Transients advisor

InfoWorks WS Pro includes a simulation option allowing production of additional results, which can be used as an indicator of potential problem areas due to transient pressures.

To turn the option on, turn on the Advise on Hydraulic Transients option on the Simulation Options dialog.

This topic discusses the mechanics of pressure wave propagation and the calculations carried out by InfoWorks WS Pro.

Hydraulic transients

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

Causes of transients in a water supply network include:

• Starting / stopping of pumps
• Opening / closing of valves
• Outage of pumps / pipes due to a power failure / burst

Transients may cause problems in the water supply network in several ways :

• Pipe bursts caused by pressures induced exceeding pressure rating of pipe
• Hydraulic oscillations resulting in weakening of pipe
• Potential water quality problems caused by low pressures causing a temporary vacuum

Example

Consider the pipe upstream of a valve that has been instantaneously closed:

 

1. As the valve is closed, the water adjacent to the valve is brought to rest with consequent compression of water upstream of the valve and expansion of the pipe cross section upstream of the valve. The increase in pressure at the valve causes the generation of a pressure wave with positive excess pressure head (DH) which is propagated upstream.
2. The pressure wave reaches the reservoir.
3. Water is forced backwards into the reservoir. Velocity in the pipe reverses and pressure drops back to static (reservoir pressure) again. The pressure wave is reversed and propagated back towards the closed valve.
4. As the pressure wave reaches the closed valve, a low pressure region forms at the valve, destroying flow and generating a pressure reducing wave which is propagated towards the reservoir.
5. Pressure reducing wave reaches the reservoir. Water flows from reservoir into pipe, velocity in pipe is reversed and a pressure generating wave is propagated towards the closed valve.

The cycle is repeated with the pressure oscillations being damped within a short period of time by friction.

Transients advice in InfoWorks WS Pro

Additional transient pressure amplitude and maximum transient pressure results may be produced when running a simulation, which can be used as an indicator of potential problem areas due to transient pressures.

To turn the transient advice option on, set the Advise on Hydraulic Transients option on the Simulation Options dialog.

The transient pressure result displays the amplitude of the pressure wave and is calculated using Joukowski's law:

Transient pressure DH at instantaneous valve closure in a rigid pipe: (1) where: g = acceleration due to gravity Dv = change in fluid velocity c = wave celerity

Transient pressure amplitude is calculated for each timestep (t) where:

• Change in fluid velocity (DV) =
• velocity at previous timestep - velocity at current timestep

Therefore the transient pressure at the first simulation timestep will be zero.

• Wave celerity (c) =
• wave celerity value defined in links grid view
• default value of 1400 m/s if value not specified in grid view

Setting wave celerity

Wave celerity is dependant on the elasticity of the fluid and of the pipe itself and on pipe external restraints.

Wave celerity can be automatically set based on pipe materials from wave celerity values set in the Pipe Materials dialog, (see Setting wave celerity for more details). However, this functionality allows only one wave celerity value per pipe material to be set from the Pipe Materials dialog.

Wave celerity values can also be entered manually in the links grid view. Wave celerity values can be determined from the graph and table below (from Fluid Transients in Pipeline Systems, Thorley ARD, 1991), which contain curves of wave celerity against diameter/wall thickness ratio for several pipe materials.

Wave Celerity Diagram

Diameter / Wall Thickness ( D/e)	Steel	Ductile Iron	Cast Iron	Concrete	Asbestos-Cement	uPVC
5	1400	1400	1360	1230	1200	550
10	1365	1355	1290	1100	1040	400
20	1310	1290	1190	930	860	290
30	1260	1230	1100	820	750	240
40	1210	1180	1040	740	680	200
50	1170	1130	980	680	620	180
60	1130	1090	930	630	580	168
70	1095	1055	890	591	540	157
80	1060	1020	848	560	510	145
90	1030	995	820	530	485	135
100	1005	965	790	510	465	125
110	985	940	765	490	440	118
120	965	920	745	475	421	110
130	940	895	720	455	405	105
140	920	880	700	440	395	103
150	900	860	680	425	385	100

Table of Wave Celerity Curves (Thorley ARD 1991)

Analysing results

The transient advisor results can be displayed as for other results: in a grid report or as a graph of the time varying result for a single network object.

Transient pressure amplitude will be graphed as smooth continuous lines joining transient pressure values. Show image

In reality transient pressure changes occur over a very short time period (seconds). Show image

The smooth graph traces produced by InfoWorks WS Pro represent an approximate bounding envelope for the real, spiky transients.