The Misunderstood Check Valve

Check valves are often misunderstood devices in the valve industry. Mentioning them to most plant personnel often prompts the response: "They don’t work." It is not uncommon for these personnel to have removed the internals or reworked the piping system to eliminate the need for check valves. Consequently, check valves are arguably among the least favored valves in contemporary use.

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This article delves into the fundamental aspects of check valves, explaining their operation, types, selection, installation, troubleshooting, and clarification of common misconceptions.

In simple terms, a check valve permits fluid flow in one direction while automatically preventing backflow when the fluid's direction reverses. These valves are unique as they self-operate without any external assistance to close or open. Some may be equipped with devices for specific situations, but most operate independently, unlike on/off valves, which require manual or external control. No matter if there’s a loss of air, power, or hydraulic pressure, check valves remain functional.

Check valves are manufactured in a wide range of sizes, materials, and connection types. Their sizes start from 1/8 inch and can exceed 50 inches. They are crafted from materials such as bronze, cast iron, plastics, carbon steel, stainless steel, and alloys like Hastelloy, Inconel, Monel, and titanium. Connection types include threaded, socket weld, butt weld, flanged, grooved, wafer, and insert types.

These valves are prevalent not just in industrial settings but also in residential applications. For instance, if you own a sump pump in your basement, a check valve is most likely situated in its discharge line. In industrial contexts, they play roles in diverse fields such as desalination, wastewater treatment, food and beverage processing, oil and gas production, and more.

Roles and Applications of Check Valves

Similar to other valves, check valves are compatible with various media including liquids, air, gases, steam, and slurries. Their applications are extensive, found in scenarios such as pump discharge, header lines, vacuum breakers, and chemical feeds, among others.

Understanding Their Operation

Check valves respond to flow; they rely on pipeline pressure to open and close. The internal disc permits forward flow, which opens the valve. A decrease or reversal in flow causes the disc to close. Their sole purpose is to prevent reverse flow, and their construction typically involves a few parts like the body, seat, disc, and cover. Depending on design, there may be various mechanisms included, such as springs or bearings.

The sealing of the disc and seat in a check valve functions due to reverse line pressure, unlike the mechanical methods utilized in on/off control valves, thus allowing for a higher acceptable leakage rate. Manufacturers use standards such as MSS SP-61 for pressure testing check valves, taking into account factors like media type and seat material, which impact leakage rates.

Special elastomers might be used for air and gas applications to ensure low-pressure sealing. When applying them, factors like service temperature and material compatibility are critical.

What Constitutes the Ideal Check Valve?

The longevity of check valves lies in their appropriate sizing for specific applications rather than just matching line size. The ideal condition is when the disc remains stable against the internal stop under flow, preventing disc chatter that might lead to early failure. Common practice tends to incorrectly size check valves based solely on line dimensions, neglecting the flow-dependent operation of these valves.

Moreover, any check valve operating without sufficient flow and pressure may encounter trim chatter, ultimately resulting in premature wear or failure. They indeed face potential failure if their internal components experience excessive contact due to inadequate flow conditions.

When a metallic part consistently rubs against another, wear is inevitable, leading to potential failure and disrupting the check valve's intended function of blocking reverse flow. In severe instances, parts may detach into the pipeline, affecting other components.

Pressure drop for check valves is typically calculated based on the assumption of a fully open valve, akin to on/off controls. However, if a valve is only partially open due to insufficient flow, the actual pressure drop could be higher than expected. This situation can lead to issues like disc chatter, bringing about failure—unlike a fully opened gate valve, which does not interfere with flow.

There are various types of check valves suited for clean media, each having distinct advantages and specially designed applications. Taking the time to engage with a manufacturer for guidance can be beneficial, especially if current installations present issues.

Types of Check Valves

Swing Check Valves

A classic swing check valve features a disc that pivots on a hinge at the top. Reverse flow, assisted by gravity, enables closing. They are suitable for a variety of media but should only be installed in horizontal positions as they do not seal effectively under low backpressure.

Piston/Poppet Check Valves

Piston or poppet check valves are designed to minimize water hammer and can be installed in any position with spring assistance aiding the disc’s closing speed.

Flange Insert Check Valves

These compact wafer-style valves serve the flanged piping sector and help prevent water hammer with their internal spring mechanisms.

Selection Criteria

Crucial considerations in selecting check valves include compatibility with media, pressure ratings, application data, installation conditions, and leakage criteria.

Troubleshooting Common Issues

When replacing a check valve, it is vital to understand the reason behind its failure. Common criticisms include noise, vibration, reverse flow, and leakage, but often the true issue relates to size, spring selection, or installation missteps.

Under-sizing can cause excessive pressure drops resulting in early wear, while oversized valves may fail to achieve full openness, leading to chatter. Additionally, ensuring sufficient straight pipe upstream helps maintain laminar flow, which is necessary for effective valve operation.

Issues like reverse flow can be catastrophic if they cause a pump to rotate backward, incurring costs that greatly outweigh those of proper initial installation. Likewise, water hammer demands quickly responding valves to prevent pressure spikes that could damage infrastructure.

Installation Tips

Follow the flow direction indicated on the valve during installation, ensuring that the appropriate check valve type is utilized. A minimum of ten diameters of straight piping before the valve is ideal to prevent turbulence.

Conclusion

Analogous to doors that open and close, check valves also need to be appropriately installed and sized for their applications. Incorrect configurations can lead to common failures, underscoring the importance of properly assessing functional needs prior to selection.

If you require further insights, especially regarding double door check valves, do not hesitate to contact LEFLOW for assistance!