10. Excess Velocity of Approach

Submerged orifices should be installed, operated, and maintained in such a way as to make the velocity of approach negligible. To prevent excess velocity of approach, the size of the approach flow area should be greater than eight times the size of the orifice opening. Generally, the requirements of section 4 of this chapter will prevent excess velocity of approach. The original tables presented by Christiansen (1935) limit velocity of approach to about 0.5 foot per second (ft/s).

To account for excess velocity of approach, the approach velocity head is frequently added to the differential head under the radical assuming that the effective coefficient of discharge does not change. However, in equation 9-1b, this correction assumes that all the correction is accounted for by an approach velocity head term alone. However, this procedure only constitutes a partial correction. The factors that cause excess velocity of approach also cause changes in the contraction and head loss. Thus, the effective discharge coefficient changes by some undefined amount. It is better to find the cause of excess velocity, and if operation changes or maintenance cannot remedy the problem, then a replacement device may be required.

Orifices can be calibrated for velocity of approach effects by comparing against another device. This comparison would require measurements at given discharges at several upstream and downstream heads. The cost of these measurements most likely would exceed the cost of a replacement measuring device.