Water Hammer Arrestor Calculation: The Ultimate Guide for Preventing Pipe Damage

Water hammer is a sudden surge of pressure in a piping system caused by the rapid change in flow velocity. This phenomenon can lead to serious damage to pipes, valves, and other system components. To prevent water hammer, water hammer arrestors are installed in the system. Calculating the correct size of a water hammer arrestor is crucial for ensuring its effectiveness. This blog post provides a comprehensive guide on how to calculate water hammer arrestors.

Understanding Water Hammer

Water hammer occurs when a valve is suddenly closed, causing the flow of water to stop abruptly. The momentum of the moving water creates a pressure surge that travels through the piping system. This surge can reach several times the normal operating pressure, potentially causing damage to the system.

Function of a Water Hammer Arrestor

A water hammer arrestor is a device designed to absorb the pressure surge caused by water hammer. It consists of a diaphragm or piston that separates a chamber filled with gas or air from the water in the piping system. When a pressure surge occurs, the diaphragm or piston moves to absorb the excess pressure, preventing it from damaging the system.

Calculating Water Hammer Arrestor Size

The size of a water hammer arrestor is determined by the following factors:

• Maximum flow rate: The maximum flow rate through the piping system.
• Pipe length: The length of the piping system in which the water hammer arrestor will be installed.
• Pipe diameter: The diameter of the piping system.
• Valve type: The type of valve causing the water hammer (e.g., gate valve, ball valve).

Step-by-Step Calculation Process

1. Determine the maximum flow rate: Measure or calculate the maximum flow rate through the piping system.
2. Calculate the pressure surge: Use the following formula to calculate the pressure surge:

“`
ΔP = ρ * V * C
“`

where:

• ΔP is the pressure surge (psi)
• ρ is the density of water (62.4 lb/ft³)
• V is the maximum flow rate (ft/s)
• C is the speed of sound in water (4,720 ft/s)

3. Determine the required arrestor volume: Use the following formula to calculate the required arrestor volume:

“`
V_arrestor = Q * ΔP / (P_max – P_min)
“`

where:

• V_arrestor is the required arrestor volume (ft³)
• Q is the maximum flow rate (ft³/s)
• ΔP is the pressure surge (psi)
• P_max is the maximum operating pressure (psi)
• P_min is the minimum operating pressure (psi)

4. Select an arrestor with the appropriate volume: Choose a water hammer arrestor with a volume that is equal to or greater than the calculated required volume.

Additional Considerations

• Location: Install the water hammer arrestor as close as possible to the source of the water hammer (e.g., valve).
• Orientation: Install the water hammer arrestor vertically with the gas or air chamber on top.
• Maintenance: Regularly inspect and maintain the water hammer arrestor to ensure proper functionality.

Wrapping Up

Calculating the correct size of a water hammer arrestor is essential for preventing damage to piping systems caused by water hammer. By following the steps outlined in this guide, you can ensure that the arrestor you select will be effective in protecting your system.

What People Want to Know

Q: How often should I replace a water hammer arrestor?
A: The frequency of replacement depends on the specific arrestor and system conditions. Generally, it is recommended to replace arrestors every 5-10 years.

Q: Can I use a smaller water hammer arrestor than the calculated size?
A: No, using a smaller arrestor may not provide adequate protection against water hammer.

Q: What is the difference between a diaphragm-type and a piston-type water hammer arrestor?
A: Diaphragm-type arrestors use a flexible diaphragm to absorb pressure surges, while piston-type arrestors use a movable piston. Both types are effective, but diaphragm-type arrestors are generally more compact.