External fire

The ASME Pressure Vessel Code [1] and the API codes or recommended procedures [10, 13, 33] recognize and set regulations and procedures for capacity design, manufacture and installation of rupture disks, once the user has established the basis of capacity requirements. 

There have been at least six different formulas proposed and used to determine the proper and adequate size of rupture disk openings for a specific relieving condition. The earlier studies of Sylvander and Katz [25] led to the development of the ASME and API recommendations. This approach assumes that a fire exists under or around the various vessels in a process. This fire may have 

For unexpected runaway or process overpressure not subject to external fire, the rupture disk set pressure, which is the bursting pressure, should be sufficiently higher than the expected under acceptable control conditions for the operation to avoid the frequent burst and shut 

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]  

Q = total heat absorption (input) to the wetted surface, in BTU per hour. 

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At first, batchwise horizontal retorts were used for smelting, and later continuous vertical retorts, both externally fired. Continuous, internally heated furnaces such as the electrothermic furnace followed, and the last important development was the Imperal Smelting blast furnace. 

The hydrocarbon gas feedstock and Hquid sulfur are separately preheated in an externally fired tubular heater. When the gas reaches 480—650°C, it joins the vaporized sulfur. A special venturi nozzle can be used for mixing the two streams (81). The mixed stream flows through a radiantly-heated pipe cod, where some reaction takes place, before entering an adiabatic catalytic reactor. In the adiabatic reactor, the reaction goes to over 90% completion at a temperature of 580—635°C and a pressure of approximately 250—500 kPa (2.5—5.0 atm). Heater tubes are constmcted from high alloy stainless steel and reportedly must be replaced every 2—3 years (79,82—84). Furnaces are generally fired with natural gas or refinery gas, and heat transfer to the tube coil occurs primarily by radiation with no direct contact of the flames on the tubes. Design of the furnace is critical to achieve uniform heat around the tubes to avoid rapid corrosion at "hot spots."... 

Understanding how sudden pressure releases can occur is important. They can happen, for example, from ruptured high-pressure tanks, runaway reactions, flammable vapor clouds, or pressure developed from external fire. The proper design of pressure rehef systems can reduce the possibility of losses from unintended overpressure. 

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... 

External fire exposure resulting in runaway reaction and/or system overpressure. 

Eliminate sources of fuel Blank unused lines at switching station Provide emergency cooling activated by external fire (e.g., fusible link, plastic tubing)... 

Chemical Reaction (this heat can sometimes exceed the heat of an external fire). Consider bottom venting for reactive liquids. ... 

Fire as a Cause of Overpressure - Equipment in a plant area handling flammable fluids is subject to potential exposure to external fire, which may lead to overpressure resulting from vaporization of contained liquids. This hazard may exist even in items of equipment containing nonflammable materials. 

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... 

Two different situations will be examined for maximum fragment range failure during testing, and failure due to an external fire. 

Determine size and specifications for all safety relief valves and/or rupture disks for process safety relief (including run-a-way reactions) and relief in case of external fire. 

It is important to understand how the over-pressure can develop (source) and what might be the eventual results. The mere sohdng of a formula to obtain an orifice area is secondary to an analysis and understanding of the pressure system. Excess pressure can develop from explosion, chemical reaction, reciprocating pumps or compressors, external fire around equipment, and an endless list of related and unrelated situations. In addition to the... 

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.

Pressure relief must be adequate to prevent internal pressures from rising over 10% above the maximum allowable working pressure, except when the excess pressure is developed by external fire or other unforseen heat source. See design details in later paragraph. Papa [85] proposes an improved technique for relief sizing. Also see Figures 7-7A and 7-7B. 

When pressure relief devices are intended primarily for protection against overpressure due to external fire or heat, have no permanent supply connection, and are used for storage at ambient temperature of non-refrigerated liquefied compressed gases, they are excluded from requirements of Par. UG-125c (1) and C (2), with specific provisions. See ASME code [1] for detailed references and conditions. 

In calculating the relief capacity to take care of external fire the following equation is used ... 

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 ... 

Example 7-5 Rupture Disk External Fire Condition... 

An uninsulated 12-ft X 36-ft horizontal storage tank containing CCI4 is to be protected from overpressure due to external fire by means of a rupture disk. The tank does not have a sprinkler system. Storage pressure is 5 psig and should not exceed 10 psig. Tank is assumed to be full, k =... 

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. 

Emergency venting of large tanks is usually associated with external fire conditions around the tanks. Under... 

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