**3.2. Emergency scenarios without anti-surge provisions**

A pressure transient analysis or surge analysis includes a number of simulations of emergency scenarios, normal operations maintenance procedures. The emergency scenar‐ ios may include:


A pump trip without anti-surge provisions causes a negative pressure wave traveling into the WSS. If the downstream boundary is a tank farm or large distribution network, then the reflected pressure wave is an overpressure wave. If the check valves have closed within the pipe period, then the positive pressure reflects on the closed check valves by doubling the positive pressure wave (Figure 4). In this way, the maximum allowable pressure may be ex‐ ceeded during a pump trip scenario.

**Figure 4.** Pressure wave propagation following a pump trip

This parameter defines the natural time scale for velocity and pressure adjustments in

Only after the pipe period the pressure wave will start to interact with other pressure waves from the boundary condition, such as a tripping pump or a valve closure. Any velocity change *Δv* within the pipe period will result in a certain "practical maximum" pressure, the

A slightly more conservative assessment of the maximum transient pressure includes the

 r

All these parameters follow directly from the basic design. The maximum rate of change in velocity is determined by the run-down time of a pump or a valve closure speed. The pump run-down time is influenced by the polar moment of inertia of the pump impeller, the gear box and motor. The full stroke closure time of valves may be increased in order to reduce

Pressure waves reflect on variations of cross-sectional area (T-junctions, diameter changes, etc.) and variation of pipe material. All these parameters must be included in a

Finally, the elevation profile is an important input, because extreme pressures typically oc‐

A pressure transient analysis or surge analysis includes a number of simulations of emergency scenarios, normal operations maintenance procedures. The emergency scenar‐

A pump trip without anti-surge provisions causes a negative pressure wave traveling into the WSS. If the downstream boundary is a tank farm or large distribution network, then the reflected pressure wave is an overpressure wave. If the check valves have closed within the pipe period, then the positive pressure reflects on the closed check valves by doubling the

(3)

(4)

D =± × ×D *p cv* r

( )*<sup>s</sup>* D =± × ×D + D *p c v gH* r

the system.

so-called Joukowsky pressure, *Δp*.

8 Water Supply System Analysis - Selected Topics

steady friction head loss *Δp<sup>s</sup> = ρgΔHs.*

the rate of velocity change.

cur at its minimum and maximum positions.

**3.2. Emergency scenarios without anti-surge provisions**

**•** Single pump trip to determine check valve requirements

hydraulic model.

ios may include:

**•** Complete pump trip

**•** Unintended valve closure; and

**•** Emergency shut-down procedures.

Check valves will generally close after pump trip. The transient closure of a check valve is driven by the fluid deceleration through the check valve. If the fluid decelerates quickly, an undamped check valve will slam in reverse flow. Fast-closing undamped check valves, like a nozzle- or piston-type check valve, are designed to close at a very small return velocity in order to minimize the shock pressure. Ball check valves are relatively slow, so that their ap‐ plication is limited to situations with small fluid decelerations.

**Figure 6.** Single and two-stage valve in 5 pipe periods (100 s)

ance on the conservative modeling approach.

protection against it (Jung and Karney 2009).

discussed in more detail in the next section.

different management options at their disposal:

**3.3. Design of anti-surge devices and emergency controls**

In general, for each scenario multiple simulations must be carried out to determine the ex‐ treme pressures and other hydraulic criteria. Scenario variations may include flow distribu‐ tions, availability of signal transfer (wireless or fiber-optic cable) for the control system and parameter variations. For example, the minimum pressure upon full pump trip will be reached in a single pipeline, if the maximum wall roughness value is used. If an air vessel is used as an anti-surge device, the minimum wall roughness and isothermal expansion must be applied to determine the minimum water level in the air vessel. Adiabatic pocket expan‐ sion in air vessels must be applied for other scenarios. The selection of input parameters so that the extreme hydraulic criterion values are computed is called a conservative modeling approach (Pothof and McNulty 2001). The proper combination of input parameters can be determined *a priori* for simple (single pipeline) systems only. Table 4 provides some guid‐

Guidelines for Transient Analysis in Water Transmission and Distribution Systems

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

11

In more realistic situations a sensitivity analysis is required to determine the worst case loading. A more recent development for complex systems is to combine transient solvers with optimization algorithms to find the worst case loading condition and the appropriate

In most cases, the emergency scenarios result in inadmissible transient pressures. Possible solutions include modifications to the system or transient event (e.g., slower valve closure), anti-surge devices, emergency controls, or a combination of the above. The solutions will be

In order to mitigate inadmissible transient pressures, hydraulic design engineers have four

**Figure 5.** Pressure wave propagation following valve closure

Emergency closure of a line valve creates a positive pressure wave upstream and negative pressure wave downstream of the valve. Although the total closure time may well exceed the characteristic pipe period, the effective closure may still occur within one pipe period, so that the Joukowsky pressure shock may still occur. The effective closure is typically only 20% of the full stroke closure time, because the valve starts dominating the total head loss when the valve position is less than 20% open (e.g., Figure 6). If a measured capacity curve of the valve is used, simulation software will deliver a reliable evolution of the discharge and transient pressures in the WSS.

Figure 6 shows an example of a butterfly valve at the end of a 10 km supply line (wave speed is 1000 m/s). A linear closure in 5 pipe periods (100 s) shows that the pressure rises only during the last 30% of the valve closure. Therefore the pressure rise is almost equal to the Joukowsky pressure. A two-stage closure, with a valve stroke from 100% to 30% open in 1 pipe period (20 s), shows a more gradual pressure rise during the closing procedure and a lower peak pressure.

**Figure 6.** Single and two-stage valve in 5 pipe periods (100 s)

Hydraulic grade line

10 Water Supply System Analysis - Selected Topics

Hydraulic grade line c

Underpressure wave

**Figure 5.** Pressure wave propagation following valve closure

and transient pressures in the WSS.

lower peak pressure.

c

Valve downstream

Valve half-way

Emergency closure of a line valve creates a positive pressure wave upstream and negative pressure wave downstream of the valve. Although the total closure time may well exceed the characteristic pipe period, the effective closure may still occur within one pipe period, so that the Joukowsky pressure shock may still occur. The effective closure is typically only 20% of the full stroke closure time, because the valve starts dominating the total head loss when the valve position is less than 20% open (e.g., Figure 6). If a measured capacity curve of the valve is used, simulation software will deliver a reliable evolution of the discharge

Figure 6 shows an example of a butterfly valve at the end of a 10 km supply line (wave speed is 1000 m/s). A linear closure in 5 pipe periods (100 s) shows that the pressure rises only during the last 30% of the valve closure. Therefore the pressure rise is almost equal to the Joukowsky pressure. A two-stage closure, with a valve stroke from 100% to 30% open in 1 pipe period (20 s), shows a more gradual pressure rise during the closing procedure and a

Overpressure wave

c

In general, for each scenario multiple simulations must be carried out to determine the ex‐ treme pressures and other hydraulic criteria. Scenario variations may include flow distribu‐ tions, availability of signal transfer (wireless or fiber-optic cable) for the control system and parameter variations. For example, the minimum pressure upon full pump trip will be reached in a single pipeline, if the maximum wall roughness value is used. If an air vessel is used as an anti-surge device, the minimum wall roughness and isothermal expansion must be applied to determine the minimum water level in the air vessel. Adiabatic pocket expan‐ sion in air vessels must be applied for other scenarios. The selection of input parameters so that the extreme hydraulic criterion values are computed is called a conservative modeling approach (Pothof and McNulty 2001). The proper combination of input parameters can be determined *a priori* for simple (single pipeline) systems only. Table 4 provides some guid‐ ance on the conservative modeling approach.

In more realistic situations a sensitivity analysis is required to determine the worst case loading. A more recent development for complex systems is to combine transient solvers with optimization algorithms to find the worst case loading condition and the appropriate protection against it (Jung and Karney 2009).

In most cases, the emergency scenarios result in inadmissible transient pressures. Possible solutions include modifications to the system or transient event (e.g., slower valve closure), anti-surge devices, emergency controls, or a combination of the above. The solutions will be discussed in more detail in the next section.
