When hydrogen is in regular or permanent contact with metal, it can penetrate the material’s microstructure, leading to hairline cracks and, eventually, mechanical failure. To minimize hydrogen embrittlement in sensors, the parts that are in direct contact with hydrogen should be made of austenitic steels like 316L or 316Ti. Special alloys such as Hastelloy C276, Inconel 718, and Elgiloy® (2.4711) are also well-suited for hydrogen applications.
- Mechanical pressure gauges and mechanical switches: 316L in the pressure port and probe element/movement
- Electronic pressure sensors: 316L or 316Ti in the pressure port and Elgiloy® (2.4711) in the sensor element
Hydrogen is the simplest and lightest element on the periodic table, and it takes very little energy to dissociate the diatomic molecule (H2) into two single atoms when it touches a metal surface. Hydrogen ions can penetrate most metals and other materials, which can lead to a signal drift of the sensor element over time. The higher the introduced energy, such as through higher process temperatures and higher pressures, the greater this effect will be.
One of the best ways to prevent hydrogen permeation is with gold plating. Gold serves as an excellent barrier and has a significantly lower permeation rate than copper, aluminum, nickel, and 316L stainless steel. Materials with a monocrystalline structure also protect sensors.
Leaks are a major issue when transporting and storing hydrogen. In addition to the economic loss, hydrogen leaks create an explosive atmosphere at levels as low as 4 mole % in air. What’s more, there are climate change issues: When H2 enters the atmosphere, it reacts with the hydroxyl radicals that would otherwise “clean up” methane, a greenhouse gas. Hydrogen also increases the concentration of water vapor, which is responsible for about half of the planet’s warming effect.
Adequate monitoring and ventilation reduces the risk of an explosion. Even better is to minimize and reduce fugitive emissions, including from measuring instruments.
• An all-welded system, in which a pressure instrument (gauge, transmitter, or switch) and a diaphragm seal are permanently welded together rather than sealed using polymer gaskets, creates a leak-proof and tamper-proof assembly.
• All instruments for hydrogen applications undergo helium-leakage testing during the final stage of production.
As hydrogen has the least mass of all known elements, it has extremely low density at atmospheric pressure. For efficient storage and transport, H2 is condensed with the following methods:
• Gas compression using pressures of 5,000–10,000 psi (350–750 bar) or more
• Cryogenic liquefaction by cooling the gas below its boiling point of −423°F (−253°C)
Due to temperature effects and safety factors, pressure sensors need to be capable of measuring up to 15,230 psi (1,050 bar) in, for example, hydrogen fueling stations. Temperature measuring instruments should be designed in such a way that they can perform with high accuracy while not hindering the hydrogen tank’s insulation.
• WIKA offers gold plating on flush sensors such as models E-11, IS-3 (flush version), and N-11, as well as on diaphragm seals.
• The special CryoTemp version of electrical thermometers can measure temperatures down to −486°F (−258°C), reliably and with repeated accuracy.