tank storage magazine - protecting storage tank bottoms - march 2009

8/18/2019 Tank Storage Magazine - Protecting Storage Tank Bottoms - March 2009

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In many locations cathodic protection is mandated, and in the absence

of legislation it is still often dictated by good engineering practices

Protecting storagetank bottomsC ontrolling corrosionrisk is critical tomaintaining theintegrity of abovegroundstorage tanks (ASTs) andmaximising their service life.

Cathodic protection (CP) istypically applied to all ASTsbuilt on a ring wall foundationto protect the external tankbottom in contact with the soil/sand foundation. It is not asrelevant for smaller tanks builton concrete slab foundations.

Galvanic versus impressedcurrent

Historically, variouscongurations of galvanicanodes, including discreetanodes and ribbon typeanodes, have been usedto protect AST bottoms.Experience has shownthat these systems do notprovide the uniform currentdistribution necessary overthe entire CP system designlife and result in prematurefailure as the galvanic anodesconsume. ASTs requiresignicant current, whichgenerally precludes the useof galvanic anodes. Almostall AST CP systems todayare designed with impressedcurrent systems to providethe current required overa long period of time.

Design information

The following informationis required to develop aCP design for ASTs:

Tank diameterThis is necessary tocalculate the surfacearea to be protected.

Tank bottom coatingTypically tank bottoms are

bare plate steel, but in somecases the plate steel may becoated on the bottom, whichreduces the current requiredfor cathodic protection. CPis still recommended fortanks with coated bottoms.

Current density requiredTypical design currentdensity requirements of1 mA/ft2 (10 mA/m 2)are sufcient to achieveNACE criteria for cathodicprotection (see discussionon operating temperature).

Depth of anodeThe separation distancebetween the anode and thetank bottom affects currentspread and anode spacing.

Sand/soil resistivityThis information is necessaryto estimate overall systemresistance necessary toproperly size the rectiervoltage. In many cases, itmay make sense to installthe anode system and testthe actual circuit resistanceusing a portable rectier orcar battery before committingto a specic rectier size.

Tank operatingtemperatureCorrosion rates increasesignicantly with elevatedtemperature, necessitatingmore current. Typicallythe following temperaturecorrection formula is usedfor CP designs for heatedtank bottoms: for every

10°C above 30°C the currentrequirement increases 25%.

Area classicationRectiers and junction boxesmust be designed withsuitable enclosures for thearea classication wherethey will be installed. Often,these can be located in non-classied areas with minimaladditional cost of cable whilesaving signicantly on thecost of the enclosures andclassied components.

Secondarycontainment linersIf polyethylene or other suchplastic liners are to be placedunderneath the tank, theseliners act as a barrier to theow of current and the anodes

Installation of concentric Matcor SPL-FBR anode rings

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must be placed between the

liner and the tank bottom. IfClaymax or other conductivetype liners are to be used,the anodes do not have tobe placed between the linerand the tank bottom.

Design lifeTypical design life for newconstruction ASTs is 25 to30 years. It is important tonote that the actual operatinglife of ASTs often exceedsthis value and depending onthe design of the tank, its

location, and the selection of acontainment liner. Replacementof the CP system may bedifcult to impossible so someconsideration should be givento the economic value ofextending the design life. For atypical concentric ring MMO/Tilinear anode conguration,the incremental anode costto go from 30-year designlife to 50-year design life isapproximately 25% additionalanode cost, with no increase ininstallation costs, making thisa very attractive alternative.

CP congurations for newconstruction ASTs

Shallow distributed anodesaround tank peripheryOne common design approachto AST bottom CP is to install ashallow distributed point anodesystem around the peripheryof the tank. These are typicallyaugured in to depths ofve to 10 feet. This designapproach only works whenthere is no electrically isolatingsecondary containmentliner under the tank.

For these designs, thecritical issue is assuring thatsufcient current reaches the

centre of the tank. Above

ground storage tank bottomsare large bare surfacesrequiring a lot of current. Toassure that current distributesproperly, the anode depth anddistance from the tank arecritical. Shallow peripheralanodes are not able to throwcurrent to the centre of allbut the smallest of ASTs.

This approach is generallynot recommended forASTs with tank diametersexceeding 20 ft (6m) dueto the quantity of anodes

required and the risk ofpoor current distributionto the centre of the tank.

Deep well anode systemsThis approach is based on

using one or more deep well

anode systems located wellbelow the tank bottom toprovide current uniformlyto the tank bottom. Thisapproach has some limitationsin heavily congested plantor terminal environmentswhere current can ow toother buried structures.When multiple deep wells areemployed to protect morethan one tank in a cluster,care must be taken to assureproper current distribution.

As with any deep well,

there are concerns withdrilling (typically 150+ feetto bottom of hole) includingaccess issues for a drill rig,environmental concerns,permitting and handling of

drilling spoils. Even with adeep well approach, whendealing with new construction,reference electrodes shouldbe installed under each tank.

Many operators prefera close coupled dedicatedCP system for each tank

rather than the blanketcoverage afforded by adeep well system.

Should the design for thenew construction tank use acontainment liner that shieldscurrent (i.e. polyethyleneliner), the deep well anodesystem cannot be used.

Distributed anodesbelow tank bottomWhile not as commontoday, many older designsused individual anodes

laid horizontally along thetank bottom and connectedparallel to header cablesexiting the ring wall. Theeconomics of this design,both in terms of installationcosts and material costs,are not favourable and thisdesign has been dropped infavour of either a grid systemor linear anodes in a parallelconcentric ring arrangement.

MMO ribbon matrix systemThis conguration involves

laying out parallel titaniumconductor bars and thenrunning mixed metaloxide (MMO) ribbon anodeperpendicular to theconductor bars. The MMO

Depth of anode“d”

NTS

Offset from tank

“t”

Shallow peripheral distributed anodes

Completed installation of anode rings prior to backlling

March 2009 • T A N K S T O R A G E CORROSION PROTECTION


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ribbon anode is eld spotwelded to the titaniumconductor bar to provide bothmechanical and electricalconnections. Wherever the

titanium conductor barscross, they too must be eldwelded together. Power feeds(preassembled cables with aat plate to connect to theconductor bar) are secured tothe titanium bar in multiplelocations and routed to thering wall penetration.

This system is a labour andQA/QC intensive installationprocess requiring signicanteld welding and on-sitetesting to assure electricalcontinuity. The attachmentof the power feeds to thetitanium grid is critical tothe system reliability. Froma design perspective, thespacing of the anodes andconductor bars must besufcient to assure evencurrent distribution. Incompetitive situations, thetemptation is to increasethe spacing to reducethe materials costs.

Concentric ring systemsUnlike the ribbon matrixsystem, the concentric ringlinear anode system forAST tanks can be factoryassembled to eliminate theneed for any eld fabrication,which greatly simplies

installation and reducesQA/QC issues, eliminatingeld welds and power feedconnections that are reliedupon with the matrix system

to assure electrical continuityand system integrity.

The principal advantageof the linear system is thateverything under the tank isfactory assembled and testedprior to installation and theonly installation effort is tolay the anode assemblies inaccordance with the designdrawings and installationinstructions. This providesfor an exceptionally simpleinstallation while assuring thehighest system reliability;

installation costs are minimal.There are two primary

congurations for linearanodes under tank bottoms.The parallel linear anode

arrangement has multipleparallel anode segments,which are fed from eachend of the anode. Theanode connections areeld spliced to loop cables,which terminate at twoanode junction boxes.

The use of concentric ringsoffers two key advantagesover parallel anode segments.The rst advantage is thatthis conguration doesrequire junction boxes onboth sides of the tank,

thus eliminating one of theanode junction boxes andreducing the cabling requiredto run from two anode

junction boxes back to thetransformer/rectier unit.The second key advantageto this conguration is that

it eliminates the need fortwo eld splices for eachanode segment. Each ringcan be manufactured withthe appropriate length ofheader cable to run each enddirectly to the single anode

junction box. These eldsplices are weak links subjectto premature failure over thelife of the anode system.

As with any system, spacingbetween anode segments isanother key design element.Matcor’s experience with ring

congurations is extensiveand it has determined throughnumerous installations andits own in-house testing thatfor tank bottom applicationswith bare bottom platesand an anode depth of 1foot, concentric rings with aspacing of 10 feet providethorough current distribution.When the anodes can beplaced deeper than 1 foot,the anode spacing can beextended. Based on the 1foot depth, typical ambient

temperature tanks can beprotected for 30+ yearswith 16 mA/ft linear anode,while a 50+ year designlife is typically achievedwith a 25 mA/ft ratedanode with a modest 25%increase in the anode cost.

Provisions for testingWith any CP system fortank bottoms, it is criticalthat provisions for testingbe installed with the anodesystem. Once the tank is

Horizontal directional drilling under an existing AST to install a leak detection tube and referenceelectrode assembly rings prior to backlling

Linear anode

Anode junction box

Field splice

Anode cable

Parallel linear segment Concentric rings

to T/R

to T/R to T/R

Linear anode congurations

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erected, making accuratepotential measurementsat various locations alongthe tank can only beaccomplished if referenceelectrodes have beeninstalled below the tank.Typically, copper-copper

sulfate (Cu-CuSO4) referenceelectrodes are installedin strategic locationsunderneath the tank.

These reference electrodesare often mistakenly calledpermanent referenceelectrodes; however, theyare not permanent as overtime the copper-coppersulfate solution becomescontaminated and ceases toprovide accurate information.Once a baseline forperformance is established

over a sufcient operatingperiod, maintaining theappropriate current outputto achieve NACE criteriais all that is required.

In some cases, clientsmay also specify dualreference electrodes suchas both zinc and copper-copper sulfate. While thezinc reference electrodesare not as consistent, theyprovide a much longeroperating life and can becalibrated against the copper-

copper sulfate electrodes.In addition to the xed

reference electrodes, it isalso common to provide areference electrode tube/conduit underneath the tankbottom to allow sliding of acalibrated reference electrodethrough the monitoring tubeto take potential readings.These can also functionas leak detection tubes.

CP congurations for retrotapplications

Provisions for testingWhen installing new tanks,the standard industrypractice is to installreference electrodes andleak detection monitoringtubes approximately 4-6inches from the tank bottombefore the nal layer ofsand cover is placed.

The reference electrodesare necessary so that theCP system can be testedto assure that the entiretank bottom is beingproperly protected and toadjust the system outputfor optimum operation.

Retrot projects, however,are not so simple. The

challenge with retrotinstallations is that it is notpossible to test the potentialof the tank, except at theperiphery, without having

some sort of referenceelectrode directly under thetank. Standard industrypractice is to use horizontaldirectional drilling (HDD) toinstall leak detection andxed reference electrodeassemblies under the tank.The leak detection andmonitoring tubes consist ofslotted or drilled/perforatedpvc tubes with referencecell assemblies that can bepulled under the tank.

For small diameter tanks

(less than 20 foot), testingaround the perimeterof the tank is sufcientand it is not necessary toinstall reference electrodesdirectly under the tankto assure proper cathodicprotection, but Matcorcannot recommend that forlarger tanks prone to poor

current distribution. If Matcorcannot test the CP system’sperformance there is noreason to have a system.

It is good engineering

practice to install monitoringtubes and referenceelectrodes under the tankusing HDD. This provides theonly means of assuring thatthe CP system is providingthorough coverage acrossthe entire tank prole.

Linear anodes installed belowtank bottom

When installed in conjunctionwith xed reference electrodesand leak detection monitoring

tubes, linear anodes placeddirectly beneath the tankbottom provide the highestdegree of design certainty.With the anodes directlybelow the tank bottom,stray current concerns areminimised and thoroughcurrent distribution across theentire tank bottom is assured.

Shallow distributed anodesaround tank periphery

One tempting designapproach to retrot ASTbottom CP is to install ashallow distributed pointanode system around the

periphery of the tank.These are typically auguredin to depths of 5-10 feet.Unfortunately shallowdistributed anodes often failto provide sufcient currentto the centre of the tank.

Above ground storagetank bottoms are large baresurfaces requiring a greatdeal of current. To assurethat current distributesproperly, the anode depth anddistance from the tank arecritical. Shallow peripheral

anodes are not able to throwcurrent to the centre of allbut the smallest of ASTs.

As with new constructionASTs, it is generally notrecommended to installshallow peripherallydistributed anodes forASTs with tank diametersexceeding 20 ft (6m) dueto the quantity of anodesrequired and the risk ofpoor current distributionto the centre of the tank.

Deep well anode systemsDeep well anodes areoften a viable solution forexisting ASTs, however caremust be taken to assurethat current from the deepwell anode system is notcausing interference withnearby buried structures.It is also very important tonote that deep well anodesystems will not providecurrent to tanks protectedwith containment liners thatshield the tank bottom suchas plastic sheeting materials.

Maintenance and inspection

Annual testing and inspectionof any tank CP system bya qualied NACE CP level 1or higher technician familiarwith testing CP systems forASTs is advised. As with anyimpressed current system,monthly rectier checksshould be performed byplant maintenance to assurethat the rectier is on andthat the voltage and currentoutputs remain stable. n

For more information:This article was written by Ted Huck,VP of sales and marketing for Matcor,[email protected] or www.matcor.com

ReferenceElectrode

Linear Anode

AnodeJunction Box

Field Splice

Anode Cableto T/R

Linear anode and reference electrode design

Common design from the 1970s

March 2009 • T A N K S T O R A G E CORROSION PROTECTION

tank storage magazine - protecting storage tank bottoms - march 2009

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