Gas Leak Detection by Dilution of Atmospheric Oxygen
Total Page：16
File Type：pdf, Size：1020Kb
sensors Article Gas Leak Detection by Dilution of Atmospheric Oxygen Armin Lambrecht *, Eric Maier, Hans-Fridtjof Pernau ID , Thomas Strahl and Johannes Herbst Fraunhofer Institute for Physical Measurement Techniques IPM, Heidenhofstr. 8, D-79110 Freiburg, Germany; [email protected] (E.M.); [email protected] (H.-F.P.); [email protected] (T.S.); [email protected] (J.H.) * Correspondence: [email protected]; Tel.: +49-7665-41904 Received: 31 August 2017; Accepted: 30 November 2017; Published: 5 December 2017 Abstract: Gas leak detection is an important issue in infrastructure monitoring and industrial production. In this context, infrared (IR) absorption spectroscopy is a major measurement method. It can be applied in an extractive or remote detection scheme. Tunable laser spectroscopy (TLS) instruments are able to detect CH4 leaks with column densities below 10 ppm·m from a distance of 30 m in less than a second. However, leak detection of non-IR absorbing gases such as N2 is not possible in this manner. Due to the fact that any leaking gas displaces or dilutes the surrounding background gas, an indirect detection is still possible. It is shown by sensitive TLS measurements of the ambient background concentration of O2 that N2 leaks can be localized with extractive and standoff methods for distances below 1 m. Minimum leak rates of 0.1 mbar·L/s were determined. Flow simulations confirm that the leakage gas typically effuses in a narrow jet. The sensitivity is mainly determined by ambient flow conditions. Compared to TLS detection of CH4 at 1651 nm, the indirect method using O2 at 761 nm is experimentally found to be less sensitive by a factor of 100. However, the well-established TLS of O2 may become a universal tool for rapid leakage screening of vessels that contain unknown or inexpensive gases, such as N2. Keywords: gas sensor; leak detection; tunable laser spectroscopy; infrared absorption; oxygen; nitrogen; methane 1. Introduction Gas leak detection is an important issue in production processes to prevent safety hazards and to ensure functionality of many products. For natural gas pipeline surveillance, many instrument developments aim at fast and sensitive remote detection of gas leaks. For these purposes gas cameras using modified thermal imaging systems, handheld tunable laser spectroscopy (TLS) instruments, and even helicopter-based systems are available [1–5]. Commercially available handheld TLS instruments in the NIR are able to detect CH4 leaks with column densities >1 ppm·m via remote detection. They can be used from a distance up to 30 m and have measurement rates up to 10 Hz [3]. Determination of leak rates is difficult for remote gas detectors since sufficient information about the gas dispersion in the surrounding environment is usually not available. Recently, a TLS remote detection device was able to identify a CH4 leak flux of 15 mL/min from a distance of 37 m [6]. However, all these systems rely on infrared absorption of the target gas, which is the case for hydrocarbons, for example. Consequently, leak detection of non-infrared absorbing gases such as N2,H2, Ar, etc., is not possible in this fashion. Sensitive leak testing of vessels, tubes, vacuum parts, refrigeration systems, etc., is usually performed via helium (He) leak testing. This detection method is based on a mass spectrometer (MS) that is tuned to He. In most cases, the examined object has to be filled with a He-containing gas. In such a leakage scenario, the He is detectable by pumping (“sniffing”) the leakage gas into the MS through a handheld nozzle located close to the leak. Very small leaks (below 10−7 mbar·L/s) can Sensors 2017, 17, 2804; doi:10.3390/s17122804 www.mdpi.com/journal/sensors Sensors 2017, 17, 2804 2 of 14 be detected by this method, but a close contact to the object is required. Apart from that, He leak testing from a distance is not possible. Further disadvantages are that the testing is a slow and manual task. A less expensive alternative to He is hydrogen or forming gas that is used in combination with sensitive electrochemical, metal oxide-, or Pd-based detectors. However, the use of hydrogen could be a safety hazard. Acoustic leak detection is another common method. This method is based on the principle that the leakage gas generates a sound wave which can be detected by a sensitive microphone. Therefore, the object needs to be pressurizing in order to obtain a turbulent flow at the orifice which is required to generate (ultra-) sound waves [7]. This may be circumvented by using an active ultrasound source within the object, but discrimination of the different sound paths could be difficult. The classical leak detection technique for rather coarse leaks is the well-known “air bubble” method. A vessel under investigation is pressurized with gas (e.g., air) and immersed into a liquid (e.g., water). Air bubbles originating from the leak indicate the leak location and leak size. Minimum leak rates ≥10−3 mbar·L/s can be detected by this method which is comparable to acoustic leak detection [8]. Obviously, this method is not practical or feasible for large objects. Furthermore, the method is, generally, a slow and manual task. However, this concept may also be transferred in the gas phase. Therefore, a background gas in the atmosphere substitutes the liquid such that the background gas is displaced and diluted by the leakage gas. Thus, remote leak detection of any target gas—also of non-IR-active gases—should be feasible by a sensitive tunable laser spectroscopy (TLS) measurement of the background gas concentration. Ubiquitous and IR-active background gases are, for instance, CO2,O2, or H2O, which should be different from the target gas. In order to detect a leak-induced ambient background concentration change, the distance has to be fixed or measured simultaneously [9]. This can be achieved with state-of-the-art laser rangefinders. In the next section, we show that O2 is the optimum candidate for a suitable background gas. Then the experimental setup is explained. Because of the visibility of the employed laser we use the term “LeakEye” for our technique. Computational fluid dynamics (CFD) simulations of the experimental setup are performed using COMSOL Multiphysics®(COMSOL Inc., Burlington, MA, USA). Then the results of experiments on extractive detection—later on, standoff detection—are given. In this work the terms non-tactile and standoff detection are synonymously used, even for rather short distances. The essential difference to extractive detection is that gas flow is not disturbed. We shall always compare experiments with direct (positive) detection of CH4 and with corresponding experiments on indirect (negative) detection of O2. 2. Modelling The fundamental relation of absorption spectroscopy is given by the Beer-Lambert law: −a(u)·d·c I(u) = I0·e with a(u) = S(T, u0)·g(p, T, u − u0) where I(υ) is the frequency-dependent transmitted intensity after passing a measurement cell with an optical path length d, c is the gas concentration, and a(u) is the frequency-dependent gas specific absorption coefficient, which is given by the product of the temperature-dependent line strength S(T,υ0) at the characteristic absorption frequency υ0, and the pressure- and temperature-dependent line shape function g(p,T,υ − υ0). As a rule of thumb we assume that a 1% reduction of the effective absorption length d by dilution of the background gas by the leakage gas has to be measured with a laser spectroscopic setup. In a typical situation an undisturbed extension of the leaking gas plume up to approximately 1 cm from the surface of the leaking object is assumed. For larger distances strong dilution by ambient air flow is expected. As a result the “LeakEye” technique is practically limited to standoff distances of approximately 1 m. For larger distances the relative measurement effect is too small. Sensors 2017, 17, 2804 3 of 14 Relevant atmospheric background gases, such as O2,H2O, and CO2, with infrared absorption data are available in the HITRAN database [10]. Suitable absorption lines of these gases for typical ambient concentrations, T = 296 K and p = 1013 mbar were selected and laser spectroscopic measurements were simulated using Mathcad (PTC, Unterschleißheim, Germany). For a total optical distance of 200 cm the transmission signal change for a 1% change of the optical distance was determined. For all gases distributed feedback (DFB) laser diodes in the near infrared (NIR) are available. In the simulation laser emission powers were set to 3 mW. Photodiodes with optimum detectivities with respect to noise equivalent powers (NEP) for the selected wavelength ranges were chosen as detectors. A maximum total transmission of 1% was assumed to account for the weak signal for standoff detection. The resulting power changes at the detector DP with respect to the relative power changes DPrel are shown in Table1. Table 1. Selection of background gases and detection wavelengths. Gas O2 H2O CO2 Wavelength(nm) 761 1392 2004 Concentration 21% + 1% H2O 1% 450 ppm + 1% H2O Detectivity D * 1013 1012 1011 (Jones) NEP (@100 kHz, nW) 0.01 0.08 0.8 DP(nW) 12 60 2 −4 −3 −4 DPrel 4 × 10 3 × 10 1.5 × 10 Suitable + − 1 − 1,2 1 ambient concentration fluctuations; 2 detector noise limitation. The measurement of O2 at 761 nm was selected as the best choice (cf. Table1): although TLS-based hygrometers could serve as an established platform [11], the expected ambient fluctuations for H2O, as well as for CO2, are too high. The first experiments with CO2 show that the measurements are strongly affected by the exhaled air of people in the lab; this effect is much stronger for CO2 than for O2.