Choosing the best check valve, which is a valve that stops the flow of liquids or gases in the reverse direction, depends on the valve materials needed and ease of maintenance. Check valves should be made of compatible materials for the product in contact with them. They should quickly stop the reverse flow of product, which is the primary function of any check valve. The valves should operate for long periods with little or no maintenance.
A check valve normally contains a disc or plate, machined to fit tightly against a seat or gasket area when closed. When liquids or gases flow against the disc, it opens and allows the product to pass through in the normal direction of flow. If the flow stops or is reversed for some reason, the product pushes the disc to a closed position and prevents backward flow.
Materials of construction for industrial applications are very important to prevent check valve failure. Copper or brass may be needed for water applications, and chemical-resistant materials may be needed for acidic or caustic products. Valves in high temperature applications may require special metal alloys or high-temperature plastics or composites.
Fast closing speed is important when choosing check valves, with reduced pressure drop also a consideration. They are primarily a safety device, because backward flow in chemical operations can cause equipment damage or hazardous conditions. Pressure drop is important because a valve that restricts or limits flow can create operating problems, and requires additional pumping power to overcome.
A simple design is a swing check, which has a disc hanging from a hinge inside the valve body or case. Normal flow keeps the disc open, but any reverse flow pushes and holds the disc closed. They are inexpensive, have few moving parts and low pressure drop. These are common on drinking water systems to prevent contamination from ground water if a pipe breaks in the system.
Tilting-disc valves are related to swing valves by the way the disc moves. The disc has a rod passing through it horizontally that is attached on each side of the valve case. The rod passes through the upper part of the disc, so more disc weight is suspended below the rod. Product flow pushes against the disc and holds it open. Tilting-disc valves have more pressure drop than swing valves, but can be adjusted for different flows using external weights added to the rod called counterweights.
Piston valves have a disc placed horizontally on top of the valve seat, like a dinner plate sitting on the opening. The product flows upward through the disc and holds it open, but shutting off the flow or reversing it pushes the disc down onto the seat. A manual shaft and valve handle can be added on top of a piston valve to push down on the disc, which allows the check valve to act as a regular stop valve.
Another type of check is a wafer valve. The valve has a thin wafer disc mounted vertically in the valve body, and held in place by a spring pushing it against the valve seat. These valves are useful when very rapid closing is needed, but will have higher pressure drop due to the spring trying to push it closed.
Maintenance includes a test to determine the valve is working correctly. Regular stop valves are installed on each side of the check valve. Small pressure fittings are installed between the stop and check valves, and with the stop valves closed, air pressure can be added to either side of the check valve. Pressure gauges can be used to determine that the valve closes properly and does not allow air to leak backward.
Some valves can also be disassembled for inspection or cleaning. This can be important if the product being handled is corrosive or contains solid material that can block the valve. They can have internal filters to block solids and prevent disc damage or leakage, or filters can be installed upstream before the product reaches the valve.