A Guide to Non Return, Check and Double Check Valves
Non-return check valves have two openings, one to let flow in and the other to let it out. They are used to ensure that liquid or gas flows in only one direction, preventing it from returning back up the valve and piping system. Non-return check valves go by a number of other names:
- Clack valves
- One way valves
- Retention valves
- Foot valves
- Reflux valves
- Stop valves
How do non return valves work?
Most non return valves come with two openings separated by a closing mechanism, which is held open by the pressure of the flow. If the gas or liquid tries to flow back upstream, the valve's closing mechanism is activated to prevent the fluid from flowing in the wrong direction. Unlike other valves, non return valves are controlled automatically by the flow, not manually by a person.
Non-return check valves come with male and female connections in various thread sizes. Some of the most popular thread sizes include.
- 12.7mm (½ inch)
- 15mm (0.59 inches)
- 19.05mm (¾ inch)
- 38.1mm (1.5 inches)
- 50.8mm (2 inches)
Types of non-return check valves
Non return valves come in different shapes and sizes for use in both domestic and industrial environments, as well as a variety of applications. Non-return check valve designs feature a variety of components including balls, tilting or swinging discs or diaphragms. With this in mind, here are some of the better-known non-return check valves.
Ball check valves
Ball valves come with a ball bearing that lowers and lifts depending on the changes in pressure inside the valve.
Swing check valves
Swing check valves come with a disc or a flapper that swings open in response to the pressure of moving fluid or gas and moves back into place to prevent reverse flow. Swing check valves are sometimes also called flapper valves or tilting disc valves.
Non-slam check valves
Similar to swing check valves, non-slam check valves feature a disc that is activated by a pressure-sensitive spring rather than backflow.
Double-check valves come with two valves that offer an extra level of protection in situations where the prevention of upstream flow is crucial. As such, they are often used in drinking water piping systems where contamination could be disastrous.
Lift check valves
Lift check valves come with a disc that lifts vertically under fluid pressure. Lift check valves are usually used in water, rather than gas, systems.
Reverse check valves
Usually used in gas lifts, reverse check valves stop backflow and are conducive to the circulation of pressure.
Low-pressure non return valves
Low-pressure non return valves are extra sensitive to fluid flow, making them ideal for low-pressure systems where standard non-return check valves might not be sufficient.
Spring non return valves
Spring non return valves come with a spring-loaded bearing, which allows fluid to flow downstream but is pushed back to block the inlet if the fluid attempts to flow upstream.
What are non-return check valves made of?
Non-return check valves are made from a range of different materials with the most popular being stainless steel, brass and PVC.
- Stainless steel check valves are sturdy enough to handle high and low temperatures. On the downside, they are more expensive than their brass and PVC counterparts.
- Brass non-return check valves are not as resistant to corrosion and hot temperatures as stainless steel check valves. They are not the best option for purified or chlorinated water. Brass check valves are commonly used in low-pressure piping systems.
- Polyvinyl chloride or PVC valves are resistant to corrosives such as solvents and seawater. However, they are not resistant to temperatures above 60 degrees Celsius.
Other valve types
Non-return check valves are not the only valves on the market. Here are some other options to control the flow of fluid.
Ball valves feature a ball with a hole that is rotated to either open or close the flow. The valve is open when the ball is parallel to the flow path and closed when it is perpendicular to the flow line. Unlike some other valves such as butterfly valves, ball valves are not usually used to regulate the flow stream. Ball valves are normally controlled manually with a lever.
Butterfly valves feature a rotating disc attached to a shaft that controls flow. Unlike ball valves or gate valves, butterfly valves are commonly used not just to open (disc is completely parallel to the flow) or close (disc blocks the flow completely) flow but also regulate it (disc blocks the flow partially).
Sometimes also called sluice valves, gate valves feature a barrier, or a gate, that is either fully lifted (fully open position) or lowered (fully closed position). Gate valves cannot remain in a partially open position to regulate flow. Gate valves are opened or closed manually by turning a handwheel 360 degrees.
Pressure relief & safety valves
While both pressure relief and safety valves are utilised to minimise the impact of some sort of system failure, they serve slightly different purposes. Pressure valves protect the system from over-pressurisation. Meanwhile, safety valves act as an emergency response in cases of complete system failures to protect both equipment and people.
Non-return check valves are used to prevent backflow of fluid or gas in a huge range of industries. They are commonly used in to keep water moving downstream in centrifugal pumps. In addition, check valves are often utilised to prevent the spread of contaminants in drinking water. Check valves are also used in heating and air conditioning to stop the coolant or steam from moving in the wrong direction. In industrial settings, non-return check valves are often used in pharmaceutical and power plant processes. Non return valves can also be found in fuel injection systems of aircrafts and spacecrafts.
Also referred to as double check valves, valves in a series are composed of two valves. Often used to prevent contamination in water pipelines, check valves in a series ensure that there is backup if one valve happens to fail. The valves also reduce the overall pressure to create a tighter seal.
While check valves and non return valves are sometimes used to refer to the same devices, they actually reference slightly different valves. Basically, check valves are a superior version of non return valves that are often used in environments where it is absolutely vital that fluid does not move in the wrong direction such as in drinking water systems. In the UK, for example, check valves are put through more testing than non return valves.
Often used in heating systems, steam boilers work by circulating steam through pipes and into radiators. Steam boilers are also sometimes used in the manufacturing and electricity industries. Non return valves are used to stop steam from moving back into a boiler, which could be potentially dangerous.
There are five main types of valves, each one with a different function. Non return valves prevent flow from backing up in a piping system. Also called way valves, directional valves are used to manage the direction of an air stream. Shut-off valves are used to open or close flow within a system. Pressure control valves are used to manage actuator force and pressure levels. Finally, flow control valves control the pressure of flow within piping systems.