Non-return valves: each to its own application

Non-return valves: what kind of hydraulic device are they? The question is extremely easy to answer, because the definition of how these mechanical components work is given by the adjective that comes before the word valve. Non-return valves are valves that allow liquid to flow in one direction only. The physical principle that regulates the operation mechanism is just as simple: when the downstream fluid pressure is higher than the upstream pressure, the flow control disk – in other words the mobile element of the valve – moves to close the valve and prevent the fluid from flowing in the opposite direction of the one that was set. This is what non-return valves, also known as check valves or one-way valves, do. This general definition, however, includes different models that are specific for different applications. Let’s take a look at some of the most common situations where non-return valves can be found, and the most suitable type for the application.

Non-return valve for sewers 

Non-return valves are often installed in sewage systems because they prevent waste water from flowing back into houses. They are a fundamental solution when the rooms are below street level, or in apartment buildings where intense drainage is necessary, but they are also useful when sewers become overloaded and in the event of cloudbursts, which can be problematic for the system.

The valve is installed between the drain and the outdoor drainage system, often in a sump pit where it is possible to intervene for maintenance. For this application non-return ball valves are preferrable, because they do not need any special maintenance and are self-cleaning. This type of valve is often used with dense, dirty liquids and with loaded liquids, as is the case of waste water.

Non-return valves for preventing hydraulic shock 

A non-return valve can be a good solution for tackling hydraulic shock. The dangerous pressure surges that are generated when a valve closes unexpectedly can damage not only the system electromechanical components, like the pumps, but also the piping. This is why installing non-return valves not only to protect the pump but also along the whole piping system is a good idea. The non-return valves most suited for this application are valves with spring and disc; if there is not much room, disc valves are a good idea, and can be installed between the flanges. (Read also How to prevent water hammer consequences and avoid damages)

Foot valves against system emptying 

Non-return valves also act as foot valves when they are installed in suction lines. They prevent the pipes from emptying when the pumping system is idle. When the system stops, in fact, gravity would cause the fluid to flow back naturally, but the foot valve fitted at suction closes because of the pressure of the fluid on the spring, stopping this occurrence from happening. As a result, when the pumping system is started again, the risk of dry running, which can cause serious damage by overheating some pump parts, can be avoided. Non-return valves with spring and disc can be used for this type of application; to prevent impurities from being sucked in, using a foot valve with filter is highly recommended.