*3.3.1. System modifications*

Measure 1 is only feasible in an early stage. A preliminary surge analysis may identify costeffective measures for the surge protection that cannot later be incorporated. If, for example, inadmissible pressures occur at a local high point that seem difficult to mitigate, the pipe routing may be changed to avoid the high point. Alternatively, the pipe may be drilled through a slope to lower the maximum elevation.

where else in the network. The control systems may have a positive or negative effect on the propagation of hydraulic transients. The distributed nature of WSS and the presence of con‐ trol systems may be exploited to counteract the negative effects of emergency scenarios.

Guidelines for Transient Analysis in Water Transmission and Distribution Systems

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If a centralised control system is available, valves may start closing or other pumps may ramp up as soon as a pump trip is detected. Even without a centralised control system, emergency control rules may be developed to detect power failures. These emergency con‐ trol rules should be defined in such a way that false triggers are avoided during normal op‐ erations. An example of an emergency control rule is: *ESD valve closure is initiated if the discharge drops by more than 10% of the design discharge and the upstream pressure falls by at least*

The above-described measures may be combined with one or more of the following anti-

**velocity change in time Pressure limiting devices** Surge vessel By-pass check valve Flywheel Pressure relief valve Surge tower Combination air/vacuum valves

An important distinction is made in Table 2 between anti-surge devices that directly af‐ fect the rate of change in velocity and anti-surge devices that are activated at a certain condition. The anti-surge devices in the first category immediately affect the system re‐ sponse; they have an overall impact on system behaviour. The pressure-limiting devices generally have a local impact. Table 3 lists possible measures when certain performance

The surge vessel is an effective (though relatively expensive) measure to protect the system downstream of the surge vessel against excessive transients. However, the hydraulic loads in the sub-system between suction tanks and the surge vessel will increase with the installa‐ tion of a surge vessel. Special attention must be paid to the check valve requirements, be‐ cause the fluid deceleration may lead to check valve slam and consequent damage. These local effects, caused by the installation of a surge vessel, should always be investigated in a detailed hydraulic model of the subsystem between tanks and surge vessels. This model may also reveal inadmissible pressures or anchor forces in the suction lines and headers, es‐ pecially in systems with long suction lines (> 500 m). A sometimes-effective measure to re‐ duce the local transients in the pumping station is to install the surge vessels at a certain

Feed tank

*0.5 bar within 60 seconds.*

*3.3.4. Anti-surge devices*

surge devices in municipal water systems.

**Devices, affecting**

**Table 2.** Summary of anti-surge devices

distance from the pumping station.

criteria are violated.

Selection of a more flexible pipe material reduces the acoustic wave speed. Larger diameters reduce the velocities and velocity changes, but the residence time increases, which may ren‐ der this option infeasible due to quality concerns.

A cost-benefit analysis is recommended to evaluate the feasibility of these kinds of options.
