Four people were injured late Thursday when storage tanks caught fire at MarkWest Energy Partners LP’s Houston Processing and Fractionation facility in southwest Pennsylvania, according to the company.

The plant, which was processing 720 MMcf/d of natural gas near full utilization in the third quarter, was not involved, a MarkWest spokesman said. Four people suffered burns at the site in Chartiers Township, about 30 miles south of Pittsburgh, and were flown by helicopter to hospitals in the city.

“Although the processing plant was not involved in the incident, it was shut down as a precaution and at this time, there are no off-site impacts,” said company spokesman Jamal Kheiry. “Agency notifications have been made and an investigation into the cause of this event is underway. Our prayers are with our injured colleagues and their families.”

Kheiry said the fire broke out at 6 p.m. ET and involved two temporary tanks that had been trucked in for routine maintenance at the plant. Aerial footage from the scene showed several other unaffected tanks that were lined up near those involved in the incident. Ryan Frazee, the emergency planner at the Washington County Department of Public Safety, told NGI’s Shale Daily that the frack tanks were holding condensate.

He added that the fire burned itself out in about an hour and said there was no evacuation of the surrounding area. Eight fire departments responded, Frazee said, mainly to ensure that there would be enough water at the rural site, if needed. Three ambulance crews also responded, he said.

MarkWest, a subsidiary of MPLX LP, is the Appalachian Basin’s largest natural gas processor. The Houston facility is one of several it operates in Pennsylvania and nearby Ohio and West Virginia. The company processed about 5.4 Bcf/d and fractionated 435,000 b/d of Marcellus and Utica shale volumes this year, according to its latest forecast.