Check Valves

Chapter One – What is a Check Valve?

A check valve is an automatic safety device designed to allow fluids and gasses to flow in one direction and prevent reverse flow. Under pressure, fluids enter the valve at the inlet where they force the disc off the seat of the valve and exit through the outlet.

Flow continues steadily as long as the pressure is maintained to keep the disc open. Once there is a pressure drop, the disc automatically returns to its seat to shut off the flow.

The pressure that opens the valve is referred to as cracking pressure, which can vary from 3 psi to 350 psi depending on the size, purpose, and function of the check valve.

Check valves ensures gasses or liquids are flowing in the right direction, and create tight seals to prevent leaks. By controlling the flow of liquids, check valves protect pumps and compressors from serious damage.

The names of check valves change according to their use and how they are manufactured. Their many names include clack, non-return (NRV), reflux, retention, and one-way valves. Regardless of the simplicity of their design and function, they are an essential factor in the protection of pumps, piping systems, and fluid movement.

The most common type of check valve, the swing valve, can be seen in the image below. It is a full port design where the disc is completely out of the way during fluid flow. The directional flow opens the disc. As the flow continues, the disc remains open. As the flow slows or stops, the disk moves, by the force of gravity, into the closed position.

The main function of a check valve is to prevent reverse flow, which can damage pumps and other mechanisms. When a check valve closes, there is a potential water hammer where the fluid in a system slams against a closed check valve. This aspect of a check valves operation requires that they be resilient and durable and be made of high quality materials.

Check valves can be noisy and need to have their noise suppressed. To that end, various forms of controls are added. To control surges and prevent the valve from slamming closed, springs, levers, or weights are designed into the valve’s structure.

Chapter Two – Types of Check Valves

There are endless varieties of check valves to fit multiple applications. There are industrial and commercial uses. In gardens, they control the flow of fertilizers and water in irrigation systems. The aerospace and aircraft industries use check valves to control corrosive fluids, hydraulic systems, and the fuel flow.

Though the function of check valves is the same regardless of where they are used, the types of check valves vary according to the flow rate, media gravity and temperature, line size, pressure, and velocity of the flow, which can be seen in the chart below.

Application of Check Valves

Check Valve Types

The choice of a check valve has to fit the needs of an application. When engineers select a check valve, they adapt to fit the type of media, the size of the pipe, the pressure gradient, the velocity of the fluid, and the type of pumping mechanism. The examination of these factors are necessary to ensure the safe operation of an application or process and to protect equipment.

There is a wide range of check valves, with standard ones that can be purchased at a home improvement store to ones that are specially designed to fit the needs of unique applications. This wide variety makes it difficult to provide an all inclusive list of check valves since every day new ones are being developed. In the midst of the many check valves, descriptive factors are common to all types.

Swing Check Valves:

A swing check valve is a self actuated valve that serves as a backflow preventer or one direction valve. They have a disc that swings on a hinge off the seat of the valve to allow forward flow. When the flow stops, it swings back into position to stop reverse flow. The weight of the disc has to be sufficient to withstand the impact of the return flow. Swing check valves have less turbulence and low pressure drop.

Lever and weight swing check valves are used when there is a chance of water hammer while ones with lever and spring are used in high pressure and high flow velocity applications. Regardless of the type of swing check valve, their disc needs to be checked and cleaned regularly.

Wafer Check Valves:

Wafer check valves have a thin disc that swings to block or allow flow. The flow of the media is allowed to flow in one direction, which lifts the disc off its seat. When the flow is in the opposite direction, the disc closes to create a seal to prevent backflow. In many cases, a spring or lever is attached to the disc for faster closing time and helps to reduce water hammer.

The thin, short, and compact design of wafer check valves makes them ideal for small pipe systems and systems that move solid or semi solid media.

Tilting Disc Check Valve:

A tilting disc check valve has its pivot point at the center of the disc so that the fluid flows over the top and bottom of the disc. They are often used where there is frequent flow reversal. When the disc is open, the flow keeps it open, which is different from a swing check valve where the velocity of the flow keeps it open. The disc is small and light with a center of gravity that is close to its pivot point. It has a low pressure drop with low flow rates and a higher pressure drop with high flow rates.

Ball:

Ball check valves use a ball that moves up and down in the valve to block the flow. The seat of the valve is designed to fit the ball and has a conically shaped chamber that guides the ball to the seat. The weight of the ball can vary depending on the pump capacity and the possibility of water hammer.

Reverse flow moves the ball back to the seat to seal off the flow. When there is enough pressure in the flow, the ball is lifted away from the flow. When the pressure decreases, it comes down and is guided to the seat.

Butterfly:

A butterfly check valve is also known as a dual, folding disc, double disc, or splits check valve. The halves of the disc open toward the centerline, as seen in the diagram, during the forward flow of the fluid. With reverse flow, the halves open and seal the pipe. The short distance that the halves have to travel lessens the slamming effect on the discs. Butterfly check valves operate very quietly making them ideal for heating, ventilation, and air conditioning systems.

Lift Check Valves:

The flow for lift check valves enters below the seat of the valve. The pressure of the flow lifts the disc or ball off the seat. When the flow weakens, slows, reverses, or stops, gravity forces the disc or ball downward onto the seat. They are ideal for systems with high flow and velocity rates.

Silent Check Valves:

Silent check valves, also known as poppet check valves, avoid water hammer or shock by closing before fluid flow reversal. The benefit of a silent check valve is that they close quickly and smoothly, which prevents damage to surrounding materials.

Duckbill Check Valves:

Duckbill check valves are made of rubber or synthetic elastomer and have the shape of a duck‘s beak. The open end of the valve is stretched over the outlet to the supply line. The other end retains its normal shape. Upstream pressure forces the duckbill lips open allowing the flow. As the pressure increases, the lips open wider. As the pressure gets lower, the lips return to a flatten and close.

Diaphragm Check Valves:

Diaphragm check valves have a rubber diaphragm or disk=c. The valve is centered in the seating area with the sealing surface on the inlet side of the valve and has an opening in the center. Backflow causes the rubber disk= to cover the sealing opening, which closes the opening to the inlet.

Foot Check Valves:

A foot check valve is at the bottom of a pipe connected to a pump. They act like a ball check valve with a screen to block debris. The opening of the valve is larger than the opening of the line. Foot valves keep the pump primed. When the pump is operating, suction pulls water up through the pipe and foot valve. When the pump is turned off, the water in the pipe is pulled back by the force of gravity. The foot check valve blocks the path of the water as it falls and is closed by the weight of the water.

Pneumatic Check Valves:

Pneumatic check valves, or air check valves, control air flow from a compressor by letting air in and preventing it from going out. They are placed in pneumatic circuits that need air flow in one direction.

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Chapter Three – How are Check Valves Used

Since the invention of the self-sealing valve, at the beginning of the 20th Century, they have become the answer to backflow problems in a wide range of industries. Any industry that transports products through pipes has a check valve somewhere in their system.

Check valves are cost saving devices that prevent waste and protect equipment. Fluids that travel through a piping network need to be prevented from being lost due to poor handling. Check valves automatically stop liquid flow when pressure drops or the flow decreases by blocking backflow and containing the fluid.

Check Valve Uses

Pumps:

Check valves in pumps is one of their more common uses.

• Single pump system – The check valve controls a flow from a suction tank and blocks it from reversing when the pump stops.

• Parallel pump system – In a parallel pump system, a check valve prevents the duty pump from pumping into the standby pump.

  

  
  

• Series pump system – A series of pumps is used when pumping materials in a pipe line over long distances. If any of the pumps fail in the series, the check valve activates to prevent losses.

• Pressure relief – Spring loaded check valves are used in gas applications for low pressure conditions. If the pressure in the system rises above the cracking pressure, the check valve safely releases the gas.

  

  
  

• Foot valve – The foot valve blocks debris and prevents water from returning to the well when the pump is turned off.

Pharmaceutical:

The pharmaceutical industry requires check valves for the movement of fluids. Check valves regulate the pressure in the fluid flow for product quality, efficiency, and production. Only sanitary check valves can be used for this process. They require easy cleaning, without the need for disassembly, and silent closing with a tight shut off.

Aircraft Hydraulic Systems:

Orifice check valves are used in the hydraulics of the landing gear actuator system. When the gear is raised, fluid flows to lift the gear. When the gear is lowered, the check valve stops the gear from dropping by controlling the flow out of the actuator. Aside from the landing gear, check valves on aircraft are used in other hydraulic systems as well as fuel and pneumatic systems.

Irrigation Systems:

In an irrigation system, check valves are located near the source of the system to prevent backflow, siphonage, of irrigation water back to the source.

Fuel Pump:

Check valves are found in vehicles that were made before the invention of the electronic injection system. Old car fuel pumps have a check valve located at the pump‘s inlet and one at the outlet, They are designed to force the fuel to flow in the correct direction. When the check valve goes bad, the fuel pressure goes down.

Water Supply Check Valves:

Water in a home should only move from the supply line and out through a fixture or from a drain to the sewer. Check valves are located in homes to prevent cross connection or backflow. They come in several varieties including ball and swing check valves. Domestic check valves can be seen below on a diagram of a hot water system for a home.

Heating Boiler:

Check valves on heating boilers prevent hot water from circulating when the thermostat is not calling for heat. They stop water from going back into the boiler, which would build up excessive pressure. In a home heating boiler, a check valve keeps boiler water from entering the domestic supply to keep it from being used for drinking, bathing, or cooking.

Liquid Fuel

Liquid fuel check valves are designed to operate with several different types of fuels including jet fuels, hydraulic oil, synthetic oil, and air. They are designed to provide stable operation from the crack of the valve to full flow. An inverted spool is used that has a built in pressure sensor to ensure balance and stability and holds a specific delta pressure in the valve. The total design allows the valve to open uniformly.

Chapter Four – How Does a Check Valve Work?

When discussing how a check valve works, it is important to first understand how they are constructed. A basic check valve has four individual parts: body, seat, disc, and cover, which can be seen in the diagram below.

Springs, balls, stems, hinge pins, and other features can be added to the basic elements according to the purpose and design of the check valve.

How Various Types of Check Valves Work

Swing:

Swing check valves have an unguided disc that moves into a fully open position when the pressure from the flow is applied. They come in several sizes and designs to fit different applications.

Lift:

Lift check valves have a guided piston that lifts when the flow enters the check valve. The seat has a barrel design and is screwed on and sealed. The opening to the check valve is the same as the same size as the inlet and outlet.

Tilting:

The design of tilting check valves is to overcome some of the problems of swing check valves, which have been the most common type. They are capable of remaining open when the flow has a low velocity and close quickly when the flow stops. They perform well in applications with high velocity.

Folding Disk:

The folding disc check valve is a split disc design with a wafer body pattern that has a soft seat. The discs are secured by a pivot rod and open with the pressure of the flow. Springs on the discs help close the valve.

Vertical Check Valves:

Vertical check valves are also known as spring check valves since a spring is used to keep the disc against the seat. The valve is held shut by the spring until a sufficient amount of pressure is applied to push the spring back and open the valve. Pressure keeps the valve open and the spring compressed. Once pressure drops, the spring forces the disc into the seat to shut off the flow. Vertical or spring check valves provide excellent backflow protection and are extremely reliable.

Materials Used to Make Check Valves:

Most check valves are made from durable materials to be able to withstand high pressure conditions. Common materials used to make check valves include PVC, CPVC, bronze, brass, iron, and stainless steel.

Polyvinyl Chloride (PVC): PVC is corrosion resistant and flexible. The smooth surface of PVC allows the check valve parts to move easily.

Chlorinated Polyvinyl Chloride (CPVC): CPVC has the same qualities as PVC but is able to endure high temperature applications.

Bronze: Bronze can be used for low and medium pressure applications, be cast in complex configurations, and is corrosion resistant.

Brass: Brass has the same abilities as bronze as well as the same machinability and is less expensive than bronze.

Cast Iron:Cast iron check valves are used for hot and cold water, HVAC, steam, gas, and utility services due to its excellent corrosion resistance.

Ductile Iron:Ductile iron has more than 3% carbon so it can be bent and shaped easily. It is stronger than cast iron and is easier to form into check valves.

Iron: Iron is used for steam, water, oil, and gas applications. It can endure a wide range of temperatures and pressures. Its excellent performance balances its high cost.

Stainless Steel: Stainless steel is corrosion resistant, durable, and can be used in harsh conditions, including chemical applications.

Polypropylene (PP):PP is used to make check valves due to its exceptional resistance to corrosion, which is superior to CPVC and PVC.

Polyvinylidene Difluoride (PVDF):PVDF plastic is used in applications where exceptional purity and resistance to acids, solvents, and hydrocarbons are a necessity.

Cast Steel:Cast steel is used to produce check valves due to its sudden impact resistance without deforming, breaking, or bending. It can also be easily shaped to any type of check valve.

Chapter Five – Advantages and Disadvantages of Check Valves

The major benefit of check valves is their ability to perform without having to be monitored or controlled. Their basic design allows them to be inserted into a pipe‘s flow and be able to control the flow without being managed.

Regardless of their many benefits, check valves are like any other type of mechanism and have drawbacks.

Advantages of Check Valves

1. Prevent backflow
2. Able to endure high and low pressure conditions
3. Perform as a backup and safety system
4. Able to be used horizontally and vertically
5. Self-actuated
6. Fast acting.
7. Protect pumps and compressors from damage caused by backflow
8. Reduce downtime and loss of production
9. Prevent water hammer
10. Reduce sudden valve failure
11. Lower maintenance costs
12. Few moving parts
13. Smaller footprint
14. Able to cope with variable flow conditions
15. Do not require power to operate

Disadvantages of Check Valves

1. Do not operate in pulsating systems
2. Closing element can slam closed causing damage and wear
3. Operate in a completely enclosed system
4. Impossible to check if they are open or closed
5. Internal parts cannot be checked
6. Disks can get stuck in the open position.
7. Noise from slamming discs
8. Water hammer
9. Reverse flow problems

Common Problems with Check Valves

Water Hammer:

Water hammer is caused by a pressure surge when there is a sudden stop in the flow of a gas or fluid and the valve suddenly closes, which causes noise and vibrations. Water hammer can damage the system and lead to costly repairs.

Water hammer can be prevented by having faster closing check valves, which stops pressure surges and shock waves. Silent check valves are one possible solution.

Reverse Flow:

Reverse flow is costly and can damage a pump by causing it to spin backwards. This problem can be corrected with tight fitting fast closing check valves. One of the benefits of spring assisted check valves is their ability to react quickly and prevent reverse flow.

Oversizing:

Some check valve systems have chattering caused by repeated opening and closing of the valve. This is caused by oversizing of the check valve. When installing a check valve, it has to be sized to fit the application. The disc has to be stable in the open position and make a complete seal when closed, which can prevent repeated opening and closing, fluttering, and failure of the check valve.

Installation

Incorrect installation and assembly of a check valve can cause future problems. The first step is to choose the correct check valve for the application. Flow capacity, positioning, and orientation are crucial factors since installing a check valve too close to a pump can cause turbidity and possible damage to the check valve.

Check valves should be mounted several straight pipe diameters upstream from circulators, elbows, tees, and strainers to prevent turbulence and rattling of the disc against the seat.

Conclusion

• Check valves are a method for controlling the flow of gases or liquids.
• Check valves are cost saving devices that prevent waste and protect equipment.
• Check valves vary according to the flow rate, media gravity and temperature, line size, pressure, and velocity of the flow, which can be seen in the chart below.
• A basic check valve has four individual parts: body, seat, disc, and cover.
• There are endless varieties of check valves to fit multiple applications.

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