Take a look at check valves
What is a check valve?
Check valves act as one-way or non-return valves in almost every industrial application. Check valves are simple devices that perform an important function by preventing reverse flow, thus preventing damage and ensuring efficient operation. For example, reverse flow can lead to water hammer. This phenomenon can be seen in the connection of piping, valves that repeat extreme pressure fluctuations and pumps that can fatally damage or rupture the system and its piping. Even if the failure does not occur immediately the repeated impacts of water hammer can promote fatigue and may also eventually lead to a loss of system integrity.
How check valves work and their applications
Check valves have a single inlet and outlet and are operated by a differential pressure. Above a certain upstream pressure, the valve will open automatically without any other intervention. This minimum operating pressure is called the opening pressure and is one of the key characteristics specified for all check valves. Chemicals, pulp and paper, food processing, water and wastewater treatment, industrial, marine and mining, pumps, pipelines, power generation and HVAC are some of the many fluid flow and pumping applications that have check valves.
Materials of check valve
Check valves are often placed in series, for example in water supply systems, to prevent contaminated water from flowing back into the clean supply line. With a wide range of potential applications, there are also many types of check valves that use many different materials, such as metals, polymers and rubber. Common designs include swing or flap check valves in which a metal disc pivots on a hinge or trunnion to prevent reverse flow. Larger check valves are usually swing or flap type. As the name implies, ball and spring-loaded ball check valves feature a ball mounted on a properly contoured seat. Duckbill check valves rely on a flexible rubber diaphragm that forms a valve that is normally closed unless positive pressure is applied.
However, unlike metal swing or flap check valves, rubber duckbill check valves do not rust, snag or jam, often increasing reliability and service life. Likewise, rubber check valves are not subject to mechanical wear, which can negatively affect check valves made of other materials.
When specifying a check valve, many factors must be considered
For example, it is important to ensure that any check valve is manufactured from a suitable material that is compatible with the liquid or gas that will be used, such as a rubber duckbill check valve where appropriate. Valve ratings, line size, type of installation (e.g., horizontal or vertical), size and connection type, maximum leakage rate, pressure drop, and any special requirements should be included as part of an in-depth evaluation.
Ensure that the correct type of valve is used for each application
However, while for most types of valves, a fully open gate does not significantly restrict flow, in check valves the degree of opening depends on the differential pressure. Nonetheless, check valves are often specified like other types of valves and are based on the maximum possible valve flow coefficient (Cv), rather than actual operating conditions. As a result, the valve may open only partially when the flow rate is lower than required. Partially open valves may experience increased resistance and pressure drop as well as valve chattering. Excessive chattering can lead to wear, which inevitably increases the likelihood of component failure. In the case of check valves, this may allow reverse flow and all the associated problems that can occur under these conditions.
For example, in check valve applications, a higher Cv value may be detrimental because it increases the likelihood of partial valve opening. Considering that pressure drop is typically calculated with the valve fully open, a high Cv valve in a low-flow application may increase the likelihood of valve component wear and higher than expected pressure drop.
Therefore, proper sizing of check valves is a critical consideration that is often misunderstood. In applications where flow is not sufficient to prevent the valve from stopping, lower Cv valves are required to prevent characteristics such as chattering. Therefore, check valves should be sized based on the specific application, not the line size, to ensure that the valve opens or closes fully under typical operating conditions. This characteristic of check valves is often overlooked, and it is common to find that such problems are ultimately caused by improperly specified check valves rather than the actual valves themselves.
Ensure that check valves are well maintained
Because check valve failure can lead to potentially significant negative results, it is critical for plant operators to ensure that check valves are well-maintained and operating correctly. For example, check valves are used in all storm and wastewater systems to prevent backflow. Failed check valves that are corroded or clogged due to contact with contaminated water can lead to standing water or flooding and associated health effects. Check valve failure modes associated with wear or poor maintenance may include problems associated with noise and vibration caused by water hammer, backflow, leakage, or damage. Stuck valves may occur when materials such as scale or debris become trapped between the valve body and moving parts, such as the valve flap or ball. This can lead to leaks where the valve seat or other components are damaged or where material is trapped. In addition to contamination, other factors that can negatively affect check valves include high temperatures, worn elastomer and seat seals, improper installation, or improper maintenance and assembly. They may also stick or leak as the valve ages and begins to break down.
As mechanical check valves age, they often give warning signs of poor condition. For example, they may begin to vibrate, make noise or chatter, and parts may fail and be lost from the mechanism. Reverse flow may also occur due to the failure of the check valve. When the valve is in the closed position, simply listening for fluid flow indicates a leak and gives a strong warning signal. It should be noted that rubber duckbill check valves are "passive" devices and are less susceptible to such deterioration. Relatively simple measures, such as minimizing debris in the line through a filter and properly lubricating the check valve assembly, can help eliminate premature failure. However, it is also worth noting that check valves do require periodic replacement. Products based on quality and experienced engineering inevitably have a significant impact on performance, while cheaper products are often falsely economical. While typical component life varies by application, manufacturers recommend that metal and plastic check valves should be replaced every 5-7 years, while check valves made of rubber can remain fully serviceable for up to 35-50 years.
Check valves are often a low-cost component and therefore are often overlooked. Given the high cost of check valve failure, this can be a high risk approach. The potential for disaster is real, and backflow can cause production to stop or even cause extensive damage to the facility. Therefore, when a check valve begins to show any signs of failure, it is imperative that plant operators immediately renew the assembly with quality replacement parts.
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