Introduction to check valves
What is a check valve?
Check valves are generally installed in pipelines to prevent backflow. A check valve is basically 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.
Where can you use check valves
Check valves are used in many different applications. For example they are 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. Also, check valves are very often used in HVAC-systems (Heating, Ventilating and Air Conditioning-systems). HVAC-systems are e.g. used in large buildings, where a coolant is pumped many storeys up. The check valves are here installed to make sure that the coolant is not flushing back down.
Considerations before a check valve is installed
When choosing a check valve it is important to make a cost-benefit analysis of the specific system. Often focus is to reduce cost and at the same time obtain the lowest possible pressure loss, but when it comes to check valves a higher safety equals a higher pressure loss. So 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. Please go to How to choose the right check valve for more details.
Different types of check valves
There are various types of check valves available for water and wastewater applications. They work in different ways but serve the same purpose. AVK offers a wide range of swing check valves, ball check valves, tilted disc check valves, slanted seat check valves, nozzle check valves and silent check valves. The most common types of check valves for water and wastewater are swing check valves and ball check valves:
- Swing check valves: 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: A ball check valve functions by means of a ball that moves up and down inside the valve. The seat is machined to fit the ball, and the chamber is conically shaped to guide the ball into the seat to seal and stop a reverse flow.
Water hammer and pressure surges
What is water hammer?
Water hammer is the result of rapid change in the velocity of a fluid in a pipeline system and is typically caused when moving fluid suddenly stops, such as by hitting an obstacle. For example, a valve which is closed too quickly, will send a shock wave along the pipe with the potential to damage both the pipe and equipment. It will continue to travel along the pipeline until the energy has dissipated, and it can cause a localized increase in pressure and produce loud noises, sometimes resembling a hammering noise, which is the noise of the expansion of the pipe under these conditions.
Why pressure surges occur in pipelines?
If a pump stops or a valve closes too quickly, the flow will continue which will create vacuum downstream to a pump or valve and a very high pressure at the other end of the pipeline. When two different pressures are present in a pipeline, the flow will go in direction of the lowest pressure, which will create a pressure surge. The flow will continue to travel back and forth until the energy has dissipated, and the pressures are equalized.
When a pressure surge occurs, although it may only last for a very short time, it can amplify the normal system pressures by up to 10 times or more. This can cause considerable damage to the system including pipe cracks, bursts, cavitation, and implosion due to vacuum pressures being formed. Apart from the resulting costs and downtime these failures cause, the health and safety risks are also considerable. These failures may not be due to one single, large pressure surge but due to repeated surges which eventually cause fatigue failure of the system.
