What Is Water Hammer and How Can It Be Prevented?
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Water hammer, also known as hydraulic shock, is a fluid dynamics phenomenon that occurs when flowing water in a pipeline is suddenly stopped (for example, by rapid valve closure) or forced to change direction. Due to the inertia of the fluid, its kinetic energy is instantaneously converted into pressure energy, generating a high-intensity pressure surge within the piping system.
This pressure surge propagates back and forth through the pipeline at a velocity close to the speed of sound in the liquid, often producing a loud knocking or banging noise. The effect is similar to striking the pipe with a hammer, which is the origin of the term “water hammer.”
I. Major Hazards of Water Hammer
● Pipeline damage: Severe pressure fluctuations can loosen pipe joints, cause cracking, or even lead to pipe rupture.● Equipment damage: Water hammer can seriously damage pumps, valves, flow meters, and other equipment connected to the piping system.
● Noise and vibration: It generates excessive noise and strong pipeline vibrations, negatively affecting system operation and the surrounding environment.
● Failure of supports: Long-term exposure to water hammer can weaken or even destroy pipe supports and hangers.
II. How to Prevent Water Hammer
The fundamental approach to preventing water hammer is to reduce the rate of change in flow velocity and absorb surge energy within the piping system. The following methods are widely used and proven effective in industrial applications.1. Operational Measures (Good Operating Practices)
Slow valve opening and closing:
This is the simplest and most critical preventive measure. Valves—especially gate valves and ball valves—should always be operated slowly during both opening and closing to avoid sudden flow interruption. For electric or pneumatic actuated valves, a longer opening and closing time should be configured.
Avoid sudden pump start-up and shutdown:
Using soft starters or variable frequency drives (VFDs) allows pump motors to ramp up or down smoothly, preventing abrupt acceleration or stoppage of flow that can trigger water hammer.
2. Equipment-Based Measures (Engineering Solutions)
These devices are specifically designed to eliminate or mitigate water hammer effects:
Installation of Water Hammer Arrestors / Surge Absorbers
● Principle:A water hammer arrestor is a pressure vessel with an internal gas bladder. When a pressure surge occurs, the gas bladder compresses, absorbing excess energy and reducing peak pressure.
● Typical applications:
Highly effective and often the first choice for resolving existing water hammer problems. Commonly installed near valves, pumps, or other locations prone to sudden flow changes.
Use of Slow-Closing Check Valves
● Principle:
Conventional check valves may close instantaneously when a pump stops, generating water hammer. Slow-closing check valves close quickly in the forward-flow direction but close gradually during reverse flow, either in stages or over a preset time, significantly reducing surge pressure.
● Typical applications:
Primarily installed at pump discharge lines to prevent backflow and water hammer during pump shutdown.
Installation of Air Valves / Vacuum Breaker Valves
● Principle:Air valves installed at high points in the pipeline release trapped air to prevent air pockets. In locations where negative pressure may occur, they admit air to prevent pipe collapse and provide an air cushion that helps dampen water hammer.
● Typical applications:
Long-distance transmission pipelines and systems with significant elevation changes.
Use of Surge Tanks / Pressure Regulating Towers
● Principle:Air valves installed at high points in the pipeline release trapped air to prevent air pockets. In locations where negative pressure may occur, they admit air to prevent pipe collapse and provide an air cushion that helps dampen water hammer.
● Typical applications:
Long-distance transmission pipelines and systems with significant elevation changes.
Ⅲ. Design-Level Prevention Measures
Proper piping system design:● Pipe diameter:
While meeting the required flow capacity, increasing the pipe diameter appropriately helps reduce flow velocity. Lower flow velocity directly results in a weaker water hammer effect.
● Pipe material:
Pipes with inherent elasticity—such as PE or PVC-U—can absorb part of the surge energy through elastic deformation, making them more effective at mitigating water hammer than rigid materials like steel or copper pipes.
● Pipeline layout:
Sharp bends and U-shaped configurations should be avoided to minimize abrupt changes in flow direction. For long-distance pipelines, proper anchoring and support spacing are essential to withstand vibration and mechanical stress induced by water hammer.
