API RP 2021:2001 pdf download

API RP 2021:2001 pdf download.Management of Atmospheric Storage Tank Fires.
6.4.2 Tank Vent Fires
Vent fires on fixed roof tanks typically are attributed to lightning. If addressed properly they can usually he extinguished with minimal damage and low risk to personnel using dry chemical or by reducing the pressure in the tank as discussed in 8.5.5.
6.4.3 Rim Seal Fires
6.4.3.1 Rim Seal Fires in External Floating Root Tanks
Rim seal fires comprise the majority of fires involving external floating roof tanks. Lightning provides the ignition source for most rim seal fires. In many cases this is attributed to an induced charge without a direct lightning hit. Success in extinguishing rim seal fires approaches 100% if there is no associated damage (such as a pontoon explosion) and if suppression efforts don’t sink the roof through excessive use of water.
Semi-fixed equipment for seal tires is described generally in Section 8.5.6.1 and in detail in NFPA I I. If permanently attached foam protection for the seal is not provided, it will he necessary to tight the fire with equipment which can he brought to the scene. Pre-incident planning and preparation can reduce delays and address potential personnel hazards when manual extinguishment is required.
6.4.3.2 Rim Seal Fires in Internal Floating Roof Tanks
Extinguishment of rim seal fires in internal floating roof tanks provides a special challenge, especially if there is no permanently attached foam system. The only access to apply a suppression agent is through small vent openings at the top of the tank, which typically have protective screens.
6.4.4 Unobstructed Full Liquid Surface Fires Without Sunken Roots
Extinguishment of full liquid surface fires where there is not a sunken roof is relatively simple in smaller tanks, but presents a major challenge in large tanks because of size and resources required. These usually involve fixed roof tanks where the roof has totally separated at a frangible (weak) seam leaving the total surface uncovered.
Fixed roof tanks have a vapor space between the liquid surface and the underside of the roof If the vapor space is in the flammable range at the time an ignition source is introduced, an explosion will occur. If the tank is constructed in accordance with API Std 650, the roof should separate from the shell at the frangible seam joint. The roof usually separates in one piece. Depending on the severity of the internal pressurization or explosion the roof will vary from a fishmouth” a few feet long to a fixed roof which is blown completely away from the tank shell. Sometimes the roof will lift into the air and fall hack into the tank. On other occasions, only pieces of the separated roof may remain intact on top of the tank. The resulting tire usually involves the entire surface area of the tank except where obstructed by the remaining root There may he fire “hiding” below roof segments (6.4.5).
When the tank contains a conventional hydrocarbon, extinguishing options include both topside application and subsurface injection of foam. The foam type must be compatible with both the fuel and the application technique. If a tire involves polar solvent with high water miscibility, the extinguishing technique is limited to topside application. For planning purposes the approach to be used should be determined based on the installed equipment on the tank, the material stored in the tank and the suppression philosophy. Then, the resources needed can be determined.
6.4.5 Obstructed Full Liquid Surface Fires with (Wholly or Partially) Sunken Roofs
Full surface fires with a full or partially sunken roof can occur where tanks have fixed roofs, internal floating roofs or external floating roofs. The roofs of internal and external floating roof tanks can sink for a variety of reasons. Where the roof is internal, gas or vapor can cock the roof causing it to buckle and sink or allowing liquid to overflow the rim. Since this introduces flammable material into the ventilated air space between the fixed and floating roofs, the result can be an explosive mixture in the vapor space below the fixed roof. If this is ignited the fixed roof may stay intact or it may separate in one of the scenarios described for fixed roof tanks without internal floating roofs. Also, if the tank seals are not vapor tight, during filling the vapor space in an internal floating roof can be in the flammable range. If lightning strikes during this time an explosion can result. Filling operations should be conducted with caution or avoided when a lightning storm is imminent in the vicinity. Vapor recovery systems may reduce the ignition hazards associated with lightning.