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Hydrogen poses unique challenges due to its ease of leaking, low-energy ignition, wide range of combustible fuel-air mixtures, buoyancy, and its ability to embrittle metals that must be addressed to ensure safe operation. Hydrogen can be explosive under specific concentration, temperature and pressure conditions. Special precautions are needed for testing, handling and plant personnel safety. Liquid hydrogen poses additional challenges due to its increased density and extremely low temperatures. Hydrogen-air mixtures can ignite with very low energy input, for reference, an invisible spark can cause ignition. The minimum energy required for spark ignition at atmospheric pressure is approximately 0.02 millijoules. The flammability limits based on the volume percent of hydrogen in air at 1 atm (101 kPa) are 4.0 and 75.0 while they are 4.0 and 94.0 for hydrogen in oxygen. The explosive limits (upper and lower limits of percentage composition of a gas mixture explodes when ignited) of hydrogen in air are 18.3 to 59 percent by volume. Hydrogen collects under roofs and overhangs, where it forms an explosion hazard and this calls for good ventilation. Hydrogen pipes should be located above other pipes to prevent explosion hazards. Hydrogen leaks can support combustion at very low flow rates, as low as 4 micrograms/s. Flames in and around pipes or structures can create turbulence that causes a deflagration or detonation. Further, hydrogen diffuses extensively and is particularly subject to leakage with high leak rate because of its low viscosity and low molecular weight (leakage is inversely proportional to viscosity). The leak rate is 50 times that of water, and 10 times that of liquid nitrogen. Hydrogen sensors allow for rapid detection of hydrogen leaks. As in natural gas, an odorant can be added to hydrogen sources to enable leaks to be detected by smell. It is difficult to see hydrogen flames with the naked eye. However, they are readily seen by UV or infrared detectors.

6.7.1. Leakage, diffusion, and buoyancy

These hazards result from the difficulty in containing hydrogen. Hydrogen diffuses extensively, and when a liquid spill or large gas release occurs, a combustible mixture can form over a considerable distance from the spill location.

Hydrogen, in both the liquid and gaseous states, is particularly subject to leakage because of its low viscosity and low molecular weight (leakage is inversely proportional to viscosity).

Because of its low viscosity alone, the leakage rate of liquid hydrogen is roughly 100 times that of JP-4 fuel, 50 times that of water, and 10 times that of liquid nitrogen.

Hydrogen leaks can support combustion at very low flow rates, as low as 4 micrograms/s.

Condensed and solidified atmospheric air, or trace air accumulated in manufacturing, contaminates liquid hydrogen, thereby forming an unstable mixture. This mixture may detonate with effects similar to those produced by trinitrotoluene (TNT) and other highly explosive materials.

Liquid Hydrogen requires complex storage technology such as the special thermally insulated containers and requires special handling common to all cryogenic substances. This is similar to, but more severe than liquid oxygen. Even with thermally insulated containers it is difficult to keep such a low temperature, and the hydrogen will gradually leak away (typically it will evaporate at a rate of 1% per day).

Hydrogen collects under roofs and overhangs, where it forms an explosion hazard; any building that contains a potential source of hydrogen should have good ventilation, strong ignition suppression systems for all electric devices, and preferably be designed to have a roof that can be safely blown away from the rest of the structure in an explosion. It also enters pipes and can follow them to their destinations. Hydrogen pipes should be located above other pipes to prevent this occurrence. Hydrogen sensors allow for rapid detection of hydrogen leaks to ensure that the hydrogen can be vented and the source of the leak tracked down. As in natural gas, an odorant can be added to hydrogen sources to enable leaks to be detected by smell. While hydrogen flames can be hard to see with the naked eye, they show up readily on UV/IR flame detectors.

Sparking and combustion 6.8.

Working with fuel gases including hydrogen in potential low concentrations requires especial prudence in the use of hand tools and electrical equipment. The worker should always be conscious of the potential for ignition and combustion. The clash of steel tools could result in a spark, electrical equipment could short and cause an arc, and dry clothing could result in static electricity which could also be an ignitor for the right mixture of fuel and oxidant.

In bubble testing, simple soap or detergent solutions in water are effective and economical indicators. If working on a line or vessel that contains, did contain or could contain oxygen, the worker needs to be aware that some detergents contain hydrocarbons and hence the danger of providing a fuel in the presence of pure oxygen could have disastrous results.

Psychological factors and safety programme 6.9.

No worker should endanger his health or his physical well-being in the course of his duties. It follows that any industrial activity should be conducted safely and with due respect to the environment. Generally there are laws and rules of good practice that govern every workplace. Inspectors often are only temporary visitors to the workplace. Although most service sites now provide site specific safety training as a condition of working on that site, the inspector still finds the occasional site where he has to apply his own common sense and the rules of his employer.

Every employee (and supervisor and manager) must be aware of their individual responsibility for their own safety. Safe work practices are documented for all activities including testing, and the worker must be aware of these practices.

Even if a formal safety programme is not required by law, it follows that the leak tester’s employer should, just as a matter of good practice and risk management, have his own programme.

A number of conventional NDT methods can be applied effectively to the detection of leaks, and thus can be considered techniques in the context of leak testing. Specific methods that can be used this way are liquid penetrant testing (PT) and acoustic emission testing (AET).

Reference should be made to Chapter 1, where an overview of the use of these methods is presented.

System reliability through leak testing 7.1.

The use of the “leak before break” design concept relies on a detectable leak being the first indication of a flaw. To the extent this concept is used, the detection of leaks as soon as they occur is critical.

Loss of contents can result in many serious consequences. A toxic gas or liquid may take lives. Even a nontoxic contaminant may spoil the environment. Leakage might result in loss of lubricant and thus machinery failure. In the packaging industry, a leak might result in product spoilage. Of course, there is always the economic loss associated with a product loss, as well as costs associated with clean up.

Leak testing related to material flaws 7.2.

Through cracks result in leakage, and through cracks may be precursors of more extensive cracking or fracture. In particular, pin holes and tiny internal breaches are detectable using leak detection techniques.

Desired degree of leak tightness 7.3.

Product specifications often recognize that perfect leak tightness is impossible, and thus specify a maximum leak rate. For many aerospace applications, for example, components and systems are typically required to have a leak rate of less than 5·10^-6 standard cubic centimeters of gas at a pressure of one atmosphere (5·10^-6 mbar-L²/sec).

Helium leak testing 7.4.

Before a leak test examination is performed, it is necessary to determine if the examination is to ascertain whether leaks are present or not, overall leak detection, or if the examination is to determine the location of a leak, localising leak detection. In some cases, an examination for overall leak detection is performed first, and if leaks are detected, the localising method is applied for pinpointing of the leak. This is however not always required nor possible.

Secondly, it is necessary to determine the leak rate which can be tolerated, as no object is 100% tight. This is the requirements of tightness of the object. If, for example, the object has to be watertight, a leak rate below 10^-4 mbar l/s will be sufficient. If the object, for example, is to be used in the chemical industry the requirements can be a leak rate below 10^-6 mbar l / s.

In leak testing, a pressure difference between the outer and the inner side of the object to be examined is produced. Subsequently the amount of gas or liquid which is passing through a leak is measured. In the helium leak test (see fig. 7.1), helium is used as a search gas. In principle, two methods are applied for leak testing and location of leaks, the "Vacuum method" and the "Overpressure method".

7. APPLICATIONS

FIG. 7.1. Helium leak test.

In the "Vacuum method", the object to be examined for leaks is evacuated and sprayed from the outside with Helium. The gas enters through any leaks present in the object and is detected by a sensor connected to the leak test instrument.

In the "Overpressure method", the object to be examined for leaks is filled with the search gas, Helium, under slight overpressure. The search gas escapes through any leaks present to the outside and is detected by a detector probe. This detector probe is in most cases called a

"sniffer" acting as a gas sampling probe.

For both methods specially developed leak detectors are available.

The object under test should, if possible, be tested according to its final mode of use, i.e. if it is used under vacuum, the vacuum method should be applied, if it is finally pressurised, the overpressure method should be adopted.

Examples of testing with the two methods are illustrated below. The Vacuum method is illustrated with the Hood Test and the Tracer Probe Test and the Overpressure method with the Hood test, the Bombing test and the Detector Probe or Sniffer test.