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CP system arrangement prevents soil-side corrosion on tank bottoms.

This article looks at two cathodic protection (CP) system projects undertaken in India to protect hydrocarbon rage tank bottoms. One conclusion from this discussion is that a properly designed, installed and maintained CP system can protect a tank-bottom plate from corrosion for the life of the structure without repair/ replacement of the bottom plate. Therefore, the employment of a CP system, which costs within 10% of the expense for bottom-plate replacement, can be justified.

A CP system provides a cost-effective means of preventing corrosion by generating a counter cell to nullify the effects of the naturally occurring galvanic corrosion cell or imposed electrolytic cell due to the proximity of a direct current (DC) source.

The CP system design should involve a proper cathode structure electrical current density. Current density figures of the order of 10mA/[m.sup.2] are normally required for uncoated tank bottoms resting on bitumen-sand carpets. Case studies included later indicate an average current density requirement as high as 20mA/ [m.sup.2] is required for protection of coated tank-bottom plates in corrosive soil.

If the operating temperature of a tank is higher than ambient temperature, a correction factor of 25% increase in the protective current density per 100[degrees]C rise in temperature up to 800[degrees]C may be considered to adequately protect the tank. At an operating temperature above 800[degrees]C, the correction factor for the protective current density may be ignored as the possibility of the presence of moisture in the soil is less at that temperature.

To arrest metal loss from the tank-bottom plate from the start of its installation, it is always advisable to install a CP system during the construction stage of the tanks. This also facilitates easy installation of suitable anodes and permanent reference cells under the tank-bottom plate to achieve efficient and effective corrosion protection and monitoring at low cost and low operating current.

Out of the anode bed types mentioned in the sidebar, the types No. 3 and No. 4 are most common. However, anodes of type No. 3 have certain advantages over anodes of type No. 4 as follows:

1. In case of type No. 4, because the primary oxidation reaction for the mixed metal oxide mesh ribbon anode material is the breakdown of water (present in the sand electrolyte) to produce oxygen and hydrogen, the dissolved oxygen thus produced acts as a cathode depolizer. In case of type No. 3, where the oxidation reaction of the anode is the oxidation of backfill carbon to oxide of carbon in addition to the breakdown of water, the depolarization effect due to the reaction product, dissolved oxygen, is either not occurring or occurring very little.

2. In case of type No. 4, the entire CP system shall become inoperative if--due to the movement of the bottom plate during loading/ unloading of tank storage contents or due to non-uniform sagging/settlement of tank foundation --there is any physical contact or electrical short circuit between the mixed metal oxide mesh ribbon anode/conductor bar and the tank bottom plate at any location. In the case of type No. 3, if there is a short and some anode string touches the bottom plate, the affected anode string can be isolated from the anode junction box and the rest of the CP system remains operative.

Reference Cells

The permanent reference cells installed under the tank-bottom plate for the purpose of monitoring the electrical potential may deteriorate over time. Therefore, it is advisable to provide an alternative facility for potential monitoring by using portable reference cells. For potential monitoring with portable reference cells, one or two perforated PVC/HDPE pipes, preferably 75-mm diameter, slotted 50-mm (2 inches) in length and 0.15-mm (0.006-inch) width, 4Kg/[cm.sup.2] pressure rating, sealed at one end, may be installed under the tank during the construction stage, for inserting a portable reference cell under the bottom plate.

Two Projects

Consider two projects undertaken in India to protect hydrocarbon storage tank-bottom plates.

A. Remote vertical deep-well anode bed CP system.

Eight NGL storage tanks, 40-m diameter each, at a gas processing complex needed to be protected against soil-side corrosion of the bottom plate in a very corrosive soil having average resistivity of 200-500 ohm-cm. The tank-bottom plates were coated with coal tar epoxy coating. The CP system contained four, vertical, deep-well anode beds, each 250 mm diameter by 50 m deep. Each anode bed contained fourteen 20-Kg tubular Hi--Si--Cr--cast-iron anodes of 50-mm diameter by 2.1-m long each. All the deep-well anode beds were installed approximately 50 m away from the tank rim and the distance between each deep well was kept to 200 m. Four transformer / rectifier (TR) units called, respectively, TR Unit No. 1, No. 2, No. 26 and No. 41--each of 50 V by 75 A capacity--were considered for feeding electrical current to deep-well anode beds. Each deep-well anode bed could be fed by a single TR Unit.

Results obtained during commissioning indicated the following individual output voltage/ current would be required to achieve adequate protection: TR Unit No. 1--25.3 V / 47 A; TR Unit No. 2--12 V / 59 A; TR Unit No. 26--10.5 V / 51 A; TR Unit No. 41--18.5 V / 54 A.

The "ON" potential achieved with respect to a Cu/CuSO4 reference cell was:--1.807 V (Max.) and--1.229 V (Min.).

Thus, electrical current of 211 A was required to protect the total surface area of 10,053 [m.sup.2] for eight 40-m diameter tank bottom plates. This worked out to be 20 mA/[m.sup.2] CP current density requirement to achieve the protection level mentioned above.

B. Close distributed long-line anode system under tank-bottom plate.

Close distributed long-line conductive polymer anode was used at a petrochemical complex for the CP of 13 storage tank-bottom plates. The bottom plates were coated with coal tar epoxy for soil-side corrosion prevention. The soil was very corrosive, having average resistivity of 500 ohm-cm. The permanent water table was also very high, ranging from 3-8 m at different locations and the load-beating capacity of the soil was poor. The tanks were installed on reinforced concrete pile foundations and a sand cushion was made between the tank-bottom plate and the concrete pile cap.

A conductive polymer anode string, prepacked with calcined petroleum coke breeze and a permanent Cu/CuSO4 reference cell for monitoring the tank-bottom plate-to-sand potential, was installed in the sand cushion. The depth of the anode string from the tank-bottom plate was kept to 1 m and spacing between the consecutive anode strings was kept to 2.5 m. To ensure that no CP system electrical current flows to the reinforced concrete structure, polyethylene fill--1.2 mm thick--was laid on the concrete surface of the pile cap.

One TR Unit, TR Unit No. 42, feeds 13 tanks through one current distribution box, having a control resistor for controlling input current for each tank. All tanks are connected to the TR Unit through dedicated anode and cathode junction boxes.

Results obtained during commissioning indicated that TR Unit output voltage 4.8 V and output current 18.2 A was required to achieve adequate protective electrical potential level for the 13 tanks. Individual tank details, tank current, tank potentials and tank-bottom plate current density requirements are given in Table 1.

Conclusions

1. Adequate awareness is still not uniformly available around the world regarding application of CP for soil-side corrosion prevention of aboveground storage tank-bottom plates. Sand bitumen pad or paint/coating alone provided for the corrosion prevention of the storage tank-bottom plates cannot prevent corrosion of the bottom plates in corrosive soil. When the sand-bitumen pad or paint/coating system for storage tanks is supplemented by a suitably designed, installed and maintained CP system, the possibility of the occurrence of leaks and possible fire hazard due to leaks can be eliminated.

[ILLUSTRATION OMITTED]

2. It is advisable to install the CP system during the construction stage of the tanks.

3. For close distributed anode beds installed under the tank-bottom plate, the tank-bottom plate is located within the voltage gradient of the anode ground bed. For such installations, in addition to "ON" potential, the 100 mV polarized potential decay criteria or the 850mV "INSTANT-OFF" tank to sand/soil potential with respect to Cu/CuSO4 reference cell criteria should be considered for monitoring of the electrical potential to ensure achievement of adequate polarization level.

4. For monitoring the potential between tank bottom plates and sand/soil, it is advisable to provide and keep provisions for measurement by both a permanent reference cell and a portable reference cell, as the permanent reference cell installed under the tank-bottom plate may deteriorate with the passage of time. For potential monitoring with a portable reference cell, one or two perforated PVC/HDPE pipes may be installed under the bottom plate from the tank rim to the tank center.

5. CP systems can protect the tank-bottom plate from corrosion for the life of the structure without repair/replacement of the bottom plate. Therefore, application of a CP system for a tank-bottom plate which costs within 10% of bottom-plate replacement can be justified.

6. Operation and maintenance of a tank-bottom CP system is simple because no rotating equipment is involved.

Anode Bed Applications For Protecting Tank Bottoms

There are various anode bed applications used for tank CP systems.

1. Remote vertical deep-well anode beds.

2. Close shallow-depth vertical distributed anode beds surrounding the tank or directionally drilled horizontal / inclined anode bed under the bottom plate. These types of anode beds are considered for applying CP systems to operating tanks.

3. Close continuous distributed long line conductive polymer anode/mixed metal oxide (MMO) wire anode piggyback connected with anode lead cable, factory prepacked with coke breeze. It is installed under the bottom plate of the tank.

4. Mixed metal oxide mesh ribbon anode installed tinder the bottom plate of the tank.

5. When the old tank bottom plate is replaced with new bottom plate, the old bottom plate might be used as a close distributed impressed current CP anode for protection of the new bottom plate.

Acknowledgements

The author thanks colleagues at Engineers India Limited for assistance in producing this article. The author also thanks NACE CORROSION 2008 Conference & Expo for publishing the full version of the technical paper from which this article is derived.

LITERATURE:

API Recommended Practice 651, section 3, paragraph 3.1.1. Cathodic Protection of Above Ground Petroleum Storage Tanks.

NACE Standard Recommended Practice RP-0193-2001, External Cathodic Protection of On-grade Carbon Steel Storage Tank Bottoms.

VDE: 0150, Specifications for the protection from corrosion by DC stray currents.

NACE Standard RP-01-69 (1983 Revision), Page 6. BS 7361, Part 1: 1991, Section 4.5.

By B. Chatterjee, Engineers India Limited, New Delhi India

B. Chatterjee is deputy general manager of Engineers India Limited (ELL), New Delhi. He joined EIL in 1980 and is responsible for design, engineering, construction supervision, troubleshooting and commissioning of CP systems. He was with Oil India Ltd. from 1975-80 where he worked in the operation and maintenance of the firm's CP systems. He holds a degree in Electrical Engineering (1974) from Jadavpur University, Calcutta, and completed the NACE International CP system design course in New Orleans, LA in 1995. He can be reached at chatterjee.b@eil.co.in.
Table 1: Commissioning results for tank CP system using distributed
long-line conductive polymer anodes under the tank bottom plate.

TANK NO. / (DIA.) TANK TANK TANK TANK "ON"
 AREA CURRENT CURRENT POTENTIAL
 (sq.m) (A) DENSITY (V)
 (mA/sq.m)

41T-003A/(10m) 78.54 1.8 22.91 -2.7
41T-005/(9m) 71.47 1.8 25.18 -2.58
41T-004A/(10.7m) 89.92 1.8 20.01 -2.41
41T-003B/(10m) 78.54 0.9 11.46 -2.52
41T-002/(10m) 78.54 2.4 30.56 -2.41
41T-004B/(10.7m) 89.92 1.6 17.79 -2.63
42T-002B/(13m) 132.73 1.8 13.56 -2.11
42T-002A/(13m) 132.73 1.6 12.05 -2.39
43T-002B/(13m) 132.73 0.9 6.78 -2.261
43T-002A/(13m) 132.73 1.6 12.05 -2.230
33T-001/(5m) 19.63 0.1 5.09 -1.982
43T-005A/(11.2m) 98.52 1.0 10.15 -2.235
43T-005B/(11.2m) 98.52 0.9 9.13 -2.360

TANK NO. / (DIA.) TANK
 INSTANT
 OFF"
 POTENTIAL
 (V)

41T-003A/(10m) -0.947
41T-005/(9m) -0.921
41T-004A/(10.7m) -0.970
41T-003B/(10m) -0.969
41T-002/(10m) -0.929
41T-004B/(10.7m) -0.966
42T-002B/(13m) -0.956
42T-002A/(13m) -0.965
43T-002B/(13m) -1.000
43T-002A/(13m) -1.030
33T-001/(5m) -0.897
43T-005A/(11.2m) -0.909
43T-005B/(11.2m) -0.941
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Article Details
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Author:Chatterjee, B.
Publication:Pipeline & Gas Journal
Geographic Code:9INDI
Date:Mar 1, 2009
Words:2125
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