position papers 1 - atmospheric storage tanks

8/10/2019 Position Papers 1 - Atmospheric Storage Tanks

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Risk Engineering Position Paper:Atmospheric Storage TanksFebruary 2011

Energy


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Section Title .....................................................................................................................................Page

1. Background ........................................................................................................................................3

2. Objective ............................................................................................................................................3

3. Scope ..................................................................................................................................................3

4. Selection of Atmospheric Storage Tanks ......................................................................................4

5. Layout and Spacing ..........................................................................................................................5

5.1 Bunds (Dykes) ....................................................................................................................................6

6. Primary Containment ......................................................................................................................7

6.1 Tank Design & Operation ................................................................................................................7

7 Secondary Containment ................................................................................................................10

7.1 Bund Integrity (leak tightness) .....................................................................................................10

7.2 Tertiary Containment ....................................................................................................................10

8 Overfill Protection ...........................................................................................................................11

9. Maintenance and Inspection ........................................................................................................1210. Leakage and Fire Detection ...........................................................................................................13

10.1 Leak Detection ................................................................................................................................13

10.2 Fire Prevention and Protection .....................................................................................................13

11. Fire Fighting Systems .....................................................................................................................15

11.1 Water Systems ................................................................................................................................15

11.2 Foam Fire Fighting Systems ..........................................................................................................15

11.3 Application Rates ............................................................................................................................18

12. References to Industry Losses ......................................................................................................18

13. References to Industry Standards ................................................................................................19

14 Appendices ......................................................................................................................................20

Appendix A - Assessment Checklist............................................................................................20

Appendix B - Mechanical Component Checklist for Floating Roof Tank Inspections .........21

Appendix C - Loss Incidents .........................................................................................................22

Contents


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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 USAthat indicate that overfilling of atmospheric storage tanks occurs once in every 3300 fillingoperations. In 2009 there were two separate incidents just days apart, one in Jaipur, India(October 29) and one in San Juan, Puerto Rico (October 23) that demonstrated the destructivecapabilities of incidents at terminals 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, moreguidance has been provided to designers and operators of facilities. Such guidance has beenincluded within this position paper.

2. ObjectiveThe objective of this position paper is to define the standards that would be expected of a verygood atmospheric storage facility in the oil, gas and petrochemical industry. These standardsare also reflected in the Marsh Energy Risk Ranking criteria. They can be used to determinerisk improvement opportunities and also to provide detailed advice to clients seeking toimprove their atmospheric storage facilities.

3. ScopeThe scope of this position paper includes:

Guidance on the appropriate selection of atmospheric tank design for class of product to bestored

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


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4. Selection of Atmospheric Storage 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 Oil

Derivative Class

Definition Recommended Storage

Tank Design

Floating Roof Fixed RoofClass 0* Liquefied petroleum gases (LPG)

Class I Liquids which have flash points below

21°C

Class II (1) Liquids which have flash points from

21°C up to and including 55°C, handled

below flash point

Class II (2) Liquids which have flash points from

21°C up to and including 55°C, handled

at or above flash point

Class III (1) Liquids which have f lash points above

55°C up to and including 100°C,

handled below flash point

Class III (2) Liquid which have flash points above

55°C up to and including 100°C,

handled at or above flash point

Unclassified Liquids which have flash points above

100°C


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5. Layout and SpacingIdeally, tank layout should be optimised to ensure that there is sufficientaccess to tanks for fire fighting and to minimise the risk of escalation in theevent of a tank fire.

Minimum spacing for tanks is specified in the table overleaf, although Marshwould advocate a minimum separation distance of 1 x the diameter of thelargest tank with an absolute minimum of 15 metres, as such a distanceprovides sufficient access for fire fighters.

Factor Minimum separation from any part of the

tank

Between adjacent fixed-roof tanks Equal 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 tanks 10m for tanks up to and including 45m diameter

15m for tanks over 45m diameter

(The spacing is determined by the size of the larger tank)

Between a floating-roof tank and a

fixed-roof tank

Equal 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 a group of small tanks and

any tank outside the group

15m

Between a tank and the site boundary,

any designated non-hazardous area,

process area or any fixed source of

ignition

15m

Source: HSG 176. The storage of flammable liquids in tanks

One notable exception is the separationbetween crude tanks, where thedestructive effects of a boil overcan extend from 5 to 10 diameters.Therefore consideration should be givento locating crude tanks at the edgeof tank farm installations and withthe largest practical separation fromadjacent tanks.

These distances should only be used

in conjunction with appropriate levelsof fire protection (see below). In the

event that tanks are existing and donot conform to the above spacing, thenadditional fire protection should beconsidered.

Floating roof tanks, with externalmetal domed roofs extending over theentire roof area (i.e. internal floatingroof tanks or tanks fitted with geodesicdomes), may be considered as fixed rooftanks for the purpose of tank location

and spacing.


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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. Thereforethe bund wall should be located so

that a reasonably close approach canbe made to a tank fire.

Fire fighting screens and stepsshould be located at various pointsaround the bund wall to assist thepositioning and protection of firefighting personnel and equipment.

Bunds would normally beconstructed from earth or preferablyconcrete and should be largelyimpervious to liquid and capableof withstanding hydrostatic andhydrodynamic pressures to whichthey could be subjected. Earthen

bunds should be fitted with a waterresistant liner and whilst grassgrowth is acceptable, it should beregularly maintained and kept short.

Tanks should ideally be locatedalone within their own bunds,although whenever tanks share a

common bund, intermediate wallsup to half the height of the mainbund walls and no more than 0.5metres high should be provided tocontrol small spillages from onetank affecting another.

Bund floors should drain to a singlelocation complete with sump forthe regular removal of water fromrainfall or firewater testing. Drainsshould normally be kept closed,with the drain isolation valvesituated outside the bund.


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It is essential that the risk of loss of containment is properly managed. This can be achieved by theproper design, operation, maintenance and inspection of tanks. Ideal practice is outlined below.

The most effective way to prevent a major accident at any site is the continued provision of suitableprimary containment of the flammable materials. This is achieved through the suitable design,construction and maintenance of the storage systems in accordance with standards.

For guidance on the most appropriate standards to use for the design and construction of atmosphericstorage tanks, refer to the UK HSE’s review of standards, Mechanical integrity management of bulkstorage tanks which can be found at the following website address:

http://www.hse.gov.uk/research/rrpdf/rr760.pdf

Outlined below is a summary of the features considered to be the most appropriate for a modern facility.

6. Primary Containment

6.1 Tank Design and OperationTanks should be designed to arelevant standard, such as API 650or BS EN 14015.

In some instances single skinbottom tanks are a better optionthan double-bottomed tanksas these provide the optimumconditions for ensuring integrityof the tank floor by the inspectionof tank floor plates. Disadvantagesof double-bottom designs includesettlement, product entrapmentand modification to nozzlecompensating plates.

Where double-bottomed tanks areprovided, additional inspectionmeasures should be provided inaccordance with a relevant standardsuch as EEMUA 183 or BS EN 14015.Leak detection should also beprovided on double-bottomed tanks.

Tank RoofsDouble deck roofs on floating rooftanks are preferred to single deckroofs with external pontoons asthey are more stable and less likelyto lose buoyancy.

For floating roof tanks, the waterdrains to remove rainwater fromthe roof should be normally closed.To ensure roofs are emptied ofrainwater, procedures should be inplace and followed for the openingand re-closure of valves.

Whenever valves are left normallyopen, then the following measuresshould be put in place:

Hydrocarbon detection indrained water with automatic

isolation Testing of operability of isolation

valves and detection systems(see overleaf)

VentsTanks should be provided witha weak seam/frangible roofconstruction, or with an emergencyvent suitably sized for the worstcase relieving scenario, to preventoverpressure under all relief

conditions. Emergency vents shouldcomply with relevant standards,such as API 2000.

Wherever a fixed roof tank is usedfor the storage of materials with aflash point below 21°C (generallyon day tanks, not recommendedfor large bulk storage tanks),then flame arresters should beprovided on the vents to preventignition of the flammable vapoursburning back into the tank. In

warmer climates, where productsurface temperatures could exceed

Double seals on floating roof tanks (NB foamdam heights should be above the heightof the upper rim seal to provide suitableprotection against a rim seal fire)

High level alarm and independent back-uphigh level instrument (with executive actiononly if appropriate)

Continuous automatic monitoring of tankcontents, including ‘rate of change’ duringfilling/emptying

Anti-rotation devices on floating roof tanks

Closed water drains

Single skin floors

Tank floor leak detection on double-floortanks. This is normally comprised of aninstrument to detect loss of vacuum in theinter-floor space.


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21°C, flame arresters should be

used wherever maximum surfacetemperatures could be within 10°Cof the flash point.

Flame arresters should be includedin preventative maintenanceroutines to ensure they do notbecome blocked by scale, paint, iceor other materials. Flame arrestersare not recommended for use whenthe material being stored is liableto polymerise or foul the arrester.Due to the potential for blockage,

conservation vents (vacuum andpressure relief) should not be fittedwith flame arresters.

Fixed roof tanks can be fitted witha gas blanket (normally nitrogen)to maintain an inert atmospherein the vapour space. Nitrogensupply pressure should be justabove atmospheric, but sufficientto displace any liquid pumped out.As a back up, a vacuum breakershould be provided in the event that

nitrogen supply is lost.Vents or vapour recovery systems(often venting back to the sourcevessel) are required. These shouldbe designed to relieve pressureslightly above that of the nitrogenand at a suitable margin below thedesign pressure of the storage tank.

Rim Seals & Foam DamsDouble rim seals (of fire-resistant

construction) are preferable tosingle seals. Due to irregularitiesduring the construction phase oflarge floating roof tanks, it is likelythat a single seal will not maintaina 100% seal between the tank roofand tank wall along the entireheight of the tank wall. Therefore,a second seal will improve thelikelihood of achieving a tight seal,will reduce emissions and minimisethe risk of rim seal fire.

When installing double rim seals,the height of the second, upper rimseal should be below the height of

the foam dam to ensure coverage

whenever rim seal pourers areactivated.

Foam dams should be providedwith intermittently-spaced gapsto allow the drainage of rainwater.However, if the gaps are too large ortoo numerous, they may affect theability to form a single continuousfoam barrier in the event of a fire.

Overflow

Consideration should be givento providing suitable overflowsystems to ensure that in theevent of a tank overfilling, the tankcontents are safely routed intosuitable secondary containment.Additionally, the overflow routeshould be designed to minimiseturbulent flow, reduce the surfacearea of flammable hydrocarbon andreduce the generation of flammablevapours, such as through splashing.Particular care needs to be taken

with strengthening rings andfirewater dispersion rails aroundtanks which are specificallydesigned to maximise spread offluids over a tank’s surface.

For existing tanks, considerationshould be given to modifications oftank top design and to the safe-re-routing of overflowing liquids.

Valves

All 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 mustbe fitted close to the tank on boththe inlet and outlet connections.Valves must either conform toan appropriate standard, suchas BS EN ISO 10497 or equivalent

international design, or should be ofintrinsically fire-safe design. Suchfeatures include:


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Metal-to-metal seats (secondary

metal seats on soft-seated valvesare acceptable)

Must not be of cast-ironconstruction

Should not be wafer-bolted(sometimes referred to as longbolt flanges)

In regions subject to a high risk ofearthquake and/or rapid ground-acceleration, the isolation valvesand actuators should also be

secured directly to the tank walland not to the ground. Anchoringthe valve to surrounding structuresor the ground could result in thevalve separating from the tankduring excessive vibrations.

Features of the ROIV should be:

Fail safe (or if not fail safe have aback-up power supply, especiallyif the emergency plan requiresthe tank to be drawn down in anemergency)

It should not be possible to auto-reset the ROIV

Adequate margin of safety forshutting off the valve, with atleast 150% torque available fromfully open to fully closed

No manual operation or override(e.g. hand wheels) which mayinhibit the operation of the ROIV

Suitable integrity andperformance to satisfy the

SIL (Safety Integrity Level)requirements

Designed to minimise pressuresurges on system pipework andcouplings, particularly ship toshore flexible 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 drain

should be achieved by the use

of two valves in series. Thesecond valve can be a temporaryinstallation.

PipingIn areas where earthquakeis a significant exposure, allassociated piping on storagetanks should have stepgeometry to allow flexingin the piping and preventpuncturing of the tank during anearthquake.

Suitably designed seismichangers should be providedin earthquake zones, and pipesupports should have fireprotection within fire hazardzones, e.g. within the bundedarea.

TestingBefore filling tanks withflammable liquids, leak testingof the installed tank andassociated pipework is required.

Hydraulic testing should onlybe used (i.e. not pneumatic) asstored energy in hydrotestingis substantially lower, andinherently safer. However, airmay be used as a means ofapplying pressure to water-filledtanks and piping.

Salt water should not be usedto hydraulically test systemscontaining stainless steel.

HousekeepingDebris within bunds should bekept at an absolute minimum.In addition to the checkswithin formalised inspectionand maintenance routines,operators should also conductfrequent checks of tanks andtheir components as part oftheir routines to ensure that thetanks are kept in a reasonablecondition. A sample of items tocheck is included in Appendix B.


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Bund penetrations should beavoided unless alternative over-wall routings are not practical. On

existing bunds, fitting steel collarsor bellows to improve fire resistanceat pipework penetrations mayintroduce local corrosion initiationsites in the pipework and is notrecommended unless corrosionprevention can be assured. Wherecorrosion can not be prevented, joints should be improved byreplacing existing sealants withfire-resistant sealants.

Bund floor penetration joints areinherently weak as failure of the

integrity is difficult to predict anddetect and may continue for sometime unnoticed. Consequently,floor penetrations should not beincorporated into new bund designs.Existing bund floor penetrationsshould be eliminated whereverpracticable. Where flexible sealantsare used in floor penetration joints,these should be replaced with fire-resistant sealants.

There have also been a numberof significant accidents resultingfrom leaks of hydrocarbons fromtanks through the base of storagetanks. Therefore the provision ofa suitable impervious base (ideallyconcrete with a membrane liner) orfloor leak detection on double floorconfigurations should be provided.

7.2 Tertiary ContainmentTertiary containment need only beconcerned if there is a significant

risk of secondary containmentbeing insufficient to prevent anescalation to a major accidentaffecting personnel, assets orthe environment. Firewatercontainment is one such example.Implementation of tertiarycontainment should therefore bea risk-based decision. However,tertiary containment should be:

Independent of secondarycontainment

Capable of fully containingforeseeable firewater and liquidpollutant volumes on the failureof secondary containment

Impermeable to water andforeseeable entrained pollutants/hydrocarbons

Of cellular configuration to allowsegregation and limit the extentof spread of pollutants and/or fire

Robust under emergencyconditions, e.g. loss of electricalsupply

Capable of allowing thecontrolled movement ofcontained liquids under normaland emergency conditions

Capable of aiding the separationof water from pollutants (e.g. oil/water separator)

Able to manage rainwater andsurface waters

More guidance on tertiarycontainment can be found indocumentation including CIRIA 164

and PPG18.

7.1 Bund Integrity (leaktightness)Bund wall and floor construction

should be leak tight. This includesthe provision of leak tightexpansion joints between differentcasts of concrete in bund walls andwherever there are penetrations inthe wall for pipes. Surfaces shouldbe maintained crack free, andwithout any discontinuities, andwithout any failed joints that mayallow liquid migration.

As observed in major incidents, suchas the Buncefield fire of 2005, the

joints in concrete bund walls are

particularly susceptible to the effectsof fire and/or subsequent cooling.Therefore, to maintain integrity,

joints should be capable of resistingfire. Ideally, steel plates (waterstops)should be fitted across the innersurface of bund joints and firesealants should be used to replaceor augment non-fire resistantmaterials. Similarly, joints to walland floor penetrations need to beprotected against the effects of fire.

Whenever designing protection plate,consideration should be given toavoiding weakening the wall structurein relation to resistance to fire,hydrostatic and hydrodynamic forces.

7. Secondary ContainmentWhilst priority should be given to preventing loss of primary containment, adequate secondary (and sometimestertiary) containment is necessary for the protection of the environment and to contain any spillages. Inatmospheric tank applications, 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 thecapacity of the largest storage vessel within the bund will normally be sufficient. When estimating bund capacity,the space occupied by other tanks should be taken into account.

Typical design of the steelplates are:

Material of construction Stainless steel

Width Minimum 200 mm

Thickness Minimum 6 mm

Fixings to bund walls Stainless steel bolts through oversized slotted holes, minimum of

30 cm intervals

Additional protectivefeatures

fireproof backing material such as cement and board;

and/or

fire resistant coating such as intumescent material to the front face


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All tanks should be fitted witha suitably high integrity overfillprotection system, which shouldbe designed to SIL 1 as a minimum,depending upon the risksassociated with failure.

Overfill protection systems shouldbe automatic and physically andelectronically separate from thetank gauging system, i.e. havean independent level gaugingsystem and back-up level switch orduplicated level gauging system.

However, justification for a singlecombined gauging and overfillprotection device could be made ifthe integrity of the instrument issufficient for the risk of failure.

Tank gauging systems should havea LAH (High Level Alarm) to alertoperators to the status of the tankwhich gives sufficient time tointerrupt the filling operation andthe subsequent activation of theback-up overfill protection deviceLAHH (High-High Level Alarm).This should also take into accountany thermal expansion of the fluidwithin the tank.

It is important that LAH should notbe used to control routine filling.

Analogue sensors are preferred todigital (switched) sensors (i.e. thereare two analogue sensing elements

on the tank), as these are able toalert whenever there is a fault inthe primary level sensor beforereaching high level.

Electro-mechanical servo gaugesshould be avoided for use withflammable materials as these areintricate devices considered to bevulnerable to a number of potentialfailure modes. Modern electronicgauge sensors, such as radargauges, should be used instead.

Where practical the LAHH shouldhave an executive action tointerrupt the filling operation byclosing the isolation valve and/orstopping the pump. Features of aLAHH are:

It should be set at or below thetank’s rated capacity

Activation of the LAHH is toinitiate a shutdown

LAHH may be limited to anaudible/visual alarm to alerta human operator to rake therequired action. Actions requiredmust be specified and welldocumented. NB – such optionsare not suitable for SIL 1 systems

The trip function should includean audible/visual alarm toprompt a check that the tripfunction has been successful.

In some cases it may be necessaryto terminate the transfer in amore gradual fashion, such as bylimiting the closure rate of theisolation valve to avoid damagingoverpressures in upstreampipework. Due allowance must begiven to the maximum filling ratebetween LAHH being activated andthe isolation valve finally closing.

The site receiving any batch ordelivery must have ultimate controland responsibility for stopping

the transfer and should have localsystems and valves to stop theoperation. This should not thereforebe under the sole control of anautomatic system or operators at aremote location.

Tank level instrumentation andinformation display systems shouldbe of sufficient accuracy and clarityto ensure safe planning and controlof product transfer into tanks.

It is preferable for tanks to becontinuously and automaticallymonitored for rate of level changeduring transfer operations to matchthe actual rate of transfer withexpected rates. This would give anearly indication that the main levelgauging system was not functioningcorrectly or that the incorrect tankwas being filled. During normalstorage operations, such a tool

could also notify operators of asignificant loss of containment.

It is not within the scope ofthis document to detail the SILdetermination or its implications.However the following pointsshould be noted:

It is the Dutyholder’sresponsibility to meet latestinternational standards, IEC61511 is a current good standard

The appropriate SIL requiredshould be specified beforedesigning and installing overfillprotection systems

LOPA (Layer of ProtectionAnalysis) is a suitablemethodology to determine SIL

As overfill protection systemsnormally function infrequently,they should be periodicallytested in their entirety to ensurethey will operate when required.

8. Overfill Protection


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All tanks must undergo regularexternal and internal (out-of-service) inspections to ensure theirintegrity and as such, a writtenscheme of examination should beprovided. Overall guidance on thesuitable methods and frequencyof such inspections are detailed inEEMUA 159 and API 653.

Individuals responsible for

the formulation of schemes ofinspection and for the inspectionof tanks should be competent andhave the appropriate qualificationfor tank inspection and forthe grade of materials used inconstruction. As a minimum,personnel should be competentto TWI’s (The Welding Institute’s)Certification Scheme for Weldingand Inspection, or equivalent.

External inspections should beconducted on a higher frequencythan internal inspections basedupon RBI (Risk Based Inspection)requirements, or as stated in therelevant codes. If active degradationmechanisms are found, then morefrequent inspections should beconducted.

Particular attention should begiven to insulated storage tanks,as corrosion under insulation, andthe quality of external coatingsapplied prior to insulation, can

have a significant effect upon tankintegrity.

Thorough internal inspection canonly be achieved by removing atank from service, cleaning it andthen conducting, as a minimum:

Full floor scan

Internal examination of shell toannular floor weld

NDE (Non DestructiveExamination) of annular plateand shell nozzles

Visual inspection

Deferral of internal inspectionsshould be risk assessed andapproved by a suitably competentperson. Particular attention shouldbe given to tanks that have hadno previous internal inspection, asthe probability of floor failure willincrease with every year that therecommended interval is exceeded.In such cases, it is unlikely that adeferral could be justified.

Failure of either a top or bottomfloor in double-bottomed tanks(as detected through the failure ofvacuum between plates) should berectified within one year. Continuedoperation in the interim periodpending repair should be supportedby a technical justificationsupporting its fitness for purpose.

Tank repairs and modifications arespecialist activities and should onlybe conducted by suitably qualifiedpersonnel with appropriatequalification (as specified above).

Water drains on floating roof tanksshould also be formally includedin routine maintenance if normallyleft open, including a check thatisolation valves can be closed. Thedetection systems for hydrocarbonsshould also be tested on a regularbasis. All tests should be formallydocumented.

Pontoons should be included informal maintenance and inspectionregimes as loss of pontoons (as aresult of corrosion or long-termremoval of pontoon cap) will affectthe stability and buoyancy of thedeck. The presence of flammablevapours within a pontoon alsoincreases the risk of vapour cloudexplosion (VCE) within a pontoon.

The functional integrity of overfillprotection systems is critical toensuring primary containment.Overfill protection systemsmay be in a normally dormantstate without being required tooperate for many years. For thisreason, periodic testing is anessential element in assuring theircontinuing integrity.

9. Maintenance and Inspection


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It should be noted that leakage,overflow and fire detectionare mitigation layers and notpreventative layers, and aretherefore of secondary priority tooverflow and leak protection.

10.1 Leak DetectionWhilst flammable gas detection instorage areas is considered to be agood measure (for highly flammablematerials), the dispersion offlammable vapours is complicatedto predict, so effective gas detectionmay not always be practical.

More effective detection of leaksis likely to be provided by liquidhydrocarbon detectors. Typicallocations would be in the bunddrain, gutter or sump, wherehydrocarbons would accumulate.Such systems may be subject to

failures or spurious alarms (e.g. dueto water collecting in the bund),therefore consideration should begiven to using multiple detectors atmore than one location. Initiationof such alarms should also initiatean interruption of any transferoperation. This is of particularimportance at terminals that arenormally unmanned.

Ideally CCTV (Closed CircuitTelevision) should be provided, with

suitable resolution and lightingin tank and bund areas to giveoperators assistance in detectingtank overflows. Operators shouldnot be expected to monitor theseconstantly, therefore CCTV shouldbe provided with systems thatdetect and respond to changes inconditions and then alert operatorsto those changes.

Fire DetectionFor floating roof tanks, rim sealsshould be constantly monitoredby the use of linear heat detection.These should be located close to

the top of the seal, and not on thefoam dam, where they are lesseffective in detecting fires.

Point heat detectors are generallyof less use for large storage tanks,other than in the vicinity of vents,given the large areas that need tobe covered. Therefore, optical flamedetectors, providing a large zoneof detection, should be used for

bunded areas.

10.2 Fire Prevention andProtectionBonding & EarthingBy nature of the storage offlammable liquids, flammableatmospheres are to be expectedin the vicinity of vents, rim sealsand within the tanks themselves.To minimise the risk of ignitionthe tank and ancillary equipment

should be fully bonded andearthed (grounded). A maximumresistance of 10 ohms to earth isrecommended.

Special attention should be madeto the earthing connection betweenthe tank wall and the roof of afloating roof tank. Sliding shunts(contactors) between the wall androof often do not maintain a goodcontact, thus overloading thoseshunts which are maintaining a

connection. Therefore, an adequatenumber of shunts should beprovided and checked to ensurethat contacts are maintained.

Due to the rising and falling ofthe floating roof, excess lengths ofearthing cable are required in thevicinity of the walkway providingaccess to the floating roof. Theearthing cable can often be severedwhen it is trapped between thewheels of the access walkway

and the rails upon which it rolls,as in example shown overleaf.Retractable grounding systems onmodern tanks would address this.

10. Leakage and Fire Detection


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One of the most common sourcesof ignition for large tanks islightning. Therefore, the provisionof suitable lightning protection withbonding and earthing would help tominimise this. NFPA 780 Standard

for the Installation of LightningProtection Systems provides moredetails on the specification oflightning protection systems.

Some installations are providedwith lightning masts intendedto act as preferential routes forlightning to discharge itself, asopposed to via the storage tank.This concept is not proven andsome parties suggest that suchmasts act to attract lightning tothe vicinity of the storage tanks.Therefore, these are not currentlyconsidered to be a robust mitigationagainst the effects of lightning.

Some materials are susceptibleto the effects of static duringsplash-filling. Therefore, measuresshould be taken in such instancesto prevent the generation ofelectrostatic charges.

It is possible that floating roofscan become unstable and sink,maybe as a result of rainwateraccumulation, or failure of flotationdevices. Often, foam is appliedto provide a blanket to suppressvapour emissions and preventignition. However, if this occurson tanks storing materials, suchas naphtha, that are susceptible toignition from splash-filling, carefulconsideration should be givenprior to the application of foam. Iffoam is to be applied, for examplein the event that electrical stormsare expected, then this should be

Damaged Earthing Cable

gently applied along the shell walland not by splash-filling to thesurface of the material.

ZoningA detailed assessment of thehazardous area classification(HAC) of the storage tank, ancillaryequipment and bunded area mustbe conducted. This should take intoaccount the class of material beingstored.

The specification of spark-inducingequipment should be suitablefor the HAC zones location andshould be made in accordance withrelevant electrical standards, suchas IP (Institute of Petroleum) ModelCode of Safe Practice Part 15, ATEX

95, ATEX 137 or BS 5345.


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Protection systems and fixed firefighting installations should beprovided and comprise fixed/semi-fixed foam systems andcooling water sprays. Additionally,access to the tanks for mobile firefighting appliances, equipment andmaterials should be provided givingmultiple points of attack with nodead-ends.

In many cases it is safer and morepractical to allow a tank fire toburn itself out; for instance, whenthere is no chance of escalation toother tanks, little environmentalexposure or where the risks offighting the tank fire are greaterthan not fighting it. Two notableexceptions are:

Rim seal fires – these will take along time to burn out

Crude tank fires – these aresusceptible to boilovers due tothe presence of water

Broad principles of water and foamapplication are given below. Moredetails are given in the EnergyInstitute’s Model Code of SafePractice Part 19: Fire precautionsat petroleum refineries and bulkstorage installations. Alternatively,NFPA 11 Standard for Low-,Medium-, and High-ExpansionFoam can be used as a reference.

As for process plant, dedicated fire-preplans should also be compiledfor storage installations. Theseshould specify type of attack,firewater and foam applicationrates, location of hydrants, semi-fixed installations and mobileapparatus, firewater containmentand worst case scenarios. It is notwithin the remit of this positionpaper to detail emergency plans,but API 2021, Management of

Atmospheric Storage Tank Fires,provides some guidance in thismatter.

Emergency response personnelshould receive training on the firepreplans, and regular exercisesshould be conducted to assess thesuitability of the preplan.

11.1 Water SystemsProvision should be made for theapplication of cooling water to fixed

roof tanks containing productsadjacent to those containing ClassI or II (petroleum) or whereverthere is less than 1 x diameter ofthe largest tank between adjacenttanks. In particular, there should becooling wherever there is less than15 metres access between tanks.

Where water is to be applied totanks, water application ratesshould be:

2 lpm/m² for exposed tanks

10 lpm/m² for engulfed tanks (tovertical and inclined surfaces)

Source:

IP 19. Model Code of Safe PracticePart 19:

Water application should beprovided by either fixed or semi-fixed systems. It is unlikelythat mobile tenders would havesufficient capacity for water storageto mount a prolonged cooling waterattack on a storage tank.

Water should not be appliedto the roof area of floating rooftanks as this may result in thedestabilisation and sinking of theroof.

11.2 Foam Fire FightingSystemsWhilst being a good coolingmedium for tanks, water is not asuitable fire extinguishing mediumfor hydrocarbon fires. Therefore,

11. Fire Fighting Systems


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foam is used for such applications.The design of foam fire-fightingsystems, be they fixed or mobile,should be suitable for the typesof fires they would be requiredto extinguish (or suitable for thecooling of adjacent equipment/tanks). Therefore each facilityneeds to have its own tailored firefighting system. Important issuesto consider when designing and

installing a fire fighting systeminclude:

Foam and the foam injection/proportioning systems must becompatible (also important if thesupplier of foam is changed)

Hydraulic design of foamsystems must be specified

Foam selection must beappropriate for the type of firelikely e.g. AFFF, (Aqueous Film

Forming Foam), should not beselected for polar solvents

Correct proportioning of foamthat may otherwise result infoam that does not flow (toolittle water) or that quicklygoes back to solution (too muchwater)

Provision of facilities to aidthe maintenance or testing offirefighting systems

Location of equipment, e.g.monitors not too far from tanksfor foam application to tanksurface

Attention to any faults in thefabrication and installationof hardware, e.g. check valvesinstalled the wrong way round,transport packing still presentaround air inductors

Once a system has been designedand installed, it is critical that

it is tested. Whilst this may beexpensive, it is critical to ensure

that the system actually completesthe job it is intended to do. Ideally,systems should be tested at themanufacturers as it is then cheaper,quicker and easier to rectify anymistakes.

Floating Roof Tank FiresRim seal fires are the mostcommon type of fire on floatingroof tanks. There is little chance of

these escalating to other tanks orturning to full surface tank fires ifthe design, maintenance and layoutof the tanks is suitable. This shouldnot be considered a rule, however,as a poorly maintained tank couldlose its integrity or be vulnerable tosubsequent explosions, perhaps asa result of vapour ingress into oneof the pontoons.

Rim seal protection should only beused on certain types of roofs (i.e.double deck, steel pontoon, etc).Pan roofs for example, should havefull surface protection.

The application of foam from aremote monitor (i.e. one outsidethe bund) would not representbest practice as it can create roofinstability and potentially escalatethe incident. Additionally, remotemonitors are wasteful of foam andcannot direct the jet so accuratelyas the objective is to apply foam tothe rim seal as quickly and gently

as possible (irrespective of the rooflevel) and to retain a complete foamseal for as long as possible.

The most common and (arguably)most effective method to controla rim seal fire is the application offoam via fixed rim seal pourers onthe top of the shell wall. Even withsimple, fixed purpose-built systems,such as rim seal pourers, there isthe danger that basic errors can bemade so that the system does not

operate as intended. Such examplesthat should be avoided are:

Top of foam dam below thetop of the rim seal – this isespecially an issue wheresecondary rim seals have beenadded to the tank to reduce tankemissions. The top of the foamdam should be at least 50 mmabove the top of the rim seal, butideally about 150 mm.

Too many or too large gaps at

the bottom of the foam dam.The purpose of the dam is tohold the foam in position aroundthe circumference of the tankto reduce the amount of foamneeded. This ensures efficientapplication to the rim seal andprevents unnecessary loadsbeing placed on the tank roof.With large gaps at the bottom ofthe dam, a significant quantityof foam could leak to the roof.

However, small, intermittentlyspaced drains should beprovided to allow rainwater todrain away.

Incorrect spacing of the foampourers. Modern foams canflow in excess of 30 metres ifproportioned correctly, althoughif there are changes to the foamtype, foam proportion or evenwater supply, then the flowcharacteristics could be affected

so that a foam seal is notpossible.

Forced/irregular addition offoam. The intention is to applyfoam gently down the side ofthe tank shell so that it formsan even blanket within the rimseal area. If the pourer has beenincorrectly positioned it may notapply the foam in such a way,e.g. catching on the top edge ofthe seal, resulting in splashing

and loss of foam to the roof.


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Foam pourers located too highon the tank, exposing themto wind and impairing theirpassage to the rim seal itself.

To overcome these issues, testsshould be conducted on thesystem to ensure it operatescorrectly. Depending upon thenumber of tanks being protected,

annual testing frequency would beappropriate.

Inspections should be conductedregularly to ensure air inlets tofoam generators are clear and thatfoam pourers are not blocked by, forexample, birds’ nests. Other goodfeatures to aid inspection or firefighting in the event of a systemfailure are:

Walkway around the top of thefloating roof tank – this can be

the wind girder with a handrailaround the circumference. Thisallows for the pourers to beaccessed for maintenance or forfire fighters to access the roofwith manual hoses.

Provide a foam hydrant at thetop of the tank (preferably atthe top of the stairs) for firefighters in the event of rim sealpourers becoming blocked. Thisalso requires less equipment

to be carried up the tank in theevent of a rim seal fire. Carefulconsideration needs to be givenas to the safety of personnelwhen attempting such a tactic.

It is important that foam can beapplied equally well when thefloating roof is at high level andwhen it is at low level.

Example of sub-surface foam

application apparatus

Other foam delivery systemsthat are more complex to varyingdegrees are:

Catenary systems – foam appliedto the rim seal by local generatorslocated directly within the foamdam through a supply line attachedto the roof via the rolling ladder

Coflexip system – foam solution

is applied in the same way as acatenary, although the supply piperuns up from low level through thetank itself via a ‘coflexip’ flexiblepipe.

‘One shot’ systems – anextinguishing agent (such asobsolete Halon) is applied atthe rim seal in a single burstextinguishing the flames. Whilstpotentially effective as an initialextinguisher, these do not preventre-ignition and require all the parts,including cylinders, being placedon the roof. For that reason, Marshdoes not recommend such anapproach.

Fixed roof tanksSimilar systems can be appliedto fixed roof tanks, with foamapplication to the surface of theproduct through a fixed or semi-fixed system, or even sub-surfaceallowing for the foam to float tothe surface of the product. NFPA 11

Standard for Low-, Medium-, andHigh-Expansion Foam provides aguide as to which products andmaterials can be protected by suchsystems.


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11.3 Application RatesA minimum of 40 minutes’ attack should be envisaged in order to have areasonable chance of success in extinguishing a fire, and foam stocks should besufficient for this duration; a duration of 60 minutes is preferred for crude oiltank fires.

Typical application rates for foam solution are shown below, although foammanufacturer’s advice on the type of foam and application should always besought:

Type Rate (l/min/m²)

Applied with mobile equipment

Pool fires 5-10*

Fixed roof tanks, Class I product 5-10

Fixed roof tanks, Class II/III product 5

Applied with fixed or semi-fixed equipment

Fixed roof tank with pourer outlet 4

Base injection (white oils only) 4

Floating roof rim seal fires 12

Floating roof tanks (roof area), projected foam 6.5

Floating roof tanks (roof area), fixed pourers 4

Source: Energy Institute’s Model Code of Safe Practice Part 19: Fire precautions at petroleum refineries

and bulk storage installations

*The higher rate may be applied to Class I products, including gasoline containing up to 10% of

oxygenates.

NFPA 11, Standard for Low-, Medium-, and High-Expansion Foam also providessimilar application rates, although there are some minor differences.

Examples of Industry losses have been included in Appendix C.

12. References to Industry Losses


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Model Code of Safe Practice Part19: Fire precautions at petroleumrefineries and bulk storageinstallations. Energy Institute,February 2007. ISBN 978-0-85293-437-1

BS EN 14015. Specification for thedesign and manufacture of sitebuilt, vertical, cylindrical, flat-bottomed, above ground, welded,

steel tanks for the storage of liquidsat ambient temperature and above.February 2005.

BS EN 61511-1:2004/IEC 61511-1:2003 Functional safety. Safetyinstrumented systems for theprocess industry sector.

BS EN ISO 10497:2004. Testingof valves. Fire type-testingrequirements. British Standard/European Standard/InternationalOrganisation for Standardisation.

HSG 176. The storage of flammableliquids in tanks. HSE. ISBN 0 71761470 0

CIRIA Report 164: Design ofContainment Systems for thePrevention of Water Pollution fromIndustrial Accidents. January 1997.Construction Industry Researchand Information Association. ISBN086017476X.

PPG 18. Planning Policy

Guidance 18: Enforcing PlanningControl. December 1991. ISBN9780117525542

HSG 244. Remotely OperatedShutoff Valves (ROSOVs) forEmergency Isolation of HazardousSubstances - Guidance on GoodPractice. October 2004. ISBN0717628035

API 650. Addendum 3 - WeldedSteel Tanks for Oil Storage. October2003. American Petroleum Institute.

API 653. Tank Inspection, Repair,Alteration, and Reconstruction,Third Edition. December 2001.American Petroleum Institute.

API 2000. Venting Atmospheric andLow-pressure Storage Tanks, SixthEdition. November 2009. AmericanPetroleum Institute.

API RP 2350. Overfill Protectionfor Storage Tanks in PetroleumFacilities, Third Edition. January2005.

NFPA 11 Standard for Low-,Medium-, and High-Expansion

Foam, 2010 Edition

Buncefield. The Final Report of theFinal Investigation Board. December2008. Buncefield Major IncidentInvestigation Board. http://www.buncefieldinvestigation.gov.uk/reports/index.htm#final

Safety and environmentalstandards for fuel storage sites.December 2009. Process SafetyLeadership Group.

http://www.hse.gov.uk/comah/buncefield/fuel-storage-sites.pdf

EEMUA PUB N° 159 - User’s guideto the inspection, maintenanceand repair of above groundvertical cylindrical steel storagetanks. January 2003. EngineeringEquipment and Materials Users’Association.

EEMUA PUB N° 183 - Guide for thePrevention of Bottom Leakage from

Vertical, Cylindrical, Steel StorageTanks. January 2004. EngineeringEquipment and Materials Users’Association.

RR760 - Mechanical integritymanagement of bulk storage tanks.HSE.

http://www.hse.gov.uk/research/rrpdf/rr760.pdf

Note: RR760 provides a summaryof relevant codes and standards

for the assurance of mechanicalintegrity in storage tanks. Theseprovide an assessment of the mostappropriate standards for differentelements of maintenance anddesign of facilities.

13. References to Industry Standards


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Appendix A - Assessment ChecklistThe following checklist is based on a very good rated atmospheric storagetank facility and can be used to assess the quality of that facility andidentify any gaps or areas for improvement..

Element/ Feature Criteria

Containment All tanks separately bunded and sized for 110% of capacity. Bund

walls sized and designed to contain and prevent bund overtopping (may

include tertiary containment).

Layout/Spacing Meets risk-based tank spacing evaluation and risk evaluation available.

Non-crude inter tank spacing equal to or greater than one diameter of

largest tank and at least 15 metres. Crude oil tanks separated by five

tank diameters to prevent risk from boilover.

Instrumentation HLA, HHLA (totally independent includes tappings, transmitters, etc.)

linked to automatic shutdown of feed. Temperature indicator. LLA, LLLA

(independent) linked to automatic shutdown of pump out and complete

with on-line diagnostic capability (i.e. not simple switch). Shutdown

system and components SIL rated in accordance with application and

location. All indicators to control room. Continuous mass balance system

for tank farm. Online blending.

Construction Double welded seals on floating roof. Weather shield. Concrete base

with impervious membrane and sides sloped away from base weld. Anti

rotation device on floaters. Earthing, lightning conductors. Good access

to pontoons, and handrails on access walk way around periphery. Auto

bottom water drain. Tank floor leak detection with double floor.

Roof drainage: rainwater receptors and spill over connections to product

(limits water level to 10”).

Selection As appropriate to class of product.

Drainage of Bunds Grading to pump pit sump. Impermeable bund material.

Equipment and

Pipework

Buried common inlet/outlet pipework and no equipment within bund

area.

Bund construction

and condition

No penetrations or gaps in the walls. Any gaps in bund walls should be

sealed appropriately with a fireproof material. Concrete faced earth wall.

Consideration given to preventing bund overtopping.

Gas Detection Bunds fitted with flammable gas detectors for low flash point materials

capable of sustaining a VCE. Detectors interlocked with tank filling

system to interrupt transfer.

Fire detection Linear Heat Detection or fusible-link/ tube (for rim seals only). Systems are

self monitoring with status indication to HMI (Human/Machine Interface).

Fixed Fire Protection

- Floating Roof

Multi-head foam pourers with foam dam for all tanks. Shell cooling water

spray, or use of multiple monitors with proof of response capability.

Supply from two separate locations. Foam for bunded area. Fixed system

supplies foam.

Fixed radiant protection on exposed walls.


- Cone roof

Subsurface foam injection (for non-polar compounds, otherwise foam

chambers). Roof and shell cooling. Foam for bunded area - dual supply.

Fixed systems.


- Cone roof with

internal floater

Foam pourers. Roof and shell cooling; bund protection with foam - dual

supply. Fixed system.

14. Appendices


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Appendix B - Mechanical Component Checklistfor Floating Roof Tank Inspections Cleanliness

Debris on roof?Ponding or rainwater on roof?

LeakageSigns of oil or holes on roof?

Roof Drains

Clogging of roof drain screen mesh?

PV Vent MeshClogging of PV vent mesh?

Emergency Roof DrainClogging of emergency drain mesh?

Weather Shields/SealsDamage or corrosion?Excessive gaps between seal and shell?

Pontoon Compartments

Presence of water or oil?Covers tight?

Earthing CableDamage on cable or connections?

Guide PolesDamage on guide pole rollers?

Rolling LadderDamage on wheel?

Roof Drain Valves

Ease of movement?

Bottom of ShellWater pooling?Corrosion?Vegetation?


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Appendix C - Loss IncidentsThe application of the measures specified in this document should

minimise the risk of a major accident occurring at a storage facility orindividual storage tank.

The table below gives some examples of incidents that have resulted inloss of containment to illustrate that they should not be considered as rareevents. Data has been compiled by a reputable operator in the USA thatindicates that overfilling occurs once every 3,300 filling operations.

Location Date Fuel released Consequence

Jacksonville,

Florida, USA

1993 Unleaded petrol/

gasoline

190 m3 released. The spill ignited, leading

to a major explosion and fire.

Coryton, UK 1997 Unleaded petrol/

gasoline

81 m3 released. Spill contained within

bund – no ignition.

Belgium 2001 Hexene Approximately 90m3 released. Spill

contained within bund – no ignition.

Sour Lake,

Texas, USA

2003 Crude oil 80 m3 released. Spill contained within


Torrance,

California, USA

2004 Jet fuel Approximately 10m3 released. Spill

contained within bund – no fire or explosion.

Bayonne, New

Jersey, USA

2004 Fuel oil 825 m3 released. Oil contained on tank

farm. – no fire or explosion.

Casper,

Wyoming, USA


gasoline

Up to 1270 m3 released. Spill contained

within bund – no ignition.

Rensselaer, NY,

USA


gasoline

0.4-4 m3 released. Spill contained within


Location Date Fuel released Cause Consequence

Fawley, UK 1999 Crude oil (400

tonnes)

Corrosion of tank

base.

No injuries or off-site effects. All

of the oil was recovered from

primary containment.

Milford

Haven, UK

2005 Kerosene (653

tonnes)

Leak from damaged

sump escaped

through permeable

floor of bund.

No injuries, but nearby

gardens, farmland, and

stream contaminated. All

wildlife killed in stream.

Antwerp,

Belgium

2005 Crude oil

(26,000

tonnes)

Catastrophic failure

of storage tank as a

result of corrosion.

Overtopping of the bund

wall occurred due to sudden

release.

Plymouth

Harbour,

UK

2005 Kerosene

(tonnage

uncertain)

Corrosion of the tank

base and a permeable

bund base

No injuries. Kerosene entered

into the ground.

Coryton,

UK

2006 Gas oil

(121tonnes)

Tank overfilled, oil

escaped from bund

by defective drain

valve.

No injuries or harm to the

environment.

Poole

Harbour,

UK

2006 Diesel oil (19

tonnes)

Diesel escaped

through damaged

base plate and

through cracks in

concrete bund floor.

No injuries. Pollution of

ground but not of the

harbour.


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Incidents with similarities with the Buncefield incident of 2005 are summarised below.

Location Date and time Comments – background Comments – explosion

Houston, Texas,

USA

April 1962 ‘Severe leak’ from gasoline tank. Almost windless

conditions. Ignition near adjacent highway.

Described as a blast, but no details are

available.

Baytown, Texas,

USA

27 January 1977 Overfilling of a ship with gasoline. Few details are available, but it is likely that

there would have been congestion.

Texaco, Newark,

New Jersey,

USA

7 January 1983

After 00.00 hrs

Overfilling of a tank containing unleaded gasoline.

114-379 m3 (80-265 tonnes) of gasoline released.

Slight wind, ignition source 300m away.

Relatively uncongested area. High overpressure

reported but not quantified. Three minor

explosions preceded the main blast.

Naples Harbour,

Italy

21 December 1985 Overfilling of a tank containing unleaded gasoline.

700 tonnes escaped. Low wind speed (2 m/s).

Relatively congested area. The tank overtopped

1.5 hours before ignition. Various overpressures

estimated from damage analysis but they are

minimum values (e.g. 48 kPa)

St Herblain,

France

7 October 1991

04:00 hours

Leak of gasoline from a transfer line into a bund.

Wind


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The information contained herein is based on sources we b elieve reliable and should be understood to be general ri sk management and insurance information only.

The information is not intended to be taken as advice with respect to any individual situation and cannot be relied upon as su ch.

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