This paper addresses a fictitious accident scenario involving an ammonia storage tank in a refinery. Ammonia is used in a deNOx system. In a selective catalytic reduction environment, a refinery stack gas containing nitric oxide and nitrogen dioxide is decomposed into nitrogen and water vapor. In this study, a cylindrical ammonia tank is "bleved" (boiling-liquid, expanding-vapor explosion (BLEVE)) with an external fire. We developed a computer model in order to determine the thermal response of a horizontal ammonia tank involved in fire engulfment accidents. The assessment of the separation distance between tanks and protection measures such as water spray cooling were also discussed. Further, we examined the relief device design and studied the open-system thermal response as well as pressure response effects in the event of a BLEVE for the ammonia tank.