Canadian Centre for Occupational Health and Safety

Canada's National Occupational Heath & Safety Resource

AN EXPLOSION IN AN ASPHALT PATCHING TRUCK

Produced by:
Professional and Specialized Services,
Ministry of Labour
Alert #I17/0196
ISSN 1195-5228

HAZARD SUMMARY

1. BACKGROUND:

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 hands.

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.

APPLICABLE REGULATIONS:

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 4.1.2.1 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
explosions) require:

adequate ventilation (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 hazardous locations).

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 procedures.

PRECAUTIONS:

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 that:
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 to them.