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How It Works
Water hammer (H = a × ΔV / g) calculates the maximum pressure surge when fluid flow is suddenly stopped or redirected. The wave velocity (a) depends on pipe material and fluid properties; in steel water pipes it ranges from 900–1,400 m/s. The resulting pressure spike can burst pipes, so engineers use slow-closing valves, surge tanks, and arrestors to mitigate it.
Example Problem
A steel water pipe has a wave velocity of 1,200 m/s. A valve closes suddenly, stopping flow from 2 m/s to 0. What is the surge pressure head?
- H = a × ΔV / g = 1,200 × 2 / 9.81
- H = 244.6 m (about 24 bar or 348 psi above normal)
This extreme pressure can easily rupture unprotected piping.
Frequently Asked Questions
What causes water hammer in pipes?
The most common cause is sudden valve closure, which converts fluid momentum into a pressure wave. Pump shutdowns, quick-acting solenoid valves, and trapped air pockets also trigger it.
How do you prevent water hammer?
Use slow-closing valves, install surge tanks or water hammer arrestors, maintain proper pipe support, and size pipes to keep velocities below 3 m/s. Variable-speed pumps with soft start/stop reduce transient pressures significantly.
What is pressure wave velocity in a pipe?
It depends on the fluid's bulk modulus and the pipe's elasticity. Steel pipes produce waves at 900–1,400 m/s; PVC pipes are slower at 300–600 m/s, which reduces surge pressure proportionally.
Related Calculators
- Fluid Pressure Calculator — calculate the static pressure component in pipe systems.
- Darcy-Weisbach Calculator — determine friction losses in the piping system.
- Pipe Flow Calculator — compute Reynolds number and flow velocity for surge analysis.
- Sound Wave Calculator — calculate the speed of the pressure wave through the pipe fluid.
- Pressure Unit Converter — convert surge pressure between psi, kPa, and bar.