AUT Check Valve, Product Insight
Check valves for water and wastewater application
Mar
General Information of check valves
Check valves, also known as non‐return, reflux or one‐way valves are automatic valve types that do not require any manual or electrical intervention to open and close. They are used to allow flow in one direction only and prevent the reversal of flow. They are also used to protect the system and it’s components from water hammer and pressure surges.
How are check valves used?
The check or non‐return valve is most commonly used in pumping stations and allows the flow in only one (and always the same) direction thus preventing back flow when the fluid in the line reverses direction. The main importance of this function is twofold:
- To prevent damage to upstream equipment that can be affected by reverse flows such as pumps and measuring equipment.
- To prevent reverse flow after system shutdown (water hammer).
Check valves are therefore safety critical valves that protect the system and its equipment from damage which can range from accelerated wear and tear to full, catastrophic system failure depending on the extent of the system design.
Selection a check valve
When selecting a check valve it is important to make a cost-benefit sizing 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. Read more Check valve for more details.
Various 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, recoil check valve, ball check valves, tilted disc check valves, slanted seat check valves, nozzle check valves and silent check valves.
Introduction to 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.
In many cases, the rate of fluid reversal is not a cause for concern and standard check valves will perform well. However, in pumped systems where fast flow reversal can occur, the selection of the correct check valve is crucial.
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. It is also important to note that these failures may not be due to one single, large surge pressure but by repeated surges which eventually cause fatigue failure of the system.
It is important to note that other factors are required to ensure a safe and trouble free system. The correct number, types and sizes of air valves, closing and opening times of isolation valves, flow control valves etc all require to be considered to protect the system from pressure surges.
To prevent the occurrence of check valve slam, the valve should close either very quickly to prevent the onset of reverse flow or very slowly once reverse flow has developed. For a check valve to close slowly, this requires additional ancillary equipment such as hydraulic dampers which act to cushion the valve door as it comes into its seated position. However, this slower closure does allow the fluid to pass through the check valve until it closes and consideration must be given to the upstream pump to ensure that it is suitable for reverse spin and flow.
