TANKS by hcj

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                  CATHODIC PROTECTION OF ABOVE GROUND STORAGE TANKS
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                      CATHODIC PROTECTION OF ABOVE GROUND STORAGE TANKS

General

The application of cathodic protection to protect the underside of tank bottom in contact with the
ground can be a complex issue depending on various parameters. Tank bottoms may be
considered quite differently to other buried structures.

Some of the conditions to be considered prior to determining the method of application are as
follows:
              Diameter of tank
              Age of tank
              Life of protection system
              Product contained within the tank
              Tank construction
              Tank foundation – material specification
              Coating if any applied to underside of tank bottom
              Resistivity of soil beneath/adjacent to tank
              Resistivity of soil for groundbed design
              Access around tank for installation equipment (excavators/boring equipment)
              Number of tanks
              Methods of tank earthing (isolated or connected, local/remote)
              Isolation of pipes and instrumentation filled to tank
              Environment
              Drainage beneath tank
              Secondary containment methods

Both impressed current and galvanic anode systems may be used to protect aboveground
storage tanks (AST’s). However galvanic systems are usually only used in low resistivity
environments or in very close Undertank systems for replacement single of double bottom
methods with an insulating liner.

Impressed current cathodic protection systems may be one of the following types
               Remote groundbed either shallow or deepwell
               Distributed anodes around the periphery of the tank
               Undertank systems –
                     o loop/lattice MMO anodes
                     o directionally drilled anodes angled under the tank

Current densities

Before fully considering the type of system to be used, it is necessary to determine the current
required to achieve protection across the entire tank bottom. Many tanks are constructed using
uncoated plates. When the plates are coated, a considerable reduction can be made bearing in
mind the uncoated areas and coating degradation with age. All this means that the current
requirement for large diameter tanks can be large. Since it is essential for tanks to be earthed
to prevent the build up of static electricity, the current drain to the earthing system/s must also
be taken into account when determining the size of each system.
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                            Suggested Minimum Allowable Surface Current Densities

                            Structure Surface                        Current Density (mA/m2)

                            Isolated from earthing by polarization cell
                            Uncoated Steel(on bitumen/sand base)            10-20
                            Coated tank bottoms (50% bare at end of life)   5-10
                            Uncoated Copper                                 40

                            Connected to earthing system
                            Uncoated Steel(on bitumen/sand base)            30-50
                            Coated tank bottoms (50% bare at end of life)   15-25

Alternatively, when tank bottoms are not isolated, particularly with large diameter bare tank
bottoms and close anodes, use figures for isolated tanks and make an allowance for current
drain to other metalwork, e.g. buried pipework, foundation steel and earthing systems

Isolation

As can be seen form the above current density figures, there is a considerable difference
between an isolated tank and an un-isolated tank. However, the decision to isolate or not is
often complicated by the operational requirements of the tanks and the safety considerations
necessary.

Where tanks contain hydrocarbons, there is a risk of static electricity charge building up during
filling of the tanks. If this is allowed to go unchecked, then there is a spark risk if the tanks are
not adequately earthed. This being the case, the cathodic protection system can be designed to
operate with the earthing connected and an allowance made for the additional current drain.
This drain may be lessened by using stainless steel earthing rods or zinc earthing rods(which
may assist the cathodic protection but may also dissipate by acting as an anode when the
cathodic protection system is switched off).

If the tank is to be isolated, all pipes connected to the tank must be filled with insulating flanges
and all electrical/instrumentation fittings must be designed to be operated without being
connected to the station earthing system. The tank must still be connected to the main earthing
system but this is carried out through a polarisation cell (chemical or solid state). This device is
rated to carry the required electrical fault currents and blocks the passage of low voltage d.c.
currents (cathodic protection current) but allows a.c. current to pass unhindered thus
maintaining the integrity of the earthing system.

Coatings

The current requirements may be considerably reduced if the tank bottoms are coated,.
However when the plates are coated, there is a border left around the edge of each plate that
will remain bare to allow for the welding of the plates and of course, these areas cannot be
coating after construction. Also even when tank bottoms are coated, the system has to allow for
the degradation of the coating with age for the entire life of the system – usually a figure of 50%
is used for coating degradation and the uncoated areas.

Monitoring

Because it is not possible with portable equipment to measure the tank/soil potentials under the
tank, permanent copper/copper sulphate saturated reference electrodes should be installed
within the foundation pad below the tank bottom, prior to tank construction, to monitor the
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potential at inaccessible and critical points. The reference electrodes shall be an IR free type.
with IR interrupter. The electrodes may be zinc electrodes, Cu/CuS04 or Ag/AgCl.

It is recommended that placement and number of permanent reference electrodes should be as
follows:

a)         For large tank (over 20m diameter) a minimum of nine reference electrodes shall, be
           located radially underneath- the tank bottom. One at the centre and four at the midway
           between centre and edge of the tank, and the other four at 900 mm (3 feet) from the
           edge of the tank.

b)         Alternatively, perforated non-conductive tube may be provided under the entire tank
           diameter with a suitable diameter to facilitate the insertion of a portable reference
           Cu/CuS04 electrode for monitoring potential.

           The reference electrode inserted in the pipe will be used to measure structure/soil
           potentials along all the entire tank bottom surface in contact with the soil. For tanks
           smaller than 20 m, the non- conductive tube should be installed under the tank at the
           middle section from side to side, and for tanks larger than 20m three Nos. non-
           conductive tubes should be installed and separated equally at 120o under the tank
           bottom.

c)         Constant distance should be maintained from the tank bottom to the reference electrode
           or the reference cell insertion tubes. Increasing the space between tank bottom and
           reference electrode will increase the IR drop.

With existing tanks, where this is not feasible, then clean potential measuring points should be
installed around the periphery of the tank. The measuring point should consist of a length of
non-metallic tubing inserted into the ground such that the lower end penetrates clean soil, i.e.
any oil contaminated soil around the tank does not affect the measured potential. The tube is
filled with clean soil and the portable reference electrode pushed into the clean soil. If the soil in
the tube dries out, water can be poured into the tube to ensure good electrolytic contact.

Cathodic Protection Systems

Cathodic protection systems divide themselves into remote and close installed anode systems.

Remote Systems

The remote systems are treated similar to pipeline systems where the total current requirement
is provided from a single or multiple groundbeds located at some distance from the tanks
(remote earth. The groundbeds may be shallow horizontally or vertically installed groundbeds
and deepwell groundbeds. Where space may be restricted, there is an advantage in using
deepwell groundbeds as the separation from the tanks and other structures may be obtained
vertically rather than horizontally. Remote groundbeds may often be used for protecting
multiple tanks of varying sizes as found on a tank farm, control of the current being obtained by
resistance control in the individual negative circuits. The power supplies can be placed close to
the groundbeds which will normally be in a non-hazardous area thus removing the need for
Zone I or Zone II equipment.

Remote groundbeds may be used for existing or new tanks. A typical deepwell installation is as
follows.
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   ANODE
  JUNCTION
    BOX                                                                 TANKS
                            TRANSFORMER
                              RECTIFIER
          + _

                                                                                  REFERENCE
                        NEGATIVE CONTROL BOX                                      ELECTRODE




    DEEPWELL
    GROUNDBED




                            T ANK PROT ECT ION USING REMOT E DEEPWELL GROUNDBED

CLOSE SYSTEMS

Close systems comprise either galvanic or ICCP installations where the anodes are installed
close to the structure being protected, i.e. by making the earth positive with respect to the tank
bottom in local areas.

Close systems can be split into several methods of application:
        Anodes installed around the periphery of the tank
        Limited number of anodes installed under the tank
        Anodes installed close to the underside of the tank and distributed over the whole
           area (only usable for new tanks or where tank bottoms are being replaced).

Galvanic Anodes

Unless the tank is a small diameter and the soil resistivities are low, individual pre-packaged
anodes have limited usage. If used they would be distributed around the periphery of the tank
or installed underneath the tank prior to laying the floor.

A more usual application would be for new tanks where magnesium or zinc ribbon is laid
beneath the tank. Again the soil resistivities need to be suitable and sufficient length installed to
a) obtain sufficient current output and b) to last the life of the system.

In the past, replacement bottoms were installed over the old bottom and welded to the old
bottom at the tank wall. This procedure created a closed corrosion circuit, making the new steel
anodic to the old bottom. This closed circuit accelerated the corrosion process of the new metal.
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Ribbon anodes is used in the space between tank bottoms in a replacement application or as in
new construction, the space between the secondary containment and the tank bottom for
cathodic protection. Clean sand is placed over a polyethylene liner used for secondary
containment and the ribbon anode is rolled out. This particular example is in a serpentine
configuration (see drawing below. The ribbon material was installed at 1 m centres.




Impressed Current systems

Impressed current systems may be installed in a number of ways, namely:
       Anodes vertically installed around the periphery of the tank
       Anodes at an angle by directional drilling under periphery of the tank
       Anodes installed under the tank ( up to 2m away from tank bottom) prior to laying the
          floor

The methods are shown in the following sketches:
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                Transformer rectifier
                                        -
                                        +

                                                                                 Anodes




                                 +          Negative connection
                                        -
                                                                                    Anodes
                              ANODES INSTALLED VERTICALLY AROUND PERIPHERY




                 Transformer rectifier
                                            -                           Anodes
                                            +




                                  +             Negative connection
                                            -
                                                                        Anodes


                               ANODES INSTALLED UNDER TANK BOTTOM AT ANGLE
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                   Transformer rectifier
                                       -                              Anodes
                                       +




                               +           Negative connection
                                       -
                                                                      Anodes
                               ANODES INSTALLED UNDER TANK BOTTOM




Anodes vertically installed around the periphery of the tank or under the tank

These systems give better control over where the current is flowing to than remote systems but
they will still lose current to other structures and earthing systems as the anodes are not that
close to the tank bottom. The more the anodes can be installed under the tank, the better to
likelihood of obtaining an even spread of current across the entire tank bottom and achieving full
protection at the centre. Also, unless the tanks are all approximately the same size, individual
systems will be required to obtain the correct flow of current to each tanks. Where there are
similar sized tanks with current requirements more or less the same, a single power source can
be used and minor resistance control can be used in the negative circuits.

Anodes Installed directly under the tank bottom

The more favoured method of applying cathodic protection to new tanks or tanks where the floor
is being replaced is to use mixed metal oxide anodes in the form of a ribbon or a wire and
distributed over the whole of the tank bottom in the form of a lattice or a series of rings as shown
in the following sketches:

The main advantage of this form of protection is that the cathodic current is evenly distributed
across the tank surface. Also, since the anodes are very close to the tank bottom, there will be
no current loss to other structures or the earthing systems. The only losses to consider may be
to the reinforcing bar within the ring wall.
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Design of System

The design of the system is determined by considering the foregoing variations. Once the
method of application is determined, the design is carried out by:

1. Determining the current required from : Surface area, coating (% bare), current density,
   number of tanks.

2. Determining the type of groundbed/anode system

3. Calculating the anode to earth resistance         and number/length of anode required

4. Determining to required output for the transformer rectifier

Again, since these are quire lengthy calculations using the formulae previously stated,
spreadsheets can be developed for automating these tasks.

Typical examples are shown for various scenarios.

Typical Loop Anode Undertank System

                                                                                  Total Total
                 Current           Design         Depth Maximum           Actual length length
    Tank Surface Density           Current Current of Spacing            spacing of       of An
Tank Dia area sq bare    Tank % Density Required anode of rings Number of rings anode cable cur
 No m       m mA/sqm Coating bare mA/sq.m Amps     m       m    of rings    m      m tails m mA
                                  TOTALS 64.80                                   896.00 435.00

ABC 50 1963.50              50   CTE 50       25     49.09    0.3     3       9      2.78   707   315   69
DEF 20 314.16               50   None 100     50     15.71    0.3    1.8      6      1.67   189   120   83
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Typical remote deepwell groundbed


                         DATA SHEET OF CATHODIC PROTECTION DESIGN PROPOSAL

                          C.P. System No.                                                        No. 25
                             Tank No.                                                           TK - 679

                                                        Current Requirements
   1         Diameter (m)                                                                            21.5
   2         Surface Area (m²)                                                                       363
   3         Current Requirement (Amps)                                                              7.26
   4         Design Contingency (%)                                                                   25
   5         Total Current Requirement (Amps)                                                        9.075



                                                     Individual Circuit Volt Drop
    6        Quantity of Anode Groundbed                                         1
    7        Quantity of MMO Anode (ea/set)                                      8
    8        Anode Groundbed Resistance (Ohms)                                 0.46
    9        Individual Groundbed Ampere Rating (Amps)                          50
   10        Length of Main Positive Cable (m)                                  40
   11        Length of Main Negative Cable (m)                                  30
   12        Total Volt Drop - Pos. and Neg Cables (Volts)                     0.538
   13        Potential Change at Cathode (Volts)                                 1
   14        Back EMF (Volts)                                                    2
  15         Total Volt Drop - (Volts)                                         3.54

                                          Recommended Rectifier Rating
  16         Required D.C Output Voltage (V)                                 18
  17         Recommended Rating of T/R                   18V x 24 A x 1 Circuit (Standard Unit Size)

(NOTES)

ITEM
          8 Anode groundbed resistance = (L) x {2.3 log(8L/d) - 1}
            = Soil Resistivity ( 10 ohm m)
            L = Length of Anode (Cokebreeze Column) (18.4m)
            d = Diameter of Anode Column = (0.254m)

        12 Total Cable Volt Drop (Volts) = 0.00032 V/A * mt (Total L Pos. & Neg Cable * Amp (24A)

        16 Required Minimum D.C. Output Voltage = The anode with highest volt drop (TK - 679 Anode 1 = 3.5Volts)

             T/R D.C. output voltage recommended by AFIC = 18 Volts based on the following :

             Voltage available = T/R D.C. Output Voltage (18v) minus Total Circuit Volt Drop (3.5 Volts)

             Therefore Actual Voltage available at TK - 679 Anode 1 is : 18 v - 3.5 v = 14.5 Volts

             Maximum Groundbed resistance to achieve 24 amps output from the groundbed = Volts available / Max Anode Current
             Ohms Law - (R= V/I) Therefore : 14.5 Volts / 24 Amps = 0.60 ohms maximum groundbed resistance
             to achieve the required 24 Amps. Therefore a 18 Volt D.C. output is recommended for this system.
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Typical Lattice Anode Undertank System

                                                       Tank Information.

Tank name:                               Crude              Tank Dia           150            ft      45.73      m
Design lifetime:                   30         Years.            Resistivity of soil under Bottom:     1,000   Ohm.m
Surface area                     1642.6           m²         Current requirement calculation:         acc. to BAC


Tank Coating.
Tank coated with:                        Epoxy                   Coating Category:                      2
Coating Efficiency                 50         % bare           Current Density Bare           25      mA/m²

                               Current Demand based on calculated surface Area.

                                                           Current Density                     Current requirement
                                  Area           Initial       Mean           Final         Initial   Mean     Final
                                   m²          mA/m²           mA/m²         mA/m²          Amp       Amp      Amp
Bare surface                       0            25              25             25           0.00      0.00     0.00
Coated surface:                  1642.6         5.0             9.4           12.5          8.21      15.40    20.53
Total:                           1,642.6                                                     8.21     15.40    20.53


                                 CATHODIC PROTECTION by LATTICE SYSTEM

                                                 MMO Ribbon Information.

                                 Ribbon Distance to Tank Bottom:              0.300           m
                                             Ribbon Type:                     MMO          Type 1
                                            Ribbon Width:                      6.35          mm
                                            Ribbon Height:                    0.635          mm
                                   Ribbon Equivalent diameter:                0.0044          m
                                        Electrical Resistance:                0.138        OHM/m
                                 Max. Ribbon Load in coke breeze                17          mA/m
                                     Nominal Ribbon Lenght:                   1,208           m
                                    Nominal Ribbon Spacing:                    1.36           m
                                            No. of ribbons:                     34           ea.
                                        Real Ribbon Spacing:                  1.345           m


                                               Conductor Bar Information.

                                           Conductor Type:                   Titanium     Grade 1
                                           Conductor Width:                    12.7          mm
                                           Conductor Height:                  0.635          mm
                                        Electrical Resistance:                0.069        OHM/m
                                   Nominal Conductor Spacing:                 4.035           m
                                          No. of Conductors:                    12           ea.
                                    Real Conductor Spacing:                   3.811           m
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                                             Feeder Cable Information.

                                 Stranded Single Cu-core:              16           mm²
                                 Ring Feeder Cable length:             100           m
                                      Electrical Resistance:          1.150      OHM/km


                                             Feeder Connection Rows.

                              Nominal Connection Row Spacing:         3.811          m
                                     No. of Connection rows:           12           ea.


                                                 Ribbon Calculation.

                              Ribbon Distance to Tank Bottom:         0.300          m
                              Resistivity of soil under Bottom:       1,000       Ohm.m
                                                           Length   Resistance   Conductance
                                                               m       ohm          mhos
                                 1            0.67         11.01     156.17        0.0064
                                 2            2.02         18.78     100.60        0.0099
                                 3            3.36         23.87      82.35        0.0121
                                 4            4.71         27.79      72.47        0.0138
                                 5            6.05         30.99      66.10        0.0151
                                 6            7.40         33.68      61.62        0.0162
                                 7            8.74         35.97      58.28        0.0172
                                 8            10.09        37.92      55.72        0.0179
                                 9            11.43        39.60      53.70        0.0186
                                 10           12.78        41.04      52.10        0.0192
                                 11           14.12        42.26      50.82        0.0197
                                 12           15.47        43.27      49.80        0.0201
                                 13           16.81        44.10      49.00        0.0204
                                 14           18.16        44.75      48.40        0.0207
                                 15           19.50        45.23      47.95        0.0209
                                 16           20.85        45.55      47.67        0.0210
                                 17           22.19        45.71      47.53        0.0210
                                 18           23.54        45.71      47.53        0.0210
                                 19           24.88        45.55      47.67        0.0210
                                 20           26.23        45.23      47.95        0.0209
                                 21           27.57        44.75      48.40        0.0207
                                 22           28.92        44.10      49.00        0.0204
                                 23           30.26        43.27      49.80        0.0201
                                 24           31.61        42.26      50.82        0.0197
                                 25           32.95        41.04      52.10        0.0192
                                 26           34.30        39.60      53.70        0.0186
                                 27           35.64        37.92      55.72        0.0179
                                 28           36.99        35.97      58.28        0.0172
                                 29           38.33        33.68      61.62        0.0162
                                 30           39.68        30.99      66.10        0.0151
                                 31           41.02        27.79      72.47        0.0138
                                 32           42.37        23.87      82.35        0.0121
                                 33           43.71        18.78     100.60        0.0099
                                 34           45.06        11.01     156.17        0.0064
                               Total                     1,223.10     1.723         0.58
                                  Maximum Ribbon Load:                16.79        mA/m
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                                            Conductor Bar Calculation.

                                                         Conductor Length m
                                  1           1.91      18.28
                                  2           5.72      30.25
                                  3           9.53      37.14
                                  4          13.34      41.57
                                  5          17.15      44.28
                                  6          20.96      45.57
                                  7          24.77      45.57
                                  8          28.58      44.28
                                  9          32.39      41.57
                                 10          36.20      37.14
                                 11          40.02      30.25
                                 12          43.83      18.28
                                Total                            434.2


                                        Voltage Drop in Ribbon Calculation.

                                            Ribbon
                                 load      Resistance   lenght      I x R Drop
                                Amp         OHM/m         m             Volt
                                0.032        0.138       1.91            0.008


                                 Voltage Drop in Conductor Bar Calculation.

                             Number of conductor bars:              12        ea.
                                  conductor                    Conductor Bar
                                     load           Resistance    lenght I x R Drop
                                Amp (average)        OHM/m          m        Volt
                                    0.544              0.069      22.87      0.858
                             Total Conductor Voltage Drop:                       0.858


                                  Voltage Drop in Feeder Cable Calculation.

                             No. of Feeders:           12       ea.
                                      Feeder Cable mm²           16
                                 load    Resistance lenght  I x R Drop
                                Amp       OHM/m        m        Volt
                               1.711      0.00115 32.865854    0.003


                                      Transformer/Rectifier Minimum Size.

                                   Min.Current Output:                   20.53   Amp
                                   Min.Voltage Output:                    36     Volt

								
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