CHAPTER 11
HYDROGEN GAS
Hydrogen gas is colorless and odorless, but it is highly flammable with a
high rate of flame propagation.
Hydrogen gas is produced by catalytic process or by electrolytic
decomposition of water separating it into hydrogen and oxygen. Oxygen will not
burn, but is a strong support of combustion. It is essential to keep the hydrogen
and oxygen or air separated since a small amount of these two gases will produce
an explosive mixture. The explosion range of hydrogen gas is 4% to 75% and
its ignition temperature 9320F.
Hydrogen gas burns instantly if ignited upon contact with air. When not
mixed with air, hydrogen gas is not explosive.
Hydrogen gas is used in many ways: 1) as a cooling medium for large
generators because it is an excellent heat conductor, and 2) in chemical process
plants. In electrical machines it is important to maintain the hydrogen gas above
atmospheric pressure in order to avoid its mixing with air. Hydrogen should be
admitted to rotating equipment only after air has been removed completely by
inert gas.
Although hydrogen gas leaks are potentially dangerous, the odorless gas may
escape entirely unnoticed because of its diffusion rate.
For cooling generators it is recommended that hydrogen gas cylinders be
located in well ventilated locations, separated from the generator. Where
hydrogen gas cooled generators are protected by a reliable source of carbon
dioxide gas, the area around the generator may be considered non hazardous.
Adequate safeguards are required to ensure prompt operation of the carbon dioxide
because the discharge of large amounts of carbon dioxide may create hazards to
personnel.
Large quantities of hydrogen gas are generally stored in a number of small
metal cylinders. Hydrogen gas in quantities above 400 cf are normally stored in
outdoor locations or separate detached buildings. (CF is cubic feet at 14.7 psia
(101 kpa) and 7O0F.)
In these outdoor and indoor locations a minimum safe clearance of 15 or 25
feet must be maintained between any general purpose electrical equipment and
storage containers. Fifteen feet is considered sufficient if the ventilating air is not
obstructed and 25 feet when the ventilating air is obstructed. If the location is
sufficiently ventilated the area within this boundary must be suitable for a Div. 2
location. However an electrical equipment located within 3 feet of a hydrogen gas
container or system must be explosion-proof and labeled Class I, Group "B".
Group "C' equipment may be used for gases of equivalent hazard such as ethylene
oxide, propylene oxide and acrolein provided that the conduits to the equipment
are sealed by conduit seals. Hydrogen gas containers should not be considered as
the actual source of hazard. Only devices such as pressure reducers, pressure
relief valves, manifolds, and valves connected to the piping system with low
integrity seal connectors are normally the actual sources of hazard (refer to Fig.
J-IO in Section II).
In small process areas the quantity of hydrogen gas is normally less than
400 cf. The hazardous boundaries required for these areas are usually much
smaller than 15 or 25 feet. Only when the process area is located indoors, and the
ventilating system consists of an electrically operated exhaust fan and ventilating
air moves from air inlet to the electrical equipment first and then to the source of
hazard, may a smaller boundary be used. If the ventilating air flows in this
sequential order, the hazardous boundary below the source of hazard may be
reduced to 3 or 5 feet. Above the source of hazard, the boundary must be much
larger than 3 or 5 feet.