Sunoco's use of deadly hydrofluoric acid at its Philadelphia refinery has come under scrutiny after a March accident that sent 13 contract workers to the hospital.

The Occupational Safety and Health Administration last month cited Sunoco Inc. for four "serious" violations related to the March 11 incident. Sunoco said the 13 workers had received "precautionary" medical treatment after a 22-pound release of hydrogen fluoride, the gaseous form of hydrofluoric acid.

The U.S. Chemical Safety Board last month announced it was investigating two other hydrogen fluoride releases during the summer at refineries in Illinois and Texas.

"Because of its high toxicity, any loss of primary containment for hydrogen fluoride is a serious matter," said John Bresland, chairman of the board, an independent federal advisory panel.

The United Steelworkers union, which represents many refinery workers, called Aug. 31 for a nationwide phaseout of the gas, known by its chemical formula, HF. Hydrogen fluoride causes skin and respiratory irritation at low exposures. In large doses, it is fatal.

"HF is a bad actor," said Jim Savage, president of Steelworkers Local 10-1, which represents workers at Sunoco's Philadelphia refinery. "This is the worst thing we deal with in the industry."

The Steelworkers say that safer alternatives are available for hydrogen fluoride, which is used in about a third of U.S. refineries, including the ConocoPhillips refinery in Trainer and Valero's Paulsboro refinery. HF serves as a catalyst in alkylation units, which produce a high-octane gasoline-blending agent.

The only commercially viable alternative for HF is sulfuric acid, which is less hazardous, but is required in far larger quantities to achieve the same result. The materials are not interchangeable - an alkylation unit is designed to use only one catalyst.

"A mandate for a refinery to switch from hydrofluoric acid to sulfuric acid would result in capital and design costs between $45 million and $150 million per refinery, and an increase in operating costs of between 200 and 400 percent," said Bill Holbrook, a spokesman for the National Petrochemical & Refiners Association.

Sunoco has experience with both catalysts at its Philadelphia refinery, a 1,400-acre complex that was formed in 1994 by the merger of the adjacent Girard Point and Point Breeze refineries. Each has an alkylation unit.

Gulf Oil Corp. built the HF alkylation unit about 30 years ago at the Girard Point refinery. The Point Breeze side has a sulfuric acid unit.

Michael G. McKee, the refinery manager, said the sulfuric alkylation unit generated much more truck traffic for the refinery - two tanker loads of acid each day, compared with one delivery a week of hydrofluoric acid. Hydrofluoric acid also produces an alkylate with slightly higher octane, he said.

Sunoco says it recognizes the potential hazards of hydrogen fluoride, but is committed to the material. It has undertaken a $125 million project to upgrade safety features on the HF alkylation unit.

Sunoco is renovating the unit to use a less volatile form of hydrogen fluoride to reduce the risk of an HF cloud during a catastrophic failure. The improvements also include new equipment to drain the alkylation unit in seven minutes if it springs a leak, averting disaster.

Thomas P. Golembeski, Sunoco's spokesman, said the improvements were not mandated by regulation, but were aimed at addressing public-safety concerns.

"It's basically a discretionary investment in community relations," he said. The project is scheduled to be completed early next year.

The contract workers installing the safety equipment were the ones evacuated on March 11 when alarms signaled a release of hydrogen fluoride.

OSHA attributed the leak to maintenance deficiencies when it cited Sunoco last month for four violations and proposed a $20,000 fine. Sunoco is contesting OSHA's findings, which also criticized the refiner for neglecting to warn the contract workers about the possibility of leaks.

Al D'Imperio, area director of OSHA's Philadelphia Office, said the Sunoco HF unit has a "history of leaks," which he partly attributed to a design change implemented in 1991, when engineers downgraded the tubing that carries the acid through a heat exchanger. The design change involved replacing expensive Monel nickel-alloy tubes with cheaper carbon-steel tubes.

OSHA said the tubes have a service life ranging from one month to 15 months between failures.

McKee, the refinery manager, said the more expensive tubes had no advantages over the carbon-steel tubes. He said that acid leaks were a recurring problem in the industry and that Sunoco anticipated them by routinely changing the tube bundles.

"We take it very seriously," he said. "We're not supposed to leak."

On March 11, hydrofluoric acid leaked into the heat exchanger, mixed with condensed steam, and then escaped as a gas into the atmosphere through a hole in the steam system.

Golembeski said 22.3 pounds of hydrogen fluoride escaped before the unit was shut down (by comparison, more than 4,000 pounds leaked during a July fire at Citgo's East Plant refinery in Corpus Christi, Texas).

Alarms alerted the Philadelphia workers to evacuate, and none of them were contaminated with HF, Golembeski said. All but one were examined and released. One worker was diagnosed with a preexisting heart condition and remained in the hospital for three days.

Golembeski said the $125 million project would add more safeguards to the system. "We are eager to complete this project and believe it shows our commitment to being a responsible neighbor," he said.

But the improvements don't impress Savage, the head of the local Steelworkers union, who is a Sunoco employee.

"I don't want a less deadly form of HF," he said. "It really just needs to go away."

Contact staff writer Andrew Maykuth at 215-854-2947 or amaykuth@phillynews.com.