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Alternative Fuels for Transportation
The potential for small continuous leaks is always present, and it is necessary to store vehicles in well-ventilated garages. LPG being a dense gas, the
use of low-level vents will allow the heavy vapors to disperse. The potential for explosions of leaked LPG to occur is known, and at least one has
occurred in Belgium and resulted in both the loss of the vehicle and of the
house above the basement garage where the vehicle was stored overnight.
The ignition source was probably the central locking system being remotely
operated, fortunately for the driver, who escaped being injured by being far
enough away from the vehicle at the time of ignition. Gross spillage of LPG
is similar to a spillage of LNG, in that rapid boil-off occurs on contact with
the ground, until the ground has cooled to the atmospheric boiling point of
the LPG. Leakage of LPG from a fractured pipe would form a large persistent flammable atmosphere, which is likely to be ignited. This would burn
back to the fracture, and if the flame were to impinge on the tank, a boiling
liquid expanding vapor explosion (BLEVE) would be likely to occur eventually. Even small cylinders are affected unless over-temperature protection is fitted. Normally the propane cylinders engulfed in fire generally did
not rupture quickly when fitted with relief valves, but continued to relieve
until the top of the cylinder became overheated and the steel weakened. The
cylinder would then rupture. However, on cylinders fitted with brass outlet
valves, the valve itself became weakened by the high temperature and blew
out, thus relieving the pressure and avoiding a rupture. Hence it would seem
prudent to fit LPG tanks with both an over-pressure relief valve and an overtemperature relief device, such as a fusible plug.
Other potential explosion hazards are hydraulic locking by overfilling,
and rupturing the cylinder due to over-pressure. This can be avoided by
careful design of the filling arrangement or a suitable relief valve, which will
of course have to vent the LPG to a safe place. The potential problems of concentration of nonvolatiles encountered with LNG is less of a problem with
LPG, as the fuel is invariably taken off in the liquid form, relying on the
storage tank pressure to maintain the fuel as a liquid until it is injected into
the engine.
For LPG refueling facilities, there are a number of hazards that are well
known, with the most significant hazards being
• Boiling liquid expanding vapor explosion (BLEVE). This results from
the loss of pressure following containment (i.e., tank) failure. In this
case the escaping liquid expands very rapidly (boils), and, if ignited
at or near the course of release, burns at a great rate. Heat radiation
is the main hazard associated with a BLEVE.
• Unconfined vapor cloud explosion (UVCE). This occurs when a
cloud of LPG, which has leaked from the tank without ignition taking place, is ignited at a later time at a source perhaps some considerable distance from the release point. The exact mechanics and
Alternative Fuels for Transportation
The potential for small continuous leaks is always present, and it is necessary to store vehicles in well-ventilated garages. LPG being a dense gas, the
use of low-level vents will allow the heavy vapors to disperse. The potential for explosions of leaked LPG to occur is known, and at least one has
occurred in Belgium and resulted in both the loss of the vehicle and of the
house above the basement garage where the vehicle was stored overnight.
The ignition source was probably the central locking system being remotely
operated, fortunately for the driver, who escaped being injured by being far
enough away from the vehicle at the time of ignition. Gross spillage of LPG
is similar to a spillage of LNG, in that rapid boil-off occurs on contact with
the ground, until the ground has cooled to the atmospheric boiling point of
the LPG. Leakage of LPG from a fractured pipe would form a large persistent flammable atmosphere, which is likely to be ignited. This would burn
back to the fracture, and if the flame were to impinge on the tank, a boiling
liquid expanding vapor explosion (BLEVE) would be likely to occur eventually. Even small cylinders are affected unless over-temperature protection is fitted. Normally the propane cylinders engulfed in fire generally did
not rupture quickly when fitted with relief valves, but continued to relieve
until the top of the cylinder became overheated and the steel weakened. The
cylinder would then rupture. However, on cylinders fitted with brass outlet
valves, the valve itself became weakened by the high temperature and blew
out, thus relieving the pressure and avoiding a rupture. Hence it would seem
prudent to fit LPG tanks with both an over-pressure relief valve and an overtemperature relief device, such as a fusible plug.
Other potential explosion hazards are hydraulic locking by overfilling,
and rupturing the cylinder due to over-pressure. This can be avoided by
careful design of the filling arrangement or a suitable relief valve, which will
of course have to vent the LPG to a safe place. The potential problems of concentration of nonvolatiles encountered with LNG is less of a problem with
LPG, as the fuel is invariably taken off in the liquid form, relying on the
storage tank pressure to maintain the fuel as a liquid until it is injected into
the engine.
For LPG refueling facilities, there are a number of hazards that are well
known, with the most significant hazards being
• Boiling liquid expanding vapor explosion (BLEVE). This results from
the loss of pressure following containment (i.e., tank) failure. In this
case the escaping liquid expands very rapidly (boils), and, if ignited
at or near the course of release, burns at a great rate. Heat radiation
is the main hazard associated with a BLEVE.
• Unconfined vapor cloud explosion (UVCE). This occurs when a
cloud of LPG, which has leaked from the tank without ignition taking place, is ignited at a later time at a source perhaps some considerable distance from the release point. The exact mechanics and