Leak Rate Detection

If the pressure rise of plot 3 is used during the first 3 h, the leak rate would appear to be 1.2 x 10 2 mbar L/s. 

This example is to show that the water desorption during applicable measuring times becomes less and less. If the pressure rise of 8 x 10 2 mbar/h in this installation is converted into leak rate, LR = 3.6 X 10 3 mbar L/s, or after 10 h it drops to 1.8 X10 3 mbar L/s. In the LR region of 10 3 mbar L/s one has to expect such variations between different measurements, since desorption depends on the history of the plant before measurements start and variations of this size disturb neither the BTM nor DR measurements. 

recorder 5, control panel sterile room 6, compressor 7, vacuum pumps 8, venting filter 9, condenser. 

Leaks and loss of power are the most unwanted events during freeze-drying (see the end of this chapter about defects). Therefore, it is recommended to make a leak test before each freeze-drying run, although this routine test does not have to follow the procedure described above. If the leak rate is measured once, the pressure as a function of time during the evacuation period can be recorded and compared with the evacuation plot of the actual run. If these plots do not deviate from each other, one can conclude that no additional leak has developed. This routine test should only be applied if the cleaning and sterilization of the plant have been the same as before the last run, to keep the desorption qualities of the surfaces as identical as possible. 

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Selection of a pressure gauge. The sensitivity of the pressure gauge will determine the minimum leak rate detectable ... 

Kirsch, L.E. Nguyen, L. Moeckly, C.S. Pharmaceutical container/closure integrity I Mass spectrometry-based helium leak rate detection for rubber-stoppered glass vials, PDA. J. Pharm. Sci. Technol. 1997, 51, 187-195. 

The leak detection sensitivity refers to the minimum top liner leak rate that can theoretically be detected, collected, and removed by the LDS. The leak detection time is the minimum time needed for liquids passing through the top liner to be detected, collected, and removed in the nearest down-gradient collection pipe. In the case of a composite top liner, the leak detection time refers to the period starting at the point when liquids have passed through the compacted soil component and ending at the point when they are collected in the collection pipe. 

U.S. EPA bases its 1 gallon/acre/day leak detection sensitivity on the results of calculations that show that, theoretically, an LDS overlying a composite bottom liner with an intact FML component can detect, collect, and remove liquids from a top liner leak rate <1 gallon/acre/day. This performance standard, therefore, can be met with designs that include a composite bottom liner. Based on numerical studies, one cannot meet the leak detection sensitivity with a compacted soil bottom liner, even one with a hydraulic conductivity of 10-7 cm/s. Therefore, the emphasis of this standard is on selecting an appropriate bottom liner system. 

Block valves are installed throughout plants to return a process to a safe condition under unusual circumstances. For example, the process shown in Figure 12-2 detects a hose leak by comparing flow rates at both ends of the hose. If the hose breaks, the leak is detected and the block valves on the reactor and sewer are immediately closed. 

The leak rates of a freeze drying plant can be measured at the empty plant with the condenser cooled and the shelves heated by measuring the pressure rise per time multiplied by the installation volume in the dimension (mbar L/s). It should be noted, that the plant has to be evacuated for several hours, e. g. down to 10-2 mbar, before the pressure rise measurements, to avoid the influence of small amounts of ice and the desorption of gas from the surfaces. Furthermore, the pressure rise should be measured up to 0.2 or 0.4 mbar to detect possible gas desorption. Only if the pressure rise has been for some time proportional with time (Fig. 2.33.1), it represents a leak rate, which is defined as... 

Fig. 5.2 Leak rate ranges for various ieak detection processes and devices...

Freon F12 loss per year (g/a) Time taken to form a gas bubble (s) Equivalent leak rate (cm3[STP]/s) Detection time using helium leak detector (s)... 

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

Method Test gas Smallest detectable leak rate Pressure range Quantitative measurement... 

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 ... 

In the calibration process proper the straight-line curve representing the numerically correct, linear correlation between the gas flow per unit of time and the leak rate is defined by two points the zero point (no display where no emissions are detected) and the value shown with the test leak (correct display for a known leak). 

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. 

The leak rates which can be detected with this process go far beyond all practical requirements. 

The integral leak test, i.e. the total leak rate for all individual leaks, facilitates the detection of microscopic and sponge-like distributed leaks which altogether result in leakage losses similar to those for a larger individual leak. 

Some of the renewable energy processes operate under high pressures (Fy, and others such as thermal solar systems circulate oils under high temperatures. In these and many other applications, leakage in tanks (under- or aboveground), valves, steam traps, or pipes must be detected. Table 3.107 lists the detectable leak rates of some common gases, including H2. 

Minimum Detectable Leak Rates of Some Common Gases Using Thermal-Conductivity-Type Leak Detectors... 

This is not normally the most effective connection point because the sensitivity is reduced enormously (SDP > SLD) and in this case the indicated leakage is close to the lowest detectable leak rate. 

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. 

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