Understanding hydraulic transients

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Hydraulic transients (also known as water hammer or surge) are pressure fluctuations caused by a change in fluid velocity.

00:11

A rapid change in flow causes elastic strain of the fluid and pipe.

00:16

Conservation of energy means that the kinetic energy of the flow converts to a pressure wave,

00:21

which then travels along the pipe at high speed.

00:24

The wave speed is determined by the stiffness of the pipe material and the viscosity of the fluid inside.

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Relatively soft pipe materials like medium-density polyethylene MDPE have lower wave speed and tend to weaken the wave energy.

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Stiffer materials like iron have high wave speeds and allow the transmission of more energy over greater distances.

00:45

The magnitude of the event is directly proportional to the change in inertia of the flowing fluid.

00:51

If a wave is powerful enough, the accompanying pressure could exceed the pressure rating of the pipeline.

00:57

This has the potential to increase its stress and shorten its life.

01:02

Sub-atmospheric drops in pressure are a concern,

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as they could collapse polyethylene pipes or draw water through perforations in the pipework.

01:10

Any event that causes the velocity of the water in a network to increase or decrease rapidly

01:16

can cause a pressure surge or a water hammer.

01:19

The most common sources of surge events are pump stations and valves.

01:24

InfoWorks TS, or Transient System, uses the Wave Characteristic Method and calculates the results at the nodes within the network.

01:33

The method of characteristics requires calculations at equally spaced interior points in a pipeline.

01:40

The objective in transient control is to reduce the magnitude (and if possible, the frequency) of the pressure disturbance.

01:47

Avoiding sudden changes will reduce the likelihood of transient generation.

01:51

Closing valves slowly, particularly during the final phase, keeps transient events under control.

01:57

Slowly starting and stopping pumps has a similar effect.

02:00

Pump, power, and valve control failures can put pipelines at risk, as can sudden high demands (like opening hydrants).

02:08

Implementing certain devices in a network can alleviate the effects of pressure waves

02:13

by releasing and replenishing excess positive and negative pressure.