Misunderstood check valve

 Check valves may be the most easily misunderstood valve in history. If you mention check valves to most factory personnel, the typical reaction is "they don't work.". In fact, these personnel are likely to have removed internal components or refurbished the system to avoid using check valves. In other words, these valves may be the least popular valves currently in use.

This article will explore the basic knowledge of check valves, how they work, what types they are, how to select and install them, how to solve their problems, and why they are not always problems.

Simply put, check valves allow one-way flow and automatically prevent backflow (backflow) when the fluid in the pipeline is reversed. They are one of the few automatic valves that do not require auxiliary opening and closing. Although some valves can be equipped with external counterweights and damping devices in special circumstances, most valves do not have open/close control or external assistance from other valves. Unlike other valves, valves can continue to function even if factory facilities lose air, electricity, or hydraulic pressure, or if humans may manually cycle valves.

Like other types of valves, check valves come in various sizes, materials, and end connections. Pipeline 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, chromium nickel iron alloy, Monel alloy, and titanium. End connections include threaded connections, socket welding, butt welding, flange connections, groove connections, clamp connections, and plug-in connections.

Check valves can be seen everywhere, including at home. If you have a sewage pump in the basement, there may be a check valve on the discharge pipeline of the pump. They are found in industries outside of the home, such as desalination, water and waste, chemicals, food and beverage, geothermal, mining, oil and gas, electricity, pulp and paper, refining, etc.

Misunderstood check valve

Like other valves, check valves are used for various media: liquid, air, other gases, steam, condensate, and in some cases, they can also be used for 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 blowing pipelines, boilers, HVAC systems, utilities, pressure pumps, sump pumps, flushing stations, and injection pipelines.

How they operate

Check valves are sensitive to flow and rely on pipeline pressure and flow to open and close. The internal valve disc allows 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 simple, with only a few components such as the valve body, valve seat, valve disc, and valve cover. According to the design, there may be other items such as valve stem, hinge pin, disc arm, spring, valve ball, elastomer, and bearing.

The internal sealing of the check valve disc and seat depends on the "reverse" pipeline pressure, rather than the mechanical force used to open/close the control valve. Therefore, the allowable seat leakage rate of the check valve is greater than that of the open/close control valve. The MSS SP-61 "Steel Valve Pressure Test" published by the Manufacturers Standardization Association is one of the standards used by manufacturers to perform seat and shell closure tests on check valves (as well as other valves). The factors that affect check valve seat leakage include reverse pressure, medium, and seat material (such as metal or elastomer). Metal and PTFE seat surfaces typically allow for some leakage, while elastomers such as Buna-N and Viton can provide bubble sealing closure (zero leakage).

Therefore, the use of elastomers should be considered for air/gas media and low-pressure seals. When using elastomers for such valves, important considerations are the operating temperature and compatibility between the elastomer and the medium.

What is the ideal check valve?

Regardless of the type or type of valve, the longest fault free operating time will come from the valve that is suitable for the application, not necessarily the same size as the pipeline. In an ideal situation, when the valve disc flows or is completely closed, the valve disc remains stable in the open position relative to the internal stop block. 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, based on pipeline size and requirements for the maximum available Cv. This ignores the fact that unlike on/off control valves with drive (manual, pneumatic, hydraulic, or electronic), only flow conditions determine the internal performance of one-way valves.

Check valves are sensitive to flow internally, unlike on/off control valves. If there is not enough flow and pressure to fully open the check valve, the valve core will vibrate inside the valve. This will lead to premature wear, potential failures, and higher pressure drops than calculated values.

Whenever a metal component rubs against another metal component, wear occurs. This will lead to the final failure of the component itself. Component failures may prevent the valve from performing its function, and for one-way valves, this is to prevent backflow. In extreme cases, a malfunction may cause components to leak into the pipeline, causing other valves or equipment in the pipeline to malfunction or become inoperable.

Usually, the pressure drop is calculated based on the 100% opening of both the check valve and the on/off control valve. However, if the flow rate is not sufficient to achieve full opening and the check valve is only partially open, the pressure drop will be higher than the calculated value. This is because when the one-way valve is partially opened, the effective Cv of the valve is less than the maximum value. In this case, a high rated Cv is actually harmful to the check valve (unlike on/off control valves). This can 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 will detach from the flow channel. Therefore, regardless of whether the flow rate is low, medium, or high, the flow through the valve will not affect the performance of the wedge.

There are various types of check valves available. Below are some more popular types. All of these can be used for cleaning media. Like other types of valves, special check valves can also be used for special applications. Although no valve is suitable for all applications, each valve has its advantages.

Swing check valve

A swing check valve is a simple design that uses a valve disc connected to a hinged arm at the top of the valve (located at the 12 o'clock position). Counterflow and gravity help to close the valve. Swing check valves can be used for most media and typically have good flow rates. They can only be installed in horizontal flow positions. This is because they cannot function properly in the vertical flow position. In low back pressure applications, they also cannot seal well. The size range of these check valves ranges from ½ From inches and below to 50 inches and above, it can be used for threaded, socket welded, flange or butt welded end connections. Swing check valves are usually easy to inspect and maintain. In most cases, valves in the pipeline can be used for maintenance. Due to the design of the swing check valve, it is not a quick closing valve due to the distance of travel from fully open to closed. 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 pipeline cleaning. There is a variant of the swing check valve called the inclined disc check valve. However, this version does not allow pipeline cleaning.

Inspection of piston/poppet valve

Pistons or lift check valves come in straight, inclined (Y-shaped), or traditional (90 degree T-shaped) body designs. All types of check valves are considered silent check valves to prevent water hammer and backflow. This is achieved by using a spring assisted valve disc, which maintains consistency with the short stroke water flow, thereby achieving rapid valve closure. As the forward speed begins to slow down, the spring assist device begins to close the valve disc. When the forward speed reaches zero, the valve disc closes tightly against the valve seat before reverse flow occurs, preventing pressure fluctuations in the pipeline and thus preventing water hammer. Most designs can be installed in any position if suitable springs are installed, including downward flow. The size of piston/poppet check valves ranges from 1/4 inch to 24 inches or larger. The selected valve body design will determine the pressure drop; Inline design will provide optimal traffic performance. The piston/poppet check valve has multiple different end connections, including threaded, flanged, weldable, etc. There are special end fittings, but you need to consult the check valve manufacturer. Some of the check valves can be inspected and repaired online. Ideally, this type of check valve should only be used for particulate free cleaning media.

Spending time contacting manufacturers to assist with selection can help you find the most suitable product. This is especially true if any type of check valve you are currently installing has problems.

Flange clamp check valve

The flange clamp type check valve is a very compact wafer type check valve used in flange pipelines. They are usually embedded, with sizes ranging from ½ Inches to 20 inches. This style is also considered a silent inspection that helps prevent water hammer. Therefore, they will have an internal spring to help close the valve. The flange clamp check valve and its compact design allow it to be added to existing systems with minimal pipeline modifications required.

Center guided check valve

A 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/poppet valve. Its specifications also belong to MSS SP125 and 126. They come in flange style, with sizes ranging from 2 to 24 inches and sometimes larger. Similarly, this type is most suitable for particle free cleaning media.

Ball check valve

A spherical check valve uses a sphere inside the valve body to control the movement of flow. This type is also considered a silent check valve. The valve ball can rotate freely, achieving uniform wear and wiping between the valve ball and the valve seat.

This function 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 in closing and for installation in a 90 degree style on a vertical line. According to the valve body design, the pressure drop of spherical 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 welding. Some valve body designs allow for online maintenance/inspection.

choice

Many factors to consider when selecting a check valve include material compatibility with the medium, valve rated pressure (ANSI), pipeline size, application data (flow rate, design/operating conditions), installation (horizontal, upward or downward flow), end connections, shell size (especially when replacing existing valves to avoid pipeline modifications), leakage requirements, and special requirements such as oxygen cleaning, NACE, CE marking, etc.

There are many different check valve designs, among which the oldest and most common is the swing check valve.

Solving problems

When replacing the check valve, asking the following simple questions can be helpful:

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 the reasons that can help solve the problem.

Common check valve problems include noise (water hammer), vibration/chatter, backflow, jamming, leakage, internal missing parts, component wear or damage. However, it is worth mentioning that the real cause is usually the incorrect size, spring, and/or style of the check valve application. In this case, the problem lies in the application, not the check valve.

The two most common problems with check valves are incorrect size or incorrect installation. There are two forms of incorrect dimensions. If the valve Cv is too small for the application, you will see a very high pressure drop, which may cause premature valve wear due to the high speed involved. A more common scenario is that if the valve Cv is too large for the application, there will not be enough pressure drop on the check valve to fully open it. Any check valve that is not fully opened is likely to experience vibration, leading to premature failure of the valve. Incorrect installation includes insufficient number of straight pipes upstream of the check valve. Ideally, the diameter of the straight pipe upstream of the check valve should be at least 10 times. This is to ensure good laminar flow through the check valve. Shorter distances may cause flow turbulence and rotation, leading to 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 quickly close the valve. Counterflow can be costly, especially when it occurs during pump discharge and the pump rotates backwards. The cost of repairing or replacing the pump, combined with factory downtime, far exceeds the cost of installing the correct check valve first. For water hammer, you need a quick closing check valve to prevent pressure surges and shock waves when the valve disc hits the valve seat, which can cause noise, vibration, and hammering, leading to pipeline rupture and damage to equipment and pipeline supports.

If internal components are missing or excessively worn, two factors may occur. Firstly, if the selected check valve does not have enough flow to stop it, a valve with a lower Cv is required to prevent internal component vibration. Secondly, if the check valve is used for the discharge of reciprocating air or gas compressors, a specialized valve or buffer with damping design is required to handle high-frequency circulation. When there is scale or dirt between the valve disc and valve body hole, it may cause jamming. Damage to the valve seat or disc, or solid substances in the pipeline, can lead to leakage. We need an elastomer to provide zero leakage.

install

When installing a check valve, point the flow arrow towards the direction of flow to enable the valve to perform its intended function. You can find flow arrows on the main text or labels. Ensure that the valve type operates in the installation position. For example, not all check valves can flow downward in vertical pipelines, and traditional or 90 degree T-piston check valves cannot operate in vertical pipelines without springs pushing the valve disc back into the flow path. When the valve is fully open, some valve discs in check valves extend into the pipeline. This may affect the performance of another valve 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 pipe fittings or other piping system components that may cause turbulence. Note, I said "if possible". After all, how many check valves have you seen bolted to the pump discharge port? A lot! The MSS SP-92 Valve User Guide published by the Manufacturers Standardization Association is a great reference source for installing check valves and other types of valves.

What are the similarities between a check valve and a door?

Finally, I like to compare check valves with doors - whether that door leads to the office or home. Usually, you open the office door at the beginning of the day and close it at the end, similar to the situation when the pump is turned on and off. However, what would happen if someone stood at your doorstep, constantly cycling on and off? In most cases, hinge pins will fail as they are weak links in door operation.

Check valves also face similar situations. Continuously circulating pins, rods, springs, or other components may malfunction. That's why it's important to choose the right check valve for its specific application. The pipeline size does not necessarily equal the size of the check valve. Check valves with high Cv in low flow applications are destined to fail from the beginning. This is not a fault with the check valve, but a fault with the wrong application selection. The selected check valve works well under appropriate flow conditions. Unfortunately, the installed check valve is believed to be the cause of the malfunction, when in reality, the cause of the malfunction is the application condition. Before purchasing a check valve, it is best to review the application and usage conditions with the manufacturer to ensure that the correct type and option are selected.