Choosing the best check valve depends on materials needed and ease of maintenance. Materials must be compatible with the product and valves should quickly stop reverse flow. Different types of valves have varying pressure drops and maintenance requirements. Regular testing and disassembly for cleaning may be necessary.
Choosing the best check valve, that is, a valve that shuts off the flow of liquids or gases in the reverse direction, depends on the valve materials needed and ease of maintenance. Check valves must be made of materials compatible with 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 run for long periods with little or no maintenance.
A check valve normally contains a disc or plate, machined to fit tightly on a seat or gasket area when closed. When liquids or gases flow against the disc, it opens and allows the product to pass in the normal direction of flow. If the flow stops or is reversed for some reason, the product pushes the disc into the closed position and prevents the reverse flow.
Materials of construction for industrial applications are very important in preventing check valve failure. Copper or brass may be required for water applications and chemical resistant materials 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 in the selection of check valves, also taking into account the reduction of the pressure drop. They are primarily a safety device, because backflow in chemical operations can cause equipment damage or a hazardous condition. Pressure drop is important because a valve that limits or restricts flow can malfunction and require 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 cheap, have few moving parts and low pressure drops. These are common on potable water systems to prevent groundwater contamination if a pipe breaks in the system.
Tilt-disc valves are related to flapper valves by the way the disc moves. The disc has a rod running horizontally through it which is fixed to each side of the valve housing. The rod passes through the top of the puck, so more of the puck’s weight is suspended below the rod. The product flow pushes against the disc and holds it open. Tilt-disc valves have a higher pressure drop than flapper valves, but can be adjusted for different flows using external weights added to the stem called counterweights.
Piston valves have a disk that sits horizontally above the valve seat, like a platter sitting on the opening. Product flows up through the disc and holds it open, but stopping or reversing the flow forces the disc down onto the seat. A manual shaft and valve handle can be added on top of a piston valve to push the disc down, which allows the check valve to act like a regular shutoff valve.
Another type of control is a wafer valve. The valve has a wafer-thin disc mounted vertically in the valve body and held in place by a spring which pushes it against the valve seat. These valves are useful when very fast closing is required, but will have higher head losses due to the spring trying to push it closed.
Maintenance includes a test to determine if the valve is functioning properly. 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 is closing properly and not allowing air to escape backwards.
Some valves can also be disassembled for inspection or cleaning. This can be important if the product to be handled is corrosive or contains solid material which can block the valve. They may have internal filters to block solids and prevent disc damage or leakage, or the filters may be installed upstream before product reaches the valve.
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