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Water Hammer Pressure Calculator

Work out water hammer pressure instantly with clear inputs, formula shown and shareable results.

About Water Hammer Pressure Calculator

The Joukowsky equation gives the maximum surge for instantaneous closure as rho x wave speed x velocity change. Closure counts as rapid if it happens within the critical time 2L/a, the time for the pressure wave to travel to the reservoir and back. Slower closure reduces the surge roughly in proportion.

Formulas

Joukowsky surge

Δp = rho x a x Δv

Critical time

Tc = 2L / a; slow closure surge = full surge x Tc / closure time

Worked example

With the default values below, Water Hammer Pressure Calculator returns Surge pressure rise of 2,000.0. Change any input above and every figure here is recalculated in your browser.

Inputs used

Initial flow velocityminimum 0.01 m/s
2 m/s
Pressure wave speedminimum 100 m/s
1,200 m/s
Pipe length to the reservoirminimum 0.1 m
500 m
Valve closure timeminimum 0.01 s
1 s

Results

Surge pressure rise
2,000.0 kPa
Surge head rise
203.94 m
Critical closure time
0.833 s
Closure assessment
Slow closure — surge reduced in proportion to closure time

Step by step

  1. 1Values used
  2. 2Joukowsky surge
  3. 3Critical time
  4. 4Surge pressure rise= 2,000.0 kPa
  5. 5Surge head rise= 203.94 m
  6. 6Critical closure time= 0.833 s
  7. 7Closure assessment= Slow closure — surge reduced in proportion to closure time

Assumptions and limits

  • The result is derived only from the 4 inputs listed above. Anything not among them is not part of the calculation.
  • In particular, this calculator does not take inflation, tax, fees or charges or compounding frequency into account.
  • Figures are rounded for display to 3 decimal places, so totals built from several rounded parts can differ slightly from the unrounded arithmetic.
  • Preliminary estimate only. Surge analysis for pumping mains and long pipelines should be carried out with transient modelling by a qualified engineer.

How these calculators are built and tested is described on the Methodology page. If a figure looks wrong, please report it.

Preliminary estimate only. Surge analysis for pumping mains and long pipelines should be carried out with transient modelling by a qualified engineer.

Frequently Asked Questions

How is the Water Hammer Pressure figure worked out?

Δp = rho x a x Δv. The Joukowsky equation gives the maximum surge for instantaneous closure as rho x wave speed x velocity change.

How do I reduce water hammer?

Slow the valve closure beyond the critical time, lower the operating velocity, shorten the pipe run, or install a surge vessel or air chamber to absorb the wave.

What values do I need to enter?

This calculator takes 4 inputs: Initial flow velocity, Pressure wave speed, Pipe length to the reservoir, Valve closure time. The pre-filled defaults are a realistic worked example — replace them with your own site or project figures.

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