Hydrocarbon fires

Typically hydrocarbon fire events can be categorized as follows  

Most fires involving gas in the oil and gas industry will be associated with a high pressure and labeled as jet fires. A jet fire is a pressurized stream of combustible gas or atomized liquid (such as a high pressure release from a gas pipe or wellhead blowout event) that is burning. If such a release is ignited soon after it occurs, (i.e., within 2 -3 minutes), the result is an intense jet 

If a combustible gas release is not ignited immediately, a vapor plume will form. This will drift and be dispersed by the ambient winds or natural ventilation. If the gas is ignited at this point, but does not explode, it will result in a flash fire, in which the entire gas cloud bums very rapidly. It is unlikely to cause any fatalities, but will damage steel structures. If the gas release has not be isolated during this time, the flash fire will bum back to a jet fire at the source of the release. A flash fire is represented by its limiting envelope, since no damage is caused beyond it. This envelope is usually taken as the LEL of the gas cloud. 

Mathematical estimates are available that can calculate the flame and heat effects (i.e., size, rate and duration) for pool, jet and flash hydrocarbon fires. These estimates are based on the assumed parameter of the material release rate. To some extent, the ambient wind speed also has a varying influence. 

All hydrocarbon fire mechanisms and estimates will be affected by to some extent of flame stability features such as varying fuel composition as lighter constituents are consumed, available ambient oxygen supplies, ventilation patterns, and wind effects. Studies into these effects have generally not progressed to the level where precise estimations can be made without scale model tests or on site measurements. 

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Double and triple bonded hydrocarbons Fire and explosion... 

Depending on the fuel involved, a specific amount of heat (i.e., calories or Btu) is released. Ordinary combustibles produce a moderate level of heat release but hydrocarbon molecules have a very high level of heat release. In ideal combustion of 0.45 kgs (1 lb.) of methane, approximately 25,157 kilo-joules (23,850 Btu) are released. The temperature of the combustion products is normally taken to be 1200 °C (2192 °F), which is a typical hydrocarbon fire temperature. 

A heat flux rate is commonly specified during consequent modeling of hydrocarbon fires. Heat flux is considered the more appropriate measure by which to examine the radiation effects from a fire. A radiant heat flux of 4.7 kw/m (1,469 Btu/ft. ) will cause pain on exposed skin, a flux density of 12.6 kw/rrfl (3,938 Btu/ft.2) or more may cause secondary fires and a flux density of 37.8 kw/m (11,813 Btu/ft. ) will cause major damage to a process plant and storage tanks. 

Smokes from hydrocarbon fires consist of liquid or solid particles of usually less than one micron in size, suspended in the combustion gases, which are primarily nitrogen, carbon monoxide and carbon dioxide, existing at elevated temperatures. At normal temperatures carbon is characterized by a low reactivity. At high combustion temperatures, carbon reacts directly with oxygen to form carbon monoxide (CO) and carbon dioxide (CO2). 

The strength of process vessel to maintain integrity during exposure to a hydrocarbon fire. 

To overcome the possibility of a vessel rupture from a hydrocarbon fire exposure several methods are available. Depressuring, insulation, water cooling or draining are usually employed in some fashion to prevent of the possibility of a vessel rupture from it s own operating pressures. A generalized method to qualitatively determine the effect of a hydrocarbon fire on the strength of vessels constructed of steel is available. With this method one can estimate the time for a vessel to rupture and therefore the need to provide protective measures. 

API conducted open pool hydrocarbon fire exposure tests (mostly naphtha and gasoline fires), on process vessels during the 1940 s and 50 s. 

Underwriters Laboratories (UL) high rise (hydrocarbon) fire test UL 1709, has an average fire temperature of 1093 °C (2,000 °F) after 5 minutes. Therefore unless the an actual fire exposure heat radiation input calculation has been made, either a worst case fire exposure temperature could be assumed or a standard temperature to the limits of UL 1709 could be applied. 

The shortest time known for a vessel to rupture from recorded incidents is thought to be 10 minutes. Rupture periods calculated for less than ten minutes should therefore not be assumed, as the historical evidence and the typical growth of a hydrocarbon fire would indicate that the immediate rupture of a vessel does not occur. Further investigations may be carried out verity if fire exposure conditions could produce such results (e.g., flange leak, gas fire exposures, etc.). 

A vessel provided with a firewater deluge system to protect against hydrocarbon fire exposures for the duration the worst case plausible incident. 

Lightning is generally considered a form of static electricity that is being discharged from particles in the atmosphere. Many instances of lightning induced hydrocarbon fires have been recorded, especially at atmospheric storage tanks. 

It has been demonstrated that steel strength decreases rapidly with temperature increases above 260 °C (500 °F). At 538 °C (1000 °F), its strength both in tension and compression is approximately half, at 649 °C (1200 °F) its strength decreases to less than one quarter. Bare steel exposed to hydrocarbon fires may absorb heat at rates from 10,000 to 30,000 Btu/hr/sq. ft., depending on the configuration of the exposure. Due to the high heat conduction properties of steel, it is readily possible for normally loaded steel members or vessels to lose their strength to the point of failure within ten minutes or less of a hydrocarbon fire exposure. 

Hydrocarbon vapors immediately bum with flame temperatures that are considerably higher than that of ordinary combustibles. For this reason damage from a hydrocarbon fire is much more severe than an ordinary combustible fire. The objective of a fire detection for the petroleum industry is to rapidly detect a fire where personnel, high value, and critical equipment may be involved. Once detected executive action is initiated to alert personnel for evacuation and while simultaneously controlling and suppressing the fire incident. 

It responds well to a wide range of hydrocarbon fires and is blind to welding arcs except when very close to the detector. It can see through smoke and other contaminates that could blind a UV detector. It generally ignores lightning, electrical arcs and other forms of radiation. It is blind to solar radiation and resistant to most forms of artificial lighting. 

Turbine Package Electrical Fire Hydrocarbon Fire Heat Optical NFPA 30, Section 5-5.5.1. 

Laboratories Hydrocarbon Fire MPS Heat NFPA 45, Section 4-1.1 4.5... 

Before the need of fire protection measures is defined, the type of hydrocarbon fire exposure should be identified. By determining the type of fire expected, the adequacy of the fire protection measures based on the philosophy of protection for the facility, can be assessed. The easiest method to arrive at the protection requirements is to identify the materials and pressures involved in the process. Once this is accomplished, the most appropriate fire control or suppression mechanism can be identified from NFPA 325M. Tables 3 and 4 provides examples of a tabular format that can be used to document the fire control mechanisms that have been chosen. 

H 0 Hydrocarbon Fire, 120 minute barrier against flame and heat passage, no temperature insulation. H 60 Hydrocarbon Fire, 120 minute barrier against flame and heat passage, 60 min. temp, insulation. 

Heat rate input is normally taken as 205 kW/m (65,000 Btu/ft2) - hr 5 minutes for hydrocarbon fires. Fire Doors... 

The scope of this book is to provide a practical knowledge and guidance in the understanding of prevention and mitigation principals and methodologies from the effects of hydrocarbon fires and explosions. The Chemical Process Industry (CPI), presents several different concerns that this book does not intend to address. However the basic protection features of the Hydrocarbon Process Industry (HPI) are also applicable to the chemical process industry and other related process industries. 

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