WO2008075066A2

WO2008075066A2 - Leak detection device in fluid filled pipelines

Info

Publication number: WO2008075066A2
Application number: PCT/GB2007/004913
Authority: WO
Inventors: Stephen Bernard Marcus Beck; Masoud Taghvaei; Joseph Barnaby Boxall; Wieslaw Jerzy Staszewski
Original assignee: The University Of Sheffield
Priority date: 2006-12-20
Filing date: 2007-12-20
Publication date: 2008-06-26
Also published as: WO2008075066A3; GB2444955A; GB0625380D0

Abstract

A leakage detection device (30) for use in water distribution systems (10) comprises a length of pipe (36), a coupling (32) on the end of the pipe adapted for connection to hydrants (14) of the system (10) intended to be tested for leaks. A pressure sensor (34) to detect pressure variations in liquid in the pipe (12) is disposed adjacent to the coupling (32). A valve (40) is in the pipe (36) spaced from the coupling (32) and arranged so that, when operated to close from an open position a detectable pressure wave is transmitted from the valve through the pipe (36). A controller (50) is arranged to operate the valve (40) and record pressure in the pipe (12) sensed by the sensor (34). The pressure wave is reflected from discontinuities in the system to which the device is attached and such reflections are detected by the sensor (34). The controller (50) includes means to analyse said recorded pressures over time and provide information including the distance from the sensor to a detected discontinuity. Such means may include performance of a cepstrum analysis of the pressure signal.

Description

Leak Detection Device in Fluid Filled Pipelines

This invention relates to a device for use in the detection of leaks in fluid filled pipelines, especially water distribution pipes.

BACKGROUND

Leakage from underground water distribution pipes is a significant problem in a number of countries. Leaks sometimes are not evident from water appearing at the ground surface, or sometimes the appearance at the surface is quite remote from the location of the leak. Frequently, water leaking from a pipe travels along the course of the pipe for a distance before finding ready access to the surface. Consequently, simply digging at the location of surface water that is evidently symptomatic of a leak does not necessarily mean that the location of a leak will quickly be identified and the leak fixed. Indeed, smaller leaks may take some time to reach, or may never be evident at, the surface.

Taghvaei et al [1] describe that leak detection techniques may be classified into twα categories: external and internal methods. The external methods detect the leak by looking for signs of it outside the pipeline; an example of this is visual inspection. The internal methods, such as internal inspection, try to find the leaks from inside the pipe. In this category are included a panoply of mathematical, computation and signal processing methods such as the volume balance. Some of these methods are based on pressure transients and use of some form of measuring instrument for data capture [2].

Any change in the steady-state operating condition of a fluid system can provide a pressure wave that propagates in a pipeline at the speed of sound in the medium. The geometric properties of the system, such as network configuration and the existence of hydraulic components and also the characteristics of the flows in the pipes determine the conditions for this wave propagation. The speed of the sound is related to_the_ density _of_ -the medium, its pressure arid temperature and the elasticity of the pipe material through which it flows. The speed can be calculated using standard formula. Both pipe flexibility and the presence of a second phase will lower the speed of sound in the pipe [3]. Wave speeds in water vary from about 1500 m s^"1 for rigid pipes to fractions of a metre per second for very flexible pipes such as blood vessels. A pressure wave can be caused by an effect known as water hammer [3]. This occurs when a valve is closed rapidly and a positive pressure wave is generated at the valve that propagates upstream. Another wave, negative this time, and known as a rarefaction wave, is also created, and will travel downstream. As any feature in the pipeline (such as a junction, a pipe end or a leak) causes discontinuities in the fluid flow, some of these pressure waves are reflected and return to the valve at the speed of sound in the medium and arrive back at the valve some time after the original wave has been caused. Each time the wave passes the feature, some of it is transmitted onwards and some of it is reflected back towards the valve. The reflected wave will be weak in comparison with the initial one which was caused by the valve closure [4]. Large leaks yield major changes in pressure gradients and these larger disturbances to the flow in the pipe are easy to detect. However, small leaks are more difficult to identify, and in these conditions various types of signal analysis can be employed to distinguish the presence of these features.

There are many techniques available for leak detection and location, for example Silva et al [5] used an on-line computational technique where the data from the transient caused by a leak occurring would be detected by a computer which displays the pressure transient plot and allows the leak location to be identified. This method relies on the fact that the pressure wave generated by a leak is accompanied by a sudden drop in pressure. Brunone and Ferrante [6, 7] investigated leak detection and location based on unsteady pressure waves initiated by the closure of an upstream valve. In their work, the analysis of the time history of pressure during a transient event allowed the location of a leak to be determined by measuring the time for a pressure wave to travel from the leak point to the measurement section. Mepsha ef a/ [8] used the transfer matrix method to carry out the frequency response analysis of several systems with and without leaks.

Liou [9] used the cross-correlation approach to locate the position of a leak using the first reflection of the pressure wave from the disturbance in the flow profile caused the leak. Beck θt al [10-12] also employed an analysis technique based on cross correlating the signal to find the length of each of the pipes in a network. By using the fact that each change in gradient of the cross correlation indicates an event, they were able to use more of the wave than that which would just identify the first reflection. With models and experimental work they showed that it was possible to find the lengths of all the pipes in a network. They also used the method to identify leaks in a network, both around bends and also when the leak was separated from the pressure transducer by junctions. One of the most powerful features of this type of work was that by analysing the signals and looking for features in the time domain, no calibration was required.

To reduce the problem of wave dispersion Taghvaei employed another analysis technique referred to as cepstrum [15]. This function originated in 1963 when Bogert et al were working on a signal containing an echo and they observed that the logarithm of the power spectrum of this signal has additive periodic components. When the Fourier transform of this logarithm was taken some peaks were seen in the signal at echo points. A short introduction to the use of this method to analyse the reflection of waves in fluid-filled pipelines has briefly been described by Beck et al [16].

Fundamentally, cepstrum is defined as the Fourier transform of the logarithm of the Fourier transform [15].

Mathematically, cepstrum of signal = FT (log(FT(the signal))) Algorithmically. signal → FT → log → FT→ cepstrum

where FT indicates the Fourier transform.

There are two types of cepstrum: power cepstrum and complex cepstrum. The latter is reversible back to the time signal and is a good tool for detection of local singularities within a pressure time history. The complex cepstrum C₄ is defined as [15]

C₄ = F-¹ (log A{f}), (1) where A{f} is the complex spectrum of α{t} . It can be represented in terms of the amplitude and phase at each frequency by

A{f} = F{α(t)} = A_R +jA₁ {f}. (2) _ _

Taking^" the complex logarithm of equation (2) gives

where j = V-T and φ(f) is the phase function.

This complex function of frequency with the logarithm of amplitude as the real part and phase as the imaginary part is inverse transformed in equation (3) to give the complex cepstrum. In the context of cepstrum analysis, the time parameter is referred to as quefrency [15]. Cepstrum can detect periodic structures in the logarithmic spectrum. It has the ability to detect families of both harmonics and sidebands with uniform spacing. Cepstrum is also capable of separating the source and transmission path effects, bringing to the signal the effect of deconvolution, as explained in [15].

Thus, the fundamental principle of detecting leaks is demonstrated, but it remains an object of the present invention to provide a simple and effect tool for use in the field.

BRIEF SUMMARY OF THE DISCLOSURE

In accordance with the present invention there is provided a leak detection device for a pressurised liquid distribution system, the device comprising: a length of pipe; a coupling on the end of the pipe adapted for connection to hydrants of the system intended to be tested for leaks; a pressure sensor to detect pressure variations in liquid in the pipe; "a valve in the pipe spaced from the coupling by at least 10 pipe diameters and arranged so that, when operated to close from an open position when there is a fluid velocity of at least 0.1 metres per second in the pipe, a detectable pressure wave is transmitted from the valve through the pipe; and, a controller arranged record pressure in the pipe sensed by said sensor, whereby said pressure wave on closing of the valve is reflected from discontinuities in the system to which the device is attached and such reflections are detected by the sensor, said controller including means to analyse said recorded pressures over time and provide information including the distance from the sensor to a detected discontinuity.

Preferably, the pressurised liquid distribution system for which the device is intended is a mains water distribution system. In a water distribution system, the term "hydrant" means a device used to access water directly from the main. In the context of other liquid distribution systems, the meaning of "hydrant" corresponds mutatis mutandis.

Preferably, said valve is a solenoid valve. Preferably, said controller includes means to operate the valve. Preferably, said length of pipe is at least 1 m long, preferably between 1 and 2 m in length, for example between 1.25 and 1.75 m in length.

Preferably said length of pipe has an internal diameter between 10 and 30 mm. Preferably said pipe is metallic. Preferably it is straight. Preferably, said sensor is positioned on the coupling.

Preferably, said controller is an integral, single unit with communication links to the valve and sensor respectively. Said communication links may comprise a wire to carry electrical signals.

Preferably said controller performs a filtering operation, such as that provided by wavelet analysis, of the signal received from the sensor over a period during which reflections of said pressure wave from discontinuities in the system are sensed by the sensor.

Preferably a cepstrum analysis of the filtered waveform is performed by the controller.

BRIEF INTRODUCTION TO THE DRAWING

The invention is further described hereinafter with reference to the accompanying drawing, in which Figure 1 shows schematically a leak detection device in accordance with the present invention.

DETAILED DESCRIPTION

In Figure 1 , a water mains distribution system 10 comprises an underground pipe 12 periodically provided along its length with hydrants 14 (only one being shown). As is well known a hydrant in a water system is provided to permit access to the pressurised water in the system 10 at ground level. Such may be needed from time to time by the fire service if a fire near the water system needs to be extinguished. Also, during drought conditions, stand-pipes may be connected to hydrants so that persons such as the occupants of nearby buildings that have been temporarily disconnected from the system 10 may be able to draw water from the system.

A hydrant 14 generally comprises a valve 16 which may or may not restrict flow along the pipe system 12. However, it certainly opens flow to an outlet 18 provided with a threaded flange for connection of standard fire hoses or stand pipes.

Leak detection device 10 comprises a connection boss 32 for connection to the outlet 18 employing the threaded flange. The boss 32 includes a pressure sensor 34 to detect the pressure of the water in the outlet 18, and thence in the system 10.

A pipe 36 extends vertically upwardly from the boss 32. In this respect "vertically upwardly" is simply a reference to the orientation of a hydrant 14 which generally is underground at the base of an access chamber (not shown) which itself may be nothing more than a large bore pipe extending (usually vertically downward) from ground surface to the hydrant. The hydrant 14 is generally not more than one metre below ground level.

For reasons to be explained, but also simply to get the pipe 36 to ground level where the pipe 12 is underground, the pipe has a length H and an internal diameter d. Length H is generally 1.5 metres, whereas d may conveniently be 15 mm. In any event, H » d and equal to at least 10 * d.

Above (or beyond) height H, pipe 36 is provided with a solenoid valve 40 which can open and close the pipe 36 on demand. The valve 40 when ooen should reduce the natural flow rate through the pipe by as little as possible. "Adequate pressure" is at least 2 bar, although less is possible. On the other hand mains distribution systems generally have a pressure in excess of between 2 and 10 bar, with a target of about 6 bar.

A drain 42 is disposed beyond valve 40 to permit delivery of the flow in the pipe 36 to a convenient discharge 44.

A controller 50 preferably comprises a single box having at least two cables 52, 54; one (52) for connection to the pressure sensor 34 and the other (54) for connection to the valve 40.

When the device 30 is installed and the valve 16 is opened button 56 on the controller is -pressed and this initiates a^" sequence of evenfs controlled by a program operating in the controller 50. Incidentally, the controller is preferably self-powered by an on-board rechargeable battery, preferably one that is rechargeable from a motor vehicle battery, as well as from mains supply.

When the sequence is initiated, the valve 40 is opened for a period of time to permit a steady flow of water through the outlet 18, pipe 36 and discharge 42, which water is driven by the mains system 10 itself. After such a delay, which typically may be 5 seconds, the controller signals the valve 40 to shut, which it does with sufficient suddenness to create a pressure pulse at the valve which travels at the speed of sound in water (at the temperature and pressure pertaining in the system 10). The speed of transmission of the pressure pulse generated also depends on the material from which the pipe 36 and, subsequently, the mains system 12 are constructed. The speed in pipe 36 is not, in fact, relevant, because the pulse is only detected when it passes the sensor 34.

Once the pulse enters the system 12 its speed is relevant and so controller 56 includes means (not shown) to enter the material of the pipe 12, which is generally either steel or plastics.

Knowing the material, as well as the temperature and pressure of the water in the system 10, the controller is able to calculate the speed of sound in the system 10. Indeed, provision to input the temperature may exist, or the temperature may simply be assumed. The pressure is, of course, measured by the sensor 34.

The controller 50 includes means to record the pressure fluctuations over time in the system 10 detected by the sensor 34. Depending on the nature of the system 12, reflections of the pressure pulse from discontinuities such as branches and leaks in the system can be experienced and detected by the sensor 34. The recording means may be any suitable storage device such as a volatile memory or magnetic or other permanent storage such as a computer hard drive.

After a delay, which again may be of the order of 5 seconds, recording of the pressure is terminated and the valve 40 may be opened again if more data is required. The procedure may be repeated in order to record several pressure signals so that spurious reflections may be eliminated from further consideration.

Such further consideration may comprise subsequent analysis employing a computer located remotely (for example in a vehicle or back in an office). However, it is preferred that the controller 50 is provided with sufficient processing power so that it can perform an analysis itself of the reflected signals detected by the sensor after the initial pulse has passed the sensor. As described above, such analysis is preferably a cepstrum analysis whereby the location of discontinuities resulting in reflections of the pressure pulse can be reported on a display 58 of the controller. Such location may simply be in the form of a distance from the sensor 34 of such discontinuity. Indeed there may be several such discontinuities defected at variously different distances and these may be accountable by known branches, for example of the system pipe work 12. Alternatively, they may be indicative of a leak.

Preferably, another hydrant is also available in the system 10 which is also employed with the device 30 so that a picture of the discontinuities in the system can be built with a knowledge of where discontinuities would be expected given the distances of them reported by the controller from difference hydrants. In this way an accurate estimate of the location of an unexplained discontinuity (that is to say, a leak) may be achieved.

Length H is arranged to be at least ten times the internal diameter d so that the pressure pulse initiated at solenoid valve 40 is clear and distinct. The standard water hammer theory assumes an infinite length of pipe, but at least 10 pipe diameters is required to produce a useful pulse.

Although pipe 36 is shown straight, this is not essential. It is useful when hydrants are deep in narrow access chambers, but is not essential from the point of view of operation of the invention.

Throughout the description and claims of this specification, the words "comprise" and "contain" and variations of the words, for example "comprising" and "comprises", means "including but not limited to", and is not intended to (and does not) exclude other moieties, additives, components, integers or steps.

Throughout the description and claims of this specification, the singular encompasses the plural unless the context otherwise requires. In particular, where the indefinite article is used, the specification is to be understood as contemplating plurality as well as singularity, unless the context requires otherwise.

Features, integers, characteristics, compounds, chemical moieties or groups described in conjunction with a particular aspect, embodiment or example of the invention are to be understood to be applicable to any other aspect, embodiment or example described herein unless incompatible therewith. The reader's attention is directed to all papers and documents which are filed concurrently with or previous to this specification in connection with this application and which are open to public inspection with this specification, and the contents of all such papers and documents are incorporated herein by reference.

All of the features disclosed in this specification (including any accompanying claims, abstract and drawings), and/or all of the steps of any method or process so disclosed, may be combined in any combination, except combinations where at least some of such features and/or steps are mutually exclusive.

Each feature disclosed in this specification (including any accompanying claims, abstract and drawings), may be replaced by alternative features serving the same, equivalent or similar purpose, unless expressly stated otherwise. Thus, unless expressly stated otherwise, each feature disclosed is one example only of a generic series of equivalent or similar features.

The invention is not restricted to the details of any foregoing embodiments. The invention extends to any novel one, or any novel combination, of the features disclosed in this specification (including any accompanying claims, abstract and drawings), or to any novel one, or any novel combination, of the steps of any method or process so disclosed.

REFERENCES

[1]. Taghvaei, M, Beck, S B M, and Staszewski, W J, "Leak detection in pipelines using

Cepstrum analysis" Meas. Sci. Technol. 17 (2006) 367-372.

[2]. Warda H A, Adam I G and Rashad A B 2004 A practical implementation of pressure^" transient analysis in leak localization in pipelines Int. Pipeline Conf., IPC04-

0551, (Canada, October)

[3]. Thorley A R D 2004 Fluid Transients in Pipeline Systems (London: Professional

Engineering)

[4]. Johnson L and Larson M 1992 Leak detection through hydraulic transient analysis Pipeline Systems ed B Couldbeck and E Evans (Dordrecht: Kluwer) pp 273-86 [5]. Silva R A, Buiatti C M, Cruz S L and Pereira J A F R 1996 Pressure wave behavior and leak detection in pipelines

Comp. and Chemical Eng. Proc. (?^h Eur. Symp. (Rhodes) 20 S491-6 [6]. Brunone B and Ferrante M 2001 Detecting leak in pressurized pipes by means of transient ASCE J. Hydraul. Res. 39 1-9

[7]. Brunone B< Ferrante M and Ubertini L 2000 Leak analysis in pipes using transients 2^nd Annual Seminar on Comparative Urban Projects (Rome, 19-23 June) [8]. Mpesha W, Chaudhry M H Gassman S L 2002 Leak detection in pipes by frequency response method using a step excitation ASCE J. Hydraul. Res. 40 55-62 [9]. Liou C P 1998 Pipeline leak detection by impulse response extraction ASME J. Fluids Eng. 120 833-8

[10]. Beck S B M, Wong C C D and Stanway R 2000 Pipe network identification through signal analysis technique 8th Int. Conf. Pressure Surges (The Hague) pp 547-56 [11]. Beck S B M, Williamson N J, Sims N D and Stanway R 2001 Pipeline system identification through cross-correlation analysis Proc. Inst. Mech. Eng. E 216 133-42

[12]. Beck S B M.Curren M D, Sims N D and Stanway R 2005 Pipeline network

Hydraul. Eng. 131 715-23

[13]. Al-Shidhani I, Beck SBM and Staszewski W J 2003 Leak monitoring in pipeline networks using wavelet analysis Key Eng. Mater. 51-8

[14]. Lighthill J 2001 Waves in Fluids (Cambridge: Cambridge University Press) [15]. Randall R B 1987 Frequency Analysis 3^rd edn (Denmark: Bruel and Kjaer) [16]. Beck S B M, Foong J and Staszewski W J 2004 Wavelet and cepstrum analyses of leaks in pipe networks Progress in Industrial Mathematics at ECMI ed A L Bucchianico et al pp 559-563

Claims

1. A leak detection device for a pressurised liquid distribution system, said device comprising: a length of pipe; a coupling on the end of the pipe adapted for connection to hydrants of the system intended to be tested for leaks; a pressure sensor to detect pressure variations in liquid in the pipe; a valve in the pipe spaced from the coupling by at least 10 pipe diameters and arranged so that, when operated to close from an open position when there is a fluid velocity of at least 0.1 metres per second in the pipe, caused by the elevated pressure in the pipeline, a detectable pressure wave is transmitted from the valve through the pipe; and, a controller arranged record pressure in the pipe sensed by said sensor, whereby said pressure wave on closing of the valve is reflected from discontinuities in the system to which the device is attached and such reflections are detected by the sensor, said controller including means to analyse said recorded pressures over time and provide information including the distance from the sensor to a detected discontinuity.

2. A device as claimed in claim 1 , wherein said valve is a solenoid valve and said controller is arranged to operate the valve.

3. A device as claimed in claim 1 or 2, wherein said length of pipe is at least 1 m long, preferably between 1 and 2 m in length, for example between 1.25 and

1.75 m in length.

4. A device as claimed in claim 1, 2 or 3, wherein said length of pipe has an internal diameter between 5 and 100 mm, preferably between 10 and 30 mm.

5. A device as claimed in any preceding claim, wherein said pipe is metallic.

6. A device as claimed in any preceding claim, wherein said length of pipe is straight.

7. A device as claimed in any preceding claim, wherein said controller is an integral, single unit with communication links to the valve and sensor respectively.

8. A device as claimed in claim 7, wherein, said communication links may comprise a wire to carry electrical signals.

9. A device as claimed in claim 7 or 8, wherein said controller performs a wavelet analysis of the signal received from the sensor over a period during which reflections of said pressure wave from discontinuities in the system are sensed by the sensor.

10. A device as claimed in claim 9, wherein a cepstrum analysis of the wavelet analysis is performed by the controller.

11. A device as claimed in any preceding claim in which said sensor is disposed on said coupling.

12. A device substantially as hereinbefore described with reference to the accompanying drawing.