Technical Aspects of Storage Tank Loss PreventionSzu-Ying Huang and M Sam MannanMary Kay OrsquoConnor Process Safety Center Artie McFerrin Department of Chemical Engineering Texas AampM UniversityCollege Station TX 77843-3122 mannantamuedu (for correspondence)

Published online 8 February 2013 in Wiley Online Library (wileyonlinelibrarycom) DOI 101002prs11550

Storage tanks are the most common chemical containingvessels within the process industries There have been manyaccidents associated within storage tank (material deficiencystructure design failures operation error etc) or among stor-age areas throughout the history Both human error and nat-ural disasters can result in devastating incidents for tanksthat are usually kept outdoors under weather influences aswell as the tank bodies and accessories are poorly designedabused or are not effectively inspected and maintained Thereare potential hazards which must be assessed carefully beforeconstruction during operation and after shutdown of anystorage areas including hazardous or flammable chemicals

This article offers an overview of the tank and tank farmwith significant incident causes about aboveground tankdesign failures operation problems and maintenance issueswith the relating discussion about the implementation of pro-cess safety management and regulations Also some innova-tive storage unit technologies including groundings roofdesign safer filling procedures and tank fire mitigation sys-tems are introduced to give a proper information session tohelp industry understand the tank hazards and implementadequate actions to prevent incidents and losses VC 2013American Institute of Chemical Engineers Process Saf Prog32 28ndash36 2013

Keywords storage tank incidents loss prevention tankdesign tank inspection hazardous material storageregulations

INTRODUCTION

Storage tanks are the fundamental for the storing of liq-uid gas and other chemical fluids Storage tanks provideaccommodation of inventory for raw materials additivesreactants intermediate materials finished products andwastes Thus the safety on tank itself is important as it isthe first layer in contact with chemical materials and con-nected with other process devices

Among the various functions design and shapes of tanksthe most discussed and applied one is the category ofaboveground storage tanks (AST) ASTs are easy to manageand maintain as long as they are above the ground andwould suffer less from the soil contaminations pressure ofstructure design and spacing with other infrastructure systemof the location of tank farm Usually crude oil productsintermediates and raw chemicals with flammable or toxichazards are likely to be stored in ASTs This work will con-sider the more general classifications of storage units byoperation conditions and containing materials with respectto the design differences on these two main categories of

tanks Also as long as most oil and gas industry pay atten-tion to low-pressure atmospheric tankage and the relatedloadingdischarging of railcars ships and road trucks foundin terminals or tank depots being a specific area of tankfarm incidents occurrence studies (BP Process Safety Series2005) reviews of previous storage tank studies will be con-sidered and the main concern of this article

To analyze random process elements there are threemain parts which shall be considered carefully design speci-fications operation conditions (pressure temperature flowparameters) and maintenance (including regular repairworks monitoring of abnormal cases and re-startup checks)The tank selections (cone roof open top internal floatingetc) are based on the operating conditions containing mate-rials and local environmental cases Each of these requiresmore concern on technology innovations for the potentialhazards due to their specifications The statistics [1] of tankand tank farm incidents expresses that overfilling has caused69 out of 242 reported tank-related incidents happening inthe US region from 1960 to 2006 while in the same timerange there were 80 lightning-caused fires destroying manystorage tanks Many aspects of tank in process or even not-in-use can go wrong The tank spacing grounding devicesrelief valves level control monitoring system and manyother layers of protection in technical point of view shall becarefully analyzed

In summary this article will provide the followinginformation to review the process safety of storage tank anddepot areas

Learn from history anything small can cause hugetroubles Tank and tank farm hazards analysis Recent technology and regulations on loss prevention of

tank and tank farms What else can we do to make tanktank farm safer

EXAMPLE CASE OF STORAGE TANK AND TANK FARM INCIDENT

Chemical plant incidents attract unwanted attention Tankand tank farm issues are not the exception In November 2008there was a catastrophic tank collapse happened in Allied Ter-minal Inc Tank storage area at Chesapeake Virginia On theday of the incident Allied employees were filling the incidentTank with liquid fertilizer UAN32 to check for leaks prior topainting the tank During the filling a welder and his helpersealed leaking rivets on the tank At a fill level about 35 inchesbelow the (calculated) maximum liquid level the tank splitapart vertically beginning at a defective weld at midway upthe tank The liquid fertilizer overtopped the secondary con-tainment partially flooding the site and adjacent neighbor-hood The collapsing tank wall seriously injured the welderVC 2013 American Institute of Chemical Engineers

Process Safety Progress (Vol32 No1)28 March 2013

and his helper Over 200000 gallons of the liquid fertilizerwere not recovered some entered the southern branch of theElizabeth River causing serious contamination

According to Chemical Safety Boardrsquos (CSB) investigationthe causes of the incident shall be traced back to 2 yearsbefore the incident tank was put into use [2] Originally thetank (numbering Tank 201) was used for petrochemical stor-age Allied modified the welding in order to make it a liquidfertilizer container Since the chemicals had different specificgravity Allied Inc contracted GampT to modify Tank 201 byreplacing the vertical riveted joints with butt-welded jointsThis was to increase the strength of the joints which wouldpermit an increase in the maximum liquid level and allowmore fertilizer to be stored in Tank 201 In 2007 While GampTwas modifying Tank 201 another contracted inspector HMTperformed an American Petroleum Institute (API) 653 Out-of-Service inspection saying that the safety filling height was2565 feet This number was increased to 2701 feet based onan April 2001 metallurgical test report provided by Allied Inc

On the day of incident as the tank was being filled whenthe tank reached a level of 2672 feet a vertical split startedmidway up the shell and rapidly ruptured to the floor and tothe roof of the tank on the side opposite the workers As thepressure of the liquid fertilizer inside the tank opened the splitthe tank shell separated from the bottom and roof quicklyspilled and released the contents and caused damages

The main issues regarding this incident had brought multi-ple layers of problems First Allied did not ensure that weldson the plates to replace vertical riveted joints met generallyaccepted industry quality standards for tank fabrication Sec-ond the company had not performed postwelding inspection(spot radiography) required for the calculated maximum liquidlevel for the tank Finally the employer had no safety proce-dures or policies for work on or around tanks that were beingfilled for the first time following major modifications anddirected contractors to seal leaking rivets while tank was beingfilled to the calculated maximum liquid level for the first time

Making the responsible statement clearer Allied Inc didnot use an authorized inspector or an engineer experiencedin storage tank design to approve the modifications to thetank Their contractor of welding GampT did not use qualifiedweld procedures or welders to perform the modification inaccordance with the ASME Boiler and Pressure Vessel CodeSection XI Welding and Brazing Qualification They did notrequire or perform spot radiography on tank while in thiscase a thorough radiography crack check is necessary forhazardous contents Last but not least the testing contractorHMT calculated a maximum liquid level for the tank usingthe requirements for weld joints that are spot radiographedHMT calculated a maximum liquid level for the tank basedon an average wall thickness in lieu of a minimum wallthickness which is likely to be overestimated

Starting from the incident this brought out the issues thatmay lead of enormous accidents On the other hand if thereis more understanding and awareness of the tank and tankfarm hazard the incidents can be more thoroughly controlled

TANK HAZARDS AND TECHNICAL PROTECTIONS

The failure of liquid storage tanks originates from inad-equate tank design construction inspection and mainte-nance Tank design and construction have already beendiscussed throughout industry facilities academic researchand various projects The failure rate for tanks already inservice can be reduced via tank maintenance and weldinspections To minimize the severity of possible tank fail-ures there should be a secondary containment such as adike surrounding the tank In terms of tank inspection priorto startup a close external inspection of leaks and corrosionshould be made External inspections should be performed

both regularly during normal operating rounds and periodi-cally by individuals who assure the mechanical integrity ofthe tanks Periodic internal inspections of the tanks are byfar the most important These inspections should includethickness readings of the walls and the tank floor

The CSB has identified hot works around tanks as signifi-cant ignition sources for tank fires (CSB 2009) [3] A properhot work permit system must be in place to prevent possibleignition sources from encountering flammable atmospheresThese hot work permits should incorporate gas testing forflammables

The followings are a series of hazards identificationregarding tank and tank farms the technical protection avail-able for loss prevention and mentioning of related regulatorystandards to guide and require the construction evaluationoperation and maintenance of storage tanks

Design Issues

Material Rupture and Corrosion

Tank body structure and material hazards The design ofthe storage tanks themselves is the first aspect of the tankfarm safety issue A good design must balance the desiredperformance with allowable costs The material selection oftank body according to tank area location compatibility ofmaterials storage ease of fabrication and resistance to corro-sion [4] heavily influences both factors The most widelyused tank materials are composed of carbon steel due tohigh corrosion resistance or climate factors Aluminum stain-less steel and fiber glass are also used as tank building mate-rials in certain situations API 650 the tank fabricationstandard details the principles on steel structure and platesstresses requirements for different types of storage Corrosionis also one of the main causes of storage tank structure fail-ures excluding the material characteristics mentioned aboveThis phenomenon occurs in (a) external surfaces exposed tothe ambient environment (b) under the tank bottom and (c)the internal liquidvapor space of the tank The atmosphericcorrosion usually takes place on the roof or shell especiallywhen storage depot is close to warm humid regions Theoxidization from the air may cause tank corrosion whilestrong winds can form rusts to scratch through tank bodyover time increase the severity of surface damage The sidebottom corrosion is usually due to ground level water layersconstruction racks linked to tank supports (especially ele-vated tanks and spherical tanks) This is likely to happen oncrude oil storage tanks with two-phase products that includecorrosive chlorides pitting from the latter will appear at thebottom part of tank surface Internal corrosion especially intwo-phase systems is very significant factor for tanks con-taining refined hydrocarbon or acidic product tanks The liq-uid-vapor interface has the most serious corrosion becauseof oxygen-moisture interaction This kind of corrosion typi-cally occurs in floating-roof as the material is usually alumi-num alloys which are relatively light and controllable buthave lower corrosion resistance

There are some related phenomena coming with corro-sion-risky conditions such as hydrostatic testing fluid pittingerosion due to filling impingment static electricity cavitationdamage thermal frettingfrictions and hydrogen embrittle-ment Along with these aspects design considerations mustalso include the storage chemical properties operating con-ditions and the environment issues to produce a functionaland safe storage tank system

Technical protection Materials grouping in API 650 canbe classified into three main factors of discussion (1) killedsemikilled (2) normalizing (3) quenching and tempering

Process Safety Progress (Vol32 No1) Published on behalf of the AIChE DOI 101002prs March 2013 29

plates [18] API 650 categorized the variables into eightgroups (six main groups and two subcategories) The com-parison of the property of these cases is listed in detail in theregulations and minimum tolerances for tank performanceare specified for industrial reference Factors affecting brittlefracture are also presented Brittle fractures can trigger hugefailures for tanks leading to accidental spills and releasesfrom the structure cracks and rupture spots Local stressallowance is important in analyzing the fracture issue API650 contains the averages of longitudinal and transverseplate Charpy V-notch test (CVN test) results for the eight cat-egories of steel materials and the acceptance requirementscan explain design specification of storage tanks Parameterssuch as operating temperature welding material composi-tions and thicknesses are also considered as well in earlydesign stage of material selection surface treatment andother analysis

Another important aspect of tank body condition can bedone by radiography and nondestructive tank shell checkingThis is significant to observe and find out the material structurefailures There are various nondestructive methods used to pre-vent tank collapse and failure on the structure design built andprestartup evaluation Radiography of tanks is one of the mostimportant widely used one among the tools The basic designis usually the sensor connected with the data acquisition systemmonitoring the level of filling with regard to the welding surfa-ces and tracks back the defects The basic categories of thetools are (a) spot radiography a radiographic technology thatallows real-time examination of the longitudinal weld (b) fullradiography with ldquoLethalrdquo material stored in the tank or tank isunder extreme operationmaintenance conditions and (c)nonintrusive tests IR image Sonics Tank Floor Electro-Mag-netic Acoustic Transducers thermal imaging

Tank Roof Failures

Roof problem hazards Except for the installation failureitself tank roofs can experience several other hazardouscases which will lead to potential damage From a incidentinvestigation study of a naphtha tank fire in Singapore [5]they found a main factor causing this incident was also rooffailure due to a combination of heavy rain maintenanceproblems and inherent design features caused the floatingroof on naphtha tank to start sinking Tank roof safety issure to be a significant issue regarding loss prevention

a Internal Beam Rupture usually triggered by material cor-rosion or pitting The welding of the roof and tank bodyis an important failure point as damage to the joint canbe the starting point of both atmospheric corrosion andinternal corrosion The structure damage of the tankbeam may even influence the rim seals the deck andthe construction pontoons

b Vent System Failures the vacuum or overpressure due tooperation problems ventilation system flaws and incor-rect setting procedures or inadequate inspection andmaintenance of the vent controls

c SnowRain Topping issue Leaks can occur as the weightand drifts of heavy snow or rain cause fittings joints andsometimes even the entire tank to shift This process canalso cause leaks and disruptions to the fittings Snow cov-ering a tank roof can prevent leaking gas from escapingand thus create a pocket of gas that can fuel a massiveexplosion Besides exhaust vents for tank top can beclogged by heavy snow or rain if the collecting anddraining systems were not properly designed accordingto the climate of that storage area

d Frangible Roof-Shell Joints a term defined in the main tankconstruction standards BSEN 14015 and API650 The con-cept of frangible roof only applies to flat bottom cone roof

tanks with limited roof apex angle A Frangible roof is a roofto shell joint or junction that is weaker than the rest of thetank and will preferentially fail if the tank is over pressur-ized Since this junction will fail before any other part of thetank such as the shell the bottom or the shell to bottomjoint the bottom and shell can be relied on to be intact

e Seismic Zone Design Failure In API 650 Appendix Ethere are series of advices mentioning the seismic issuesabout minimum requirement available in tank foundationdesign Failure of tanks during Chilean earthquake of1960 and Alaska earthquake of 1964 led to beginning ofmany investigations on seismic analysis of liquid storagetanks Following two aspects came to forefront

Due consideration should be given to sloshing effectsof liquid and flexibility of container wall while evalu-ating the seismic forces on tanks It is recognized that tanks are less ductile and have

low energy absorbing capacity and redundancy com-pared to the conventional building systems

The failure modes of seismic design are likely to be tearpiping base separated from shell floating roof collapse andsloshing wave damages [23] Seismic motions caused tankroofs to lose buoyancy and go to the direction of gravitywith relative velocity on seismic force Even though API 650Appendix E has been updated continuously the API ASTcommittees still consider these recommendations to be awork in progress The earth activity zone regarding soil layerstructures earthquake scales and seismic waves resultingsloshing waves had been causing many serious accidents toliquid storage tanks containing flammable chemicals such as1998 Turkey Ismit Tank farm incidents due to earthquakeand 2002 Japan Hokkaidorsquos Chemical Tank Incidents [6]

Technical protections There are various ways to preventtank roof failures From a study done in 2004 by AlyeskaPipeline Service Company [7] the roof failure location con-ditions and possible consequence if the safeguard does notexistfunction are listed as Tables 1 and 2

From the study we can clearly see that the safeguardshall be put in place especially in terms of overpressure andoverfilling prevention better basic structure of tank roofgood welding practices and avoiding objects collision suchas heavy snow rain or other external loads In this logicthe ventilation of excess pressure storage contents and roof-top loads shall be carefully designed and put in service Theinstruments which monitor the level control and pressurerise due to various reasons need to be kept in function andconnected to relief system and alarms More examples willbe given in later sections about operation errors

Walkways Another attempt to create a bond betweenthe roof and the shell relies on the tankrsquos walkway Nearlyall tanks have a walkway or ladder where the upper end isattached to the rim of the tank and the lower end is restingon the floating roof The quality of this electrical connectionis questionable The upper connection is a bolted hinge sub-ject to loosening corrosion and surface-covering paints Thelower end is a pressure connection with only two wheelsresting on rails and is also subject to corrosion and surface-covering paints This can also be a significant considerationwhile dealing with roof failure prevention

Potential Ignition Source and Flammable Vapor

Tank fire hazards With existence of ignition sourcesfrom nearby area of the tanks the failures from design issuesmentioned above will be the source of significant hazardsThe possible ignition sources are everywhere in tank areawelding equipment cables static electricity and other

DOI 101002prs Process Safety Progress (Vol32 No1)30 March 2013 Published on behalf of the AIChE

Table 2 Cone roof tank hazards (Adapted from Ref [12])

Operating Mode Potential Hazards Cause Effects if No Safe Guards in Place

Startup Flammable environment in headspace

On initial fill this shouldnormally occur

Possible fire or explosion

Maintenance Minor fire during maintenance Maintenance in tank Minor fire during maintenanceNormal operations High snow load High snow loads but less benefit

from product heat vents andopen manways open to theenvironment

Snow removal on the cone roofpotential for worker injuryexposure

Normal operations Overfill Operator error level indicationand alarm failure

Spill crude to the environmentpotential for pool fire

Normal operations Static electricity and inducedcharge differentials

lightning Provides an ignition sourcewhich can ignite vaporsthrough an opening in the tank

Maintenance Mixer motor replacement leads toflammable environment in tank

Mixer motor failure Possible tank fire

Normal operations Process safety hazard from vaporrecovery system

Typical causes of leaks inrotating equipments

Effect range from seal fire toexplosion

Inspection Release of hydrocarbon vapors tothe environment duringinternal inspection

Internal inspection as requiredby regulation

Environmental release

Table 1 Internal floating roof tank hazards (Adapted from Ref [12])

Operating Mode Potential Hazards Cause Effects if No Safe Guard in Place

Startup Filling the tank with legs landed On initial fill this shouldnormally occur

Effect range from no impact topossible fire or environmentalrelease

Normal operations Excessive fill rate Operator error Environmental release possibleroof sunken or pool fire

Maintenance Release of hydrocarbon vaporsfrom unexpected source inroof seal

Maintenance in tank Worker injury

Inspection Release of hydrocarbon vapors toenvironment during internalinspections

Internal inspection as requiredby regulations

Environmental release

Startup Flammable environment in headspace

On initial fill this shouldnormally occur

Effect range from no impact topossible fire or environmentalrelease

Normal operations If operators are not aware offloating roof level mayinadvertently land the roofcreating vapor in head space

Instrument error operator error Effects ranging from seal fire toexplosion

Maintenance Minor fires during maintenance Maintenance in tank maybemore difficult to clean IFRtank prior to maintenance

Minor fire during maintenance

Normal operations Product on the roof Roof hangs up on columnsearthquake

Fire in head space

Normal operations Roof hangs up causing internalfire

Hangs up on columnsearthquake turbulence duringfill wax build-up

Flammable space below floatingroof or burning liquid abovefloating roof

Normal operations High snow loads with less heatavailable in IFR design

High snow loads but less benefitfrom product heat area aboveroof open to environment

Increase snow removal on thecone roof with commensurateworker injury exposure

Normal operations Static electricity and inducedcharge differentials

Fluid flow improper groundingwax buildup lightning

Provides an ignition sourcewhich can ignite vaporsstemming from a seal leakleading to seal fire

Maintenance Mixer motor replacement createsflammable head space

Mixer motor failures that requirestank entry

Possible fire worker injury

Maintenance Worker injury due to reduceworking space from floatingroof

Inherent IFR tank design Possible fire worker injury

Process Safety Progress (Vol32 No1) Published on behalf of the AIChE DOI 101002prs March 2013 31

miscellaneous sources from human error Static electricity isdependent on two properties generation and accumulationGeneration of static electricity occurs whenever two objectsare rubbed against each other With the existence of flamma-ble fuel vapor on the top of storage tank near leaking orrelease points and even the more serious vapor cloud formednear tank farm the ignition sources may cause majorincidents

The generation of static electricity cannot be completelyprevented but should be reduced for tank safety API 2003mentions several means of static electric generation flowgeneration pumping changes in vortex or pipeline diameterflow through filters or fittings splashing spraying and tankfilling Under ideal conditions though the charge immedi-ately dissipates The generation of static electricity can behandled without hazard if accumulation does not occurAccumulation occurs when the generation of electricity isgreater than the dissipation 016 of tanks have a rim fireevery year 95 of those are caused by lightning The sparksare due to the accumulation of electricity and can lead tohuge fires and losses

Technical protection Inerting [16] [17] The elimina-tion of flammable fuel vapors in commercial aircraft fueltanks is a principal safety priority Proper inerting of a fueltank can significantly decrease the risk of explosions andfires Liquefied Natural Gas (LNG) carriers and ground leveltank farms are required to have inerting systems to preventsuch incidents Inerting refers to the rendering of the ullage(the air above the fuel) unable to propagate a reaction givenflammable conditions and an ignition source In this case itrefers specifically to reducing the oxygen concentration inthe tank This effectively eliminates one side of the fire trian-gle Usually a pump is used to exhaust the oxygen richwaste to ambient pressure an inert gas generating system inorder to improve the performance of the permeable mem-brane air separator component and to insure sufficient gen-eration of inert gas when available source air pressure islow Nitrogen inerting systems are another safeguard whichis often employed Chemical plants that have fixed roof tanksand are concerned about flammable atmospheres or the in-halation of moisture may relay on nitrogen pads Com-pressed nitrogen is piped into the vessels and maintains afew ounces of pressure This avoids the potential for air tobe sucked out of the tank as the internal pressure drops dueto cooling or liquid being pumped out

Shunts for floating roof tanks NFPA 780 requires thatstainless steel shunts be spaced no more than 10 feet apartaround the roof perimeter These shunts are bolted to theedge of the floating roof and connect with the inside of theshell Unfortunately shunts do not bond well for several rea-sons First some components of heavy crude oil such as waxtar and paraffin tend to coat the inside of the tank wall form-ing an isolating barrier between the shell and the shunts Sec-ond rust on the inside of the shell creates a high-resistanceconnection between the shells and the shunts Third 10 to25 of tanks are painted on the inside typically with an ep-oxy-based paint which insulates the shell from the shuntsFinally large tanks may become elliptically distort by severalinches which can cause the shunts to pull away from theshell Another method uses shunts submerged in the storedproduct These submerged shunts may provide some benefitswhen arcing occurs since no air is present however the sub-merged shunts still rely on pressure contact that is subject toall the conditions outlined above In addition submergedshunts are exceedingly difficult to inspect and maintain

Roof-shell bonding cable Another method is to install acable from the top of the shell to the middle of the roof typ-ically on the order of 250 to 500 MCM The cable is con-

nected to the top of the rim near the top of the internalladder suspended along the bottom of the ladder andbonded to the center of the roof The cable must be longenough to connect to the roof at its lowest position Althoughat 60 Hz this cable has low impedance at lightning frequen-cies it has very high impedance For example at 100 kHz theimpedance of 100 feet of 250 MCM cable is estimated at over32 ohms Therefore when thousands of amps of electricityflow across the tank the impedance of the roof-shell bondingcable is too high to prevent sustained arcing at the shunts

A Retractable Grounding Assembly device (RGA) wasdeveloped by Lightning Eliminators amp Consultants Inc cre-ates a permanent electrical bond between the roof and shellWhen properly applied multiple RGAs provide low-imped-ance pathways to safely discharge the long duration currentresponsible for many tanks fires The RGA attaches betweenthe roof and shell with a wide spring-loaded cable con-structed from 864 strands of 30 AWG tinned copper wirebraided to form a wide flat strap 1625 inches wide by 011thick This has been studied and proved to be applicable formost grounding and current suspending case

Loading rates Loading of liquid contents as mentionedin the hazard section is also an important factor of staticelectricity generation and accumulation There are multipleways to eliminate this charge gathering A previous study [8]on the IEC TC31101 JWG29 new model for IEC TechnicalSpecification 60079-32-1 Explosive AtmospheresndashPart 32-1Electrostatic hazards Guidance ldquoBritton and Smith modelrdquowas introduced and was able to address range of flow ratefor transfer of single-phase static-accumulating flammableliquids into vertical cylindrical tanks containing no significantsediment or water bottom The research had put together aseries of theoretical analysis on factors having impact ondielectric constant threshold surface potential and maximumvelocity of fillingloading to storage tanks They also pro-vided comparisons between smaller and larger size tanks invarious chemical properties and filling depth This shall be areferable model development in the static electric reductionfor practical use

Operation Issues

Common Operational Hazards

Tank overpressure and vacuum Operations and situa-tions which may result in tank overpressure are (1) pumpingrate failure (2) temperature change and (3) physical statechange due to failure of venting systems or poor practicesOSHA Standard 45 CFR 3920-11 has mentioned the hazardand protection of the overpressure Cargo tanks venting sys-tem should be capable to ventilate 125 times the maximumtransfer rate while the pressure in the vapor space of eachtank connected to the vapor collection system does notexceed the maximum designed working pressure of thistank

Overpressure is usually caused by excessive operatingpressure resulting in rupture of hoses or tank or release ofcargo through the pressure relief valves (if rupture disk andfittings do not properly adjunct) with possible risk of injuryand spillage Vacuum on the other hand created in tank dur-ing pumping out or as a result of tank cooling after systemcleaning or after discharge of heated cargo Vacuum couldresult in damage to tank shell [11] Workers must ensure thattank is vented (via manhole or air-line) during cargo dischargeor after steam cleaning or discharge of heated cargo

Tank level measurement and overfilling Hazards associ-ated with operating procedures are important to considerwhen designing tank farms Proper management of the

DOI 101002prs Process Safety Progress (Vol32 No1)32 March 2013 Published on behalf of the AIChE

miscellaneous sources from human error Static electricity isdependent on two properties generation and accumulationGeneration of static electricity occurs whenever two objectsare rubbed against each other With the existence of flamma-ble fuel vapor on the top of storage tank near leaking orrelease points and even the more serious vapor cloud formednear tank farm the ignition sources may cause majorincidents

The generation of static electricity cannot be completelyprevented but should be reduced for tank safety API 2003mentions several means of static electric generation flowgeneration pumping changes in vortex or pipeline diameterflow through filters or fittings splashing spraying and tankfilling Under ideal conditions though the charge immedi-ately dissipates The generation of static electricity can behandled without hazard if accumulation does not occurAccumulation occurs when the generation of electricity isgreater than the dissipation 016 of tanks have a rim fireevery year 95 of those are caused by lightning The sparksare due to the accumulation of electricity and can lead tohuge fires and losses

Technical protection Inerting [16] [17] The elimina-tion of flammable fuel vapors in commercial aircraft fueltanks is a principal safety priority Proper inerting of a fueltank can significantly decrease the risk of explosions andfires Liquefied Natural Gas (LNG) carriers and ground leveltank farms are required to have inerting systems to preventsuch incidents Inerting refers to the rendering of the ullage(the air above the fuel) unable to propagate a reaction givenflammable conditions and an ignition source In this case itrefers specifically to reducing the oxygen concentration inthe tank This effectively eliminates one side of the fire trian-gle Usually a pump is used to exhaust the oxygen richwaste to ambient pressure an inert gas generating system inorder to improve the performance of the permeable mem-brane air separator component and to insure sufficient gen-eration of inert gas when available source air pressure islow Nitrogen inerting systems are another safeguard whichis often employed Chemical plants that have fixed roof tanksand are concerned about flammable atmospheres or the in-halation of moisture may relay on nitrogen pads Com-pressed nitrogen is piped into the vessels and maintains afew ounces of pressure This avoids the potential for air tobe sucked out of the tank as the internal pressure drops dueto cooling or liquid being pumped out

Shunts for floating roof tanks NFPA 780 requires thatstainless steel shunts be spaced no more than 10 feet apartaround the roof perimeter These shunts are bolted to theedge of the floating roof and connect with the inside of theshell Unfortunately shunts do not bond well for several rea-sons First some components of heavy crude oil such as waxtar and paraffin tend to coat the inside of the tank wall form-ing an isolating barrier between the shell and the shunts Sec-ond rust on the inside of the shell creates a high-resistanceconnection between the shells and the shunts Third 10 to25 of tanks are painted on the inside typically with an ep-oxy-based paint which insulates the shell from the shuntsFinally large tanks may become elliptically distort by severalinches which can cause the shunts to pull away from theshell Another method uses shunts submerged in the storedproduct These submerged shunts may provide some benefitswhen arcing occurs since no air is present however the sub-merged shunts still rely on pressure contact that is subject toall the conditions outlined above In addition submergedshunts are exceedingly difficult to inspect and maintain

Roof-shell bonding cable Another method is to install acable from the top of the shell to the middle of the roof typ-ically on the order of 250 to 500 MCM The cable is con-

nected to the top of the rim near the top of the internalladder suspended along the bottom of the ladder andbonded to the center of the roof The cable must be longenough to connect to the roof at its lowest position Althoughat 60 Hz this cable has low impedance at lightning frequen-cies it has very high impedance For example at 100 kHz theimpedance of 100 feet of 250 MCM cable is estimated at over32 ohms Therefore when thousands of amps of electricityflow across the tank the impedance of the roof-shell bondingcable is too high to prevent sustained arcing at the shunts

A Retractable Grounding Assembly device (RGA) wasdeveloped by Lightning Eliminators amp Consultants Inc cre-ates a permanent electrical bond between the roof and shellWhen properly applied multiple RGAs provide low-imped-ance pathways to safely discharge the long duration currentresponsible for many tanks fires The RGA attaches betweenthe roof and shell with a wide spring-loaded cable con-structed from 864 strands of 30 AWG tinned copper wirebraided to form a wide flat strap 1625 inches wide by 011thick This has been studied and proved to be applicable formost grounding and current suspending case

Loading rates Loading of liquid contents as mentionedin the hazard section is also an important factor of staticelectricity generation and accumulation There are multipleways to eliminate this charge gathering A previous study [8]on the IEC TC31101 JWG29 new model for IEC TechnicalSpecification 60079-32-1 Explosive AtmospheresndashPart 32-1Electrostatic hazards Guidance ldquoBritton and Smith modelrdquowas introduced and was able to address range of flow ratefor transfer of single-phase static-accumulating flammableliquids into vertical cylindrical tanks containing no significantsediment or water bottom The research had put together aseries of theoretical analysis on factors having impact ondielectric constant threshold surface potential and maximumvelocity of fillingloading to storage tanks They also pro-vided comparisons between smaller and larger size tanks invarious chemical properties and filling depth This shall be areferable model development in the static electric reductionfor practical use

Operation Issues

Common Operational Hazards

Tank overpressure and vacuum Operations and situa-tions which may result in tank overpressure are (1) pumpingrate failure (2) temperature change and (3) physical statechange due to failure of venting systems or poor practicesOSHA Standard 45 CFR 3920-11 has mentioned the hazardand protection of the overpressure Cargo tanks venting sys-tem should be capable to ventilate 125 times the maximumtransfer rate while the pressure in the vapor space of eachtank connected to the vapor collection system does notexceed the maximum designed working pressure of thistank

Overpressure is usually caused by excessive operatingpressure resulting in rupture of hoses or tank or release ofcargo through the pressure relief valves (if rupture disk andfittings do not properly adjunct) with possible risk of injuryand spillage Vacuum on the other hand created in tank dur-ing pumping out or as a result of tank cooling after systemcleaning or after discharge of heated cargo Vacuum couldresult in damage to tank shell [11] Workers must ensure thattank is vented (via manhole or air-line) during cargo dischargeor after steam cleaning or discharge of heated cargo

Tank level measurement and overfilling Hazards associ-ated with operating procedures are important to considerwhen designing tank farms Proper management of the

DOI 101002prs Process Safety Progress (Vol32 No1)32 March 2013 Published on behalf of the AIChE

storage area can prevent incidents Level measurement isvital to preventing overfilling The loss of level control hascontributed to three significant industrial incidents

1 In Australia the Esso Longford explosion in September1998 resulted in two fatalities eight injuries and morethan $1 billion in losses

2 In the United States the BP Texas City explosion inMarch 2005 caused 15 fatalities and more than 170 inju-ries profoundly affected the facilityrsquos production formonths afterward and created losses exceeding $16billion

3 In the United Kingdom the Buncefield explosion inDecember 2005 injured 43 people devastated the Hert-fordshire Oil Storage Terminal and led to total losses ofas much as $15 billion

Tank filling requires proper procedures and protectionsystems Overfilling usually leads to major accidents [9] Thekey causes of this failure are lack of hazard recognitionunderestimation of overfilling frequency insufficient trainingfor operators ill-defined safety fill limits and lack of applica-ble mechanical integrity

Atmospheric tanks (internal design pressure gt25 psig)should be vertical cylinders constructed above ground A ver-tical fixed-roof tank consists of a cylindrical metal shell with apermanently attached roof that can be flat conical or dome-shaped among other styles [13] Fixed-roof tanks are used tostore materials with a true vapor pressure less than 15 psiaThese tanks are less expensive to construct than those withfloating roofs and are generally considered the minimum ac-ceptable for storing chemicals organics and other liquids

Technical Protection for Overpressure and Overfilling

Overpressure and vacuum protection Each fixed rooftank should be provided with proper over pressure and underpressure devices These devices must be properly sized forthe worse possible conditions and be periodically inspectedto assure that they are serviceable Vents must be tamperproof Vents must be checked for buildup or choking Noalterations must be made to the vents or the relief deviceswithout the use of Management of Change policies

Overfilling and tank capacity Before designing or select-ing a tank the capacity needs to first be determined Thetotal capacity is the sum of the inactive capacity and theoverfill protection capacity The inactive working capacity isthe volume below the bottom invert of the outlet nozzlenormally at least 10 inches above the bottom seam to avoidweld interference [10] The net working capacity is calculatedas the volume between the low liquid level and the high liq-uid level For an in-process tank the net working capacity iscalculated by multiplying the required retention time of theliquid by its flow rate In some cases the requirednetworking capacity may be divided up into multiple tanks ifthe size of a single tank is physically unrealistic or if sepa-rate tanks are needed for other reasons such as dedicatedservices or rundowns

Tank Spacing and Facility Siting [11]When discussing the safety of tank farms it is of upmost

importance to take into account the siting of the entire areaThe relative positions distances and levels control for eachvessel are just some of the factors that should be taken intofurther considerations The impact of potential incidents mayalso be addressed by the following factors among others

Adequately separating tanks Segregating different risks Minimizing the potential for an impact or explosion

Minimizing the potential for and exposure to toxicreleases Maintaining adequate spacing for emergency personnel

including firefighting Minimizing the exposure to fire radiation Considering the prevailing wind directions in site layout Considering potential future expansions during site

layout

The relative elevation of a site area is important to considerwhen designing site layout Whenever practical locate openflames (process units with heaters direct fired utility equip-ment) at higher elevations than bulk quantities of flammables(tanks and storage) this minimizes the potential for the igni-tion of vapor releases or liquid spills as spills will migratedownhill [22] Where it is not feasible to locate storage tanks atelevations lower than process areas increased protectionmeasures may be required to offset the increased potential forignition These measures may include dikes high-capacitydrainage systems vapor detection increased fire protectionshutdown systems and other safety systems NFPA 30 [14] hasmentioned series of tank spacing according to the category oftanks tank body diameter and operating conditions For spe-cifically large oil tanks the spacing shall be paid special atten-tion to From a study by Meng et al [15] that when thecapability of oil tanks significantly increases the risks substan-tially increase due to high property density and energy densitythus people cannot ignore the risk anymore The Dominoeffects regarding the safety performance increases as the tankcapability increases Standards still require updates as the tankspacing and siting issues come to ultra-large oil tanks

As a summary the hazards of tanks and storage area canbe drawn into a fishbone figure as Figure 1 The operationinstallation and related designmaintenance are important inthe hazard analysis and loss prevention

REGULATIONS ABOUT TANK AND TANK FARMS

Among the previous sections we had mentioned the haz-ards and technical protections regarding tank and tank farmWe also referred to regulation requirements pointing out fail-ures and guiding inspection procedures In this section thestudy offered more overall regulatory standards Tank stand-ards are written by committees comprised of individualsfrom tank operating companies tank building companiesand consultants The multiple standards that apply to aboveground storage tanks are as follows for reference

API Standard Related to TanksAPI Standards 650 653 and 620 are the primary industry

standards by which most aboveground welded storage tanksare designed constructed and maintained These standardsaddress both newly constructed and existing ASTs used inthe petroleum petrochemical and chemical industries API650 is considered to be the most general and widely usedstandards for inspector certification and procedure of tankdesign installation operation and evaluation compliance

There are other related standards which are compensationof the instrumentation guides such as 2000 2517 and 2550[19] These guidelines point out significant issues for meas-uring tank capacity containments and construction work ofASTs

API-650 Welded Steel Tanks for Oil StorageAPI-651 Cathodic Protection for Above Ground Petroleum

Storage TanksAPI-652 Lining of Above Ground Petroleum Storage

TanksAPI-653 Tank Inspection Repair Alteration and

Reconstruction

Process Safety Progress (Vol32 No1) Published on behalf of the AIChE DOI 101002prs March 2013 33

API-620 Design and Construction of Large Welded LowPressure Storage Tanks

API-2000 Venting Atmospheric and Low-Pressure StorageTanks

API-2517 Evaporating Losses from External Floating RoofTanks

API-2519 Evaporating Losses from Internal Floating RoofTanks

API-2350 Overfill Protection for Petroleum Storage TanksAPI-2015 Cleaning Petroleum Storage TanksAPI-2550 Measurements and Calibration of Petroleum

Storage Tanks

For Individual Certification Program of inspectors thereare three mostly required API items

API-510 Pressure Vessel Inspection CodeAPI-570 Piping Inspection CodeAPI-653 Tank Inspection Repair Alteration and

Reconstruction

Non-API Codes Related to TanksExcept for API codes other institutes consulting tank and

tank farm sustainability also provide reference tools and sug-gestions for handling hazardous materials regarding storagetanks NFPA-30 mentions important factors of tank spacing anddepot arrangements CFR standards include mostly oil or chem-icals pollution prevention and bulk material storing issues

NFPA-30 National Fire Protection Association Flammableand Combustible Liquids Code

EPA-40 CFR 112 Oil Pollution Prevention and ResponseNon-transportation related onshore and offshore facilities

DOT-33 CFR 154 Facilities transferring oil or hazardousmaterials in bulk

DOT-33 CFR 155 Oil or hazardous material pollution pre-vention regulations for vessels

DOT-33 CFR 156 Oil and hazardous materials transferoperations

DOT-49 CFR 172 Hazardous materials provisions com-munications emergency response training requirementsand security plans

OSHA Standards

Frequently Cited

The following standards were the most frequently citedby Federal OSHA from October 2009 through September2010 in Oil And Gas Field Services (SIC code 138)

Guarding floor and wall openings and holes Permit-required confined spaces Mechanical power-transmission apparatus Logging operations Specifications for accident prevention signs and tags Abrasive wheel machinery

Other Highlighted Standards

General Industry (29 CFR 1910)

1910 Subpart D Walking-working surfaces Fixed industrial stairs 1910 Subpart E Exit routes emergency action plans and

fire prevention plans Emergency action plans 1910 Subpart G Occupational health and environment

control 1910 Subpart H Hazardous materials Flammable and combustible liquids Process safety management of highly hazardous

chemicals Hazardous waste and emergency response Standard

In general ASTs are primarily regulated by states laws aswell as the overall design rules regulated by federal guide-lines The requirements can usually found in environmentalregulations andor the fire code There may be a require-ment to paint the tank or a fill pipe a certain color to

Figure 1 Fishbone risks about tank farm (Adapted from Ref [1])

DOI 101002prs Process Safety Progress (Vol32 No1)34 March 2013 Published on behalf of the AIChE

identify its contents provide alarms in addition to thosefederally required and registration of the tank

FUTURE WORK ON TANK LOSS PREVENTION

The important good work practices are important to bothfacility and surrounding area A good safety integration pro-gram with strict installation code review operation mainte-nance and follow-up evaluation shall be done by thefacilitators With the need of new technology in the futurethere are several parts needed to be taken into consideration

New Corrosion-Proof and Chemical-ResistantMaterials

ASTM standards include discussion concerning appropri-ate corrosion-resistant materials Many studies also focus ontank design and material selections ASTM D3299 mentionedthe specifications of commercial storage tanks and theirbuilding materials glass-fiber-reinforced polyester or vinyl-ester thermoset resin fabricated by filament winding forabove-ground vertical installation which were likely chosento contain aggressive chemicals at atmospheric pressurewhich is the usual operation area conditions

The corrosion study is not only important for tank bodybut also for the surrounding dikes connected piping sys-tems and coatings near monitoring devices Choosingadequate tank body materials is the first step The materialshall be compatible to the chemicals stored inside What canbe done and controlled except for original materials shall bethe exterior addition of corrosion prevention Anodes are thebetter direction for most of the storage tanks They areusually long-line flexible cable-like anodes placed in con-tinuous close proximity to the target structure and provideuniform distribution of current The anodes allow current toflow long distances down the center conductor while allow-ing sufficient cathodic protection current to continuouslypass through the conductive polymer all along the length ofthe anode The fabric materials better conductive polymerswhich can prevent being corroded and a tight adhesion tothe target tanks are the issues future research shall focus on

Good Measuring Methods and Tools toPrevent Overfilling

There are many facilities and companies trying to find outa better solution for tank gauging with the instrumentationproved for least interruption to the system Radar tank gaugeis one potentially useful method which can be further devel-oped into quicker more accurate reading values There aretwo defined methodologies initiated by TankGauging Inc asin Figure 2 which are to improve measurement for overfill-ing single point measurement and continuous measurementInstallation and integration of third-party equipment fromindustry leading manufacturers into a central system of con-trol for remote monitoring and alarm in addition to therequired local alarms at the crucial process value point

Single point measurement devices can be installed at thehigh level Common technologies used are mechanicalswitches electronic conductor or optical sensors For contin-uous measurement a standard level gauge is a commonsolution Any of the major gauging technologies can beused float and tape servo radar or magneto-astrictive tech-nologies The process shall be kept in a smooth surfacewhile taking measurement which is usually in the prestartupcheck or normal pressure conditions

Optimization Control [19]Facilities with storage tank area shall consider a well-distrib-

uted system including control center response system monitor-ing devices and unit siting Especially in tank farms the

production storage transport and evaluation of tanks and con-taining chemicals shall be considered as a target for an optimiza-tion control program The proven solution for liquid bulk storageand distribution is an uprising issue nowadays in tank field Thenew technology gives tank farm the automation at most but thenear-miss events or actually incidents still happen even with theleast portion of human error participation This is why we haveto consider a best optimized way to deal with bulk storages

Optimization in tank farm can be separated into severalaspects optimized safety optimized reliability optimized tanklife sustainability and optimized automation with communica-tion and management assurance Safety integrated level is usedand studied as a criterion of tank management in automationinstruments Flow-weighbridges hydraulic fluid piping leakdetection (pump stations sensor and alarm) and pressure con-trol (valves fittings and indicators) should be combined to-gether as a whole program to design and run a storage fieldEach ldquonoderdquo of the control line should be clearly defined andcommunicated to employees while all the online workers getadequate training and proper work shifts The automation sys-tem should also consider the fire protection sprinklers andother related emergency response that a facility needs to have

LITERATURE CITED

1 JI Chang and C-C Lin A study of storage tank acci-dents J Loss Prevention Process Ind 19 (2006) 51ndash59

Figure 2 Overfilling prevention measurements with radardevices (a) single point measurement (b) continuous mea-surement (TankGauging Inc Tank System InstrumentationDesign) [Color figure can be viewed in the online issuewhich is available at wileyonlinelibrarycom]

Process Safety Progress (Vol32 No1) Published on behalf of the AIChE DOI 101002prs March 2013 35

2 CSB Seven Key Lessons to Prevent Worker Deaths Dur-ing Hot Work In and Around Tanks February 2010

3 CSB CSB Conducting Full Investigation of Massive TankFire at Caribbean Petroleum Refining Available athttpwwwcsbgovnewsroomhtm Accessed No-vember 17 2009

4 P Myers Aboveground Storage Tanks McGraw-Hill NewYork 1997 519ndash562

5 TV Rodante Investigation of a naphtha storage tank fireProcess Saf Prog 24 (2005) 98ndash107

6 J Lieb Updated on API 650 App E Seismic ZoneDesign September 2006 API meeting proceedings

7 Alyeska Pipeline Service Company Fire Hazard Assess-ment for Valdez Crude Tank Internal Floating Roofs FinalProject Report 2004

8 LG Britton and HL Walmsley Static electricity Newguidance for storage tank loading rates Process Saf Prog31 (2012) 219ndash229

9 A Summers Donrsquot Underestimate Overfilling Risks SIS-TECH Solutions Chemical Processing article 143 2010

10 FM Davie PF Nolan and TWS Hoban Case historiesof incidents in heated bitumen storage tanks J Loss Pre-vention Process Ind 7 1994 pp 217ndash221

11 BP Exploration amp Production Inc BP Tank Farm and(Un) Loading Safe Operations Manual Booklet ICheme2004

12 Capstone Engineering Valdez Crude Tank Internal Float-ing Roofs for Alyeska Pipelines Anchorage Alaska FinalReport Rev 0 January 29 2004

13 Yacine Amrouche Chaitali Dave Kamal Gursahani Rosa-bella Lee and Lisa Montemayor General rules for above-

ground storage tank design and operation ChemicalProcessing 2002

14 NFPA 30 Flammable and Combustible Liquids Code 2000Edition National Fire Protection Association 2000

15 M Yifei Z Dongfeng L Yi and W Wendong Study onperformance-based safety spacing between ultra-large oiltanks Process Saf Prog 31 (2012) 398ndash401

16 C Daniel and Joseph F Louvar Chemical Process SafetyFundamentals with Applications 2nd ed Prentice HallUpper Saddle River NJ 2002

17 SA Manatt Fuel tank inerting system United States Pat-ent 4556180 1985

18 WR Kanne Jr DA Lohmeier KA Dunn and MH Tos-ten Metallographic analysis of helium-embrittlementcracking of repair welds in nuclear reactor tanks MaterCharacterization 30 (1993) 23ndash34

19 RRY03 Societal RiskmdashInitial Briefing to Societal Risk Ad-visory Group HSE Research Report 2009 UK

20 The Buncefield Incident Final Report of the Major Inci-dent Investigation Board Vol 1 2008

21 S Mannan A Technical Analysis of the Buncefield Explo-sion and Fire Proceedings of the HAZARDS XXI Confer-ence Manchester Conference Centre Manchester UnitedKingdom November 10ndash12 2009

22 M Sawyer Tank Farm Safety Presentation to theMKOPSC Steering Committee August 3 2010

23 OR Jaiswel DC Rai M and SK Jain Review of CodeDivisions on Design Seismic Forces for Liquid StorageTanks IITK-GSDMA-EQ01-V10 IITK-GSDMA Project onBuilding Codes

DOI 101002prs Process Safety Progress (Vol32 No1)36 March 2013 Published on behalf of the AIChE