Leak Detection in Pipelines
Transcript of Leak Detection in Pipelines
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LEAK DETECTION IN PIPELINES.
I. B. Macias, S. L. Cruz*, and J . A. F. R. Pereira
DESQ / FEQ / UNICAMP
Abstract. Pipeline networks are an essential part of the infrastructure of modern society. When transportingchemical products they frequently cross highly populated regions, water supplies or natural reserves.
Therefore leak detection and location methods play a key role in the overall integrity manegement of apipeline system.A leak detection technique based on the analysis of pressure transients has been experimented with pipelinestransporting gas, liquid and gas-liquid mixtures. Pressure transients caused by leaks were detected andanalysed in a 1250 m long pipeline operating with liquid and gas-liquid mixtures, and in a 60 m long pipelineoperating with gas. The detection equipment consisted of pressure transducers, placed along the pipeline andconnected to a PC computer equipped with an ADA converter. Leaks were simulated through side outletsfitted with solenoid valves placed along the pipeline. In the gas pipeline leaks were simulated through an
orifice which size varied from 0.7 mm to 5 mm in diameter. Leaks were generated by the computer throughthe D/A converter coupled to the solenoid valves. The characteristics of pressure transients were analysed forvarious operation conditions: liquid Reynolds number (Re) ranging from 4000 to 12000 and leak magnituderanging from 10% to 50% of the nominal flow.
The obtained results show that leaks as small as 5% of the liquid flow are readily detected. When operatingwith gas-liquid mixtures the pressure wave caused by the leak is partially absorbed by the gas. Pressure wavevelocities ranged from 43 to 76 m/s, being on average 9 times lower than those obtained with liquid flowonly. In the gas pipeline leaks could not be detected with an orifice small as 2mm and the pressure wavevelocity was determined to be about 375 m/s.
Keywords: Pipeline Networks, Leak Detection, Pressure Wave Velocity.
1. Introduction
Pipeline networks are an essential part of the infrastructure of modern society, frequently crossing highly
populated regions, water supplies or natural reserves. Sudden pressure changes, corrosion and weld failure could
origin leaks. A small percentage loss can give rise to events with considerable economic impact, environmental
damage, and injuries, which could be avoided through careful pipeline supervision and appropriate pumping
shutdown. Therefore leak detection and location methods play a key role in the overall integrity management of
a pipeline system.
When a leak occurs in a pipeline it causes a sudden decrease in pressure, this pressure pulse travels upstreamand downstream the pipeline as a wave. The detection and analysis of pressure transients generated by leak
occurrence together with the measured pressure wave velocity, allow the detection and location of leaks in
pipelines.
________________________________* To whom all correspondence should be addressed.Address: Depto Engenharia de Sistemas Qumicos, Faculdade de Engenharia Qumica, UNICAMP
13083-970, Campinas, SP, Brazil.
E-mail: [email protected]
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Silva et al. (1996) detected and located leaks in a 1250 m long pipeline operating under steady-state
conditions. The detection was based on the analysis of pressure transients through on-line computer techniques.
The pressure transient profiles generated by leak occurrence presented a sudden drop in pressure, followed by acertain recovery, which depended on liquid flow rate and leak magnitude. Leaks, as small as 5 % of the nominal
liquid flow, were readily detected and located with an error smaller than 5 metres.
Belsito et al. (1998) report the development of an Artificial Neural Network based system for leak detection
and location. The main database was generated by using a well-validated computer code, by starting from the
transducer characteristics and then using available field data. The ANNs was able to detect even 1% leak in
transient conditions and the location of large (5 %, 10 %) leaks was predicted very accurately even when noisy
signals were used.
Zhang (2001) examines the application of a statistical system, based on flow and pressure measurements,
which calculates the probability of leak and estimates the leak size and location. Variations generated by
operational changes are registered, ensuring that a leak alarm is generated only when a unique pattern of
changes in flow and pressure exists.
Fukushima et al. (2000) reported a leak detection system implemented in a 250 km pipeline. The leak
detection is performed by measuring pressure and temperature at the valve stations and by measuring gas flow
rate at the inlet and outlet. The mass balance was modeled along the pipeline. The leak detection system
automatically determines the leaking point and the leak rate in real-time basis. Leaks of about 1 % have been
detected.
Jonsson and Larson (1992) studied the characteristics of pressure wave propagation in hydraulic system,after pump stop both with and without leak occurrence. By applying spectral analysis to the measured pressure
time series leak detection was possible through the identification of reflected wave from the leak.
Ferrante and Brunone (2003) report that unsteady-state tests can be used for pipe diagnosis and leak
detection. The transient flow equations for the pressurized pipes are solved directly in the frequency domain by
means of impulse response method. Leaks can be detected by comparing the experimental transfer function with
the theoretical one corresponding to an intact system or comparing transfer functions obtained uder different test
conditions.
In this work a leak detection technique based on the analysis of pressure transients has been experimented in
pipelines transporting gas and gas-liquid mixtures. The propagation velocities of the pressure waves generated
by the leak were also determined.
2. Experimental Work
Pressure transients caused by leaks were detected and analysed in a 1250 metre long PVC pipeline, operating
with water and air-water mixtures (Figure 1). The detection equipment consisted of four pressure transducers,
placed along the pipeline (at 494 m, 744 m, 994 m and 1244 m from the entrance) and connected to a PC
computer through an ADA converter. Leaks were simulated through side outlets fitted with solenoids valves at
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250 m and 750 m from the entrance. Leaks were generated by the computer through the D/A converter coupled
to the solenoid valve.
Experiments were performed for various operation conditions. Liquid Reynolds number (Re) ranged from4000 to 12000. Air was introduced in the pipeline either as single slugs in a steadily flowing water stream or as a
continuous stream which flow rates ranged from 0.042x10-3 m3/s to 0.97 x10-3 m3/s. Leak magnitude ranged
from 10% to 50% of the nominal liquid flow.
Figure 1. Liquid and gas-liquid mixtures pipeline scheme
The experiments with gas (air) were performed in diameter, 60 metres long, galvanized iron pipeline
(Figure 2). A 0.7 mm orifice was settled at the end of the pipeline to keep it pressurized. Leaks were simulated
through a side outlet, fitted at the entrance of the pipeline, having an orifice which size was varied from 0.7 mm
to 5 mm in diameter. Gas flowrate corresponded to Re nunber from 55000 to 175000 and leak magnitude from
10% to 50 % of the nominal gas flow rate.
The detection equipment consisted of two pressure transducers, placed at the inlet and at the outlet of the
pipeline and connected to a computer fitted with an ADA converter.
Figure 2. Gas pipeline scheme
A computer program to be run on-line was developed to read and filter all the pressure transducers signals
and to display the pressure profiles plots. In the liquid and gas-liquid mixture pipeline the program allows to
operate the solenoid valves, to provoke the leak and to inject the air, through the D/A converter which operates
an optically coupled circuit to drive an on-off relay. In the gas pipeline the leak was provoked manually,
however the valve was connected to the computer so that it was possible to determine the exact instant at it was
opened.3. Results and Analysis
T1T2
Pressuregauge
Converter
Computer
L
1-5V
psig0-100
Compressor
Air feed
ConverterComputer
494 m
1-5V
psig0-100
T4T3T2T1
mixer
air
994 m744 m 1244 m
1250 m
leak 1(250m)
leak 1(250m)
reservoir
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3.1. Leak Detection in the Liquid and Gas-L iquid Pipeline
When a leak occurs a pressure wave travels along the pipeline upstream and downstream the leak position.
The analysis and detection of pressure pulses generated by leak occurrence may be used as a technique for
detection and location of leaks in pipelines (Silva et al., 1996).
Figures 3(a) to 3(c) show the pressure transients caused by a leak when air was introduced in the pipeline as
a single slug. All the leaks in Figure 3 occurred at 750 m from the entrance. In all cases the propagation of
pressure waves generated by the leak was clearly detected by the transducers nearer to the leak.
The pressure profiles show a sudden reduction, followed by a partial pressure recovery depending on leak
magnitude and on air flow rate. As the air flow rate and the leak magnitude increased leak occurrence was
clearly indicated by rapid oscillation of pressure, which continued to decrease. The profiles obtained through the
transducers located nearer and downstream the leak presented higher degree of oscillation of the pressure pulse.
A different behaviour was observed in the profiles generated by transducer T1 (at 256 m upstream the leak) for
the higher air flow rates and the leak magnitude indicating a higher degree of weakening of the pressure wave
when propagating in the opposite direction to the gas-liquid flow.
The profiles on Figure 3(b) when compared with those generated by a leak, under the same operation
conditions, but in a pipeline operating with liquid only (Figure 3(d)), indicate that the presence of air causes
reflection of the pressure waves as it is observed by the pressure oscillations on the profiles presented on Figure
3(b).Figures 3(e) and 3(f) show the pressure transients caused by leak of an air-water mixture in the pipeline. In
all cases shown on Figures 3(e) and 3(f) leak occurrence was clearly detected only by the transducer located
nearest to the leak, and when leak magnitude was 30% and above.
3.2. Leak Detection in the Gas Pipeline
Figures 4(a) and 4(b) shows the pressure profiles caused by a gas leak through a 5 mm orifice with the
pipeline operating under a pressure of 550 kPa. In Figure 4(a) the the leak was provoked and stoped after a few
seconds. Figures 4(b) to 4(d) show the pressure profile in the pipeline during leak occurrence. Figure 4(c) shows
the pressure transients generated by leakage through various orifices (1 mm to 5 mm) for the same initial
pressure (550 kPa) and Figure 4(b) shows the pressure transients for a 4 mm orifice under different pipeline
pressure values.
At the instant that the leak occurs the pressure falls, with a magnitude that depends on the orifice size,
remaining constant during leak occurrence. Leaks could be detected only for orifices larger than 2 mm in
diameter, which corresponded to 11 % of nominal gas low. As seen from Figures 4(a) and 4(b) the pressure
transients detected by transducers T1 (located beside the leak) an T2 (at the end of the pipeline) presented the
same characteristics, with the pressure wave reaching transducer T2 with a time delay of 0.16 s.
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0 10 20 30 40 50 60 70time (s)
0.0
50.0
100.0
150.0
pressure
(kPa)
Re =10000leak =10 %air flow rate =0.042x10 m /s
y (m)
(-256 m)
(-6 m)
(+244 m)
(+494 m)3-3
(a)
0 10 20 30 40 50 60 70time (s)
-50.0
0.0
50.0
100.0
150.0
pressure
(kPa)
y (m)
(-256 m)
(-6 m)
(+244 m)
(+494 m)Re =10000leak =30 %air flow rate =0.30x10 m /s
3-3
(b)
0 10 20 30 40 50 60 70time (s)
-50.0
0.0
50.0
100.0
150.0
pressure
(kPa)
Re =6000leak =30%air flow rate =0.30x10 m /s
(-256 m)
(-6 m)
(+244 m)
(+494 m)
y (m)
-3 3
(c)
0 10 20 30 40 50 60 70time (s)
-50.0
0.0
50.0
100.0
150.0
pressure
(kPa)
Re =10000leak =30 %y (m)
(-256 m)
(-6 m)
(+244 m)
(+494 m)
(d)
0 10 20 30 40 50 60time (s)
0.0
50.0
100.0
150.0
200.0
250.0
300.0
pressure
(kPa)
Re =10000leak =30 %air flow rate =0.028x10 m /s
(-256 m)
(-6 m)
(+244 m)
(+494 m)
y (m)
-3 3
(e)
0 10 20 30 40 50 60time (s)
0.0
50.0
100.0
150.0
200.0
250.0
300.0
pressure
(kPa)
Re =10000leak =50 %air flow rate =0.32x10 m /s
(-256 m)
(-6 m)
(+244 m)
(+494 m)3-3
y (m)
(f)Figure 3. Pressure transients in air-water mixtures
It may be observed that the pressure change remains practically constant for the same orifice size as the inlet
pressure is increased. This occurs for all the orifices since the air flow release through an orifice to the
atmosphere is a chocked flow, the sonic velocity is reached and higher pressures do not influence the flow rate.
3.3 Pressure Wave Velocity
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The pressure wave velocity (v) was determined, when a leak occurs, by processing the signals generated by
the transducers in the pipeline. By knowing the distances between the transducers and the detected pulse time
delay between them, the propagation velocity of the pressure wave could be determined. For a pipelinetransporting liquid only the pressure wave velocities ranged from 470 m/s to 530 m/s (Silva et al., 1996).
(a)
(b)
(c) (d)
Figure 4. Pressure transients in the gas pipeline
Table 1 shows the determined pressure wave velocity in an air-water mixture. The propagation velocities
ranged from 43 to 76 m/s, being on average 9 times lower than those obtained with liquids only. The results
show that the impact of the pressure wave caused by the leak is partially absorbed by the air flowing in the
pipeline and also that the wave velocity is lower when the wave propagates in the opposite direction to the air-
water flow (cases where y =(-) 256 m and y =(-) 6 m)).
Table 1. Pressure wave velocities in air-water mixturesRe Qair x 10
3 leak (%) y (m) * v (m/s)
0 2 4 6 8 10
0
100
200
300
400
500
T1
T2
Orifice 5 mm
P0=550 kPa
P
ress
ure
(kP
a)
Time (s)
0 2 4 6 8 10
0
100
200
300
400
500
Orifice 5 mm
P0=550 kPa
%LEAK =74%
T1
T2
P
ress
ure
(kP
a)
Time(s)
0 2 4 6 8 10
0
100
200
300
400
500
Orifice 4 mm
T1
T2
Pressure
(kPa)
Time (s)
0 2 4 6 8 10
400
500
Pressure
(kPa)
Time (s)
T2 P0552 kPa
o1 o2 o3 o4 o5
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(m3/s)10000 0.028 50 (+) 244 7610000 0.028 50 (-) 256 56
10000 0.15 50 (+) 244 6010000 0.028 30 (+) 244 5710000 0.028 30 (-) 6 43
* y =transducer position; (-) downstream, (+) upstream the leak
In all the experiments performed in the gas pipeline the time delay between the transducers was about 0.16s,
with an error of 0.002 s which is the time between two acquired points during the experiment. This value
represents the time required to flow through the distance of 60 metres that separates the transducers. So, the
experimental pressure wave velocity was determined to be about 375 m/s.
4. Conclusions
A leak detection technique based on the analysis of pressure transients has been tested in pipelines operating
with gas, liquid and gas-liquid mixtures. The detection equipment consisted of pressure transducers placed along
the pipeline and connected to a PC computer equipped with ADA converter.
The presence of air caused reflection of the pressure waves generated by the leak, the impact of the pressure
wave being partially absorbed by the air flowing in the pipeline. When operating with gas-liquid mixtures the
pressure wave velocities ranged from 43 to 76 m/s, being on average 9 times lower than those obtained with
liquid flow only. Leaks were readily detected when air flowed as a single slug, otherwise when the air streamwas continuos leakage was detected only by the transducers located nearest to the leak and when leak magnitude
was 30% and above.
In the gas pipeline leaks could be detected only for 2 mm diameter orifices, corresponding to 11 % leak, and
higher. The pressure wave velocity was determined to be about 375 m/s.
5. Notation
leak =leak magnitude based on the nominal liquid or gas flow (%)o =orifice size ( mm)
Po =pressure (kPa)
Qair =volumetric air flow rate (m3/s)
Re =Reynolds number based on liquid flow
v =pressure wave propagation velocity (m)
y =distance between the leak and transducer (m)
References
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Belsito, S., Lombardi, P., Andreussi, P., Banerjee, S. (1998). Leak Detection in Liquefied Gas Pipeline by Artificial Neural
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Analysis.Advances in Water Resources, 26, 95.
Fukushima, K., Maeshima, R., K inoshita, A., Shiraishi, H., and Koshijima, I. (2000). Gas Pipeline Leak Detection System
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Academic Publishers, Dordrecht, Netherlands.
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Zhang, J. (2001). Statistical Pipeline Leak Detection for All Operating Conditions.Pipeline & Gas JournalOnline, February.