The explosion happened at the electricity generation plant at the refinery. The power plant works on gas and was undergoing maintenance.
The BPCL refinery is crucial to the supply of oil products such as petrol, diesel and LPG to Kerala as a whole.
Two workers have been injured, including one who has seen serious injuries.
UPDATE: Sources suggest that two people lost their lives in the incident.
The incident happened at 10 AM.
The fire has been brought under control, but the explosion could force operations some units to remain suspended.