Non-Return Valves in Pump Systems: Preventing Water Hammer and Backflow

Non-return valves, also known as check valves, play a vital protective role in pump systems by preventing reverse flow that could damage pumps, cause water hammer, and disrupt upstream process equipment. When a pump stops or a pipeline experiences a sudden pressure reversal, the potential for destructive backflow events is significant. Selecting and sizing the correct check valve for each pump system application is an important engineering task that requires understanding of both hydraulic transients and valve dynamic characteristics.

Water Hammer Fundamentals

Water hammer is a pressure surge or wave that occurs when fluid flow is suddenly stopped or changed direction. In pump systems, the most common cause is pump shutdown, either planned or due to power failure. When a pump stops, the flowing column of liquid decelerates rapidly, and the momentum of the moving fluid creates a strong pressure wave that travels back and forth through the piping until dissipated by friction. The peak pressure of this wave can be several times the normal operating pressure, and if check valves slam shut too late (after significant reverse flow has developed), the impact creates an additional pressure spike known as check valve slam.

• Water hammer pressure rise: approximately 0.1 MPa per 1 m/s of sudden velocity change in water

• Check valve slam occurs when the valve disc is still traveling to close as reverse flow impacts it

• Surge analysis (hydraulic transient simulation) should be performed for all large pump systems

• Slow-closing check valves reduce slam but may allow more reverse flow and pump reversal

• Fast-closing (spring-assisted or non-slam) check valves close before significant reverse velocity develops

Check Valve Types for Pump Applications

Several check valve types are commonly used in pump discharge applications. Swing check valves are simple and provide low pressure drop when fully open, but their heavy disc swings to close only when reverse flow begins, making them prone to slam in fast-closing systems. Dual plate (wafer) check valves use two semi-circular spring-loaded plates that close rapidly due to their low mass and spring assistance, making them excellent for reducing water hammer. Axial flow check valves use a spring-loaded disc or cone that closes very quickly as flow decelerates, providing the best protection against slam and reverse rotation of the pump.

• Swing check: low pressure drop when open, high risk of slam in pump system applications

• Dual plate (wafer) check: fast closing, compact, suitable for most pump discharge applications

• Axial flow (nozzle) check: fastest closure, lowest pressure drop, preferred for large pumps

• Tilting disc check: intermediate performance, used where both low pressure drop and moderate slam protection are needed

• Lift check: high resistance, good for vertical applications with upward flow only

Sizing and Selection Criteria

Check valves must be sized correctly to operate with the disc or plate in a stable, fully open position at normal operating flow. A check valve that is oversized for the actual flow will flutter (oscillate between open and partially closed positions), causing rapid wear and noise. The minimum cracking pressure and the flow velocity required to hold the disc fully open must both be verified against actual operating conditions. For pump systems, a surge analysis performed by a hydraulic engineer is recommended for any system with significant elevation changes, long pipelines, high pump heads, or multiple parallel pumps.