Preventing Water Hammer With Variable Speed Actuators

Water hammer can be a major concern in pumping systems and should be a consideration for designers for several reasons. If not addressed, it can cause a host of issues, from damaged piping and supports to cracked and ruptured piping components. At worst, it may even cause injury to plant personnel.

Water hammer happens when there is a surge in pressure and flow rate of fluid in a piping system, causing rapid changes in pressure or force. High pressures can result in piping system failure, such as leaking joints or burst pipes. Support components can also experience strong forces from surges or even sudden flow reversal. Water hammer can occur with any fluid inside any pipe, but its severity varies depending upon the conditions of both the fluid and pipe. Usually this occurs in liquids, but it can also occur with gases.

Increased pressure occurs every time a fluid is accelerated or impeded by pump condition or when a valve position changes. Normally, this pressure is small, and the rate of change is gradual, making water hammer practically undetectable. Under some circumstances, many pounds of pressure may be created and forces on supports can be great enough to exceed their design specifications. Rapidly opening or closing a valve causes pressure transients in pipelines that can result in pressures well over steady state values, causing water surge that can critically damage pipes and process control equipment. The importance of controlling water hammer in pump stations is widely recognized by utilities and pump stations.

Typical water hammer triggers include pump startup/shutdown, power failure and sudden opening/closing of line valves. A simplified model of the flowing cylindrical fluid column would resemble a metal cylinder suddenly being stopped by a concrete wall. Solving these water hammer challenges in pumping systems requires either reducing its effects or preventing it from occurring. There are many solutions system designers need to keep in mind when developing a pumping system. Pressure tanks, surge chambers or similar accumulators can be used to absorb pressure surges, which are all useful tools in the fight against water hammer. However, preventing the pressure surges from occurring in the first place is often a better strategy. This can be accomplished by using a multiturn variable speed actuator to control the speed of the valve's closure rate at the pump's outlet.

The advancement of actuators and their controls provide opportunities to use them for the prevention of water hammer. Here are three cases where addressing water hammer was a key requirement. In all cases, a linear characteristic was essential for flow control from a high-volume pump. If this had not been achieved, a hammer effect would have resulted, potentially damaging the station's water system.

Design Challenge

The East Cherry Creek Valley (ECCV) Southern Booster Pump Station in Colorado was fitted with high-volume pumps and used pump check valves for flow control. To avoid water hammer and potentially serious system damage, the application required a linear flow characteristic. The design challenge was to obtain linear flow from a ball valve, which typically exhibits nonlinear flow characteristics as it is closed/opened.

Solution

By using a variable speed actuator, valve position was set to achieve different stroke positions over intervals of time. With this, the ball valve could be driven closed/open at various speeds to achieve a more linear fluid flow change. Additionally, in the event of a power failure, the actuator can now be set to close the valve and drain the system at a predetermined emergency curve.

The variable speed actuator chosen had the capability to control the valve position based on preset times. The actuator could be programmed for up to 10 time set points, with corresponding valve positions. The speed of valve opening or closing could then be controlled to ensure the desired set position was achieved at the correct time. This advanced flexibility produces linearization of the valve characteristics, allowing full port valve selection and/or significantly reduced water hammer when closing the valves. The actuators’ integrated controls were programmed to create linear acceleration and deceleration of water during normal pump operation. Additionally, in the event of electrical power loss, the actuators ensured rapid closure via backup from an uninterruptible power supply (UPS). Linear flow ratechange was also provided, and this ensured minimum system transients and easy calibration/adjustment of the speed-time curve.

Due to its variable speed capability, the variable speed actuator met the challenges of this installation. A travel dependent, adjustable positioning time provided by the variable speed actuators generated a linear flow through the ball valve. This enabled fine tuning of operating speeds through ten different positions to prevent water hammer.

Design Challenge

In the area of Oura, Australia, water is pumped from multiple bore holes into a collection tank, which is then pumped into a holding tank. Three pumps are each equipped with 12-inch butterfly valves to control the water flow.

To protect the valve seats from damage caused by water cavitation or the pumps from running dry in the event of water loss, the butterfly valves must be capable of rapid closure. Such operation creates huge hydraulic forces, known as water hammer. These forces are sufficient to cause pipework damage and must be avoided.

Solution

Fitting the valves with part-turn, variable speed actuators allows different closure speeds to be set during valve operation. When closing from fully open to 30% open, a rapid closure rate is set. To avoid water hammer, during the 30% to 5% open phase, the actuator slows down to an eighth of its previous speed. Finally, during the final5% to complete closure, the actuator speeds up again to reduce cavitation and consequent valve seat damage. Total valve operation time from open to close is around three and a half minutes.

The variable speed actuator chosen had the capability to change output speed based on its position of travel. This advanced flexibility produced linearization of valve characteristics, allowing simpler valve selection and reducing waterhammer. The valve speed is defined by a maximum of 10 interpolation points which can be precisely set in increments of 1% of the open position. Speeds can then be set for up to seven values (n1-n7) based on the actuator type.

Design Challenge

In Mid Cheshire, United Kingdom, a chemical company used several hundred brine wells, each using pumps to transfer brine from the well to saturator units. The flow is controlled using pump delivery recycle butterfly valves driven by actuators.

Under normal operation, when a reduced flow is detected, the actuator which controls the valve is opened over a period of 80 seconds. However, if a reverse flow is detected, then the valve needs to be closed in 10 seconds to protect the pump. Different actuation speeds are required for opening, closing and emergency closure to ensure protection of the pump.

Solution

The variable speed actuator is able to provide up to seven different opening/closing speeds. These can be programmed independently for open, close, emergency open and emergency close.

Improving valve modulation is one solution to consider when addressing water hammer concerns in a pumping system. Variable speed actuators and controls provide pump system designers the flexibility to continuously control the valve's operating speed and accuracy of reaching setpoints, another task apart from closed-loop control.

Additionally, emergency safe shutdown can be provided using variable speed actuation. With the capability of continuing operation using a pump station emergency generator, the actuation technology can offer a failsafe option.

In other words, if a power failure occurs, the actuator will close in emergency mode in various speeds using power from a UPS system, allowing for the system to drain. The positioning time curves can be programmed individually for close/open direction and for emergency mode.

Variable speed, multiturn actuators are also a solution for open-close duty situations. This design can provide a soft start from the beginning position and soft stop upon reaching the end position. This level of control avoids mechanical pressure surges (i.e., water hammer) that can contribute to premature component degradation. The variable speed actuator's ability to provide this control positively impacts maintenance intervals and extends the lifetime of system components.

Craig Mizer is the engineering manager at AUMA Actuators, Inc. located in Canonsburg, Pennsylvania. Mizer oversees the engineering activities supporting AUMA's North American sales and manufacturing. For more information, visit auma.com.