What a Duckbill Check Valve Is
A duckbill check valve is also known as a rubber check valve, a flexible back pressure check valve or an elastomer check valve. It is a one way valve used to prevent reverse flow of gas or liquid, and it derives its name from the shape of the outlet: two lips formed in the shape of a duck's bill. The valve has no disc, no hinge, no spring and no operating mechanism, so it belongs to the simplest class of automatic non return valves.
Because the flow path never passes a metal seat, the valve suits media that would damage or jam a conventional check valve, including raw sewage, slurries and water carrying grit or debris.
Structure and Materials
The body is produced from elastic rubber reinforced with fabric, bonded by vulcanisation into a single component. The rubber flange, the valve body and the duckbill outlet are one piece, and the unit is bolted to a matching metal flange with a gasket. Because the whole wetted part is elastomer, the valve is inherently corrosion resistant.
| Elastomer | Typical characteristics | Notes on service |
|---|---|---|
| Natural rubber | High elasticity and good abrasion resistance | Suited to water and sewage at moderate temperature |
| Neoprene | Good weather and ozone resistance | Common for outfall and drainage duty |
| Nitrile rubber | Resistance to oils and hydrocarbons | Selected where the medium contains oil |
| EPDM | Excellent resistance to water, ozone and ageing | Preferred for drinking water and general wastewater |
| Fluorocarbon rubber | Resistance to chemicals and higher temperature | Used for aggressive chemical service |
The elastomer choice fixes the usable temperature range and the chemical compatibility, so the medium must be defined before the compound is selected. A reinforced fabric layer carries the pressure load, and the wall thickness is matched to the line pressure and the size of the valve.
Working Principle
With no internal pressure, the two lips rest in contact and the outlet stays closed under the natural elasticity of the rubber. As pressure builds inside the valve, the lips separate progressively and open into a wider slit, which allows the medium to discharge at a high flow rate with very little head loss. When the flow stops and the internal pressure falls, the outlet closes again by itself and the external liquid cannot pass back through it, which is what stops reverse seepage.
Because the seal is produced by elastic preload rather than by a loaded seat, the valve cracks open at a very low differential head, and the head loss across it is small compared with a metal seated check valve of the same size. Closing is also hydraulic rather than mechanical, so there is no slamming action and no water hammer from the valve itself.
Features and Advantages
All elastomer construction. No metal wetted parts, so a wide range of corrosive duties can be met simply by changing the rubber compound.
No blockage and reliable sealing. There are no moving parts to jam and no electrical signals or manual operation are needed; the valve operates silently.
Low maintenance. With no vulnerable mechanism, there is little to wear out and maintenance costs are low.
Low opening pressure. The lips open at a very small differential head, so pumping energy is not wasted.
Flexible installation. The flange pattern can be adapted to several mounting arrangements, and the valve can be supplied for buried, submerged or wall mounted service.
Applications
Municipal and coastal engineering. Outfalls on sea walls, beaches, docks and reservoirs, and municipal flood or storm drainage, where the valve keeps flood water and sewage from flowing back into street drainage and basements. The same duty applies to tide flap replacement, where the elastomer valve closes against a rising sea level and resists silt and debris.
Wastewater treatment. Discharge lines and pump station outlets, where reverse flow would spread odorous or contaminated fluid, and where the valve must remain operable in a heavily fouled stream.
Industrial service. Chemical, petroleum, food, pharmaceutical and aviation installations that require reliable backflow protection in a line carrying solids or aggressive fluids.
Other fields. Irrigation networks, aeration and vacuum systems, and equipment such as ventilators and pumps, where a small, silent, corrosion free non return element is needed.
Selection, Limits and Installation
The size of the valve is set by the line bore, and the elastomer is set by the medium, the temperature and the required abrasion resistance. A duckbill check valve is a low pressure non return device: it seals against reverse flow by elasticity, so it should not be used as a shutoff valve and should not be relied on to hold a high reverse differential pressure for long periods without a mechanical back up.
Install the valve with the flange tightened evenly and the lips unobstructed, so that the outlet can open and close freely without contact with the pipe wall or with accumulated debris. On submerged outfalls, the outlet should be arranged so that the lips close against external water rather than being held open by a fouled seat area, and the assembly should be accessible for periodic inspection and cleaning.
FAQ
Q: How does a duckbill check valve close without a spring or a disc?
The rubber lips are moulded with an elastic preload, so they touch when there is no internal pressure. Forward pressure opens them, and when the pressure falls the elasticity returns them to the closed position.
Q: What is the advantage of an all rubber construction?
There are no metal wetted parts and no mechanisms to corrode or jam, so the valve works in sewage, slurry and corrosive media that would soon damage a conventional metal check valve.
Q: How much head is needed to open the valve?
Very little, because the lips are opened by internal pressure against their own elasticity rather than by lifting a weighted disc. This is why head loss across the valve is low.
Q: Can a duckbill check valve hold reverse pressure like a metal check valve?
No. It is a low pressure non return element that seals by elasticity. Where a high reverse differential pressure must be held, a mechanical check valve should be used or provided as back up.
Q: Which elastomer should be chosen for a sewage outfall?
EPDM or neoprene is common for sewage and seawater duty. Nitrile rubber is selected when the medium contains oil, and fluorocarbon rubber is used for aggressive chemicals or higher temperature.
Q: What maintenance does the valve need?
Little, because it has no moving parts. Periodic inspection of the lips for cuts, hardening or debris is normally sufficient, and the valve can be unbolted and replaced as a single piece if it is damaged.