arpadveress 
A very interesting reading about hydrogen and more. Even in terms of Ex it is a nice summary. Thanks to Authors.
“Primary Explosion Protection
Primary explosion protection (avoiding explosive mixtures) means that the formation of undesired gas mixtures is suppressed from the very beginning. One way to achieve this is to prevent the unintentional escape of flammable gases from their vessels or pipes. This can be achieved by designing the devices “technically tight”, reducing make-and-break connections to a minimum, connecting pipes and other parts by soldering or welding if possible etc. Regular inspection of the installations, scheduled replacement in regular intervals of seals and other parts subject to wear and comparable measures fall below this chapter.
Primary explosion protections means also that in case of an unintentional escape of gas this event is detected in due time and the gas is removed as quickly and as safely as possible, for example through self-opening windows or fans, before the mixture concentration exceeds the lower explosion limit. The necessary air intake. Mmust be calculated on the basis of the amount of gas which can escape during incidents or accidents like leaks or pipe ruptures. Diffuse leaks should also be taken into account. For such leaks and also for gas trapped during plant shut down ventilation should be provided which is appropriate to handle the gas volumes which could escape.
Quick (and automatic, if possible) interruption of the gas supply should reduce the amount of gas which can escape to a minimum.
Since hydrogen has no characteristic colour or odour, but the supervision for leaks has to be permanent, sensors are indispensable. They should be arranged in vicinity of the likely escape sites, but due to the properties of hydrogen also at the highest point of the room or at least as high as possible.
At places where hydrogen is handled commercially and an escape cannot be excluded, the facilities are usually protected by stationary gas alarm devices. They frequently give a pre-alarm at about 10 % of the lower explosion limit which is 0.4 % H2 in air, and a general alarm at 25 % of the lower explosion limit, which is 1%H2 in air (these limits are not prescribed and are at the discretion of the operator). So there is enough time for countermeasures or evacuation of the facility.
What exactly are the appropriate measures must always be determined on the basis of the local conditions. Important is how much hydrogen is ever involved, in what form it is stored and used, the size of the room or building, whether it houses other dangerous devices, the properties and conditions of surroundings of the building etc.
Secondary Explosion Protection
Secondary explosion protection is to prevent the ignition of a gas mixture which might form by avoiding the presence of ignition sources which could provide the necessary energy. There are a number of possible ignition sources. Electrical sparks are among the most important, but hot surfaces or mechanically generated sparks can ignite as well. Correspondingly numerous and diverse are the options for secondary explosion protection. It consists essentially in explosion-proof design of the electrical and other installations in rooms in which the existence of an ignitable gas mixture cannot be excluded completely (in spite of primary explosion protection).
The electrical installation is usually in the focus of considerations. Normal switches and sockets are not allowed in an explosion risk area. If possible, electrical components should be avoided at all and replaced by different ones. For example, pneumatic valves may be used instead of solenoid valves.
Explosion protection is of particular importance when pumps or compressors are to be used in a hazardous area. These are not only electrical devices with considerable power ratings of a few 100 kW or MW, but they can have hot surfaces due to friction..
Electrical sparks are not only generated in the electrical installation, but also by discharge of static electricity. These sparks can ignite just as well. Expert grounding of all relevant parts of the installation is an important part of this chapter. This is true not only for the gas containing parts of the facility but also for e.g. radiators, water and gas pipes and other metallic parts. Also conductive floors can be very useful.
Mechanically generated impact or friction sparks may ignite just as well as electrical sparks. Even spark-proof tools are not a perfectly safe means. This is an important issue in the construction of fans which are located in an explosion risk area to remove an ignitable gas mixture if necessary. If in case of a malfunction a contact between rotor blades and casing sparks might be generated if unsuitable pairs of material interact with each other. One possible countermeasure is to cover the tips of the rotor blades with organic material or to make them completely from such material.
Constructive Explosion Protection
Constructive (“tertiary”) explosion protection does not attempt to prevent the explosion itself but is about limiting the damage caused by it.
One way to achieve this is to design all parts of the equipment in which an explosion might occur sufficiently pressure proof. Whether this is feasible depends on the operating pressure under normal conditions. In the event of an explosion it has to be anticipated that the pressure in a closed vessel may rise by a factor of at least 10. If a detonation occurs a factor 20 may be reached or even exceeded.
This method is usually practical only for small vessels or pipes, not for large vessels or even rooms or buildings. In this case, a means must be provided to release the overpressure in such a way that it does not cause damage, at least not more than acceptable.
For this purpose vessels or pipes are usually equipped with pressure release devices (safety valves, burst disks). Opening pressure and flow rate must be selected on the basis of the danger to be coped with. The outlet must be designed so that the escaping fluids do not cause further damage. This is especially true if these fluids are hot, flammable, harmful or otherwise dangerous.
Constructive explosion protection for rooms or buildings usually means that certain elements of the building (windows, outer door, wall) are designed in such a way that they open or fall out under a low overpressure so that the pressure can be released. Here again it is important that the environment is not endangered by a pressure wave, fragments, or escaping fluids.”
Keep up good work!
Arpad
