Overview
- Water hammer occurs when rapidly closing downstream valves in pipes, causing pressure surges that may burst pipes. Its severity depends on water flow and head difference.
- Not limited to water, any fluid pipeline, including gas and liquid, can experience water hammer. Mitigation methods include water towers and pressure relief valves.
- The process involves pressure wave propagation and water flow oscillations, ultimately halting flow. To reduce risk, avoid sudden changes in water velocity during operations.
Cause and effect
Rapid Valve Closure
When a pipe is suddenly closed at the outlet (downstream), the mass of water before the closure is still moving, thereby building up high pressure and a resulting shock wave. In domestic plumbing this is experienced as a loud banging resembling a hammering noise. Water hammer can cause pipelines to break if the pressure is high enough. Air traps or stand pipes (open at the top) are sometimes added as dampers to water systems to absorb the potentially damaging forces caused by the moving water.
Vacuum Creation from Valve Closure
On the other hand, when an upstream valve in a pipe closes, water downstream of the valve attempts to continue flowing creating a vacuum that may cause the pipe to collapse or implode. This problem can be particularly acute if the pipe is on a downhill slope . To prevent this, air and vacuum relief valves or air vents are installed just downstream of the valve to allow air to enter the line to prevent this vacuum from occurring.
Pump Failure and Check Valve Slam
Other causes of water hammer are pump failure and check valve slam (due to sudden deceleration, a check valve may slam shut rapidly, depending on the dynamic characteristic of the check valve and the mass of the water between a check valve and tank). To alleviate this situation, it is recommended to install non-slam check valves as they do not rely on gravity or fluid flow for their closure. For vertical pipes, other suggestions include installing new piping that can be designed to include air chambers to alleviate the possible shockwave of water due to excess water flow.
Mitigating measures
- Reduce the pressure of the water supply to the building by fitting a regulator. Lower fluid velocities.
- To keep water hammer low, pipe-sizing charts for some applications recommend flow velocity at or below 1.5 m/s (4.9 ft/s)
- Fit slowly closing valves.
- Air valves often remediate low pressures at high points in the pipeline. Though effective, sometimes large numbers of air valves need be installed. These valves also allow air into the system, which is often unwanted.
- Shorter branch pipe lengths.
- Shorter lengths of straight pipe, i.e. add elbows, expansion loops. Water hammer is related to the speed of sound in the fluid, and elbows reduce the influences of pressure waves.
- Arranging the larger piping in loops that supply shorter smaller run-out pipe branches. With looped piping, lower velocity flows from both sides of a loop can serve a branch.
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Positive Water Hammer and Negative Water Hammer
Water hammer can be classified into positive water hammer and negative water hammer. The commonly discussed water hammer effect refers to the positive water hammer, as introduced in the previous section. Negative water hammer, on the other hand, occurs when the pressure inside the pipeline drops below atmospheric pressure, causing reverse water flow or the formation of low-pressure zones within the pipeline. Below is a simple comparison of the two:
Positive Water Hammer
Positive water hammer occurs when water flow is suddenly stopped, converting the kinetic energy of the water into pressure. This creates a strong pressure wave that can be imagined as water "crashing" against the inner walls of the pipeline when its flow is abruptly blocked, generating a large pressure surge. Positive water hammer not only produces loud noises but may also cause pipe ruptures, valve damage, or equipment failure.
Common scenarios:
• Opening or closing household water taps in plumbing systems
• Closing valves in industrial pipeline systems
• Sudden shutdowns of pumps in municipal water supply systems
Negative Water Hammer
Negative water hammer occurs when water flow stops or changes direction suddenly, and the pressure inside the pipeline rapidly drops below atmospheric pressure. This creates a negative pressure wave, leading to a sharp decrease in internal pressure. It may cause cavitation, where dissolved gases or water vapor in the water form bubbles. These bubbles collapse abruptly when the pressure recovers, generating powerful shock waves that can erode or damage the inner walls of the pipeline and potentially cause ruptures.
Common scenarios:
• Siphonic drainage systems
• Water supply systems in high-rise buildings
• Agricultural irrigation systems experiencing pressure drops, reverse flow, or cavitation formation