Non-Slam Check Valves: Preventing Water Hammer and Protecting Piping Systems

Non-Slam Check Valves: Preventing Water Hammer and Protecting Piping Systems

Water hammer caused by check valve slam is one of the most damaging phenomena in industrial piping systems. When flow reverses through a slowly closing check valve, the reverse flow develops momentum before the valve finally slams shut, creating a sudden pressure surge that can rupture pipes, damage supports, break valve bodies, and destroy pump casings. Non-slam check valves are designed to close rapidly enough to prevent significant reverse flow from developing, eliminating the pressure surge that causes water hammer damage. Selecting the correct non-slam check valve for each application is essential for protecting the integrity of the piping system.

Wofer Valve supplies a comprehensive range of non-slam check valves including dual plate check valves with spring-assisted closing, axial flow nozzle check valves, and non-slam silent check valves for pump discharge, compressor discharge, and pipeline applications where water hammer prevention is critical.

Understanding Check Valve Slam

Check valve slam occurs when the valve disc closes after reverse flow has already started. The mechanism is as follows: when the forward flow decelerates (due to a pump trip, pump shutdown, or valve closure upstream), the check valve disc begins to close under the influence of gravity and/or a reverse pressure differential. If the disc closes slowly, reverse flow passes through the valve before the disc reaches its seat. When the disc finally seats against the rapidly moving reverse flow, the kinetic energy of the fluid is converted into a pressure surge that travels through the pipeline at the speed of sound in the fluid. This pressure surge can reach many times the normal operating pressure, causing catastrophic damage. The severity of water hammer is proportional to the velocity of the reverse flow at the moment the valve closes and the speed of sound in the fluid.

Dual Plate Check Valves for Non-Slam Service

Dual plate check valves (covered in detail in our dedicated article) are the most widely used non-slam check valve for general industrial applications. The two spring-loaded plates close very quickly (typically in 0.1-0.3 seconds) when forward flow ceases, preventing significant reverse flow from developing. The spring preload is selected to ensure that the plates begin closing before forward flow reaches zero, which means the plates are already partially closed when the flow reverses, minimizing the volume of reverse flow before full closure. For the most demanding non-slam applications, torsion spring designs provide faster closing than compression springs because the spring force increases progressively as the plates rotate toward the closed position.

Axial Flow Nozzle Check Valves

Axial flow nozzle check valves (also called axial flow check valves or nozzle check valves) use a spring-loaded disc that moves axially along the flow path. In the open position, the disc is withdrawn into a streamlined nozzle-shaped body, providing very low pressure drop (typically 30-50% lower than equivalent dual plate check valves). When flow reverses, the spring and reverse flow pressure rapidly drive the disc onto the seat. Axial flow check valves provide the fastest closing of any check valve type, making them the preferred choice for applications where water hammer is a critical concern, such as pump discharge lines in long pipelines, compressor discharge lines, and water distribution systems. Their slim, in-line design is particularly advantageous where space is limited.

Sizing Non-Slam Check Valves

Proper sizing is critical for non-slam check valve performance. An undersized check valve creates excessive pressure drop and velocity, increasing the risk of water hammer if reverse flow occurs. An oversized check valve may not close reliably because the flow velocity through the oversized valve may be too low to keep the disc fully open, causing the disc to flutter and potentially chattering against the seat. The general sizing guideline for non-slam check valves is to select the size based on maintaining the minimum flow velocity required for stable disc opening (typically 5-8 feet per second for dual plate and axial flow types) while not exceeding the maximum velocity that could cause excessive pressure drop or disc instability (typically 25-35 feet per second). For pump discharge applications, the check valve should be sized for the pump's rated flow, not the pipe size.

Installation Best Practices

Correct installation is essential for non-slam check valve performance. A minimum of 5 pipe diameters of straight pipe upstream and 3 diameters downstream of the check valve should be provided to ensure uniform flow across the disc. For pump discharge installations, the check valve should be installed between the pump discharge nozzle and the discharge isolation valve, so that the check valve is protected from any reverse flow from the discharge piping. For dual plate check valves, the correct orientation must be observed: horizontal (discs vertical), vertical up (flow upward, discs horizontal with spring closing), or vertical down (flow downward). Some dual plate check valve designs are not suitable for vertical down flow and may require special springs for this orientation. The manufacturer's installation instructions must always be followed.