Leak detection limitations

For leak detection, the sensor must be sensitive and fast enough to provide early leak detection so that action can be taken before the explosive limit in air is reached. Utilizing a fiber-optic sensor configuration could provide the best chance of meeting fast response and inexpensive and reliable goals. 

Present formula gasolines contain approximately 300 ppm. No. 2 fuel oil contains 2,200 to 2,600 ppm by weight of sulfur. Even pipeline gas contains sulfur-containing odorants (mercaptans, disulfides, or commercial odorants) for leak detection. Metal catalysts in the fuel reformer can be susceptible to sulfur poisoning and it is very important that sulfur in the fuel reformate be removed. Some researchers have advised limiting the sulfur content of the fuel in a stream reformer to less than 0.1 ppm, but noted the limit may be higher in an autothermal... 

Compiled in Figure 5.2 are the nature and detection limits of frequently used leak detection methods. 

This leak rate represents the detection limit for good halogen leak detectors (=0,1 g/a). ... 

Halogen leak detection Substances containing halogens 10- (10- ) 7.10-= (10- ) Positive pressure (vacuum) With limitations... 

The detection of a test gas using mass spectrometers is far and away the most sensitive leak detection method and the one most widely used in industry. The MS leak detectors developed for this purpose make possible quantitative measurement of leak rates in a range extending aaoss many powers of ten (see Section 5.2) whereby the lower limit = 10 mbar l/s, thus making it possible to demonstrate the inherent gas permeability of solids where helium is used as the test gas. It is actually possible in principle to detect all gases using mass spectrometry. Of all the available options, the use of helium as a tracer gas has proved to be especially practical. The detection of helium using the mass spectrometer is absolutely ( ) unequivocal. Helium is chemically inert, non-explosive, non-toxic, is present in normal air in a concentration of only 5 ppm and is quite economical. Two types of mass spectrometer are used in commercially available MSLD s ... 

If compounds are detected in lower concentrations than desirable because of detection limits and no leaks have occurred, sample size, gas flow rate, and isolation time can be adjusted to increase the total volatile quantity collected. The method can be optimized efficiently using trial runs in which one parameter is changed at a time. Be aware that volatile ratios will likely be affected by adjustments in these parameters. 

Measurement and control of low-flow rates are a requirement in such applications as fuel cells, purging, bioreactors, leak testing, and controlling the reference gas flow in chromatographs or in plasma-emission spectrometers. The most traditional and least expensive low-flow sensor is the variable-area flowmeter. It has a high rangeability (10 1) and requires little pressure drop. Due to its relatively low accuracy, it is limited to purge and leak-detection applications. 

Work by Coyne and Cobb provided a more effective list of discharge colors because the colors were observed in the act of actual leak detection. They are presented in Table 7.16. Their work demonstrated that spraying gases had limited success in leak indication, but moderate success when the item being tested was enclosed within the gas (by way of a bag). Vapors from applied liquids provided better indicators and more easily demonstrated the specific location of a leak. 

Most helium leak detectors will not operate with pressures above lO"4 torr to 10 5 torr. At these greater pressures, the main element to the mass spectrometer will bum out. Fortunately most, if not all, helium leak detectors have various safety check mechanisms that automatically shut off the current to the main filament if the pressure goes above a set limit. So, you must depend on alternate leak detection methods, or use the detector-probe technique to discover large leaks. Once large leaks have been discovered and closed, you can concentrate on the smaller leaks that can be found with the tracer-probe technique. 

Evacuated stainless steel canisters are widely used collection devices for ambient air samples.6 Sample canisters have smooth and inert internal surfaces and few or no active sites that adsorb volatile analytes or catalyze chemical reactions. The 1-6 L canisters are easily transported to a remote or a field laboratory. Canisters are leak tested and cleaned in the laboratory prior to use, evacuated to about 5 X 10-2 Torr or less, and transported to the sampling site where samples are taken by opening the sampling valve. Composite samples can be taken over time and/or space and an in-line pump can be used to pressurize the container with either additional sample air or pure air if sample dilution is required. Pressurized samples are useful when longer-term composite samples are taken or when larger samples are needed to lower detection limits. 

Other methods of leak detection include using soap fluid with a little water in it, ultrasonic detectors, stethoscopes, feathers and electronic gas leak detectors. The reality of leak detection is that each method has its place, but each method is limited to certain leak characteristics. If one type of sensor does not show a leak, another may find it. 

Contamination of the reaetor by leaks and permeation of laboratory air contaminants is minimized by continuously flushing the enclosure that houses the reactors with purified air. NOx and formaldehyde levels in the enclosure before or dining irradiations were generally less than 5 ppb and PM coneentrations are below the detection limits of oin instrumentation. Introduction of contaminants into the reactor is also minimized by use of pressure control to assure that the reactors are always held at slight positive pressines with respect to the enclosure, so leaks are manifested by reduction of the reactor volume rather than dilution of the reaetor by enelosure air. The leak rate into the chamber was tested by injecting —100 ppm of CO into the enelosure and monitoring CO within the reactor. No appreciable CO (below the 50 ppb deteetion limit) was obtained for this experiment. 

Johansson and Granat (1984) conducted research on pressure differences between the outside atmosphere and air within a chamber using a tilted water manometer, which indicated that pressure differences were below detectable limits (0.2 mm H2O). Arkinson (2003) conducted research on temperature difference between the outside atmosphere and air within the chamber. The results indicate that temperature differences are < 0.4°C. The out-flowing gas also flows through a Teflon FEP sample line and is directed into a temperature controlled mobile laboratory, which houses the analytical instruments and data acquisition system. Here, the sample is analyzed continuously for content of the gas of interest. A vent line is fitted to the exiting sample line to prevent pressurization and was periodically bubble tested to check for under pressurization and/or leaks in the enclosed system. Sample lines do not exceed 10 meters. 

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