Water hammer is the pressure spike that travels through a pipe when flowing liquid is suddenly stopped or redirected. The moving fluid has momentum; when a valve slams shut, that momentum has nowhere to go and converts into a pressure wave that hammers back and forth between the closure and the nearest reservoir or expansion. The bang you hear in old plumbing when a washing machine valve snaps closed is water hammer in action — and at industrial scale, it ruptures pipes, cracks pump casings, and destroys check valves.

The physics: The Joukowsky equation gives the maximum pressure rise for an instantaneous valve closure:

ΔP = ρ · a · Δv

• ρ = fluid density (~1000 kg/m³ for water)
• a = wave speed in the pipe (~1200 m/s in steel, ~400 m/s in PVC)
• Δv = change in fluid velocity (m/s)

Rule of thumb: In a steel water line, every 1 m/s of velocity change produces roughly 1.2 MPa (175 psi) of pressure spike if closed instantly. A 3 m/s flow stopped abruptly creates ~525 psi above static pressure — easily enough to exceed Schedule 40 steel's working rating.

Closure timing matters. A closure is "sudden" only if it happens faster than the pipe's critical time: T_c = 2L/a, where L is pipe length to the nearest reservoir. For a 600 m steel main, T_c ≈ 1 second. Close the valve in 0.5 s and you get the full Joukowsky spike; close it over 5 seconds and the pressure rise drops dramatically because the reflected wave returns before closure completes.

Real-world example: Municipal pump stations are notorious for water hammer during power failure. A 200 HP pump pushing 2 m/s through a 1 km cast iron main loses power — the check valve slams shut on reverse flow, generating a spike that has snapped cast iron mains and destroyed entire stations. Mitigation: surge tanks (air cushion absorbs the spike), slow-closing check valves (controlled closure over several seconds), and pressure relief valves set above working pressure but below burst.

Design implications:

• Specify valve closure times based on pipe length, not convenience
• Avoid quick-acting solenoid valves on long high-velocity lines
• Add air chambers or arrestors near washing machines, dishwashers, and any fast-acting solenoid
• For pump discharge lines, always analyze the worst-case power-loss transient — that's when hammer kills systems