# Water Hammers

## Rapidly closing or opening valves may cause pressure transients in pipelines known as water hammers

A Water Hammer is a pressure change caused by a variation of the flow rate in a pipe or tube. Waterhammers are created by suddenly starts or stops of liquid flows. Since water flows are restricted inside pipes, shock waves of incompressible water will travel back down the pipes deflecting everything in the path.

Pressure transients in pipe lines caused by a shock wave when closing or opening a valve can be calculated as

dp = 0.070 dv l / t                                      (1)

where

dp = increase in pressure (psi)

dv = change in flow velocity (ft/s)

t = valve closing time (s)

l = upstream pipe length (ft)

• 1 ft (foot) = 0.3048 m
• 1 ft/s = 0.3048 m/s
• 1 psi (lb/in2) = 6894.8 Pa (N/m2)

### Example - Water Hammer generated when closing a Solenoid Valve

The pressure increase (water hammer) in a 100 ft water pipe where water flow velocity is reduced from 6 ft/s to 0 ft/s when a solenoid valve close in 0.1 s can be calculated as

dp = 0.070 ((6 ft/s) - (0 ft/s)) (100 ft) / (0.1 s)

= 420 (psi)

With a closing time of 1 s (solenoid valve with damper) - the pressure increase (water hammer)  can be calculated as

dp = 0.070 ((6 ft/s) - (0 ft/s)) (100 ft) / (1 s)

= 42 (psi)

Note! - it is important to open and close valves slowly and use soft starters to start and stop pumps to avoid damaging water hammers in piping systems.

## Related Topics

• Water Systems - Hot and cold water service systems - design properties, capacities, sizing and more
• Pressure Ratings - Pressure ratings of pipes and tubes and their fittings - carbon steel , stainless steel, plastic, copper and more

## Search the Engineering ToolBox

- "Search is the most efficient way to navigate the Engineering ToolBox!"

## Engineering ToolBox - SketchUp Extension - Online 3D modeling!

Add standard and customized parametric components - like flange beams, lumbers, piping, stairs and more - to your SketchUp model with the Engineering ToolBox - SketchUp Extension/Plugin - enabled for use with the amazing, fun and free SketchUp Make and SketchUp Pro . Add the Engineering ToolBox extension to your SketchUp from the Sketchup Extension Warehouse!

Translate the Engineering ToolBox!