*3.3.4. Anti-surge devices*

**1.** System modifications (diameter, pipe material, elevation profile, etc.);

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

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‐

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

A reduction of the rate of velocity change will reduce the transient pressure amplitude. A variable speed drive or soft start/stop functionality may be effective measures for normal operations, but their effect is negligible in case of a power failure. A flywheel increases the polar moment of inertia and thereby slows down the pump trip response. It should be veri‐ fied that the pump motor is capable of handling the large inertia of the flywheel during pump start scenarios. Experience shows that a flywheel is not a cost-effective option for

If inadmissible pressures are caused by valve manipulations, the valve closure time must be increased. The velocity reduction by a closing valve is not only influenced by the valve char‐ acteristic, but also by the system. The valve resistance must dominate the total system resist‐ ance before the discharge is significantly reduced. Therefore, the effective valve closure time is typically 20% to 30% of the total closure time. A two-stage closure, or the utilization of a smaller valve in parallel, may permit a rapid initial stage and very slow final stage as an ef‐ fective strategy for an emergency shut down scenario. The effective valve closure must be spread over multiple pipe periods to obtain a significant reduction of the peak pressure. Ex‐ isting books on fluid transient provide more detail on efficient valve stroking (Tullis 1989;

Since WSS are spatially distributed, the power supply of valves and pumps in different parts of the system is delivered by a nearly-independent power supply. Therefore, local con‐ trol systems may continue operating normally, after a power failure has occurred some‐

**2.** Moderation of the transient initiation event;

through a slope to lower the maximum elevation.

der this option infeasible due to quality concerns.

*3.3.2. Moderating the transient initiation event*

pumps that need to start and stop frequently.

Streeter and Wylie 1993; Thorley 2004).

*3.3.3. Emergency control procedures*

**3.** Emergency control procedures; and/or

12 Water Supply System Analysis - Selected Topics

**4.** Anti-surge devices.

*3.3.1. System modifications*

The above-described measures may be combined with one or more of the following antisurge devices in municipal water systems.


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

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 criteria are violated.

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 distance from the pumping station.


15

gency valve closure.

One of the disadvantages of a surge tower is its height (and thus cost and the siting chal‐ lenges). If the capacity increases, so that the discharge head exceeds the surge tower level, then the surge tower cannot be used anymore. A surge tower is typically installed in the vi‐ cinity of a pumping station in order to protect the WSS downstream. A surge tower could also be installed upstream of a valve station to slow down the over pressure due to an emer‐

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15

Pump trip

Closing valve

Another device that reduces the velocity change in time is the flywheel. A flywheel may be an effective measure for relatively short transmission lines connected to a tank farm or distribu‐ tion network. A flywheel can be an attractive measure if the following conditions are met:

**2.** The pump motor can cope with the flywheel during pump start-up, which means that the motor is strong enough to accelerate the pump impeller - flywheel combination to the pump's rated speed. If the polar moment of pump and flywheel inertia is too large for the motor, then a motor-powered trip may occur and the rated speed cannot be reached.

**Figure 8.** Surge tower near pumping station or valve station.

**1.** Pump speed variations are limited.

**Table 3.** Possible mitigating measures in case of violation of one or more performance criteria

Be- en ontluchter

One of the disadvantages of a surge tower is its height (and thus cost and the siting chal‐ lenges). If the capacity increases, so that the discharge head exceeds the surge tower level, then the surge tower cannot be used anymore. A surge tower is typically installed in the vi‐ cinity of a pumping station in order to protect the WSS downstream. A surge tower could also be installed upstream of a valve station to slow down the over pressure due to an emer‐

""vented"

**Figure 7.** Non-aerated surge vessel

**Figure 7.** Non-aerated surge vessel

gency valve closure.

One of the disadvantages of a surge tower is its height (and thus cost and the siting chal‐ lenges). If the capacity increases, so that the discharge head exceeds the surge tower level, then the surge tower cannot be used anymore. A surge tower is typically installed in the vi‐ cinity of a pumping station in order to protect the WSS downstream. A surge tower could also be installed upstream of a valve station to slow down the over pressure due to an emer‐ gency valve closure.

**Figure 8.** Surge tower near pumping station or valve station.

Another device that reduces the velocity change in time is the flywheel. A flywheel may be an effective measure for relatively short transmission lines connected to a tank farm or distribu‐ tion network. A flywheel can be an attractive measure if the following conditions are met:

**1.** Pump speed variations are limited.

**Operation Criterion Violation Improvement**

valve throttling pressure instability use multiple valves

**Table 3.** Possible mitigating measures in case of violation of one or more performance criteria

Air vent

""non vented"

**Figure 7.** Non-aerated surge vessel

""vented"

**Figure 7.** Non-aerated surge vessel

gency valve closure.

compressor

Be- en ontluchter

One of the disadvantages of a surge tower is its height (and thus cost and the siting chal‐ lenges). If the capacity increases, so that the discharge head exceeds the surge tower level, then the surge tower cannot be used anymore. A surge tower is typically installed in the vi‐ cinity of a pumping station in order to protect the WSS downstream. A surge tower could also be installed upstream of a valve station to slow down the over pressure due to an emer‐

rate of fluid deceleration through check valve (high pressure due to valve closure)

pump trip high pressure air vessel with check valve and throttled by-pass pump trip reverse flow in pump increase (check) valve closure rate by choosing an

bypass pipe, flywheel larger pipe diameter air vessel, accumulator surge tower, surge vessel, feed tank air valve(s) at low pressure points in the system other pipe material with lower Young's modulus

appropriate fast-closing check valve (e.g. nozzle type)

apply spring to reduce check valve closing time apply spring or counter weight with damper to increase check valve closing time and allow return flow

air vessel slower valve closure pressure relief valve or damper at high pressure points higher pressure rating

> air vessel slower valve closure air valves at low pressure points

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Guideline for Transient Analysis in Water Transmission and Distribution Systems

adjust control settings

use air valves prevent drainage on shut-down

pump trip low pressure

14 Water Supply System Analysis - Selected Topics

valve closure high pressure (upstream)

valve closure low pressure (downstream)

drainage, filling entrapped air

pump trip

**2.** The pump motor can cope with the flywheel during pump start-up, which means that the motor is strong enough to accelerate the pump impeller - flywheel combination to the pump's rated speed. If the polar moment of pump and flywheel inertia is too large for the motor, then a motor-powered trip may occur and the rated speed cannot be reached.

**Figure 9.** Effect of flywheel on transient pressure after power failure in the pumping station

A by-pass check valve is effective at sufficient suction pressure, which becomes available au‐ tomatically in a booster station. Wavefront steepness is not affected until the by-pass check valve opens. A similar reasoning applies to the other pressure-limiting devices. Further‐ more, the release of air pockets via air valves is an important source of inadmissible pres‐ sure shocks. Air release causes a velocity difference between the water columns on both sides of the air pocket. Upon release of the air pocket's last part, the velocity difference *Δv* must be balanced suddenly by creating a pressure shock of half the velocity difference (Fig‐ ure 10). The magnitude of the pressure shock is computed by applying the Joukowsky law:

$$
\Delta p = \pm \rho \cdot c \cdot \Delta v / 2 \tag{5}
$$

**Figure 10.** Pressure shock due to air valve slam.

**3.** Commissioning tests.

**9.** Switch-over procedures.

**6.** Normal, scheduled, valve closure.

**3.4. Design of normal procedures and operational controls**

also appendix C.2.2. in standard NEN 3650-1:2012): **1.** Start of pumping station in a primed system.

**2.** Normal stop of single pump or pumping station.

**4.** Priming operation or pump start in partially primed system.

**5.** Procedure to drain (part of) the system for maintenance purposes.

**10.** Risk assessment of resonance phenomena due to control loops.

The following scenarios may be considered as part of the normal operating procedures (see

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17

**7.** Stop of one pumping station or valve station and scheduled start of another source.

**8.** Other manipulations that result in acceleration or deceleration of the flow.

A large inflow capacity is generally positive to avoid vacuum conditions, but the outflow capacity of air valves must be designed with care.

Guidelines for Transient Analysis in Water Transmission and Distribution Systems http://dx.doi.org/10.5772/53944 17

**Figure 10.** Pressure shock due to air valve slam.

## **3.4. Design of normal procedures and operational controls**

The following scenarios may be considered as part of the normal operating procedures (see also appendix C.2.2. in standard NEN 3650-1:2012):


c

Hydraulic grade line, steady state

2 (5)

c

pump without flywheel

16 Water Supply System Analysis - Selected Topics

flywheel with pump

**Figure 9.** Effect of flywheel on transient pressure after power failure in the pumping station

A by-pass check valve is effective at sufficient suction pressure, which becomes available au‐ tomatically in a booster station. Wavefront steepness is not affected until the by-pass check valve opens. A similar reasoning applies to the other pressure-limiting devices. Further‐ more, the release of air pockets via air valves is an important source of inadmissible pres‐ sure shocks. Air release causes a velocity difference between the water columns on both sides of the air pocket. Upon release of the air pocket's last part, the velocity difference *Δv* must be balanced suddenly by creating a pressure shock of half the velocity difference (Fig‐ ure 10). The magnitude of the pressure shock is computed by applying the Joukowsky law:

> D =± × ×D *p cv* r

capacity of air valves must be designed with care.

A large inflow capacity is generally positive to avoid vacuum conditions, but the outflow

Hydraulic grade line, steady state


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 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‐

min. pressure

Guidelines for Transient Analysis in Water Transmission and Distribution Systems

max. pressure due to cavity

min. pressure and min. surge tower level

> max. pressure, max. surge tower level

min. air vessel level

min. pressure (close to air vessel)

min. pressure (downstream part)

max. air vessel level

max. pressure (close to air vessel)

(air vessel present) max. pressure (upstream part)

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

Single pump trip, while others run max. rate of fluid deceleration high friction and

pump trip min. pressure high friction and

**Model Parameters (conservative approach)**

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19

high wave speed or low wave speed, high vapour pressure

low suction level

high friction and high suction level

low friction and low suction level

low friction and high suction level

(conservative approach)

low friction and low suction level and isothermal air behaviour

low friction and low suction level and adiabatic air behaviour

high friction and low suction level and adiabatic air behaviour

low friction and high suction level and isothermal air behaviour

low friction and high suction level and adiabatic air behaviour

high friction and high suction level and adiabatic air behaviour

low suction level

implosions high vapour pressure

criterion model parameters

**Scenario Output Criterion**

any operation (cavitation not allowed) max. pressure and

downstream valve closure max. pressure

rameter choice in the conservative modelling approach.

**Critical**

upstream valve closure or pump trip (cavitation allowed from process requirements)

upstream valve closure or

upstream valve closure or pump trip (surge tower present)

downstream valve closure (surge tower or present)

> critical operation

upstream valve closure or pump trip (air vessel present)

upstream valve closure or pump trip (air vessel present)

upstream valve closure or pump trip (air vessel present)

downstream valve closure (air vessel present)

downstream valve closure (air vessel present)

downstream valve closure

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.

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

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

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 pressure gradients.
