Static is a frequent source of ignition attributed to various hydrogen releases. Low levels of static
electricity are sufficient to ignite hydrogen – air mixtures. Static charges can be created by the
atmospheric disturbances and storms, high velocity particles entrained by the gas impacting stationary
objects, and human activity. Grounding of equipment and operators is important to lower the probability
of static discharges.

Because hydrogen leaks frequently ignite, and because about half the time the ignition source is not identified, when evaluating hazards with hydrogen leaks, many people just assume the leak will be ignited. Note that consideration needs to be made for what may happen with immediate ignition (jet fire) and what may happen with delayed ignition (explosion). 

It is still important to minimize the probability of ignition and to minimize the consequences if it does ignite. We do that by properly characterizing hydrogen with all its unique properties, including flammability and low MIE, and by providing the appropriate safeguards prescribed by codes and standards specific to hydrogen. These safeguards include minimum quantities, using proper materials of construction, leak prevention practices, proper ventilation, proper disposal to safe areas, and ignition source control such as the use of non-sparking, electrical grounding, and classified electrical equipment.

Yes, numerous incidents have occurred where frozen air (which contains oxygen) has built up within a hydrogen process or vent system. These incidents with vent systems incorporate more than just a vent stack, but include a vent system consisting of additional atmospheric equipment (such as a tank) where the equipment stays cold and allows air into the system in contact with a cold hydrogen stream. 

Vent systems are at risk since they are “open” to the atmosphere and certain flow conditions might result in the aspiration of air into a cold hydrogen flow which then leads to the freezing of the air. 

A small quantity of solid air can create an explosive hazard which then leads to cascading failures from the initial incident. Solid air in hydrogen is also shock sensitive which can lead to unexpected ignition.

This may be able to be accomplished as a method for ignition. We have not seen it.

As flaring is not usually recommended, especially when timing is an issue, a sparker that takes time to ignite would not be recommended.

It is not possible to define ignition potential by just velocity without more data (i.e. pressure, materials involved, direction of impact). Due to the multiple methods of developing an ignition source (friction, impact, electrical charge) and the low ignition energy, it is assumed that hydrogen in the air will ignite (between 4 -74%), as it does 30-40% of the time with no known ignition source (see GH2 chart below). Therefore, to try and manage impingement by velocity as an ignition source is not a practical method to assure no ignition.

Other Information:
The ignition energy of hydrogen is .02 millijoules. By definition, a joule is equal to the kinetic
energy of a kilogram mass moving at the speed of one meter per second.

From “Mechanical Sparks as an Ignition Source of Gas and Dust Explosions” from The Italian Association of Chemical Engineering Online:

Mechanical sparks are small particles, which due to the impact between two objects are torn loose from the surface of one of the two colliding objects. The kinetic energy is turned into heat and deformation work. Mechanical spark generation is dependent on the pressure with which the one object is working against the other, the relative speed between the objects, the friction coefficient and the hardness of the materials involved.

Extrapolation of the experimental results using a model it could be shown that incendive hot surfaces can be generated also at relative speeds of < 1 m/s.

Additionally, tests were performed using a file traveling at 1m/s against a metal surface, and the ignition of hydrogen over many concentrations was observed.

Several organizations published a paper together on this topic in 2017 (see attached). Based on comparisons with tests and CFD simulations, the following conclusions were drawn:

1. The gas concentration for vapor cloud explosion blast load calculations for H2 jets can be limited to approximately 10% to 75%. Note that testing for H2-air VCEs in congested environments has been performed by organizations such as Baker Risk and concluded that 10% is the lowest H2 concentration that needs to be considered. This published this as well.
2. For ignition of the H2-air jet at 30%H2, a mass release rate of about 0.5 kg/s is needed to get above a TNO Muli-Energy Severity Level of 4 (i.e., where VCE blast load perspective starts getting significant with a maximum overpressure of 0.1 bar). This corresponds to a flame speed of about 100 m/s and is shown in Figure 13 of the attached paper.
3. Ignition of the H2-air jet at 60%H2 (worst-case ignition location) requires a mass release rate of about 0.1 kg/s (100 g/s) to get above a TNO Muli-Energy Severity Level of 4. More testing on this has been done and is being done, so these might get refined in the future, but it is not expected that there will be major changes in the “threshold” mass release rate
   needed to produce a jet that (if ignited) can represent a VCE hazard. Of course, the blast loads from a hydrogen jet won’t extend a long distance because the explosion energy (i.e., flammable cloud size) is limited compared to traditional VCE cases (e.g. where a large flammable cloud fills all of a refinery process unit). Lastly, if a facility owner defined a hazard level of concern (e.g., greater than 0.5 psig at 100 feet), then a mass release rate of concern could be calculated.

It depends on the construction and location of the lights. Assuming the typical practice of using non-classified lighting on vehicles, operation of those lights during a delivery could provide an ignition source if located within the classified area near the delivery or venting activity. A more modern LED lighting systems may present a lower risk.

Yes, although not as common as high-pressure gas releases, high-velocity cold H2 gas has ignited during rupture disc and relief valve activation.