First Experimental Results

Water Distribution System Analysis 2010 – WDSA2010, Tucson, AZ, USA, Sept. 12-15, 2010

FAST TRANSIENTS AS A TOOL FOR PARTIAL BLOCKAGE DETECTION IN PIPES: FIRST EXPERIMENTAL RESULTS S. Meniconi, B. Brunone, M. Ferrante, and C. Massari Dipartimento di Ingegneria Civile ed Ambientale, University of Perugia Perugia, Italy [email protected]

Abstract Partial blockages in pipes cause minor head losses that reduce the carrying capacity and increase operating costs due to energy consumption. The same effect – i.e., an undesirable minor head loss – can additionally be due to negligently partially closed in-line valves. Early detection of these singularities is of interest since prompt remedial actions have to be taken to keep on the requested performance level of the pipe. In the last decade, a number of promising transient test-based techniques were presented to detect blockages and to check the position and asset of in-line devices. Particularly for supply systems, these techniques are attractive for two main reasons: their very short duration and the possibility of using low cost pressure probes. This paper shows the first results of an extensive experimental campaign carried out at the Water Engineering Laboratory (WEL) of the University of Perugia, Italy, on a long high-density polyethylene (HDPE) pipe system where the partial blockage is simulated by means of a smaller diameter HDPE pipe.

Keywords Transients, partial blockage, in-line valve, pipe diagnosis.

1.

INTRODUCTION

Partial blockages in pipes cause minor head losses that reduce the carrying capacity and increase operating costs due to energy consumption. Blockages can be due to deposition of excess calcium carbonate scale when water is supersaturated with calcium (Photo 1), deposit of sediment and water freezing in cold climates (Walski, 1984). They may also be formed by agents such us paraffins, asphaltenes, hydrates and other chemicals in the petroleum and chemical industry (Scott and Satterwhite, 1998). The same effect – i.e., an undesirable minor head loss – can additionally be due to negligently partially closed in-line valves. Early detection of these singularities is of interest since prompt remedial actions have to be taken to keep on the requested performance level of the pipe system. In the last decade, a number of promising transient test-based techniques were presented to detect singularities (e.g., blockages, partially closed in-line valves, and leaks). These techniques are attractive because of their very short duration – which significantly reduces both the interference with the regular functioning of the system and the personnel costs – and the possibility of using low cost pressure probes. With regard to test duration, unlike acoustic techniques, which require a time consuming survey of pipes, during transient tests pressure waves travel fast along the pipe pointing out singularities. For what it concerns the standard quality of pressure transducers, it has to be pointed out that when transient tests are carried out to localize a singularity, attention is focused on pressure changes instead of the pressure value itself. As for leak detection purposes (e.g., Brunone 1989; 1999; Mpesha et al. 2001; Wang et al. 2002; Lee et al. 2005), in order to evaluate the position and characteristics of singularities, different approaches can be followed for the analysis of the pressure signal (i.e., the time-history of the piezometric head) at selected measurement sections. They can be classified with respect to the domain – time domain, frequency domain or by means of wavelet functions – and the duration of the analysis, short and long period analysis, respectively. If the analysis of the pressure signal is focused on the first characteristic time of the pipe, as in this paper, it is defined as short period analysis with respect to the long period analysis in which a much longer period of time is taken into account. Within the long period analysis, in the case of transients generated by the fast closing of a valve, by means of a discrete Fourier transform algorithm, Wang et al. (2005) analytically prove that the blockage-induced damping depends on the blockage’s magnitude and position but they do not produce a single value for the location of the blockage. Transients generated by the periodic opening and closure of a valve show that the presence of a blockage produces a change in the


pressure response by decreasing the amplitude of pressure oscillations at the odd harmonics and increasing the amplitude of pressure oscillations at even harmonics. Specifically, in Mohapatra et al. (2006a; 2006b) the pattern and number of peaks in the peak pressure frequency response is used to predict the location of a partial blockage in single or branched pipe systems; furthermore the mean peak pressure fluctuation is used to determine its size. In Lee et al. (2008) the properties of blockage-induced oscillations are numerically determined using a Fourier transform of the inverted peak magnitude in the frequency response diagram. Sattar et al. (2008) reveal that the increase of amplitude of the pressure oscillations at even harmonics has a periodic pattern whose amplitude and frequency depend on the location and size of the partial blockage. This paper shows the first results of an experimental campaign carried out at the Water Engineering Laboratory (WEL) of the University of Perugia, Italy, in order to show the mechanism of interaction between a pressure wave and a partial blockage simulated by means of a smaller diameter pipe.

Photo 1. Partial blockage in a ten years old steel pipe that caused a 60% diameter reduction.

Figure 1. RLBV system: experimental set-up.

2.

EXPERIMENTAL SET-UPS

Experimental tests at WEL are executed in two different high-density polyethylene (HDPE) pipe systems supplied by a constant head reservoir. In the RBV system (Reservoir-in line Ball valve-end Valve) the pipe has a length L = 164.93 m, an internal diameter D = 93.30 mm, a nominal diameter DN110, with a ball valve – DN90 and internal diameter, db = 82.50 mm – located at a distance L’ = 75.97 m from the end valve. The other pipe system (Figure 1) – RLBV system (Reservoir-Long Blockage-end Valve) – is more complex since it consists of three pipes in series. In such a system, the upstream main pipe, that links the reservoir with the second pipe, has a diameter equal to D and a length equal to 62.23 m. The second pipe − simulating a long partial blockage (BL) − has a diameter dBL = 38.30 mm, DN50 and a length LBL = 100 m. The downstream main pipe of diameter D and length L’ = 110.44 m, connects the long blockage to the end valve. Then, according to the steady-state flow direction, the Upstream Connection (UC) (Figure 1) − i.e. the connection between the upstream main pipe and the long blockage − is a sudden contraction whereas the Downstream


Connection (DC) is a sudden enlargement (Photo 2). Transients are generated by the fast and complete closure of the end valve, a DN50 ball valve. The main measurement section, hereafter referred to as section M, is placed immediately upstream of the maneuver valve. Furthermore, in order to study the mechanism of interaction between pressure waves and the long blockage, in the RLBV system further pressure measurement sections were added (sections N, P and S placed at a distance LN = 102.77 m, LP = 130.90 m, and LS = 190.90 m from the maneuver valve, respectively, Figure 1). The pressure signal, h, is acquired by piezoresistive transducers with a frequency acquisition of 1024 Hz. The steady-state discharge and minor losses are measured by means of a electromagnetic flowmeter and a variable reluctance differential pressure transducer, respectively.

Photo 2. The connection between the main pipe and the partial blockage in the RLBV system.

3. HYDRAULIC BEHAVIOR OF THE BALL VALVE AND THE DOWNSTREAM CONNECTION (DC) In order to determine the hydraulic behavior of both the ball valve and the DC connection, steady-state tests were carried out. The minor loss across the ball valve, ζ0,B, was measured for different values of the discharge Q0 and the opening degree AB/A, with AB = ball valve cross-sectional area, A = main pipe area and the subscript “0” denoting the steady-state. A second series of tests concerned the evaluation of the minor loss across the DC connection, ζ0,DC, for different values of Q0. According to usual flow conditions in real pipe systems, experiments concerned wholly turbulent flow. Then the constant value of the minor head loss coefficient, χi, was determined through the following equation:

Q02 ζ 0,i = χ i 2gA2

(1)

in which i = B denotes the ball valve whereas i = DC denotes the DC connection, with g = gravity acceleration. The dependence of the experimental values of χB and χDC on the steady-state Reynolds number, N0 = V0D/ν, with V0 = Q0/A = mean flow velocity and ν being the kinematic viscosity of the fluid, is reported in Figure 2.

4.

LONG BLOCKAGE DETECTION BY MEANS OF TRANSIENT TESTS

4.1 Effects of the interaction between a pressure wave and a long blockage on the pressure signal The mechanism of interaction between a pressure wave and a blockage has been poorly investigated experimentally. As an example, Figure 3 shows the pressure signals acquired at measurement sections M, N, P, and S in the RLBV system in the case of Q0 = 1.80 l/s. In order to highlight pressure waves clearly, in the plot h is the difference between the transient and the steady-state value, h0; t is the time relative to the beginning of the maneuver. Each sharp variation in the


pressure signals corresponds to a pressure wave crossing the measurement sections. Particularly, at t1 the wave generated by the closure of the end valve passes through section M causing the first sharp pressure rise (ΔhM,V = 10.09 m). This wave proceeds toward the reservoir, arriving at section N at t2 (see the increase in the dashed line). When at t3 the pressure wave generated by the maneuver reaches the DC connection, it is partially reflected and returns to sections N and M. Since the maneuver is not instantaneous and there is a small distance between section P and the DC connection (about 20 m), the value of pressure increase (= 14.23 m) at t2 is due to the combination of the arriving of the wave generated by the maneuver and the wave reflected by the DC connection. Furthermore this reflected pressure wave reaches section M at t7 and causes a pressure rise (= 12.45 m). It has to be noted that such a rise is approximately double the wave reflected by the DC connection, ΔhM,DC, because at t7 the end valve is fully closed. On the other hand, the pressure wave that continues beyond the DC connection reaches sections along the long blockage P and S, at t4 and t5, respectively. Moreover, at t6 this wave arrives at the UC connection and it is partially reflected and transmitted. This reflected pressure wave reaches section M at t8 and causes a sharp decrease (= -3.67 m).

Figure 2. Minor head loss coefficient of the ball valve and DC connection vs. steady-state Reynolds number.

Figure 3. RLBV system: pressure signals at sections M, N, P and S (Q0 = 1.8 l/s).


4.2 Localization of the long blockage For pressure signals of Figure 3, the detection of the abovementioned instants of time has been done by means of wavelet analysis, which is a powerful tool for edge detection (Ferrante et al. 2007; 2009) that improves the precision of the procedure with respect to the straightforward time domain analysis (Stoianov et al. 2001; Al-Shidhani et al. 2003). An example of the wavelet analysis’ performance is given in Figure 4, where the wavelet analysis properly points out all discontinuities of the pressure signal hM of Figure 3.

Figure 4. RLBV system: a) pressure signal at section M of Fig. 3; b) corresponding wavelet analysis. On the basis of these times, the pressure wave speed of the main pipe, a, and the long blockage, aBL, can be evaluated using the following relationships, respectively:

a=

LN L − LP ; aBL = S t2 − t1 t5 − t4

(2)

which links the distance from the measurement section M, LN, LS and LP, to times t1, t2, t4 and t5. It is worthy of noting that, according to Eqs. (2), the localization of singularities coincides with the evaluation of the times at which pressure waves pass through the measurement section M. The pressure wave speed of the main pipe and the long blockage from Eqs. (2) are equal to 380.00 m/s and 408.16 m/s, respectively. Moreover, the position of the long blockage, i.e. the location of the DC connection, is given by:

L′ =

t7 − t1 a 2

(3)

In this case the distance L’, calculated from Eq. (3), is equal to 108.87 m, with a relative error of 1.4%. In addition, it is possible to evaluate the length of the long blockage by means of the following relationship:

L'BL =

t8 − t7 aBL 2

(4)

with a relative error equal to 2.6%. It is worthy of noting that – with respect to time domain analysis – the use of wavelet functions not only improves the precision in the evaluation of times in Eqs. (2), (3) and (4) and consequently in the localization of the singularities, but also furthers the impartiality of the pressure signal analysis.


4.3 A long blockage vs. a partially closed in-line valve Figure 5 compares the pressure signals acquired at section M in the RBV and RLBV systems during transient tests with the same steady-state discharge, Q0 = 2.60 l/s – and thus the same pressure wave originated by the closure, ΔhM,V (= 13.69 m) – and the relative area AB/A = ABL/A = 0.1685 (ABL = long blockage area). As numerically demonstrated by Wylie and Streeter (1993) and experimentally by Meniconi et al. (2009; 2010), the status of an in-line valve, i.e., the valve opening degree, can be evaluated by means of transient tests since the value of the pressure wave reflected by the valve, approximately 0.5ΔhM,B, is directly proportional to ζ0,B (for further details, see Meniconi et al., 2010). In the case of the long blockage, the steady-state minor head loss does not play a similar crucial rule. As an example (Figure 5), the same ΔhM,V determines very different values of the pressure wave reflected by the DC connection (≈ 0.5ΔhM,DC = 7.92 m) and the in-line ball valve (≈ 0.5ΔhM,B = 1.10 m). This higher value of 0.5ΔhM,DC with respect to 0.5ΔhM,B is not due to a higher value of ζ0,DC with respect to ζ0,B (ζ0,DC = 0.058 m and ζ0,B = 1.18 m).

Figure 5. RBV and RLBV systems: pressure signals at section M (Q0 = 2.60 l/s and AB/A = ABL/A = 0.1685).

4.4 Laboratory results vs. simple numerical models: sizing of the long blockage Simple numerical models, simulating the interaction between a change of diameter and a pressure wave, are advantageous in practical applications because they allow taking a prompt decision. They are based on the assumptions of instantaneous closure of the maneuver valve and negligible friction losses. Specifically, by integrating the differential equations governing frictionless transients, Swaffield and Boldy (1993) obtained the following relationship:

δDC = cΔV

(5)

where δDC is the pressure wave reflected by the DC connection, whereas ΔV is the incident pressure wave and c = (aBL/ABL-a/A)/(aBL/ABL+a/A). By means of Eq. (5), the value of dBL can not be straightforwardly evaluated. In fact, due to the distance L’ between the measurement section and the DC connection, there is a difference between ΔV and ΔhM,V, and between δDC and 0.5ΔhM,DC. Figure 6 shows the comparison between the experimental values of 0.5 ΔhM,DC and the corresponding numerical values of δDC given by Eq. (5), vs. the steady-state Reynolds number, N0. According to Covas et al. (2004) and Brunone et al. (2004), differences between 0.5 ΔhM,DC and δDC are mainly due to the effect of pipe viscoelasticity that causes a damping of the


travelling pressure waves. According to Ramos et al. (2004), the related damping factor k has been experimentally estimated in a straight pipe (Meniconi et al., 2010). The best fitting relationship that gives k as a function of the Reynolds number is:

k = 4 10-7 N

(6)

In order to consider this attenuation, Eq. (5) has been modified in the following relationship:

ΔhMk ,DC = c(ΔhM ,V − k0L')− k1L'

(7)

where k0 and k1 are given by Eq. (6) with N = |ΔV0|D/ν – with ΔV0 = (0-V0) being the velocity change associated to the incident pressure wave – and N = |ΔVr|D/ν – with ΔVr = (Vr - 0) being the velocity change associated to the pressure wave reflected by the DC connection – respectively, with Vr = gΔhM,DC/a.

Figure 6. Pressure wave reflected by the DC connection: numerical models vs. experimental data.

The good agreement between numerical data modified by means of such a damping factor and experimental data is shown in Figure 6. By means of Eq. (7), it is possible to derive straightforwardly the value of dBL. Data of Figure 5 generate an estimated value of dBL = 38.08 mm, with a very small relative error (= 0.55%).

5.

CONCLUSIONS

This paper presents the results of the experimental tests carried out at the Water Engineering Laboratory (WEL) of the University of Perugia that allow the detailed analysis of the interaction between a pressure wave – generated by the fast and complete closure of a valve – and a long partial blockage. The difference between a long partial blockage and a partially closed in-line valve has been pointed out. Within the transient test-based approach, if the analysis of the pressure signal is focused on the first characteristic time of the pipe, wavelet analysis can be used to localize and measure the length of the blockage, confirming the reliability of this tool. Furthermore it is shown that the dimension of the blockage can be evaluated reliably by means of simple numerical models.

Acknowledgments. This research has been supported by the Italian Ministry of Education, University and Research (MIUR) under the Project of Relevant National Interest “Innovative criteria for the sustainable management of water resources in the water distribution systems”.


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