Water Hammer is a pressure surge that occurs in a piping system due to a sudden change in the flow rate of fluid. This change can produce a shock wave that travels back and forth inside the pipeline resulting in physical damage to the system (pumps, piping, valves, or fittings). In addition, on-going maintenance demands will increase due to costly damages to the pump equipment and pipe work. The phenomenon can be easily detected by the noise it makes. It usually exists when the pump starts and stops, or during the sudden closure of a valve.
Non-Return Valves (Check Valves) can be the main cause of a pipeline water hammer. Normally, such valves are opened by the fluid flow and are closed either by the reverse fluid flow or by using a spring. Unless correctly chosen, check valves can suddenly slow the liquid as it reaches the closed position. Hence, if the valve’s door is slow to operate, a significant reverse flow rate can increase, causing the door to slam shut onto the seat, thus producing a water hammer. Fortunately, AVK has recognized this dilemma and produced the suitable check valve that closes quickly and prevents the door from slamming.
Unlike other products that attempt to prevent slamming, the AVK Recoil Check Valve (Series 641) does not accelerate closure by using springs, levers, dashpots, or other extraneous equipment. Such equipment need maintenance and contribute to the possibility of failure. Everything necessary for non-slam operations have been included in the innovative design of the recoil valve itself, from the shape of the body to the design of the door.
AVK has a dedicated hydraulics laboratory, where an elaborated piping system has been built to create an actual flow condition in the check valve, to study what happens in the pipeline when water hammer occurs. Analysis of the experimental data, generated in this manner, displayed the requirements that must be met to achieve non-slam closure. The recoil valve has been proven to reduce water hammer pressures, caused by check valves slamming, by up to 97%. This holds perfectly with branch velocities up to 3 m/sec and in conditions of less severity this might increase to 5 m/sec, compared to the 2-2.5m/sec limitation on standard swing check valves.
The valve is available today as the 641/11 and 641/21 in sizes from DN100 to DN1600 to serve the Water and Wastewater applications. The design was developed more than 70 years ago. There are many examples of valves that have been in operation for around 70 years that are still providing reliable service today. In fact, some have exceeded their minimum design life of 25 years by almost two times over. Compare that with a few, usually cheaper, present-day products purporting to do the same job but are failing well within the expected 25 years.