Head Loss Characteristics of Flap Gates at the Ends of Drain Pipes

Flap gates are commonly used at the end of pipe drains and pump outlets to prevent backflows of water and entry of small animals. Flap gates are relatively inexpensive, with low maintenance costs, but can trap debris in their hinge systems. Many texts refer to studies performed on flap gates at the University of Iowa in 1936, which may be limited in value because they are for specific “lightweight” gates. More recent studies in England have attempted to generalize the characteristics for pin–hinged flap gates and place the Iowa studies into a broader perspective. The flap–gate backpressure effect on a gate with free outfall appears to be small. However, under certain submerged conditions, a flap gate will increase the upstream backwater levels. This may be critical in sewerage situations and some surface land drainage cases. Recently, at least one manufacturer sells a rubber–coated steel gate cover with a flexure hinge made of the same rubber material. While it reduces the opportunity for trash to catch, such as may happen on a pinned–hinge type of pivot, this rubber hinge arrangement essentially becomes a spring–loaded gate with the force of closure due to both the weight of the gate and the elastic properties of the hinge. Users have questioned whether this arrangement introduces significant backpressure. We therefore tested a rubber–hinged flap gate to verify whether these gates fit into the general pattern of the limited previous studies on pin–hinged gates, which is to exhibit a continuous decrease in backpressure with increasing flow rate, and hence gate opening. The design information for pin–hinged flap gates is also updated to make it more readily available for design uses. The rubber hinge resulted in a slight deviation towards more head loss, approximately 3 mm (0.1 in.) at larger gate openings compared to about 1 mm (0.025 in.) for pin–hinged gates, which is attributed to the flexure strength of the rubber hinge. Thus, for free–flow outlet applications, flap gates of either the pinned–hinge or the flexure style add small head losses that amount to about 1% to 2% of the pipe diameter. These studies and the review of previous work allow users to evaluate whether flap gates of either the pinned hinge design or the rubberized flexure design can cause detrimental backpressure on a drainage system under free