Pressure relief considerations

Protection from Fire Exposure and Pressure Relief Considerations - Pressure relief valves cannot protect a vessel that becomes locally overheated on an unwetted surface, although they do prevent the pressure from rising beyond the accumulation pressure of the valve. However, in such a case the vessel may be effectively protected against failure by either one of two methods for mitigating the effect of fire ... 

Consideration should be given to the effects of thermal expansion of liquids and pressure-relief valves installed unless ... 

While the above permitted variations are not used for selecting the pressure setting of pressure relief devices, these variations may be used for piping considerations such as startup, shutdown and emergency conditions, provided that the pressure vessel limits are not exceeded and that such variations are permitted by local codes. 

The required relieving rate for a pressure relief valve is determined from consideration of the contingencies which can cause overpressure. Basic... 

In the case of a balanced bellows pressure relief valve, to the maximum pressure permitted by considerations of bellows and bellows bonnet flange mechanical strength. This maximum pressure may be obtained by applying the following correction factor to the maximum back pressure listed for 38 °C. 

Where, of necessity, the vent pipe rises to a considerable height, excessive internal pressure on the tank may result, due to the pressure head of oil should an overflow occur. To prevent any possible tank failure due to such an occurrence a vent pipe pressure-relief device must be provided. It should be self-draining to reduce the risk of blockage, particularly when using heavy oil fuel. Codes require that these devices should not place any restriction on oil flow and must discharge within the bund area. 

The compressor intake air must be clean and free from solid and gaseous impurities abrasive dust and corrosive gases are particularly harmful. Exhaust fumes present a hazard if compressed air is required for breathing purposes. The possibility of contamination of the intake by discharge from pressure-relief devices of other plant must be taken into consideration and changes of wind direction must not be overlooked. 

New systems or processes may also need to be qualified from an operational safety perspective. This is particularly relevant in the case of chemical synthesis involving exothermic reactions. Critical safety aspects are usually identified using hazard operability or HAZOP assessments and studies. For example, a HAZOP analysis of an exothermic reaction vessel would involve consideration of the consequence of failure of the motors for mixers or circulation pumps for cooling water. Thus, the qualification of such a system would involve checks and assessment to ensure that the system/process can be operated safely and that pressure relief valves or other emergency measures are adequate and functional. 

Gravity Separator Safety Considerations and Features A pressure relief device may be required on vapor-liquid gravity separators, based on the following assumptions ... 

Each vault housing a pressure-limiting, pressure-relief, or pressure-regulating station shall be examined to determine its condition each time the equipment is examined and tested in accordance with para. GR-5.23.3. For any vault which personnel enter, the atmosphere shall be tested for combustible gas. If the atmosphere is hazardous, the cause shall be determined. The vault shall be examined for adequate ventilation. The condition of the vault covers shall be carefully examined for hazards. Unsatisfactory conditions disclosed shall be corrected. No welding maybe done if a combustible gas mixture is present. Maintenance work performed in the vault shall be in accordance with procedures developed per para. GR-5.23.3, with particular consideration given to the monitoring of the atmosphere and safety protection for personnel in the vault. 

A-2.11.1 Storage Vessel Failure. The release of GH2 or LH2 may result in ignition and combustion, causing fires and explosions. Damage may extend over considerably wider areas than the storage locations because of hydrogen cloud movement. Vessel failure may be started by material failure, excessive pressure caused by heat leak, or failure of the pressure-relief system. 

Examples of common safe practices are pressure relief valves, vent systems, flare stacks, snuffing steam and fire water, escape hatches in explosive areas, dikes around tanks storing hazardous materials, turbine drives as spares for electrical motors in case of power failure, and others. Safety considerations are paramount in the layout of the plant, particularly isolation of especially hazardous operations and accessibility for corrective action when necessary. 

Besides the safety aspect another consideration is that about 30% of process industry losses can be at least partially attributed to deficient pressure relief systems. These losses alone could subsidize a closer look at the pressure relief systems in a process plant or a better upfront selection of the safety systems. 

An accelerating rate calorimeter (ARC) can be used to provide design values for emergency pressure-relief flow requirements of runaway systems. The ARC is a device used to obtain runaway history of chemical reactions in a closed system (DeHaven, 1983 Huff, 1982,1984a Townsend and Tou, 1980). The experimental technique is fairly straightforward, but considerable engineering expertise is required to do the calculations needed to design a relief system from the ARC data. 

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