Water Hammer by the Numbers

About this book

Close a valve on a moving column of water and the water cannot stop all at once. The front of the column stops first, its pressure jumps, and that jump runs back up the pipe as a wave at over a kilometre a second. A few seconds later it has reflected, reversed and returned. Pressure surges of this kind, called water hammer, burst pipes, split fittings, lift pipes off their supports, slam check valves and pull water apart into vapour pockets that collapse with a second, harder blow.

This book treats water hammer as what it is: a pressure wave in a long, elastic tube of slightly compressible liquid. It starts from the wave speed, uses the Joukowsky relation to size the instant surge, adds reflections and the pipe period to judge valve closure and pump trip, and then draws the head envelopes along a real profile to find where pressures go too high or too low. The last chapters cover protection, pulsation in branches and the method of characteristics that a full surge study uses.

It serves mechanical, process and maintenance engineers, pump and pipework designers and project engineers who specify, commission or troubleshoot pumped water lines. You need comfortable algebra, SI units, the idea of hydraulic head, and a spreadsheet. The book uses metres of head throughout, with pressures in kPa or MPa alongside where it helps; one metre of water is about 9.79 kPa at the temperatures used here.

The running case

Every chapter works on one fictional pipeline. A pump station at a dam lifts process water to a header tank on a hill, 900 m away, through a steel main of 168.3 mm outside diameter and 7.1 mm wall, giving a 154.1 mm bore. The pump sits at reduced level (RL) 100 m and the tank’s water level is RL 142 m. The line rises steeply at first, flattens near a high point at chainage 600 m, and ends at a motorised inlet valve at RL 141 m beside the tank. At 30 L/s the water moves at 1.61 m/s, and the pump delivers a head of 58 m. Valves and flanged fittings on the line are rated PN16.

This is an illustration. All dimensions, levels, flows, ratings, times and results are invented for teaching and checked by calculation. They are not KEVOS® work, a real site, or a measured system. The book’s transient results come from a short method-of-characteristics model written for this case and described in Chapter 10. It simplifies the pump as a fixed head and omits some effects real studies include.

Source limits and professional responsibility

Surge in a real pipeline depends on details this book simplifies: the pump’s full characteristics and inertia, valve curves, air content, pipe restraint, friction during transients, and vapour cavities. Hand methods give a first estimate and show which questions matter. They do not replace a surge analysis by a competent engineer using a validated program, with the actual pipe, pump and valve data.

This book is educational. Pressure pipelines store large amounts of energy and fail violently. Design, modification and commissioning of pumped mains, surge vessels and pressure equipment are governed by the relevant Australian Standards, the requirements of the water authority or asset owner where they apply, and the model Work Health and Safety laws as adopted in your state or territory, which include registration requirements for certain pressure equipment. Testing a pipeline’s transient behaviour on purpose, for instance by tripping a pump, must be planned and approved, with people kept clear of the line.