Antislam Air Valves

Many bursts can be directly attributed to air valve operation. As air is released from an air valve the upstream and downstream fluid columns can develop a differential velocity. Once the air valve closes, these two moving columns impact and there is a pressure rise. This propagates upstream and downstream from the checkvalve. While this pressure rise may be very large and cause and air valve to "explode", it is more common that the reflecting waves cause a cavitation event marginally upstream and downstream of the air valve and bursts are often found in these areas as a result of cavitation damage.

In response to this, many valve manufacturers produce an antislam device for their air valves. These operate by determining when air expulsion rates are high, and then throttling this expulsion. Their intention is to "cushion the blow"  by retaining a volume of air in the valve or the pipeline. 

The philosophy of operation works if the discharge rate triggers the antislam device while there is sufficient cushioning volume in the pipeline. However, this is not always the case. Their effectiveness can only be determined by reliable modelling. In many circumstanced it is possible to experience pressure surges large enough to damage the pipe without the antislam device being able to operate.

If these devices are to be used it is essential that their operation is properly understood, and modelled. It is important that the discharge rates at which they act are known and modelled by the designers and the full spectrum of surge events they are likely to see and/or generate are considered fully. They are by no means a device that can be used to solve all pressure surge problems.