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Equipment-based Mitigation: Check Valves

Equipment-based Mitigation: Check Valves

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Check valves are an essential component in any system. Whenever a pump is present, there is the potential for the pump’s driving force to be removed, and for flow to reverse direction through the system. A check valve works to ensure the system does not see sustained reverse flow.

 

A key factor in how a system responds to a check valve closure is the maximum reverse velocity the valve sees before it closes. A common misconception is that a check valve will prevent all reverse flow in a system. However, since the check valve disk must travel from open to closed, many check valves allow limited amounts of reverse flow.

 

The reverse flow allowed by the check valve directly relates to the magnitude of any waterhammer events caused by the valve. Reverse flow typically increases up to the moment when the check valve fully closes. A higher reverse flow velocity will cause larger valve slam events, meaning it is important to minimize reverse flow through the system and select a check valve best suited for the system.

 

Minimizing Check Valve Slam

 

Two main approaches exist to minimize the amount of slam caused by the check valve: lengthen the time taken to close the valve or reduce the fluid velocity when the valve closes.

 

Waterhammer events can be reduced at any valve by allowing the valve to slowly change position, stopping flow over a longer time. A check valve can be forced to close slowly using an external device to restrain the check valve disk.

 

One such device is an oil dashpot. Here, the valve disk is connected to a piston via a connecting rod or cylinder. The dashpot restricts the flowrate of oil entering or exiting the reservoir behind the piston, thus controlling the rate of disk travel. Figure 1, below, shows a schematic of this device.

Schematic of a check valve oil dashpot

Figure 1: Schematic of a check valve oil dashpot.

 

For this approach to work, the pump associated with the check valve needs to withstand reverse flow and potentially reverse rotation. However, the reverse flow will be present for a reduced amount of time when compared to the system without a check valve.

 

When the pump cannot withstand any reverse flow, the check valve cannot be closed slowly, and a different waterhammer mitigation approach is needed. Reducing the fluid velocity at the valve closure means the valve causes less waterhammer. Closing the check valve faster typically means the fluid is closer to a zero velocity when the valve fully closes. 

 

A fast check valve closure can be accomplished by reducing the inertia of the check valve, reducing the distance the disk must travel to close the valve, or by assisting the disk closure with a spring. A Silent Check Valve is a commonly used model that assists disk closure with a spring and features a short travel distance. That combination helps the valve close quickly, before substantial reverse flow develops, minimizing valve slam.

 

Matching the check valve with the overall system

 

Closing a check valve as rapidly as possible may sound like the perfect solution to avoiding check valve slam. However, it is not a perfect approach. Instead, the engineer must find a valve that best matches the pumping system.

 

An important characterization of the system is its fluid deceleration. The average deceleration of the fluid is calculated from the starting fluid velocity and the time for flow to stop. If the fluid is travelling at 10 ft/s and measured data shows flow will stop in 0.5 seconds, the average deceleration is 20 ft/s2.

 

Each type of check valve is meant for different fluid deceleration conditions. Figure 2 below shows how several types of check valves operate on a plot of Maximum Reverse Velocity vs. Fluid Deceleration.

amic characteristics of various check valves.

Figure 2: Dynamic characteristics of various check valves. (BCV: Ball Check Valve, SWCV: Swing Check Valve, RHCV: Resilient Hinge Check Valve, TDCV: Tilted-Disk Check Valve, DDCV: Dual-Disk Check Valve, RHCV-S: Resilient Hinge Check Valve With Spring, SCV: Silent Check Valve)

 

Figure 2 plots the maximum reverse velocity the check valve closure will see against the average fluid deceleration for a given closure. Valves that close slowly – like a Ball or Swing Check Valve – see large reverse velocities at a given fluid decelerations. Valves that close more quickly – like a Dual-Disk or Silent Check Valve – see low reverse velocities at a given fluid deceleration. On this plot, maximum reverse velocities over 1 ft/s are considered severe slam events.

 

Choosing a check valve that best matches the fluid deceleration seen in a system allows the engineer to effectively prevent reverse flow without seeing check valve slam and waterhammer events.

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