This work deals with the safety of high-pressure vessels by blowdown analysis in oil and gas industry using Aspen HYSYS simulation tool. During the design of high pressure vessel API 521 standards are implemented. The required parameters governing the depressurisation technique and the selection of thermodynamics property package, heat transfer model selection are explained. All physically significant effects of the process which governs the safety of the high-pressure vessel are predicted. These findings are highly useful in the safe design of high-pressure vessels in oil and gas industry. The predicted results are also useful in the selection of the material of construction and reduce the cost and reduce the risk of failure of the equipment. This work guarantees the life of people working in the industry and reduces the process equipment cost and environmental problems. This work is useful to process design engineers and safety engineers in mitigating the chemical disasters in oil and gas industries. Copyright © 2018 Inderscience Enterprises Ltd.