Pressure relief devices bursting discs

The mass flow rate from a rounded nozzle branch is greater than through a conventional pipe branch but it is the latter which is generally used. Further possibilities are leaks from drain and sample points, pressure relief devices, bursting discs, seals and flanges, and pipe ends. 

A rupture disc device is a non-reclosing pressure relief device actuated by inlet static pressure and designed to function by the bursting of a pressure containing disc. 

Within this Workbook, the maximum pressure required to fully open the pressure relief device will be referred to as the "relief pressure". (Caution some papers on relief sizing refer to "set pressure" but mean "relief pressure"). For a bursting disc, the relief pressure will be the maximum specified bursting pressure and for a safety valve, it will be the set pressure plus 10% overpressure (or whatever percentage overpressure the valve has been certified at). 

A diaphragm designed to burst at a predetermined pressure differential. A simple rupture disc device that is composed of a non-reclosing pressure relief device that relieves excessive static inlet pressure via a rupture disc. 

A bursting or rupture disk is a pressure relief device that protects a vessel or system from excess pressures. They have been commonly used in aerospace, aviation, defense, nuclear, and oilfield applications often as a backup device for a conventional safety valve. In this instance, if the pressure increases and the fitted safety valve fails to operate, the rupture disk will burst as required. The discs are usually made from thin metal foil, and gold has been used in some instances because of its ductility and resistance to corrosion. Gold discs fitted to liquid ammonia tanks, for example, have shown good durability in this application compared to other metals. The use of gold in this application was reviewed in the 1970s when this industrial application for gold was more common [10]. 

Pressure relief of a runaway reaction is likely to be via a bursting disc or a safety valve, or a combination of both these items. Further information about these is given in Chapter 9. For relief system sizing, it is important to know the pressure at which a relief device will open. 

If solids are allowed to accumulate on the underside of a bursting disc or safety valve, then it is likely that the relief device will not operate when required, at least not at. the required set pressure. Safety valves may be more vulnerable in this respect than bursting discs, and it is common practice to fit a bursting disc upstream of the safety valve to protect it. Further information is given in 10.2.1 and reference 1. 

CEN - European Committee for Standardization Brussels wvwv.cenorm.be EN/ISO 4126- Safety Devices for Protection Against Excessive Pressure Part 1 - Safety Valves Part 2 - Bursting Disc Safety Devices Part 3 - Safety Valves and Bursting Disc Safety Devices in Combination Part 4 - Pilot Operated Safety Valves Part 5 - Controlled Safety Pressure Relief Systems Part 6 - Application, Selection and Installation of Bursting Disc Safety Devices Part 7 - Common Data... 

The term set pressure is used to denote the pressure at which the relief device is open fully. This is not the definition used by some — for example, British Standards. For a safety valve the set pressure is often 10% above the set pressure as specified by the manufacturer or measured in the workshop, since this higher pressure is needed to open the valve fully. For a bursting disc, the set pressure is the nominal burst pressure plus any tolerance (or alternatively it is the maximum specified bursting pressure). The term overpressure is used here to mean the difference between the (redefined) set pressure and the maximum pressure reached during the venting process. 

Corrosive fluids are less likely candidates for the use of pressure relief valves. Whether by corrosion of the valve itself or of surrounding pipework, they often create a problem with deposits on the valve element or its seat. These deposits can prevent proper sealing and allow the slow escape of fluid. To combine the advantages of rupture discs and relief valves, the two devices can be installed in series, with the disc before the valve. Even if the valve is not precisely seated, there is no release from the system until the disc bursts. The same overpressure then also opens the valve. Once the piessme has dissipated and dropped below its set point, the valve reseats and prevents further release. 

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