Water Hammer Calculator 🔨

Joukowsky Equation and Wave Speed

In rigid steel pipe the wave travels at about 1,200–1,400 m/s, so stopping 2 m/s of flow instantly adds about 25–28 bar. In flexible PVC or HDPE the wave is far slower (250–450 m/s), which is why plastic pipes see lower surges. The full Joukowsky surge applies whenever the closure is faster than the critical time 2L/a; slower closures reduce it roughly in proportion (Δp ≈ 2ρLv/t).

How to Control Water Hammer

• Close valves slowly (slow-closing or gear-operated valves, soft-start pumps or VFD ramp-down).
• Use silent (spring) check valves on pump discharges.
• Install water hammer arresters near quick-closing solenoid valves and flush valves (PDI-WH201 sizing).
• Keep velocities moderate; see the Water Pipe Pressure Drop Calculator.

FAQ

How do you calculate water hammer pressure?

With the Joukowsky equation Δp = ρ × a × Δv: water density × pressure wave speed × change in velocity. For water in steel pipe, each 1 m/s stopped suddenly adds roughly 12–14 bar.

What is the critical closure time?

2L ÷ a, the time for the pressure wave to travel to the pipe end and back. Closing a valve faster than this produces the full surge; slower closures reduce it.

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