Nine Petroleos Mexicanos employees reported minor injuries after a storage tank at a refinery in Ciudad Madero caught fire late on Tuesday night.

Seven workers have been treated for dehydration and two for minor burns, according to a Pemex press official who declined to be named because of company policy.

Emergency services crews continue to work to extinguish the fire at the state-owned oil producer’s refinery 500 kilometers (311 miles) north of Mexico City in the Tamaulipas state. No serious injuries have been reported.

The refinery has production capacity of 190,000 bpd.

The blaze is at least the third accident at the site this year. A fire at the unit’s coker plant limited operations for five days in May, less than a month after a boiler exploded because of an excessive pressure build-up in April. No injuries were reported in either incident.

The Ciudad Madero refinery, the smallest of Pemex’s six national refineries, processed 129,763 bpd in 2013, and 128,293 bpd in May.