EN 16299:2013
(Main)Cathodic protection of external surfaces of above ground storage tank bases in contact with soil or foundations
Cathodic protection of external surfaces of above ground storage tank bases in contact with soil or foundations
This European Standard defines the conditions necessary for an effective application of the cathodic protection method to mitigate corrosive attacks on the external surfaces of above ground storage steel tank bottoms in contact with soil, cushion or foundations.
This European Standard specifies the requirements for the design, implementation, commissioning, operation and maintenance of such a cathodic protection system.
This European Standard applies both for new and existing tanks.
NOTE 1 This European Standard is not applicable to reinforced concrete above ground storage tanks for which EN ISO 12696 applies.
NOTE 2 Detailed information concerning measurement techniques of cathodic protection given in EN 13509 are referred to in the present standard.
NOTE 3 Cathodic protection of internal surfaces of above ground storage steel tanks storing waters is addressed in EN 12499.
NOTE 4 Cathodic protection of external surfaces of buried tanks is addressed in EN 13636.
Kathodischer Korrosionsschutz für erdberührte und gegründete Außenflächen von oberirdischen Lagertanks aus Stahl
Diese Europäische Norm definiert die Bedingungen, die für eine effektive Anwendung kathodischer Schutzverfahren notwendig sind, um Korrosionsangriffe auf die Außenflächen der Böden in Kontakt mit Erdboden, Bettungen oder Fundamenten von Stahltanks für oberirdische Lagerung zu verringern.
Diese Europäische Norm legt Anforderungen an Konzipierung, Umsetzung, Inbetrieb¬nahme, Betrieb und Wartung kathodischer Schutzsysteme fest.
Diese Europäische Norm gilt sowohl für neu zu errichtende als auch für bereits bestehende Tankanlagen.
ANMERKUNG 1 Die vorliegende Europäische Norm ist nicht auf oberirdische Lagertanks aus Stahlbeton anwendbar; für diese Lagertanks gilt EN 12696.
ANMERKUNG 2 Ausführlichere Angaben zu den Messtechniken für den kathodischen Korrosionsschutz, die in EN 13509 gegeben sind, werden in dieser Norm in Bezug genommen.
ANMERKUNG 3 Der kathodische Korrosionsschutz für die Innenflächen oberirdischer Lagertanks für Wasser wird in EN 12499 behandelt.
ANMERKUNG 4 Der kathodische Korrosionsschutz der Außenflächen von unterirdischen Tankanlagen wird in EN 13636 behandelt.
Protection cathodique des surfaces externes des fonds de réservoirs de stockage aériens en contact avec le sol ou les fondations
La présente Norme européenne définit les conditions nécessaires à une application efficace du procédé de protection cathodique visant à atténuer les attaques corrosives sur les surfaces externes des fonds de réservoirs en acier de stockage aériens (AST) en contact avec le sol, la couche d'assise ou les fondations.
La présente Norme européenne spécifie les exigences concernant la conception, la mise en œuvre, la mise en service, l’exploitation et la maintenance d’un tel système de protection cathodique.
La présente Norme européenne s'applique aussi bien aux nouveaux réservoirs qu’aux réservoirs existants.
NOTE 1 La présente Norme européenne n’est pas applicable aux réservoirs de stockage aériens en béton armé pour lesquels l’EN ISO 12696 s’applique.
NOTE 2 La présente Norme fait référence à des informations détaillées concernant les techniques de mesure de la protection cathodique données dans l’EN 13509.
NOTE 3 La protection cathodique des surfaces internes des réservoirs en acier de stockage aériens contenant des eaux est traitée dans l'EN 12499.
NOTE 4 La protection cathodique des surfaces externes des réservoirs enterrés est traitée dans l'EN 13636.
Katodna zaščita zunanjih površin nadzemnih nosilcev skladiščnih cistern, ki so v stiku z zemljo ali temelji
Ta evropski standard določa pogoje, ki so potrebni za učinkovito uporabo metode za katodno zaščito za preprečevanje korozije zunanjih površin jeklenih nadzemnih nosilcev skladiščnih cistern, ki so v stiku z zemljo, oblazinjenjem ali temelji. Ta evropski standard določa zahteve za načrtovanje, izvajanje, zagon, delovanje in vzdrževanje takih katodnih zaščitnih sistemov. Ta evropski standard velja za nove in obstoječe cisterne. OPOMBA 1 Ta evropski standard se ne uporablja za nadzemne skladiščne cisterne, ki so ojačane z betonom in za katere velja standard EN ISO 12696. OPOMBA 2 Podrobne informacije o tehnikah merjenja katodne zaščite iz standarda EN 13509 so navedene v tem standardu. OPOMBA 3 Katodna zaščita notranjih površin nadzemnih jeklenih skladiščnih cistern za shranjevanje vode je obravnavana v standardu EN 12499. OPOMBA 4 Katodna zaščita zunanjih površin zakopanih cistern je obravnavana v standardu EN 13636.
General Information
Standards Content (Sample)
2003-01.Slovenski inštitut za standardizacijo. Razmnoževanje celote ali delov tega standarda ni dovoljeno.Kathodischer Korrosionsschutz für erdberührte und gegründete Außenflächen von oberirdischen Lagertanks aus StahlProtection cathodique des surfaces externes des fonds de réservoirs de stockage aériens en contact avec le sol ou les fondationsCathodic protection of external surfaces of above ground storage tank bases in contact with soil or foundations77.060Korozija kovinCorrosion of metals23.020.10UH]HUYRDUMLStationary containers and tanksICS:Ta slovenski standard je istoveten z:EN 16299:2013SIST EN 16299:2013en,fr,de01-junij-2013SIST EN 16299:2013SLOVENSKI
STANDARD
EUROPEAN STANDARD NORME EUROPÉENNE EUROPÄISCHE NORM
EN 16299
April 2013 ICS 77.060 English Version
Cathodic protection of external surfaces of above ground storage tank bases in contact with soil or foundations
Protection cathodique des surfaces externes des fonds de réservoirs de stockage aériens en contact avec le sol ou les fondations
Kathodischer Korrosionsschutz für erdberührte und gegründete Außenflächen von oberirdischen Lagertanks aus Stahl This European Standard was approved by CEN on 23 February 2013.
CEN members are bound to comply with the CEN/CENELEC Internal Regulations which stipulate the conditions for giving this European Standard the status of a national standard without any alteration. Up-to-date lists and bibliographical references concerning such national standards may be obtained on application to the CEN-CENELEC Management Centre or to any CEN member.
This European Standard exists in three official versions (English, French, German). A version in any other language made by translation under the responsibility of a CEN member into its own language and notified to the CEN-CENELEC Management Centre has the same status as the official versions.
CEN members are the national standards bodies of Austria, Belgium, Bulgaria, Croatia, Cyprus, Czech Republic, Denmark, Estonia, Finland, Former Yugoslav Republic of Macedonia, France, Germany, Greece, Hungary, Iceland, Ireland, Italy, Latvia, Lithuania, Luxembourg, Malta, Netherlands, Norway, Poland, Portugal, Romania, Slovakia, Slovenia, Spain, Sweden, Switzerland, Turkey and United Kingdom.
EUROPEAN COMMITTEE FOR STANDARDIZATION
COMITÉ EUROPÉEN DE NORMALISATION EUROPÄISCHES KOMITEE FÜR NORMUNG
Management Centre:
Avenue Marnix 17,
B-1000 Brussels © 2013 CEN All rights of exploitation in any form and by any means reserved worldwide for CEN national Members. Ref. No. EN 16299:2013: ESIST EN 16299:2013
Groundbed data . 39 A.1 General considerations . 39 A.2 Types of ground beds . 39 A.3 Anodes types . 40 Bibliography . 42
This European Standard applies only for external corrosion prevention, which is independent of internal corrosion issues. During the design of any new tank, a complete corrosion control study including the use of cathodic protection methods should be performed for preventing external corrosion of the surfaces in contact with soil, cushion or foundations. When cathodic protection is adopted, it is an effective method if designed, implemented, operated and maintained in accordance with this standard. The pre-requisites for the cathodic protection should be taken into account from the basic design. In case cathodic protection is not adopted, a documented technical justification on the equivalent effectiveness of alternative methods should be given. For existing tanks, corrosion risks of external surfaces of tank bottoms may be important and possibly cause leaks, depending on the nature of soil, cushion, foundations, design of tank, and other equipment electrically connected to it such as an earthing system. Depending on the design and environmental conditions of the tank as detailed in the present standard, cathodic protection may be effective to mitigate corrosion when designed, implemented, operated and maintained in accordance with this standard.
Cathodic protection is aimed at supplying a direct current (d.c.) to the steel surface such that the steel-to-electrolyte potential is lowered to values where corrosion becomes insignificant. Cathodic protection of above ground storage steel tanks is normally used in combination with a compatible protective coating system to protect the external surfaces of above ground storage steel tank bottoms from corrosion.
This European Standard applies both for new and existing tanks. NOTE 1 This European Standard is not applicable to reinforced concrete above ground storage tanks for which EN ISO 12696 applies. NOTE 2 Detailed information concerning measurement techniques of cathodic protection given in EN 13509 are referred to in the present standard. NOTE 3 Cathodic protection of internal surfaces of above ground storage steel tanks storing waters is addressed in EN 12499. NOTE 4 Cathodic protection of external surfaces of buried tanks is addressed in EN 13636. 2 Normative references The following documents, in whole or in part, are normatively referenced in this document and are indispensable for its application. For dated references, only the edition cited applies. For undated references, the latest edition of the referenced document (including any amendments) applies. EN 13509, Cathodic protection measurement techniques
EN 14015, Specification for the design and manufacture of site built, vertical, cylindrical, flat-bottomed, above ground, welded, steel tanks for the storage of liquids at ambient temperature and above EN 14505, Cathodic protection of complex structures
EN 50162, Protection against corrosion by stray current from direct current systems EN 60079-0, Explosive atmospheres — Part 0: Equipment — General requirements (IEC 60079-0) EN 60079-1, Explosive atmospheres — Part 1: Equipment protection by flameproof enclosures "d" (IEC 60079-1) EN 60079-2, Explosive atmospheres — Part 2: Equipment protection by pressurized enclosures "p" (IEC 60079-2) EN 60079-5, Explosive atmospheres — Part 5: Equipment protection by powder filling "q" (IEC 60079-5) EN 60079-7, Explosive atmospheres — Part 7: Equipment protection by increased safety "e" (IEC 60079-7) EN 60079-10-1, Explosive atmospheres — Part 10-1: Classification of areas — Explosive gas atmospheres (IEC 60079-10-1) EN 60079-11, Explosive atmospheres — Part 11: Equipment protection by intrinsic safety "i" (IEC 60079-11) EN 60079-14, Explosive atmospheres — Part 14: Electrical installations design, selection and erection (IEC 60079-14) SIST EN 16299:2013
(IEC 60079-15) EN 60079-18, Explosive atmospheres — Part 18: Equipment protection by encapsulation "m" (IEC 60079-18) EN 60079-25, Explosive atmospheres — Part 25: Intrinsically safe electrical systems (IEC 60079-25) EN 60587, Electrical insulating materials under severe ambient conditions — Test methods for evaluating resistance to tracking and erosion (IEC 60587) EN 61558-1, Safety of power transformers, power supplies, reactors and similar products — Part 1: General requirements and tests (IEC 61558-1) EN ISO 8044, Corrosion of metals and alloys — Basic terms and definitions (ISO 8044) 3 Terms and definitions For the purposes of this document, the terms and definitions given in EN ISO 8044 and EN 14015 and the following apply. 3.1 cushion material in contact with the bottom of an aboveground storage tank
3.2 foundations buried construction aimed at mechanically supporting locally the above ground storage tank 3.3 hazardous area area in which an explosive atmosphere is present, or may be expected to be present, in quantities such as to require special precautions for the construction, installation and use of equipment
Note 1 to entry: For the purposes of this standard, an area is a three-dimensional region or space.
3.4 IR drop voltage that is the product of all currents flowing through the cathodic protection circuit and the resistance of the current path (mainly the electrolyte and the tank bottom) Note 1 to entry: This is derived from Ohm’s law (U = I x R). 3.5 IR free potential polarised potential structure to electrolyte potential without the voltage error caused by the IR drop due to the protection current or any other current 3.6 local earthing system local earthing system for the structure under consideration which is electrically separated from any other general earthing 3.7 shield conductive or non conductive structure or object, which modifies the protection current distribution on a structure to be protected SIST EN 16299:2013
MMO Mixed Metal Oxides T Temperature t Thickness of cushion ρ Resistivity
5 Competence of personnel Personnel who undertake the design, supervision of installation, commissioning, supervision of operation, measurements, monitoring and supervision of maintenance of cathodic protection systems shall have the appropriate level of competence for the tasks undertaken.
EN 15257 constitutes a suitable method of assessing and certifying competence of cathodic protection personnel which may be utilised.
Competence of cathodic protection personnel to the appropriate level for tasks undertaken should be demonstrated by certification in accordance with EN 15257 or by another equivalent prequalification procedure. SIST EN 16299:2013
6.1 General The corrosion rate of a metal in soil or water is a function of the potential, E, of the material in its surrounding electrolytic environment. The corrosion rate decreases as the potential is shifted in the more negative direction. This negative potential shift is achieved by feeding direct current from anodes via the soil or water to the metal surface of the structure to be protected. In the case of coated structures, the current mainly flows to the metal surface at coating pinholes and holidays. The protection current can be provided by impressed current systems or galvanic anodes. The corrosion risks and the need and feasibility of cathodic protection for a given above ground storage tank are directly linked to the mechanical design of the storage tank bottoms and to its environment. The cushion material in contact with the tank has a significant effect on external corrosion of the tank bottom and can influence the effectiveness and applicability of external cathodic protection. During the design of any new tank, a complete corrosion control study including the use of cathodic protection shall be performed. In case cathodic protection is not adopted, a documented technical justification on the equivalent effectiveness of alternative methods shall be given.
Cathodic protection is an effective method of corrosion prevention if electrical protection current reaches the whole surface of the tank bottom. Factors that may reduce or prevent the flow of current are: concrete foundations (shielding effect depending on anode location); asphalt or oiled sand made cushions; impervious membranes (depending on anode location); new tank bottoms installed above the original tank bottom and therefore not in contact with the cushion; excessive current demand caused by external structures such as earthing networks, especially when made of bare copper, and contact with steel reinforcement; interference from foreign cathodic protection systems; trapped air between the tank bottom and the cushion. 6.2 Corrosion risks of external surfaces of above ground storage tank bottoms in contact with soil or foundations 6.2.1 General The external bottom of a tank is in contact with the cushion and, therefore, just like any other structure in contact with an electrolyte, it is subject to corrosion. The major factors to consider for an assessment of the corrosive conditions are the corrosivity of the cushion in contact with the bottom, galvanic effects, differential aeration and environmental conditions.
The tank bottom is subjected to large load variations caused by loading and discharge of the tank. It is also subject to temperature changes with the seasons and the stored product. The tank bottom is flexible, if not well supported by the soil and/or the cushion, except at its periphery where it is very rigid due to the weight of the shell on the concrete ring foundation. To a lesser degree, some rigidity exists at the periphery of each plate due to overlapping and lap-welds. This can result in vertical motions with possible introduction of moist air, which promotes corrosion. SIST EN 16299:2013
retention of precipitation in the bund area; resistivity measurements of the cushion and surrounding soil; likelihood of sulphate reducing bacteria development. Soil conditions contributing to the so-called “corrosion load” (sum of all the effects on a steel structure due to a corrosive medium) are developed in EN 12501-1 and EN 12501-2. This standard enables classification of the risks of corrosion in three groups (low, medium and high “corrosion load”). It gives some guidance for assessing “corrosion load” from pH and resistivity of soil samples. Figure 1 reproduces Table 1 of
EN 12501-2:2003 for pH less than 9,5.
Key 1 high 2 medium 3 low * Minimum resistivity value after adding deionised water (Ω.m) Figure 1 — Corrosivity of soil (free corrosion without concentration cell from EN 12501-2:2003) NOTE 1 API Standard 651 third edition [2] recommends the use of resistivity for a qualitative classification of soil “potential corrosion activity” as follows: • resistivity < 5 Ω.m: very corrosive; • resistivity from 5 Ω.m to 10 Ω.m: corrosive; • resistivity from 10 Ω.m to 20 Ω.m: moderately corrosive; • resistivity from 20 Ω.m to 100 Ω.m: mildly corrosive; • resistivity > 100 Ω.m: progressively less corrosive. NOTE 2 API Standard 651 second edition [1] related to chloride and sulphate concentrations: chlorides between 0,03 % to 0,1 % or sulphates between 0,1 % to 0,5 % were considered corrosive and chlorides higher than 0,1 % or sulphates higher than 0,5 % were considered very corrosive.
6.2.3 Galvanic effects Galvanic cells can develop on the bottom of the tank itself due to different potentials between the steel of the plates and the weld metal, and possibly due to metallurgical differences in the metal of adjacent plates that make up the tank bottom. The lower sides of the tank bottom are often coated with a bituminous solution. When this product is burnt during welding operations it can turn into conductive carbon particles, which will form a galvanic cell with the tank bottom. Galvanic cells can exist if the tank is directly electrically linked to earthing systems made of copper or other metals or alloys such as stainless steels which are more cathodic than steel. The bottom surface can be corroded as illustrated in Figure 2. Galvanic cells can exist if the tank is in contact with steel reinforcement of a concrete ring beam.
NOTE Generally, on new tanks, the presence of a waterproof plastic membrane spread out under the tank cushion before its construction minimises ingress of water or other contaminants from the native surrounding soil towards the cushion material and suppress, by shielding effect, galvanic cells caused by earthing systems. See 7.2.4 for more detailed information.
Key 1
above ground storage tank 4
soil 2
cathode 5
anodic area 3
concrete ring beam 6
bare copper earthing system 7
wet soil Figure 2 — Galvanic corrosion of external surface of tank bottom due to coupling with copper earthing system 6.2.4 Stray currents
The presence of stray currents may result in local accelerated corrosion. When cathodic protection is applied, greater current requirements than those required under natural conditions may be necessary. 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. More details on risks and their mitigation are given in EN 50162. SIST EN 16299:2013
The detailed design of the cathodic protection system shall take into account the type of cushion, foundations and earthing connections to the tank. The effectiveness of the applied cathodic protection system can be adversely affected by the construction methods and materials. The effectiveness of the cathodic protection system shall be demonstrated by conforming to the criteria specified in Clause 8. Measurement of the tank bottom potentials is best achieved by the use of permanent reference electrodes and coupons placed close to the tank bottom during construction. Other techniques can be used, such as drilled tubes filled with an electrolyte and a movable reference electrode, but due consideration should be taken of the effectiveness of the measurements. For existing tanks, cathodic protection can be effective in mitigating the corrosion although there may be limitations imposed by the nature of the cushion, availability of correct locations for the installation of anodes and the difficulties imposed by earth connections made to the tanks. Criteria in Clause 8 shall be applied. 7.2 New tanks 7.2.1 Tank cushion material The tank cushion material is required to provide a soft base for the tank bottom with a low corrosion loading and allowing efficient flow of a cathodic protection current. The best material for this is cleaned and washed sand. The sand shall be of uniform small grain size and free from stones and other debris. This is discussed in EN 14015. After the laying of the sand, it is important that no construction debris, such as welding rods or other metallic objects, is present before compacting the sand. Stones and debris will cause localised corrosion when in contact with the bottom of the tank, may damage anodes laid in the sand beneath the tank and will impede the flow of cathodic protection current. The sand shall be washed, and screened to ensure uniform particle size. In particular the sand should have a chloride content of less than 0,01 % with a pH higher than 6.5 and shall not be impregnated with any form of oil or coagulant, e.g. coal-tar enamel, asphalt, crude oil, oiled-sand cushions. The addition of oil and coagulant will prevent cathodic protection current reaching the tank bottom and will provide no benefits in preventing corrosion Samples of the sand shall be subject to soil resistivity measurement, chloride content measurement and pH measurement.
Air introduced due to vertical motions of tank bottoms during loading and discharge can prevent access of cathodic protection current and consequently have an adverse effect on the effectiveness of the cathodic protection. In order to avoid this, it is recommended that when sand is used for the cushion its granular size is selected such that it maintains a good contact with the plates even at the level of the laps. 7.2.2 Ring beam of the tank
The main purpose of the concrete ring beam is to support the shell and to avoid the spread of the cushion. Direct electrical contact between the tank and the rebar creates a galvanic cell between the bottom plates and the ring beam steel reinforcement.
This contact may be avoided, or at least limited, by sufficient concrete thickness over the external layer of steel reinforcement bars (rebars) and/or the use of insulation, or high duty coatings on the steel plate and/or on the concrete ring.
Additional cathodic protection current may be required to compensate for the current losses to the steel reinforcement and to eliminate the galvanic corrosion cells. SIST EN 16299:2013
Key 1
shell: 20 mm to 40 mm 5
flexible sealing 2
lap-weld 6
concrete ring beam with rebar 3
external plate: 10 mm to 12 mm 7
natural soil 4
sandy cushion
Figure 3 — Importance of sealing to prevent water ingress 7.2.3 Bottom external coatings To reduce the levels of cathodic protection current the tank bottom plates can be externally coated. Regardless of any coatings applied to protect against atmospheric corrosion during transit and storage, the plates should have a qualified coating applied before their installation. The coating should be properly specified to suit the installation and operating conditions, particularly temperature, and compatibility with cathodic protection. 7.2.4 New tank with membrane Plastic membranes are often installed to contain leaks. The membrane will also electrically isolate the tank bottom from the ground, which has the advantage of preventing external galvanic couples but has the disadvantage of restricting the space available for the cathodic protection anodes and references electrodes or other monitoring systems. The anode system shall be installed between the membrane and the bottom of the tank as shown by Figure 4. Any other solution will not work. A minimum distance between the tank bottom and the membrane shall be maintained in order to allow proper and reliable installation of the cathodic protection anode and monitoring systems. NOTE A typical distance between tank bottom and membrane is 600 mm.
Any kind of metals for earthing system may be used because there is no risk of galvanic coupling. SIST EN 16299:2013
If it is not possible, earthing with metals and alloys more cathodic than steel, such as copper or stainless steels, should not be used. Alternative earthing materials such as galvanised steel or zinc should be selected that fulfil the safety requirements and do not create detrimental galvanic couples.
Where earthing systems made of metals and alloys more cathodic than steel are used, the adverse effect of the directly connected earthing systems can be overcome either by an increase of the cathodic protection current (typically the current will need to be at least doubled) or by connecting to the tank via a d.c. decoupling device such as a polarisation cell. Polarisation cells do not impede the flow of alternating current but they block direct current and therefore have no adverse effect on the electrical safety and lightning protection provisions. A local earthing may be used in order to reduce the cathodic current demand.
Care should be exercised when applying isolation in this manner to ensure that the isolation integrity is not compromised by instrumentation earths or inadvertent short circuiting of the isolation by other pipework. In a large plant the complexities are such that it may not be possible to achieve electrical isolation. The installation of electrical equipment (e.g. pumps, electrically controlled valves, telemetric measuring devices.) in contact with the tank to be protected can affect the electrical separation between this tank and the general earthing system. Depending on national regulations, separation can be achieved by:
a) the isolation of electrical equipment from the cathodically protected structure. In this case the equipment is not protected by the cathodic protection system of the tank; b) the use of electrical equipment of protection classes II or III defined in EN 61140; c) the installation of a fault current breaker, if necessary in conjunction with a local earthing system;
d) the use of an isolating transformer (safe isolation, see EN 61558-1); e) the installation of d.c. decoupling devices between the electrical equipment and the general earthing system. Where isolation devices are to be used they shall be fully specified within the design both in terms of performance and location. Typical devices include polarisation cells (or any electronic equivalent), surge divertors, lightning arrestors, and insulated flange kits. These should be installed in such a way that accidental contacts of the isolated parts of the structure to the general earthing systems are avoided. They have to be protected against damage caused by atmospheric and mechanical influences. SIST EN 16299:2013
Key 1
rectifier 6
earthing system 2
reference electrode 7
wet soil 3
concrete ring beam 8
plastic membrane 4
soil 9
sump 5
anode
Figure 4 — Cathodic protection applied in conjunction with the use of a plastic membrane If any electrical isolation is installed between the storage tank and the piping, it shall be compatible with: national applicable regulations, general safety requirements in accordance with codes and standards. In addition, all isolating joints, whether monobloc or flanges, shall be fitted with lightning protection devices to protect them from damage by high voltage.
All isolating joints should preferably be installed above ground and, for measurement purposes, should be easily accessible from both sides. If buried, they shall be coated. The requirements for isolating joints in hazardous areas are given in 9.9.3. The anode system necessary to protect the tank may be installed under the bottom of the tank. Remote anodes can work if properly designed, particularly with respect to the earthing system and piping systems connected to the tank. 7.3 Existing tanks 7.3.1 Extent of corrosion
Cathodic protection and repair strategies should be developed from Information on the degree and location of tank-bottom corrosion.
Field procedures for determining the extent of existing corrosion can include: a) visual inspection, sampling of removed bottom plate; SIST EN 16299:2013
7.3.2 Electrical isolation Electrical isolation facilities shall be compatible with electrical earthing requirements conforming to applicable codes and safety requirements. If the tank bottom is to be cathodically protected, the use of alternative electrical earthing materials, such as galvanised steel and galvanic anodes, shall be considered. Alternatively, a local earthing system may be used in order to reduce the cathodic current demand. In this case, it shall be checked that no short-circuit exists with other structures in contact with the general earthing circuit system or other local earthing systems. The 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. Electrical 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. Polarisation cells, lightning arrestors, 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. 7.3.3 Secondary steel bottoms The problem of corroded tank bottoms are sometimes overcome by installing a new steel tank floor above the existing, corroded, tank bottom. In such cases, it is not possible to achieve cathodic protection of the bottom of the new floor unless the void between the original and the secondary bottom is filled with an electrolyte, such as clean sand, and a cathodic protection system installed between the two bottoms. Figure 5 illustrates such a system. It does not show permanent reference electrodes, which are necessary to monitor the effectiveness of cathodic protection. SIST EN 16299:2013
Key 1
overground tank cathodic protection with rectifiers and anodes 5
anodes 2
rectifier 6
earthing system 3
concrete ring beam 7
old steel floor 4
soil 8
new steel floor Figure 5 — Cathodic protection applied to a secondary steel bottom 7.4 Tank design and feasibility of cathodic protection
The feasibility of installing a cathodic protection system for protecting the external side of tank bottoms is summarised in Table 1 for new tanks and Table 2 for existing tanks.
Table 1 — Tank design and cathodic protection feasibility – New tanks Tank design Feasibility of cathodic protection Membrane under bottom with sand cushion, with or without connection to earthing systems Feasible if minimum distance between membrane and bottom observed, excellent conditions for ensuring effectiveness of protection. No membrane, with clean sand cushion, no direct connection to earthing systems
Feasible, excellent conditions for ensuring effectiveness of protection. No membrane, with any kind of cushion other than clean sand, no direct connection to earthing systems
Feasible, but effectiveness depends on type and quality of cushion. No membrane, connection to an earthing system
Feasible, but effectiveness depends on type and quality of cushion with higher current demand to apply. SIST EN 16299:2013
Feasible excellent conditions for ensuring effectiveness of protection. No membrane, with any kind of cushion other than clean sand, no direct connection to earthing systems
Feasible, but effectiveness depends of type and quality of cushion. Especially questionable where soil or cushion contain oily materials.
No membrane, connection to an earthing system
Feasible, but effectiveness depends on type and quality of cushion with higher current demand to apply. Especially questionable where soil or cushion contain oily materials. Secondary steel tank bottom Feasible only if design adapted to ensure protection in the void between the old and new bottoms
8 Criteria for cathodic protection and measurement techniques 8.1 General For any new tank, effectiveness of cathodic protection shall be demonstrated in accordance with the criteria specified in 8.2.
For existing tanks, effectiveness should be verified in accordance with the criteria specified in 8.2 whenever possible. If this is not possible because of contact with other buried structures or earthing systems, alternative criteria and methods as defined in 8.3 are acceptable.
8.2 Criteria
8.2.1 Primary criteria
The metal to electrolyte potential at which the corrosion rate is < 0,01 mm per year is the protection potential, Ep. This corrosion rate is sufficiently low so that during the design life time unacceptable levels of corrosion damage cannot occur on the structure. The criterion for cathodic protection is therefore: E ≤ Ep The most common protection potentials (Ep) for above ground storage tank bottom protection are given in Table 3. The criterion is to be chosen according to the resistivity and the temperature of the medium beneath the tank, and to the possible presence of water and aeration conditions. The free corrosion potential (Ecor) measurement can sometimes be a guide to determine what kind of conditions are existing below the tank. The criterion selected is also dependent on the type of the tank and the ty
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