System Monitoring of Cathodic Protection in Plant Facilities
1. Tank Bottoms
1.1 A saturated cupper/Cupper sulphate reference electrodes shall be installed under tank bottoms as detailed in Para 1.7.
1.2 Remote monitoring and control system shall be supplied in accordance with SABIC Standard L02-S01. Contractor shall provide details of their proposed system for review and approval.
1.3 A perforated high density PVC pipe (1 ½ inch dia) properly encapsulated in the textile fiber shall be placed under the tank bottom. This pipe shall be crossing the center of the tank bottom and extended either side of the concrete ring wall. This monitoring tube shall not run parallel to the anode ribbons. If tank diameter is more than 18 meters, two monitoring tubes shall be installed.
1.4 Calibration tubes consisting of a 1.5 inch dia. shall be provided for each buried reference electrodes.
1.5 The Suitable number (minimum 4 no.) of soil access holes (1½ inch in diameter) under the bottom through the ring-wall shall also be provided for potential measurements using portable reference electrodes.
1.6 Reference electrodes (RE) shall be positioned away from the anode feeder points. One RE shall be placed at the center of the tank base and the balance at the perimeter.
1.7 The number of buried reference electrodes per tank depends on the tank diameter and shall be as follows:
2. Tank Internals
2.1 Silver/silver chloride (Saturated KCl) reference electrode shall either be installed permanently or arrangement to be made at the top of the anode roof for monitoring the CP potentials inside the tank using Ag/AgCl reference electrode.
9.2.2 Remote monitoring and control system shall be supplied in accordance with SABIC Standard L02-S01. Contractor shall provide details of their proposed system for review and approval.
3 Buried Pipelines/Piping & Isolated Metallic Structures
3.1 Remote monitoring and control system shall be supplied in accordance with SABIC Standard L02-S01. Contractor shall provide details of their proposed system for review and approval.
3.2 Buried reference electrodes shall be a saturated copper/copper sulfate electrode if the soil resistivity is greater than 2,000 ohm.cm or silver/silver chloride (Saturated KCl) if soil resistivity is less than 2,000 ohm-cm.
3.3 Coupon assemblies with buried reference electrodes shall be installed within the piping network for true CP polarized potential measurement.
3.4 A number of big fink test stations with soil access holes (for every 70 m pipeline/piping length) shall be provided in each area of the pipeline/piping network for potential measurement using portable reference electrode. These test stations shall be located midway between CP anodes. Each big fink test stations shall be provided with buried reference electrodes and coupons.
3.5 Soil access holes along with big fink test stations provided with shunt resistance shall be provided for 25% of the isolated structure protected by SACP for potential measurements.
4. Power Supplies and Remote Monitoring
4.1 DC power supply units are classified into two categories as follows:
a. DC power supply unit with manual monitoring/adjustment facility
b. DC power supply unit with remote monitoring/adjustment facility
4.2 The transformer rectifiers shall be oil cooled installed outdoor or air cooled installed in air conditioned room or container.
4.3 For existing plant structures, DC power supply or TR units shall be supplied with remote monitoring/adjustment facility for all tanks that are requiring bottom placement and pipelines/piping, where the length is greater than 1,000 m.
4.4 For new plants and expansion projects, remote monitoring and adjustment of CP systems shall be mandatory in general. However, in case of small and/or only a few CP systems (where the total pipelines/piping length is less than 1,000 m, manually controlled TRs may be used but only after SABIC STC approval.
4.5 For new plants and expansion projects, CP systems of all structures shall be grouped together (large and small structures, and also including all concete structures if possible & feasible) and designed to have DC power supply or TR units with remote monitoring and adjustment facility using a common remote monitoring software & hardware from one central location in the plant or complex. However, the onwer can select a separate remote monitroing and control system for each plant.
4.6 The new remote monitoring and control system shall be linked, if possible & feasible, to the existing remote monitoring and control system if any.
4.7 The voltage and current output of each pipeline/piping network DC power supply or TR unit shall not exceed 100 V and 100 A. However, this condition is not applied for tanks,
where each tank shall have its own TR.
4.8 The rated current output of each DC power supply or TR unit shall not be less than the maximum anode current capacity.
4.9 For submerged metallic structures (sea walls, anchors, gates, screens…etc) in submerged areas of the seawater structures, cooling towers and basins, the rectifiers for metallic structures and the rectifiers of concrete structures should be synchronized and connected to one remote monitoring system (RMS). This will enable taking the instant off potentials simultaneously for both structures in order to eliminate the CP interference between these structures and CP of nearby concrete structures during the instant Off potential measurements.
4.10 For detailed design and specification of oil and air cooled power supplies or TR units, software & hardware for remote monitoring/ adjustment, refer to SES L02-S01.
5. Stray Current Interference
5.1 Stray current interference currents can cause corrosion on metallic structures buried in soil. This form of corrosion differs from other causes of corrosion damage in that the DC current, which causes the corrosion, has a source foreign to the affected structure. Appropriate tests shall be conducted in areas where these interference currents are suspected.
5.2 All buried metal structures influnenced by CP systems shall be bonded through bond boxes to insure electrical continuty.
9.5.3 Jumper rods of the thermit welding type shall be made across each pipe joint where electrical continuity is to be maintained.
6. Documentation
6.1 Detailed design drawings shall be prepared as specified in Para of this article Cathodic Protection Design of Plant Facilities , to clearly show the overall layout of the structure to be protected for review and approval.
6.2 Detailed calculations, as specified in Para Cathodic Protection Design of Plant Facilities, to support the design shall be submitted to for review and approval.
6.3 Design calculations shall be produced on computer excel sheets for each CP design so that all above items given in Cathodic Protection Design of Plant Facilities can be checked, verified and approved.
6.4 Full QA/QC document shall be produced. Document shall include details of:
a. Qualified project personnel, and their specific areas of responsibility
b. Materials specification, procurement and handling
c. Continuity inspection
d. Hold point installation procedures
e. Inspection and test certificates
6.5 Commissioning procedure.
6.6 Operation and Maintenance manuals.
6.7 As built drawings.