ATMOSPHERIC STORAGE TANKS - Marsh
Energy PracticeRISK ENGINEERING POSITION PAPER – 01ATMOSPHERICSTORAGE TANKS
ii Atmospheric Storage Tanks
CONTENTSSectionTitle.Page1.Background . 12.Objective . 13.Scope. 14.Selection of Atmospheric Storage Tanks . 25.5.1Layout and Spacing . 3Bunds (Dykes) . 46.6.1Primary Containment . 5Tank Design and Operation . 57.7.17.2Secondary Containment . 8Bund Integrity (leak tightness) . 8Tertiary Containment . 98.Overfill Protection . 109.Maintenance and Inspection . 1110.10.110.2Leakage and Fire Detection . 12Leak Detection . 12Fire Prevention and Protection . 1211.11.111.211.3Fire Fighting Systems . 14Water Systems . 14Foam Fire Fighting Systems . 14Application Rates . 1612.References to Industry Losses . 1713.References to Industry Standards . 1714.Appendices . 19Appendix A - Assessment Checklist . 19Appendix B - Mechanical Component Checklist for Floating Roof Tank Inspections . 20Appendix C - Loss Incidents. 21Marsh iii
1. BACKGROUNDThere have been numerous incidents in the oil, gas, and petrochemical industry involvingatmospheric storage tanks. Data has been compiled by a reputable operator in the USA that indicatesthat overfilling of atmospheric storage tanks occurs once in every 3300 filling operations. In 2009there were two separate incidents just days apart, one in Jaipur, India (October 29), and one in SanJuan, Puerto Rico (October 23) that demonstrated the destructive capabilities of incidents atterminals and tank farms.The Buncefield incident in the UK in December 2005 resulted in an independent investigationcommissioned by the Health and Safety Commission in the UK. As a result of this, more guidance hasbeen provided to designers and operators of facilities. Such guidance has been included within thisposition paper.2. OBJECTIVEThe objective of this position paper is to define the standards that would be expected of a very goodatmospheric storage facility in the oil, gas, and petrochemical industry. These standards are alsoreflected in the Marsh Energy Risk Ranking criteria. They can be used to determine risk improvementopportunities and also to provide detailed advice to clients seeking to improve their atmosphericstorage facilities.3. SCOPEThe scope of this position paper includes: Guidance on the appropriate selection of atmospheric tank design for class of product to be stored. Guidance on layout and spacing. Guidance on appropriate means of ensuring primary containment. Suitable design of secondary containment. Detection arrangements for loss of primary containment and fires. Fire protection arrangements. Examples of loss incidents involving atmospheric storage tanks.1 Atmospheric Storage Tanks
4. SELECTION OF ATMOSPHERICSTORAGE TANKSThe layout and general design of a storage facility should be based uponconsiderations of safety, operational efficiency, and environmental protection. Aprimary consideration is ensuring that the design of the storage tanks themselves issuitable for the classification of the hydrocarbon being stored.The table below summarises the design of tank suitable for each class ofhydrocarbon:CRUDE OILDERIVATIVECLASSDEFINITIONRECOMMENDEDSTORAGE TANK DESIGNFloatingRoofFixed RoofClass 0Liquefied petroleum gases (LPG)88Class ILiquids which have flash points below21 C98Class II (1)Liquids which have flash points from 21 Cup to and including 55 C, handled belowflash point99Class II (2)Liquids which have flash points from 21 Cup to and including 55 C, handled at orabove flash point98Class III (1)Liquids which have flash points above55 C up to and including 100 C, handledbelow flash point89Class III (2)Liquid which have flash points above 55 Cup to and including 100 C, handled at orabove flash point89UnclassifiedLiquids which have flash points above100 C89Marsh 2
5. LAYOUT AND SPACINGIdeally, tank layout should be optimised to ensure that there is sufficient access to tanks for fire fightingand to minimise the risk of escalation in the event of a tank fire.Minimum spacing for tanks is specified in the table below, although Marsh would advocate a minimumseparation distance of 1x the diameter of the largest tank with an absolute minimum of 15 metres, as sucha distance provides sufficient access for fire fighters.FACTORMINIMUM SEPERATION FROM ANY PART OFTHE TANKBetween adjacent fixed-roof tanksEqual to the smaller of the following:(a) The diameter of the smaller tank.(b) Half the diameter of the larger tank.(c) Not less than 10m (Marsh advocate 15m).Between adjacent floating-roof tanks10m for tanks up to and including 45mdiameter.15m for tanks over 45m diameter.(The spacing is determined by the size of thelarger tank).Between a floating-roof tank and a fixed- Equal to the smaller of the following:roof tank(a) The diameter of the smaller tank.(b) Half the diameter of the larger tank.(c) Not less than 10m (Marsh advocate 15m).Between a group of small tanks and anytank outside the group15mBetween a tank and the site boundary,any designated non-hazardous area,process area or any fixed source ofignition15mSource: HSG 176. The storage of flammable liquids in tanks.One notable exception is the separation between crude tanks, where the destructive effects of a boil overcan extend from 5 to 10 diameters. Therefore consideration should be given to locating crude tanks at theedge of tank farm installations and with the largest practical separation from adjacent tanks.These distances should only be used in conjunction with appropriate levels of fire protection (see below).In the event that tanks are existing and do not conform to the above spacing, then additional fireprotection should be considered.Floating roof tanks, with external metal domed roofs extending over the entire roof area (i.e. internalfloating roof tanks or tanks fitted with geodesic domes), may be considered as fixed roof tanks for thepurpose of tank location and spacing.3 Atmospheric Storage Tanks
5.1 BUNDS (DYKES)Above-ground tanks should becompletely surrounded by bund walls(see also Secondary Containment).These should be designed to offerprotection to fire fighters. Therefore thebund wall should be located so that areasonably close approach can be madeto a tank fire.Fire fighting screens and steps shouldbe located at various points around thebund wall to assist the positioning andprotection of fire fighting personnel andequipment.Bunds would normally be constructedfrom earth or preferably concrete andshould be largely impervious to liquidand capable of withstanding hydrostaticand hydrodynamic pressures to whichthey could be subjected. Earthen bundsshould be fitted with a water resistantliner and whilst grass growth isacceptable, it should be regularlymaintained and kept short.Tanks should ideally be located alonewithin their own bunds, althoughwhenever tanks share a common bund,intermediate walls up to half the heightof the main bund walls and no morethan 0.5 metres high should beprovided to control small spillages fromone tank affecting another.Bund floors should drain to a singlelocation complete with sump for theregular removal of water from rainfall orfirewater testing. Drains shouldnormally be kept closed, with the drainisolation valve situated outside thebund.Marsh 4
6. PRIMARY CONTAINMENTIt is essential that the risk of loss ofcontainment is properly managed.This can be achieved by the properdesign, operation, maintenance, andinspection of tanks. Ideal practice isoutlined below.tanks, refer to the UK Health andSafety Executive’s review ofstandards, Mechanical integritymanagement of bulk storage tanks,which can be found at the followingwebsite address:The most effective way to prevent amajor accident at any site is thecontinued provision of suitableprimary containment of theflammable materials. This is achievedthrough the suitable design,construction, and maintenance of thestorage systems in accordance /rr760.pdfFor guidance on the most appropriatestandards to use for the design andconstruction of atmospheric storage6.1 TANK DESIGN ANDOPERATIONTanks should be designed to arelevant standard, such as API 650 orBS EN 14015.In some instances single skin bottomtanks are a better option than doublebottomed tanks as these provide theoptimum conditions for ensuringintegrity of the tank floor by theinspection of tank floor plates.Disadvantages of double-bottomdesigns include settlement, productentrapment, and modification tonozzle compensating plates.Where double-bottomed tanks areprovided, additional inspectionmeasures should be provided in5 Atmospheric Storage TanksOutlined below is a summary of thefeatures considered to be the mostappropriate for a modern facility: High level alarm and independentback-up high level instrument (withexecutive action only ifappropriate). Continuous automatic monitoringof tank contents, including “rate ofchange” during filling/emptying. Anti-rotation devices on floatingroof tanks. Closed water drains. Single skin floors. Double seals on floating roof tanks(NB foam dam heights should beabove the height of the upper rimseal to provide suitable protectionagainst a rim seal fire). Tank floor leak detection on doublefloor tanks. This is normallycomprised of an instrument todetect loss of vacuum in the interfloor space.accordance with a relevant standardsuch as EEMUA 183 or BS EN 14015.Leak detection should also beprovided on double-bottomed tanks.Whenever valves are left normallyopen, then the following measuresshould be put in place: Hydrocarbon detection in drainedwater with automatic isolation.TANK ROOFSDouble deck roofs on floating rooftanks are preferred to single deckroofs with external pontoons as theyare more stable and less likely to losebuoyancy.For floating roof tanks, the waterdrains to remove rainwater from theroof should be normally closed. Toensure roofs are emptied of rainwater,procedures should be in place andfollowed for the opening andre-closure of valves. Testing of operability of isolationvalves and detection systems (seeoverleaf).VENTSTanks should be provided with a weakseam/frangible roof construction, orwith an emergency vent suitably sizedfor the worst case relieving scenario,to prevent overpressure under allrelief conditions. Emergency ventsshould comply with relevantstandards, such as API 2000.
Wherever a fixed roof tank is used forthe storage of materials with a flashpoint below 21 C (generally on daytanks, not recommended for largebulk storage tanks), then flamearresters should be provided on thevents to prevent ignition of theflammable vapours burning back intothe tank. In warmer climates, whereproduct surface temperatures couldexceed 21 C, flame arresters shouldbe used wherever maximum surfacetemperatures could be within 10 C ofthe flash point.Flame arresters should be included inpreventative maintenance routines toensure they do not become blockedby scale, paint, ice or other materials.Flame arresters are notrecommended for use when thematerial being stored is liable topolymerise or foul the arrester. Due tothe potential for blockage,conservation vents (vacuum andpressure relief) should not be fittedwith flame arresters.Fixed roof tanks can be fitted with agas blanket (normally nitrogen) tomaintain an inert atmosphere in thevapour space. Nitrogen supplypressure should be just aboveatmospheric, but sufficient todisplace any liquid pumped out. As aback up, a vacuum breaker should beprovided in the event that nitrogensupply is lost.Vents or vapour recovery systems(often venting back to the sourcevessel) are required. These should bedesigned to relieve pressure slightlyabove that of the nitrogen and at asuitable margin below the designpressure of the storage tank.RIM SEALS & FOAM DAMSDouble rim seals (of fire-resistantconstruction) are preferable to singleseals. Due to irregularities during theconstruction phase of large floatingroof tanks, it is likely that a single sealwill not maintain a 100% sealbetween the tank roof and tank wallalong the entire height of the tankwall. Therefore, a second seal willimprove the likelihood of achieving atight seal, will reduce emissions, andminimise the risk of rim seal fire.When installing double rim seals, theheight of the second, upper rim sealshould be below the height of thefoam dam to ensure coveragewhenever rim seal pourers areactivated.Foam dams should be provided withintermittently-spaced gaps to allowthe drainage of rainwater. However, ifthe gaps are too large or toonumerous, they may affect the abilityto form a single continuous foambarrier in the event of a fire.OVERFLOWConsideration should be given toproviding suitable overflow systemsto ensure that in the event of a tankoverfilling, the tank contents aresafely routed into suitable secondarycontainment. Additionally, theoverflow route should be designed tominimise turbulent flow, reduce thesurface area of flammablehydrocarbon, and reduce thegeneration of flammable vapours,such as through splashing. Particularcare needs to be taken withstrengthening rings and firewaterdispersion rails around tanks whichare specifically designed to maximisespread of fluids over a tank’s surface.For existing tanks, considerationshould be given to modifications oftank top design and to the safe-rerouting of overflowing liquids.VALVESAll important valves on atmospherictanks should be labelled and theirfunction indicated.Fire-safe shut off valves should beprovided, preferably automatic andremotely operated shut off/isolationvalves (ROSOV/ROIV). They must befitted close to the tank on both theinlet and outlet connections. Valvesmust either conform to anappropriate standard, such as BS ENISO 10497 or equivalent internationaldesign, or should be of intrinsicallyMarsh 6
fire-safe design. Such featuresinclude: Metal-to-metal seats (secondarymetal seats on soft-seated valvesare acceptable). Must not be of cast-ironconstruction. Should not be wafer-bolted(sometimes referred to as long boltflanges).In regions subject to a high risk ofearthquake and/or rapid groundacceleration, the isolation valves andactuators should also be secureddirectly to the tank wall and not to theground. Anchoring the valve tosurrounding structures or the groundcould result in the valve separatingfrom the tank during excessivevibrations.Features of the ROIV should be: Fail safe (or if not fail safe have aback-up power supply, especially ifthe emergency plan requires thetank to be drawn down in anemergency). It should not be possible to autoreset the ROIV. Adequate margin of safety forshutting off the valve, with at least150% torque available from fullyopen to fully closed. No manual operation or override(e.g. hand wheels) which mayinhibit the operation of the ROIV. Suitable integrity and performanceto satisfy the safety integrity level(SIL) requirements.7 Atmospheric Storage Tanks Designed to minimise pressuresurges on system pipework andcouplings, particularly ship to shoreflexible pipes.Tank drainage valves should beblanked off when not in use.Whenever operations to removeaccumulations of water fromunderneath the product are to beconducted, isolation of the drainshould be achieved by the use of twovalves in series. The second valve canbe a temporary installation.PIPINGIn areas where earthquake is asignificant exposure, all associatedpiping on storage tanks should havestep geometry to allow flexing in thepiping and prevent puncturing of thetank during an earthquake.Suitably designed seismic hangersshould be provided in earthquakezones, and pipe supports should havefire protection within fire hazardzones, e.g. within the bunded area.TESTINGBefore filling tanks with flammableliquids, leak testing of the installedtank and associated pipework isrequired. Hydraulic testing shouldonly be used (i.e. not pneumatic) asstored energy in hydrotesting issubstantially lower, and inherentlysafer. However, air may be used as ameans of applying pressure to waterfilled tanks and piping.Salt water should not be used tohydraulically test systems containingstainless steel.HOUSEKEEPINGDebris within bunds should be kept atan absolute minimum. In addition tothe checks within formalisedinspection and maintenance routines,operators should also conductfrequent checks of tanks and theircomponents as part of their routinesto ensure that the tanks are kept in areasonable condition. A sample ofitems to check is included inAppendix B.
7. SECONDARY CONTAINMENTWhilst priority should be given to preventing loss of primary containment, adequate secondary (and sometimes tertiary)containment is necessary for the protection of the environment and to contain any spillages. In atmospheric tankapplications, secondary containment will be provided by bund walls.Bund capacity should be sufficient to contain the largest predictable spillage. A bund capacity of 110% of the capacity ofthe largest storage vessel within the bund will normally be sufficient. When estimating bund capac
3 Atmospheric Storage Tanks 5. LAYOUT AND SPACING Ideally, tank layout should be optimised to ensure that there is sufficient access to tanks for fire fighting and to minimise the risk of escalation in the event of a tank fire. Minimum spacing for tanks is specified in the table below, although Marsh would advocate a minimum
TANK & EQUIPMENT CO. Stationary Tanks Farm Tanks UL 142 Horizontal & Vertical Tanks API 650 Tanks UL 142 Double Wall Tanks UL 2085 “Protected“ Tanks UL 2245 At/Below Grade Tanks Insulated & Jacketed Storage Tanks Chemical Storage & Blending Tanks P.O. Box 2014 Tyler,
atmospheric ammonia storage tanks, which operate at -33 C. 3.2 Types of ammonia storage tanks Illustrations of different types of storage tanks are shown in Appendix 2. The main types of atmospheric tanks operating at -33 C are: A Single wall tanks, which are tanks with one steel bottom and wall designed to contain the full liquid level of .
Annex L : API Standard 650 Storage Tank Data Sheet Annex M : Requirements for Tanks Operating at Elevated Temperatures Annex P : Allowable External Loads on Tank Shell Openings Annex S : Austenitic Stainless Steel Storage Tanks Annex V : Design of Storage Tanks for External Pressure Hossein Sadeghi WELDED TANKS FOR OIL STORAGE (Rev. 0) 12 STANDARD INTRODUCTION. Hossein Sadeghi WELDED TANKS FOR .
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