Vessel failure, catastrophic

Arulanantham and Lees (1981) have studied pressure vessel failures in process plants such as olefins plants. They define failure as a condition in which a crack, leak or other defect has developed in the equipment to the extent that repair or replacement is required, a definition which includes some of the potentially dangerous as well as all catastrophic failures. In olefins plants fired heaters have failure rates of about 0.4 failures/year, while process pressure vessels have 0.0025 failures/year and heat exchangers 0.0015 failures/year. It is noticed that fired heaters are much unsafer than process pressure vessels, which are a little unsafer than heat exchangers. 

The effect of fire exposure is predictable for pressure vessels, such as, spheres, spheroids or horizontal vessels. If no fire protection is provided or is not adequate or inoperative, the vessel will probably fail catastrophically in a prolonged fire. Vessel failure typically results from excessive metal temperature weakening the tank wall above the liquid level of its contents. This weakening can occur within a few minutes if the initial liquid level is significantly belowthe maximum flame height and the flames impinge on the shell. 

There are two possible mechanisms that can lead to catastrophic vessel failure upon rapid depressurization, which is typically caused by metal failure or relief device actuation (Melhem et al., 1994) ... 

The superheat limit was first proposed in the late 1970s as a possible mechanism for explaining catastrophic vessel failures. Reid (1979), Jones (1985), Martinsen et al. (1986), Davenport (1988), and Dunn (1988) suggest that BLEVEs (Boiling Liquid Expanding Vapor Explosions) are superheat explosions and therefore are easily predicted by assessing the superheat limit for any pressurized liquid material. 

Catastrophic vessel failures have also been reported at temperatures lower than the superheat limit. The experiments conducted by Melhem et al. (1993), Birk et al. (1993), and Ogiso et al. (1972) clearly show that vessel... 

In a beyond-design-basis accident, it is assumed that the air-cooled passive decay-heat-cooling system has failed and that significant structural failures (vessel failure, etc.) have occurred. Decay heat continues to heat the reactor core but decreases with time. To avoid the potential for catastrophic accidents (accidents with significant release of radionuclides), the temperature of the fuel must be kept below that of fuel failure by (1) absorption of decay heat in the reactor and silo structure and (2) transfer of decay heat through the silo walls to the environment. For the modular high-temperature gas-cooled reactor (MHTGR), the maximum size of reactor that can withstand this accident without major fuel failure is -600 MW(t). 

A BLEVE has been defined as an explosion resulting from the failure of a vessel containing a liquid at a temperature significantly above its boiling point at normal atmospheric pressure (CCPS, 1994). This section advances possible explanations for the very complex fluid stmc-ture interactions (ESI) observed in the BLEVE event and supports the hypotheses with detailed reexaminations of recent experimental data (Roberts et al., 1995a-d), and new physical interpretation and metallurgical appraisals of these same trials. The detailed reanalyses of the catastrophic failures of these four 4.5-ton water capacity LPG vessels with various fills subjected to jet fire attack indicates that the severity of the event and the intensity of the fireballs formed is not necessarily a function of the superheat of its contents but may have more to do with the initiating mode of vessel failure and the thermohydraulic state of the contents at final failure. 

The mechanism of vessel failure appears to be a two-step process The formation of an initiating overpressure crack in the high-temperature, vapor-wetted walls of the vessel, followed by the final catastrophic unzipping of the containment and a nearly instantaneous release of its contents. The distribution and hashing of the lading causes a fireball if the contents are flammable. The failure of the vessel and the surface emissive power of the BLEVE fireball do not appear to be directly related to the superheat of the contents at failure and indeed may be most severe for conditions when the vessel fails while undergoing a pressure reduction at low superheat. 

Vessel failure was initiated by a small longitudinal rupture near the top of the tank. This crack, which was 290 mm long, commenced forming about 880 mm to the left of the eenter weld and about 50 mm circumferentially forward of the top. Visual and video observations indicated that when the PRV operated, this gave a vertical jet of flame approximately 10 m high with a lift-off distance of about 2 m. After 254 seconds, the tank failed catastrophically. 

MAWP(maximum allowable working pressure) This is the legal maximum pressure that a process vessel is allowed to experience. Above this pressure, a relief valve should open to protect the vessel from catastrophic failure. 

Table 4.3.2.2.- Sequence SGTR. Material Distribution at Time 5h 35. After Catastrophic Vessel Failure.

Welded structures often have to be tested nondestructively, particularly for critical application where weld failure can he catastrophic, such as in pressure vessels, load-bearing structural members, and power plants. 

It is worrying that a vessel which is safe when it enters service may become unsafe by slow crack growth - either by fatigue or by stress corrosion. If the consequences of catastrophic failure are very serious, then additional safety can be gained by designing the vessel so that it will leak before it breaks (like the partly inflated balloon of Chapter 13). Leaks are easy to detect, and a leaking vessel can be taken out of service and repaired. How do we formulate this leak-before-break condition ... 

Catastrophic failure of containers as cryogen evaporates to cause pressure build-up within the vessel beyond its safe working pressure (e.g. pressures <280 000 kPa or 40 600 psi can develop when liquid nitrogen is heated to ambient temperature in a confined space). 

The original steam generators were simple pressure vessels that were prone to caiasirophic failures and loss of life. Due to better boiler design, tube-fired boilers, and boiler inspections, the incidence of catastrophic failure is now to a rare event (about once every 100,000 vessel-years). In Great Britain in 1866, there were 74 steam boiler explosions causing 77 deaths. This was reduced to 17 explo.sions and 8 deaths in 1900 as a result of inspections performed by the Manchester Steam User Association. In the United States, the American Society of Mechanical Engineers established the ASME Pressure Ves.sel Codes with comparable reductions. 

A credible spill for Catastrophic Loss Potential. For a catastrophic loss potential, the spill size should be based on the contents of vessels or connected vessel train. The existence of shutoff valves between vessels should not be considered. In addition, the catastrophic failure of major storage tanks should be considered. Leaks in pipelines carrying materials of concern from large-capacity, off-site, remote storage facilities must be considered. For this purpose, it must be assumed that the pipeline is completely severed and that the spill will run for 30 minutes. 

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