Cracking Pressure Of Swing Check Valve

[Meghan Beas]

Checkvalves are used in a wide range of industrial processes. While selecting the correct check valve design and materials for your application is important, it can be equally important to pay attention to what differential pressure is required to open the check valve.

Spring-loaded check valves have a spring that is activated by the pressures in the media, which open and close the valve. In spring-loaded check valves the cracking pressure is determined by the spring setting and orientation of flow through the check valve. Swing check valves; however, use a flapper that swings off the seat to allow forward flow and then swings back to the seat when the flow is stopped (Learn more about the differences in spring-loaded check valves and swing check valves). With spring-loaded check valves it may be possible for the spring (and the resulting net cracking pressure) to be tailored to the application. Typically, with swing check valves you are limited to the weight of the flapper or checking mechanism.

Spring loaded check valves can be built with springs to allow flow in any orientation while most swing checks are not suitable for flow vertical up or vertical down through the valve. Additionally, the use of a spring check valve minimizes water hammer.

Most spring-loaded check valves have a minimum recommended net cracking pressure; however, the cracking pressure is affected by vertical flow orientation. If the flow direction is vertical down, gravity pulls the weight of the trim components against the valve spring. This reduces the net cracking pressure, which may cause the valve to open or fail to return closed. In these cases, the spring selected must be heavy enough to support the weight of the trim, any column of liquid or media desired to be retained and achieve the minimum net cracking pressure. If you install a spring-loaded check valve in a vertical flow up orientation, then the weight of the trim components would increase the amount of force required to open the check valve. Your net cracking pressure will be the combination of the trim weight, the spring, and any fluid column above the check valve.

Check valves, regardless of the individual design, serve the specific purpose of permitting the flow of the service medium in one direction while intentionally preventing, or at least limiting, the flow of said medium in the reverse direction. Whether an inline check valve, wafer check valve, swing check valve or lift check valve, each version will necessarily have a dynamic element, whether disc, door, flapper, or ball, whose movement directly corresponds to the opening or closing of the subject flow path.

Depending upon the specific type of check valve under consideration, gravity or spring force can play a role in keeping this dynamic element closed against the valve seat when the upstream pressure or flow in the desired direction proves insufficient to push or lift the disc, door, flapper, or ball into the open position.

In many cases, a smaller check valve will have a notably lower cracking pressure than a larger version of a similar design. Typically, as the size of the valve increases, the size and mass of the aforementioned dynamic element also increase. With a more massive disc, door, flapper, or ball, that the process must displace in order to permit a detectable flow, the minimum pressure required to achieve that displacement typically proves elevated as well. Likewise, the required installation orientation of the valve in service can also influence the cracking pressure.

Take the example of check valves that prove suitable for both horizontal and vertical installations. The role that gravity plays in opposing the movement of the dynamic element of the valve necessarily changes as the orientation changes, as a result, the positioning of the said valve can potentially influence the cracking pressure required to permit flow.

While the designed cracking pressure of the valve adheres to an initially established value, within certain tolerances, extended or harsh service can also conceivably impact cracking pressure. Wear on the seats, galling on wetted surfaces, fatigue on a spring, deterioration of elastomers owing to caustic or rough service, and particulate contamination in the line offer just several examples of concerns that can certainly erode the predictability of the value of the upstream pressure at which the valve will allow a minimum detectable flow.

In some cases, this may decrease the cracking pressure as flow leaks past the seats even at nominal pressures. In other instances, the compromised valve may prove more resistant to opening thus increasing the cracking pressure. Neither condition represents an ideal situation and serves instead as reminders of the virtues of routine maintenance and replacement.

Certain valves that may offer a particularly low cracking pressure may also require some measure of backflow in order for the valve to reseat and close. Additionally, some of these low cracking pressure designs may not offer a bubble-tight shut-off, In other words, while they may generally restrict flow in the reverse direction when sitting in the closed position, the design may tolerate some volume of permissible leakage from the downstream side of the valve back through the seat to the upstream side.

Given the frequent employment of check valves on water and air applications, one must consider this potential for contamination when selecting the appropriate valve design and inherent shut-off class. Selecting a valve with a slightly higher cracking pressure but that offers positive shut-off in the reverse direction may prove more ideal.

Though minimizing the cracking pressure proves fruitful in certain circumstances, like those low- flow and low-pressure applications previously discussed, the class of valves known as restrictor checks typically elevates the cracking pressure to achieve specific values.

By utilizing interchangeable springs within the same inline check valve design, restrictor checks can purposefully manipulate the cracking pressure of the valve. This proves particularly useful in simple relief applications where the check valve principally serves to bleed excess media only when the upstream process exceeds a specific pressure threshold as opposed to conventional applications where the valve primarily controls process flow direction.

A restrictor check can typically offer a smaller length of stroke to achieve full lift and lower available cracking pressures than that of the typically coded safety relief valve. Importantly, though they may crack at specific pressures, restrictor checks do not take the place of safety relief valves. The design of the restrictor check necessarily anticipates a high cycle life where the valve will open and close frequently to address minor excesses of upstream pressure.

Alternatively, a safety relief valve will open only under emergency conditions to specifically accommodate the relief of a calculated capacity of the service medium. The intended use of a safety relief valve would not involve anticipated routine cycling to mitigate minor spikes in upstream process pressure, a task better left to the likes of back pressure regulators and the aforementioned restrictor checks.

Russ Bailey is a seasoned professional with an impressive 35-year career in valve sales. He serves as an Outside Sales Representative for both ValveMan.com and FS Welsford, where his deep industry knowledge and exceptional salesmanship enable him to provide tailored solutions for a diverse clientele. Russ is known for his ability to build robust relationships and his unwavering dedication to customer satisfaction. With a strong commitment to excellence, Russ continues to drive business growth and contribute significantly to the success of both companies.

A spring check valve is a valve that ensures unidirectional flow and prevents reverse flow. They have a single inlet and outlet and must have proper spring selection to perform effectively. On the side of a spring check valve, and all check valves, is an arrow that points in the flow direction. Spring loaded check valves are referred to as non-return or one way valves. The purpose of a spring check valve is to stop back flow using a spring and pressure placed on a disc to close the valve.

For a check valve to function properly, it has to have differential pressure with flow moving from high to low pressure. High pressure on the inlet side, or cracking pressure, allows the flow to move through the valve and overcome the strength of the spring in the valve.

Check valves, in general, are devices that allow any form of media to flow in one direction. The checking mechanism can be shaped like a ball, disc, piston, or poppet, a mushroom shaped head. When the pressure in a system begins to lower, slow, stop, or reverse, spring check valves prevent reverse flow as a method of protecting pumps, equipment, and machinery.

Swing check valves allow flow in one direction and automatically close when the cracking pressure decreases. They are a form of butterfly valve with a disc that covers the valve opening. The disc is attached to a hinge such that the disc can swing open or close when it is struck by the flow of the media. On the side of the valve body is an arrow that indicates the direction the flow must be as it enters and exits the valve.

The pressure level of the flow pushes the disc or door open, allowing the flow to pass through. When the flow moves in the wrong direction, the disc closes due to the force of the liquid or media pushing against it.

Swing check valves do not require external power. Fluid or media passes through them unobstructed by their presence. They are installed horizontally in pipelines but can be installed vertically as long as the flow is upward.

For a spring loaded check valve to function properly, it must have upstream pressure, known as cracking pressure, to keep it open. The amount of cracking pressure required varies depending on the valve type, its construction, spring characteristics, and its orientation in the pipe. The specifications for the cracking pressure are in pounds per square inch gauge (PSIG), pounds per square inch (PSI), or bars, the metric unit of pressure that equals 14.5 psi.

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