What a Lift Check Valve Is
A lift check valve is a non-return valve that allows flow in one direction only. Inside the body, a disc is guided in a bore perpendicular to the seat. When fluid flows in the forward direction, the pressure lifts the disc off the seat; when the flow stops or reverses, the disc settles back onto the seat and blocks the passage. The flanged version has flanged end connections per ASME B16.5 or the applicable standard, which makes it suitable for piping sizes and pressures where threaded ends are not practical. Lift check valves are used on clean liquids and gases where a tight, quick shut-off against reverse flow is required.
Operating Principle
The disc is lighter than the differential pressure it must seal, so the forward flow opens the valve with very little resistance. In a free-lift design, the disc is lifted by the flow and returns by gravity and reverse pressure; this type must be installed in the correct orientation, with the disc axis vertical or per the manufacturer's instruction. In a spring-loaded design, a spring holds the disc on the seat and assists closure as soon as the flow velocity falls, which reduces the slam and the water hammer that occur when reverse flow hits the disc at speed. The spring also allows installation in other orientations and makes the valve suitable for pulsating systems.
Construction and Materials
The valve body is typically cast or forged carbon steel, stainless steel, or a copper alloy, with the seat and disc either integral or fitted with renewable inserts. The trim, including the disc, seat, and spring, is selected for the fluid: stainless steel trim for water, steam, and mild chemicals, and alloy trims for aggressive media. The bonnet may be bolted or screwed, and the valve is normally supplied with a threaded or flanged end per the order. Face-to-face dimensions follow ASME B16.10 or the manufacturer's series, and the pressure-temperature rating follows the flange class, such as Class 150 or 300.
Applications
Lift flanged check valves are used in pump discharge lines to protect pumps from reverse rotation and backflow, in boiler feed and steam systems, in compressed air and gas lines, and in water and process piping where the fluid is clean and free of solids. Because the disc travels a short distance and the closure is fast, the valve is not the best choice for slurry or viscous service, where a swing check valve is more suitable. The media characteristics should be checked against the trim material before selection.
Installation Rules
The valve must be installed with the flow arrow aligned with the pipe flow, and with sufficient straight pipe upstream and downstream to avoid turbulence affecting the disc. Non-spring lift check valves should be installed with the disc axis vertical, or in the orientation recommended by the manufacturer. The valve should be sized so that the disc is fully lifted at the operating flow rate; a valve that runs partially lifted wears quickly and can chatter. For systems with rapid pump starts and stops, a spring-loaded or silent check design should be selected to prevent slamming.
Selection Considerations
Select the valve by the pipe size, the pressure class, the temperature, the fluid, and the allowable pressure drop. Check the minimum opening pressure of spring-loaded designs, which must be lower than the differential pressure of the system at the minimum flow. Confirm the end connection standard, the face-to-face dimension for the flange class, and the body and trim materials against the service. For fire-safety or pressure-vessel applications, the valve should carry the required certification, such as a PED declaration or the applicable national approval.
Frequently Asked Questions
Q: How does a lift check valve work?
A disc is lifted off its seat by the forward flow and falls back to close when the flow stops or reverses. Spring-loaded versions use a spring to speed closure and prevent slamming.
Q: What is the difference between a lift and a swing check valve?
A lift check disc moves perpendicular to the seat along a guide, giving a fast, tight closure for clean fluids. A swing check disc pivots on a hinge, suits larger sizes and lower velocities, and tolerates some solids.
Q: Can a lift check valve be installed horizontally?
Spring-loaded designs can be installed in any orientation. Free-lift (non-spring) designs must be installed with the disc axis vertical, or per the manufacturer's instruction.
Q: What causes water hammer in check valves?
Water hammer occurs when reverse flow slams the disc onto the seat. Spring-loaded and silent check designs close before reverse flow develops, preventing the slam.
Q: What standards apply to flanged check valves?
End flanges follow ASME B16.5, face-to-face dimensions follow ASME B16.10, and the pressure-temperature rating follows the flange class of the valve.
Q: Is a lift check valve suitable for dirty fluids?
No. The close-tolerance disc and seat are damaged by solids. For dirty or viscous service, a swing check or a specifically designed non-return valve is preferred.
