Non-Slam Check Valves

When a pump trips, the forward flow in the discharge main decelerates rapidly. In a conventional swing check valve, the disc remains open until the flow reverses, then slams shut under the pressure of the returning fluid. The resulting pressure spike can be severe enough to fracture pipe joints, damage valves and fittings and cause structural damage to pump station walls and floors. Non-slam check valves are specifically engineered to prevent disc slam and the pressure surges it produces.

The axial-flow nozzle check valve achieves non-slam closure through two design features: a spring that begins closing the disc as soon as forward flow decelerates, and a short disc travel that brings the valve to full closure before the flow reverses. Because the disc closes before the reverse velocity builds, there is no significant pressure transient on closure. This design is now widely specified by hydraulic engineers as the preferred non-return valve for pump stations and rising mains where surge analysis indicates slam risk.

The NSCV range from AFC Valves Europe covers DN50 to DN1200 in two size groups: the NSCV small-bore range (DN50 to DN350) and the NSCV large-bore range (DN400 to DN1200). Both are constructed with a ductile iron body and cover, stainless steel internals, EPDM seat rings and a grade 316 stainless steel spring. The full-bore flow path minimises head loss and the straight-through design allows pigging in transmission main applications.

The tilting-disc non-slam check valve uses a disc pivoting about a centreline axis to reduce closing travel. While not as definitively non-slam as the nozzle design, tilting-disc valves offer a practical compromise where head loss is a concern and water hammer risk is moderate. They are often used on pump stations handling wastewater and sludge, where the nozzle design's small-bore internals would be vulnerable to fouling.

Non-slam check valves should be sized for the full operating flow range, not just the design flow. An oversized check valve may partially close under normal operating conditions, leading to disc flutter and premature seat wear. Our technical team routinely assists with check valve sizing calculations and can provide recommendations based on pump station flow and pressure data.

All NSCV valves are supplied with manufacturer's test certificates, material certificates and installation and maintenance manuals. Third-party inspection and witnessed pressure testing can be arranged on request for critical infrastructure projects.