Canadian Centre for
Occupational Health and Safety
Canada's National Occupational
Heath & Safety Resource
EXPLOSION IN AN ASPHALT PATCHING TRUCK
Professional and Specialized Services,
Ministry of Labour
A method of patching asphalt that has
been used in the USA has now come into use in Canada.
A truck made in California has a heating system that keeps
the asphalt hot (at about 120 degrees C (250 degrees F))
and liquid until it can all be used for patching. Formerly,
hot asphalt was delivered in unheated dump trucks or other
vehicles, so that some asphalt often cooled before it
was used and had to be discarded.
The new vehicle has an insulated hopper
and both an indirect propane heater and an electric exterior
heater. The exterior heater is in the layer of insulation
around the hopper. Doors on
top of the hopper are used for loading the asphalt, and
a conveyor belt on the bottom interior unloads it through
an opening at the rear.
When the hopper is empty, there is still
some residual asphalt that must be cleaned from its interior
walls. The operator does this by spraying diesel fuel
from a wand onto the asphalt.
Diesel fuel is widely used for cleaning residual asphalt
from road maintenance equipment because it is effective
in liquefying and removing asphalt and because, compared
with other solvents,
it has a relatively high flash point and low toxicity.
Cleaning is carried out with the heaters
turned off but usually before the hopper is fully cooled
because warm asphalt can be removed more easily.
2. ACCIDENT DESCRIPTION
Recently there was an explosion in one
of these vehicles while the operator was cleaning it.
It was probably caused by a spark of static electricity
igniting diesel vapours.
The operator was standing at the rear
of the truck, holding the nozzle of the wand inside the
opening for the conveyor belt. It is believed that a static
electrical charge produced by the flow
of fuel through the hose accumulated on the metal spray
wand. Since the hose between the metal wand and the metal
diesel tank was of non-conductive material, the charge
could not be grounded through the hose. (Also, the operator
was wearing rubber boots, which would keep any static
electrical charge on his body and on the wand he was holding.)
The charge resulted in a spark jumping
from the wand to the interior surface of the metal hopper
while atomized diesel fuel was being sprayed. The temperature
inside the hopper is thought
to have been above 40 degrees C (105 degrees F), which
is the flash point of diesel fuel. Because the doors on
top of the hopper were closed, the force of the explosion
was directed mainly through the rear opening. The operator
was blown backwards and suffered burns to his arms and
It should be noted that ignition sources
other than a static electrical spark could also have caused
this explosion: for example, if the operator was smoking
during the spraying of diesel fuel.
LOCATION AND SECTORS:
Transportation/road maintenance departments:
provincial, regional and municipal.
Diesel fuel has a flash point above 38
degrees and below 93 degrees C (above 100 degrees and
below 200 degrees F) and is classified as a combustible
liquid in the National Fire Code of
Canada, 1990. Section 18.104.22.168 of the Code requires that
"when a combustible liquid . . . is being processed,
stored, handled or used at a temperature at or above its
flash point, it shall be
treated as a flammable liquid." Also, when diesel
fuel has been turned into a mist (atomized) it can be
ignited at well BELOW its flash point as a liquid.
Sections 22 and 23 of the Regulations
for Industrial Establishments (on the storage and dispensing
of flammable liquids) and section 63 (on processes involving
the potential for
(to prevent hazardous concentrations of flammable vapours);
static electrical bonding and grounding; and
that no potential sources of ignition are present (for
example, electrical equipment that is not suitable for
Sections 81 and 84 of the regulations
require eye and hand protection when a worker is exposed
to the hazard of injury to these parts of the body (as
in an explosion of diesel fuel or from skin contact with
it). Section 85 of the regulations requires fall protection
equipment when a worker is exposed to the hazard of falling
more than 3 metres (10 feet).
Sections 25(2)(a) and (d) and 27(2)(a)
of the Occupational Health and Safety Act require the
employer and supervisor to advise workers of proper work
Replace the spray wand hose with one
made of conductive material.
Add a mechanism for mechanically grounding the truck and
hopper before cleaning begins.
Post signs saying "NO SMOKING WITHIN 10 FEET"
on all four sides of the hopper.
Prepare a written hopper cleaning procedure, specifying
the worker who cleans the truck should consciously contact
it immediately before spraying begins to make sure there
will not be a charge on his or her body;
the truck and hopper must be grounded;
the interior of the hopper should be allowed to cool to
below 30 degrees C (86 degrees F) before spraying diesel
fuel (install a thermometer, if necessary);
the residual asphalt should be removed, as far as possible,
with a long-handled mechanical tool, to minimize the quantity
of diesel fuel needed for cleaning;
the doors on top of the hopper should be open during diesel
spraying (to increase both natural ventilation and the
explosion venting area);
the operator should spray the diesel fuel through the
top opening, which is larger, and not through the rear
opening; fall protection equipment will be required if
he or she could fall more than 3 metres (10 feet); and
proper personal protective equipment (eye and hand protection)
must be worn during spraying.
Train workers adequately in hopper
cleaning and enforce the procedure outlined above.
NOTE: An alternative to diesel fuel in
cleaning ashpalt trucks is being investigated. It is a
water-soluble product that is sprayed on the inner walls
of the hopper to minimize the amount of asphalt that adheres