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REFINING

ALUMINUM TANK COMPONENTS—1

Oil & Gas Journal --- TEMCOR Aluminum Domes - 29484 Aluminuma good alternativeto steel for fixed-roof tanks

Philip E. Myers Chevron Products Co.San Ramon, Calif. George L. Morovich Tank & Environmental Technologies Inc.The Woodlands, Tex. Earl J. Crochet Plantation Pipe Line Co.Atlanta

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luminum, used formajor aboveground- storage tank (AST)components, has advantages and disadvantages whencompared to the conventionalsteel materials of construc- tion.For fixed-roof tanks, refin-ers can save money and gain improved quality if they con- sider the advantages of alu- minum over steel.This article is the first of atwo-part series that discusses the use of aluminum structur-al components for ASTs. Thefirst article presents a compar- ison of covered tanks vs. un- covered tanks (tanks with a floating roof but no fixed- roof) as well as a comparison of aluminum-dome roofs (ADRs) vs. steel-cone roofs (SCRs). The second article compares various types ofaluminum and steel internal-floating roofs.For large ASTs, fixed roofsshould be aluminum because the roof-support columns can be eliminated when using alu- minum domes (Fig. 1). Large SCRs require columns. Alu- minum domes maximize tank capacity, reduce coating costs,and have fire-safety advan-tages.For internal-floating roofs,aluminum and steel each has its own merits. Instead of just bid prices, tank managers and operators should consider the long-term operational, safety, environmental, and economic issues of aluminum and steelcomponents in ASTs.Although API 650 definesa minimum standard of qual- ity, in some cases, when safe- ty and total cost of ownershipare considered, the standard may not be adequate for de- sired long-term performance. Aluminum domes maximize usable tank capacity, avoidcoating costs, and avoid column problems. Photo courtesyof Temcor (Fig. 1).

Covered vs. uncoveredtanks Overloading external-floating roofs

External-floating roofs arevulnerable to overloading. The fixed roof eliminates the intrusion of rainwater, snow, ice, and dirt into the tank.In EFRTs, precipitation cansink the floating roofs during heavy rainfalls or prolonged winter storms. Historically, several external-floating roofs sink each year in the U.S. dur- ing the winter or as a result of extreme rainfall events. This number may be underesti- mated because these incidentsare not required to be report-ed through any public chan- nels.Snow, ice, dirt, and debrisaccumulation can render the drains inoperable, resulting in overloading. Overloading can result when drain systems freeze close or when drain ca- pacities are inadequate.In addition, heavy snow-fall can result in an unbal- anced load condition if the snow collects on the wind- ward and shaded surfaces. Inthe Middle East, where rain-water, snow, and ice are not a concern, some users require IFRTs to avoid accumulations of windblown sand.External-floating roofs areespecially vulnerable to over- loading when the product level is low or while the float- ing roof is landed on its sup- ports. There are several rea-sons for this:•Per API 650-AppendixC.3.4, the design buoyancy is based on supporting 10 in. of rainwater (or 52 psf); howev- er, the supports are only de- signed to support 25 psf, per API 650-Appendix C.3.10.2.•When the roof is float-ing at a low elevation, the head pressure forcing rainwa-ter through the drains is re-duced. This head reduction increases the amount and weight of water on the deck during rainfall.•In an annular-pontoonfloating roof (flat-center deck), the center deck is high- ly flexible and will rise whenvapors evaporate as the deckis heated by solar energy. The deck also deflects downward at the center as it is loaded with water. When the roof is floating near the bottom, loaded with water and sag- ging at the center, the center deck supports can contact the bottom and fail as a result ofbeing overloaded. The float-ing roof supports have also been known to puncture the An AST that includes afloating roof and is covered by a fixed roof is considered an internal-floating roof tank (IFRT). An existing external-floating roof tank (EFRT) canbe converted to a covered IFRT by retrofitting it with a fixed roof or cover.Although the EFRT mayhave a lower initial cost than the IFRT, it may not have the lowest total ownership cost when long-termoperatingcosts, maintenance costs, andpotential risks are considered. The factors which favor build- ing or retrofitting EFRTs with fixed roofs are incident pre- vention, environmental pro- tection, safety, and reduced long-term maintenance and operating costs. Table 1 summarizes theconsiderations for the use of covered tanks (IFRTs) vs. un- covered tanks (EFRTs). Reprinted with revisions to format, from the May 18, 1998 edition of OIL & GAS JOURNAL Copyright 1998 by PennWell Corporation