Flow Measurement by Square-Edged Orifice Plates: Pipe Roughness Effects

This paper presents the results of experimental work carried out recently to determine the effects on the discharge coefficient of various degrees of pipe roughness adjacent to and at different distances from an orifice plate. Experiments were carried out for air flow in 12-, 6-, and 3-inch pipes and water flow in 3-inch pipes.

Downstream, severe incrustation has no significant effect. Upstream, the effects vary with degree of roughness, pipe diameter, m value, and Reynolds number. For an m value of 0·5 and severe conditions of roughness, the combined effects of roughness and pipe diminution produce disturbance factors of the order of 1·40, 1·17, and 1·08 respectively for 3-, 6-, and 12-inch pipes. A length of two diameters of rough pipe immediately upstream from an orifice plate gives an effect close to that which would be obtained for a very long rough pipe. It has been found possible to extrapolate the results to very large pipes.

One of the most important findings is that cleaning a rough upstream pipe adjacent to an orifice plate for a relatively short distance reduces errors which may be otherwise very considerable to within known small limits. A table detailing the extent of such cleaning for various pipe sizes, mvalues, and degrees of roughness is given.

It is recommended that, where reasonable accuracy is required in the flow measurement of dirty fluids, installations should be designed for easy periodic inspection and cleaning of the upstream pipe adjacent to the orifice plate.