1. Definition and Classification of Orifices

An orifice used as a measuring device is a well-defined, sharp-edged opening in a wall or bulkhead through which flow occurs. Orifices may be used to measure rates of flow when the size and shape of the orifices and the heads acting upon them are known. For irrigation use, orifices are commonly circular or rectangular in shape and are generally placed in vertical surfaces, perpendicular to the direction of channel flow.

A submerged orifice and the same orifice discharging freely have nearly the same coefficient of discharge. Submerging an orifice provides the capability to measure relatively large flows with a small drop in water surface, conserving delivery head compared with weirs. However, the submerged orifice requires head measurements upstream and downstream. The difference between the two heads is used in the orifice equation (equation 9-1). A free- flow measurement requires measurement of only one upstream head.

Using an orifice as a water measuring device demands attention to dimensions and craftsmanship, and the orifice must closely simulate conditions existing when it was calibrated. Equation coefficients depend on these details and can vary considerably. For full contraction, the orifice plate should be machined around the entire perimeter of the opening to a clean, straight, and sharp upstream edge and corner in the direction of the flow. Rounding of an upstream face corner can partially or fully suppress contraction. For example, 1-percent rounding of the upstream face corner of the opening perimeter in terms of minimum orifice opening dimension causes about a 3-percent increase of the contraction coefficient. A well-designed bellmouth will eliminate all contraction (Schuster, 1970).

Orifices may be partially contracted in two senses. One is the amount of curvature of the jet in the direction of flow and the other in the amount of orifice opening perimeter which produces no curva-ture of the jet passing through the opening. The latter is called suppression. An example of a partially suppressed orifice is a sluice gate, where the approach boundary on the sides and bottom continues past the sharp edge of the gate leaf above. This arrangement eliminates the jet contraction along part of the orifice opening perimeter, increasing the effective coefficient of discharge.

Coefficients are used in equations to account for: neglecting the approach velocity head, the approach velocity distribution, the decrease of jet velocity caused by friction, and the amount of jet contraction caused by the flow curving around the corner of the orifice perimeter.

The proximity of the upstream water surface to the top of the orifice opening affects the amount of contraction. For full contraction, the water surface upstream from the orifice must always be well above the top of the opening. Similar to weirs, if the bottom or the side walls of the approach channel are too close to the orifice edges, then the sides and bottom of the orifice jet are not fully contracted. If the upstream water surface drops below the top of the opening, the opening, in effect, becomes a weir.