A plastic check valve, like all check valves, prevents backflow by allowing process streams such as water, corrosives, compressed air, liquid metal, hydraulic fluid, fuel, gasoline, syrup, acids, adhesives, slurry and sludge, to name a few, to flow freely in a downstream direction while simultaneously sealing itself should the direction of flow reverse.
Plastic check valves provide this security with relatively simple devices. The body of the valve is a molded, extruded or die cast cylindrical element that is placed midstream in the plumbing. Because they require no external activation or actuation mechanism, these non return valves are easily installed into pre-existing pipeline configurations. The body holds in place a ring-like plastic element known as the seat. This extends from the inner walls toward the center-line of the pipe but does not fully close.
A plugging mechanism, such as a piston or ball rests in the seat. Another plugging device is a swinging disk. When pressure is great enough, and going in the desired direction, the stopper is lifted away from the seat of the valve opening the gap to allow process stream to flow freely downstream. Reverse flow inversely pushes the plug back into the hole, creating an air tight seal that will not be opened again until forward flow pressure is greater than the back pressure.
As aforementioned, plastic check valves come into frequent contact with a number of different process streams. In order to provide reliable stream control, it is important to select a valve material that is chemically compatible with the flow composition. A number of different plastics are therefore available to suit check valve needs under any circumstances.
PVC check valves are common as they provide good flexibility, smooth surfaces and are nontoxic. Polypropylene is also used and is similar to PVC but with additional benefits of resistance to UV, weathering and ozone exposure. PTFE is suited for use with highly corrosive substances while CPVC maintains integrity in high heat applications such as boiling water or molten metal transfer. Each of these acetal polymers offers high fatigue resistance and lubricity which help to increase product longevity.
While metals may also be used in valve design, plastics are easily fabricated into precise shapes and offer many of the same qualities as metals with lower overhead cost. In addition to material considerations, manufacturers should consider plastic check valve dimensions, pressure ratings, thermal capabilities, connection types and curing technology, all of which can have a significant impact on the plastic check valve performance.