- Email: [email protected]
An improved method of soil-gas sampling for pipeline leak detection: Flow model analysis and laboratory test
Journal of Natural Gas Science and Engineering 42 (2017) 226e231
Contents lists available at ScienceDirect
Journal of Natural Gas Science and Engineering journal homepage: www.elsevier.com/locate/jngse
An improved method of soil-gas sampling for pipeline leak detection: Flow model analysis and laboratory test Y. Chen, T. Kuo*, W. Kao, J. Tsai, W. Chen, K. Fan Department of Mineral and Petroleum Engineering, National Cheng Kung University, Tainan, Taiwan
a r t i c l e i n f o
a b s t r a c t
Article history: Received 19 November 2016 Received in revised form 25 January 2017 Accepted 3 March 2017 Available online 14 March 2017
A flow model is presented in this short communication to design a horizontal sampling method of soilgas for pipeline leak detection. The soil-gas flow model is verified by air-extraction tests. A laboratory method is also designed in this short communication to evaluate the effective detection radius of organic volatile compounds (VOCs) using the horizontal sampling method. Results of laboratory tests demonstrate that the effective detection radius using the horizontal sampling method for pipeline leak detection is at least 30 m and 20 m for gasoline and diesel, respectively. © 2017 Published by Elsevier B.V.
Keywords: Pipeline Leak detection Effective radius of detection Soil gases
Contents 1. 2.
3.
4.
Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 226 Materials and methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 227 2.1. Description of horizontal soil-gas sampling method . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 227 2.2. Laboratory leak-detection tests . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 227 Results and discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 228 3.1. Modeling and verification of soil-gas flow distribution through porous probes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 228 3.2. Leak-detection tests . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 230 Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 230 Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 231 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 231
1. Introduction Pipeline leaks of gasoline and diesel represent one of the most common environmental problems in shallow aquifers contaminated with petroleum hydrocarbons liquids. The leak sources continue to dissolve into aquifers as active sources of groundwater contamination (Kim and Corapcioglu, 2003; Kuo et al., 2016). It is essential to detect pipeline leaks as early as possible. This short communication presents a soil-gas flow model to design an
* Corresponding author. E-mail address: [email protected] (T. Kuo). http://dx.doi.org/10.1016/j.jngse.2017.03.008 1875-5100/© 2017 Published by Elsevier B.V.
improved sampling method of soil-gas for pipeline leak detection. Soil-gas techniques have been used to survey leaks of organic volatile compounds (VOCs) from underground storage tanks and pipelines (Kerfoot and Mayer, 1986; Marrin and Thompson, 1987; Thompson and Marrin, 1987; Marrin and Kerfoot, 1988; Liang and Kuo, 2006). Traditional soil-gas method installs vertical probes in the vadose zone to take gas samples. The effective sampling radius of a vertical soil-gas probe is only around 5 m (Liang and Kuo, 2006). Therefore, a large quantity of vertical probes and gas samples are required to conduct a soil-gas survey for a long-distance pipeline. An improved sampling method of soil-gas was proposed by Liang and Kuo (2006) to enhance the effective radius of soil-gas
Y. Chen et al. / Journal of Natural Gas Science and Engineering 42 (2017) 226e231
227
Fig. 1. An improved method of soil-gas sampling. (a) Schematic diagram of pipeline leak detection. (b) A basic section unit. For Qpe, see Eqs. (2) and (4) in text. (c) Cross section of a porous probe. For Qpor, see Eq. (3) in text. (from Liang and Kuo, 2006).
sampling for pipeline leak detection. Fig. 1a shows the improved sampling method which is a horizontal sampling line of soil gas running above and nearby the pipeline. The horizontal sampling line consists of intermittent porous sampling probes connected in series. To design the improved method of soil-gas sampling for pipeline leak detection, a mathematical model is required to predict the flow distribution of soil-gas through each porous probe in the horizontal sampling line. An incorrect derivation of flowing pressure was found in the soil-gas flow model presented by Liang and Kuo (2006). One can refer to Fig. 1b for the materials balance and mathematical symbols used in the above-mentioned equation as follows.
Pout;next
section
¼ Pw þ
Qpor ðPatm Pw Þ Qpe
(1)
Liang and Kuo (2006) assumed that the pressure drop between the inlet and center of the porous tube was negligible. Originally, this assumption was thought not to be serious. However, the value of gas viscosity at 20 C and 1 atm absolute used in their model calculations (m ¼ 0.07 cp) had to be notably higher than the literature value (m ¼ 0.012 cp, McCabe and Smith, 1976). In this short communication, the above-mentioned assumption is removed from the soil-gas flow model. The revised model predictions are also verified with experimental data obtained from air-extraction tests. Currently, experimental data using the above improved sampling method for soil-gas is scarce in the literature. A laboratory method is designed in this short communication to evaluate the effective detection radius of VOCs for the horizontal sampling method. The objectives of this short communication were to (1) present a soil-gas flow model to design the horizontal sampling method, (2) verify the soil-gas flow model using air-extraction tests, and (3) evaluate the effective leak-detection radius of VOCs for the horizontal sampling method by the laboratory method.
2. Materials and methods 2.1. Description of horizontal soil-gas sampling method The improved sampling method is a horizontal sampling line of soil gas running above and nearby the pipeline (Fig. 1a). The sampling line is made up of intermittent porous sampling probes connected in series by impermeable flow lines. Fig. 1b shows a basic section unit consisting of a porous sampling probe and an impermeable flow line. The specifications of materials used for constructing the horizontal sampling line of soil gas in this study are as follows. Each impermeable flow line is a polyethylene tube 100 cm long with an outside and inside diameter equals to 1.2 cm and 0.9 cm, respectively. Each porous sampling probe made from rubber is 5 cm long with an outside and inside diameter equals to 1.6 cm and 1.2 cm, respectively. The measured permeability of the porous sampling probes used in this study is 0.0619 ± 0.0095 Darcy.
2.2. Laboratory leak-detection tests Fig. 2 shows the schematic diagram of the laboratory leakdetection experiment with a sampling line. One end porous probe of the sampling line is installed inside a glass column above a reservoir of gasoline or diesel. At the other end of the sampling line, an extraction pump is employed to take soil gas samples with Tedlar bag in a vacuum box. The glass column is backfilled with coarse sands. The permeability of backfill using coarse sands (in the order of 100 Darcy) is much greater than that of the porous tubes. The high-permeability backfill allows sufficient soil-gas to flow through each porous tube during soil-gas extraction. Each leak-detection experiment starts with a fresh column of coarse sands and a cleaned detection system either 20-m or 30-m long. Background soil-gas samples are taken before filling the reservoir with gasoline or diesel. After taking the background
228
Y. Chen et al. / Journal of Natural Gas Science and Engineering 42 (2017) 226e231
Fig. 2. Schematic diagram of the laboratory leak-detection tests.
samples, the reservoir is filled with 300-mL gasoline or diesel and the glass column is allowed to stand still at least for 9 days for the soil-gas to attain equilibrium in the coarse sands. At least nine days after filling the reservoir with gasoline or diesel, soil-gas samples are taken for analyses. Soil gas samples were measured using a gas chromatograph (HP 6890N GC) with a DB624 capillary column (60 m length and 0.25 mm inside diameter) and mass selective detector (HP 5973N MSD). 3. Results and discussion 3.1. Modeling and verification of soil-gas flow distribution through porous probes An efficient design of an improved leak-detection method requires understanding the flow distribution of soil-gas through the porous probes at various positions on the horizontal sampling line. The soil-gas flow model consists of two parts: (1) gas flow from soil atmosphere through the porous probes, and (2) gas flow inside the impermeable flow lines. Darcy's law and Poiseuille's equation are used in the mathematical model to describe the gas flow behavior through the porous probes and inside the impermeable flow lines, respectively. Poiseuille's equation can be used to calculate the flow rate and pressure drop of soil-gas inside an impermeable flow line (Bird et al., 1960). Fig. 1b shows the mathematical symbols used in Poiseuille's equation as follows.
2 P2 4 p Pin out R Qpe ¼ 3:8 10 Pb mL 11
(2)
where Qpe is the soil-gas flow rate in an impermeable flow line, L/ min; R and L are the inside radius and length of an impermeable flow line, respectively, m; m is the soil gas viscosity, cp; Pin and Pout are the soil-gas flowing pressures at the inlet and outlet of an impermeable flow line, respectively, atm absolute; Pb is the reference pressure (¼ 1 atm absolute). The calculations start from the 1st impermeable flow line near the extraction pump. For the 1st impermeable flow line, Pout and Qpe are also the pressure and total flow rate observed at the inlet of the extraction pump. Darcy's law can be utilized to calculate the flow rate and pressure drop of gas flow from soil atmosphere through the porous probes (Amyx et al., 1960). Fig. 1c shows the cross section of a porous probe and the mathematical symbols used in Darcy's law as
follows.
Qpor ¼ 6:0
2 2 pkh Patm Pw
mPb ln
(3)
re rw
where Qpor is the soil-gas flow rate through a porous sampling probe, L/min; k is the permeability of a porous sampling probe, Darcy; h, re , and rw are the length, outside radius, inside radius of a porous sampling probe, respectively, m; Patm is the atmospheric pressure outside of a porous sampling probe (¼ 1 atm absolute); Pw is the pressure inside of a porous sampling probe, atm absolute. The flow rate of soil-gas inside the next section of impermeable flow line can then be calculated from materials balance as follows.
Qpe; next section ¼ Qpe Qpor
(4)
The flow distribution of soil-gas through each porous probe in the leak-detection system during soil-gas extraction can be calculated by repeating the calculations using Eqs. (2)e(4). By using the above soil-gas flow model, the calculated distributions of soil-gas flow through each porous probe for the 20-m and 30-m long sampling lines are shown in Fig. 3. The parameters used in the model calculations consist of two groups according to their sources (literature values, and laboratory-measured values). The literature values used in the model calculations are reference pressure (Pb ¼ 1 atm absolute) and soil gas viscosity at 20 C and 1 atm absolute (m ¼ 0.012 cp, McCabe and Smith, 1976). The laboratory-measured values used in the model calculations are length of an impermeable flow line (L ¼ 1 m), inside radius of an impermeable flow line (R ¼ 0.0045 m), outside radius of a porous sampling probe (re ¼ 0.008 m), inside radius of a porous sampling probe (rw ¼ 0.006 m), length of a porous sampling probe (h ¼ 0.05 m), and permeability of a porous sampling probe (k ¼ 0.0619 Darcy). For the 20-m long system, the total extraction rate of soil-gas and the extraction-pump pressure measured are Qtotal ¼ 42.2 L/min and Pout ¼ 0.9210 atm absolute, respectively. For the 30-m long system, the total extraction rate of soil-gas and the extraction-pump pressure measured are Qtotal ¼ 41.8 L/min and Pout ¼ 0.9340 atm absolute, respectively. Air extraction tests were conducted to measure the flow distribution of air through each porous probe in both 20-m and 30-m long sampling lines. Details of air-extraction tests were provided by Liang and Kuo (2006). Fig. 3 also verifies the calculated distributions of air flow with experimental data measured in air extraction
Cumulative airflow distribution (L/mim)
Y. Chen et al. / Journal of Natural Gas Science and Engineering 42 (2017) 226e231
229
45 40
model prediction (30 meter system) observation (30 meter system) model prediction (20 meter system) observation (20 meter system)
35 30 25 20 15 10 5 0 0
5
10
15
20
25
30
Distance of individual porous tube from the extraction point (m) Fig. 3. Distributions of air flow in a 20-m long system (total air flow rate ¼ 42.2 L/min; extraction pressure ¼ 0.9210 atm absolute) and a 30-m long system (total air flow rate ¼ 41.8 L/min; extraction pressure ¼ 0.9340 atm).
Fig. 4. GC Spectrum measured for a soil-gas sample taken at 20-m away from a leak-source. (a) gasoline; (b) diesel.
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Fig. 5. GC Spectrum measured for a soil-gas sample taken at 30-m away from a leak-source. (a) gasoline; (b) diesel.
tests. The verified model of soil-gas flow distribution is useful for the optimal design of an improved sampling method of soil-gas for pipeline leak detection.
3.2. Leak-detection tests The main objective of the leak-detection tests was to evaluate the effective leak-detection radius of the horizontal sampling method for both gasoline and diesel by laboratory tests. Fig. 4 and Fig. 5 present the results of the laboratory leak-detection tests. Fig. 4a and b show the gas-chromatograph (GC) spectrums measured for soil-gas samples taken from the 20-m system using the gasoline leak-source, and diesel leak-source, respectively. Fig. 4a and b clearly show that both the gasoline and diesel leaks can be detected at 20 m away from the leak source. The peaks with retention times greater than 6 min are the volatile organics from gasoline and diesel. Fig. 4a and b indicate that the gasoline leak is easier to be detected than the diesel leak because gasoline is more volatile than diesel. Notice that the toluene concentration measured from the gasoline leak (25 ppm) is higher than that measured from the diesel leak (13 ppm).
Fig. 5a and b show the GC spectrums measured for soil-gas samples taken from the 30-m system using the gasoline leaksource, and diesel leak-source, respectively. Fig. 5a shows that a gasoline leak can be detected at 30 m away from the leak source. The toluene concentration measured from the gasoline leak is 8 ppm (Fig. 5a). However, due to the lower volatility of diesel than gasoline, Fig. 5b shows that a diesel leak is not detectable at 30 m away from the leak source. As shown in Fig. 5b, the toluene peak disappears in the GC spectrum. The effective detection radius of VOCs using the conventional sampling method using vertical soil-gas probes is only around 5 m (Liang and Kuo, 2006). Results of the above laboratory leakdetection tests indicate that the effective detection radius of VOCs using the improved sampling method is at least 30 m for gasoline leakage from a long-distance pipeline.
4. Conclusions A mathematical model to predict the flow distribution of soilgas samples has been presented for the horizontal sampling method. Darcy's law and Poiseuille's equation were utilized to
Y. Chen et al. / Journal of Natural Gas Science and Engineering 42 (2017) 226e231
model the flow behavior of soil-gases through the porous sampling probes and inside the impermeable lines, respectively. The soil-gas flow model has been verified by the air-extraction tests. The verified model is useful to design the horizontal sampling method of soil-gas for pipeline leak detection. Results of laboratory leak-detection tests corroborate that the horizontal sampling method enhances the effective sampling radius of soil gas for pipeline leak detection. The effective detection radius of organic volatile compounds (VOCs) using the horizontal sampling method is at least 30 m and 20 m for gasoline and diesel, respectively. On the other hand, the effective detection radius of VOCs using the conventional vertical soil-gas probe is only around 5 m. Acknowledgments This research has been supported by Environmental Protection Administration and National Science Council of Taiwan (MOST 1042116-M-006-002, MOST 105-2116-M-006-006).
231
References Amyx, J.W., Bass, D.M., Whiting, R.L., 1960. Petroleum Reservoir Engineering. McGraw-Hill, Inc., New York. Bird, R.B., Stewart, W.E., Lightfoot, E.N., 1960. Transport Phenomena. John Wiley & Sons, Inc., New York. Kerfoot, H.B., Mayer, C.L., 1986. The use of industrial hygiene samples for soil-gas surveying. Ground Water Monit. Remedia 6 (4), 74e78. Kim, J., Corapcioglu, M.Y., 2003. Modeling dissolution and volatilization of LNAPL sources migrating on the groundwater table. J. Contam. Hydrol. 65 (1e2), 137e158. Kuo, T., Chen, Y., Lin, C., Chen, J., 2016. Oil recovery from a fluctuating water table. Pet. Sci. Technol. 34 (17e18), 1562e1567. Liang, K.F., Kuo, M.C.T., 2006. A new leak-detection system for long-distance pipelines utilizing soil-gas techniques. Ground Water Monit. Remedia 26 (3), 53e59. Marrin, D.L., Kerfoot, H.B., 1988. Soil-gas surveying techniques. Environ. Sci. Technol. 22 (7), 740e745. Marrin, D.L., Thompson, G.M., 1987. Gaseous behavior of TCE overlying a contaminated aquifer. Ground Water 25 (1), 21e27. McCabe, W.L., Smith, J.C., 1976. Unit Operations of Chemical Engineering, third ed. McGraw-Hill, Inc., New York. Thompson, G.M., Marrin, D.L., 1987. Soil gas contaminant investigations: a dynamic approach. Ground Water Monit. Remedia 7 (3), 88e93.
Contents lists available at ScienceDirect
Journal of Natural Gas Science and Engineering journal homepage: www.elsevier.com/locate/jngse
An improved method of soil-gas sampling for pipeline leak detection: Flow model analysis and laboratory test Y. Chen, T. Kuo*, W. Kao, J. Tsai, W. Chen, K. Fan Department of Mineral and Petroleum Engineering, National Cheng Kung University, Tainan, Taiwan
a r t i c l e i n f o
a b s t r a c t
Article history: Received 19 November 2016 Received in revised form 25 January 2017 Accepted 3 March 2017 Available online 14 March 2017
A flow model is presented in this short communication to design a horizontal sampling method of soilgas for pipeline leak detection. The soil-gas flow model is verified by air-extraction tests. A laboratory method is also designed in this short communication to evaluate the effective detection radius of organic volatile compounds (VOCs) using the horizontal sampling method. Results of laboratory tests demonstrate that the effective detection radius using the horizontal sampling method for pipeline leak detection is at least 30 m and 20 m for gasoline and diesel, respectively. © 2017 Published by Elsevier B.V.
Keywords: Pipeline Leak detection Effective radius of detection Soil gases
Contents 1. 2.
3.
4.
Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 226 Materials and methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 227 2.1. Description of horizontal soil-gas sampling method . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 227 2.2. Laboratory leak-detection tests . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 227 Results and discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 228 3.1. Modeling and verification of soil-gas flow distribution through porous probes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 228 3.2. Leak-detection tests . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 230 Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 230 Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 231 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 231
1. Introduction Pipeline leaks of gasoline and diesel represent one of the most common environmental problems in shallow aquifers contaminated with petroleum hydrocarbons liquids. The leak sources continue to dissolve into aquifers as active sources of groundwater contamination (Kim and Corapcioglu, 2003; Kuo et al., 2016). It is essential to detect pipeline leaks as early as possible. This short communication presents a soil-gas flow model to design an
* Corresponding author. E-mail address: [email protected] (T. Kuo). http://dx.doi.org/10.1016/j.jngse.2017.03.008 1875-5100/© 2017 Published by Elsevier B.V.
improved sampling method of soil-gas for pipeline leak detection. Soil-gas techniques have been used to survey leaks of organic volatile compounds (VOCs) from underground storage tanks and pipelines (Kerfoot and Mayer, 1986; Marrin and Thompson, 1987; Thompson and Marrin, 1987; Marrin and Kerfoot, 1988; Liang and Kuo, 2006). Traditional soil-gas method installs vertical probes in the vadose zone to take gas samples. The effective sampling radius of a vertical soil-gas probe is only around 5 m (Liang and Kuo, 2006). Therefore, a large quantity of vertical probes and gas samples are required to conduct a soil-gas survey for a long-distance pipeline. An improved sampling method of soil-gas was proposed by Liang and Kuo (2006) to enhance the effective radius of soil-gas
Y. Chen et al. / Journal of Natural Gas Science and Engineering 42 (2017) 226e231
227
Fig. 1. An improved method of soil-gas sampling. (a) Schematic diagram of pipeline leak detection. (b) A basic section unit. For Qpe, see Eqs. (2) and (4) in text. (c) Cross section of a porous probe. For Qpor, see Eq. (3) in text. (from Liang and Kuo, 2006).
sampling for pipeline leak detection. Fig. 1a shows the improved sampling method which is a horizontal sampling line of soil gas running above and nearby the pipeline. The horizontal sampling line consists of intermittent porous sampling probes connected in series. To design the improved method of soil-gas sampling for pipeline leak detection, a mathematical model is required to predict the flow distribution of soil-gas through each porous probe in the horizontal sampling line. An incorrect derivation of flowing pressure was found in the soil-gas flow model presented by Liang and Kuo (2006). One can refer to Fig. 1b for the materials balance and mathematical symbols used in the above-mentioned equation as follows.
Pout;next
section
¼ Pw þ
Qpor ðPatm Pw Þ Qpe
(1)
Liang and Kuo (2006) assumed that the pressure drop between the inlet and center of the porous tube was negligible. Originally, this assumption was thought not to be serious. However, the value of gas viscosity at 20 C and 1 atm absolute used in their model calculations (m ¼ 0.07 cp) had to be notably higher than the literature value (m ¼ 0.012 cp, McCabe and Smith, 1976). In this short communication, the above-mentioned assumption is removed from the soil-gas flow model. The revised model predictions are also verified with experimental data obtained from air-extraction tests. Currently, experimental data using the above improved sampling method for soil-gas is scarce in the literature. A laboratory method is designed in this short communication to evaluate the effective detection radius of VOCs for the horizontal sampling method. The objectives of this short communication were to (1) present a soil-gas flow model to design the horizontal sampling method, (2) verify the soil-gas flow model using air-extraction tests, and (3) evaluate the effective leak-detection radius of VOCs for the horizontal sampling method by the laboratory method.
2. Materials and methods 2.1. Description of horizontal soil-gas sampling method The improved sampling method is a horizontal sampling line of soil gas running above and nearby the pipeline (Fig. 1a). The sampling line is made up of intermittent porous sampling probes connected in series by impermeable flow lines. Fig. 1b shows a basic section unit consisting of a porous sampling probe and an impermeable flow line. The specifications of materials used for constructing the horizontal sampling line of soil gas in this study are as follows. Each impermeable flow line is a polyethylene tube 100 cm long with an outside and inside diameter equals to 1.2 cm and 0.9 cm, respectively. Each porous sampling probe made from rubber is 5 cm long with an outside and inside diameter equals to 1.6 cm and 1.2 cm, respectively. The measured permeability of the porous sampling probes used in this study is 0.0619 ± 0.0095 Darcy.
2.2. Laboratory leak-detection tests Fig. 2 shows the schematic diagram of the laboratory leakdetection experiment with a sampling line. One end porous probe of the sampling line is installed inside a glass column above a reservoir of gasoline or diesel. At the other end of the sampling line, an extraction pump is employed to take soil gas samples with Tedlar bag in a vacuum box. The glass column is backfilled with coarse sands. The permeability of backfill using coarse sands (in the order of 100 Darcy) is much greater than that of the porous tubes. The high-permeability backfill allows sufficient soil-gas to flow through each porous tube during soil-gas extraction. Each leak-detection experiment starts with a fresh column of coarse sands and a cleaned detection system either 20-m or 30-m long. Background soil-gas samples are taken before filling the reservoir with gasoline or diesel. After taking the background
228
Y. Chen et al. / Journal of Natural Gas Science and Engineering 42 (2017) 226e231
Fig. 2. Schematic diagram of the laboratory leak-detection tests.
samples, the reservoir is filled with 300-mL gasoline or diesel and the glass column is allowed to stand still at least for 9 days for the soil-gas to attain equilibrium in the coarse sands. At least nine days after filling the reservoir with gasoline or diesel, soil-gas samples are taken for analyses. Soil gas samples were measured using a gas chromatograph (HP 6890N GC) with a DB624 capillary column (60 m length and 0.25 mm inside diameter) and mass selective detector (HP 5973N MSD). 3. Results and discussion 3.1. Modeling and verification of soil-gas flow distribution through porous probes An efficient design of an improved leak-detection method requires understanding the flow distribution of soil-gas through the porous probes at various positions on the horizontal sampling line. The soil-gas flow model consists of two parts: (1) gas flow from soil atmosphere through the porous probes, and (2) gas flow inside the impermeable flow lines. Darcy's law and Poiseuille's equation are used in the mathematical model to describe the gas flow behavior through the porous probes and inside the impermeable flow lines, respectively. Poiseuille's equation can be used to calculate the flow rate and pressure drop of soil-gas inside an impermeable flow line (Bird et al., 1960). Fig. 1b shows the mathematical symbols used in Poiseuille's equation as follows.
2 P2 4 p Pin out R Qpe ¼ 3:8 10 Pb mL 11
(2)
where Qpe is the soil-gas flow rate in an impermeable flow line, L/ min; R and L are the inside radius and length of an impermeable flow line, respectively, m; m is the soil gas viscosity, cp; Pin and Pout are the soil-gas flowing pressures at the inlet and outlet of an impermeable flow line, respectively, atm absolute; Pb is the reference pressure (¼ 1 atm absolute). The calculations start from the 1st impermeable flow line near the extraction pump. For the 1st impermeable flow line, Pout and Qpe are also the pressure and total flow rate observed at the inlet of the extraction pump. Darcy's law can be utilized to calculate the flow rate and pressure drop of gas flow from soil atmosphere through the porous probes (Amyx et al., 1960). Fig. 1c shows the cross section of a porous probe and the mathematical symbols used in Darcy's law as
follows.
Qpor ¼ 6:0
2 2 pkh Patm Pw
mPb ln
(3)
re rw
where Qpor is the soil-gas flow rate through a porous sampling probe, L/min; k is the permeability of a porous sampling probe, Darcy; h, re , and rw are the length, outside radius, inside radius of a porous sampling probe, respectively, m; Patm is the atmospheric pressure outside of a porous sampling probe (¼ 1 atm absolute); Pw is the pressure inside of a porous sampling probe, atm absolute. The flow rate of soil-gas inside the next section of impermeable flow line can then be calculated from materials balance as follows.
Qpe; next section ¼ Qpe Qpor
(4)
The flow distribution of soil-gas through each porous probe in the leak-detection system during soil-gas extraction can be calculated by repeating the calculations using Eqs. (2)e(4). By using the above soil-gas flow model, the calculated distributions of soil-gas flow through each porous probe for the 20-m and 30-m long sampling lines are shown in Fig. 3. The parameters used in the model calculations consist of two groups according to their sources (literature values, and laboratory-measured values). The literature values used in the model calculations are reference pressure (Pb ¼ 1 atm absolute) and soil gas viscosity at 20 C and 1 atm absolute (m ¼ 0.012 cp, McCabe and Smith, 1976). The laboratory-measured values used in the model calculations are length of an impermeable flow line (L ¼ 1 m), inside radius of an impermeable flow line (R ¼ 0.0045 m), outside radius of a porous sampling probe (re ¼ 0.008 m), inside radius of a porous sampling probe (rw ¼ 0.006 m), length of a porous sampling probe (h ¼ 0.05 m), and permeability of a porous sampling probe (k ¼ 0.0619 Darcy). For the 20-m long system, the total extraction rate of soil-gas and the extraction-pump pressure measured are Qtotal ¼ 42.2 L/min and Pout ¼ 0.9210 atm absolute, respectively. For the 30-m long system, the total extraction rate of soil-gas and the extraction-pump pressure measured are Qtotal ¼ 41.8 L/min and Pout ¼ 0.9340 atm absolute, respectively. Air extraction tests were conducted to measure the flow distribution of air through each porous probe in both 20-m and 30-m long sampling lines. Details of air-extraction tests were provided by Liang and Kuo (2006). Fig. 3 also verifies the calculated distributions of air flow with experimental data measured in air extraction
Cumulative airflow distribution (L/mim)
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229
45 40
model prediction (30 meter system) observation (30 meter system) model prediction (20 meter system) observation (20 meter system)
35 30 25 20 15 10 5 0 0
5
10
15
20
25
30
Distance of individual porous tube from the extraction point (m) Fig. 3. Distributions of air flow in a 20-m long system (total air flow rate ¼ 42.2 L/min; extraction pressure ¼ 0.9210 atm absolute) and a 30-m long system (total air flow rate ¼ 41.8 L/min; extraction pressure ¼ 0.9340 atm).
Fig. 4. GC Spectrum measured for a soil-gas sample taken at 20-m away from a leak-source. (a) gasoline; (b) diesel.
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Y. Chen et al. / Journal of Natural Gas Science and Engineering 42 (2017) 226e231
Fig. 5. GC Spectrum measured for a soil-gas sample taken at 30-m away from a leak-source. (a) gasoline; (b) diesel.
tests. The verified model of soil-gas flow distribution is useful for the optimal design of an improved sampling method of soil-gas for pipeline leak detection.
3.2. Leak-detection tests The main objective of the leak-detection tests was to evaluate the effective leak-detection radius of the horizontal sampling method for both gasoline and diesel by laboratory tests. Fig. 4 and Fig. 5 present the results of the laboratory leak-detection tests. Fig. 4a and b show the gas-chromatograph (GC) spectrums measured for soil-gas samples taken from the 20-m system using the gasoline leak-source, and diesel leak-source, respectively. Fig. 4a and b clearly show that both the gasoline and diesel leaks can be detected at 20 m away from the leak source. The peaks with retention times greater than 6 min are the volatile organics from gasoline and diesel. Fig. 4a and b indicate that the gasoline leak is easier to be detected than the diesel leak because gasoline is more volatile than diesel. Notice that the toluene concentration measured from the gasoline leak (25 ppm) is higher than that measured from the diesel leak (13 ppm).
Fig. 5a and b show the GC spectrums measured for soil-gas samples taken from the 30-m system using the gasoline leaksource, and diesel leak-source, respectively. Fig. 5a shows that a gasoline leak can be detected at 30 m away from the leak source. The toluene concentration measured from the gasoline leak is 8 ppm (Fig. 5a). However, due to the lower volatility of diesel than gasoline, Fig. 5b shows that a diesel leak is not detectable at 30 m away from the leak source. As shown in Fig. 5b, the toluene peak disappears in the GC spectrum. The effective detection radius of VOCs using the conventional sampling method using vertical soil-gas probes is only around 5 m (Liang and Kuo, 2006). Results of the above laboratory leakdetection tests indicate that the effective detection radius of VOCs using the improved sampling method is at least 30 m for gasoline leakage from a long-distance pipeline.
4. Conclusions A mathematical model to predict the flow distribution of soilgas samples has been presented for the horizontal sampling method. Darcy's law and Poiseuille's equation were utilized to
Y. Chen et al. / Journal of Natural Gas Science and Engineering 42 (2017) 226e231
model the flow behavior of soil-gases through the porous sampling probes and inside the impermeable lines, respectively. The soil-gas flow model has been verified by the air-extraction tests. The verified model is useful to design the horizontal sampling method of soil-gas for pipeline leak detection. Results of laboratory leak-detection tests corroborate that the horizontal sampling method enhances the effective sampling radius of soil gas for pipeline leak detection. The effective detection radius of organic volatile compounds (VOCs) using the horizontal sampling method is at least 30 m and 20 m for gasoline and diesel, respectively. On the other hand, the effective detection radius of VOCs using the conventional vertical soil-gas probe is only around 5 m. Acknowledgments This research has been supported by Environmental Protection Administration and National Science Council of Taiwan (MOST 1042116-M-006-002, MOST 105-2116-M-006-006).
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