Pipeline Network Features and Leak Detection by Cross-Correlation Analysis of Reflected Waves

This paper describes progress on a new technique to detect pipeline features and leaks using signal processing of a pressure wave measurement. Previous work (by the present authors) has shown that the analysis of pressure wave reflections in fluid pipe networks can be used to identify specific pipeline features such as open ends, closed ends, valves, junctions, and certain types of bends. It was demonstrated that by using an extension of cross-correlation analysis, the identification of features can be achieved using fewer sensors than are traditionally employed. The key to the effectiveness of the technique lies in the artificial generation of pressure waves using a solenoid valve, rather than relying upon natural sources of fluid excitation. This paper uses an enhanced signal processing technique to improve the detection of leaks. It is shown experimentally that features and leaks can be detected around a sharp bend and up to seven reflections from features/leaks can be detected, by which time the wave has traveled over 95?m. The testing determined the position of a leak to within an accuracy of 5%, even when the location of the reflection from a leak is itself dispersed over a certain distance and, therefore, does not cause an exact reflection of the wave.     

Leaks and Water Use Representation in Water Distribution System Models: Finding a Working Equivalence

The challenge of water demand representation in water distribution systems is revisited with a brief exploration of the relationship between a pressure-dependent leak and a fixed legitimate demand. Specifically, the idea that a leak can be modeled as an increment to legitimate demand in such a way that it entails an equivalent impact on both water loss and energy consumption is explored. Conversely, the representation of demands as leaks is briefly considered. The effectiveness of pressure reduction and demand curtailment as leak management schemes are compared for a single pipe system. The influence of pipe resistance on this relationship is assessed, suggesting that such schemes are more important in rougher pipes. In general, the notion that leakage and demand analysis/management are two sides of the same coin, and that pressure/demand management is essentially conservation, is put forth.     

Experimental Evidence of Hysteresis in the Head-Discharge Relationship for a Leak in a Polyethylene Pipe

The relationship between leak outflow from a damaged pipe and flow condition inside the pipe plays a crucial role in pressurized pipe systems management. As an example, this relationship is used in leakage reduction techniques based on pressure control and in leak detection techniques based on inverse analysis. To explore the relationship between total head inside the pipe and leak outflow for a single leak in a polyethylene pipe, tests were carried out at the Water Engineering Laboratory of the University of Perugia. These tests point out that the viscoelastic nature of the pipe material gives rise to a hysteretic behavior of the investigated relationship, i.e., the outflow depends not only on the synchronous total head but also on the total head time history and variation rate.     

Wavelets for the Analysis of Transient Pressure Signals for Leak Detection

Transient tests can be reliably used for the diagnosis of pressurized pipe systems. As a matter of fact, the pressure signals acquired during these tests can reveal the presence of anomalies, e.g., leaks, since any irregularity in the pipe gives rise to reflected waves which in turn create discontinuities in the observed signal at the measurement section. In order to make the most of the interpretation of the pressure signals, as well as to improve the effectiveness of the location of leaks, wavelet analysis—a powerful tool within the realm of harmonic analysis—can be used. It aids in the diagnosis of pressure pipe systems by better exposing pressure signal discontinuities and precisely determining the arrival time of the pressure waves reflected by leaks, thus locating the leak itself. Since many wavelet transforms are available and the choice is driven by the specific application, both continuous and discrete wavelet transforms are considered in the paper, comparing different mother wavelets. Then the reliability of the technique with respect to the noise effects is tested on numerically simulated and experimental pressure signals. Specifically, it is shown that the wavelet analysis of numerical signals, with and without superimposed white noise, facilitates testing the ability of this tool to recognize small step variation—corresponding to small leaks—using low cost transducers.     

Waterhammer Analysis—Essential and Easy (and Efficient)

For most piping systems the maximum and minimum operating pressures occur during transient operations. Therefore it is essential to good design and operation to perform a transient analysis for normal startup and shutdown and for unplanned events such as a pump trip associated with a power outage. This author also claims that waterhammer (transient) analysis is easy. Hydraulic engineers who have studied the traditional approach to transient analysis might dispute this claim but, in fact, carrying out an analysis using the concept of pressure wave action provides an accurate, intuitive, and simple method for transient pipe system analysis of simple or complex pipe systems. Not only is this approach simple, it is extremely efficient producing accurate solutions with far fewer calculations making this approach suitable for analyzing large pipe distribution systems.     

Range of Validity of the Transient Damping Leakage Detection Method

In a recent paper, an elegant, efficient, and easy to apply transient-based leakage detection method was proposed. The method exploits the fact that friction and leakage damp the modes of transient waves in a different manner. The method involves six major assumptions. These are: (1) the periodic motion in time of each mode is linearly independent of all other modes; (2) the amplitude of the induced transient is small; (3) the magnitude of the leak is small in comparison with the flow rate; (4) the wall friction can be represented by the Darcy–Weisbach equation; (5) the transient is initiated by an instantaneous small amplitude disturbance; and (6) the pipe system is a simple reservoir–pipe–valve type system or reservoir–pipe–reservoir type system. These six assumptions are relaxed and the validity of the transient damping method is assessed. The analysis shows that the first four assumptions do not pose any serious restriction to the applicability of the damping rate method provided that the mathematical model, used to generate the transient head trace in the leak-free pipe, accurately represents the frictional damping in the system. On the other hand, Assumptions (5) and (6) restrict the applicability of the method to systems that do not involve internal boundary conditions, such as junctions and pumps, and to transients triggered by impulses whose duration is smaller than the wave travel time. Extension of this method to complex pipe systems requires that the linearized waterhammer equations are solved under more general initial and boundary conditions. In addition, more investigation in relation to the frequency content of the input signal and its importance in leakage detection is warranted. The general framework used to derive the damping rate method has led to an efficient and direct algorithm for identifying leaks and future research should seek ways to adapt this framework to more complex pipe systems.     

Leak Detection in Pipes by Frequency Response Method

The frequency response method is used to determine the location and rate of leakage in open loop piping systems. A steady-oscillatory flow, produced by the periodic opening and closing of a valve, is analyzed in the frequency domain by using the transfer matrix method, and a frequency response diagram at the valve is developed. For a system with leaks, this diagram has additional resonant pressure amplitude peaks (herein referred to as the secondary pressure amplitude peaks) that are lower than the resonant pressure amplitude peaks (herein called primary amplitude peaks) for the system with no leaks. Several piping systems are successfully analyzed for all practical values of the friction factor to detect and locate individual leaks of up to 0.5% of the mean discharge. The method, requiring the measurement of pressure and discharge fluctuations at only one location, has the potential to detect leaks in real-life pipe systems conveying different types of fluids, such as water, petroleum, and so on.     

Use of Supervisory Control and Data Acquisition for Damage Location of Water Delivery Systems

Urban water delivery systems can be damaged by earthquakes or severely cold weather. In either case, the damage cannot easily be detected and located, especially immediately after the event. In recent years, real-time damage estimation and diagnosis of buried pipelines attracted much attention of researchers focusing on establishing the relationship between damage ratio (breaks per unit length of pipe) and ground motion, taking the soil condition into consideration. Due to the uncertainty and complexity of the parameters that affect the pipe damage mechanism, it is not easy to estimate the degree of physical damage only with a few numbers of parameters. As an alternative, this paper develops a methodology to detect and locate the damage in a water delivery system by monitoring water pressure on-line at some selected positions in the water delivery systems. For the purpose of on-line monitoring, emerging supervisory control and data acquisition technology can be well used. A neural network-based inverse analysis method is constructed for detecting the extent and location of damage based on the variation of water pressure. The neural network is trained by using analytically simulated data from the water delivery system with one location of damage, and validated by using a set of data that have never been used in the training. It is found that the method provides a quick, effective, and practical way in which the damage sustained by a water delivery system can be detected and located.     

Address-Oriented Impedance Matrix Method for Generic Calibration of Heterogeneous Pipe Network Systems

The generic evaluation of pipeline parameters is one of the most demanding technological tasks in the efficient management of a water distribution system. Information about current pipeline status is feasible by monitoring the pressure variation online. Conventional methods of transient computation and parameter calibration for a heterogeneous pipeline network suffer from cost issues both in time and storage as well as several other constraints associated with the numerical representation of a real-life system. As an alternative approach, an extension of the impulse response method, namely the address-oriented impedance matrix method (AOIMM), has been developed for a more robust calibration of a heterogeneous and multilooped pipe network system. The genetic algorithm was incorporated into the AOIMM for generic calibration of several parameters, such as the location and quantity of leakage, friction factor, and wave propagation speed. The potential of the proposed calibration algorithm over other conventional approaches was demonstrated when it was applied to a hypothetical heterogeneous pipe network system.     

Leak Detection in Pipelines using the Damping of Fluid Transients

Leaks in pipelines contribute to damping of transient events. That fact leads to a method of finding location and magnitude of leaks. Because the problem of transient flow in pipes is nearly linear, the solution of the governing equations can be expressed in terms of a Fourier series. All Fourier components are damped uniformly by steady pipe friction, but each component is damped differently in the presence of a leak. Thus, overall leak-induced damping can be divided into two parts. The magnitude of the damping indicates the size of a leak, whereas different damping ratios of the various Fourier components are used to find the location of a leak. This method does not require rigorous determination and modeling of boundary conditions and transient behavior in the pipeline. The technique is successful in detecting, locating, and quantifying a 0.1% size leak with respect to the cross-sectional area of a pipeline.     

In-Line Pipe Device Checking by Short-Period Analysis of Transient Tests

In this paper, the results of laboratory transient tests concerning the interaction between a pressure wave and an in-line device are discussed. Transients are generated by means of a fast and complete closing of an end valve, and pressure measurements are carried out just upstream of the maneuver valve. The main effect of the device on the pressure time-history—hereafter referred to as pressure signal—is a sharp increase caused by the pressure wave reflection. Three experimental setups are considered, in which different in-line devices are installed (i.e., a ball valve, a butterfly valve, and different-sized orifices). The pressure signal is analyzed in the time domain, and a reliable evaluation of the device location is obtained by means of wavelet functions. Furthermore, on the basis of the value of the pressure increase, the status of the device can be determined; experimental results are synthesized in a dimensionless diagram. Finally, criteria for using the obtained results in other pipe systems are discussed.     

Numerical Modeling for Hydraulic Resonance in Hydropower Systems Using Impulse Response

An effective numerical method to compute hydraulic resonance in pressurized piping of hydropower systems is presented. For this purpose, the impulse response method is used, i.e., a unit pressure impulse is introduced at the downstream waterway as an exciter. The method of characteristics is used to solve the governing equations for the hydraulic transient and to get the pressure response of the system in the time domain. The discrete Fourier transform is used to compute the frequency response of the system which gives the resonance frequencies in the hydropower plant system. To increase the accuracy of the results, unsteady friction is incorporated into the methodology. The influence of the unsteady friction, wave speed, and power on the pressure response diagram is investigated for the waterway of a multiunit hydropower plant which has been recently installed. Computed results agree very well with those obtained from the standard method of the characteristics.     

Numerical Sensitivity Study of Unsteady Friction in Simple Systems with External Flows

This paper investigates the importance of unsteady friction effects when performing water hammer analyses for pipe systems with external fluxes due to demands, leaks, and other system elements. The transient energy equation for a system containing an orifice-type external flow is derived from the two-dimensional, axial momentum equation. A quasi-two-dimensional flow model is used to evaluate the relative energy contribution of total friction, unsteady friction, and the external flow, in a 1,500?m pipeline, with orifice flows ranging from steady-state flows of 2–70% of the mean pipe flow, and a Reynolds number of 600,000. It is found that for initial lateral flows larger than around 30% of the mean flow, unsteady friction effects can probably be neglected, whereas for external flows smaller than this, unsteady friction should generally be considered. Overall, the relative role of unsteady friction is found to diminish as the external flux increases, implying that unsteady friction is not critical for systems with large external flows. These results imply that unsteady friction may have a significant impact on the validity of transient leak detection techniques that have been derived assuming quasi-steady friction. To demonstrate this point, an existing transient leak detection method, originally derived under quasi-steady conditions, is tested with unsteady friction included.     

Partial Blockage Detection in Pipelines by Frequency Response Method

A new technique is presented utilizing the frequency response for the detection of partial blockages in a pipeline. In the system frequency response, a partial blockage increases the amplitude of the pressure oscillations at even harmonics. Such an increase in amplitude has an oscillatory pattern, the frequency and amplitude of which may be used to predict the location and size of a partial blockage. In this technique, the pressure transient history at only one location is sufficient, and the history of the transient in the pipe prior to blockage is not needed, which is an advantage over a number of other available techniques, in addition to being simpler to use. It is shown that the technique successfully detects the location of a blockage in a number of simple systems with blockage size as small as 10%. The technique is verified by comparing the computed results with those computed by the method of characteristics and with measurements from simple laboratory setups. A number of practical issues and limitations for field implementations are discussed.     

Impulse Response Method for Pipeline Systems Equipped with Water Hammer Protection Devices

Surge protection devices, such as surge tanks and air chambers, have been modeled with the impulse response method for transient analysis of water distribution systems. The lumped inertia model and continuity equation are used to represent nonpipe hydraulic elements. Results of pressure or discharge variations obtained by using the impulse response method and the method of characteristics are in good agreement. The impulse response method provides total pressure and discharge along any pipeline segment by direct integration of the ratio of complex head or complex discharge to a complex downstream discharge, respectively. A modification is proposed so that transition between turbulent and laminar flows can be considered. The representation of hydraulic devices has been incorporated into the impedance matrix method, which was developed for heterogeneous and multilooped pipe network systems. The potential advantages of the proposed method over other conventional approaches were investigated by applying the proposed method to hypothetical pipe network systems.     

Extensive Development of Leak Detection Algorithm by Impulse Response Method

The oscillatory flows in pipeline systems due to excitation by valve operation are efficiently analyzed by the impulse response method. The impact of leakage is incorporated into the transfer functions of the complex head and discharge. Frequency-dependent friction is used to consider the impact of unsteady friction for laminar condition. Extensive development of the impulse response method was made by considering the sources of friction associated with the local and convective acceleration of velocity for turbulent flow. The genetic algorithm was integrated into the impulse response method to calibrate the location and the quantity of leakage. The calibration function for leakage detection can be made using the pressure-head response at the valve, or the pressure-head and flow response at the section upstream from the valve. The proposed leak detection algorithm shows the potentials for being applied to a simple pipeline system with a single leak or multiple leaks.     

Vapor Leak Detection for an Underground Storage Tank System

This paper presents a mathematical model for the pressure inside the vapor space of an underground storage tank (UST). Using mass balances based on dispensing activities, vapor recovery, evaporation, leaks, and the safety vent, the model resulted in a system of nonlinear differential equations with unknown parameters, leak rate and evaporation rate. Given the leak rate and evaporation rate, the pressure inside the UST can be obtained by solving the system of differential equations numerically. Given the pressure inside a UST over a period of time, one can find the evaporation and leak rates such that the calculated pressure from our model best fits the given pressure data. This optimal value of leak rate can allow us to determine whether the UST system is leaking and if the leak rate is above the EPA standard. Numerical results and statistical analysis of our model are also presented.     

Dynamic Memory Computation of Impedance Matrix Method

The computational efficiency of the impedance matrix method has been greatly improved for large pipe networks with various dimensions and complexity. Several numerical methods for solving linear system were modified to deal with the complex domain operation and used into impedance evaluation. Two different memory reduction schemes were developed based on one-dimensional storage and implemented with the biconjugate gradient method and the Gaussian elimination scheme, respectively. A new implementation of the impedance matrix method, namely, the dynamic memory allocation scheme, was introduced to efficiently model hydraulic transients in pipeline systems that have large topological structures. Three hypothetical pipe networks, the multiseries system, the multilooped system, and the multiblock system, were used to test the performance of the developed schemes. The impact of randomizing pipeline parameters, i.e., friction factor, length, and wave speed, on computation efficiency was evaluated and compared. The dynamic memory allocation scheme not only reduces costs substantially in CPU execution time and memory space compared to other schemes but also shows significant potential as a real-time unsteady flow predictor for large pipe networks.     

Effects of different triggering systems and external PEEP on trigger capability of the ventilator

OBJECTIVE: The triggering capability of both the pressure and flow triggering systems of the Servo 300 ventilator (Siemens-Elema, Sweden) was compared at various levels of positive end-expiratory pressure (PEEP), airway resistance (R(aw)), inspiratory effort and air leak, using a mechanical lung model. DESIGN: The ventilator was connected to a two bellows-in-series-type lung model with various mechanical properties. Lung compliance and chest wall compliance were 0.03 and 0.121/cmH2O, respectively. R(aw) was 5, 20 and 50 cmH2O/l/s. Respiratory rate was 15 breaths/min. To compare the triggering capability of both systems, the sensitivity of pressure and flow triggered pressure support ventilation (PSV) was adjusted to be equal by observing the triggering time at 0 cmH2O PEEP and 16 cmH2O of pressure support (PS) with no air leak. No auto-PEEP was developed. In the measurement of trigger delay, the PS level ranged from 16 to 22 cmH2O to attain a set tidal volume (V(T)) of 470 ml at a R(aw) of 5, 20 and 50 cmH2O/l/s. The PEEP level was then changed from 0, 5 and 10 cmH2O at a PS level of 17 cmH2O and R(aw) of 5 and 20 cmH2O/l/s, and the trigger delay was determined. The effect of various levels of air leak and inspiratory effort on triggering capability was also evaluated. Inspiratory effort during triggering delay was estimated by measurements of pressure differentials of airway pressure (Paw) and driving pressure in the diaphragm bellows (Pdriv) in both systems. MEASUREMENTS AND RESULTS: There were no significant differences in trigger delay between the two triggering systems at the various PEEP and R(aw) levels. At the matched sensitivity level, air leak decreased trigger delay in both systems, and additional PEEP caused auto-cycling. A low inspiratory drive increased trigger delay in the pressure sensing system, while trigger delay was not affected in the flow sensing system. The Paw and Pdriv differentials were lower in flow triggering than in pressure triggering. CONCLUSIONS: With respect to triggering delay, the triggering capabilities of the pressure and flow sensing systems were comparable with and without PEEP and/or high airway resistance at the same sensitivity level, unless low inspiratory drive and air leak were present. In terms of pressure differentials, the flow triggering system may require less inspiratory effort to trigger the ventilator than that of the pressure triggering system with a comparable triggering time. However, this difference may be extremely small.     

Hydraulic Analysis and Direct Design of Multiple Outlets Pipelines Laid on Flat and Sloping Lands

Adequate hydraulic analysis of a multiple outlets pipelines is very important for the design and evaluation of irrigation systems. In this paper, an analytical direct design procedure for a single multiple outlets pipelines is presented. The proposed equations, taking into consideration the influence of local energy loss, are suitable for designing laterals and manifolds in both trickle and sprinkler irrigation systems, and can be applied for various types of outlet, different flow regimes, and uniform line slope ranges. In this analytical procedure, for any desired uniformity level and given design slope range with remaining known parameters, the pipe diameter and the pipe length can then be directly designed. For any desired uniformity level, the procedure also provides an opportunity to evaluate the influence of local energy loss, as well as the influence of different uniform line slopes on the pipe geometric characteristics (pipe size and length), and on the corresponding hydraulic variables (operating inlet pressure head, downstream end pressure head, and total energy loss). Comparison test with a revised step-by-step numerical method for various slope combinations indicated that the presented methodology produces sufficiently accurate results for various design cases in both trickle and sprinkler lateral design. The methodology is simple, easy to apply, and useful for hydraulic analysis and direct design of a multiple outlets pipelines in irrigation subunits.