Q: How does a check valve work?
A: Cracking pressure
A check valve requires a minimum upstream pressure (pressure differential between inlet and outlet) to open the valve and pass the media through it. This minimum upstream pressure at which the valve opens is called the check valve's ‘cracking pressure.’ The specific cracking pressure changes based on the valve design and size, so ensure that the system’s pressure can generate the cracking pressure of the chosen check valve(s).
Closing
If the upstream pressure falls below the cracking pressure or there is back pressure (flow attempting to move from the outlet to the inlet), the check valve will close. Typically, check valves have a gate, ball, diaphragm, or disc that is pressed against a seal to close the check valve. Gravity or spring can assist the closing process. As the inlet pressure decreases below the cracking pressure or there is backpressure, the valve closes by gravity, spring, and/or by using the backpressure.
Installation orientation
As a one-way valve only allows flow in one direction, it is crucial to know the correctinstallation orientation. Typically, an arrow on the valve’s housing indicates the flow direction. If there isn’t an arrow, examine the valve to ensure it is installed in the intended flow direction. If the valve is installed backward, media will not be able to move through the system, and the resulting pressure build-up can cause damage.
Normally open and normally closed check valves
A normally open check valve allows the medium to flow freely but shuts off the flow in case of backflow. A normally closed check valve prevents the flow of media through it until the cracking pressure builds up, at which point the valve opens.
Q: What materials is the Check Valve made of?
A: Brass check valve
Brass check valves are excellent for air, water, oil, or fuel applications. However, they are not resistant to seawater, purified water, or chlorinated water. They are less resistant to heat and corrosion compared to stainless steel and are typically used for applications with low pressure.
Stainless steel check valve
Stainless steel check valves have superior resistance against corrosion, heat, and low temperature and have excellent mechanical properties. For applications that don’t require high durability or resistance, stainless steel is typically not a cost-effective solution compared to PVC or brass check valves. Stainless steel wafer check valves are typically high-quality check valves used for high-temperature and high-pressure applications.
PVC (Polyvinyl Chloride) check valve
Plastic check valves like PVC or CPVC are frequently used in irrigation and water management systems. They are corrosion resistant to most corrosive media like seawater, acids, bases, chloride solutions, and organic solvents. However, they are not immune to aromatic and chlorinated hydrocarbons and typically have a max temperature resistance of around 60°C.
Polypropylene (PP) check valve
Polypropylene check valves are used for water, aggressive media, and liquid food products. They are resistant to most corrosive media like inorganic acids, bases, and aqueous solutions that rapidly corrode metals. However, they are not resistant to concentrated acids and oxidizing agents and typically have a max temperature resistance of around 80°C.
Cast iron check valves
Cast iron check valves are typically used as high-temperature check valves. Cast iron is extremely strong and is immune to vibrations. The material has excellent resistance to wear and tear and temperature tolerance. But cast iron is not ductile in nature. Hence, any bending can cause the cast iron material to crack and become useless. Cast iron can work at higher temperatures when compared to PVC but gets corroded over time. These valves find applications in sugar industries, paper industries, and oil lubrication systems.
Q: Where is Check Valve used?
A: Due to how check valves function, they are typically used for one of four different reasons in various applications:
To protect equipment downstream from backflow damage
To prevent contamination due to reverse flow
To prevent siphoning
To keep a vacuum seal
Due to their function, they are used in almost every industry. They are used on common household appliances, like dishwashers, washing machines, and wastewater lines. For industrial purposes, they are used on boilers, furnaces, gas systems, pumping applications, or vacuum systems. They are also frequently used as aquarium check valves on water and CO2 lines. Also, a miniature check valve is a popular choice where the space is limited, yet a reliable operation is essential. Two of the most common check valve applications are for water and air, which are discussed in more depth below.
Check valves for water
A water check valve is used in numerous water applications, like drinking water and wastewater. These valves are simply called one-way water valves. For drinking water applications, they ensure that no media from the environment (outlet side of the valve) can enter the system with the safe, clean drinking water and contaminate it. For wastewater applications, they ensure that the waste water cannot re-enter the system and cause an overflow or additional contamination. For water pumping applications, a foot valve is often used to ensure no debris will enter the line and to keep internal pressure for priming purposes. Duckbill valves can also be used for discharges on water lines. Sump pump check valves ensure that the discharged water does not come back into the sump pump with gravity when the pump is turned off.
Pneumatic check valve
A pneumatic check valve, or air check valve, allows airflow and prevents it from going out. They are often simply called one-way air valves. The most common application is for an air compressor. A pneumatic check valve allows the compressor to keep certain parts pressurized and other parts de-pressurized. They can be located on an air receiver, discharge pipe, or as a piston check valve on the piston compressor's inlet and outlet sides.
Q: What materials are used to make Check Valve?
A: 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.
Q: What are the common problems when using Check Valve?
A: 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.
Q: What is the ideal check valve?
A: Regardless of type or style of valve, the longest trouble-free service will come from valves sized for the application, not necessarily the line size. Ideally, the disc is stable against the internal stop in the open position when flowing or fully closed when no flow or checking. When these conditions are met, no chattering of the disc will occur, thereby preventing premature valve failure. Unfortunately, most check valves are selected in the same way on/off control valves are selected, by line size and the desire for the largest Cv available. This ignores the fact that unlike on/off control valves that have actuation (manual, pneumatic, hydraulic or electronic), only the flow conditions determine the internal performance of the check valve.
Check valve internals are flow sensitive, unlike on/off control valves. If there is not enough flow and pressure to fully open the check valve, trim chatter occurs inside the valve. This results in premature wear, potential for failure and a higher pressure drop than calculated.
Whenever a metal part rubs against another metal part, wear is a result. That leads to eventual failure of the component itself. A component failure can result in the valve not performing its function, which in the case of a check valve is to prevent reverse flow. In extreme cases failure could result in the component(s) escaping into the line, causing failure or nonperformance of other valves or equipment in the line.
Typically, pressure drop is calculated based on the check valve being 100% open as with on/off control valves. However, if the flow is not sufficient to achieve full open and the check valve is only partially open, the pressure drop will be higher than what’s calculated. This is due to the effective Cv of the valve being less than maximum when the check valve is partially open. In this situation, a large rated Cv actually becomes detrimental to the check valve (unlike with on/off control valves). This results in chattering of the disc and eventual failure. Such is not the case with some other valves. For example, with a gate valve that is fully open, the wedge is out of the flow path. Therefore, the flow through the valve does not affect the performance of the wedge whether that flow is low, medium or high.
Various types of check valves are available. Some of the more popular types are included below. All these can be used for clean media. As with other types of valves, specialty check valves can be found for unique applications. While no one type of valve is good for all applications, each has its advantages.
Taking time to contact the manufacturer to assist in selection can help you find the best fit. This is especially true if you are having problems with whatever type of check valve is presently installed.
Q: What are the common types of Check Valve?
A: Various types of check valves are categorized according to different characteristics such as design, mode of operation, valve material, and size, with the most popular characteristic for classifying check valves being design/build. The following are the commonly used types of check valves.
Swing check valves
A swing check valve is built with a disc hinged, at one edge, to the valve body. When fluid flows into the valve with sufficient pressure, it forces open the disc, allowing flow. The disc shuts off when there is reverse flow or when the inflow pressure is no longer sufficient. These valves are suitable as water check valves and are widely used in water distribution systems.
Tilting disc check valves
These valves are built with a double-eccentric disc and are usually designed to be installed between two flanges. They are suitable for large diameter flow and require relatively high pressure to open. Tilting disc check valves are not suitable as wastewater check valves as debris can get stuck in the portions of the valve that are in the flow path.
Ball check valves
Ball check valves feature a ball as the closing mechanism. The inflow pressure lifts the ball off its seat and into a dedicated chamber. When the inflow pressure drops, the ball returns to its seat, shutting off the flow and preventing reverse flow. A ball check valve will perform excellently as a sewage check valve because the closing mechanism doesn't interfere with the flow in the open position.
Foot valves
Foot valves are commonly used in water pumping lines to prevent water from draining in the event of a pump stop. They are installed in the pump's suction line and usually feature a screen that filters out debris from the reservoir. This feature makes the foot valve a suitable check valve for pump applications. Since this valve type works with gravity, the installation must be vertical in the pipeline.
Axial Silent Check valves
These valves close with the help of a spring. Axial silent check valves are fast closing valves and prevent pumps from rotating in the reverse direction when the backflow occurs, as this can damage the pump.
Q: What To Consider When Selecting a Check Valve?
A: Several factors should be considered when selecting a check valve for a specific application. Here are some of the key considerations:
Flow Rate and Pressure Drop Considerations
One of the primary things to consider when selecting a check valve is the flow rate and pressure drop of the fluid. The type of check valve selected should be able to handle the anticipated flow rate and pressure drop while minimizing head loss.
Material Selection Considerations
The material of the check valve is also an important factor, as it affects the compatibility of the valve with the fluid being transported. In addition, different materials have different chemical resistance and durability properties, and the selected material should be able to withstand the specific environment and conditions of the application.
Temperature and Pressure Limitations
Check valves also have specific temperature and pressure limitations that must be considered when selecting a valve. The valve should be able to withstand the maximum temperature and pressure of the fluid without failure or damage.
Installation, Maintenance, and Cost Considerations
Finally, when choosing a check valve, it’s important to account for the initial purchase cost, assembly expenses, maintenance fees, and ongoing running costs. Cost is typically a significant factor, following quality and functionality. While low purchase prices may seem attractive, they can result in high maintenance and running expenses. Also, a low-cost check valve that fails to perform its intended function can cause serious issues at a pumping station.
Check valves must be installed properly to ensure optimal performance and prevent failure. Certain types of check valves may require expensive modifications to the pipeline, which can increase assembly costs. Maintenance and repair expenses also tend to rise as the valve’s complexity increases. In some cases, complex solutions may necessitate the assistance of a professional.
Q: What is the role of Check Valve in various systems?
A: Hydraulic Systems
In hydraulic systems, a check valve performs critical functions, including:
Pressure Control: Direct-acting and pilot-operated check valves serve as pressure relief valves in hydraulic systems. Direct-acting valves use a spring that is normally closed until the system attains the valve’s cracking pressure. The higher the system pressure, the more the valve opens until it reaches the full relief valve pressure, where it is fully open. As for pilot-operated check valves, they are two-staged valves typically operating at higher pressures than their direct-acting counterparts. In the first stage, the system’s pressure must overcome a smaller spring-loaded piston to allow fluid to flow through the valve. Then, this creates a pressure drop that opens a larger piston to allow full flow through the valve at the second stage.
Pump Protection: Often, hydraulic systems utilize multiple pumps. These pumps may be on simultaneously when operating at full capacity. However, this does not occur for too long, with a few pumps operating most of the time. In these situations, check valves prevent operating pumps from moving fluid into non-operational pumps, which could damage them. Also, hydraulic systems typically use accumulators to keep the system pressurized when pumps are not operating. Check valves prevent the return of fluid to the pump, thereby avoiding backspin.
Keeping Prime: Pumps require the presence of fluid inside at all times to avoid damage. Check valves are used at both entrances and exits of pumps to ensure this.
Water Pump
A check valve performs several functions in water pumps and water distribution systems in general. To ensure that there is always water in the pump for safe starting, check valves are always located at the pump entrance and exit lines. The presence of check valves at a pump isolates it from backflow and the harmful effects of water hammer. In water treatment facilities, it is common to find check valves in a series, so that wastewater cannot reenter the system. Also, check valves prevent elements from the environment from contaminating clean water.
Boiler
Ball check valves are one of the most important components when controlling water flow from the feed pump to the boiler. As boiler pressure increases beyond the pump pressure, these valves prevent backflow from the boiler to the pump. Also, they prevent backflow when the feed pump is not in operation.
Q: What are common uses of check valves?
A: Check valves are used in a wide range of products, from industrial to domestic applications. If a liquid, gas or steam is being moved through a pipe, you’ll likely find a check valve in operation, ensuring the material is moving in the desired direction. These devices are used to protect equipment from backflow, which can cause damage, system shutdowns and costly repairs.
One of the most common uses of check valves is in pump systems where they keep fluids, gases or steam flowing in one direction. In the oil and gas industry, large check valves prevent the flow from reversing. They are often installed on the discharge end of the pump, where they automatically shut off the flow after the pump is disengaged, preventing the system from draining.
In industrial processes where gas and another material, such as an oxidizer, are mixed together in one stream, check valves are used to ensure the gas cylinders remain separate, adding a level of safety.
Check valves help maintain sanitary conditions when used in domestic applications. In sewer pipes, they allow wastewater to exit a structure while preventing it from re-entering in the event of a backup in the sewer system. Dishwashers and clothes washers use check valves to prevent contaminated water from re-entering the domestic water supply.
Water pump systems use check valves to maintain pressure when the pump is off, as well as prevent backflow. For example, on a home water heater, a check valve will prevent hot water from returning to the cold inlet waterline when there is a drop in pressure on the cold side. HVAC systems in large buildings also have check valves to control the flow of coolant through multiple floors.
Whether used in domestic or industrial applications, check valves play an important role in safeguarding equipment and maintaining performance.
Q: When To Replace Check Valves?
A: With the potentially significant negative outcomes that can result in response to a failed check valve, ensuring check valves are well maintained and operate correctly is critical for plant operators. For example, check valves are used in all stormwater and wastewater systems where they prevent backflow. Failed check valves which are corroded or jammed can result in standing water or flooding and associated health impacts because of contact with contaminated water. Check valve failure modes associated with wear or poor maintenance can include problems related to noise and vibration from water hammer, reverse flow, leakage or damage. Sticking valves can occur when material like scale or debris is trapped between the valve body and the moving parts such as the disc or ball. Where the valve seat or other elements become damaged or material is lodged this can result in leaking. Along with contamination, other factors which may negatively affect check valves include high temperatures, worn elastomers and seat seals, incorrect installation or poor maintenance and assembly. Valves may also stick or leak as they age and begin to break down.
As mechanical check valves deteriorate, they typically give warning signs of their poor condition. For example, they may start to vibrate, emit noises or chatter and it is also possible that components may fail and be lost from the mechanism. As check valves fail reverse flows may also occur. Simply listening for fluid flow when the valve is in the closed position is indicative of leakage and a strong warning sign to act. Relatively simply measures such as minimizing debris in the line through filters and correctly lubricating check valve components can help to eliminate premature failure. However, it is also worth noting that check valves do need to be replaced on a regular basis. Products that are based on high quality and experienced engineering inevitably has a significant impact on performance with cheaper products often a false economy.
Check valves are typically a low-cost component and as such are frequently overlooked. This can be a high-risk approach given that the cost of check valve failure is significant. The potential for catastrophe is real where reverse flow can halt production or even extensively damage a facility. Consequently, where check valves begin to show any sign of trouble plant operators must promptly renew the component with a high-quality replacement.
Q: What is Meant by Check Valve Sizing?
A: Sizing a center guided valve is not difficult. Along with the pipe size, pressure class and type of valve required (flanged, wafer, etc.), users need the actual working pressure, flow rate, media type, temperature and the specific gravity of the media. It may be as simple as building the valve with a lighter spring to allow the valve to fully open. In order for the valve to reach the full open position, it may need a lift limiter to reduce how far the disc travels. When the valve is 100 percent open, it will be stable in the flow and will result in reduced premature wear and failure by eliminating the effects of chatter. It is very important to remember that these valves are designed to the actual flow values and not to the line size. A properly sized valve will be in either the full open or closed positions.
A: Benefits of using a properly sized and positioned silent check valve:
It can protect a system from costly failures and downtime of a production facility.
The valve lifespan can be greatly increased by reducing the risk of parts breaking off and damaging equipment downstream.
It protects the pumps that are upstream by not allowing backflow, which can cause the pump to spin in the reverse direction and cause severe damage.
It leads to better pump and compressor protection.
It results in less piping vibration.
There is a reduction in water hammer issues.
It will work in the vertical down flow direction.
Q: What are the benefits of Check Valve?
A: protect pumps and compressor equipment from damage caused by backflow / reverse flow.
reduce down time and loss of production due to failure of unsuitable valves.
low pressure drop increases energy savings.
check valves are very effective in preventing water hammer.
eliminate chatter and reduce the possibility of sudden valve failure.
lower maintenance costs; fewer moving parts.
smaller footprint than conventional check valves.
flexibility to cope with variable flow conditions.
interchangeability with most conventional swing check valves without expensive piping modification.
