API RP 2023:2001 pdf download
API RP 2023:2001 pdf download.Guide for Safe Storage and Handling of Heated Petroleum-Derived Asphalt Products and Crude-Oil Residua.
4.6 FIRE PREVENTION
Before there is the possibility of a storage tank fire there must be a flammable mixture in the tank. Some actions to help prevent flies and explosions by controlling the vapor space in heated storage tanks are:
a. Ventilating to remove vapors and keep vapor space in the ‘lean” range (for tanks with low volatility materials stored at least 50°F below their open cup flash point).
b. Venting limited to a single vent that is properly sized (see
API 2000].
c. Preventing restriction caused by condensed, solidified deposits building up in the vent.
d. Keeping gauge hatches closed during normal operation.
e. Regulating the storage temperature to keep the evolved vapors outside the flammable range (either too lean or too rich—see Appendix D.3).
f. Careful control if lines are blown with air (see 6.4).
g. Careful control if using air fur mixing (see 7.3).
h. Inert blanketing the vapor space to reduce oxygen content.
i. Temporanly inerting the vapor space with flue gas or another suitable medium when tanks are emptied ai rates subsianrially above normal.
Ignition of hydrocarbon vapors is possible when the oxygen level is about 11% (by volume) or greater. To maintain a safety margin. SOLAS calls for a maximum of 8% (by volume) oxygen in inert blanketed tanks on board crude oil tankers. This is achieved using systems supplying inert gas with a maximum of 5% oxygen. Inert gas is lighter (lower molecular weight) than hydrocarboniair mixtures. If blanketing is used, the potential time effectTM of the inerting technique should be recognized. If the inert gas is introduced near the top there can be stratification in the tank, with the heavier air/fuel mixture near the surface of the stored hydrocarbon. Diffusion and thermal currents will eventually provide dilution of the original atmosphere. If an empty tank is being inert blanketed this stratification effect can be used to advantage by venting the tank of I-IC vapors from the bottom with inert gas introduced near the top. This will tend to displace (instead of dilute) the heavier hydrocarbonJair mixture. Potential personnel exposure and environmental issues should be addressed. Tanks which have been inert blanketed may require special procedures when being removed from service (see 612) because inerting can cause a buildup of pyrophoric iron sulfide.
Precautions against exposure to sources of ignition should be emphasized and observed. If ignition occurs inside a tank there may be an internal explosion. The resulting overpressure may cause the tanks roof to fail and separate from the shell. The result will be a full surface (or partially impeded) tank fire. Suppression is discussed briefly in 4.7 and Appen-dix K,and in more depth in API RP 2021. See Appendix Afor some handling suggestions.
4.6.1POTENTIAL FUEL SOURCES
Hydrocarbons must be in the vapor phase to burn and be intheir flammable range. Temperature control and knowledgeof material in tanks become very significant.This is espe-cially true if a tank is being switched from one service toanother, particularly where hot heavy product is being mixedwith cool product containing a light fraction (e.g. cutbacks).As the temperature in the tank goes higher there will be morevapor generated. If the temperature goes very high (overapproximately 450°F), or there is localized overheating, thenlight vapors may be generated as a result of cracking. In thesecases, the light components released can increase the fire haz-ard.
Some situations in which tanks may have a flammablemixture include:
a. Nearly empty tanks containing a product with an apprecia-ble vapor pressure at the tank temperature.(Air will be drawninto vented atmospheric tanks as product is withdrawn).
b.Tanks being filled from a relatively low level (with air inthe headspace) with a product which has an appreciable vaporpressure at the rundown temperature.
c. Tanks at a temperature higher than normal for low vaporpressure products.
d.Tanks with a non-functioning inert gas blanket.e. While air-blowing asphalts.
f. Mixing or contamination with a small amounts of lightsolvent
g. Material heated above the flash point.
h. Inadequate ventilation (normal vents do not draw offvapors)
From a practical standpoint it is useful to recognize thatsmall quantities of absorbed gases and low molecular weightcomponents can accumulate in the tank vapor space in con-centrations higher than yielded by an intuitive judgementbased on percentage. Since hydrogen sulfide is heavier thanair and very flammable (4% to 44%) it can accumulate in thevapor space of tanks and represents fuel for a potential fire.
