In the thermal spray coating process, materials are deposited onto a surface at high velocity. Sporting goods, medical devices, aerospace components – all may have thermal spray-applied coatings.
Hydrogen, oxygen, nitrogen, argon and helium drive these coating processes – the gases are used for combustion or inerting – and each gas has special properties in liquid and gaseous form. Understanding their properties, storage, application equipment and maintenance per industry standards is key to using the gases safely.
Bulk hydrogen, nitrogen, argon and oxygen are stored as cryogenic liquids at thermal spray facilities and are extremely cold, e.g. hydrogen is -423°C. The liquid or an uninsulated portion of pipe carrying liquid can cause a severe low-temperature burn on exposed skin. Extremely hot or cold temperatures, flammable gases, asphyxiants and high pressures all pose safety hazards in thermal spray operations.
Commonly used in high velocity oxygen fuel, hydrogen is extremely flammable in air at compositions between 4 and 74 percent. Per the fire triangle, fuel, oxygen and heat or other ignition source are needed for a fire. The amount of energy required to ignite a hydrogen fire is miniscule. So, it’s important to bond and ground (connect with heavy copper wire to the same grounding grid) every piece of hydrogen supply station equipment – tank, vaporizers, manifold and fence – to eliminate any difference in electrical potential that could result in a spark.
Consider the following foundational safety measures for using hydrogen:
Oxygen
At oxygen concentrations above 23.5 percent, materials that do not readily ignite in air (about 21 percent oxygen) can burn rapidly. Again, consider the fire triangle: fuel and oxygen are always present with oxygen supply systems. If supply piping itself contacts an enriched oxygen atmosphere or heat, it can serve as the fuel. Carbon steel, for example, burns more readily than stainless steel.
Consider the following foundational safety measures when using oxygen:
The first step to reducing risk is to understand common ignition sources in oxygen-based thermal spray systems. Common ignition mechanisms are mechanical impact, particle impact, mechanical friction, flow friction, and adiabatic compression.
Following are a few ways to reduce the risk of associated with these ignition sources during the oxygen system design and fabrication process:
Consider these standards and foundational practices when designing oxygen systems for thermal spray and performing maintenance.
Inert Gases — Nitrogen, Argon, Helium
Inert gases, which can leave the thermal spray gun at pressures as high as 2000 lb/in2, may be supplied as bulk liquid. At their simplest, bulk liquid supply systems consist of a vacuum jacket-insulated liquid storage tank, ambient air vaporizers (i.e., heat exchangers), and a pressure-regulating manifold to regulate the final line pressure of the gas delivered to the spray booths.
Depending on pressure requirements, thermal spray companies may use a pump in conjunction with ASME storage vessels. The storage vessels are carbon steel, so they need low-temperature protection to prevent low-temperature embrittlement, a common occurrence below –20°F. So does carbon steel used as process piping, for the same reason. CGA P-56, Cryogenic Vaporization Systems – Prevention of Brittle Fracture of Equipment and Piping provides guidelines for protecting carbon steel vessels and process piping.
Safety Training
At Messer, we are committed to helping our customers train plant personnel to safely operate bulk industrial gas supply equipment, properly handle gases, and deploy firefighting techniques. In fact, our “Taking the Lead in Safety” series of videos highlights the unique properties, safe handling, emergency procedures and use of cryogenic gases including hydrogen, oxygen, nitrogen, argon and helium. (838)
Oxygen Systems for Thermal Spray: Standards and Best Practices
Compressed Gas Association
American Society of Mechanical Engineers
National Fire Protection Agency
American Society for Testing Materials