The oil refinery explosion and fire that rocked Superior, Wis., in April could have been prevented if the plant's equipment had been properly maintained, a federal investigation has found.

The Occupational Safety and Health Administration levied an $83,150 fine for 13 violations including outdated safety procedures, failures to address certain hazards, and failed inspections and testing, among other problems.

"Ensuring the mechanical integrity of critical equipment used during the refinery shutdown operation could have prevented this incident," OSHA spokesman Mark Hysell said in a statement.

The explosion occurred while the refinery was shut down for maintenance.

The findings, delivered to Superior Refining Co. on Tuesday by OSHA's Eau Claire, Wis., office, come two months after the U.S. Chemical Safety and Hazardous Investigation Board said the massive explosion could have been caused by a worn valve that allowed air to leak and mix with combustible hydrocarbons.

One safety violation cited by OSHA dealt directly with the valve in question. The company didn't correct equipment deficiencies in a timely manner, including addressing the failure of what's called the "spent catalyst valve," according to OSHA.

A spokeswoman for Calgary-based Husky Energy Inc., which owns Superior Refining, said the company plans to meet with OSHA to discuss the findings.

"It is our policy to continuously improve our process safety programs. Superior Refining remains committed to this facility, our employees and the community," spokeswoman Kim Guttormson said in an e-mail.

OSHA also found that the refinery had failed to identify "critical hazards" in its own manufacturing process, hadn't developed a written procedure with "clear instructions" for an emergency shutdown, and hadn't adequately trained employees on certain aspects of shutdown procedures.

Superior's mayor, fire chief, and City Council president could not be reached or did not immediately respond Wednesday to requests for comment.

The Twin Ports Action Alliance, a citizen's group created in the days after the refinery explosion that has called for closer scrutiny of the plant, denounced the size of the OSHA fine as insufficient.

"The refining industry is self-regulating with no regard to the fenceline community or [its] workers," TPAA founders Ginger Juel and Kevin Swanberg said in a joint statement.

The April 26 blast sent shrapnel into a tank of asphalt, which eventually caught fire and burned for hours, sending a plume of black smoke and ash far across northwestern Wisconsin. City officials feared that a tank of highly toxic hydrogen fluoride at the plant was also vulnerable, and ordered a near citywide evacuation.

An Environmental Protection Agency worst-case scenario for the plant states that a massive leak of the hydrogen fluoride tank would put 180,000 people at risk, or most of the Twin Ports population. In the days following the explosion and fire, the mayors of Superior and Duluth asked the company to use alternative chemicals and remove the hydrogen fluoride from the site.

Eleven workers were hurt in the blast, and 25 other people sought medical attention. The workers filed a lawsuit in August, alleging they had to run for their lives to escape. A class-action lawsuit filed that same month by some residents of Superior seeks payment for the costs they faced due to fleeing the city.

The OSHA penalties are the largest fines the safety agency has levied on the Superior refinery in a decade. In 2008, the refinery's then owner, Murphy Oil, agreed to pay $179,000 in fines for numerous safety violations, including for safety alarms that had been deactivated or improperly monitored.

Murphy, which had owned the Superior refinery for decades, sold it in 2011 to Calumet Specialty Products, which in turn sold it last year to Husky. In 2015, Calumet paid $16,800 in fines to OSHA for violations involving flammable liquids, hazardous waste operations and emergency response.

Husky Energy, Inc., says it doesn't expect the Superior refinery to resume normal operations until 2020.