**4. Modelling of water supply systems for transient analyses**

This section provides some guidelines on the modelling of a pipeline system with respect to pressure surge calculations.

It is recommended to model the top of the pipes in computer models, because the dynamic behaviour may change significantly at low pressures due to gas release or cavitation.

The modelling and input uncertainties raise the question of which model parameter values should be applied in a particular simulation. The simulation results may be too optimistic if the model parameters are selected more or less arbitrarily. The model parameters should be selected such that the relevant output variables get their extreme values; this is called a con‐ servative modelling approach. The conservative choice of input parameters is only possible in simple supply systems without active triggers for control procedures. Table 4 lists the pa‐ rameter choice in the conservative modelling approach.

Normal operating procedures should not trigger emergency controls. If this is the case, the con‐ trol system or even the anti-surge devices may have to be modified. As a general rule for normal operations, discharge set-points in control systems tend to exaggerate transient events while pressure set-points automatically counteract the effect of transients. Two examples are given. The first deals with a single pipeline used to fill a tank or supply reservoir. Suppose a down‐ stream control valve is aiming for a certain discharge set-point to refill the tank or reservoir. If an upstream pump trip occurs, the control logic would lead to valve-opening in order to maintain the discharge set-point. This will lower the minimum pressures in the pipe system between the pumping station and the control valve. On the other hand, if the control valve aims for an up‐ stream pressure set-point, the valve will immediately start closing as soon as the downsurge has

arrived at the valve station, thereby counteracting the negative effect of the pump trip.

speed, so that the loss of one pumping stations is compensated by the three others.

during commissioning may indicate whether the system is properly de-aerated.

**4. Modelling of water supply systems for transient analyses**

pressure gradients.

pressure surge calculations.

18 Water Supply System Analysis - Selected Topics

The simulation of the normal operating procedures provides detailed knowledge on the dy‐ namic behaviour of the WSS. This knowledge is useful during commissioning of the (modi‐ fied) system. For example, a comparison of the simulated and measured pressure signals

It is emphasized that a simulation model is always a simplification of reality and simulation models should be used as a decision support tool, not as an exact predictor of reality. The design engineer of complex WSS must act like a devil's advocate in order to define scenarios that have a reasonable probability of occurrence and that may lead to extreme pressures or

This section provides some guidelines on the modelling of a pipeline system with respect to

It is recommended to model the top of the pipes in computer models, because the dynamic

The modelling and input uncertainties raise the question of which model parameter values should be applied in a particular simulation. The simulation results may be too optimistic if

behaviour may change significantly at low pressures due to gas release or cavitation.

The second example is a distribution network in which four pumping stations need to main‐ tain a certain network pressure. The pumping stations have independent power supply. Suppose that three pumping stations follow a demand prediction curve and the fourth pumping station is operating on a set-point for the network pressure. If a power failure oc‐ curs in one of the discharge-driven pumping stations, then the network pressure will drop initially. As a consequence the pump speed of the remaining two discharge-driven pumping stations will drop and the only pressure-driven pumping station will compensate tempora‐ rily not only the failing pumping station, but also the two other discharge-driven pumping stations. If all pumping stations would be pressure-driven pumping stations, then the fail‐ ure of a single pumping station will cause all other pumping stations to increase their pump


**Table 4.** Overview of conservative modelling parameters for certain critical scenarios and output criteria.

If control systems are triggered to counteract the negative effect of critical scenarios (pump trip, emergency shut down), then the extreme pressures may occur at other combinations of input parameters than listed in Table 4. Therefore, a sensitivity analysis or optimisation rou‐ tine is strongly recommended to determine extreme pressures in these kind of complex wa‐ ter supply systems.

[2] Jung, B. S. and B. W. Karney (2009). "Systematic surge protection for worst-case tran‐ sient loadings in water distribution systems." Journal of Hydraulic Engineering

Guidelines for Transient Analysis in Water Transmission and Distribution Systems

http://dx.doi.org/10.5772/53944

21

[3] NEN (2012). Requirements for pipeline systems, Part 1 General. NEN, NEN.

[4] Pejovic, S. and A. P. Boldy (1992). "Guidelines to hydraulic transient analysis of

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[6] Pothof, I. W. M. and G. McNulty (2001). Ground-rules proposal on pressure transi‐ ents. Computing and Control for the Water Industry. Leicester, RSP Ltd, England.

[7] Streeter, V. L. and E. B. Wylie (1993). Fluid transients in systems. New York, Pren‐

[8] Thorley, A. R. D. (2004). Fluid Transients in Pipeline Systems. London, UK, Profes‐

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