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Marketed alongside the steel water pipes are the high quality heat shrink sleeves Canusa jointing sleeves , which form a permanently bonded watertight seal on all diameters of pipe. 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 application is limited to situations with small fluid decelerations. 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. 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. Therefore the pressure rise is almost equal to the Joukowsky pressure.
In general, for each scenario multiple simulations must be carried out to determine the extreme pressures and other hydraulic criteria. Scenario variations may include flow distributions, 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 expansion 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 The proper combination of input parameters can be determined a priori for simple single pipeline systems only. Table 4 provides some guidance 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 In most cases, the emergency scenarios result in inadmissible transient pressures.
Possible solutions include modifications to the system or transient event e. The solutions will be discussed in more detail in the next section.
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In order to mitigate inadmissible transient pressures, hydraulic design engineers have four different management options at their disposal:. Measure 1 is only feasible in an early stage. A preliminary surge analysis may identify cost-effective measures for the surge protection that cannot later be incorporated.
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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. 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 render this option infeasible due to quality concerns. 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 flywheel increases the polar moment of inertia and thereby slows down the pump trip response.
It should be verified 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 pumps that need to start and stop frequently. 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 characteristic, but also by the system.
The valve resistance must dominate the total system resistance before the discharge is significantly reduced. 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 effective 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. Existing books on fluid transient provide more detail on efficient valve stroking Tullis ; Streeter and Wylie ; Thorley 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 control systems may continue operating normally, after a power failure has occurred somewhere else in the network.
The distributed nature of WSS and the presence of control systems may be exploited to counteract the negative effects of emergency scenarios. 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 control rules should be defined in such a way that false triggers are avoided during normal operations. The above-described measures may be combined with one or more of the following anti-surge devices in municipal water systems. An important distinction is made in Table 2 between anti-surge devices that directly affect 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 response; 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.
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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 installation of a surge vessel.
Special attention must be paid to the check valve requirements, because 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. A sometimes-effective measure to reduce the local transients in the pumping station is to install the surge vessels at a certain distance from the pumping station.
One of the disadvantages of a surge tower is its height and thus cost and the siting challenges. 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 vicinity 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 emergency valve closure.
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 distribution network. A flywheel can be an attractive measure if the following conditions are met:. 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.
A by-pass check valve is effective at sufficient suction pressure, which becomes available automatically 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. Furthermore, the release of air pockets via air valves is an important source of inadmissible pressure shocks. Air release causes a velocity difference between the water columns on both sides of the air pocket. The magnitude of the pressure shock is computed by applying the Joukowsky law:. A large inflow capacity is generally positive to avoid vacuum conditions, but the outflow capacity of air valves must be designed with care.
The following scenarios may be considered as part of the normal operating procedures see also appendix C. Normal operating procedures should not trigger emergency controls. If this is the case, the control 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 downstream 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 upstream 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 maintain 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 occurs 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 temporarily 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 failure 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.
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The simulation of the normal operating procedures provides detailed knowledge on the dynamic behaviour of the WSS. This knowledge is useful during commissioning of the modified 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.
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.