Vacuum Breaker Valves

It was suggested that installing vacuum breaker/air release valves at the high points in the system was one way of controlling pressure surges arising due to cavity separation.

The scenario is as follows. The clack open deluge valve receives a signal to open, and opens because it is actuated by instrument air or some other means. The fire pump receives a signal to start at the same time. However, it takes time for the fire pump to prime the caisson and during that time it is unable to maintain the pressure in the ringmain. This would result in vacuum pressures developing in the higher parts of the system which could lead to cavity formation.

Vacuum breaker/air release valves are often used to counter this cavity formation. Basically before the fire pump has primed they allow air to come into the system fast so that the pressure at the higher levels of the ringmain remain near atmospheric pressure. When the pump has primed the system the air release valve, which is a much smaller valve, allows the ingressed air to go out relatively slowly. It is important that the air release valve is small, otherwise water can move up the riser pipe very fast and this can create a surge when it reaches the air release valve and closes it.