Orifice Plate:

The orifice plate is the simplest of the flowpath restrictions used in flow detection, as well as the most economical. An Orifice plates are flat plates 1/16 to 1/4 inch thick. They are generally mounted among a pair of flanges and are installed within a straight run of smooth pipe to prevent disturbance of flow patterns from fittings and valves. Three categories of orifice plates are used:  eccentric, concentric, and segmental (as display in below Figure).

Figure: Orifice Plates

The concentric orifice plate is the most general of the three types. As displays, the orifice is equidistant (concentric) to the inside diameter of the pipe. Flow by a sharp-edged orifice plate is features through a change in velocity. As the fluid passes by the orifice, the fluid converges, and the velocity of the fluid increases to a maximum value. At that point, the pressure is at a minimum value.  As the fluid diverges to fill the whole pipe area, the velocity decreases back to the original value. A pressure increases to about 60 percent to 80 percent of the original input value. The pressure loss is irrecoverable; thus, the output pressure will always be less than the input pressure. A pressure on both sides of the orifice is measured, concluded in a differential pressure that is proportional to the flow rate.

Segmental and eccentric orifice plates are functionally identical to the concentric orifice. A circular section of the segmental orifice is concentric along with the pipe. A segmental portion of the orifice will eliminates damming of foreign materials on the upstream side of the orifice while mounted in a horizontal pipe. It Depending on the category of fluid, the segmental section is placed on either the top or bottom of the horizontal pipe to raise the accuracy of the measurement.

Eccentric orifice plates shift the edge of the orifice to the within of the pipe wall. This design also avoids upstream damming and is used in the similar way as the segmental orifice plate.

Orifice plates have two distinct disadvantages; that cause a high permanent pressure drop (outlet pressure will be 60 percent to 80 percent of inlet pressure) and those are subject to erosion, that will eventually cause inaccuracies within the measured differential pressure.