Three persons were injured in a boiler blast at a private steel melting firm in Thirunallar on Monday. The blast is said to have occurred at around 8 p.m. A First Information Report has been filed against five persons, of whom, a supervisor, a manager and a labour contractor have surrendered, while the owner and manager are at large. The injured were male migrant labourers in the age group of 20 to 30 years. Most of the workers are from Orissa. The firm has been ordered closed till further investigations.