2.3.1 Propensity to leak

2.3.1.1 Low viscosity

Hydrogen gas has a very low viscosity and so it is very difficult to prevent hydrogen systems from developing leaks. Pipe work that was ‘leak tight’ when pressure-tested with nitrogen will often be found to leak profusely when used on hydrogen duty.
Hydrogen leakage through welds, flanges, seals, gaskets, etc is an important consideration and an important design and operational issue for hydrogen systems.
The use of suitable sealing interfaces and appropriate components within a hydrogen system, however, will significantly reduce the likelihood of this occurring if fitted by a competent person. For high-pressure storage systems, hydrogen would leak nearly three times faster than natural gas and over five times faster than propane. However the low energy density of hydrogen means that it produces substantially lower energy leakage rates.

2.3.1.2 Extremely high diffusivity

Hydrogen is very much lighter than air and is also very diffusive. Thus, unlike heavier gaseous fuels, if a hydrogen leak occurs in an open or well-ventilated area its diffusivity and buoyancy will help to reduce the likelihood of a flammable mixture forming in the vicinity of the leak. However, as with other gases when leaks occur within poorly ventilated or enclosed areas, the concentration may rapidly reach dangerous levels. Due to its lightness, hydrogen will concentrate in elevated regions of an enclosed space, whereas other gases, dependent upon their relative mass, will concentrate at ground level (LPG) or at elevation (CNG).  If unprotected electrical equipment or other sources of ignition are present, the risk from explosion could be considerable.

As hydrogen diffuses more rapidly through air and through solid materials compared to other fuel gases such as methane or propane, it will usually disperse more rapidly if released, although buoyancy effects are less significant for high momentum releases from high-pressure hydrogen systems. When harnessed through intelligent equipment design and layout, this buoyancy and hydrogen’s rapid dispersion rate can become a significant safety asset.

2.3.1.3 High buoyancy

The buoyancy of hydrogen can also be used to manage the risk normally associated with fuel handling by segregating the hydrogen from foreseeable sources of ignition using internal partitions and bulkheads and differential pressurisation. This can also be done by locating all potential sources of ignition well below the level of the equipment from which hydrogen may leak and accumulate, and ensuring adequate ventilation and safe discharge of the exhaust.  Table 1 compares some of the properties of natural gas, LPG and hydrogen.