These are used in walls or on the ends of pipes to prevent reversal of flow, and consist of a frame with a gate or flap which is hinged at the top. In order to ensure proper closure against the frame, the gate is double hung, by means of stainless steel pins in bronze bushes.

The smaller sizes, leading dimensions of which are tabulated below, are generally of cast iron, but can also be cast in stainless steel, and have bronze or stainless steel sealing faces.

Larger sizes, having square or rectangular openings, are fabricated from stainless, corrosion-resistant, or mild steel, and are usually supplied with Neoprene seals.