NACE RP0193-2001

NACE RP0193-2001
NACE RP0193-2001

Standard

Recommended Practice

External Cathodic Protection of On-Grade Carbon

Steel Storage Tank Bottoms

This NACE International standard represents a consensus of those individual members who have reviewed this document,its scope,and provisions.Its acceptance does not in any respect preclude anyone,whether he has adopted the standard or not,from manufacturing,marketing,purchasing,or using products,processes,or procedures not in conformance with this standard.Nothing contained in this NACE International standard is to be construed as granting any right,by implication or otherwise,to manufacture,sell,or use in connection with any method,apparatus,or product covered by Letters Patent,or as indemnifying or protecting anyone against liability for infringement of Letters Patent.This standard represents minimum requirements and should in no way be interpreted as a restriction on the use of better procedures or materials.Neither is this standard intended to apply in all cases relating to the subject.Unpredictable circumstances may negate the usefulness of this standard in specific instances.NACE International assumes no responsibility for the interpretation or use of this standard by other parties and accepts responsibility for only those official NACE International interpretations issued by NACE International in accordance with its governing procedures and policies which preclude the issuance of interpretations by individual volunteers.

Users of this NACE International standard are responsible for reviewing appropriate health,safety,environmental,and regulatory documents and for determining their applicability in relation to this standard prior to its use.This NACE International standard may not necessarily address all potential health and safety problems or environmental hazards associated with the use of materials,equipment,and/or operations detailed or referred to within this https://www.360docs.net/doc/705784290.html,ers of this NACE International standard are also responsible for establishing appropriate health,safety,and environmental protection practices,in consultation with appropriate regulatory authorities if necessary,to achieve compliance with any existing applicable regulatory requirements prior to the use of this standard.

CAUTIONARY NOTICE:NACE International standards are subject to periodic review,and may be revised or withdrawn at any time without prior notice.NACE International requires that action be taken to reaffirm,revise,or withdraw this standard no later than five years from the date of initial publication.The user is cautioned to obtain the latest edition.Purchasers of NACE International standards may receive current information on all standards and other NACE International publications by contacting the NACE International Membership Services Department,1440South Creek Dr,Houston,Texas 77084-4906(telephone +1[281]228-6200).

Revised 2001-06-15Approved October 1993NACE International 1440South Creek Drive Houston,Texas 77084-4906

+1281/228-6200ISBN 1-57590-014-9?2001,NACE International

NACE Standard RP0193-2001

Item No.21061

RP0193-2001

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Foreword

It is extremely important to maintain the integrity of on-grade carbon steel storage tanks for both

economic and environmental reasons.The proper design,installation,and maintenance of

cathodic protection(CP)systems can help maintain the integrity and increase the useful service

life of on-grade carbon steel storage tanks.

The purpose of this standard recommended practice is to outline practices and procedures for

providing cathodic protection to the soil side of bottoms of on-grade carbon steel storage tanks

that are in contact with an electrolyte.Recommendations for both galvanic anode systems and

impressed current systems are included.Design criteria for the upgrade of existing tanks as well

as for newly constructed tanks are included.This standard is intended for use by personnel

planning to install new on-grade carbon steel storage tanks,upgrade cathodic protection on

existing storage tanks,or install new cathodic protection on existing storage tanks.

This NACE standard was originally prepared by Task Group T-10A-20,a component of NACE Unit

Committee T-10A on Cathodic Protection,in1993.It was technically revised by Task Group013

in2001.Task Group013is administered by Specific Technology Group(STG)35on Pipelines,

Tanks,and Well Casings and sponsored by STGs03on Protective Coatings and Linings—

Immersion/Buried and STG05on Cathodic/Anodic Protection.This standard is issued by NACE International under the auspices of STG35on Pipelines,Tanks,and Well Casings.

In NACE standards,the terms shall,must,should,and may are used in accordance with the

definitions of these terms in the NACE Publications Style Manual,4th ed.,Paragraph7.4.1.9.Shall

and must are used to state mandatory requirements.The term should is used to state something

good and is recommended but is not mandatory.The term may is used to state something

considered optional.

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RP0193-2001

________________________________________________________________________

NACE International

Standard

Recommended Practice

External Cathodic Protection of On-Grade Carbon Steel

Storage Tank Bottoms

Contents

1.General (1)

2.Definitions (1)

3.Preliminary Evaluation and Determination of the Need for Cathodic Protection (2)

4.Criteria for Cathodic Protection (5)

5.General Considerations for Cathodic Protection Design (7)

6.Design Considerations for Impressed Current Cathodic Protection (9)

7.Design Considerations for Galvanic Anode Cathodic Protection (11)

8.Design Considerations—Cathodic Protection for Tanks with Replacement Bottoms or

Release Prevention Barriers (12)

9.Installation Considerations (15)

10.Energizing and Testing (17)

11.Operation and Maintenance of Cathodic Protection Systems (18)

12.Recordkeeping (19)

References (19)

Bibliography (20)

Figures

Figure1:Soil Resistivity Testing(Four-Pin Method) (3)

Figure2:Temporary Groundbed for Current Requirement Testing (5)

Figure3:Stray Current Corrosion (6)

Figure4:Vertically Drilled Anode CP System (9)

Figure5:Angled Anode Cathodic Protection System (10)

Figure6:Deep Anode Groundbed (10)

Figure7:Horizontally Installed Anode Groundbed (10)

Figure8:Typical Double-Bottom Cathodic Protection Layout(Impressed or Sacrificial)13

Figure9:Typical Double-Bottom Galvanic Anode Design (14)

Figure10:Typical New Tank or Double-Bottom Impressed Current Anode Design (14)

Figure11:Perforated Pipe Installed for Reference Electrode (16)

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RP0193-2001 ________________________________________________________________________

Section1:General

1.1This standard presents guidelines for the design, installation,and maintenance of cathodic protection for the exterior bottoms of on-grade carbon steel storage tanks. Cathodic protection can be installed to protect new or existing tanks,but cannot protect carbon steel surfaces that are not in contact with an electrolyte.

1.2This standard is applicable to welded,bolted,and riveted carbon steel tanks that are either field-or shop-fabricated.

1.3It is understood in this standard that cathodic protection may be used alone or in conjunction with protective coatings.1.4All cathodic protection systems should be installed with the intent of conducting uninterrupted,safe operations. When cathodic protection is applied,it should be operated continuously to maintain polarization.

1.5The criteria for cathodic protection are based on current industry standards.

1.6Corrosion control must be a consideration during the design of on-grade carbon steel storage tanks.

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Section2:Definitions

Amphoteric Metal:A metal that is susceptible to corrosion in both acid and alkaline environments.

Anode:The electrode of an electrochemical cell at which oxidation occurs.Electrons flow away from the anode in the external circuit.Corrosion usually occurs and metal ions enter the solution at the anode.

Backfill:Material placed in a hole to fill the space around the anodes,vent pipe,and buried components of a cathodic protection system.

Cathode:The electrode of an electrochemical cell at which reduction is the principal reaction.Electrons flow toward the cathode in the external circuit.

Cathodic Protection(CP):A technique to reduce the corrosion of a metal surface by making that surface the cathode of an electrochemical cell.

Cell:See Electrochemical Cell.

Current Density:The current to or from a unit area of an electrode surface.

Deep Groundbed:One or more anodes installed vertically at a nominal depth of15m(50ft)or more below the earth’s surface in a drilled hole for the purpose of supplying cathodic protection.

Differential Aeration Cell:An electrochemical cell,the electromotive force of which is due to a difference in air (oxygen)concentration at one electrode as compared with that at another electrode of the same material.

Electrical Isolation:The condition of being electrically separated from other metallic structures or the environment.Electrochemical Cell:A system consisting of an anode and a cathode immersed in an electrolyte so as to create an electrical circuit.The anode and cathode may be different metals or dissimilar areas on the same metal surface.

Electrolyte:A chemical substance containing ions that migrate in an electric field.

External Circuit:The wires,connectors,measuring devices,current sources,etc.,that are used to bring about or measure the desired electrical conditions within an electrochemical cell.It is this portion of the cell through which electrons travel.

Foreign Structure:Any metallic structure that is not intended as a part of a system under cathodic protection. Galvanic Anode:A metal that provides sacrificial protection to another metal that is more noble when electrically coupled in an electrolyte.This type of anode is the electron source in one type of cathodic protection.

Groundbed:One or more anodes installed below the earth’s surface for the purpose of supplying cathodic protection.For the purposes of this standard,a groundbed is defined as a single anode or group of anodes installed in the electrolyte for the purposes of discharging direct current to the protected structure.

Impressed Current:An electric current supplied by a device employing a power source that is external to the electrode system.(An example is direct current for cathodic protection).

On-Grade Storage Tank:A tank constructed on sand or earthen pads,concrete ringwalls,or concrete pads.

RP0193-2001

Oxidation:(1)Loss of electrons by a constituent of a chemical reaction.(2)Corrosion of a metal that is exposed to an oxidizing gas at elevated temperatures.

Piping:For the purposes of this standard,this term refers to all piping associated with the transfer of products in and out of storage tanks.

Reduction:Gain of electrons by a constituent of a chemical reaction.Reference Electrode:An electrode whose open-circuit potential is constant under similar conditions of measurement,which is used for measuring the relative potentials of other electrodes.

Stray-Current Corrosion:Corrosion resulting from current through paths other than the intended circuit,e.g.,by any extraneous current in the earth.

________________________________________________________________________ Section3:Preliminary Evaluation and Determination of the Need for Cathodic Protection

3.1This section outlines the information that should be considered prior to designing a cathodic protection system to protect on-grade carbon steel storage tank bottoms in contact with an electrolyte.

3.2Site Assessment Information

3.2.1Prior to designing a cathodic protection system,

the following information should be obtained:

(a)Tank,piping,and grounding construction

drawings,including dimensions,etc.

(b)Site plan and layout

(c)Date of construction

(d)Material specifications and manufacturer

(e)Joint construction(i.e.,welded,riveted,etc.)

(f)Coating specifications

(g)Existing or proposed cathodic protection systems

in the area

(h)Location of electric power sources

(i)Electrochemical properties of the tank bedding or

padding material

(j)History of the tank foundation(i.e.,whether the tank has been jacked up/leveled,etc.)

(k)Unusual environmental conditions

(l)Operating history of the tank,including leak information(internal and external)

(m)Maintenance history of the tank

(n)Containment membranes/impervious linings

(o)Secondary bottoms

(p)Water table and site drainage information

(q)Liquid levels maintained in the tank

(r)Nearby foreign structures

(s)Type of liquid stored

(t)Operating temperature

(u)Electrical grounding 3.3Predesign Site Appraisal

3.3.1Determining the Extent of Corrosion on Existing

Systems

3.3.1.1Information regarding the degree of

tank-bottom corrosion is useful because

considerable bottom damage may require

extensive repairs or replacement prior to the

installation of cathodic protection.

3.3.1.2Field procedures for determining the

extent of existing corrosion may include:

(a)Visual inspection

(b)Tank bottom plate-thickness measurements

(ultrasonic testing,coupon analysis,etc.)

(c)Estimation of general corrosion rates

through the use of electrochemical procedures

(d)Determination of the magnitude and

direction of galvanic or stray current transferred

to or from the tank through piping and other

interconnections

(e)Determination of soil characteristics

including resistivity,pH,chloride ion

concentration,sulfide ion concentration,and

moisture content

(f)Estimation of the degree of corrosion

deterioration based on comparison with data

from similar facilities subjected to similar

conditions

3.3.1.3Foundation characteristics are also

important factors in the assessment of the extent

of existing corrosion.The pad material of

construction,thickness of ringwalls,and water

drainage should all be considered.

3.3.1.4Data pertaining to existing corrosion

conditions should be obtained in sufficient

quantity to permit reasonable engineering

judgments.Statistical procedures should be

used in the analysis,if appropriate.

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3.3.2Electrical Isolation

3.3.2.1Electrical isolation facilities must be

compatible with electrical grounding requirements conforming to applicable codes and safety requirements.If the tank bottom is to be cathodically protected,the use of alternative electrical grounding materials,such as galvanized steel and galvanic anodes,should be considered.

3.3.2.2The designer of a cathodic protection

system should consider the possible need for electrical isolation of the tank from piping and other interconnecting structures.Isolation may be necessary for effective cathodic protection or safety considerations.

3.3.2.3Electrical isolation of interconnecting

piping can be accomplished through the use of isolating flanges,dielectric bushings or unions,or other devices specifically designed for this purpose.These devices shall be rated for the proper operating pressure and be compatible with the products being transported.

3.3.2.4Polarization cells,lightning arresters,

grounding cells,and other decoupling devices may be useful in some situations for maintaining isolation under normal operating conditions and providing protection for an isolating device during lightning strikes,power surges,and other abnormal situations.

3.3.2.5Tests to determine tank electrical

characteristics include:

(a)Tank-to-earth resistance tests

(b)Tank-to-grounding system resistance and

potential tests

(c)Tank-to-electrolyte potential tests

(d)Electrical continuity tests for mechanical

joints in interconnecting piping systems

(e)Electrical leakage tests for isolating fittings

installed in interconnecting piping and between the tanks and safety ground conductors

3.3.3Cathodic Protection Type,Current Require-ments,and Anode Configuration

3.3.3.1Soil resistivity tests should be

performed in sufficient quantity as to aid in determining the type of cathodic protection (galvanic or impressed current)required and the configuration for the anode system.Figure1 illustrates the four-pin method of soil resistivity testing.

3.3.3.2Resistivities can be determined using

the four-pin method described in ASTM(1)G57,1 with pin spacings corresponding to depths of at least that expected for the anode system,or by using an equivalent testing method(in very dry environments,electromagnetic conductivity testing may be used to measure resistivities).2 The resistivity measurements should be obtained in sufficient detail to identify possible variations with respect to depth and location.As a general guideline,resistivity data should be obtained at a minimum of two locations per tank.

Figure1:Soil Resistivity Testing(Four-Pin Method)

Note:a=Depth of interest for the soil resistivity measurement. ___________________________

(1)American Society for Testing and Materials(ASTM),100Barr Harbor Dr.,West Conshohocken,P.A.19428.

a a a

RP0193-2001

3.3.3.3If deep groundbeds are considered,

resistivities should be analyzed using procedures described by Barnes3to determine conditions on a layer-by-layer basis.On-site resistivity data can be supplemented with geological data including subsurface stratigraphy,hydrology,and lithology.

Sources for geological information include water well drillers,oil and gas production companies,the U.S.Geological Survey Office,(2)and other regulatory agencies.

3.3.3.4Cathodic protection current require-

ments can be estimated using test anode arrays simulating the type of groundbed planned.Test currents can be applied using suitable sources of direct current.Test groundbeds can include driven rods,anode systems for adjacent cathodic protection installations,or other temporary structures that are electrically separated from the tank being tested.Small-diameter anode test wells may be appropriate and should be considered if extensive use of deep anode groundbeds is being considered.Figure2 illustrates a temporary groundbed for current requirement testing.

3.3.4Stray Currents

3.3.

4.1The presence of stray earth currents

may result in cathodic protection current requirements that are greater than those required under natural conditions.Possible sources of stray current include DC-operated rail systems and mining operations,other cathodic protection systems,welding equipment,and high-voltage direct current(HVDC)transmission systems.3.3.4.1.1Field tests to determine whether stray currents are a concern include those that provide tank-to-electrolyte and structure-to-electrolyte potential measure-ments on adjacent structures,earth gradient measurements,and current flow measure-ments on tank piping and safety grounding conductors.

3.3.

4.1.2Possible interference effects caused by adjacent cathodic protection systems should be determined by interrupting the current output using a known timing cycle.Structure-to-electrolyte potentials and other parameters should be monitored over a minimum24-hour period in areas where dynamic stray currents or transient effects are expected to be a concern.Recording instruments can be used for this purpose.Figure3illustrates stray current corrosion.

3.3.

4.1.3Cathodic protection designs should incorporate every practical effort to minimize electrical interference on structures not included in the protection system. Predesign test results can be analyzed to determine the possible need for stray-current control provisions in the cathodic protection system.

___________________________

(2)U.S.Geological Survey Office,P.O.Box25046.Federal Center,Denver,CO80225.

RP0193-2001

Figure 2:Temporary Groundbed for Current Requirement Testing

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Section 4:Criteria for Cathodic Protection

4.1This section lists criteria for cathodic protection that,if complied with either separately or collectively,indicate that cathodic protection of an on-grade carbon steel storage tank bottom has been achieved.4.2General

4.2.1The objective of using cathodic protection is to control the corrosion of an on-grade carbon steel storage tank bottom in contact with an electrolyte.The selection of a particular criterion for achieving this objective depends,in part,on prior experience with

similar tank bottoms and environments in which the criterion has been successfully used.

4.2.2The criteria in Paragraph 4.3were developed through laboratory experiments or were determined empirically by evaluating data obtained from successfully operated cathodic protection systems.It is not intended that personnel responsible for corrosion control be limited to operating under these criteria if it can be demonstrated by other means that the control of corrosion has been achieved.

RP0193-2001

Figure 3:Stray Current Corrosion

4.2.3Potential measurements on storage tanks shall be made with the reference electrode located as close as possible to the tank bottom.On most tanks,measurements should be taken at the perimeter,near the center of the tank bottom,and at various points in between.Consideration must be given to voltage drops other than those across the structure-to-electrolyte boundary,the presence of dissimilar metals,and the influence of other structures.These factors may interfere with valid interpretation of potential measurements.Also,measurements made with a reference electrode located on asphalt pavement or a concrete slab or outside the concrete wall may be in error.

4.3Criteria for Corrosion Control of Carbon Steel Tank Bottoms

4.3.1Corrosion control can be achieved at various levels of cathodic polarization depending on environmental conditions.However,in the absence of specific data that demonstrate that cathodic protection has been achieved,one or more of the following must apply to the system:

4.3.1.1A negative (cathodic)potential of at least 850mV with the cathodic protection current applied.This potential shall be measured with

respect to a saturated copper/copper sulfate reference electrode (CSE)contacting the electrolyte.Consideration must be given to voltage drops other than those across the structure-to-electrolyte boundary for valid interpretation of this voltage measurement.

4.3.1.1.1Consideration is understood to mean the application of sound engineering practice in determining the significance of voltage drops by methods such as:

(a)Measuring or calculating the voltage drop(s),

(b)Reviewing the historical performance of the cathodic protection system,

(c)Evaluating the physical and electrical characteristics of the tank bottom and its environment,and

(d)Determining whether or not there is physical evidence of corrosion.

4.3.1.2A negative polarized potential of at least 850mV relative to a CSE.

4.3.1.3A minimum of 100mV of cathodic polarization between the carbon steel surface of the tank bottom and a stable reference electrode

RP0193-2001

contacting the electrolyte.The formation or

decay of polarization may be measured to satisfy

this criterion.

4.4Reference Electrodes

4.4.1Other standard reference electrodes may be

substituted for the CSE.Two commonly used reference electrodes are listed below.The voltages given are equivalent(at25°C[77°F])to a negative850 mV potential referred to a CSE:

(a)Saturated silver/silver chloride reference

electrode:a negative780mV potential

(b)High-purity zinc(99.99%):a positive250-mV

potential(see Paragraph7.3.4)

4.4.2Stationary(permanently installed)reference

electrodes may assist in measuring potentials under the tank.Stationary electrodes may be encapsulated in an appropriate backfill material.4.5Special Considerations

4.5.1Special cases,such as stray currents and stray

electrical gradients,that require the use of criteria different from those listed above may exist.

4.5.2Coupons and electrical resistance probes may

be useful in evaluating the effectiveness of the cathodic protection system.

4.5.3Conditions in which cathodic protection is

ineffective or only partially effective sometimes exist.

Such conditions may include the following:

(a)Elevated temperatures

(b)Disbonded coatings

(c)Shielding

(d)Bacterial attack

(e)Unusual contaminants in the electrolyte

(f)Areas of the tank bottom that do not come into

contact with the electrolyte

(g)Dry tank cushion

4.5.4Rocks,clay deposits,or clumps under tank

bottom plates can promote the formation of localized corrosion activity,which is difficult to monitor or evaluate.

________________________________________________________________________ Section5:General Considerations for Cathodic Protection Design

5.1This section recommends procedures and considerations that apply to the design of cathodic protection systems for on-grade,single-and double-bottom carbon steel storage tanks.

5.2Cathodic Protection Objectives and System Characteristics

5.2.1The major objectives for the design of a cathodic

protection system are:

(a)To protect the tank bottom from soil-side

corrosion

(b)To provide sufficient and uniformly distributed

current

(c)To provide a design life commensurate with the

design life of the tank bottom or to provide for periodic anode replacement

(d)To minimize interference currents

(e)To provide adequate allowance for anticipated

changes in current requirements for protection

(f)To locate and install system components where

the possibility of damage is minimal

(g)To provide adequate monitoring facilities to

permit a determination of the system’s performance (see Paragraph11.2)

5.2.2General characteristics of impressed current and

galvanic current cathodic protection systems are listed in Table 1.Impressed current systems are usually used if the service temperature is elevated,or if other factors require higher current densities.Impressed current systems are also used if higher driving potentials are needed due to the presence of high-resistance electrolytes,or if the economic benefit of such a system is considered significant to the project.

5.2.3An impressed current cathodic protection system is powered by an external source of direct current.The positive terminal of the direct current source is connected through insulated conductors to the anode system.The negative terminal of the direct current source is electrically connected to the tank bottom to be protected.Anode systems for on-grade storage tanks can include shallow groundbeds around or under the tank and/or deep anode groundbeds.

5.2.3.1Satisfactory anode materials include

mixed-metal oxides,polymer carbon,graphite, high-silicon chromium-bearing cast iron,platinized niobium(columbium),platinized titanium,scrap metal,and belowgrade metallic structures that have been removed from service and cleaned of contaminants.Anode selection should be based on soil chemistry,contaminants,and the compatibility of the anode with the environment.

RP0193-2001

5.2.4Galvanic current cathodic protection systems operate on the principle of dissimilar-metals corrosion. The anode in a galvanic current system must be more electrochemically active than the structure to be protected.Cathodic protection using a galvanic system is afforded by providing an electrical connection between the anode system and the storage tank bottom.Typical galvanic current anode materials for storage tank bottom applications include magnesium and zinc.

TABLE1

Cathodic Protection System Characteristics

Galvanic Current Impressed Current No external power required External power required

Fixed,limited driving voltage Driving voltage can be varied Limited current Current can be varied

Satisfies small current requirements Satisfies high current requirements Used in lower-resistivity environments Used in higher-resistivity environments Usually no stray current interference Must consider interference with other structures

5.3In the design of a cathodic protection system,the following shall be considered:

(a)Recognition of hazardous conditions prevailing at the site and the selection and specification of materials and installation practices that ensure safe installation and operation

(b)Compliance with all applicable governmental codes and owner requirements

(c)Selection and specification of materials and installation practices that ensure dependable and economic operation of the system throughout its intended operating life

(d)Design of proposed installation to minimize stray currents

(e)Avoiding excessive levels of cathodic protection, which may cause coating disbondment and possible damage to high-strength and special alloy steels

(f)If amphoteric metals are involved(i.e.,lead,tin, aluminum),avoiding high or low pH conditions that could cause corrosion

(g)Presence of secondary containment systems

5.4Current Requirement

5.4.1The preferred method of determining the current

requirements for achieving a given level of protection on an existing tank bottom is to test the tank bottom using a temporary cathodic protection system.

Alternately,a current density can be used for design purposes based on a current density successfully used at the same facility or at a facility with similar characteristics.

5.4.2For design purposes,current requirements on

new or proposed tank bottoms may be established by calculating surface areas and applying a protective current density based on the size of the tank,the electrochemical characteristics of the environment,the service temperature,and the parameters of the groundbed.Design current densities of10to20 mA/m2(1to2mA/ft2)of bare tank bottom surface are generally sufficient.Systems exposed to chemistry involving chlorides,sulfides,or bacteria or to elevated

service temperatures require more current.The history of other tanks in the same environment should be considered when choosing a design current density.

5.4.3Care must be exercised to ensure that anode

type and placement result in uniform distribution of protective current to the tank bottom surfaces.

5.4.4Liquid levels within tanks must be sufficient to

ensure that the entire tank bottom is in intimate contact with an electrolyte while establishing current requirements and testing applied protection levels.

Adequate liquid levels are important to maintaining polarization.

5.4.4.1As the liquid level increases(and more

of the tank bottom contacts the electrolyte),the

protective current requirement increases and the

potential measured may decrease due to the

increased surface area of steel contacting the

electrolyte.

5.5Tank System Configuration

5.5.1Design,materials,and construction procedures

that do not create shielding conditions should be used.

5.5.2Nonwelded mechanical joints might not be

electrically continuous.Electrical continuity can be ensured by bonding existing joints.

5.5.3If electrical isolation is required,care must be

taken to assure that the isolation is not shorted, bypassed,etc.

5.6Special consideration should be given to the presence of sulfides,chlorides,bacteria,coatings,elevated temperatures,shielding,pH conditions,treated tank padding material,soil/groundwater contamination,dissimilar metals,and pad/concrete/metal interface at the ringwall,as well as any heating or refrigeration coils under tank bottoms.Clean,fine sand is the preferred tank pad material.

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5.7On-grade tanks that are set on solid concrete or asphalt pad foundations generally require specialized measures for corrosion protection,because cathodic protection may be ineffective.In this circumstance,the external surface of the tank bottom should be coated.In all cases,steps should be taken to ensure that water does not migrate between the tank bottom and the pad.

5.7.1Providing electrical isolation between the

reinforcing steel and the tank bottom should be considered if a concrete ringwall or pad is used.5.8Design Drawings and Specifications

5.8.1Specifications should be prepared for all

materials and installation procedures that are used during construction of the cathodic protection system.

5.8.2Suitable drawings should be prepared to show

the overall layout of the tank bottoms to be protected and the cathodic protection system and associated appurtenances.

________________________________________________________________________ Section6:Design Considerations for Impressed Current Cathodic Protection

6.1This section recommends procedures and considerations that specifically apply to the design of impressed current cathodic protection systems for on-grade carbon steel storage tank bottoms.

6.2Impressed Current Anode Systems

6.2.1Impressed current anodes shall be connected

with an insulated cable,either singularly or in groups, to the positive terminal of a direct current source such as a rectifier or DC generator.The tank bottom shall be electrically connected to the negative terminal.

Cable insulation should be selected based on the anticipated environmental conditions and should be resistant to oil and water.6.2.2Anode groundbed configurations may be vertical,angled,deep,or horizontal,as illustrated in Figures4through7.Anodes may be installed in a distributed fashion under tank bottoms.The selection of anode configuration is dependent on environmental factors,current requirements,the size and type of tank bottom to be protected,whether the tank is of new or existing construction,and whether it is a single-or double-bottom tank.

Figure4:Vertically Drilled Anode CP System

Sand

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Figure 5:Angled Anode Cathodic Protection System

Figure 6:Deep Anode Groundbed

Figure 7:Horizontally Installed Anode Groundbed

6.2.3Deep anode systems should be designed and

installed in accordance with NACE Standard RP0572.4

6.2.4Anode materials have varying rates of deterioration when discharging current.Therefore,for

a given output,the anode life depends on the environment,anode material,anode weight,and the number of anodes in the cathodic protection system.Established anode performance data should be used to calculate the probable life of the system.

Sand

Sand

Deep Groundbed Anodes in Coke

Breeze-Filled Column

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NOTE:Platinized niobium(columbium)and polymeric anodes should not be used in hydrocarbon-contaminated environments.

6.2.5The useful life of impressed current anodes can be lengthened by the use of special backfill around the anodes.The most commonly used backfill materials are metallurgical coal coke and calcined petroleum coke.Because coke is noble compared to carbon steel,coke should not be allowed to come into contact with the tank bottom.

6.2.6In the design of an extensive,distributed-anode impressed current system,the voltage and current attenuation along the anode and the anode-connecting (header)cable should be considered.In such cases, the design objective should be to optimize anode system length,anode size and spacing,and cable size in order to achieve effective corrosion control over the entire surface of each tank bottom.

6.2.7Suitable provisions for venting the anodes should be made in situations in which it is anticipated that entrapment of gas generated by anodic reactions could impair the ability of the impressed current groundbed to deliver the required current.Venting systems must be designed to prevent contaminants from getting into the venting system.6.3Safety

6.3.1All impressed current systems must be designed

with safety in mind.Care must be taken to ensure that all cables are protected from physical damage and the possibility of arcing.

6.3.2Rectifiers and junction boxes must meet

regulatory requirements for the specific location and environment in which they are installed.Such locations shall be determined by reviewing regulatory agency and prevailing industrial codes.

6.3.2.1Consideration should be given to

locating isolating devices,junction boxes,and

rectifiers outside hazardous areas in case sparks

or arcs occur during testing.

6.3.3In order to prevent arcing,care must be

exercised when working on piping attached to tanks with cathodic protection applied.When cathodic protection systems are turned off,sufficient time must be allowed for depolarization before opening connections.Bonding cables must be used when parting piping joints.

________________________________________________________________________ Section7:Design Considerations for Galvanic Anode Cathodic Protection

7.1This section describes the factors that should be considered in the design of external corrosion protection of on-grade carbon steel storage tank bottoms without secondary containment that are protected by galvanic anode cathodic protection.

7.2Galvanic protection systems may be applied to a tank bottom if the carbon steel surface area exposed to the electrolyte can be minimized through the application of a dielectric coating,the surface area is small due to the tank size or configuration,or no power source or impressed current source is available.

7.2.1Galvanic anodes should be connected to the

tank bottom through a test station so that anode performance and voltage drops can be monitored.

7.2.2In applications for which the tank bottom is either

uncoated or large due to the tank size or configuration, the use of impressed current cathodic protection should be considered to minimize the cost of the protection system.Section6provides more information regarding the design considerations for impressed current cathodic protection systems.7.3Galvanic Anode Selection

7.3.1The three most common types of galvanic

anodes effective in soil environments are standard magnesium,high-potential magnesium,and high-purity zinc.

7.3.2The selection and use of these anodes should

be based on the current requirements of the tank bottom,the soil conditions,the temperature of the tank bottom,and the cost of the materials.

7.3.3The current available from each type of anode

depends greatly on the soil conditions,the anode shape(whether bar,block,or ribbon),and the driving potential of the anode.

7.3.4If high-purity zinc anodes are employed,care

should be exercised to ensure that the anodes meet the requirements of ASTM B4185Type II anode material.The purity of the zinc can greatly affect the performance of the material as a galvanic anode for soil applications.

7.3.5Zinc anodes should not be used if the

temperature of the anode environment is above49°C (120°F).Higher temperatures can cause passivation of the anode.The presence of salts such as carbonates,

RP0193-2001

bicarbonates,or nitrates in the electrolyte may also affect the performance of zinc as an anode material. 7.3.6Galvanic anode performance may be enhanced in most soils by using special backfill material. Mixtures of gypsum,bentonite,and sodium sulfate are the most common.7.3.7Galvanic anodes(except for rebar-type anodes) should be supplied with adequate lead wire attached by the anode supplier.

7.3.7.1Lead wire should be at least2mm in

diameter(#12AWG[American Wire Gauge.]) Cable insulation should be selected based on the anticipated environmental conditions and should typically be resistant to oil and water.

________________________________________________________________________ Section8:Design Considerations—Cathodic Protection for Tanks with Replacement Bottoms or

Release-Prevention Barriers

8.1Introduction

8.1.1Release-prevention barriers and replacement

tank bottoms can be used together or separately.

8.1.2Release-prevention barriers and/or secondary

carbon steel tank bottoms may shield the carbon steel surface of the primary tank bottom from the flow of cathodic protection current,resulting in a lack of adequate cathodic protection.

8.1.3Any impact(i.e.,corrosiveness)that the fill

material beneath or between the tank bottoms could have on the cathodic protection system should be considered.

8.2Release-Prevention Barriers

8.2.1Impervious membranes or liners constructed of

a nonconductive material used as a release-prevention

barrier can prevent the flow of cathodic protection current from anodes located outside the barrier envelope.Anodes must be placed between the barrier and the carbon steel tank bottom so that current flows to the surfaces requiring protection.

If release-prevention barriers made of conductive material are used with a cathodic protection system with anodes outside the space contained by the barrier, the barrier must maintain a resistance low enough for sufficient cathodic protection current to flow to the tank bottom.

8.2.2Stationary reference electrodes and/or portable

reference electrode insertion tubes must be located between the carbon steel tank bottom and the barrier or between the bottoms to obtain accurate structure-to-electrolyte data.

8.3Replacement Tank Bottoms

8.3.1If a replacement tank bottom is installed in an

existing tank over an original bottom so that there is an electrolyte between the two tank bottoms,galvanic corrosion activity can develop on the new bottom, resulting in premature failure.

8.3.2Cathodic protection should be considered for the

primary(new)bottom.The anodes and reference electrodes or nonconductive reference electrode insertion tubes must be placed in the electrolyte between the two bottoms.Figure8illustrates a typical double-bottom cathodic protection layout.

8.3.3The installation of a nonconductive,impervious

membrane or liner above the original bottom reduces galvanic corrosion activity on the replacement bottom, reducing the current required for cathodic protection.

8.3.4If the original tank bottom is removed and

replaced with a new bottom,the cathodic protection design should be that utilized for a standard,single-bottom tank.

RP0193-2001

Figure 8:Typical Double-Bottom Cathodic Protection Layout (Impressed or Sacrificial)

8.4Cathodic Protection Anodes

8.4.1Either impressed current or galvanic anode cathodic protection may be used.

8.4.1.1Galvanic anodes may be magnesium or zinc.Figure 9illustrates a typical double-bottom galvanic anode design.

8.4.1.2Anode materials that may be used for impressed current systems include mixed-metal oxides,polymer carbon,graphite,high-silicon chromium-bearing cast iron,platinized niobium (columbium),platinized titanium,scrap metal,and below-grade metallic structures that have been removed from service.Figure 10illustrates a typical new tank or double-bottom impressed current anode design.

8.4.1.3Due to the depolarizing effect of oxidation by-products (typically chlorine,oxygen,

or carbon dioxide)migrating from the anode to the steel cathode,the current density for protection with an impressed current system may be higher than that required for a galvanic anode system.

8.4.2Adequate space must be provided between the two tank bottoms to allow for installation of a cathodic protection system with uniform current distribution from the anodes.Due to limited space between bottoms,close anode spacing may be required to improve current distribution.

Impressed current anodes must not contact the carbon steel surfaces of the tank.

8.4.4Anodes must be installed in a conductive electrolyte.The electrolyte must be sufficiently compacted as to prevent settlement of the replacement tank bottom.

RP0193-2001

Tank Shell

Figure9:Typical Double-Bottom Galvanic Anode Design

Tank Shell

Wire Anode

Figure10:Typical New Tank or Double-Bottom Impressed Current Anode Design

RP0193-2001 ________________________________________________________________________

Section9:Installation Considerations

9.1This section recommends elements to consider during the installation of cathodic protection systems for on-grade carbon steel storage tank bottoms.

9.2Preparation

9.2.1Materials should be inspected prior to installation

in order to ensure that specifications have been met.

9.2.2Installation practices shall conform to all

applicable regulatory agencies codes and requirements.

9.3Anode Installation

9.3.1Anodes should be installed as designed.Care

must be taken to ensure that the anodes do not come into electrical contact with any piping or tankage during installation.

9.3.2Slack should be allowed in the anode lead wires

to avoid possible damage due to settlement of the tank and surrounding soils.Anodes,lead wires,and connections should be handled with care to prevent damage or breakage.

9.3.3The anode lead wires should be extended to the

side of the tank away from the construction to minimize possible damage.After the tank foundation has been prepared and the tank set in place,the wires should be terminated in a test station or junction box,which may include shunts for measuring anode current outputs.

9.4Reference Electrodes

9.4.1Stationary reference electrodes or noncon-

ductive,perforated tubes for temporary installation of a portable reference electrode should be installed under all tanks regardless of the groundbed type and location.

9.4.1.1Stationary reference electrodes may be

prepackaged in a backfill and placed in the soil

under the tank bottom or positioned inside the perforated reference electrode access piping.

Reference electrodes placed inside access piping should be surrounded with a backfill material designed to provide contact between the electrode and the electrolyte outside the pipe.If practical,provisions should be made for future verification of all stationary reference electrode potentials with portable reference electrodes.

9.4.1.2Reference electrode access piping must

have some means of contact with the electrolyte and should have at least one end accessible from outside the tank shell.This contact can be through the use of holes,slits,or not capping the end of the piping beneath the tank.Perforations and slots should be designed to minimize entry of tank pad material.Portable reference electrodes shall be inserted through the inside diameter of the access pipe with a nonmetallic material such as small-diameter polyvinyl chloride(PVC)pipe.Inserting a reference electrode with metallic tape,bare wires,etc.,may adversely affect potential readings.If necessary, water should be injected inside the access pipe to establish continuity between the electrode and the electrolyte.Deionized water should be used for double-bottom tanks or tanks with secondary containment.

9.4.2For existing tanks,reference electrode access piping should be installed under the tank with horizontal drilling equipment capable of providing guidance and directional control to prevent tank bottom damage and to ensure accurate placement of the piping. Consideration must be given to the structural aspects of the tank padding and foundation to ensure that support capabilities are not adversely affected.Figure 11illustrates the placement of perforated pipe installed for a reference electrode.

RP0193-2001

Figure11:Perforated Pipe Installed for Reference Electrode

NOTE:Special consideration must be given during the design and installation of access pipes to assure that any tank-containment system is not breached.

CAUTION:Extreme caution must be exercised when boring or water jetting under tanks.

9.4.2.1A constant distance should be

maintained from the tank bottom to the reference

electrode.Increasing space between tank

bottom and reference electrode increases the

voltage drop.

9.5Test Stations and Junction Boxes

9.5.1Test stations or junction boxes for potential and

current measurements should be provided at sufficient locations to facilitate cathodic protection testing.

9.5.2The test station or junction box should be

mounted on or near the side of the tank in an area that is protected from vehicular traffic.

9.5.3The test station or junction box should allow for

disconnection of the anodes to facilitate current measurements and potential measurements for voltage drop as required to evaluate the protection level.If desired,test leads from buried reference electrodes can be terminated in the same test station as tank bottom test wires.

9.5.4Junction boxes can be used to connect

continuity bonds or protective devices.

9.5.5The test station or junction box in a galvanic

system may be equipped with calibrated resistors (shunts)in connections between the anodes and the tank to measure the anode current output and thus the estimated anode life.Shunts are typically rated between0.001and0.1ohm.

9.5.6The test station or junction box should be clearly

marked and accessible for future monitoring of the tank bottom and,if possible,should be attached to the tank.

9.5.7All lead wires to the test station or junction box

should be protected from damage by a minimum46-cm(18-in.)burial and/or placement within a conduit.

Warning tape may be installed over direct-buried cables to prevent the possibility of damage during future excavation.

9.6Safety Considerations

9.6.1All personnel to be involved in the installation of

the cathodic protection system should participate in a thorough safety-training program.

9.6.2All underground facilities,including buried

electric cables and pipelines in the affected areas, should be located and marked prior to digging.

9.6.3All utility companies and other companies with

facilities crossing the work areas should be notified and their affected structures located and marked prior to digging.

9.6.4All areas with low overhead wires,pipelines,and

other structures should be located and noted prior to any construction.

9.6.5Operations and maintenance personnel should

be notified of pending construction to coordinate necessary shutdowns or emergency considerations.

Reference Electrode

RP0193-2001 ________________________________________________________________________

Section10:Energizing and Testing

10.1This section discusses factors that should be considered when energizing and testing a cathodic protection system for on-grade carbon steel storage tank bottoms.If the tank has a secondary containment system, suitable access ports through the ringwall must be provided for testing.

10.2Design Parameters

10.2.1Knowledge of the performance criteria

considered during the design of a cathodic protection system as well as the operational limits of cathodic protection devices and hardware should be available to the personnel setting operating levels for the cathodic protection system.

10.3Initial Data

10.3.1Verification of cathodic protection devices

and hardware,such as the following,should be done prior to energizing:

(a)Location of anodes

(b)Ratings of impressed current sources

(c)Location of reference electrodes

(d)Location of test facilities

(e)Location of cathodic protection system cables

10.3.2Prior to energizing the cathodic protection

system,the following data and information should be collected:

(a)Tank bottom-to-electrolyte potentials

(b)Pipe-to-electrolyte potentials on connected

piping

(c)Verification of dielectric isolation

(d)Foreign structure-to-electrolyte potentials

(e)Test coupon data

(f)Fluid level in the tank during testing,and

(f)Corrosion-rate probe data.

10.3.3All initial baseline data should be

documented and the records maintained for the life of the cathodic protection system or the on-grade storage tank.Any deviations from the design or as-built documentation should be noted and included with the initial baseline data.

10.3.4When measuring the structure-to-electrolyte

potential,the portable reference electrodes should be placed at sufficient intervals around the perimeter and under the tank to ensure the potentials measured are representative of the entire tank bottom.The potential measured at the perimeter of a large-diameter tank does not represent the potential at the center of the tank.10.4Current Adjustment

10.4.1The desired operating level of a cathodic

protection system must often be determined by a series of trial tests at various operating levels.The specific operating level depends on the criterion for cathodic protection used for the on-grade storage tank(s).Section4defines the various criteria for achieving cathodic protection of on-grade carbon steel storage tank bottoms.Time required to achieve polarization on a bare tank bottom can be different from tank to tank.

10.4.2When the operating levels of cathodic

protection systems are adjusted,consideration must be given to the effect of stray current on adjacent structures.Owners of these structures should be notified of the installation of a new cathodic protection system.

10.4.2.1Among the structures that should be

considered as being possibly affected by stray

current are:

(a)On-grade and buried storage tanks

(b)Piping separated from the tank(s)by high-

resistance fittings

(c)Buried electric facilities

(d)Buried fire-protection piping

(e)Buried water piping

(f)Transmission or distribution piping serving

storage tank(s)

(g)Municipal or public utility structures serving

the facility in which a storage tank(s)is located

(h)Fencing

10.4.2.2Structures that may contain

discontinuous fittings or joints,such as cast iron

systems,ductile iron piping systems,or piping

with mechanically connected fittings,require

special attention to ensure that stray current

effects are detected and mitigated.

10.4.3The final operating level of a cathodic

protection system should be established to achieve the cathodic protection criterion established by the design documents as set forth in Section4,or by the operating policies of the facility owner.

10.5Documentation

10.5.1Documentation of all operating parameters

should be completed after the system is energized.

Those parameters should include:

(a)Initial baseline data

(b)As-built drawings

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