Vessels External fires

A common cause of a BLE T] in plants of the hydrocarbon-chemical industry is exposure to fire. With an external fire below the liquid level in a vessel, the heat of vaporization provides a heat sink, as with a teakettle evolved vapors exit tnrough the relief valve. But if the flame impinges on the vessel above the liquid level, the metal will weaken and may cause the vessel to rupture suddenly, even with the relief valve open. The explosive energy for a BLE T] comes from superheat. This energy is at a maximum at the superheat hmit temperature. (SLT is the maximum temperature to which a hquid can be heated before homogeneous nucleation occurs with explosive vaporization of the hquid and accompanying overpressure.) The SLT... 

The following is a procedure for caleulating the required relieving rates for a vessel that is exposed to an external fire. 

Fire. The relief valve must be sized to handle the gases evolving from liquids il the equipment is exposed to an external fire. A procedure for calculating this is presented in API RP 520. This condition may be critical for large, low-pressure vessels and tanks but does not normally govern for other pressure vessels. 

BLEVEs are more commonly associated with releases of flammable liquids from vessels as a consequence of external fires. Such BLEVEs produce, in addition to blast and fragmentation ejects, buoyant fireballs whose radiant energy can bum exposed skin and ignite nearby combustible materials. A vessel may rupture for a... 

Vessel Expansion. In most cases, vessels rupture without significant expansion. In most cases in which a vessel is exposed to external fire, the vessel wall temperature distribution is very uneven. Then, typically, only a small bulge is produced before... 

Figure 7-7A. Pressure level relationship conditions for pressure relief valve installed on a pressure vessel (vapor phase). Single valves (or more) used for process or supplemental valves for external fire (see labeling on chart). Reprinted by permission, Sizing, Selection and Installation of Pressure Relieving Devices in Refineries, Part 1 Sizing and Selection, API RP-520, 5th Ed., July 1990, American Petroleum Institute.

For this condition, the API-RP-521 Code [13] (Figure 7-7A) shows an allowable 16% maximum accumulation relieving pressure above the set pressure. For external fire conditions on a vessel, the maximum allowable accumulation pressure is 21% above the set pressure [13] for both single or multiple relieving devices (Figure 7-7A). 

Due to gas expansion from external fire, the API code [10] provides for calculation of the pressure relief valve orifice area for a gas containing vessel exposed to external fire on die unwetted surface ... 

Outbreathing conditions are usually established when (a) the tank is being filled and the vapor space is being displaced with liquid, (b) thermal expansion and evaporation of the liquid, and (c) external fire on the vessel creating additional heat input to the contents. 

The amount of heat absorbed by a vessel exposed to an open fire is markedly affected by the size and character of the installation and by the environment. These conditions are evaluated by the following equivalent formulas, in which the effect of size on the heat input is shown by the exponent of Aw, the vessel wetted area, and the effect of other conditions, including vessel external insulation is included in a factor F [33] ... 

To provide some external protection against the damage that an external fire can do to a pressure relief valve or rupture disk, this author recommends that these devices be insulated after installation in such a manner as not to restrict their action but to provide some measure of reliable performance, even if the vessel is not insulated. 

Q = total heat absorption from external fire (input) to the wetted surface of the vessel, Btu/hr Q = liquid flow, cu ft/scc... 

External fires, 450 Unwetted gas only vessels, fire, 454 Viscosity correction, centrifugal pumps, 203-207... 

Boiling liquid expanding vapour explosions occur when there is a sudden release of vapour, containing liquid droplets, due to the failure of a storage vessel exposed to fire. A serious incident involving the failure of a LPG (Liquified Petroleum Gas) storage sphere occurred at Feyzin, France, in 1966, when the tank was heated by an external fire fuelled by a leak from the tank see Lees (1996) and Marshall (1987). 

Boiling-liquid expanding-vapor explosion (BLEVE) A BLEVE occurs if a vessel that contains a liquid at a temperature above its atmospheric pressure boiling point ruptures. The subsequent BLEVE is the explosive vaporization of a large fraction of the vessel contents possibly followed by combustion or explosion of the vaporized cloud if it is combustible. This type of explosion occurs when an external fire heats the contents of a tank of volatile material. As the tank contents heat, the vapor pressure of the liquid within the tank increases and the tank s structural integrity is reduced because of the heating. If the tank ruptures, the hot liquid volatilizes explosively. 

Figure 9-11 Heating of a process vessel as a result of an external fire. Venting is required to prevent vessel rupture. For most fires only a fraction of the external vessel is exposed to fire.

Abnormal heat addition to the reactive material or mixture, such as by an external fire or the injection of steam to a vessel jacket or directly into the material or mixture... 

Pressure vessels (including heat exchangers and air coolers) in a plant handling flammable fluids are subject to potential exposure to external fire. A vessel or group of vessels which could be exposed to a... 

Acrylic monomers, in particular, are inclined to polymerisation in the absence of oxygen which serves as a chain-breaker in their radical polymerisation. Most such monomers are also flammable and may therefore be directed to be stored under a nitrogen blanket. If nitrogen purging is complete, the risk of fire within vessels may be zero, but the risk of explosive polymerisation, tank-rupture and external fire is increased. Some suspect that accidents of this type have occurred already [6],... 

LPG vessels may be directly buried below grade or mounded above grade to reduce exposure to an external fire. Both of these methods require special precautions, careful preparation, and special design features, since they introduce other substantial hazards. These hazards include undetected corrosion, undetected leaks, and the potential for mechanical damage when the vessel is unearthed for inspection. 

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