Bryan W. Karney

Climate Variability and the Frequency of Extreme Temperature Events for Nine Sites across Canada: Implications for Power Usage

To study the impact of incremental climatic warming on summer extreme temperature event frequency, the historical record of daily maximum June, July, and August temperatures was analyzed for nine sites across Canada. It was found that all of these sites are well modeled by a first-order autoregressive process using three parameters: the mean, variance, and first-order autocorrelation coefficient. For slight changes in the mean or variance there are increases in the frequency of both single days and runs of 2‐5 consecutive days with daily maximum temperatures over a threshold value. For example, for a 38C increase in the mean daily maximum temperature at Toronto, the frequency of a 5-day consecutive run over 308C rose by over a factor of 8 to 7.1%. Sites with less variability are more sensitive to an increase in the mean summer temperature than sites with higher variability. Analysis of simulated series indicates that when two parameter values change simultaneously the change in the frequency of a given event is usually greater than the sum of the individual changes. Output from the Canadian Climate Centre GCMII model for the nine sites for both the current and 2 3 CO2 atmosphere indicate an average increase in the daily maximum temperature of 4.28C. Changes in the standard deviation and autocorrelation were usually less pronounced. For Toronto, a positive correlation (R2 5 0.718) between daily peak power demand and the cube of the current and previous 2 days daily maximum temperature was found. A sensitivity analysis was performed on daily peak power demand by first generating temperature time series and then using the derived regression relationship. Results follow a predictable pattern and indicate that the standard deviation of the peak power series increases proportionally more than the mean for increases in the mean daily maximum temperature. For example, for a 38C increase in mean daily maximum temperature, the increase in mean peak power demand was 7% (1200 MW) while the increase in the standard deviation of peak power demand was 22%. Changes in the autocorrelation of the temperature time series do not lead to significant changes in the mean or standard deviation of daily peak power demand. These results indicate that, while the average peak power demand is not moved drastically, the number of high energy consumption days may increase appreciably due to higher variability, placing stress on the provincial power utility to meet this higher demand.

Frequency domain analysis of pipe fluid transient behaviour

ABSTRACT Pipe transient signals are hyperbolic in nature where key features of the signal repeat periodically and are well suited to the analysis in the frequency domain. For this reason, a number of studies have been conducted on the use of frequency domain approaches for a variety of purposes, from fault detection to the prediction of the unsteady system response. Despite the number of papers on the topic over the past decades, there are no detailed review of the developments in the frequency domain analysis of pipe transient signals. This paper provides an assessment and review of the relevant research and provides a critical discussion of both the strengths and weaknesses of this approach. A method for extracting a systems frequency response function using conventional valve closure signals is proposed and the influence of various faults, friction and pipe wall viscoelasticity on this response function are compared with the corresponding impacts in the time domain. This study shows that most changes on the transient trace in time manifest as changes to the resonant responses in the frequency domain and the resonant responses encapsulate the essence of the system behaviour.

Fluid transients and pipeline optimization using GA and PSO: the diameter connection

This paper describes the optimal selection of pipe diameters in a network considering steady state and transient analysis in water distribution systems. Two evolutionary approaches, namely genetic algorithms (GA) and particle swarm optimization (PSO), are used as optimization methods to obtain pipe diameters. Both optimization programs, inspired by natural evolution and adaptation, show excellent performance for solving moderately complex real-world problems which are highly nonlinear and demanding. The case study shows that the integration of GA or PSO with a transient analysis technique can improve the search for effective and economical hydraulic protection strategies. This study also shows that not only is the selection of pipe diameters crucially sensitive for the surge protection strategies but also that more global systematic approaches should be involved in water distribution system design, preferably at an early stage in the design process.

Optimal design and operation of irrigation pumping stations using mathematical programming and Genetic Algorithm (GA)

For many water authorities worldwide, one of the greatest potential areas for energy savings is in pump selection and in the related effective scheduling of daily pump operations. The optimal control and operation of an irrigation pumping station is achieved here by first solving the nonlinear governing model using Lagrange Multipliers (LM) and then through Genetic Algorithm (GA) approach. Computation in both methods is driven by an objective function that includes operating and capital costs subject to various performance and hydraulic constraints. The LM approach first specifies the annual energy costs and minimizes the total cost for all sets of pumping stations; the method then selects the least-cost pumps from among the feasible sets. The GA model simultaneously determines the least total annual cost of the pump station and its operation. The solution includes the selection of pump type, capacity, and the number of units, as well as scheduling the operation of irrigation pumps that results in minimum design and operating cost for a set of water demand curves. Application of the two models to a real-world project shows not only considerable savings in cost and energy but also highlights the efficiency and ease of the GA approach for solving complex problems of this type.

Rigid-plug elastic-water model for transient pipe flow with entrapped air pocket

Pressure transients in a rapidly-filling pipe with an entrapped air pocket are investigated analytically. A rigid-plug elastic-water model is developed by applying elastic-water hammer to the majority of the water columns while applying rigid-water analysis to a small portion near the air–water interface. The proposed model is validated by the full elastic-water model and experimental data. It effectively avoids the interpolation error of the method of characteristics reducing its complexity when tracking the air–water interface. Moreover, the current model has the same accuracy as the full elastic-water model.

Leak Size, Detectability and Test Conditions in Pressurized Pipe Systems

The relationships between leak geometry and detectability are explored with a distinction between steady- and unsteady-state based techniques. Various criteria to evaluate the size and detectability of a leak are first discussed. These criteria can be useful for the benchmarking and the comparison of different techniques. Since the test conditions play a crucial rule in leak detectability, the proposed criteria take this effect into account. Furthermore, they show that while in steady-state conditions increases in system pressure enhance leak detectability, in transient state, by contrast, higher pressures tend to decrease detectability. This effect is also confirmed by experimental tests carried out at the Water Engineering Laboratory of the University of Perugia.

Phenomenon of White Mist in Pipelines Rapidly Filling with Water with Entrapped Air Pockets

The phenomenon of white mist in a rapidly filling pipeline containing an entrapped air pocket is numerically and experimentally investigated. The air-water flow patterns, pressure, and temperature histories are synchronously recorded to illustrate their interrelations. The white mist phenomenon is particularly observed during fast transients, especially during the first compression of the air pocket. Comparisons between calculations and experiments indicate that the white mist primarily reflects a condensation process. More specifically, the air temperature increases because of rapid compression of an entrapped air pocket, and the high temperature could cause water to adhere to vapor at the pipe surface. For fast transients, the first compression causes a near-adiabatic air compression, but heat exchange effects become more significant in the subsequent compression and expansion cycles. As the initial air length decreases, the maximum pressure first increases and then declines, with the most dangerous air length occurring when about 3.4% is initially occupied by air. The ratio of the maximum pressure to the driving pressure increases approximately linearly with respect to the upstream pressure. A local-interpolation elastic-water model is developed by considering air-temperature change and its validity is confirmed by comparing the model and experimental results.

Modelling the advection equation under water hammer conditions

The quality of water delivered by a distribution network may degrade for many reasons. This research considers one of these, focusing attention on the connection between water quality and the hydraulic events in a pipe system. More specifically, pressure and velocity variations associated with hydraulic transients or water hammer conditions, particularly through leaks and rapid device adjustments, have the potential to degrade water quality. In most previous applications, numerical transport schemes have been coupled to quasi-steady hydraulic models. By contrast, the current contribution couples a finite difference solution of the advection-reaction equation to a fully unsteady, method of characteristics (MOC) based, hydraulic solution. Depending on system properties, the effects of advection, compressibility and reaction may be evident in the modelled response. The numerical properties of consistency, stability and convergence of the proposed model are investigated both analytically and numerically. Although some case studies have revealed important water quality implications associated with dynamic conditions, particularly in cases of contaminated water intrusion, it should be admitted that many transient simulations exhibit few differences compared with quasi-steady results.

Stochastic Analysis of Water Hammer and Applications in Reliability-Based Structural Design for Hydro Turbine Penstocks

The randomness of transient events, and the variability of its associated dependencies, ensures that water hammer and surges in a pressurized pipe system are inherently stochastic. To improve reliability-based structural design, a stochastic transient model is developed for water conveyance systems in hydropower plants. The statistical characteristics of key factors in boundary conditions, initial states, and hydraulic system parameters are analyzed on the basis of a large record of observed data from hydro plants in China; the probability distributions of annual maximum water hammer pressures are then simulated by using a Monte Carlo method and verified with an analytical probabilistic model for a simplified pipe system. The key loading characteristics (annual occurrence, sustaining period, and probability distribution) are introduced and discussed. By using an example of penstock structural design, it is shown that the total water hammer pressure should be split into two individual random variable loads—the steady/static pressure and the water hammer pressure rise during transients—and that different partial load factors should be applied to individual loads to reflect specific physical and stochastic features. Particularly, the normative load (usually the unfavorable value at a 95-percentage level) for steady/static hydraulic pressure should be taken from the probability distribution of its maximum values over a pipe’s design life, whereas for the water hammer pressure rise, as the second variable load, the probability distribution of its annual maximum values determines its normative load.

The Challenge of Air Valves: A Selective Critical Literature Review

AbstractOne key alternative for removing, preventing, and effectively coping with the often vexing presence of air in water pipelines is the combination air-vacuum valve. Despite their often effective role, such valves can be highly problematic if not well designed and maintained. This paper critically reviews the current design, application, functionality, and simulation of air valves and the associated shortcomings, with a primary focus on air/vacuum valves (AVVs). It is argued that the efficient number of air valves along undulating pipelines is yet to be fully articulated. Air-valve simulations should expressively consider their dynamic behavior, the physical behavior of any air pockets that might form below an air valve, and the varying character of the water surface at the air valve location. There is a pressing need for a comprehensive and systematic study on the proper sizing and location of AVVs for the transient protection of systems. Overall, the efficient application of AVVs requires broad res...