4.4.1 Prevention of explosive atmospheres

The first line defence in preventing an explosion is to ensure an explosive atmosphere never exists, either as a result of a leak generating an external explosive atmosphere, air ingress forming an explosive atmosphere inside the equipment, or having a process that operates with gas mixtures in the explosive range. 

Hydrogen, due to its low viscosity, is particularly prone to leakage from piping, vessels, etc and therefore special attention should be paid to ensuring gas tight connections in any equipment containing hydrogen.  The requirements for hydrogen containment and piping are discussed in section 4.2.  For processes that operate at sub-atmospheric pressures, leakage of hydrogen will not be an issue but the possibility of air ingress, resulting in the formation of an internal explosive atmosphere, needs to be considered.

Ventilation can be used to prevent small leaks generating an explosive atmosphere by ensuring the escaping gas cannot accumulate to concentrations above the LEL.  Ventilation is the air movement leading to replacement of a potentially dangerous atmosphere by fresh air. The following principles should be used to ensure that any foreseeable release of a dangerous substance cannot accumulate to a concentration that affects the safety of people and property:

• Wherever possible locate hydrogen storage/handling equipment outside;
• Estimate the maximum foreseeable release rate;
• Provide adequate high and low ventilation;
• Beware of low ceilings, canopies, covers and roofs;
• Ensure the dilution air is drawn from a safe place;
• Ensure vents and purges discharge to a safe place;
• Use computational fluid dynamics (CFD) for complex ventilation requirements.

It is always best to locate hydrogen storage/handling equipment in the open air, however precautions still need to be taken to ensure that a flammable atmosphere cannot accumulate:

• Avoid the use of low, impervious roofs, canopies or bulkheads;
• Avoid locations below eaves or other overhanging structures;
• Use a suitable, non-combustible security fence rather than a wall;
• Ensure adequate high- and low-level ventilation apertures where a wall around the storage system in unavoidable.

The size of any foreseeable leak into an enclosed or partially enclosed area should be used as the basis for any calculations of the ventilation requirements. The ventilation regime should be sufficient to ensure that the hydrogen concentration is normally maintained below 10% of the LEL (0.4% v/v for hydrogen), with only occasional temporary increases to 25% of the LEL. Some basic equations for a calculating degrees of ventilation are described in BS EN 60079-10:2003 [32]. Further details are also given in Appendix 5.

Two main types of ventilation are recognised:

a)Passive or natural ventilation: the flow of air or gases is created by the difference in the pressures or gas densities between the outside and inside of a room or enclosed space.

b)Active or forced (mechanical) ventilation: the flow of air or gas is created by artificial means such as a fan, blower, or other mechanical means that will push or induce an air flow through the system. The artificial ventilation of an area may be either general or local.

Natural ventilation can be provided by permanent openings. The location of the openings shall be designed to provide air movement across the room or enclosed space to prevent the unwanted quantities of hydrogen-air mixtures. Inlet openings for fresh air intakes should be located near the floor in exterior walls (and only in such a way so that they do not reintroduce air previously evacuated from the process area). Outlet openings should be located at the high point of the room in exterior walls or roof. Inlet and outlet openings shall each have a minimum total set area of the room volume (UNIPI experiments). In Safety Standard for Hydrogen and Hydrogen Systems, NASA [33], a minimum total ventilation area of 0.003 m2/m3 of room volume was set for the inlet and outlet openings. Discharge from outlet openings shall be directed or conducted to a safe location. Ventilation openings shall be designed so that they will not become obstructed during normal operation by dust, snow or vegetation in accordance with the expected application. In open air situations, natural ventilation will often be sufficient to ensure dispersal of any explosive gas atmosphere which arises in the area. For outdoor areas, the evaluation of ventilation should normally be based on an assumed minimum wind speed of 0.5 m/s which will be present virtually continuously (EN 60079-10:2003 [32]).

The effect of wind should be borne in mind when deciding vent orientation. Depending on the position of the vents, wind may impede or enhance the ventilation efficiency (Appendix A6).

If it can be verified, natural ventilation should be permitted to provide all required ventilation and makeup air. If mechanical ventilation is required, the ventilation system shall be interlocked to the hydrogen process equipment to prevent process equipment from working in the absence of ventilation, and therefore shut it down upon loss of ventilation. It shall also be equipped with an audible and visual alarm in order to give a warning in case of failure. The ventilation unit shall be constructed and installed in such a way as to preclude the presence of mechanical and electrical sparking.

The forced ventilation of an area may be either general or local and, for both of these, differing degrees of air movement and replacement can be appropriate. Although forced ventilation is mainly applied inside a room or enclosed space, it can also be applied to situations in the open air to compensate for restricted or impeded natural ventilation due to obstacles. As in the case of natural ventilation, the dilution air used to artificially ventilate the area should enter at low level and be taken from a safe place. The ventilation outflow should be located at the highest point and discharge to a safe place outdoors. Furthermore, the mechanical means used to ventilate the enclosure should be suitable and in particular, the electrical motor(s) should not be located in the potentially contaminated exhaust air stream.

Suitable arrangements should be in place to detect when the ventilation system is failing to provide adequate ventilation. This may be based on the measurement of flow or pressure. This should raise an alarm and safely isolate the electricity supply outside the enclosure and the hydrogen supply outside the building with a normally closed (fail safe) valve. The fuel cell system should shut down safely upon loss of adequate ventilation.
The cooling/air supply fan or compressor present in many fuel cell modules may sometimes be suitable to provide effective ventilation. Where this approach is used, the air must be drawn from a safe place and the direction of the forced airflow must be compatible with the expected movement of any hydrogen release as a result of buoyancy, thermal effects etc.

Where differential pressure is used to prevent the ingress of hydrogen into adjoining compartments, the pressurisation air should drawn from/discharged to a safe place. Also, suitable fail safes should be in place to raise alarms/cause shutdown in the case of any detected loss of ventilation or differential pressure.

The dilution airflow and the number and location of flammable atmosphere detectors should be appropriate in complex systems or congested areas. An appropriate modelling technique should be used in these situations to ensure that pockets of flammable mixture will not accumulate and remain undetected.

In situations where other fuels such as methane, LPG etc are present in addition to hydrogen, the different densities and diffusivities need to be taken into account to ensure that the ventilation arrangements provided are adequate.

Ventilation is not recommended as a prevention measure for large leaks, for example from the catastrophic failure of pipe, as ventilation systems are unlikely to be able disperse large leaks quickly enough to prevent an explosive atmosphere accumulating. If ventilation is used as a prevention measure, then the reliability of the system has to be guaranteed and if the ventilation is only activated when a leak occurs then there must also be a reliable method, e.g. gas detectors, of detecting the leak.  Guidance on the selection and location of gas detectors is given in Appendix A7.

There is a higher risk of an explosive atmosphere being present in equipment during commissioning, when items of equipment will initially contain air before assembly, or during maintenance when equipment is opened up for inspection/repair allowing air ingress.  For these operations, inerting can be employed to prevent an explosive atmosphere forming.  Inerting is a technique by which the equipment is purged with an inert gas, such as nitrogen or carbon dioxide, until the oxygen concentration falls below the level required for flame propagation to occur.  This is called the limiting oxygen concentration (LOC). The LOC depends on the inert gas being used, inerts with higher heat capacities being more efficient and giving higher values of LOC for a given flammable gas.  For inerting with nitrogen the LOC for hydrogen is 5% v/v, while for inerting with carbon dioxide it is 6% v/v.  Guidance on the application of the inerting technique can be found in the ISO published document PD CEN/TR 15282:2006 [34]. 

Even if the formation of an explosive atmosphere cannot be prevented, then at a minimum, measures should be implemented to limit the extent of the explosive atmosphere.  Such measures could include ventilation, use of gas tight seals on doors, pipe entry points, etc to prevent gas migration between rooms and compartments, and the use of a soft barrier.  An example of a soft barrier is a curtain, made from polythene sheeting for example, that would allow easy access to the area where the gas source is, but would restrict the flow of gas to the surrounding areas.