined by the following equation:
Q is the flow rate in m3/s
C is the flow coefficient (C = 0.80 for a sharp-edged orifice and 0.60 for a square-edged orifice)
A is the area of the orifice opening in m^
S is the specific gravity of the flowing fluid and
AP = P1-P2 is the pressure drop across the orifice in psi or kPa.