Mamidi Srinivas (23), Krishna Chand (40) and A Appa Rao (48), who were undergoing treatment at separate hospitals here, died today, refinery's director B K Namdev said.
Yesterday, two more private workers had succumbed to burn injuries at a hospital.
At present 23 patients are undergoing treatment at various hospitals in the city while eight injured persons have been shifted to burns hospital in Mumbai, where they are responding well, sources said.
Prima facie a gas leak in the sea cooling water system was attributed to the massive blaze even though probe is on into the incident.
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Out of the 12 persons killed in the mishap so far, only Ch Murali, in-charge of the cooling tower, was an employee of HPCL, while the other victims were private workers engaged on contract by the company.
HPCL officials had earlier said that altogether 102 workers, including eight employees of HPCL, were at the site at the time of the accident. Out of the total workers present at the site, 56 workers, including seven HPCL staffers, escaped unhurt when the blaze started.