Anatomy of an Accident Plant Explosion

THE 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 exhausted.

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 pump.

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 failed.

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.

LESSONS LEARNED

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 irreversibly altered.
Are your men using diesel at your asphalt plant?