Pressure relief and safety devices

Almost all compressed gas containers are fitted with pressure relief devices. A pressure relief device is a pressure- and/or temperature-activated device used to prevent the pressure from rising above a predetermined maximum, and thereby prevent rupture of a normally charged cylinder when subjected to a standard fire test as required by Title 49 of the U.S. Code of Federal Regulations (49 CFR 173.34(d)), or equivalent regulations of Transport Canada. [1], [2], and [3] 

This chapter deals with the various types of pressure relief devices used in the compressed gas industry and the standards developed to describe their design criteria, performance characteristics, and applications. The sections covered are  

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All valves, fittings, accessories, pressure relief and safety devices, gauging devices, and the like must be adequately protected against mechanical damage by a protective housing or must be recessed and under a cover plate. 

For more detailed information on pressure relief and safety devices, refer to Chapter 7. 

Pressure relief and safety devices have uniquely contributed to the safe utilization of compressed gases. Chapter 7 will provide insights into the evolution and systematic application of the various types of pressure relief devices now in use. Likewise, the systematic use of cylinder valve connections, connections to ancillary equipment, and bulk transfer connections has evolved into a methodology that is intended to help ensure that the proper gas is actually connected for the intended application. This methodology,... 

Pressure Relief and Safety Devices Table 7-2. Continued)... 

Design and selection philosophy for use of safety devices for pressure relief and alarm. 

Caution The pressure relief rupture disk device is a primary safety component and the following guidelines should be followed ... 

The vital subjects of pressure relief and the use of safety valves and other protective devices would normally be considered in the context of pressure vessel safety. Since this chapter does not discuss vessels directly, some considerations on pressure relief and a list of applicable standards are included in Section 3.4.2 on piping. 

The new Bayview Terminal had a design operating pressure of 740 pounds per square inch (50 atmospheres or 50 bar) and the accident pipeline was designed for much higher pressures, so it was necessary to install pressure-reduction equipment where the pipeline joined the new terminal. There were three layers of control and safety devices (i) a control valve to throttle the incoming flow, (ii) a pilot-operated spring-loaded relief valve, and (iii) three motor-operated isolation valves. This triple-barrier approach is sound design - a control system, backed-up by two protection systems (the relief valve and the isolation valves). In the event of a problem with the control valve, the relief valve should operate quickly, and only in the event of a fault with the relief valve should the isolation valves ever have been required to operate. 

Pressure Relief Valves Definitions for pressure relief valves, relief valves, pilot-operated pressure relief valves and safety v alves, are found in the ASVIE Boiler and Pressure Assel Code, Section Division 1, Rules for Construction of Pressure Assels, Paragraphs UC-125 and UC-126, The pressure-relief valve is an automatic pressure relieving device designed to open when normal conditions are exceeded and to close again when normal conditions are restored. Within this class there are relief valves, pilot operated pressure relief valves, and safety valves. 

Pressure-relief-device requirements are defined in Subsec. A. Set point and maximum pressure during relief are defined according to the service, the cause of overpressure, and the number of relief devices. Safety, safety relief, relief valves, rupture disk, breaking pin, and rules on tolerances for the reheving point are given. 

Safety Devices Pressure relief devices, flame arresters, and methods for handhng effluent from controlled releases provide control of accidental undesirable events. Special equipment should be considered for highly toxic chemical service. The following matters are considered ... 

Pressure Relief Devices The most common method of overpressure protection is through the use of safety rehef valves and/or rupture disks which discharge into a containment vessel, a disposal system, or directly to the atmosphere (Fig. 26-13). Table 26-8 summarizes some of the device characteristics and the advantages. 

The pressure relief device used most often in refinery and chemical plant equipment is the spring-loaded, top-guided, high-lift, nozzle-type safety relief valve, which is illustrated in Figure 2. The spring is usually external and enclosed by a bonnet for weather protection, and the bonnet chamber is vented through an internal passage to the valve outlet. 

The most important safety devices in a production facility are the pressure relief valves, which ensure that pipes, valves, fittings, and pressure vessels can never be subjected to pressures higher than their design pressures. Relief valves must be designed to open rapidly and fully, and be adequately sized to handle the total flow of gas and liquids that could potentially cause an overpressure situation. They relieve the pressure by routing this stream to a safe location where it can be vented to atmosphere or burned. 

As long as pressure, level, and temperature control devices are operating correctly, the safety system is not needed. If the control system malfunctions, then pressure, level, and temperature safety switches sense the problem so the inflow can be shut off. If the control system fails and the safety switches don t work, then relief valves are needed to protect against overpressure. Relief valves are essential because safety switches do fail or can be bypassed for operational reasons. Also, even when safety switches operate correctly, shutdown valves take time to operate, and there may be pressure stored in upstream vessels that can overpressure downstream equipment while the system is shutting down. Relief valves are an essential element in the facility safety system. 

The set pressure tolerances of pressure relief valves are not to exceed +2 psi for pressures up to and including 70 psig and +3% for pressures above 70 psig. Indirect operation of safety valves, for example, by pilot valve, is not acceptable unless the primary unloading valve will automatically open at not over the set pressure and will operate fully in accordance with design relieving capacity conditions if some essential part of the pilot or auxiliary device should fail [1]. 

The capacity of the combination of the rupture disk device and the spring loaded safety or safety relief v alve may be established in accordance with the appropriate paragraphs of UG-132, Certification of Capacity of Safety Relief Valves in Combination with Non-reclosing Pressure Relief Devices. 

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