Water Hammer Pressure Surge Calculator
Estimate the pressure surge when a valve closes quickly or a pump trips, using the Joukowsky equation with the wave speed corrected for pipe elasticity.
- c = √[(K/ρ) ÷ (1 + K D ÷ (E e))] = 1,278 m/s
- ΔP = ρ × c × Δv = 1,000 × 1,278 × 2 = 25.56 bar
- Closures faster than 2L/c = 0.7823 s produce the full surge
Joukowsky Equation
c = √[(K/ρ) ÷ (1 + K D ÷ (E e))]
tc = 2L ÷ c
K is the bulk modulus of the liquid (2.2 GPa for water), E the elastic modulus of the pipe wall, D the diameter and e the wall thickness. Closing slower than tc reduces the surge roughly in proportion.
Worked example
Stopping water flowing at 2 m/s in a 200 mm steel pipe (6.35 mm wall): c ≈ 1,280 m/s and ΔP ≈ 25.6 bar, on top of the working pressure.
Frequently Asked Questions
What causes water hammer?
A sudden change in flow velocity, such as fast valve closure or a pump trip, sends a pressure wave along the pipe.
How fast can I close a valve without water hammer?
Slower than the critical time 2L/c, and preferably several times slower. Long pipelines need very slow final closure.
Can water hammer damage instruments?
Yes. Pressure transmitters and gauges can be overranged or damaged; use snubbers and choose an adequate overpressure rating.
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