A check valve is a one-way valve, in which the flow can run freely one way,but if the flow turns the valve will close to protect the piping,other valves,pumps etc.If the flow turns and no check valve is installed,water hammer can occur.Water hammer often occur with an extreme force and will easily damage a pipeline or components.Therefore check valves are generally installed in pipelines to prevent backflow.
Check valves are used in many different applications.
Example as - A check valve is often placed on the outlet side of a pump,to protect the pump from backflow.Centrifugal pumps, the most common type of water pumps, are not self-priming, and therefore check valves are essential for keeping water in the pipes.Check valves are frequently used in HVAC-systems (Heating, Ventilating and Air Conditioning-systems).HVAC-systems are used in large buildings, where a coolant is pumped many storeys up.The check valves here,are installed to make sure that the coolant is not flushing back down.While choosing a check valve it is of prime importance to make a cost-benefit analysis of the specific system.Mostly the focus is to reduce cost and at the same time to obtain the lowest possible pressure loss,but when it comes to check valves a higher safety equals a higher- pressure loss.Therefore in order to make sure the check valve protects the system properly,each system has to be assessed individually,and factors such as the risk of water hammer,acceptable pressure loss,and the financial consequence of installing a check valve with a too high safety margin against water hammer have to be considered.
What is a water hammer?
In a pumped system,the water is forced from a lower level to a higher level by means of a pump.The fluid flows in one direction only when the pump is in operation.When the pump stops, the flow of fluid will reduce until it also stops.Because the overall pipeline will be rising, when the fluid stops,it will then return back down the pipe.To prevent this flow reversal entering into the pump,well or intake,a check valve is installed.
If a pump stops and the forward flow reverses back down the line towards the pump before the check valve has fully closed,the flow will force the valve door to slam onto its seat.This scenario can almost instantaneously stop the reverse flow and it is this instantaneous stoppage which results in pipeline water hammer. This can produce loud hammer noises which is not the noise of the valve coming into its seated position but is the stretching of the pipe under these conditions.The consequent pressure wave (surge) can cause considerable damage to the system including pipe cracks,bursts,cavitation and implosion due to vacuum pressures being formed.
Types of Check valves :–
Swing Check Valves
AVK swing check valves feature very easy access to maintenance and can be installed in both horizontal and vertical positions. A swing check valve is mounted with a disc that swings on a hinge or shaft. The disc swings off the seat to allow forward flow and when the flow is stopped, the disc swings back onto the seat to block reverse flow. The weight of the disc and the return flow has an impact on the shut-off characteristics of the valve.
Ball Check Valves
AVK ball check valves are self-cleaning, as the ball rotates during operation which eliminates the risk of impurities getting stuck on the ball. The standard ball is designed with a NBR rubber lined metal core, and the rubber hardness is optimized to prevent the ball from getting stuck in the seat. Balls of polyurethane are suitable for abrasive media and when different balls weights are needed to prevent noise and water hammer. A full and smooth bore ensures full flow with low pressure loss and eliminates the risk of deposits at the bottom that could prevent tight closure.