(101e) Overpressure Scenario Analysis in Chemical Plants

Process safety measures have been a great concern in most chemical and petrochemical industries that make use of pressurized systems. Numerous historical relates show that accidents involving such systems can be catastrophic, resulting in deaths and damage of valuable properties. The frequency and magnitude of such accidents have made official regulations and internal company standards become more stringent. Accurate design and appropriate operation and maintenance of pressurized systems and their protective devices are mandatory.

In spite of the rigorous specifications, pressurized vessels have the potential for rupture and leakage, and explosions still occur throughout the world. The most common cause of such accidents is the failure of a pressure relief valve.

Pressure relief devices are used to protect pressurized systems from exceeding the design pressure. The pressure may be imposed by an external or internal source. The protective devices include safety valves, relief valves and safety relief valves.

A safety valve is a spring-loaded pressure relief valve, which is designed to open to relieve excess pressure and to reclose to prevent further flow after normal conditions have been restored.

In this contribution, the results of analysis carried out on more than a thousand safety devices and pressurized equipment in plants of major chemical and petrochemical sites in the USA are summarized. The analysis consisted of a systematic evaluation of existing pressure relief valves, vents and rupture disks, including overpressure scenario identification and evaluation as well as sizing calculations.

The procedures used for analyzing the safety devices, individual pieces of equipment and system of equipment are presented. Global analyses including the overall system, for instance total and partial power failure, cooling water supply failure, air instrument failure, etc, are also conducted. The approach for sizing a header/flare system is also presented.

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