Pressure Relief Device Process Specification

For most processes, the optimum operating point is determined by a constraint. The constraint might be a product specification (a product stream can contain no more than 2 percent ethane) violation of this constraint causes off-specification product. The constraint might be an equipment hmit (vessel pressure rating is 300 psig) violation of this constraint causes the equipment protection mechanism (pressure relief device) to activate. As the penalties are serious, violation of such constraints must be very infrequent. 

Where hazardous conditions can develop within a process, a protective system of some type must be provided. Sometimes this is in the form of process hardware such as pressure relief devices. However, sometimes logic must be provided for the specific purpose of taking the process to a state where the hazardous condition cannot exist. The term safety interlock system is normally used to designate such logic. 

Depending on the contents of a mixture, one or more pressure relief devices may be required on the container. Most pressure relief devices are an integral part of the valve. However, some devices are installed directly into the bottom or top of the container. For an in-depth discussion on pressure relief devices for mixtures, see CGA S-7, Method for Selecting Pressure Relief Devices for Compressed Gas Mixtures in Cylinders, [10] Also consult Chapter 7 of this Handbook for a presentation on pressure relief devices. The selection of the proper pressure relief device for a mixture involves a knowledge of the contents of the mixture, the final pressure, its physical state, and the type of container which will be used. To simplify the process of selecting the proper pressure relief device for a specific mixture, refer to the mixture safety selection algorithm in Fig. 2. Consult CGA S-7 for complete information on the use of this algorithm. [10]... 

This accident occurred on March 23, 2005 at the British Petroleum (BP) Texas City Refinery, the third largest oil refinery in fhe United States, when a hydrocarbon vapor cloud exploded at the isomerization process unit [8]. More specifically, on March 23, 2005 at the refinery, an isomerization unit s start-up whose raffinate tower was overfilled resulted in the raffinate overheating and pressure relief devices opening, and then consequently led to a flammable liquid geyser from a blow down stack unequipped with flare and then an explosion and fire [1,9]. The accident killed 15 workers and injured over 170 others [1,8,9]. 

A regulator is a compact device that maintains the process variable at a specific value in spite of disturbances in load flow. It combines the functions of the measurement sensor, controher, and final control element into one self-contained device. Regulators are available to control pressure, differential pressure, temperature, flow, hquid level, and other basic process variables. They are used to control the differential across a filter press, heat exchanger, or orifice plate. Regulators are used for monitoring pressure variables for redundancy, flow check, and liquid surge relief. 

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