31 May 2001
In the line of fire
by Bob Felton
Coker unit refinery blast points to safety issues.
Coker units, like the one at the Tosco Corp. petroleum refinery that sent 100,000 of its neighbors scurrying indoors on 23 April to escape a cloud of toxic smoke boiling 3,000 feet into the sky, have been involved in several fires in recent years. A coker unit typically is the last stop in processing a batch of crude oil and allows refineries to improve their profitability by squeezing more salable product out of each barrel.
Although there were no injuries in the April fire, less than a month before the Chemical Safety Board (CSB) had criticized Tosco management for failures that led to four deaths in a February 1999 refinery fire near San Francisco.
The April fire began at 4:50 p.m. (PDT) and burned for approximately five hours. A Tosco spokesman estimated the blaze reached a temperature of 2,000°F at its most intense. Five area fire departments helped the refinery's fire-fighting crew. Most of the 230-acre facility was unaffected by the fire and continued production without interruption.
Around the plant, however, the effects ranged from a massive snarl on the San Diego Freeway to cancellation of Little League baseball games, as a film of black soot covered childrens' uniforms. About 100,000 residents of Orange County and the cities of Carson and Long Beach were told to go indoors and stay put, keeping doors and windows closed.
The plant processes about 130,000 barrels of petroleum per day, and the price of May gasoline futures spiked sharply on news of the fire, closing at $1.09 per gallon, slightly below last year's peak. Additionally, the Tosco refinery is among just a few refineries producing gasoline that complies with the Environmental Protection Agency's clean-air requirements for the California marketplace.
Though a plant spokesperson told reporters that the Carson plant had a good safety record, the California Occupational Safety and Health Administration fined the plant $15,935 in December 2000 for safety violations it described as "serious."
Inside a coker
A black, heavy, tarry paste with some amount of light, trapped components is all that remains, once most of the petroleum products have been removed from the crude. The refinery then heats the paste to about 900°F at pressures on the order of 550 pounds per square inch and pumps it into the bottom of the coker tank. The coker tanks typically have diameters of 15 to 30 feet and are 55 to 90 feet high.
The light materials escape from the paste as it cools in the relatively low-pressure and low-temperature environment, and they are then collected. The remaining material is carbon, which is gathered and sold as fuel. The coking tanks process successive batches until filled with carbon. It takes on a hard consistency that requires a drill-riglike apparatus to bore through the middle of the coke, which is then "sawed" into smaller pieces using water jets.
Anton Riecher, editor of Industrial Fire World, addressed the subject of coker fires in a May 1999 article: "It is almost certain," he wrote, "that fires and explosions involving coker units will continue into the new millennium with disturbing frequency." The reason is that the crude stock now sent to coking units has a greater sulfur and heavier metal content than in years past. At high temperatures, these materials promote corrosion of the tanks.
Additionally, the coking process is inherently destructive of tanks. Though catastrophic structural failures of coking units are practically unknown, the frequent heating and cooling cycles tend to promote cracking and bulging at discontinuities. When a leak occurs and the superheated, pressurized product escapes from the tanks, it expands rapidly and is easily ignited.
Plant engineers will have to implement a broad range of strategies to eliminate coker fires. Jerry Craft, a onetime refinery fire chief and now a consultant, emphasized the importance of corrosion control in a 1999 talk. "Metallurgy has to be upgraded," he said, "in order to compensate for the [crude] bottoms being processed today. Standard carbon steel is not an acceptable metal for surfaces where you are feeding hot, heavy products that are heated at a very high temperature. The severity factor gets to be so critical that you've got a high potential for corrosion acceleration."
Further, Craft recommended that plant engineers identify the steel used throughout the refining process and take steps to ensure that the correct steel is used everywhere. Similarly, corrosion inspection must be frequent and comprehensive.
Prior fire killed four
The 1999 fire occurred at a Tosco refinery in Martinez, Calif., near San Francisco. Four people were killed and one critically injured in that fire, which began when workers inadvertently released naphtha onto a hot fractionator while attempting to replace piping attached to the 150-foot-tall fractionator tower while the process unit was in operation. The subsequent fireball engulfed five workers then working on the tower.
The CSB released its report of its investigation in Martinez on 28 March and was critical of Tosco management. The board recommended, among other things, that Tosco undertake a companywide, systematic assessment of nonroutine maintenance procedures at its refineries, including the degree of management oversight. IT