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No Cause Set in ‘Whistling’ Tank Blast

Friday, March 18, 2011

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A Detroit-area tank lining contractor will not be cited in the explosion of a 27,000-gallon tank during lining removal because authorities are unable to determine what caused the blast.

Duratech Systems

Duratech Systems LLC

Duratech Systems provides shop and field tank
lining, equipment painting and other services.

The Michigan Occupational Safety and Health Administration has concluded its investigation into the Jan. 11 explosion at Duratech Systems LLC without finding a cause of the accident.

Two workers were removing a rubber liner from the storage tank—34 feet tall and 12 feet in diameter—when they heard a whistling noise from the unit and fled the scene. The tank exploded moments later.

Thus, MI OSHA issued no citations but recommended that Duratech “develop and implement engineering controls, training and written procedures for employees to prevent over-pressurization of a tank being worked on.”

Accident Account

MI OSHA documents gave this account of the accident:

Tank owner PVS Technologies (formerly known as  Pressure Vessel Service Inc. and PVS Chemicals Inc.) of Detroit had sent the carbon steel storage tank to Duratech in December for stripping and relining of the internal rubber coating. PVS used the tank to manufacture Ferric Chloride, commonly used in wastewater treatment, by chlorinating Ferrous Chloride.

PVS told Duratech that the tank had developed a small leak Nov. 10 in the discharge flange at the bottom, so the tank was drained and taken out of service. The tank needed some residual insolubles removed before it could be spark tested. A company called Inland Water removed the solids and pressure-washed the tank, using water only, on Nov. 22 and Nov. 30.

Duratech then spark tested, found “numerous cracks in the lining,” and PVS opted to have the tank stripped and relined, the documents say.

Power Washing

When the tank was delivered to Duratech, it “had been steam cleaned and contained no explosive gases,” “had its flanges sealed off” and had been “empty and dry for several weeks,” the report says.

When it was time to remove the lining, the tank was loaded onto rollers and moved into “the outside blast room” of a steel outbuilding at Duratech, where it was stored on its side.

Exterior Heating

On Monday, Jan. 10, the report said, employees “checked the tank for an oxygen-enriched or explosive atmosphere and then blanked off all the flanges to prevent a fire….”

As they had done on several previous occasions, the workers then used propane torches to heat the outside of the tank “to help release the bonding agent used to secure the rubber coating to the inside of the tank to aid in the removal process.”

The coating—Enduraflex VE-824-WN—is made by Blair Rubber Company, of Seville, OH. Blair describes the coating on its site as a Food and Drug Administration-compliant “white, semi-hard natural lining for water and chemical treatment service.”

Blair Rubber Company

Blair Rubber Company

The tank lining, Enduraflex, was manufactured
by Blair Rubber Company.

The heating procedure involved moving the torches “in an S motion until the paint bubbles or flakes and then move on,” documents said. Two workers used two propane torches on each side. The workers had followed the same procedure for a total of about 29 hours over four previous days.

On Jan. 10, the work continued for three or four hours (the documents list both times), then the workers ran out of propane.

‘Squealing’ and Steam

On Jan. 11, the employees resumed heating the tank with the torches about 7:45 a.m. About 9:10 a.m., they saw “smoke or steam pouring out of one of the top flanges and heard a whistling noise.” (One of the workers described it as a “squealing” noise.)

Feeling that “something wasn’t right,” the workers turned off the torches and fled. The tank exploded about 15 seconds later.

The blast destroyed the vessel, seriously damaged the building and a nearby truck, and shook foundations several hundred yards away. The workers were unhurt.

Pressure Issues

Investigators later found that the nine-inch-diameter flange that had been smoking “was blocked off with a rounded blank and bolted down.”

The investigation found that “all flanges were covered, which wouldn’t allow any pressure to be released.” One worker told investigators that “flanges were blocked to prevent oxygen from getting into [the] tank to prevent the rubber from catching fire while they heat[ed]the tank.”

Either PVC or Water Inland—the identity is blacked out in the documents—told investigators that “the tank is not a pressure vessel and did not have a pressure relief device installed when it left there [sic] site.”

‘No One Knows’

A fire official said at the time that he had been told by people on the scene that the site may have been insufficiently ventilated, but that was not addressed in the MI OSHA report.

“There was nothing that [name redacted] knows of that would cause an explosion other than the heating [of] a tank that is sealed causing pressure to develop,” investigators wrote.

Blair Rubber officials told investigators that heating the lining “would cause no explosive gases to be released and that the bonding agent after drying would also release no explosive gases.”

The investigator wrote: “No one knows the cause of combustion.”

   

Tagged categories: Accidents; Explosions; Health and safety; Linings; OSHA; Tank interiors

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