Check Valves

Introduction

Check valves, also known as non-return valves or one-way valves, are devices used to allow flow of fluids along a single direction. By construction, these valves have two ports, meaning that they have one port for medium entry and one for medium exit. They are used to prevent backflow in pipes.

These valves have a very simple design and generally operate without the need for automation or human intervention. They instead rely on the inlet fluid pressure to open or close the valve. Unlike other valves, they do not need a handle or lever to operate.

The main purpose of a check valve is to prevent damage to equipment by means of stopping reverse flow. They are generally small and simple in construction, although larger sizes are available. At Tameson we offer, besides other sizes, the most common sizes: from 1/8 inch to 2 inches.

Typical inline spring-loaded check valve

Typical inline spring-loaded check valve

Cracking Pressure

Since check valves allow unidirectional flow, they need a minimum upstream pressure to open, so flow can occur. This minimum upstream pressure at which valve opening occurs is called “Cracking Pressure”. For a given size and type of check valve, its design is centered around a specific cracking pressure. For some of the valves, we have on stock the cracking pressure ranges from 0,03 bar to 0,1 bar.

Spring loaded Inline- and Y-check valves

Different types of check valves are available to handle a variety of media and use various forms of construction. The most common check valve however is the in-line check valve.

Working principle

Inline spring-loaded check valves, also known as nozzle check valves or silent check valves, are the most commonly used check valves with a conventional and fairly simple design. They have a spring assisted disc in line with the flow and have a short travel distance, leading to quicker closing action. Due to their compact design, they occupy less space compared to other check valves. The valve opens when the inlet fluid pressure overcomes the spring tension which is normally the ‘cracking pressure’ of the valve. The spring pressure causes the disc to close when the inlet fluid velocity reduces and thereby prevents reverse flow. It prevents pressure surges in the line and thus prevents water hammer or fluid hammer.

These designs can be used with many orientations, including down flow if a proper spring is installed. Depending on the housing and seal materials, these valves can be used to handle a variety of media including steam and corrosive chemicals. They have to be removed from the line to perform internal inspections or repair.

Y-check valves operate basically in the same way as in-line check valves. The spring is positioned at an angle, which makes it easier to service and clean, because the valve does not have to be removed from the line.

Typical y-check valve

Typical y-check valve

Applications of spring loaded in-line- an Y-check valves

Typically, they are used to:

  • Protect equipment from backflow damage
  • Prevent contamination due to reverse flow
  • Prevent siphoning
  • Sealing and holding vacuum

Spring-loaded check valves are mainly used in Pneumatic, Hydraulic and Gas applications.

There are many applications in almost any industry and even at home where spring-loaded check valves are used. Some application examples are:

Liquids: Washing machines, dishwashers, engines, water treatment plants, boilers, furnaces etc.

Gasses: Mostly they are used for air such as air compressor check valve or to protect equipment from backflow in pneumatic systems, but also in passenger cars, public transport, gas systems etc.

For vacuum systems, spring-loaded check valves with a very low cracking pressure (~0,03 bar) are used. Accordingly, the vacuum system can reach its maximum vacuum level before the check valve seals of the system.

Foot Valves

Typical foot valve with inlet strainer

Typical foot valve with inlet strainer

A Foot valves is a check valve type installed at the bottom of a pump suction line of a water well, fuel tank or any other application where the suction line is situated below the pump. There are two major purposes for using food valves:

  1. Keep liquid pumps primed. A food valve prevents liquid from flowing back out of the pumps suction line when the pump is stopped. Due to the clearances and displacement ratios of liquid pumps and the compressibility of air, most pumps are unable to draw air out of the inlet line. Therefore, in order to start the pump, the inlet line needs to be filled with liquid. With an inlet line full of liquid, the pump is able to develop suction pressure. This process is called priming the pump and usually needs to be done manually. A foot valve however prevents the liquid from flowing back down into the suction line and keeps the pump primed and ready to be started and work at all times. This is one of the major purposes for foot valves.
  2. Prevent liquid from siphoning back. When pumping liquid from a lower tank to, for example, a higher positioned tank there is a great risk that the liquid siphons back and empties the higher positioned tank after the pump has been turned off. A food valve in de bottom of the suction line will prevent this.

Foot valves are usually combined with an inlet strainer which prevents large debris ingress into the foot valve, which could clog the valve in its open position or damage other downstream components.

Other Check valve types

For specialty purposes and bigger flows there are a lot of different and usually less common check valves:

Swing Check valves or Tilting-disc Check valve

Swing check valves consist of a disc which is the movable part and opens to allow flow of fluid in one direction and closes to prevent reverse flow. The disc normally swings on a hinge or trunnion, which in turn is suspended from the valve body by means of a hinge pin. As inlet fluid pressure increases, the disc slowly swings open and allows flow through the valve. When the inlet fluid pressure drops, it causes the disc to close and block flow.

Lift Check valves

A Lift Check valve consists of a guided disc or “lift” which is raised up off its seat by inlet fluid pressure which thereby allows flow towards the outlet side. The guide keeps the motion of the lift along a vertical line such that the valve can be re-seated with correct alignment. When the inlet pressure is no longer higher, the lift is lowered back onto its seat, shutting down the valve and stopping reverse flow. These valves are also available with horizontal or angular configurations

Ball Check valves

A Ball Check valve uses a free-floating or spring-loaded ball as the closing element. This ball rests on a seat which is normally conically tapered in order to guide the ball into the seat and create a positive seal thereby stopping reverse flow. When the pressure of the fluid in the inlet side exceeds the cracking pressure, the ball is dislodged from its seat and allows flow to occur. For designs without springs, reverse flow moves the ball back towards the seat and closes the valve.

Stop Check valves

A Stop Check valve is a type of check valve with override control to stop flow irrespective of the flow direction or inlet pressure. By closing in response to backpressure or low inlet pressure, it performs the function of a normal check valve. In addition, it can also be deliberately shut off by an external mechanism such as an actuator, lever or hand wheel. Thus, these types of valves act as two valves in one.

Stop check valves are commonly used in power plants in applications such as boiler circulation, steam generators and boiler feed water, turbine cooling, starter water and safety systems.

Diaphragm Check valves

They consist of a rubber diaphragm that flexes open and closes when the inlet pressure is increased or reduced inside a pipeline.

Butterfly Check valves

They are similar in construction and flow characteristics as normal butterfly valves. They consist of a butterfly shaped discs which allows flow in one direction and closes to stop reverse flow.

Duckbill valves

Duckbill valves allow flow to proceed through a soft tube of which the end has a natural flattened shape. Its shape reminds of a duck beak.The flow opens the flattened end of the duckbill, permitting fluid to pass. When pressure is removed, the duckbill end returns to its flattened state, thereby cutting off the flow.

Wafer Check valves

Wafer check valves feature a wafer shaped design which is two half circle discs hinged together. These discs fold together to allow flow in one direction and retract to form a full circle to prevent reverse flow. It is thinner and lighter compared to conventional check valves for large volumes and occupies less space due to the compact wafer design.

Valve Materials

Stainless steel check valve Typical inline check valves from stainless steel and brass

Typical inline check valves from stainless steel and brass

Check valves are normally made using the following materials:

Stainless steels

Stainless steel valves have superior corrosion resistance, heat resistance, low-temperature resistance and excellent mechanical properties. For some applications, that do not require such excellent parameters, for example corrosion protection, there are other, more cost-effective alternatives like brass.

Brass / Bronze

Brass or bronze have excellent machinability and properties for air, water, oil, fuels, etc. It is however not resistant to seawater, purified or chlorinated water. They are usually used for relatively small valve sizes in low pressure situations.

PVC

PVC is the abbreviation for PolyVinyl Chloride. PVC check valves are frequently used in irrigation and water management systems. PVC is corrosion resistant to most corrosive media, such as seawater, acids, bases, chloride solutions and organic solvents. It is not resistant to aromatic and chlorinated hydrocarbons. The maximum temperature of the medium is about 60°C.

Polypropylene

Polypropylene or PP is often used for water, aggressive media and liquid food products. PP is corrosion resistant to most corrosive media, such as most inorganic acids, bases, and aqueous solutions which rapidly corrode metals, although it is not resistant to some concentrated acids and oxidizing agents, especially at elevated temperatures. The maximum temperature of the medium is about 80°C.

Valve selection

The basic criteria for valve selection are:

  1. Material compatibility with the medium.
  2. Valve rating (ANSI).
  3. Line size.
  4. Application data-flow, design and operating conditions.
  5. Installation-horizontal, vertical flow up or down.
  6. End connection.
  7. Envelope dimensions.
  8. Leakage requirements.

Additional Information

Click one of the links below for more information: