Check Valves

Check valves are mechanical valves that permit gases or liquids to flow in only one direction, preventing flow from reversing down the line. They are classified as one-way directional valves. Fluid flow in the desired direction opens the valve, while undesirable reverse flow forces the check valve to close. Most check valves contain a flat disc that is slightly larger in diameter than that of the port. Flow indicators on the side of the body indicate the direction of the flow for correct installation.

Spring check valves have a disc held in place by a spring, which means they and can operate in either a vertical or horizontal mounting position because they do not rely on flow pressure or direction to function. Spring check valves are available with many different types of seats, the most common being nylon, which tends to wear quickly, requiring replacement more often than more durable brass seats.

Swing check valves have a hinged disc which swings into position, and can operate in either horizontal or vertical (upward flow) positions. Movement through the line automatically swings the disc open to allow full flow, so swing check valves must be installed with normal flow opening the disc. Should reverse flow occur, the reversed pressure and the weight of the disc closes the disc back against the seat and back-flow is stopped immediately. This type of check valve offers minimal flow restriction, allowing full flow to be achieved through the valve.

A swing check valve is a type of check valve with a variety of uses in plumbing and water pumping systems; in fact, any application where water needs to be allowed into a system, but not back out again.

That’s because a swing check valve (also known as a ‘tilting disc’ check valve) consists of a disc on a hinge that swings open with the pressure of entering water and swings closed when there is backflow.

When water is travelling in the required direction, the flow pressure pushes the disc open and when it travels in the opposite direction, the force of the backflow pushes the disc against its seat and closes the valve.

That’s why it’s important to install these valves correctly and if you find there is no water being allowed to enter, it means you have mounted the valve the wrong way around (look for the arrow on the housing that indicates the flow direction).

Because swing check valves require no external power to operate, other than the pressure of the flowing water, they are ideal for a wide range of applications from light PVC plumbing and irrigation systems to heavy industrial applications.

A variation of the swing check valve is the spring check valve, which as the name implies, uses a spring to close the disc against backflow. Because of the spring, it also requires more pressure to open the disc, so this type of valve is more suited to heavy duty applications where the water pressure is stronger.

Lift check valves work automatically with horizontal line pressure as the bonnet is positioned vertically above the valve, lifting up with forward flow. Like a globe valve, with its indirect line of flow, lift check valves are flow-restricting, and so are generally used as a companion to globe valves. Line pressure lifts the disc, and flow is allowed to occur. When flow reverses, the disc falls back onto its seat and immediately cuts off the flow, preventing back-flow from occurring.

If accurately sized and correctly installed, a check valve can deliver many years of reliable operation. The components it is made from will also help to determine its longevity, with metal more likely to outlast a less resilient material.

Basically, a check valve is comprised of a valve (ball, disc, diaphragm etc) installed in a pipeline, which opens when water flows one way through it and closes when water tries to flow back the other way (hence ‘checking’ or stopping the flow), but despite its simplicity of design, every check valve will eventually fail and some warning signs to look out for can include:

• Vibrating noises indicating that the valve has broken
• Water hammer (shock wave) causing the disc to slam into the valve
• Chattering after repeated opening and closing.

Premature valve failure can be caused by a number of things including:

Incorrect installation – if the valve is oversize, this can lead to chattering and eventual failure. If it is not suited to the application and is slow to close each time, this will cause some backflow leakage, which can damage your pump as well as the valve.

Lack of maintenance – if no filter is installed in the pipeline or the system is not flushed regularly, dirt, debris and contaminants can catch and stick in the valve, preventing full closure and causing wear and tear.

Adverse operating conditions – high temperatures can cause check valves to fail early, so make sure the one you install is suited to the water temperature it is operating in and is checked regularly and replaced if required.

Because check valves will eventually fail, it’s important to conduct regular inspections of pipelines where they are employed and when signs of damage or wear are detected, to replace them with new ones before they fail altogether.

A check valve is a one-way valve, which opens to allow water to flow in one direction and closes if the flow is reversed.

Check valves are used in every industry from household appliances such as dishwashers and washing machines to industrial equipment such as boilers, pumps and furnaces.
They are used in many different applications and perform a variety of tasks including:

• Protecting pumps from damaging backflow
• Keeping centrifugal pumps primedPreventing waste from re-entering waste water systems such as sewer lines
• Preventing coolant from flushing back down when pumped up to high-rise Heating, Ventilating and Air Conditioning (HVAC) systems
• Preventing backflow in reverse osmosis filters.

Particular types of check valves are also used in particular situations including:

In-line spring loaded check valves - quick closing to prevent pressure surges and water hammer

Diaphragm check valves – ideal for low-pressure or vacuum applications due to their Normally Open (NO) nature.

Stop check valves – with a manual override, they can function as two valves in one, making them cost-effective in power plants, steam generators, turbine cooling and safety systems.

Foot valves (check valves with strainers) – installed on the end of suction lines in wells or fuel tanks, they keep pumps primed, prevent liquid from siphoning back and keep debris out of the lines.

Check valves made from different materials also have different applications including:

Brass check valves - ideal for small, low pressure applications using air, water, oil or fuels.

Stainless steel check valves – ideal for applications requiring high durability, thanks to their superior corrosion and heat resistance and excellent mechanical properties.

PVC check valves – ideal for use in irrigation and water management systems due to their lower cost and corrosion resistance.

Polypropylene check valves - used for water, aggressive media and liquid food products thanks to their resistance to most corrosive media.

A one way check valve is simply another name for a check valve that is used in situations involving water, such as drinking water and wastewater applications.

For drinking water, they are installed to ensure that no dirt, debris or foreign bodies enter the system and contaminate the clean drinking water. For wastewater, one way check valves ensure that waste flows out of the system, but cannot re-enter it to cause further contamination.

For water pumping applications, a one way check valve known as a foot valve is used to keep the pump primed and keep debris from entering the suction line. And in the case of sump pumps, a sump pump check valve will ensure that discharged water doesn’t flow back with the aid of gravity when the pump is switched off.

So how does a one way check valve work? Basically, it is a ball, disc, diaphragm or other type of check valve that, when installed in a pipeline, opens when water flows through it one way and closes when water flows back through it the other way (thereby ‘checking’ or stopping the flow).

Types of one way check valves include:

Swing check valves - a hinged disc swings off the valve seat to allow forward flow of water and back onto the seat to block reverse flow.

Ball check valves - a ball moves up and down inside the valve’s conically-shaped chamber allowing forward flow and sealing to stop reverse flow.

Globe check valves - a movable disk and stationary ring seat in a globe-shaped body.

Butterfly check valves – a hinged disc with two doors that can hinge open to allow forward flow or arrest backflow.

Piston check valves - a piston valve is opened or closed by the pressure (or lack of it) passing through the pipe.

Stop check valves – an override control allows the valve to be manually controlled to allow or prevent flow as needed.