Pedro J. Lee

Experimental verification of the frequency response method for pipeline leak detection

This paper presents an experimental validation of the frequency response method for pipeline leak detection. The presence of a leak within the pipe imposes a periodic pattern on the resonant peaks of the frequency response diagram. This pattern can be used as an indicator of leaks without requiring the “no-leak” benchmark for comparison. In addition to the experimental verification of the technique, important issues, such as the procedure for frequency response extraction and methods for dealing with frequency-dependent friction are considered in this paper. In this study, transient signals are generated by a side-discharge solenoid valve. Non-linearity errors associated with large valve movements can be prevented by a change in the input parameter to the system. The optimum measuring and generating position for two different system boundary configurations—a symmetric and an antisymmetric system—are discussed in the paper and the analytical expression for the leak-induced pattern in these two cases is derived

Unsteady friction and visco-elasticity in pipe fluid transients

The current waterhammer models coupled with friction models cannot adequately represent the pressure wave attenuation observed in real-world pipe systems, because the pressure wave damping is affected by additional effects not accounted for. One such effect is pipe visco-elasticity from the material behaviour of pipe-wall to be investigated herein. The numerical results indicate that the pressure head attenuation attributable to unsteady friction is comparable to the visco-elastic effect during the initial transient stage, while the visco-elastic effect becomes dominant both in terms of damping and phase shift at later stages. An analytical analysis shows that the visco-elastic effect is more critical if the visco-elastic retardation time is less than the wave travel time along the entire pipeline length. In addition, it is demonstrated that the visco-elastic term in waterhammer models is wrongly referred to in the literature as energy dissipation instead of energy transfer between fluid and pipe-wall by the work done by the pressure force.

Leak detection in complex series pipelines by using the system frequency response method

This research investigates the applicability of the transient-based frequency-response function (FRF) method for detecting leaks in complex series pipelines. The behaviour of transient waves in these pipelines with internal series junctions indicates that junction reflections modify the system resonant frequencies but have a small effect on the leak-induced information contained within the system frequency responses. The analogous method previously developed for single pipelines is extended to complex series pipe systems by using the analytical transfer matrix method herein and the extended method is validated by two simple numerical experimental cases consisting of 3-series pipes and 10-series pipes, respectively. The applied results indicate that the extended FRF method can be applied to detect single and multiple leaks in complex series pipelines as long as the location and size of the resonant peaks of system frequency responses are accurately determined.

Experimental Investigation of Coupled Frequency and Time-Domain Transient Test–Based Techniques for Partial Blockage Detection in Pipelines

AbstractPartial blockages commonly exist in pressurized pipeline systems, and the rapid remediation of such faults is required to reduce the wastage of energy as well as to maintain the serviceability of the pipe network. Numerous transient test–based techniques (TTBT) have been developed for detecting pipe defects, with each technique providing different advantages. Two previously developed techniques—pressure signal analysis (PSA) and frequency response analysis (FRA)—are experimentally tested in this study on systems of different pipe material and characteristics. Each method is validated using the experimental data, and the results show that PSA is most accurate for locating the blockage while FRA is most accurate for determining the radial constriction and length of the blockage section. To take advantage of the different strengths of the techniques, a coupling of the two methods is proposed. Experimental application results reveal that both detection accuracy and calculation efficiency are improved ...

Extended Blockage Detection in Pipelines by Using the System Frequency Response Analysis

This paper proposes a technique for the detection of extended blockages in pressurized water pipelines by using the system frequency responses under transient conditions. The impact of extended blockages on the system frequency responses is illustrated by using the analytical transfer matrix method, and the results indicate that extended blockages cause the resonant frequencies of the system to shift, and the blockage locations and sizes can be determined by analyzing the occurrences of the resonant peaks in the frequency axis. The analytical derivations of the blockage effect on the resonant frequencies are validated through different numerical experiments, and the applied results demonstrate that the extended blockages in pipeline can be detected by the proposed method as long as the system resonant frequencies are accurately determined. Practical implementations of the findings are discussed at the end of this paper.

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.

Extended Blockage Detection in Pipes Using the System Frequency Response: Analytical Analysis and Experimental Verification

Extended blockages in a pipeline system are expected to impose changes onto the system resonant frequencies where the size and nature of the frequency shifts can be used to determine the blockage characteristics. Although a theoretical method for detecting and locating extended blockages in pipeline systems using these changes in the system frequency response (SFR) was developed by the authors in a previous paper, the impact of an extended blockage on SFR has yet been verified experimentally and is the topic of this paper. The impact of six different extended blockages under a range of different Reynolds numbers on the frequency response is used to confirm the theoretical behavior of an extended blockage. These experimental tests are conducted in the pipeline hydraulic laboratory at the University of Canterbury, New Zealand. An analytical simplification of the original SFR-based method is used to identify the key block- age parameters governing the frequency shifts and shows that the magnitude of the frequency shift increases with severity of blockages and is related to the changes in characteristic impedance and wave propagation coefficient of pipeline (pipe diameter, thickness, and/or wavespeed) imposed by the blockage. The experiments show that the length and location of potential extended blockages in the pipeline can be accurately predicted by the proposed method. Significant error exists, however, in the prediction of the pipe constriction diameter and is a result of the nonlinear operations in the experiments such as full valve closure in this study and the inability of existing models for perfectly replicat- ing transient events in pipes with severe constrictions. DOI: 10.1061/(ASCE)HY.1943-7900.0000736.

System Response Function–Based Leak Detection in Viscoelastic Pipelines

The possibility of leak detection in a viscoelastic pipe system is studied. The frequency response function method (FRFM) is used for leak detection, and the analytical expression for the FRFM originally developed for an elastic pipeline is extended to the viscoelastic situation in this study. The extended FRFM is validated from numerical experiments with one-dimensional viscoelastic transient models. The analysis shows that the pipe-wall viscoelasticity effect has significant impact on the amplitude damping and phase shift of the pressure wave, but little influence on the leak-induced patterns of pressure head peaks in transient system frequency responses. The results indicate that the extended FRFM is applicable to the viscoelastic situation. The impact of viscoelastic parameters and incident wave bandwidth on the applicability of the extended FRFM i also investigated in this paper, and the results imply that transient input signals with rapid changes in time are preferable for leak location in viscoelastic pipelines.

Leak location in pipelines using the impulse response function

Current transient-based leak detection methods for pipeline systems often rely on a good understanding of the system—including unsteady friction, pipe roughness, precise geometry and micro considerations such as minor offtakes—in the absence of leaks. Such knowledge constitutes a very high hurdle and, even if known, may be impossible to include in the mathematical equations governing system behavior.An alternative is to test the leak-free system to find precise behavior, obviously a problem if the system is not known to be free of leaks. The leak-free response can be used as a benchmark to compare with behavior of the leaking system. As an alternative, this paper uses the impulse response function (IRF) as a means of leak detection. The IRF provides a unique a relationship between an injected transient event and a measured pressure response from a pipeline. This relationship is based on the physical characteristics of the system and is useful in determining its integrity. Transient responses of completely different shapes can be directly compared using the IRF. The IRF refines all system reflections to sharp pulses, thus promoting greater accuracy in leak location, and allowing leak reflections to be detected without a leak-free benchmark, even when complex signals such as pseudo-random binary signals are injected into the system. Additionally, the IRF approach can be used to improve existing leak detection methods. In experimental tests at the University of Adelaide the IRF approach was able to detect and locate leaks accurately.

Relevance of Unsteady Friction to Pipe Size and Length in Pipe Fluid Transients

This paper investigates the importance of pipe system scale—specifically pipe length and diameter—on unsteady friction in pipe transients. A dimensionless analysis is conducted for the one-dimensional (1D) water-hammer model in this study and the analytical ex- pression for the relative importance of unsteady friction damping to the total friction damping is obtained in the frequency domain. In addition, a two-dimensional (2D) waterhammer model coupled with a 2D κ-e turbulence model is applied to a reservoir-pipe-valve system. A parametric study covering a number of pipe diameters, pipe lengths, and initial Reynolds numbers is conducted. The investigation spans a range of water-hammer travel time and turbulent radial diffusion timescales. In each case, the transient is generated by a sudden and complete valve closure. The analytical solution for the importance of unsteady friction is verified by the numerical simulations and the results show that unsteady friction damping has less effect on the damping rate of the transient envelope as (1) the ratio of the wave travel timescale to the radial diffusion timescale increases and (2) the product of the initial friction factor and Reynolds number increases. Furthermore, the findings of this study arevalidated by both laboratory and field experiments from literature. The implication of the findings is that the role of unsteady friction on the damping rate of the transient envelope diminishes with the scale ratio of pipe length and pipe diameter and that laboratory experiments, which are usually limited to relatively small scale ratios of pipe lengths and diameters, have lead researchers to overestimate the importance of unsteady friction on the damping of the transient envelope in real large pipe-scale systems. DOI: 10.1061/(ASCE)HY.1943-7900.0000497.