Selecting Non-Slam Check Valves for Pump Rising Mains

Standard swing check valves fail on pump rising mains because they close too late. As the pump decelerates following a trip, flow in the rising main slows and then reverses. A swing check disc moves with the reverse flow, accelerating before slamming onto the seat at speed. The resulting pressure transient can be many times the steady-state working pressure and can fracture pipe joints, split fittings and damage the check valve itself. The correct response is a valve that closes before significant reverse flow has developed.

How slam occurs

When pump flow reduces, the water column in the rising main continues to decelerate due to gravity and friction. In the period between pump trip and disc closure, the column reverses: water flows backward through the check valve. The disc accelerates in this reverse flow, building momentum, before hitting the seat at velocity. The impact generates a pressure wave that travels up the rising main.

The severity of slam depends on the reverse flow velocity at the moment of closure. This is determined by the pipeline profile, the static head, the time constant of the pump-motor combination and the proximity of the check valve to the pump. On long mains and steep grades, reverse velocities at closure can be high enough to produce transient pressures well above the pipe rating. Standard spring-free swing checks have no mechanism to close before reverse flow develops.

Spring-loaded axial design: DN50 to DN350

Spring-loaded non-slam check valves use a coaxial spring to hold the disc in near-closed position during pump deceleration, so the disc reaches the seat before significant reverse flow has developed. When pump flow begins, the disc opens against the spring; when flow drops, the spring closes the disc progressively rather than waiting for reverse flow to push it. Closure is smooth and the disc velocity at seating is low.

For DN50 to DN350, the spring-loaded axial design is compact enough to fit within a standard flanged spool. No external pipework or pilot connections are required. The spring rate can be specified for the required cracking pressure, which must be matched to the expected minimum pump flow in continuous service to prevent unwanted partial closure under low-flow conditions.

Headloss is higher than a swing check at full flow because the coaxial disc creates more obstruction than a swung-aside disc. For most rising main applications this penalty is acceptable. On very long mains where pumping energy cost dominates whole-life cost, a larger body size for lower velocity may be justified.

Nozzle check valves: DN400 to DN1200

For DN400 and above, the spring-loaded axial disc becomes impractical: the spring force required to close a large disc at safe velocity is too high to maintain low headloss. Nozzle check valves use a different geometry. A nozzle-shaped body contracts flow through a venturi section, and a conical disc on a central guide rod closes against the nozzle throat.

The venturi geometry converts kinetic energy to pressure recovery downstream of the disc, producing lower overall headloss than any other check valve design. Combined with a light closing spring, the disc closes gently as flow decelerates without waiting for reverse flow. The result is non-slam performance with a headloss coefficient lower than a well-designed swing check, which is valuable on large mains where pumping energy costs over a 30-year asset life are significant.

Installation must allow for the extended face-to-face dimension of the nozzle body, which is longer than a standard flanged spool of the same diameter. The valve must be installed on a horizontal or slightly upward-inclined axis to allow drainage on pump shutdown.

Installation requirements

Non-slam check valves should be installed as close as practical to the pump discharge: within five to ten pipe diameters is ideal. The further the valve from the pump, the more time reverse flow has to develop before the disc closes.

A minimum of five pipe diameters of straight pipe upstream is required to avoid swirl reaching the disc. Elbows, tees and partially open isolation valves within this distance will create non-uniform velocity profiles that cause disc flutter during low-flow operation, eventually fatiguing the disc guide or spring.

The valve must be capable of fully closing against the maximum back pressure the system can impose, including the static head of the rising main at full column height. Specify the maximum differential pressure at closure to the manufacturer, as spring selection must accommodate this load.