Methods of Leak Detection

A review of the common leak detection methods in chemical engineering is given in Table 8.2. 

Method Test fluid Threshold leakage rate (mbar 1 s ) System pressure Leak localisation Quantitative leakage rate statement 

The presented methods can be divided basically into two categories  

1) pressure change methods (without specific tracer gas) 

A guide to select appropriate test methods can be found in the European standard EN1779 (a description of pressure change methods and tracer gas methods can be found in the standards EN 13184 and EN 13185, respectively). 

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Air leaks are another source of trouble in the MS. A simple method of leak detection is to squirt a small volume of acetone on flanges and other areas where leaks could occur. Caution is advised not to use this procedure near hot surfaces because of the flammability of acetone. A second way to test for small leaks is to tune the MS to m/z 40 and to use argon to test for leaks. The m/z 40 peak will increase if argon enters the source. Helium m/z 4) is a better choice, except when helium carrier gas is used in conjunction with the GC. A small stream of the gas is aimed at all seals where a leak can occur. If a leak is detected at a seal, it can sometimes be stopped by tightening the seal, but it is better to replace the seal than to overtighten it. 

With vacuum systems, the accepted method ( vacuum method) of leak detection/measurement is to evacuate the object/system under test and with the MSLD coupled via its inlet port, apply He externally and observe the response from the MSLD. 

A typical indication of leakage is the inability of the leaking system to achieve the expected working pressure. Pressure-rise tests are frequently carried out to assess the magnitude of the problem (Examples 4.3, 4.5-4.7,4.9). A convenient and highly sensitive method of leak detection and quantification uses He as a tracer gas and a He leak detector (MSLD). Some aspects of practical leak detection using MSLD were discussed (Examples 4.10-4.12). 

The closures of BFS containers are formed within the automated process by the head mold set which closes around the top of the severed section of parison following filling. The integrity of the container and closure is generally tested by a manual or automated method of leak detection performed outside of the filling environment following removal of excess plastic (deflashing) from the filled product units. 

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. 

A turkey feather, or any good-sized downy feather, is a good visual indicator for leaks, but they can be tricky to use because they are so sensitive that any breathing or very slight movement of air from any source can also ruffle the down. It s not exactly high tech, but I have found it to be a very effective method of leak detection. 

An Engineering Assessment of Acoustic Methods of Leak Detection in Aboveground Storage Tanks... 

Various methods of leak detection have been verified ... 

Most of the activity from any such steam and water leakage is associated with activation products which have been retained by the pipework lagging. This, incidentally, has given a useful method of leak detection when the reactor is shut down and depressurized. The feed heater cell is classified as a restricted radiation area principally because it contains bled st earn pipes and the primary circuit polishing plant. However, access has been possible at all times for visual inspection, and some maintenance work has been performed with the reactor at full power. 

Workers using LPG for any reason must be given training. This should Include the approved method of leak detection,... 

There are three methods of release detection that are associated with modem tank systems.18,22 The first approach is to conduct an annual tank or line tightness test to detect small releases and to use more frequent monitoring by another method to detect large releases. All tank and line tightness tests must be performed at least once a year and must be able to detect leaks of 0.38 L/h (0.1 gal/h). In all cases where annual tightness tests are used, the regulation requires an additional form of leak detection in which tests on tanks are conducted at least monthly and those on pressurized lines at least hourly this ensures the detection of excessively large releases. For tanks, daily inventory records must be reconciled monthly, for pressurized lines, leaks of up to 11.4 L/h (3 gal/h) must be reliably detected. 

Fig. 4-5. Principle of radiotracer pig method radiotracer injection device (up, left), pig container (up, middle), pig introduction in pipe (up, right), schematic principle of leak detection in buried pipe (bottom) and typical signal obtained from datalogger (middle).

Spanish NPPs with extensive degradation phenomena in the steam generators have in operation a detection system, based on the monitoring of the short-lived N-16, installed in the steam lines. This system has been introduced into the Technical Specifications. The previous method of leak measurements, based on the monitoring of the steam generators blowdown, is used to confirm the N-16 readings. 

Superfluid helium can pass easily through openings so small that they caimot be detected by conventional leak detection methods. Such leaks, permeable only to helium II, are called supedeaks. They can be a source of fmstrating difficulties in the constmction of apparatus for use with helium II. 

Different combinations of stable xenon isotopes have been sealed into each of the fuel elements in fission reactors as tags so that should one of the elements later develop a leak, it could be identified by analyzing the xenon isotope pattern in the reactor s cover gas (4). Historically, the sensitive helium mass spectrometer devices for leak detection were developed as a cmcial part of building the gas-diffusion plant for uranium isotope separation at Oak Ridge, Tennessee (129), and heHum leak detection equipment is stiU an essential tool ia auclear technology (see Diffusion separation methods). 

Spill Prevention and Detection. It is far better to prevent a leak or a spik than to clean one. The fundamental rule of leak and spik prevention is to reduce the possibkity for contamination by directing resources as close to the source as possible (Fig. 11). In addition to increasing the effectiveness of a spik and leak prevention program, the costs are lower if the focus is placed on preventing the occurrence in the first place. Regulatory trend, however, is to requite methods that respond to leaks after they occur. In addition to being more costly, this type of requirement is often a disincentive to prevent the leaks in the first place, because of the additional cost. 

Out-of-tank leak detection ground penetrating radar and inventory methods... 

Use of Liners. The use of impermeable liners and membranes, often called release prevention barriers (RPBs) under tanks, may be the most effective leak detection and prevention method. On new tanks, it is relatively easy to install these systems, and large numbers of tanks are being built with this type of system in the 1990s. For existing tanks, however, it would be very costiy if not impractical to install liners. For existing tanks, the combination of other methods as well as an effective inspection program can be more effective as a substitute for a release prevention barrier. 

In order to minimize the number of leaking units, it is important that the mold sets are correctly aligned. Even very slight misalignment may potentially lead to the production of imits with very slight leaks which may be difficult to detect by routine methods. Correct molding is therefore of key importance and can usually be simply checked by careful and experienced visual examination of units. 

EPA. 1987a. U.S. EPA evaluation of volumetric UST leak detection methods. In Proceedings of the 13th Annual Research Symposium, Cincinnati, OH, May 6-8,1987. Cincinnati, OH U.S. Environmental Protection Agency, 164-171. 

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

The fact that the pressure reading at vacuum gauges (see Section 3.3) is sensitive to the type of gas involved can, to a certain extent, be utilized for leak detection purposes. Thus it is possible to brush or spray suspected leaks with alcohol. The alcohol vapors which flow into the device - the thermal conductivity and ionizablity of which will vary greatly from the same properties for air - will affect and change pressure indication to a greater or lesser extent. The availability of more precise, easy-to-use helium leak detectors has, however, rendered this method almost completely obsolete. 

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 ... 

These units are the most sensitive and also provide the greatest degree of certainty. Flere certain is intended to mean that there is no other method with which one can, with greater reliability and better stability, locate leaks and measure them quantitatively. For this reason helium leak detectors, even though the purchase price is relatively high, are often far more economical in the long run since much less time is required for the leak detection procedure itself. 

Here the points suspected of leaking at the pressurized test specimen (see Fig. 5.4, d) are carefully traced with a test gas probe which is connected with the leak detector by way of a hose. Either helium or hydrogen can be detected with the INFICON helium leak detectors. The sensitivity of the method and the accuracy of locating leaky points will depend on the nature of the sniffer used and the response time for the leak detector to which it is connected. In addition, it will depend on the speed at which the probe is passed by the leak points and the distance between the tip of the probe and the surface of the test specimen. The many parameters which play a part here make it more difficult to determine the leak rates quantitatively. Using sniffer processes it is possible, virtually independent of the type of gas, to detect leak rates of about 10 mbar l/s. The limitation of sensitivity in the detection of helium is due primarily to the helium in the atmosphere (see Chapter 9, Table VIII). In regard to quantitative measurements, the leak detector and sniffer unit will have to be calibrated together. Here the distance from the specimen and the tracing speed will have to be included in calibration, too. 

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