Orifice Meter

  • The equation of venturi meter applies here as well, with \(C_o\) (orifice coefficient) replacing \(C_v\).

  • The orifice opening is usually circular, but can be other shapes as well: square, oval, triangular, etc.

  • The orifice plate can easily be changed to accommodate widely different flow rates, where as the throat diameter of a venturi meter is fixed.

  • The orifice meter has a large permanent loss of pressure because of the presence of eddies on the downstream side of the orifice plate. The shape of the venturi meter prevents the formation of these eddies and greatly reduces the permanent loss.

  • There are a number of customary positions of the pressure taps which lead to the manometer such as pipe taps, flange taps and vena contracta taps out of which, flange taps are the most common.

  • The coefficient of discharge of orifice \(C_o\) is dependent on the location of the pressure taps.

  • For \(\text{Re} > 10000\), \(C_o\) of orifice meter is constant at 0.61. At lower Reynolds numbers the orifice coefficient becomes a strong function of \(\text{Re}\).

  • The orifice discharge coefficient is significantly affected by flow disturbances which originates in valves, bends, and other fittings located upstream from the orifice. It is less affected by downstream disturbances. As a general rule, the meter should be placed 50 pipe diameters downstream and 10 pipe diameters upstream from any disturbances. The upstream distance can often be reduced by placing straightening vanes in the pipe.

  • The orifice opening can be ‘sharp’ (beveled) for better accuracy. But beveled orifice is not suitable if it is used to measure flow in both the directions.