What is the most reliable type of check valve?

Advantages and Disadvantages of Two Types of Check Valves - Crane Engineering

Check valves are vital components in water and wastewater pumping systems. They automatically open when pumps are operational and close to prevent reverse flow when they are not. This functionality minimizes energy usage and protects the pumping infrastructure from pressure surges and reverse flow damage.

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When choosing check valves for your application, it can be compared to selecting hardware for new cabinetry. Once you’ve settled on a knob or handle, there’s a multitude of styles and price ranges to contemplate beyond that. Similarly, while it's evident that your process requires a check valve, the question arises, which type? Each check valve design serves a specific function, so let’s explore the advantages and disadvantages of the lift check valve and the swing check valve.

Lift Check Valves

Lift check valves are often used in high-rise buildings, power plants, and water and wastewater industries. They lack external moving parts, making them economical and dependable. However, their long-term energy costs might be high as the disc remains in the flow stream during operation.

Silent Check Valves

Silent check valves are recognized for their quiet operation. Flow causes the disc to open for forward movement. When the pump halts, a compression spring pushes the disc against the seat, ensuring silent closure.

This design is predominantly used in clean water applications involving high pressure.

Nozzle check valves, similar to silent check valves, cater to high-pressure, industrial, and power applications.

Ball Check Valves

Ball check valves are frequently employed in water and wastewater scenarios. They exhibit uncomplicated operation and are compact and cost-effective. These valves feature a rubber-coated ball that moves in and out of the seat as flow occurs.

However, due to the extensive travel distance required when the pump stops, these valves can slam in high-pressure situations.

Swing Check Valves

Swing check valves are among the most commonly used check valves in water and wastewater pumping systems. They are widely available, relatively inexpensive, and operate automatically without the need for external power—simply guided by the flow direction.

Dual-Disc Check Valve

This valve incorporates a wafer body design with two D-shaped discs that rotate aside as flow enters. They possess good non-slam characteristics, although they are ill-suited for wastewater due to their susceptibility to wear and vibration.

Traditional Swing Check Valves

These are the most recognizable check valves in water and wastewater applications. Typically, they feature metal or resilient seats and a swing range of 60-90 degrees. However, they too can experience slamming, attributed to the long stroke, packing friction, and the disc’s inertia.

Some use air cushions to mitigate slamming, while others argue that a lever with a spring is an effective accessory allowing for quicker closure.

Tilted Disc Check Valves

Unlike traditional swing check valves, tilted disc ones allow flow to pass on either side of the disc, significantly reducing head loss.

This type of valve is not recommended for wastewater applications due to the potential for debris collection on the pins that extend into the flow stream. They are better suited for clean water or treated effluents.

Do you have the correct check valve for your application? Is there a more suitable option? Always consult an engineer familiar with various check valve types for advice. They can assist in finding a valve that offers better energy efficiency or reduces slamming.

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How to Select the Right Check Valve?

Tilting Disc and Slanted Seat Check Valves

The slanted seat check valve provides enhanced resistance to water hammer. It features a double eccentric shaft and an increased seating angle, resulting in a shorter valve stroke which reduces the closing time.

Hydraulic dampers are advisable, particularly in pumping stations where frequent valve openings and closures occur. The valve disc closes quickly during the initial 85% of its angular travel before engaging with the hydraulic damper, which absorbs the disc’s kinetic energy and prompts it to open slightly. This controlled closure reduces the likelihood of water hammer, thanks to the softened approach to sealing.

Nozzle Check Valves

Nozzle check valves have a disc connected to the stem that is guided along the central horizontal axis. A spring situated between the disc and diffuser sleeve compresses as flow enters, allowing the valve to open. Conversely, when the flow ceases, the spring returns the disc to its closed position.

Due to their spring-assisted closure and short linear stroke, nozzle check valves act quickly, making them suitable for systems where water hammer could be an issue. However, because the disc is always aligned with the flow, head loss characteristics are higher compared to conventional swing check valves.

Factors to Consider When Choosing Check Valves

When selecting the proper check valve, several criteria must be considered. Importantly, there’s no one-size-fits-all valve type, as the selection criteria may hold varying importance based on specific applications. Factors include fluid compatibility, flow characteristics, head loss, non-slam properties, and overall ownership costs.

Fluid Compatibility

All check valves discussed in this article are intended for water and treated wastewater. However, using them for raw sewage can present challenges due to potential solid content affecting performance.

Flow Characteristics

If a check valve closes too rapidly, it might not only prevent slamming against upstream equipment but can also trigger surges from pump operation. A balance must be struck between opening and closing speeds to avoid exacerbating surge issues.

Head Loss

Head loss’s magnitude depends on fluid velocity and is determined by the valve’s internal design and the degree to which it opens. Valves with narrower openings increase velocity through the valve, leading to greater head loss compared to those designed without such restrictions.

Total Cost of Ownership

The costs associated with check valves extend beyond mere purchase price. Depending on the context, installation costs might be the most significant. However, maintenance and operational energy expenses can also play critical roles. Analyzing the total costs over the valve’s lifecycle will provide better insights.

Non-Slam Characteristics

Check valve slamming can exacerbate pressure surges. The process initiates as pump cessation starts to create a pressure surge, which is further accentuated if the flow reverses and hits a closing check valve. Fast closures may convert kinetic energy into pressure spikes, stressing pipelines and generating noise.

Slamming produces noise not from the actual closing but from sound waves arising from pressure fluctuations in the pipeline. To avoid slamming, the valve should close more slowly near the seated position, aided by auxiliary components like hydraulic dampers that cushion the disc as it seals. Swing check valves, with discs in the flow, typically offer better non-slam characteristics. Modern frequency-converted pumps can also modify start-up and closure rates to mitigate water hammer issues.

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