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The check valve is perhaps the most easily misunderstood valve in history. If you mention check valves to most factory workers, the typical response is “they don’t work.” In fact, it is likely that these personnel have removed the internal components or refurbished the system in order to avoid using check valves. In other words, these valves are probably the least popular ones in use today. This article will explore the basic knowledge of check valves, how they work, what types they are available, how to select and install them, how to resolve issues with them, and why they are not always the cause of problems. In simple terms, a check valve allows flow in one direction and automatically prevents backflow when the fluid in the pipeline moves in the opposite direction. They are among the few automatic valves that do not require assistance to open and close. Although some valves can be equipped with external weights and damping devices under special circumstances, most valves lack on/off control or other external aids for the valve. Unlike other valves, it can continue to function even if the factory facility loses air, power, or hydraulic pressure, or even if humans attempt to manually operate the valve. Like other types of valves, check valves are available in various sizes, materials, and end connections. Pipe sizes range from 1/8 inch or smaller to 50 inches or larger. They are made of bronze, cast iron, plastic, carbon steel, various grades of stainless steel, and alloys such as Hastelloy, nickel-chromium iron alloy, Monel alloy, and titanium. End connections include threaded connections, socket welding, butt welding, flange connections, grooved connections, clamp connections, and plug-in connections. Check valves can be found everywhere, including at home. If you have a sewage pump in the basement, there may be a check valve on the pump’s discharge pipeline. They are found in industries other than the household sector, such as desalination, water and waste treatment, chemicals, food and beverages, geothermal energy, mining, oil and gas, power generation, pulp and paper, and petroleum refining. The misunderstood check valve: Like other valves, check valves are used with various media: liquids, air, other gases, steam, condensate; in some cases, they can also be used with liquids containing particles or slurries. Applications include pump and compressor discharge pipes, header pipelines, vacuum circuit breakers, non-standard pressure relief pipes, steam pipelines, condensate pipelines, chemical feed pumps, cooling towers, loading racks, nitrogen purge pipes, boilers, HVAC systems, utilities, pressure pumps, sump pumps, flushing stations, and injection pipes. How do they work? Check valves are sensitive to flow rate; they rely on pipeline pressure and flow to open and close. The internal valve disc allows the fluid to flow forward, thereby opening the valve. As the forward flow decreases or reverses, the valve disc begins to close the valve, depending on the design. The function or purpose of a check valve is to prevent backflow. The structure is usually very simple, consisting of only a few components such as the valve body, valve seat, valve disc, and valve cover. According to the design, there may be other components as well, such as valve stems, hinge pins, valve disc arms, springs, valve balls, elastomers, and bearings. The internal sealing of the check valve’s disc and seat relies on the \"reverse\" pipeline pressure, rather than the mechanical force used to open/close the control valve. Therefore, the allowable seat leakage rate for check valves is greater than that for on/off control valves. MSS SP-61, “Pressure Testing of Steel Valves”, published by the Manufacturers Standardization Society, is one of the standards used by manufacturers to conduct seat and shell closure tests on check valves (and other valves). Factors affecting the leakage of a check valve seat include back pressure, the medium, and the seat material (such as metal or elastomer). Some leakage is generally acceptable on the surfaces of metal and PTFE valve seats, while elastomers such as nitrile rubber (Buna-N) and fluorocarbon rubber (Viton) can provide a bubble-tight seal (zero leakage). Therefore, elastomers should be considered for use in air/gas media and low-pressure seals. When using elastomers for such valves, important considerations are the operating temperature and the compatibility of the elastomer with the medium. What is an ideal check valve? Regardless of the type of valve, the longest fault-free operating time will be achieved with a valve that is suitable for the application, not necessarily one with the same pipe size. Ideally, when the valve disc is flowing or fully closed, it remains stable in the open position relative to the internal stop. When these conditions are met, the valve disc will not vibrate, thereby preventing premature failure of the valve. Unfortunately, most check valves are selected in the same way as on/off control valves—namely, based on pipe size and the required maximum Cv value. This ignores the fact that, unlike on/off control valves with actuation (manual, pneumatic, hydraulic, or electronic), it is only the flow conditions that determine the internal performance of a check valve. The check valve is sensitive to flow rate inside, unlike on/off control valves. If there is not enough flow and pressure to fully open the check valve, flutter of the valve disc will occur inside the valve. This will lead to premature wear, potential failures, and a higher voltage drop than calculated. Wear occurs whenever a metal component rubs against another metal component. This will lead to the eventual failure of the component itself. Component failure can prevent the valve from performing its function; in the case of check valves, this is to prevent backflow. In extreme cases, a failure can cause components to leak into the pipeline, leading to failures or malfunctions in other valves or equipment within the pipeline. Typically, the pressure drop is calculated assuming that the check valve is 100% open, just like the on/off control valve. However, if the flow rate is insufficient to achieve full opening and the check valve opens only partially, the pressure drop will be higher than the calculated value. This is because when the check valve is partially open, the effective Cv of the valve is less than its maximum value. In this case, a high rated Cv is actually harmful to check valves (unlike on/off control valves). This will cause the valve disc to vibrate, ultimately leading to failure. Some other valves are not like this. For example, when the gate valve is fully open, the wedge disengages from the flow path. Therefore, whether the flow rate is low, medium, or high, the flow passing through the valve does not affect the performance of the wedge. There are various types of check valves. Below are some of the more popular types. All of these can be used as cleaning media. Like other types of valves, special check valves can also be used for special applications. Although no single valve is suitable for all applications, each type has its advantages. Taking the time to contact the manufacturer for assistance in making a choice can help you find the most suitable product. This is especially true if there are problems with any type of check valve that you currently have installed. Swing check valve: A swing check valve features a simple design; it uses a valve disc that is attached to an arm hinged at the top of the valve (located at the 12 o’clock position). Backflow and gravity help to close the valve. Swing check valves can be used with most media and generally offer good flow rates. They can only be installed in a horizontal flow position. This is because they cannot function properly in a vertical flow position. In low backpressure applications, they also cannot seal well. The size range of these check valves spans from ½ inch and below to 50 inches and above, and they can be connected using threaded, socket weld, flange, or butt weld methods. Swing check valves are usually easy to inspect and maintain. In most cases, the valves in the pipeline can be used for maintenance. Due to the design of the swing check valve, and the distance the valve must travel from fully open to closed, it is not a valve that closes quickly. This means they are highly susceptible to water hammer problems. Most swing check valves comply with ANSI B16.10 face-to-face dimensions, allowing for line cleaning. There is a variant of the swing check valve called the inclined disk check valve. However, this version does not allow pipeline pigging. Piston/lift valve inspection: Piston-type or lift-type check valves come in straight-through, inclined (Y-type), or traditional (90-degree T-type) valve body designs. All types of check valves are considered silent check valves, capable of preventing water hammer and backflow. This is achieved by using a spring-assisted valve disc that moves in line with the short-stroke water flow, thereby enabling rapid closure of the valve. As the forward speed begins to slow down, the spring-assisted device starts to close the valve disc. When the forward velocity reaches zero, the valve disc closes tightly against the valve seat before backflow occurs, preventing pressure fluctuations in the pipeline and thus avoiding water hammer. Most designs can be installed in any position, including downward flow, as long as the appropriate springs are used. The sizes of piston/lift valve check valves range from 1/4 inch to 24 inches or larger. The selected valve body design will determine the pressure drop ； Inline design will provide the best traffic performance. Piston/lift valve check valves come in a variety of end connections, including threads, flanges, and weldable types. There are special end fittings, but you need to consult the check valve manufacturer. Some of these check valves can be inspected and repaired online. Ideally, this check valve should be used only with particle-free clean media. Flange-clamped check valve: The flange-clamped check valve is a very compact wafer-type check valve used for flanged pipelines. They are usually built-in, with sizes ranging from ½ inch to 20 inches. This style is also considered a form of silent inspection that helps prevent water hammer. Therefore, they will have an internal spring to help close the valve. The flange-mounted check valve and its compact design allow it to be added to existing systems with minimal pipeline modifications required. The flange-clamped check valve features a compact clamp design. Center-guided check valve: The center-guided check valve is another type of silent check valve. They are also designed to prevent water hammer and backflow. This type is similar to a piston/lift valve. Its specifications also fall under MSS SP125 and 126. They are available in flange type, with sizes ranging from 2 to 24 inches, and sometimes larger. Similarly, this type is most suitable for particle-free cleaning agents. Spherical check valve: A spherical check valve uses a sphere within the valve body to control the flow of fluid. This type is also considered a non-silent check valve. The valve ball can rotate freely, thereby ensuring even wear and facilitating wiping between the valve ball and the valve seat. Spherical check valves or silent check valves are suitable for viscous medium applications. This feature makes the spherical check valve suitable for viscous media. The size of spherical check valves is usually less than or equal to 2 inches. Some designs include a spring to assist with closing, and are designed for a 90-degree orientation mounted on a vertical line. Depending on the valve body design, the pressure drop of ball check valves may be higher than that of other types of check valves. Spherical check valves can be used for various end connections, including threaded and socket weld. Some valve body designs allow for online maintenance/inspection. Among the various factors to consider when selecting a check valve are material compatibility with the medium, the valve’s rated pressure (ANSI), pipeline size, application data (flow rate, design/operating conditions), installation orientation (horizontal, upward flow, or downward flow), end connections, housing size (especially when replacing existing valves to avoid pipeline modifications), leakage requirements, and special requirements such as oxygen cleaning, NACE compliance, CE marking, etc. There are many different designs of check valves, among which the oldest and most common is the swing check valve. Solving problems: When replacing a check valve, it helps to ask the following simple question: Why do I need to replace this valve? What is the problem? Sometimes we are too busy or focused on other things, and we forget that reasons can help solve problems. Common check valve problems include noise (water hammer), vibration/flutter, backflow, sticking, leakage, internal defects, and worn or damaged components. However, it is worth noting that the actual reason is usually an incorrect size, spring, and/or design of the check valve used. In this case, the problem lies with the application, not the check valve. The two most common problems with check valves are incorrect size or improper installation. There are two forms of incorrect dimensions. If the valve’s Cv is too small for the application, you will experience a very high pressure drop, which may lead to premature wear of the valve due to the high speeds involved. More commonly, if the valve Cv is too large for the application, then insufficient pressure drop will be generated across the check valve to fully open it. Any check valve that is not fully open is highly likely to experience flutter, leading to premature failure of the valve. Incorrect installation includes insufficient straight pipe upstream of the check valve. Ideally, the diameter of the straight pipe upstream of the check valve should be at least 10 times larger. This is to ensure good laminar flow through the check valve. Shorter distances can lead to turbulent flow and rotation, thereby causing premature wear of any type of check valve. Other issues with check valves include backflow and water hammer. In both cases, it is necessary to close the valve quickly. Backflow can be costly, especially when it occurs during pump discharge and the pump rotates in reverse. The cost of repairing or replacing the pump, along with the downtime of the factory, far exceeds the cost of installing a proper check valve from the beginning. For water hammer, you need a quickly closing check valve to prevent pressure surges and shock waves from occurring when the valve disc hits the valve seat, which can result in noise, vibration, and hammering sounds, and may lead to pipe rupture as well as damage to equipment and pipe supports. If the internal components are missing or excessively worn, two factors may arise. First, if the selected check valve does not have sufficient flow capacity to prevent it from stopping, a valve with a lower Cv is required to avoid vibration of the internal components. Secondly, if a check valve is used for the discharge of reciprocating air or gas compressors, a dedicated valve or buffer with a damping design is required to handle the high-frequency cycles. Sticking may occur when scale or dirt is trapped between the valve disc and the valve body bore. Damage to the valve seat or valve disc, or solid substances in the pipeline, can all cause leaks. An elastomer is needed to ensure zero leakage. Installation: When installing the check valve, orient the flow arrow in the direction of flow so that the valve can perform its intended function. Flow arrows can be found in the main text or labels. Ensure the valve type works at the installation location. For example, not all check valves can handle flow in a vertical pipeline downward, and traditional or 90-degree T-type piston check valves cannot operate in vertical pipelines without a spring to push the valve disc back into the flow channel. When the valve is fully open, the valve discs in some check valves extend into the pipe. This may affect the performance of another valve that is directly bolted to the check valve. As mentioned earlier, if possible, install the check valve at a location at least 10 times the pipe diameter downstream of any fittings or other pipeline system components that could cause turbulence. Note, I said “if possible”. After all, how many check valves have you seen that are bolted to the pump’s discharge port? Many! The MSS SP-92 \"Valve User Guide\" published by the Manufacturers’ Standardization Association is an excellent reference for installing check valves and other types of valves. What are the similarities between check valves and doors? Finally, I like to compare check valves to doors—whether that door leads to an office or a home. Normally, you open the office door at the start of the day and close it at the end, similar to how a pump is turned on and off. However, what would happen if someone stood at your door, continuously opening and closing it in a loop? In most cases, the hinge pins fail because they are the weak points in door operation. Check valves face a similar situation. Pins, rods, springs, or other components that are in continuous circulation may fail. This is why it is very important to choose the right check valve for its specific application. The pipe size does not necessarily equal the check valve size. Check valves with a high Cv in low-flow applications are doomed to fail from the start. This is not a fault with the check valve, but rather a fault due to incorrect application selection. The selected check valve performs well under appropriate flow conditions. Unfortunately, the installed check valve was considered to be the cause of the failure, when in fact the real cause was the operating conditions. Before purchasing a check valve, it is advisable to review the application and operating conditions with the manufacturer to ensure the correct type and options are selected.