Fire, exposure

The general guidelines outlined in API RP 521 [2] should be followed for the design of protechon against fire. Both liquid- and gas-filled vessels are to be analyzed for protection against fire. For liquid wet vessels, the required 

Process engineering and design using Visual Basic 

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Dimerization of acrolein is very slow at ambient temperatures but it can become a mnaway reaction at elevated temperature (ca 90°C), a consideration in developing protection against fire exposure of stored acrolein. 

Flammability = 4, ie, very flammable gas, very volatile, and materials that in the form of dusts or mists form explosive mixtures when dispersed in air Health = 2, ie, hazardous to health, but may be entered freely with self-contained breathing apparatus Reactivity = 0, ie, is normally stable when under fire-exposure conditions and is not reactive with water... 

Liquid Hazards. Pure liquid ethylene oxide will deflagrate given sufficient initiating energy either at or below the surface, and a propagating flame may be produced (266,267). This requites certain minimum temperatures and pressures sensitive to the mode of initiation and system geometry. Under fire exposure conditions, an ethylene oxide pipeline may undergo internal decomposition either by direct initiation of the Hquid, or by formation and subsequent decomposition of a vapor pocket (190). 

Materials that on exposure under fire conditions would offer no hazard beyond that of ordinary comhiistihle material 0 Materials that will not burn 0 Materials that are normally stable, even under fire exposure conditions, and which are not reactive with water... 

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

NFPA 80A Recommended Practice for Protection of Buildings from Exterior Fire Exposures. National Fire Protection Association, Quincy, MA. 

Fire Hazards - Flash Point Non flammable solution Flammable Limits in Air (%) Not pertinent Fire Extinguishing Agents Not pertinent Fire Extinguishing Agents Not To Be Used Not pertinent Special Hazards of Combustion Products Not pertinent Behavior in Fire Exposure to heat can cause polymerization to a combustible, viscous material Ignition Temperature Not pertinent Electrical Hazard Not pertinent Burning Rate Not pertinent. 

The basis for design overpressure described in this section is related to the ASME Boiler and Pressure Vessel Codes and ANSI B31.3, Code for Petroleum Refinery Piping. Compliance with these codes is a requirement, or is recognized as the equivalent of a requirement in many locations. Where more stringent codes apply, the local requirements must be met. Therefore, local codes must be checked to determine their requirements. For example, some countries do not permit the use of block valves underneath pressure relief valves, unless dual valves with interlocks are installed. Also, in some cases, 20% accumulation under fire exposure conditions is not permitted, and accumulation allowed may be lower than the ASME Codes. In the United States, the ASME Code is mandatory, since it is a requirement under the Occupational Safety and Health... 

Except for special sihiations, pressure relief devices are not provided for fire exposure of heat exchangers, air fins, or piping, nor are the exposed surfaces of such items included for calculating the fire exposure heat input. Special situations may be congestion and substandard spacing, or unusually large equipment with normal liquid holdup over about 4 m and/or which represents over 15% of the total wetted surface of the system to which it is directly connected for pressure relief. 

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 ... 

In cases where vessels are provided with PR valves for protection against overpressure from fire exposure or an operating failure contingency, additional thermal expansion protection is not required. 

As the vessel pressure continues to rise above set pressure, the spring is further compressed until the disc is at full lift. The valve is designed to pass its rated capacity at the maximum allowable accumulation (10% for contingencies other than fire, 16% if multiple valves are used or 21 % for fire exposure). 

Figure 7. Pressure conditions for safety relief valve installed on a pressure vessel (vapor phase) supplemental valve used for fire exposure only.

All heat absorption from fire exposure is considered as latent heat and no credit is taken for the sensible heat capacity of the fluid in the vessel. The vapor release rate W is calculated from ... 

Maximum Allowable Accumulated Pressure for Multiple-valve Installation (Other Than Fire Exposure)... 

MAXIMUM ALLOWABLE ACCUMULATED PRESSURE [Pg.359]

FOR MULTIPLE-VALVE INSTALLATION (OTHER THAN FIRE EXPOSURE)... 

Each input source is protected by a PSV set no higher than the maximum allowable working pressure of the vessel, and a PSV is installed on the vessel for fire exposure and thermal expansion. 

If failure is due to fire exposure, the vessel s overpressure results from external overheating and can reach a maximal value of 1.21 times the opening pressure of the safety valve. This maximal value is called the accumulated pressure. As overheating reduces the vessel s wall strength, failure occurs at the point at which its strength is reduced to a level at which the accumulated pressure can no longer be resisted. If vessel failure is due to corrosion or impact, it can be assumed that pressure at failure will be the operating pressure. 

An important case of gas discliarge is tlie flow from pressure relief valves and rupture disks. Wlien relief is required due to fire exposure in a nonreacting... 

Maximum allowable accumulated pressure (fire exposure only)... 

Maximum allowable accumulated pressure for Installation of a multiple rupture disk device (other than fire exposure)... 

Maximum allowable burst pressure for supplemental (fire exposure) rupture disk device (see Note 6)... 

Figure 7-38. Curve for determining requirements for emergency venting during fire exposure. Reprinted by permission, The American Petroleum institute, API Std.-2000, 3rd Ed. 1987, Venting Atmospheric and Low Pressure Storage Tanks.

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