Technical Details of the Orifice Plate

2019-06-25

The concentric orifice plate flow meter has a sharp concentric bore that provides an almost pure line contact between the plate and the fluid, with negligible friction drag at the boundary. The diameter ratios of concentric orifice plates range from 0.25 to 0.75. The max velocity and min static pressure occurs at some 0.35 to 0.85 pipe diameters downstream from the orifice plate. That point is called the vena contracta. Measuring the differential pressure at a location close to the orifice plate minimizes the effect of pipe roughness, since friction has an effect on the fluid and the pipe wall. 

Flange taps are predominantly used in the United States and are located 1 inch from the orifice plate's surfaces. They are not recommended for use on pipelines under 2 inches in diameter. Corner taps are predominant in Europe for all sizes of pipe, and are used in the United States for pipes under 2 inches . With corner taps, the relatively small clearances represent a potential maintenance problem. Vena contracta taps which are close to the radius taps, are located one pipe diameter upstream from the plate, and downstream at the point of vena contracta. This location varies with beta ratio and Reynolds number from 0.35D to 0.8D.