HOTWORK hired for curing of refractory. 10AM Alternate Energy vaporizer pressurized, leak checked and lit-then Hotwork burners lit. Fittings checked on vapor outlet & inlet lines. 10:15AM Hercules plant worker let know that vaporizor was afire-2.5' flame - Pressure outlet valve shut off & flame went out. Exam of piping shows that elbow was not tight at connection.