C. M. Consulting
P.O. Box 407
Odell, Oregon 97044
C. M. CONSULTING
A Division of Cliff Mansfield Incorporated
Anatomy of an Accident
It had been a long day, in a month of long days. The Astec portable drum
mix plant had produced nearly 5,000 tons of asphalt since daylight. The operator
had shut the plant down and loaded out what he hoped was the last truck of the
day ten minutes earlier. He sat at his desk, head down on his forearms, nearly
The radio crackled to life. “21 to plant, we’ve got enough mix, go ahead
and clean out. Same schedule for tomorrow.”
The plant operator keyed the mike. “Ok, thanks.” He keyed his walkie-talkie and
said to his ground man, “Switch the dieseling valves on the asphalt pump and
then diesel out the silo.” As he spoke, the operator switched off the asphalt
The ground man went to the asphalt tank and saw that the pump was stopped. He
closed the suction line from the pump to the tank and opened another valve which
led to a 55 gallon drum full of diesel. This done the ground man climbed the
ladder to the silo top and grabbed a dieseling hose installed to allow him to
clean the asphalt out of the corners of the silo. He heard the drag conveyor and
drum start up as he directed the diesel to the corner of the silo.
Inside the control room the plant operator reached down and turned on the
‘inject’ valve to send the diesel to the drum, then started his asphalt pump.
Immediately he saw a small puff of white smoke at the drum discharge. It
dissipated quickly, but was soon followed by a massive explosion. The blast
traveled out the drum discharge, up the drag conveyor and down into the silo
where the ground man was spraying the silo with his dieseling hose. He was
engulfed in a huge fireball, igniting the diesel in the silo and covering him in
flames. He surged back to escape the flames and all but fell down the ladder to
the ground. He screamed in agony as the flames seared his skin.
A water truck happened to be making his rounds when he saw the explosion.
Thinking fast he raced to the plant with his truck and directed a stream of
water onto the victim writhing on the ground. The flames subsided and as the
water cooled his skin the ground man's cries diminished. It was a difficult 15
minutes for the ground man while he waited for the ambulance to arrive. At the
hospital the extent of his injuries was realized. He had been severely burned
over a large portion of his body. The burns ended the man’s career, he would be
on disability the rest of his life.
The damage to the asphalt plant was also severe. The baghouse was
completely destroyed. Down each side of this unit a 14” x 3/8” thick beam ran
the full length. The explosion blasted these cross-welded beams 3’ out of
straight. The lids on top of the baghouse landed nearly a mile away. The flames
burned away most of the wiring to the asphalt tank, drum and silo. As a result
of this accident this asphalt plant was out of action for many months.
WHAT WENT WRONG
I was called in by an attorney for the plant owner’s insurance company,
as part of an accident investigation team. My job was to try and shed some light
on what went wrong. Was there a design flaw in the asphalt plant which caused
the explosion or did the plant’s personnel’s actions cause the accident?
The first thing that jumped out at me when I read the accident report was
the fact that the plant’s personnel were using diesel in their clean-out
routine. Diesel is a very volatile fuel with a flash point around 165 degrees.
Why would you inject it into a 300 degree environment such as the mixing chamber
of a portable asphalt plant?
To answer this question we need to understand how the asphalt injection
system works on the contemporary asphalt plant. Liquid asphalt is held in a
storage tank where it is maintained at an elevated temperature, somewhere around
300 degrees. If the liquid falls below 270 degrees, depending on the asphalt, it
becomes too viscous to pump. When this happens, nothing short of heat will lower
the viscosity enough to pump.
When we want to make hotmix we heat our rocks to the desired temperature,
then we use a pump to inject the liquid asphalt into the mixing chamber. This
process requires a pump and a series of pipes in which the temperature of the
asphalt must be maintained above the 275 degree threshold we talked about
earlier. To do this, factories like Astec ‘jacket’ all the oil lines and the
pump, the force a heated liquid, heat transfer oil, through the system. At the
end of the mixing session, most plant operators simply reverse the direction of
the pump rotation which sucks the asphalt out of the lines and the pump.
With this in mind, the question becomes- Why were these men injecting diesel
rather than simply sucking out the oil by reversing the pump? To answer this
question I began by examining the asphalt injection system. The first thing I
noticed was where the plant personnel had added a 55 gallon drum onto the pump
pallet and plumbed it into the suction side of the pump. This was the device the
men used to add diesel to the pump and introduce it into the injection system.
The next thing that I saw was that the Astec factory asphalt metering system had
been replaced with a ‘Micro Motion’ metering system. I also noticed that the new
metering unit and the pipes added during its installation had not been jacketed.
I talked with the plant operator and was told that the installation of the
‘Micro Motion’ system was hurried affair, the men concentrating on getting the
plant running again as quickly as possible. Once the installation was complete,
the plant’s personnel discovered that the new metering system had enough
internal restriction that the pump could no longer be simply reversed to clean
it out. This Astec plant was the first drum mix plant owned by this particular
company. All of their other plants had been batch plants. It was a common
practice to use diesel to clean out batch plants, so it is not hard to see why
the plant personnel would revert to diesel when normal clean out protocols
The causal factors of this accident were now evident. Some time in the
past the asphalt plant had been modified from its factory configuration. During
these modifications a simple oversight led to the necessity to use diesel to
clean the asphalt out of the system. This set the stage for the coming tragedy.
The first and foremost lesson that this incident teaches us is that there
is no circumstance that diesel should be pumped through a heated asphalt
injection system and into the heated, enclosed environment of a drum mix asphalt
plant. Its not hard to imagine the diesel heating up as it travels through the
sections of pipe that were jacketed. As it passed its flash point of 165 degrees
the only thing it needed was room to expand and oxygen to turn it into a
vaporous cloud of explosive. It got that needed room as it was injected into the
heated atmosphere of the mixing chamber. The only thing missing was a source of
ignition. Perhaps it was a rock caught between a mixing paddle and the floor, or
maybe a piece of metal rubbing against another piece of metal. Whatever the
source, the diesel ignited catastrophically, and peoples’ lives were
Are your men using diesel at your asphalt plant?
For additional information on this subject
or help with any problems encountered
contact Cliff Mansfield,
7:30am to 9:00pm Pacific Standard Time.
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