Check valve slam is a major contributor to pressure surges in pipeline systems and one which is poorly understood. The major damage caused by check valves is related to delayed closure of the check valve and not rapid closure as many would presume. Delayed closure will allow a reverse velocity to develop before the valve closes. When the valve closes on a moving column of fluid, it sets up a pressure surge event up stream and downstream of the valve.
The demand for rapid closure of a check valve increases if there is an air vessel downstream of a pump installation. In this instance, the reverse acceleration on the fluid can be very high. The check valve is required to operate very rapidly in this situation to prevent very large reverse velocities, and resulting pressure rises.
Proper selection of check valves for this type of installation is essential.
The picture is of a nozzle check valve. This type of check valve is generally offers the best performance, but cannot be used on sewage applications.