what it is, how to prevent it, how to calculate the max pressure (2024)

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1. Introduction
2. Consequences of a water hammer

1. Introduction

What is the "water hammer" phenomenon ?

Water hammer, also known as hydraulic shock or surge, is a sudden pressure increase or shockwave that occurs within a fluid-carrying pipe system when there is a rapid change in flow velocity. It happens due to the inertia of the moving fluid and the compressibility of water. The phenomenon often results from abrupt valve closures (including check valves), pump starts and stops, or sudden changes in flow direction.

Figure 1 : Water hammer phenomenon

2. Consequences of a water hammer

When a flow is suddenly halted or redirected, the kinetic energy of the moving water transforms into pressure energy, causing pressure spikes that can damage pipes, valves, and other system components. The resulting shockwave travels through the piping system, creating a distinct banging noise.

3. Water hammer pressure rise calculation

The pressure rise that will happen due to water hammer when there is a sudden stop of the flow (very quick valve or check valve closure for instance) can be calculated thanks to the following formula [Hall] :

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ΔP_h = ρ * c_s * u

With :

ΔP_h = increase of pressure due to water hammer (Pa)
ρ = fluid density (kg/m3)
c_s = velocity of sound in the fluid (m/s) = 1439 m/s for water (change if another fluid)
u = fluid velocity (m/s)

It's important to note that water hammer can have serious consequences, including pipe bursts, equipment damage, and system failures.

Note the formula above assumes that the velocity of the fluid will go to 0 *(hence expressing with u), other sources are considering a difference in velocity (Δu) that could be useful for change of directions for instance.

Another formula is proposed when slow closure of a valve is happening :

P₂ = 2 * ρ * L * u / t + P₁

With :

P₂ = pressure reached during water hammer phenomena (Pa)
P₁ = inlet pressure when the closure happens (Pa)
ρ = fluid density (kg/m3)
u = fluid velocity (m/s)
L = upstream pipe length (m)
t = valve closure time (s)

This formula is approximate [Perry], apply safety factors.

4. Prevention of water hammer

The following designn considerations can help to prevent the water hammer to happen :

• Respect recommended fluid velocities (please check those resources : max recommended velocity in pipe)
• Control the speed at which valves are closing
• Install surge tanks that will help to stay at a given flowrate and will also absorb the pressure shocks
• Employing air chambers, typically in the form of tee-fittings with air-filled chambers, can act as shock absorbers, reducing the shock's impact on the pipeline during sudden flow changes
• Chosing the right type of check valve. It's crucial to consider the type of check valve used in the system. Swing, tilting disc, or piston-style check valves, which rely on gravity and flow reversal to close, can contribute to water hammer by causing water to slam into the valve mechanism, generating pressure waves. Silent or spring-assisted check valves, equipped with internal springs, offer a quieter solution by closing before flow reversal, minimizing water hammer risk
• Install Steam Lines with a Gradual Slope : The water hammer effect in steam lines occurs from condensation accumulation and pooling. You can get a steam line installed with a gradual slope toward the flow as a way to combat the water hammer effect.

5. Water Hammer Pressure rise Excel calculator

How to calculate the pressure rise during water hammer ?

You can access to an Excel calculation tool in order to calculate the pressure rise in a pipe when the phenomenon of water hammer happens : Water Hammer Pressure rise Excel calculation tool (click here)

Warning : this calculator is provided to illustrate the concepts mentioned in this webpage, it is not intended for detail design. It is not a commercial product, no guarantee is given on the results. Please consult a reputable designer for all detail design you may need.

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