Non-Return Valves are often referred to as check valves since in fulfilling this function they prevent reverse flow.

In many installations, the system deceleration gradient is severe and hence valves must possess low reverse velocity capability.

This means simply that valves must close very quickly, before reverse velocity of the fluid column is established i.e. valves must close at the instant that forward flow ceases.

There are a number of means used to give valves this rapid closing capability, such as:

• Lever and weight
• Springs
• Dashpots and dampers

Whilst these can be effective, the simplest, most reliable and most flexible method is to fit a non slam valve. The onerous systems are those where branch velocities are high, multi pump delivery is used, vertical lift is involved and where surge vessels or similar are employed.