The HT analysis

Figure 6-10 shows a typical signal from a simulated network. By using the HT, the instantaneous frequency and phase angle of the original signal was obtained. Figure 6- 11(dash line) shows the instantaneous characteristics of the signal. Both instantaneous phase and frequencies clearly highlight the presence of a reflection. The peak of the analyzed signal matched up with the time taken for the wave to travel along the pipe network to the reflection point and return to the measurement point. The distance of the reflection point is calculated by multiplying the time delay corresponding to the peak by the speed of sound in the pipe network (a=493 m/s) and halving this value to account for the return journey.

The instantaneous characteristics of the analysis for the network with a leak positioned approximately 27m from the measurement point are also shown in Figure 6-11(solid line).

121 A new peak is now present, which indicates a leak. The two cases analyzed above reveal the relationship of the instantaneous characteristics in the signal and features of the simulated pipe network. In the first case, the change of phase and peaks in the instantaneous frequency are observed at a distance of about 80m. This distance corresponds to the location of the pipe end. In the second case where leaking point was introduced; additional change of phases and instantaneous frequency occur approximately 27m from the valve.

122 Figure 6-11: Instantaneous phase (top) and instantaneous frequency (bottom) analysis using HT of simulated networks without leak (dash line) and with a leak (solid line) located 27m from the valve. The first experiment was conducted without the present of a leak. The data captured by pressure transducer and the processed result using the HT of the signal without a leak is shown as Figures 6-12 and 6-13 respectively. From the first of these, it can be seen that the pressure in the pipeline goes up when the solenoid valve is closed also the reflections of the pressure wave in the pipe. From Figure 6-13, the instantaneous phase and frequency show the signature of reflection point, which occurred approximately 0.323 seconds after the valve shut. This is the time taken for the waves to propagate along the pipeline network to the reflection point and return to the measuring section. If this time is multiplied by the speed of sound and divided by two, it gives 79.6m, which corresponds to the distance to the outlet of the pipe located 80m from the measuring section, with the error of about 0.5%.

Outlet Leak

Outlet

123 Figure 6-12: Experimental pressure signal without the present of leak.

Figure 6-13: Instantaneous phase (top) and instantaneous frequency (bottom) from HT analysis of experimental data without the present of leak.

Outlet

124 The second experiment was conducted with the presence of a leak 27m from the measuring point. Figure 6-14 shows the experimental pressure signal for one complete cycle of the system. By using the HT, we can obtain the instantaneous characteristics of the signal. Figure 6-15 shows the instantaneous frequency and phase angle of the measured signal. It can be clearly seen in the results from the HT analysis that there are two peaks, which are the signature of the reflection points corresponding to the leak, and the end of the pipe. The time corresponding to the first reflection point is 0.11s which gives an analyzed length of 27.12m. Compared to the actual leak (27m from the measurement point), the error is about 0.44%. For the outlet (80m from the measurement point), the time is 0.32s which corresponds to an analyzed length of 79.6m, giving an accuracy of about 0.5%.

125

Figure 6-15: Instantaneous phase (top) and instantaneous frequency (bottom) from HT analysis of experimental data with a leak located 27m from the measuring point.

For the next two experiments leaks were introduced 35m and 74.5 from the measuring point. The pressure signal data for these experiments are shown as Figure 6-16.

Figure 6-16: Experimental pressure signal with leak located at 35m (top) and 74.5m (bottom) from measuring point.

Leak Leak

Outlet

126 The signal data has again been analysed using HT and the processed results are shown as Figures 6-17 and 6-18. As it can be seen in Figure 6-14, two peaks appear at 0.145s and 0.320s. This value corresponds to the location of the leak (35m) and also the exit of pipe (80m) with errors of about 1.91% and 1.49% respectively. Moreover, Figure 6-18 shows the occurrence of a reflection at 0.297s and 0.321s which also corresponds to the locations of the leak (74.5m) and the outlet of the pipe (80m), the error is about 0.98% for the leak and 1.06% for the outlet of the pipe. It can be noted that in Figure 6-15, the presence of noise is also obvious; this can be filtered out using a suitable filtering method which will be discussed in the next section.

Figure 6-17: Instantaneous phase (top) and instantaneous frequency (bottom) from HT analysis of experimental data with a leak located 35m from the valve.

Outlet Leak

127 Figure 6-18: Instantaneous phase (top) and instantaneous frequency (bottom) from HT

analysis of experimental data with a leak located 35m from the valve.

Table 6-1 summarises the results of all the tests. In general, both experimental and simulated results confirm that the HT can provide simple and clear results, which indicate that this analysis technique can locate leaks and features in simple pipe systems with acceptable errors.

Table 6-1: Summary results of the tests with and without leaks using HT.

Test Analysed time (s)

Corresponding Analysed Distance

(m)

Measured

Distance (m) Error (%)

Leak Outlet Leak Outlet Leak Outlet Leak Outlet No Leak - 0.323 - 79.6 - 80 - 0.5 Test1 0.11 0.323 27.12 79.6 27 80 0.44 0.5 Test2 0.145 0.32 37.74 78.8 35 80 1.91 1.49 Test3 0.297 0.321 74.21 79.15 74.5 80 0.98 1.06 Outlet Leak Outlet Leak

128

6.4.2 The HHT analysis

In this section, the signal is analysed using a newly proposed method from Huang [119]; the Hilbert Huang transform (HHT). As briefly discussed in Chapter 4, the best representation of the results obtained using the HHT for the given data analysis is the Hilbert-Huang spectrum (HHS). Firstly, the signal needs to be decomposed using EMD before it can be presented in the form of the Hilbert spectrum. The processed results presented in this section use the distance in metres for the x-axis instead of the time in seconds as shown in the previous section. It is simple to convert from time to distance by multiplying the time taken that wave to travel by the speed of sound divided by two. Figure 6-19 shows the eleven IMFs and their residue from the simulated signal without the existence of a leak. The first three IMFs contain the highest frequency while higher levels of IMF correspond to low frequencies. The offset and the basic response of the system are in the low frequency range; these levels were therefore discarded. By selecting IMF4-IMF7 and summating them generates a pressure signal without noise. Consequently, Figure 6-20 shows the HS of the pressure response of the pipe network without a leak. As expected, it can be seen that there is only one high peak at a location about 80m from the valve, showing the reflection of the pipe end. Figure 6-21 shows the HS for signals of selected IMF4-IMF7 of pipe network with a leak present. Figure 6-21 also shows a peak at about 80m, in addition, we also have a peak at approximately 27m, which corresponds to the leak position. The analysis using HHT produced a good result for a leak and also can identify the end point in the simulated pipeline network. Incidentally, the HS also generates unwanted IMFs in the low frequency region, indicating a disadvantage of the EMD method, which is not encountered by the HT.

129

130 Figure 6-20: HHT spectrum analysis of IMF4-IMF7 of simulated network without leak.

Figure 6-21: HHT spectrum analysis of IMF4-IMF7 of simulated network with leak located 27m from the valve.

Outlet

Outlet Leak

131 The experimental data from the rig described above was then was analyzed using the HHT. The signal with the leak located 27m from the measuring point was decomposed into 11 IMFs and its residue as shown in Figure 6-22. As we can see from this, IMF1 to IMF3 contain the highest frequencies (and highest energy), which is mostly noise. The other IMFs are in the low frequency range. In addition, EMD clearly displays spatial and temporal information from the pressure wave signal records. These features could not have been extracted using classical Fourier transform analysis. By removing IMF1 to IMF3 and summing IMF4-IMF8, a pressure signal without noise was generated and when compared with original signal (as shown in Figure 6-23), it looks similar but more clean. As explained by Flandrin et al [144], the EMD technique can be worked as the filter to remove the noise from the data. Furthermore, Figure 6-24 shows the HS of the selected IMF. From the results shown in the HS, clearly it can reveal the reflection points corresponding both to the location of the leak and the end of the pipe, which are located 27m and 80m, respectively from the measurement point.

132 Figure 6-22: The IMF’s and its residue of the experimental data.

133

Figure 6-23: Experimental pressure signal for leak at 27m from measuring point and filtered signal without noise using EMD.

Figure 6-24: HHT spectrum analysis of IMF4-IMF8 of experimental data with leak located 27m from the valve.

Outlet

134

Meanwhile, for the test of the leak that located 35m and 74.5m from the measurement point IMF4- IMF9 have been utilized for both analysis. The processed result using HHT for the both experiments is shown in Figure 6-25 to Figure 6-28.

Figure 6-25: Experimental pressure signal for leak at 35m from measuring point and filtered signal without noise using EMD.

Figure 6-26: Experimental pressure signal for leak at 74.5m from measuring point and filtered signal without noise using EMD.

135 Figure 6-27: HHT spectrum analysis of IMF4-IMF9 of experimental data with leak located

35m from the measuring point.

Figure 6-28: HHT spectrum analysis of IMF4-IMF9 of experimental data with leak located 74.5m from the measuring point.

Leak Leak

Outlet Outlet

136 As shown in Figure 6-25 and 6-26 the pressure signals have been filtered using the EMD technique to remove the noise. The reconstructed signal using IMF4-IMF9 was then analysed using HS. The result is shown as Figures 6-27 and 6-28. As it can be seen from the figures, the reflection corresponding to both the leak and outlet of the pipe can be clearly captured using the HHT method. On the other hand, there is an unnecessary ripple which still exists in the low frequency region caused by the EMD method. There is also visible in the presence of noise. As explained by Huang [119] It is impossible to remove all the noise that is buried in the signal.

Table 6-2 summarises the results of all of the tests. In general, both experimental and simulated results confirm that the HHT can provide simple and clear results, which indicate that this analysis technique can locate leaks and features in simple pipe systems with acceptable errors.

Table 6-2: Summary results of the tests with and without leaks using HHT.

Test Analysed time (s)

Corresponding Analysed Distance

(m)

Measured

Distance (m) Error (%)

Leak Outlet Leak Outlet Leak Outlet Leak Outlet Test1 0.111 0.326 27.35 80.32 27 80 1.28 0.4 Test2 0.144 0.325 35.44 80.13 35 80 1.24 0.16 Test3 0.303 0.326 74.68 80.31 74.5 80 0.24 0.4

137

6.5 Summary

The instantaneous phase and frequency of pressure waves through fluid-filled pipelines produced using both simulated and experimental signal were analyzed in order to identify leaks and other features. These two characteristics were extracted using the HT and the HHT.

Neither method had previously been applied in a systematic way to the problem of identifying features in pipeline systems. The results confirmed that HT analysis can identify features in simple pipeline systems for both simulated and experimental signals. The features that were identified as causing a reflection were the leak and the pipe end. The location of the leak by the HT approach was excellent with a low percentage of error. Furthermore, the similar HHT analysis worked well to identify features if experimental data is used.

138

Chapter 7

Comparative Study of Instantaneous

Frequency Based Methods for Leak

Detection in Pipeline Networks.

7.1 Introduction

Yorkshire Water is a water supply company servicing South Yorkshire, West Yorkshire, the East Riding of Yorkshire, part of North Lincolnshire, most of North Yorkshire and part of Derbyshire, in England with 1.7 million household. They provide about 1291m litres through 31062 km of mains pipeline. The geographical position of Yorkshire Water compared to other water companies in England and Wales is shown as Figure 7-1.

139 As reported by the UK industry regulator (OFWAT) the total daily water loss in England and Wales due to occurrence of leakage is about more than 1.8 billion litres, or 12% of the water that the companies put into the distribution system [27]. It will cost about £100 billion to replace all of existing pipeline networks and every customer’s supply pipe [27]. As a result, the total costs and the impact on customers’ bills would be several times higher. As well as renewing pipelines, leak reduction can be effectively implemented by fixing specific leaks. A variety method as explained in the Chapter 2 have been developed and implemented by the researchers in order to pinpoint the location of the leak. However, there a very few study that have been validated with the field data [164].

For the real systems filtering is required to remove unwanted features such as noise, trends or frequency components prior the instantaneous frequency (IF) analysis. Many methods of filtering could be used to remove noise; the current study utilizes the empirical mode decomposition (EMD) filtering procedure due to nonstationary nature of the analysed data. The data are decomposed to different levels with different frequency bands. The low levels contain the high frequency (system noise) and high levels of this decomposition contain low frequency elements (the basic system response in this case). Summing the signal without the low and high frequency components allows a filtered signal to be produced. The work presented here will demonstrate its applicability in live water distribution networks and then compared with existing methods of analysing instantaneous frequency. The next section discussed the leak detection scheme and tested via simulation followed by implementation of the proposed method using field test data.