Sharp-edged weir

Sharp-edged weirs have edges like those of square or sharp-edged orifices (see subsection Orifice Meters ). Notched weirs are or(iuarily sharp-edged. Weirs not in the sharp-edged class are, for the most part, those described as broad-crested weirs. 

In this case, no end corrections are apphed even though the formula apphes only for sharp-edged weirs. See Schoder and Dawson, Hydraulics, McGraw-Hill, New York, 1934, p. 175, for further details. 

So far in this chapter and in the vast majority of problems in pipes, channels, ducts, etc., we assume that the velocity is practically uniform across the pipe, duct, or channel so that we may associate one velocity with the entire flow at one area perpendicular to the flow. In most flows of practical interest to chemical engineers, this simplification introduces negligible errors. However, there are somej very simple and common flows for which this is not the case. The simplest arid most illustrative example is the flow over a sharp-edged weir. 

Figure 5.18 shows schematically the flow in an open channel, which passes over a sharp-edged weir. You can study a very similar flow in the kitchen sink by pouring water out of a pot or a cup, at a high enough velocity that the flow does not dribble down the side of the pot or cup, but rather flows freely away from the edge, as shown in Fig. 5,18. The flow over the weir in Fig. 5.18,is much simpler than the flow out of a cup, because the weir is assumed to be straight an ld to extend a long way into and out of the page, so the complications where it meets the walls of the channel and the complications due to the curvature of the cup or pot can be ignored.

Water flows in a channel 50 m wide at lOOm /s, and it spills over a sharp-edged weir. 

The width of the thin-plate rectangular weir is the same as the flume, Weir plates for sharp edges on the top and contact with water in a line (see Fig. 1), the overflow is freedom without... 

The flume must be fixed-width, flume wall should be perpendicular and smooth, weir plate should be vertical orthogonal to the flume, weir crest sharp edges thickness should be no greater than 1 mm, the difference between the water level at the lower tail and the weir crest elevation should be greater than 7 cm, to ensure the flow free. 

The upstream face of the weir plate should be smooth, and the plate should be strictly vertical. The crest should have a sharp, square upstream edge, a top width of He to % in, and a bevel on the downstream side, so that the nappe springs clear, making a line contact for all but the very lowest heads. If it does not spring clear, the flow cannot be considered as true weir flow, and the coefficients do not apply. If the crest extends downstream so that it supports the nappe, the weir is broad-crested. The velocity at any point in the nappe can be determined. The approach channel should be long enough so that normal velocity distribution exists, and the surface should be free of waves. It may sometimes be necessary to install baffles to ensure a quiet flow of approach. All deviations from proper weir construction affect the flow in the same way. That is, they increase it over the computed value. 

Sharp-Crested Weir Obstruction with a thin, sharp upper edge, used to measure flow rate in an open channel. 

The weir crest must be sharp or at least square-edged. Steel is the best construction material, but tempered wood is also used. 

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