Leak Detection in Pipelines Using Hydraulic Transients L

Leak detection in pipelines using hydraulic transients

L. Jonsson

Department of Water Resources Engineering, University of Lund, S-221 00 Lund, Sweden

Abstract

Analysis of pressure transients in long, single pipelines for water transport has a potential of providing information on hydraulic features such as air/gas pockets,leaks. Thus the transient measured in a pipeline for sewage water had an oscillation period compatible with the location of a local high point of the pipe profile. The effect of leaks on transients was studied by means of computer modelling. It was found that leaks - even small ones of the order of

1 % - interacted significantly with the basic transient. One effect is a more or less strong attenuation of the pressure oscillations indicating the existence of a leak. Secondly the leak gives rise to extra pressure wave reflexions the time scale of which can be used for locating the leak.

1 Introduction

Conveyance of water often takes place in single pipelines of considerable length. A good hydraulic performance of the pipeline is important from technical, economical and environmental points of view. Sewage water leaks could for instance cause pollution problems in the ground whereas leaks in water supply pipelines might involve a significant economical loss but also a health hazard if polluted groundwater is drawn into the pipe during transient, low pressure periods. Air/gas pockets could reduce the capacity of a pipeline but also give rise to surge-like flow due to interactive effects. There is thus a need for monitoring the hydraulic status of such pipelines with the purpose of indicating the existence and possibly the approximate location of different kinds of "irregular" hydraulic features such as leaks, air/gas pockets, unexpected constrictions etc.

This paper will discuss the use of pressure transients due to normal flow changes such as pump stop and/or valve closure as a means of obtaining

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information on possible hydraulic irregularities, mainly leaks but also gas pockets. The latter will be illustrated by a simple field case whereas the former is discussed on the basis of computer model results.

2 Background

The use of pressure transients as a means of obtaining information on some hydraulic properties of conduits is based on the following idea. A pressure transient generated at a point of a pipeline will propagate back and forth in the pipeline at the same time being affected by certain properties of the pipeline and the flow. Thus the pressure waves might be considered as a "probe" propagating through the pipeline. Measurement and subsequent analysis of the pressure transient could provide some information on the pipeline such as gas pockets, single or extended leaks, changes in pipe material, pipe diameter. One effect could be that part of the pressure wave is reflected thus changing the appearance of the measured pressure transient in comparison with the case with no "flow disturbance". An analysis of the measured transient could potentially

- in the first place - provide an indication of the existence of a flow irregularity. Secondly a closer study of the transient might give a clue to the approximate location of the irregularity.

length (m) 1000 2000

Figure 1. Pipe profile for sewage conduit and measured pressure transient for simultaneous stop of two pumps with check valves. Oscillation period T = 23 s.

The potential of pressure transients analysis will first be illustrated by a field case with a probable gas pocket. Fig 1 shows the profile of a 225x11 mm

PVC sewage water conduit 4400 m long with a pumping station at the

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upstream end equipped with two pumps in parallel and check valves. Fig 1 also shows the measured transient at the stop of two pumps simultaneously. The measured period of the pressure oscillations were found to be T = 23 s. Using the relation T = 4L/a, with L=length of the conduit, a = wave velocity one obtains a —765 m/s which is far too high for a PVC conduit. Introducing a realistic wave velocity instead - say a = 390 m/s - gives L =23-390/4=2260 m. As there is a distinct local peak of the pipe profile at about this point one could expect a gas pocket to exist there - especially as sewage water is pumped according to the on/off principle implying long periods between pump operation. Thus the gas pocket - if large enough - will act as a total reflector for the pressure waves.

3 Effect of a leak on a pressure transient - general discussion and computer model

The possibility of detecting a leak by analysing a pressure transient is based on the assumption that a (small) leak will cause (small) extra reflexions which will modify the basic transient propagating back and forth in the pipeline. An analysis of the modification should supposedly lead to an indication of the existence of a leak and also an approximate location for a spatially concentrated leak. An obvious example of the effect of a leak is sketched in Fig 2 where a transient is generated by tripping the pump and subsequently closing the valve. If the pump inertia is small the pressure will drop instantaneously and part of this negative pressure wave will be reflected at the leak a distance L from the valve and will reach the valve again after the time 2LVa. This can be seen on the pressure registration (computed) as an abrupt change. In order to study the effects of different kinds of leaks on the pressure transient more systematically computer model calculations were used. Two different models were used - one for investigating distributed leaks and one for a single leak. Fig 3 shows the pipeline used for the former case. A pump with a check valve is delivering water, Q=0.008 rrrVs, to a L=1000 m long conduit, diameter D=0.1 m, frictional coefficient f=0.02, geometrical head Az = 30 m and wave velocity a= 1000 m. Computations were based on the standard one-dimensional, unsteady equations for transient pipe flow, Wylie [1], using the method of characteristics. Leakage was simulated at every second nodal point (number of points N=21) with the leakage described by:

= 0 * < 0

346 Hydraulic Engineering Software with k=loss coefficient with the same value for all the leaks. The transient was caused by pump trip and subsequent valve closure. The pump was described in a simplified manner assuming no inertia - after the trip the pump was supposed to work as a concentrated loss. The computational scheme was based on a consideration of every leakage point as an inner boundary point on the basis of the equation of continuity, an assumption of a constant pressure head in the vicinity of the leakage point (Hli^+=Hl^') and the two characteristics.

pressure change due to leak

9 11 13 13 17 19 21 22 25 T1HE (S)

Figure 2. Pipeline with a concentrated leak and the effect on the transient generated by pump stop and automatic valve closure. Extra reflexion at leak visible as a small pressure "hump" registered (computed) at

the pump

The case with the concentrated leak also referred to a similar conduit with a pump but with a controllable shut-off valve instead. The leak could be located arbitrarily along the line and the leak was described computationally in the same way as described above. The steady-state flow was Q=0.008 mVs, the conduit length L=1000 m and diameter D=0.1 m, frictional coefficient f=0.02 and wave velocity a=1000 m/s.

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\^r T" 'AZ = 30m

S7 1=12 345 N-4 N-2 T4

* 1 2 t | "•—Hi =k-Qut — -*-

Figure 3. Conduit with pump and check valve used for transient compputa- tions. Leakage at i = 3,5,...,N-4,N-2. Same leakage coefficient at all

these points

4 Distributed leak

Computations on distributed leakage were performed on the conduit described above, Fig 3. The conduit was divided into 20 parts using 21 equidistant nodal points. Leakage took place at every second point, i = 3,5,...,19- altogether 9 points. Fig 4 shows two examples of the calculated transient at the valve due to pump stop and check valve closure. The upper case corresponds to a negligable leakage whereas the lower case represents a leakage of 4.8 % in relation to the pipe flow. It is obvious that the effect of leakage is to attenuate the transient pressure oscillations significantly. As the leakage is distributed it is of course not possible to locate it. Using the amplitude of the fourth and seventh oscillation respectively the computations gave the following results:

Table 1. Amplitude attenuation due to distributed leaks

Ampl 4th osc Ampl 7th osc LLeakage(( % of pipe flow) 1 1 0 1 0.98 0.05 0.88 0.86 0.5

0.,76 0.63 1.5 0.,45 0.24 5.0 0.,06 0 14.8

Thus one finds that the leakage rate is correlated with the attenuation of the transient and that a small leakage of the order of a few percent will decrease

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0 20 40 60 80 100 120 14O TIME (S)

Figure 4. Pressure transient at pump when the pump is stopped. Top : Negligable leakage

Bottom : Distributed leakage 4.8 %

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the pressure amplitude significantly. Secondly the attenuation effect is more pronounced the larger the order of the oscillation period is.

5 Concentrated leak

Computations for a concentrated leak were performed on a conduit as described earlier with the transient generated by pump stop and controlled valve closure. Pump behaviour after tripping was described considering the (small) inertia, Wylie [2]. The 1000 m long conduit was divided into a grid with 51 equidistant grid points. The effect of a concentrated leak on the pressure transient was studied in different ways. The first and most obvious way is illustrated in Fig 2. The combination of pump stop and small inertia gives rise to a very steep negative wave part of it being reflected at the leak situated 280 m from the pump and with a leakage discharge of 6.4 %. The return of the reflected wave is readily visible as a small, abrupt pressure change in the computed pressure transient at the pump. A detailed study reveals that the time between pump stop and the small, abrupt pressure change is At = 9. 05-8. 50 = 0. 55 s giving for the location L of the leak:

f\ . 7 1 n . r 1 = 0.55; I* = 275 m 1000

i.e. a good agreement with the real location of the leak. The results of some similar computations are given in Table 2 (initial pressure drop at the pump « 30 m H?O.

Table 2. Concentrated leak. Pressure change due to reflected leak

Pressure change Leakage Derived location Real location (m H2O) (%) leak (m) leak (m) 0.3 1.9 800 780 1.2 5.9 800 780 1.5 6.4 275 280 1.6 8.4 720 720 2.7 19. 750 780 2.7 19. 840 840

3.0 19. 280 280

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•in - 63 65 67 69 71 73 75 TIME (S)

Figure 5. Transient due to pump stop and automatic valve closure. Concentra- ted leak at U = 180 m, leakage rate 0.6 %

Top : Overall computed transient. Notice the growing distortion of the peaks Bottom : Enlargement of pressure peak. Abrupt change at t = 71.46 s due to reflection at leak

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It is obvious that a small reflected wave is produced and that the analysis based on its time scale locates the leak correctly. However, the leakage has to be rather large to produce a significant pressure change. The second way of studying the effect of a leak on the transient is related to the behaviour of the pressure oscillations after valve closure. Fig 5 gives an example with a very small leak located 180 m from the valve and with a leakage rate of 0.6 %. The top figure shows an overall view of the transient and the bottom figure an enlarged portion of the latter part of the transient. It is obvious that a small leak will cause a growing disturbance of the pressure peaks making even small leaks visible provided that a large enough number of oscillations occur. Secondly one could use the appearance of the disturbance to locate the leak, Fig 5 bottom. It can be seen very clearly that the rate of change of the linearly decreasing pressure in a peak is suddenly changed

(lowered). Assuming that this is due to reflections at the leak one obtains At=71.46-71.10=0.36 s giving for the location of the leak:

9 • 1 * 9 . / 1 ^—— = Af; ±—A_ = 0.36; L* = 180 m a 1000

which is in full agreement with the real location. Further computational examples, which cannot be described here, show that the rather small, concentrated leaks have the above-mentioned qualitative effect on the transient and that the location of the leak can be determined on the basis of a careful analysis of the pressure changes. Larger leaks, say 2 %, will even cause the basic pressure transient to deteriorate. A third way of detecting and locating a leak was tested but without success at least until now. The idea was that the reflexion at the leak should produce an oscillation with a smaller cycle period (4LVa) and superimposed on the basic cycle period. Even if this oscillation should be rather small and not directly visible it was assumed that spectral analysis of the transient should be a useful tool for revealing the extra oscillation. However, computational tests up to now have not showed that this is the case. Spectral analysis of transients with a leak have revealed the basic frequency (a/4L) and odd harmonics of it

(not the even harmonics). Studies will be continued as to the possibility of using spectral analysis.

6 Discussion and conclusion

Computational experiments on transients in a single pipeline due to pump stop and valve closure with a small leak have demonstrated that the leak will affect the transient - for instance registered at the valve - in different ways. In the first place both a concentrated leak and a distributed leak will tend to attenuate the pressure oscillations after valve closure. Even small leakage flows - 1-2 %

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- will cause a significant attenuation. Thus an unusual attenuation might indicate the presence of a leak. This fact brings forward two points. The first point is the importance of being able to compute the attenuation of pressure oscillations in a pipeline without leaks in a realistic way - the use of a steady- state frictional coefficient will normally underestimate the attenuation. The second point is the usefulness of having a databank of transient recordings for normal (i.e. no leak) conditions for comparisonal purposes.

Secondly a leak will cause extra pressure reflexions which can be seen on the pressure recording immediately after stopping a pump with low inertia but before valve closure. The temporal occurrence for the leak-induced small pressure change can be directly related to the location of the leak on the basis of the known wave velocity. The computations also showed that the pressure oscillations after valve closure will be distorted due to the leak - the large the number of oscillations the stronger the distortion making the effect of even very small leaks visible. A careful analysis of such a distortion can also provide information on the location of the leak.

References

1. Wylie, B., Streeter, V. Fluid transients, McGraw-Hill, 1978

2. Wylie, B., Streeter, V. Fluid transients, Chapter 6, Transients caused by

turbopumps, McGraw-Hill, 1978