The Misunderstood Check Valve

The Misunderstood Check Valve

Check valves may be the most misunderstood valves ever invented. If you mention check valves to most plant personnel, the typical response is “they don’t work.” In fact, those personnel may well have taken out the internals or repiped the system to avoid utilizing check valves. In other words, these valves may be the least popular valve in use today.

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This article will explore the fundamentals of check valves, their operation, types available, selection and installation guidelines, troubleshooting, and why check valves are often unfairly blamed for system issues.

How Check Valves Operate

Simply put, a check valve allows flow in one direction and automatically prevents backflow (reverse flow) when fluid in the line reverses direction. They are one of the few self-automated valves that do not require assistance to open and close. While some can be fitted with externally weighted and dampened devices for special circumstances, the majority do not have any outside assistance as found with on/off control or other valves. Unlike other valves, they continue to work even if the plant facility loses air, electricity, hydraulic pressure, or the human operator typically required to cycle them.

Like other types of valves, check valves are available in a full range of sizes, materials, and end connections. The line sizes range from 1/8 inch or smaller to 50 inches and larger. They are made of materials like bronze, cast iron, plastics, carbon steel, various grades of stainless steel, and alloys such as Hastelloy, Inconel, Monel, and titanium. End connections include threaded, socket weld, butt weld, flanged, grooved, wafer, and insert type.

They are found everywhere, including in homes. If you have a sump pump in the basement, a check valve likely exists in the discharge line of the pump. Outside the home, they are common in industries such as desalination, water and wastewater treatment, chemical, food and beverage, geothermal, mining, oil and gas, power, pulp and paper, refining, and more.

Understanding How Check Valves Are Used

Check valves handle a variety of media: liquids, air, other gases, steam, condensate, and even liquids with particulate or slurries. They are used in applications like pump and compressor discharge, header lines, vacuum breakers, non-code pressure relief, steam lines, condensate lines, chemical feed pumps, cooling towers, loading racks, nitrogen purge lines, boilers, HVAC systems, utilities, pressure pumps, sump pumps, wash-down stations, and injection lines.

How They Operate

Check valves are flow-sensitive and rely on the line pressure and flow to open and close. The internal disc allows flow to pass forward, which opens the valve. The disc begins closing the valve as forward flow decreases or is reversed, depending on the design. The function or purpose of a check valve is to prevent reverse flow. Construction is normally simple with a few components such as the body, seat, disc, and cover. Depending on the design, there may be additional items like a stem, hinge pin, disc arm, spring, ball, elastomers, and bearings.

Internal sealing of the check valve disc and seat relies on "reverse" line pressure as opposed to the mechanical force used for on/off control valves. Consequently, allowable seat leakage rates are greater for check valves than for on/off control valves. The MSS SP-61 standard, "Pressure Testing of Steel Valves," published by the Manufacturers Standardization Society, is one standard used by manufacturers to perform seat and shell closure tests for check valves. Factors affecting check valve seat leakage include reverse pressure, media, and the seat material (such as metal or an elastomer). Metal and PTFE seating surfaces generally allow some leakage, while elastomers like Buna-N and Viton provide bubble-tight shutoff (zero leakage).

Because of this, elastomers should be considered for air/gas media and low-pressure sealing. When using elastomers, service temperature and compatibility with the media are important considerations.

What is the Ideal Check Valve?

Regardless of type or style, the longest trouble-free service will come from valves sized for the application, not necessarily the line size. Ideally, the disc is stable against the internal stop in the open position when flowing or fully closed when there is no flow or checking. When these conditions are met, no chattering of the disc will occur, thereby preventing premature valve failure. Unfortunately, most check valves are selected like on/off control valves, by line size and the desire for the largest Cv available. This overlooks the fact that unlike on/off control valves that have actuation (manual, pneumatic, hydraulic, or electronic), only the flow conditions determine the internal performance of the check valve.

Check valve internals are flow-sensitive, unlike on/off control valves. If there is not enough flow and pressure to fully open the check valve, trim chatter occurs inside the valve, resulting in premature wear, potential for failure, and a higher pressure drop than calculated.

Whenever a metal part rubs against another metal part, wear is the result, leading to eventual failure of the component. In such cases, valve failure means it no longer prevents reverse flow, potentially leading to the component(s) escaping into the line and causing the failure of other valves or equipment.

Pressure drop is typically calculated based on the check valve being 100% open, as with on/off control valves. However, if the flow is insufficient to reach full open and the check valve is only partially open, the pressure drop will be higher than calculated because the effective Cv is less than the maximum Cv when partially open. This makes a large-rated Cv detrimental to the check valve, unlike with on/off control valves. The result is chattering of the disc and eventual failure. Other valves like gate valves do not face this issue; when fully open, the wedge is out of the flow path.

Types of Check Valves

Various check valve types are available. Some of the more popular include:

Swing Check

Swing check valves use a disc attached to an arm hinged at the top of the valve. Reverse flow and gravity assist in closing the valve. Swing checks can be used for most media and generally provide good flow capacity, but should only be installed in horizontal flow positions as they won't operate properly in vertical positions. They range in size from 1/2 inch and smaller to 50 inches and larger, with threaded, socket weld, flanged, or butt weld end connections. They are typically easy to inspect and maintain, often repairable in place. Due to their design, swing checks are not fast-closing valves and are susceptible to water hammer issues. They usually meet ANSI B16.10 face-to-face dimensions.

Piston/Poppet Check

Piston or poppet style check valves come in inline, inclined (Y-pattern), or conventional (90-degree T-pattern) body designs. Considered silent check valves, they prevent water hammer and reverse flow with a fast-closing spring-assisted disc. Available from 1/4 inch to 24 inches and larger, they come in various end connections, are ideal for clean media service, and can often be inspected and repaired in-line.

Flange Insert Check

Flange insert check valves are compact wafer-style valves for flanged piping systems, available from 1/2 inch to 20 inches. They are another type of silent check valve, featuring an internal spring to assist with closing, minimizing piping alterations for system integration.

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Center Guided Check

Another silent check valve type, center guided check valves prevent water hammer and reverse flow. They come in flanged styles from 2 to 24 inches, adhering to MSS SP125 & 126 specifications. Best suited for clean media, they offer reliable performance.

Ball Checks

Ball check valves use a rotating ball inside the body to control flow, providing even wear and a wiping action. This makes them suitable for viscous media. Typically found in sizes up to 2 inches, some designs include a spring for vertical installations. Though they may incur higher pressure drops, their end connections include threaded and socket weld, often permitting in-line repair or inspection.

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