CHAPTER 9 - SUBMERGED ORIFICES
4. Conditions for Accuracy of Fully Contracted Submerged Rectangular Orifices
The standard submerged rectangular orifice described on figure 9-1 and in table 9-1 has four sides, consisting of thin-edged plates, far enough removed from the sides, bottom, and top of the water prism in the approach channel that essentially full jet contraction occurs. The contraction which occurs is about equal to the maximum that could be obtained with the sides of the orifice at infinite distances from the water prism boundaries.
The following conditions are required to accurately replicate equation 9-1 coefficients and use table 9-1:
(1) The upstream edges of the orifice should be straight, sharp, and smooth.
(2) The upstream face of the orifice wall should be vertical.
(3) The top and bottom edges of the orifice opening should be level.
(4) The sides of the opening should be truly vertical.
(5) The inset orifice plates must be flush, and the upstream face of the supporting bulkhead with the fasteners must be countersunk on the upstream side.
(6) The distance from the opening edges to the boundary and the water surface, both on the upstream and downstream sides, should be greater than twice the least dimension of the orifice opening.
(7) The face of the plates must be free of grease and oil.
(8) Avoid orifice plate knife edges because they are a safety hazard and can damage easily; orifice opening plate perimeter should be between 0.03 and 0.08 inch (in) thick.
(9) If the plates are thicker than condition (8), the plate edges should be reduced to the required thickness by chamfering the downstream edge of the orifice plates to an angle of at least 45 degrees.
(10) Flow edges of plates require machining or filing perpendicular to the upstream face to remove burrs or scratches and should not be smoothed off with abrasive cloth or paper.
(11) The edges of the supporting bulkhead wall cutout to receive the orifice opening plate should be located at least one wall thickness from the orifice opening edges.
(12) For submerged flow, the effective head on the orifice is the actual difference in elevation between the water surfaces upstream and downstream from the orifice wall. The differential head should be at least 0.2 foot (ft).
(13) For free flow, the effective head on the orifice is the difference in elevation between the upstream water surface and the center of the orifice opening.
(14) The cross-sectional area of the water prism 20 to 30 ft upstream from the orifice should be at least eight times the cross-sectional area of the orifice.
(15) The selected type of head measuring device must be compatible with required project accuracy and the amount of head loss that is acceptable.
If all these requirements are satisfied, then the effective discharge coefficient is good to +/-2 percent. However, the accuracy of discharge measurement depends strongly on the secondary head measurement system selected and the magnitude of h. Chapter 8 contains error values for different head measurement systems.