Type 1 steel tanks can be cut with a welding torch, so they can definitely be impacted by flame impingement. Above 400-500 C° the material properties of steel will start to degrade.
It is challenging to protect a vessel, either Type 1 steel or composite Type 2, 3, or 4 from an impinging fire since the safety devices may not see the elevated temperatures in the area of impingement. Devices such as thermally activated pressure relief devices (TPRDs) or rupture discs are usually the best for fire exposure since when activated, they fully depressurize the contents to relieve stresses on the vessel walls. However, TPRD’s will not activate properly to relieve the contents unless exposed to the heat of the localized impinging fire. Similarly, a rupture disc may not activate unless the pressure increases to its setpoint or unless it’s weakened by the impinging fire. Relief valves may activate if the pressure rises but will maintain pressure until the walls potentially weaken below the stress imparted by the pressure even if at or below the MAWP.
Composite tanks are even more susceptible to impingement fire since the materials are not as robust as steel. In particular, the resin used for the composite material may weaken at temperatures as low as 100 C°. The resin and fiber materials also may be flammable and result in further weakening as the fire continues. Composite vessels are usually protected with TPRDs but have the limitation mentioned above. Generally, Type 1 steel vessels are likely to withstand a hydrogen jet flame longer than a composite vessel because of the difference in materials and the lower mass and thickness.
Impingement may best be addressed by ensuring that the hardware design of the project largely eliminates likely sources of a non-isolatable leak/jet fire. A good system design would not have any hardware beyond the cylinder shut-off valve itself that could create this scenario by utilizing good pipe routing/geometry. A solution could be a fire barrier either directly applied to the vessel (such as a coating system) or as a stand-off shield or wall to block the fire impingement. In addition, for some projects fire suppression systems such as sprinklers may be appropriate. They won’t put out the jet fire, but they can help keep other surfaces cool. A thorough hazard analysis should be part of any project design.
