How to Select the Right Check Valve

There is no single check-valve design that is the best choice for every application, and the importance of individual selection criteria varies from one system to another. Selecting the correct check-valve type, size and specification is essential for smooth, trouble-free operation and long service life.

Swing check valves

Swing check valves are among the most widely used check-valve designs. They are cost-effective and self-acting, so they require no external power or control system; the direction of flow determines valve operation. A swing check valve with an enclosed shaft does not provide a direct open/closed indication, but versions fitted with a lever and counterweight or a lever and spring allow visual confirmation of valve movement.

Ball check valves

Ball check valves are generally simple and economical. Their compact design and lack of external moving parts help keep cost low and reliability high. A possible disadvantage in some applications is that they do not provide a direct open/closed position indication.

Recoil check valves

Recoil check valves are a well-established high-performance check-valve design. They are used in pumping systems where rapid flow reversal can occur. Typical installations include pumping stations with high branch velocities, automatic start/stop operation and multiple pumps discharging into a common rising or vertical main.

The recoil check valve is designed to provide both mechanical and hydraulic assistance so that the valve closes rapidly.

• Mechanical assistance is achieved through low inertia, concentrated moving mass and optimized suspension of the moving elements.
• Hydraulic assistance is provided by the way forward and reverse flow acts within the valve body and around the valve doors.

For large valve sizes, a multi-door design can be used. Dividing the flow across two, three or even four doors reduces the angular travel required by each door and therefore shortens closing time.

Recoil check valves use metal seats and therefore have a defined permissible leakage rate.

Tilting-disc check valves

A tilting-disc check valve has an external appearance similar to a double-eccentric butterfly valve. The body is double flanged and relatively short. The disc is supported by a shaft positioned eccentrically to the body centerline in both the horizontal and vertical directions.

This double eccentricity causes the lower part of the disc to occupy a larger area in the flow path, allowing the disc to begin opening at relatively low flow velocity. Tilting-disc check valves are therefore commonly used in pumping systems with low or fluctuating flow rates.

During flow reversal, gravity starts to close the disc while the portion of the disc above the shaft centerline is acted on by the reverse flow. This produces a hydraulic braking effect and helps damp the disc as it returns to the closed position. Tilting-disc check valves can also be supplied with a lever and counterweight to adjust closing characteristics, making them well suited to reducing the risk of water hammer.

Because the internal shafts are located in the flow path, tilting-disc check valves are typically used for water and treated-wastewater applications.

Slanted-seat tilting-disc check valves

A slanted-seat design further improves resistance to water hammer. The valve combines a double-eccentric shaft position with an increased seat angle, resulting in shorter disc travel and therefore a shorter closing time. As an option, the valve can be equipped with an internal hydraulic damper.

A hydraulic damper is particularly recommended for pumping-station installations where the valve opens and closes frequently. Under these conditions the damper helps protect the valve from accelerated wear of the internal moving components.

The slanted-seat design also helps the valve achieve tight shut-off at lower reverse pressure.

Selection criteria for a check valve

Many check-valve designs are available for water and wastewater systems. When a check valve is required, the correct type and size should be selected so that the system can:

• Operate smoothly at optimum efficiency.
• Reduce maintenance, repair work and shutdowns.
• Achieve a longer service life.

The following factors should always be considered.

Medium

For wastewater applications, it is important that the valve has a full-bore flow path so that solids can pass through without obstruction.

Flow rate: minimum and maximum

Minimum and maximum flow rates are required to size the valve correctly. Different check-valve designs reach the fully open position at different flow velocities.

For optimum system efficiency, the valve should be fully open under normal flow conditions. This minimizes head loss through the valve, reduces wear and damage to internal moving components and provides more stable operation.

Maximum flow velocity is often governed by applicable international standards, such as BS EN 1074-1. A maximum velocity of approximately 4 m/s is commonly used as a design reference.

Upstream and downstream working pressure: minimum and maximum

These values determine the valve pressure rating, for example PN10, PN16 or PN25.

It is also important to know the valve's cracking pressure, meaning the upstream pressure at which flow begins to pass through the valve.

Reverse pressure on the back of the disc must provide sufficient force to press the disc against the sealing surface. This is especially important in low-pressure systems.

Valves with high pressure loss also increase the system's energy consumption.

Head loss

Head loss is related to flow velocity, while pressure loss across the valve is influenced by system flow conditions and by the valve's internal surfaces. The geometry of the valve body and the design of the closure element determine the available flow area and therefore also affect head loss.

Total head is commonly considered as a combination of static head, caused by elevation difference, and friction head, caused by losses in pipes and valves. Several calculation methods can be used. A common approach is to use the flow coefficient, which represents the quantity of water passing through a valve at a specified pressure loss. For comparative purposes, the Kv value is a useful reference.

Total cost of ownership

The cost of a check valve involves more than its purchase price. In some installations, the largest costs may be purchase and installation; in others, maintenance or energy consumption can be equally important or even more significant. When cost is used as a selection criterion, the total cost over the valve's service life should therefore be considered. As a general rule, simpler valve designs tend to require less maintenance.

Non-slam characteristics

Check-valve slam can create a pressure surge in the system. The process begins when a pump stops and the flow starts to reverse. Some reverse flow may pass through the valve before it reaches the fully closed position. When that reverse flow is suddenly stopped, the change in velocity converts the kinetic energy of the fluid into pressure.

The resulting noise can sound as if the disc or ball is striking the seat. In reality, much of the sound is caused by the pressure wave stressing the pipe wall. To eliminate slam completely, a check valve would need to close before any reverse velocity developed; in practice this is difficult to achieve. Valve geometry determines how much reverse flow occurs, so a faster-closing valve generally produces less slam.

Valve designs in which the closing element responds quickly to the changing flow can provide improved non-slam performance. Modern variable-speed pump systems can also control pump starting and stopping times to reduce water-hammer risk.

Check valves can be installed in horizontal or vertical pipelines where the valve design permits. Slam can be a greater concern in vertical installations because flow can reverse rapidly, although gravity can also assist rapid valve closing depending on the configuration.