Impedance Method for Leak Detection in Zigzag Pipelines

Transcription

1 10.478/v MEASUREMENT SCIENCE REVIEW, Volume 10, No. 6, 010 Impedane Method for Leak Detetion in igzag Pipelines A. Lay-Ekuakille 1, P. Vergallo 1, A. Trotta 1 Dipartimento d Ingegneria dell innovazione, University of Salento, Via Monteroni, Lee, Italy, [email protected] Dipartimento di Elettrotenia ed Elettronia, Polytehni of Bari, Via Orabona, Bari, Italy Transportation of liquids is a primary aspet of human life. The most important infrastruture used aordingly is the pipeline. It serves as an asset for transporting different liquids and strategi goods. The latter are for example: hemial substanes, oil, gas and water. Thus, it is neessary to monitor suh infrastrutures by means of speifi tools. Leakage detetion methods are used to reveal liquid leaks in pipelines for many appliations, namely, waterworks, oil pipelines, industry heat exhangers, et.. The onfiguration of pipelines is a key issue beause it impats on the effetiveness of the method to be used and, onsequently, on the results to be ounterheked. This researh illustrated an improvement of the impedane method for zigzag pipeline by arrying out an experimental frequeny analysis that has been ompared with other methods based on frequeny response. Hene, the impedane method is generally used for simple (straight) pipeline onfigurations beause ompliated pipelines with many urves introdue diffiulties and major unertainties in the alulation of harateristi impedane and in the statement of boundary onditions. The paper illustrates the ase of a water pipeline where the leakage is aquired thanks to pressure transduers. Keywords: Pipeline leak detetion, magneti sensors, impedane method, transient analysis, pressure transduers T 1. INTRODUCTION HE BUILDING of strategi infrastrutures as pipelines is a matter of interest beause it is a multidisiplinary field that involves finanial resoures as muh as human ones. The problem of proteting natural resoures has beome important beause resoures are limited. In partiular, water is one of the most important natural resoures that is present in many loations all over the world as an environmental omponent. Its rational and lever use is a matter of onern for thousand of millions of people all over the world. Sine sweet water is not unlimited, its use in loations where it is not present authorizes its transportation towards areas where it must be used, thanks to pipelines and artifiial reservoirs and natural basins. For the above reasons, the ontrol and the detetion of leakages in pipelines for water usage is basially an interesting topi for researh. Leak Detetion is aomplished in two phases. During the first phase, the entire system is surveyed for "leak sounds." When a sound is heard, the loation is noted as a potential leak site. Atually, any ondition whih interferes with the normal flow of water an produe vibrations similar to the vibrations aused by leaks. During the seond phase, eah loation is further investigated. If neessary, a omputerized leak orrelator that works on soni transmission (speed of sound) priniples is used to pinpoint the exat loation of the leak. The orrelator eliminates the need for extensive hit-or-miss exavation, and the unneessary destrution of expensive pavement. Without the orrelator, finding many leaks would be like searhing for the proverbial needle in a haystak [1]. Sine urrent leak detetion tehniques rely on vibrations that travel from moleule to moleule along the wall of the pipe, the most important fator affeting leak detetion is the pipe material itself. The denser the wall of the pipe, the greater the distane leak sounds will travel. Density is a funtion of moleular proximity. Cast iron, dutile iron, galvanized steel, and opper pipes are all extremely dense and exhibit exellent transmission qualities. Asbestos-onrete pipe (AC), or transite as it is often alled, is not as dense and dampens vibrations muh quiker than metalli pipes. Due to their lak of density, PVC and polypipe absorb, or attenuate, vibrations rather quikly. As a result, leak sounds do not travel great distanes on these plastis. To ompensate for transmission shortomings, the leak detetion operator will, if at all possible, hoose aess point intervals appropriate for the pipe material []. Pipe diameter also affets sound transmission harateristis. Large diameter pipes tend to attenuate vibrations. Thus, a six-inh iron main will transmit leak sounds farther than a 1-inh iron main. In addition, the degree of soil ompation around a pipe often alters its transmission harateristis. When the soil around a pipe is firmly ompated, the pipe wall loses some of its elastiity, and sound transmission is improved. Generally, the degree of ompation is a funtion of pipe depth [3]. Fig.1 illustrates an overview of leak detetion methods urrently used. Fig.1. Leak detetion method overview. 09

2 MEASUREMENT SCIENCE REVIEW, Volume 10, No. 6, 010. IMPEDANCE METHOD The impedane method is generally used for linear pipelines with less urves and bends, that is, straight pipelines. If we want to use an analytial model, we have to point out the motion equations whih desribe transient onedimensional flow in a ylindrial pipe aording to the Hooke law that an be modified from the model implemented by some authors [4]. Applying the onservation laws of mass and momentum yields the following equations of ontinuity and motion: Q ga H + 0 x t = (1) H 1 Q λqq + + = 0 () x ga t gda where Q is the fluid disharge, H=z+p/ρg is the head, C is the pressure wave elerity, ρ is the fluid density, λ is the oeffiient of frition, A is the ross setion area of the pipe, D is the irle pipe diameter, t is the time and x is the distane along the pipe. The integration of Eqs. (1) and (), utilizing the method of harateristis [5], permits to desribe the pressure time-history at eah omputational point as originated by the propagation of positive and negative pressure waves. Following this approah, first of all, we an briefly desribe two methods: leak analysis using transients and the impedane method [6]..1 Analysis of transients in pipelines The method of harateristis is used to transform the system of partial differential (1) and () into a system of ordinary differential equations that an be integrated numerially without diffiulty. Hene, ga dq ± dh + gajdt = 0, dx =± Cdt (3) C the + indiates the waves are oming from the upstream while the - stands for the waves oming from downstream. ( ga )/ Q J=(λ/D)Q is the linear head loss by unit of the length of the pipe. Equation (3) determines the evolution of the head pressure and the disharge aording to the time and the spae. Its solutions onstitute the solutions to the original system of (1) and (). In an experimental pipeline, when the valve is suddenly losed, an abrupt inrease in pressure wave is then refleted bak by the leak provoking a partial redution of the inoming pressure wave [7]. Then, the wave is refleted bringing on a negative wave, if for instane a reservoir is onneted to the pipe. Analogously, disontinuities in the pipe flow onditions suh as leaks, provoke partial refletion of the inoming pressure wave. Thus, through orretly interpreting these head and leak disharge-time histories at the valve setion, it is possible to extrat information on the leak loation.. Impedane method The instantaneous hydrauli impedane line elevation H and the disharge Q are divided into two parts, the average magnitudes and Q and the osillatory H = H + h and Q= Q+ q [8]. The hydrauli impedane (x) is defined as the ratio of the head flutuation h to the disharge q. Hene h iγ C Ce + Ce Ce + Ce x ( ) = = = q ωga Ce C e Ce C e x γ x γx γx γx 1 1 γ x γx γx γ 1 1 having assumed the following: h=h(x)e jωt and q=q(x)e jωt, where ω is the angular frequeny; γ=α+iβ is generally referred to as the propagation onstant; C 1 and C are the integration onstants from h and q. The harateristi impedane is defined by (4) = γ C C ( β iα ) iωga = ωga (5) Partiular boundary onditions of the pipeline must be envisaged in order to assess the integration onstants C 1 and C. The terms used in (3) are the following: C as water speed, g stands for aeleration of free fall and A is the pipe setion. If the pipe frition is negleted, the final formulation of the pipe impedane with a leak ould be: 1 tan ( πωrl /) + r sx = + tan ( πωrlr / ) / L The leak position l r is alulated from the above relationship by alulating L for eah point where a leak ould be deteted. The system equation without leaks is given by the following: 1+ tan ( πωrl /) r + s = tan ( rlr / ) tan ( r/ ) πω πω L (7) 1 + tan ( πωrlr/ ) tan ( πωr / ) tan ( πωrlr/ ) + L 1 tan ( πωrlr/ ) tan ( rlr/ ) tan ( r/ ) πω πω L + tan ( πωrlr / ) 1+ tan ( πωrlr / ) 3. EXPERIMENTAL FACILITY DESCRIPTION The method has been applied to the hydrauli iruit of Fig., used in previous work [9] in order to make a omparison. The length of the iruit is 59 m while the setion is 1 inh. The inner part of the pipe is in opper with a plasti sheath. 5 water taps are used to simulate leaks and a 1/ (6) 10

3 MEASUREMENT SCIENCE REVIEW, Volume 10, No. 6, 010 series of pressure transduers is loated on the zigzag pipeline. Fig.3 illustrates the signal transmission as a loop departing from the omputer aross different aforementioned devies and apparatuses and arriving to the same omputer. The MATLAB-based interfae is onneted to the hardware thanks to a RS-3 onnetion the user utilizes. The experimental setup inludes, as depited in Fig.4: tank (100 liters); flexible rubber pipe (1 inh); eletri pump (Ebara CMA 1.00T, 0.75kW, 3000 rpm, impeller diameter 160 mm); multilayer pipe in opper, oated with a plastibased sheath, having 11 valves; Gems sensors 00A pressure transduer (output 4-0 ma wires, supply voltage sensitivity 0.01% FS/Volt, supply voltage 1.5 VDC above span to 35 6mA); and various other items suh as eletroni ontrol. remove noise at high frequenies related to analog input of the miroontroller of the PICPLC16 ard. The entire wall mounted pipeline is illustrated in Fig.4. Fig.4. System mounted on the wall. 4. DISCUSSION The algorithm works as follows: alulating the harateristi impedane of all the pipelines (on 59 m), analyzing the suitable frequeny for eah loation of the water tap, alulating L and sx for eah water tap position, it is possible to determine leak position l r thanks to (4). Further explanations are derived from Fig.5. The frequenies are experimentally reovered from plots of Fig. and Fig.3 for eah position of presumed leaks by transforming the time domain in frequeny one versus pressure. Only the time (then the frequeny) that orresponds to a high peak is onsidered for the impedane method. The proedure is performed by alulating the impedane at the presumed leak as indiated in Fig.6 and Fig.7. Fig.. Experimental plant layout. The eletroni ard PICPLC16 with the miroontroller has analog inputs, PWM (Pulse Width Modulation) outputs and a RS-3 onnetion. The suessive ative lowpass iruit transforms PWM signal into a signal proportional to the duty yle of PWM. Fig.5. Impedane method proedure. Fig.3. Experimental plant data arhiteture. The water flow variation is aomplished by ating on the pump voltage thanks to miromaster 410 inverter (0.55kW); afterwards, on the above desribed pressure transduer. The suessive differential amplifier allows the transformation of differential voltage oming from the pressure transduer into one amplified signal referred to the system potential GND, where GND stands for ground. The last blok illustrates a lowpass iruit with an appropriate gain to Fig.6. Opening and losing trend for leak1. 11

4 MEASUREMENT SCIENCE REVIEW, Volume 10, No. 6, 010 Fig.7. Opening and losing trend for leak 4. Fig.11. Time analysis for leak 6. The results of the aforementioned algorithm are illustrated in Fig.8 - Fig.11 for eah leak of the experimental pipelines. In absene of leaks, the system displays the plot of Fig.8 in aord with the relationship (7), while the other plots show the detetion of leaks aordingly. Simultaneously, during the aquisition of data for the appliation of the impedane method, FFT and STFT algorithms were applied for a omparison, as depited in Fig.1. Tab.1 illustrates the final results with appropriate omparisons among the different methods. Fig.8. Trend without leaks. Fig.9. Time analysis for leak 1. Fig.1. FFT and STFT algorithms. Fig.10. Time analysis for leak 3. Tab.1 also illustrates the differene between zigzag pipe and a straight pipe. Hene, the modified impedane method demonstrates the possibility of a good appliation for a 1

5 MEASUREMENT SCIENCE REVIEW, Volume 10, No. 6, 010 double zigzag pipeline. The experimental hydrauli iruit has a ounter-urrent flow in a vertial diretion after the 6 th leak. So, the ondition of the appliation of the proposed method is very onservative with unexpeted diffiulties with respet to ommon plants enountered in reality. Table 1 Experimental results. 5. CONCLUSIONS As we have already mentioned in the introdution, water is beoming one of the most preious natural resoures. Human ativities require huge amounts of water and thousands of ubi meters of water are wasted during its transportation with pipelines and distribution by means of urban waterworks. Implementing water leak detetion is a hallenging task for many researhers. Impedane method is one of them. The appliation of (4) for all leaks, with a good and preised frequeny analysis has yielded the results depited in Tab.1. In that table a omparison is arried out between the impedane method and the STFT (Short-time Fourier Transform) and FFT (Fast Fourier Transform). The proedure that leads to STFT and FFT has been explained in [10]. In the same table, a simulation has been exeuted by onsidering the pipeline as a straight one. Thus, the results are, of ourse, better than for the zigzag pipe. This last performane is due to the nature of the impedane method for straight pipeline. However, the frequeny analysis proedure used in this paper has shown good and omparable results with respet to STFT and FFT. That is the desired improvement found in this researh. REFERENCES [1] Water & Waste Pipe Testing In. (1999). A Report for Conduting a Water Leak Detetion Survey on the Entire Water Distribution System for the Town of Ipswih. Wakefield, MA, USA. [] Georgia Enviromental Protetion Division. (007). Water Leak Detetion and Repair Program. Atlanta, USA: Georgia EPD. [3] New Mexio Environmental Finane Center. (006). Analysis of Fluid Conservation System s Leak Detetion Tehnology. Albuquerque, NM, USA. [4] Wylie, E.B., Streeter, V.L., Suo, L. (1993). Fluid Transients in Systems. Prentie Hall. [5] Fox, J.A. (1977). Hydrauli Analysis of Unsteady Flow in Pipe Networks. Halsted Press. [6] Al-Khomairi, A.M. (005). Use of the steady state orifie equation in the omputation of transient flow pipe leaks. The Arabian Journal for Siene and Engineering, 30 (1B), [7] Mpesha, W., Chaudry, M., Grassman, S.L. (001). Leak detetion in pipes by frequeny response method using a step exitation. Journal of Hydrauli Researh, 40 (1), [8] Lee, P., Vítkovský, J. (006). Disussion of Detetion of partial blokage in single pipelines by P.K. Mohapatra, M.H. Chaudhry, A.A. Kassem, J. Moloo. Journal of Hydrauli Engineering, 13 (), [9] Brunone, B., Ferrante, M. (1999). On leak detetion in single pipes using unsteady-state tests. In Hamza, M.H. (ed.) IASTED International Conferene on Modeling and Simulation, May 5-8, Anaheim, CA: Ata Press, [10] Lay-Ekuakille, A., Vendramin, G., Trotta, A., Vanderbemden, P. (009). STFT-based spetral analysis of urban waterworks leakage detetion. In Girao, P.S., Ramos, P.M. (eds.) IMEKO XIX World Congress: Fundamental and Applied Metrology, September 6-11, 009. Lisbon. Portugal. Reeived November 17, 010. Aepted Deember 30,