Prior to starting the detailed CP design, the CP designer (consultant or contractor) shall develop a conceptual CP design which shall be submitted to Client for review and approval. The CP system shall provide a cost effective design life commensurate with the required life of the structure to be protected. The specialist shall consider, as a minimum, lifetime requirements, service environment and installation sequence and constraints.
Cathodic Protection Design of Plant Facilities
The specialist shall familiarize with operation of the unit attached to the structure to be protected, and shall ensure that CP system installation works will not obstruct these operations significantly. The specialist shall ensure that location and size of CP system components shall not cause obstruction to operation of the existing instruments.
The specialist shall ensure that CP system components are specified, manufactured and installed in accordance with the hazardous area classification requirements of plant. The detailed CP design package shall include the following as a minimum, which shall be submitted in one packagefor SABIC STC review and approval:
a. The scope of work
b. Design report
c. Pre design survey data
d. Detailed calculations that shall include but not limited to the following:
i. Structure surface area to be protected
ii. Cathode current requirement.
iii. Potential shift analysis.
iv. Attenuation calculations
v. Circuit resistance’s of each component.
vi. Power supply sizing.
c. Bill of materilas
d. Installation drawings, which show all CP component locations using appropriate symbols and organized numbering system. The drawings shall show and include but not limited to the following:
i. The overall layout of the structure to be protected.
ii. Anode layout with details of anode feeder connections.
iii. Layout of structure (system negative) connections, buried / embedded reference electrodes, coupons, test stations, soil access holes and associated structure signal connections with typical details of each.
iv. A CP general arrangement drawing showing locations of anodes, positive and negative junction boxes (JBs), bond boxes and Transformer Rectifiers (TRs).
v. A complete circuit diagram for all positive, negative and system monitoring connections between the structure and the TR through all positive and negative junction boxes.
vi. Typical details (sketches) for the installation of JBs and TRs.
vii. All cables, cable routing and termination points shall be identified on drawings.
e. Detailed material specifications from the original manufacturer.
f. Installation Procedures.
g. Inspection and test plan (ITP).
h. Inspection forms.
6.2 Design review approval
The Contractor shall submit all proposed design packages including drawings for every design package to the proponent organization (Affiliate) and the Material & Corrosion Section of Client for review and approval.
The contractor shall not issue drawings for construction that have not been approved by the MCS/STC-J.
The design package submitted for review and approval shall contain all the requirements listed in Para. 6.1.6.
Protection Criteria
All representative monitoring point (access hole, accessable open area and buried/ embedded reference electrode) for structures to be porotected shall meet the following criteria:
a. An instant off potential (measured between 0.1 s and 1 s after switching the DC circuit open) more negative than -850 mV and less negative than -1,200 mV with respect to a saturated copper/copper sulphate (Cu/CuSO4) reference electrode.
b. An instant off potential (measured between 0.1 s and 1 s after switching the DC circuit open) more negative than -800 mV and less negative than -1,150 mV with respect to a silver/silver chloride (Ag/AgCl/Sat. KCl) reference electrode.
c. Where anaerobic bacteria are active, an instant off potential (measured between 0.1 s and 1 s after switching the DC circuit open) more negative than -950 mV and less negative than -1,300 mV with respect to a saturated copper/copper sulphate (Cu/CuSO4 ) reference electrode or more negative than -900 mV and less negative than -1,250 mV with respect to a silver/silver chloride (Ag/AgCl/Sat. KCl) reference electrode.
d. For tank internal only, ON potential shall be more negative than -900 mV and less negative than -1,250 mV with respect to silver/silver chloride (Ag/AgCl/Sat. KCl) reference electrode. This criterion is only applicable to tank internal when protected by sacrificial anode cathodic protection system. For tank internal protected by impressed current cathodic protection system, follow para. a and b.
e. Instant off potential shall not be more negative than -900 mV vs. copper/copper sulphate reference electrode for pre-stressed concrete cylinder pipes to avoid over protection problems.
6.4 Design Criteria
- CP system shall be capable of delivering sufficient protective current to polarize the structure satisfactorily so that selected criterion for CP is attained efficiently.
- CP system type shall be as specified in Para. 6.6.
- Minimum design life of the impressed current and sacrificial anode cathodic protection systems and minimum current densities shall be as specified in Para. (Design Life and Current Density).
- The current distribution to different parts of the structures shall be adequately uniform and under good control.
- The minimum design steel current density for structures shall not be less than the values specified in Para. (CP System Type).
- The CP designer shall consider in his design the current drainage to unintended structures such as grounding system and adjasent structures affected by the CP system.
Extent of Protection
The installation of the CP system and extent of cathodic protection of steel in different types of structures shall be as follows:
CP System Type
The CP system type for different sections and/or parts of the structures to be protected shall be as given in the Table below:
Design Life and Current Density
The CP system design life and minimum design current density shall be as follows:
Service Conditions
CP components will be installed outdoor in either hazardous or non-hazardous areas. Electrical CP components in hazardous locations shall be suitable for installation in classified area. The CP components shall be suitable for the following conditions:
- Minimum Temperature 0 degree C (32 degree F)
- Maximum Temperature:
- Sheltered 55 degree C (131 degree F)
- Exposed to Sun 90 degree C (194 degree F)
- Minimum Humidity 0%
- Maximum Humidity 100%
Onshore Environment
All equipment shall be protected from the following airborne contaminants:
a. The normal air-borne concentration shall be considered as 1 mg/cubic m.
b. During sand storm conditions, dust concentration reaches to 500 mg/cubic m. with 95% of dust has a particle size of less than 20 micrometers with which 50% particles are less than 1.5 micrometers in size. The winds may gust to 112 km/hr.
c. Other pollutants including H2
S, CO, NO(X), SO(2) and hydrocarbon may be expected in atmosphere worst case.
CP Design of Pipelines/Piping and Plant Facilities
- All buried pipelines/piping shall be cathodically protected by distributed impressed current systems. Electrically isolated buried sections of normally above-grade piping inside fenced areas or buried sections of normally above-grade pipelines/piping such as road crossings can be cathodically protected with galvanic anodes.
- Plant facilities shall be cathodically protected with impressed current systems. Short buried sections of piping, small isolated coated structures etc. may be protected with
galvanic anodes. - Short pipes, which are less than 300 meters, can be protected by SACP if the piping is not electrically continuous with any other buried structures and if all piping parts are 75
meters away from the nearest impressed anodes. - Buried fire hydrants, risers, valves, gas accumulators, electrically isolated pipe casings, sleeves, fence crossings and pipe vents, thrust anchors, thrust bored, shall be protected by SACP.
- Impressed anodes shall be used for the above structures if the use of galvanic anodes are not practical, especially, if other impressed current distributed anodes are within 75 meters from the structure to be protected.
- Provide additional impressed current capacity at the ends of pipelines/piping terminating at plants, at pipelines/piping junctions, etc. to offset current loss to the underground metallic plant facilities. Special design considerations (e.g., additional CP current) are also required for buried pipelines/piping at and inside the fence for plants. Coordinate these designs with, and obtain approval from the SABIC STC.
- Special design considerations are required for the pipelines/piping at and inside the security limits for plants. Additional galvanic or distributed impressed current anodes shall be installed for these sections. This design shall be coordinated and approved by SABIC STC.
- Provide all normally below ground pipelines/piping with an integrated impressed current cathodic protection system irrespective of the type of coating on the new or existing pipelines/piping. New cathodic protection systems shall be designed to be compatible with existing systems. Do not use existing impressed current CP system spare capacity for the protection of new pipelines/piping without the written concurrence of the proponent organization.
- Distributed sacrificial and impressed anodes for pipelines/piping shall be installed horizontally and/or vertically.
- Anode spacing normally varies from 15 to 30 m, with a 15 m spacing or less in congested areas, and up to a 30 m spacing in less congested areas.
- Adjacent anode beds protecting other structures and powered from separate power sources shall be separated by a minimum distance of 50 m.
- Where pipeline/piping cathodic protection system has a mixture of vertical and horizontal anodes, different symbols or colors shall differentiate between the two. .
- Cathodic protection shall be provided within 15 days of pipeline/piping burial.
- Design new cathodic protection systems to be compatible with existing systems.
- Magnesium or zinc anodes, based on soil resistivity, and test stations are required for the following list of structures:
- Buried valve: two-60 lbs Mg or Zn anodes.
- Buried fire hydrant two-60 lbs Mg or Zn anodes.
- Pipeline/piping thrust anchor: four-60 lbs Mg or Zn anodes.
- Electrically isolated sleeves or casing: two-60 lbs Mg or Zn anodes each side.
- At plant fence crossings: two-60 lbs Mg or Zn anodes at each patrol road.
- Thrust bored crossings: two-60 lbs Mg or Zn anodes each side.
- Buried gas accumulators: four-60 lbs Mg or Zn anodes.
- Note: Other similar structures shall be protected by SACP, and quantity and anode type shall be approved by SABIC STC.
- The net weight of each magnesium or zinc anode shall be 60 Ibs excluding the anode backfill. if Mg or Zn anodes are not practical, impressed current can be used for the above structures in Para. 6.10.15.
- If SACP will be used, zinc anodes shall be used if the layer soil resistivity is less than 1,000 ohm.cm. Magnesium anodes shall be used when the layer soil resistivity is greater
than 1,000 ohm.cm. - Structures protected or influenced by CP systems shall be electrically continuous. Bonds shall be installed (where required) to ensure electrical continuity.
- Polymeric anodes shall be installed for the protection of buried pipelines/piping in the very congested area, where the distributed anodes can’t be installed due to shielding by concrete foundations, parallel piping/pipelines, etc. The CP contractor shall locate the very congested area and must obtain approval from SABIC STC, which have the right to accept or reject the contractor’s proposal.
- When using polymeric anodes, attenuation calculations shall be provided.
- For buried pipelines/piping, impressed anode system shall be high silicon cast iron anodes type TA4 or TA5.
- Temporary sacrificial CP system shall be installed for all new pipeline/piping during the construction work till energizing the ICCP.
6.11 CP Design of Above-ground Tank Bottoms & Underground Tanks
- All tank bottoms and underground tanks in contact with soil with or without ring wall regardless of the soil or backfill material resistivity shall be cathodically protected. Tank
foundations don’t require CP. - Impressed current cathodic protection system, utilizing mixed metal-oxide (MMO) coated titanium ribbon (grade 1 titanium) in grid pattern shall be installed for all new tanks and
all existing tanks requiring bottom replacement to protect from soil side corrosion. - MMO anodes shall be placed in clean sand above the dielectric secondary containment liner.
- MMO anode shall be free from discoloration (black type).
- The spacing of ribbon anode shall not be more than 1200 mm. Attenuation calculations shall be provided for each tank.
- Tank pad shall be clean sand and the resistivity shall be between 10,000 – 30,000 ohm.cm. For calculating anode to earth resistance, 30,000 ohm.cm shall be used while 10,000 ohm.cm shall be used for attenuation calculations.
- 6.11.7 Secondary containment liners shall be installed underneath all tanks.
- Existing aboveground storage tanks with no bottom replacement shall be protected by a distributed impressed current anode system.
- If existing tanks don’t require bottom placement, High Silicon Cast Iron anodes type TA4 or TA5 shall be equally spaced around the tank.
- Distributed anodes for tanks can be installed horizontally or vertically.
- The distributed anodes shall be equally spaced around the tank perimeter and shall be installed a minimum of ¼ the tank diameter from the tank wall and a maximum of one tank diameter from the tank wall and a maximum spacing of 20 m (center-to-center).
- Sand mixed with oil or bituminous material shall not be used under the tank bottom.
- Cathodic protection is not required for tank bottom installed on full reinforced concrete foundation.
- Suitable member of soil access holes (1½ in in diameter) under the bottom through the ring-wall shall be provided for potential measurement.
6.12 Tank Internals
- Cathodic protection is required for the internal surfaces of metallic tanks if the resistivity of the contents of the tank shall be 2000 ohm-cm or less at any time during the life of the tanks.
- Cathodic protection shall be provided by galvanic or impressed current system for tank internals. Type of CP system and anodes of tank internals shall only be approved by Client after reviewing the CP contractors’ proposal.
- An impressed current system utilizing inert anodes shall be used where undesirable contamination of the contained liquid would occur due to the corrosion product of other anode materials.