Ever since the Hindenburg blew up hydrogen has had a bad rap. But hydrogen did not cause that disaster- the coating on the canvas shell was highly flammable, kin to rocket fuel, and a static charge set it alight (most of the passengers survived, and those that died jumped prematurely, or suffered diesel fuel burns).
Hydrogen has its little ways, and safety precautions are of course vital, just as they are for gasoline or propane. It is no more intrinsically dangerous. The advantage of dealing with hydrogen is that unfamiliarity encourages appropriate caution, as opposed to the casual attitude with which we sometimes handle, say, gasoline. The disadvantage is that we are scared away from hydrogen when we shouldn't be.
Hydrogen is highly flammable (2H2 + O2 → 2H2O). It can ignite in combination with as little as 23% air (≈5% oxygen). By comparison, propane needs a mix of almost 90% air to ignite. This means that even small amounts of air in lines or storage tanks are potentially dangerous. In addition, the friction caused by high pressure gas passing through a narrow valve could theoretically create enough heat to ignite hydrogen. Therefore, it is vital that no air be in lines or storage tanks used for hydrogen. This comes down to the simple matter of purging with nitrogen (see Purging).
Hydrogen also burns with an invisible flame, which creates the risk of discovering a fire by walking into it. Again, awareness and simple precautions (such as use of a straw broom to find suspected flames) can address the risk.
Hydrogen is highly reactive, and combines readily with metals, leading to corrosion and fatigue (hydrogen embrittlement). Thus all pipes and fittings have to be of stainless steel. This raises the price of installations, but not the difficulty.
Hydrogen gas is a small, high energy molecule, prone to escape. Leaks can be tenacious, more so than with other gases. Plumbing can thus be a more fussy business.
On the plus side, hydrogen is so light that given an escape route it will dissipate very quickly, greatly reducing the danger of accumulated gases exploding. Compare propane, which is heavier than air and can thus collect in low spots.
The relevant codes are not hard to follow (see Hydrogen Codes).
One of the various safety systems: If the hydrogen sensor detects H2 it shuts off the fuel cell and electrolyzers via relay (see Balance of Plant).
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