DEEP ANODE GROUNDBED

DESIGN AND INSTALLATION GUIDELINES

BY: ELTECH Systems Corporation

March, 2000

1.0 Current

1.1 Choice of LIDA^® Tubular Anodes
1.2 ELTECH LIDA^® ONE Anode
1.3 Anode cable or lead wire

2.0 Soil resistivity

2.1 Guidelines for obtaining resistivity values

3.0 Desired total current resistance and/or rectifier voltage

4.0 Goundbed hole or column dimensions

5.0 Backfill current density

5.1 Guidelines for choice of backfill current density

6.0 Carbonaceous backfill

6.1 Advantages of proper backfill use
6.2 Carbonaceous backfill categories
6.3 Impact of backfill resistivity
6.4 Carbonaceous backfill consumption
6.5 Design options to maintain adequate backfill
6.6 Example calculations to assure backfill availability and supply

7.0 Vent pipes

8.0 Ventralizers

9.0 Anode bottom weight

10.0 Casing

11.0 Non-conductive backfill

12.0 Power supply

13.0 Site installation

1.1 Choice of LIDA^® Tubular Anode: ^(1,2)
The designer or installer of ELTECH LIDA^® deep groundbeds has 3 different tubular anode sizes to choose from in order to achieve a given current. Description of these anodes with maximum rated current output in amperes (in soil) is as follows:

 

TABLE  I
ELTECH TUBULAR LIDA® ANODE DIMENSIONS AND CURRENT OUTPUT IN SOIL
	MAXIMUM RATED OUTPUT IN AMPS
ANODE TYPE	20 YEAR DESIGN
FW/ST 2.5/50	5.5
FW/ST 2.5/100	11.0
FW/ST 2.5/152	16.9
ELTECH LIDA® ONE 1" x 45"	3.4
ELTECH LIDA® ONE 1" x 60"	4.5
ELTECH LIDA® ONE 1" x 90"	6.8

TRUE DIMENSIONS
ANODE TYPE	DIAMETER		LENGTH		WEIGHT
	cm.	in.	cm.	in.	kg.	lbs.
2.5/50	2.5	1.0	50	19.7	0.18	0.40
2.5/100	2.5	1.0	100	39.4	0.35	0.77
2.5/152	2.5	1.0	152.4	60	0.53	1.42
LIDA® ONE 1" x 45"	2.5	1.00	114	45.0	0.64	1.40
LIDA® ONE 1" x 60"	2.5	1.00	152	60.0	0.73	1.60
LIDA® ONE 1" x 90"	2.5	1.00	229	90.0	0.91	2.00

1.2 ELTECH LIDA^® ONE Anode
The ELTECH LIDA^® ONE anode is the newest in the family of mixed metal oxide tubular anode products specifically tailored to the cathodic protection of buried structures. Designed for installations where the user prefers multiple single anodes, each on its own cable, the mixed metal oxide coating is specially formulated for use in carbonaceous backfill.

1.3 Anode cable or lead wire
The purpose of the lead wire is to deliver current to the anodes from the power supply. The deep groundbed designer must give high priority to the choice of cable or lead wire insulation that will be used. The insulation must not be destroyed by the anodic reaction products, which form down hole when the anodes are energized. Strong acidity in combination with evolving chlorine is the principal cause of failure with many cable insulation materials. The dissociation of water and backfill is responsible for the strong acidic environment. Soils or ground waters rich in chloride are the source of chlorine when the anodes are energized.

The LIDA^® applicator should make every attempt to select a cable insulation known or proven to be compatible with the choice of groundbed area. As a convenience, the listed cables are categorized as to their resistance to chlorine:

A. Non-resistant to chlorine:
HMWPE (high molecular weight polyethylene)
EPR/CSPE (ethylene propylene rubber under chloro-sulfonated rubber)

B. Resistant to chlorine:
PVDF/HMWPE (KYNAR^®/FLUOROPOLYMER + HMWPE)
ETFE/ HMWPE (TEFZEL^® - Fluoropolymer + HMWPE)

Table II lists cable sizes and types that are available for use with conventional LIDA^® tubular and LIDA^® ONE anodes.

TABLE  II
AVAILABLE CABLE TYPE AND SIZE
CABLE TYPE	WIRE SIZE	CURRENT OUTPUT
PVDF/HMWPE	6 AWG	50 AMPS
PVDF/HMWPE	8 AWG	27 AMPS
ETFE/HMWPE	6 AWG	50 AMPS
HMWPE	6 AWG	50 AMPS
HMWPE	8 AWG	27 AMPS

TEFZEL is a Registered Trademark of E.I. du Pont de Nemours & Co. Inc.     KYNAR is a Registered Trademark of Ausimont USA, Inc.

2.1 Guidelines for obtaining resistivity values
Many times the designer may obtain an estimate of average soil resistivity from a knowledge of rectifier output from the groundbed that is to be replaced. The following example illustrates this procedure.

Assume that it is desired to obtain soil resistivity in an area where another groundbed is operating at 22 volts and 40 amperes. In this technique, it is preferred to use operating data when the groundbed was first installed since the size and condition of the bed is known more precisely. It is known that the active column length of the operating bed is 49 m (160 ft.)with a diameter of 0.152 m (6 inches). Experience has shown that the resistance of the backfill column to earth is approximately 85% of the total circuit resistance. In the example above the total circuit resistance by Ohm's law is:

RT =

V I

=

22 volts 40 amps

= 0.55 ohm   {1}

Then the backfill to earth resistance R_A is approximately:

R_A = 0.55 ohm x 0.85 = 0.47 ohm   ^{2}

The H. B. Dwight relationship can be used to determine the backfill to earth resistance   ⁽⁴⁾

where

R_A = groundbed resistance (ohm)
r = soil resistivity, (ohm - m)
L = active bed length, (m)
d = active bed radius, (m)

Since R_A is known, the above equation can be rearranged to solve for the resistivity, r.

The value of soil resistivity can be determined by substituting the known values for groundbed resistance indicated above as 0.47-ohm, the groundbed length and diameter.

r  =
	p(0.47)(49)(2)     =   23.5 ln ( (4)(49) ) - 1 0.152

The estimated value of 23.5 ohm-m (2350 ohm-cm) can then be used in the new design with reasonable confidence.

If existing groundbed data is not available, soil resistivity must be estimated by other techniques. Soil resistivities generally relate to the salinity or purity of the water or moisture, which historically has permeated the soil and remains there to one degree or another.

The following broad guidelines relate U.S.A. areas and soil type conditions with its resistivity:

Area and/or Soil Type	Resistivity Range (ohm-cm)
Brackish water lowlands, poor or slow drainage, coastal areas	150 -1,200
Coastal plains, low elevation	600 - 1,500
Central coastal areas, satisfactory to good drainage	1,200 - 5,000
South central, Midwest and central, Farm and range lands	3,500 - 10,000
West central desert plains, mountains	5,000 - 25,000
Eastern and northeast high country, Excellent drainage, dry & arid	10,000 - 25,000
Western high country, excellent drainage, Dry and arid	10,000 - 50,000

Large variations in resistivities within short distances are often seen in higher resistivity areas. It is not unusual to have variations in resistivity from less than 10,000 to more than 100,000 within 152 m (500 ft.).

3.0 Desired circuit resistance and/or operating rectifier voltage
The designer of a LIDA^® deep anode groundbed may choose either total circuit resistance or operating rectifier voltage as a basis of design. Generally, when a depleted groundbed is to be replaced and the rectifier remains dependable and operable, its current and voltage characteristics may dictate parameters for the new installation. Values of expected circuit resistance for the replacement bed or new beds in the vicinity can be estimated from prior rectifier operation. Lower circuit resistance may be desired to reduce power wattage to the rectifier. Common design values for circuit resistance range from 0.5 to 3.0 ohm.

4.0 Groundbed hole or column dimensions
It was stated previously that soil resistivity has an influence on selection of hole diameter as well as height of the backfill column. In this circumstance the backfill length or height is the sensitive variable with respect to groundbed resistance. Diameter is less sensitive.

When selecting a hole or column diameter, the designer must remember that a 2.5 cm (1.0 in.) diameter vent pipe and one or more lead wires as well as the LIDA^® anodes must occupy space within the backfill column. ELTECH recommends that LIDA^® tubular strings and LIDA^® ONE anodes in deep groundbeds utilize a minimum hole diameter of 20 cm (8 in.). Smaller hole diameters are possible but design and installation practices must be strictly monitored. Larger diameters are recommended for higher current holes.

5.0 Backfill current density
The designer must not arbitrarily choose the length of the backfill or active groundbed column. Hole depth varies with a number of important factors, a principal one being the current density at the backfill-to-earth or backfill ground interface.

Definition of backfill current density: Total anode bed current divided by the area of the backfill to soil interface.
Backfill current density:

Amps/m2  =
	groundbed current (amps)     p(hole diameter)(active depth)

where diameter is in meters, depth is in meters, and p = 3.1416.

Experience indicates that the current density at the backfill/ground interface should be limited. The limitation results from the following. ⁽⁶⁾

1. Moisture must exist at the backfill/ground interface to provide a conductive path between the backfill and ground. The application of current to an anode produces an electro-osmotic force that drives moisture away from the anode. The strength of the force depends on the soil type and the force increases as anode current density increases.

2. Anodic oxidation reactions produce gases and consume moisture. Either gas accumulation or lack of moisture on the anode surface prevents current flow. The rate of gas generation and moisture consumption is directly related to current density.

3. Under certain conditions the temperature at the backfill/ground interface could increase and dry the soil. Temperature rise is related to current density, soil thermal conductivity, and backfill dimensions.

5.1 Guideline for choice of backfill current density
Table III is a list of recommended backfill current densities.

Table III
BACKFILL CURRENT DENSITY - DESIGN VALUES
Environment	Amps/m2	mA/ft2
VERY DRY SOIL	1.08	100
DRY SOIL	1.61	150
PARTIALLY DRY - NOT IN WATER TABLE	2.15	200
MOIST, IN THE WATER TABLE	3.22	300
OPEN HOLE	4.95	460

Once the backfill current density is selected, the designer user may then calculate the active backfill length. The active length for the particular application may then be exceeded but not reduced. The following example shows how the active backfill length may be calculated. Assume that a deep groundbed is to be installed in a dry soil. Using Table III, a backfill current density of 1.61 A/m² (150 mA/ft² ) is selected. The design calls for a current of 40 amperes. Hole diameter was chosen as 0.20m (8 in.). The minimum active depth is then calculated using equation {7}:

Length (m)  =
	40 amps           p(0.20m)(1.61 A/m2)

Length is approximately 40 meters (131 ft.)

The active groundbed length is optimized for the LIDA^® groundbed designer when using the ELTECH software.

6.0 Carbonaceous backfill
The choice of carbonaceous backfill is the next important task for the designer of a LIDA^® deep groundbed. The correct carbonaceous backfill improves anode performance because it provides an electronic path for current flow. When the anodes are properly installed inside of the backfill, the oxidation reaction, which would have taken place on the LIDA^® surface, is largely transferred to the backfill surface.

6.1 Advantages of proper backfill use
Advantages of proper backfill use may be summarized as follows:

1. Anode to earth resistance is decreased since the backfill column functions as the anode rather than just the anode material itself.
2. Gas blockage and drying tendencies are decreased by the increased anodic reaction surface area.
3. Anode life is increased by the shift of the oxidation reaction from the anode surface to the backfill surface.
4. The backfill maintains hole integrity and prevents cave-in.

Since the purpose of the carbonaceous backfill is to provide an electronic path for current flow, resistivity, particle size, and specific gravity are important backfill properties. Low resistivity favors electronic current flow to the backfill/ground interface. Small particle size and high density favors electronic current flow between the anode material and backfill by improving contact between the anode material and the backfill.

6.2 Carbonaceous backfill categories
Petroleum and metallurgical coke are the two primary categories of carbonaceous backfill. Petroleum coke when calcined at high temperature results in a highly conductive coke and provides uniform performance. It is recommended for LIDA^® deep groundbeds. It is recommended that the carbonaceous backfill be pumped from the bottom in a deep anode system. Recommended commercial pumpable backfills include:

LORESCO® Types SC-2, SC-3
Asbury Carbons Type 251, 251 P
           (LORESCO® is a trademark of Cathodic Engineering Equipment Company, Hattiesburg, MS.)

The properties of metallurgical coke vary because of the non-uniformity of coal and variation of coking temperature. Metallurgical coke is not recommended for LIDA^® deep groundbeds. Table IV lists minimum characteristics of petroleum calcined coke backfills for use with LIDA^® anodes.

Table IV
BACKFILL SPECIFICATIONS
RESISTIVITY	MAX. 10 OHM-CM
PARTICLE SIZE	NOMINAL 200 TO 20 MESH
CARBON CONTENT	MINIMUM 90%
BULK DENSITY	NOMINAL 1120 kg/m3 (70 lb/ft3)

 

6.3 Impact of backfill resistivity
It cannot be over emphasized that low resistivity backfill greatly improves current flow from the LIDA^® anodes in the backfill column to the backfill-earth interface. When this occurs, fewer anodes are required to produce a given current over the same active groundbeds length. Table V clearly illustrates this point. In Table V, 5 different backfills are used to fill an active column of 58m (190 ft). Hole diameter in each instance is 20 cm (8 in.). Resistivity of each backfill is indicated. In each case, the deep groundbed will deliver 40 amperes in a soil of 5000 ohm-cm. The data shows that as the backfill resistivity increases, the anodes must be spaced closer together so a larger number of anodes are required to fill the active length. Ultimately this reduces the output requirement for each anode.

It is highly recommended that a conservative resistivity value of 2.0 ohm-cm be used when using a calcined petroleum coke backfill in deep groundbeds. Commercial backfill brochures will correctly list values of resistivity much lower than 2.0 ohm-cm since they were obtained at test pressures of 10.57 kg/cm² (150 psi). The above value allows for varying densities of materials in the inactive column length, the presence or absence of water in the active column length and bore hole irregularities. Values of backfill resistivity are pressure sensitive and are reduced with increased pressure. This is due largely to the shape and sizing of the carbon particulate and the bulk density of the backfill. Thus, spherical carbonaceous particles are preferred over flat, irregular shaped particles. Spherical particles making up a high-density backfill will settle compactly lowering contact resistance. The beneficial end result is a shift in the corrosion reaction from the anode surface to the carbon backfill surface.

TABLE  V
IMPACT OF BACKFILL RESISTIVITY ON ANODE SPACING EXAMPLE DEEP GROUNDBED
CURRENT	40 Amps
ACTIVE DEPTH	57 m (185 ft.)
HOLE DIAMETER	20.3 cm (8 in.)
SOIL RESISTIVITY	5000 ohm-cm
BACKFILL CURRENT DENSITY	1.61 A/m2 (150 mA/ft2)

BACKFILL RESISTIVITY OHM-CM	ANODE SPACING		NUMBER OF ANODES REQUIRED	OUTPUT PER ANODE AMPS
	m	ft.
1	11.3	37	5	8
3	11.3	37	5	8
10	7.0	23	8	5
25	4.6	15	12	3.3
50	3.0	10	18	2.3

6.4 Carbonaceous backfill consumption
As current is applied to anodes in the active groundbed column, the carbonaceous backfill is slowly depleted to form gases and consume moisture. Allowing for inefficiency of reaction, one may assume that the backfill is depleted at the rate 1.0 kg. for every ampere of current flowing through the column per year. If consumed backfill is not replenished, the critical length of the active bed column is shortened and finally the diameter of the hole suffers collapse or cave-in since backfill is no longer present in that space. A result is increased groundbed resistance and reduced anode life.

The best deep groundbed design will assume that the carbonaceous backfill will remain at a level always above the top anode in the active groundbed column during the design life of the bed.

6.5 Design options to maintain adequate backfill
The groundbed designer has a number of options at his disposal to maintain adequate carbonaceous backfill level.

a. Calculate the amount of backfill that will be consumed during the life of the bed and place that amount in the column above the top anode in the string or array.
b. Case the column down to the top of the active groundbed height and add a pre-determined amount of carbonaceous backfill each year or every other year.

Table VI has chosen hole diameters 20.3 and 25.4 cm. (8 in. and 10 in.) with backfill consumption expressed as meter per ampere-year.

TABLE  VI
Carbonaceous Backfill Consumption as a Function of Hole Diameter and Depth per Ampere-Year
Hole Diameter		Backfill volume per Meter Depth		Backfill wt* per Meter Depth		Backfill Consumption**
cm.	in.	m3	ft3	kg.	lb.	m/Ay	lb/Ay
20.3	8	0.0324	1.146	38.5	84.9	0.026	0.085
25.4	10	0.0507	1.79	60.2	132.7	0.017	0.056

* Density of backfill @ 1187 kg/m3 (74 lb./ft.3)      ** Consumption of backfill @ 1.0 kg/amp-year (2.2 lb/amp-yr)

6.6 Example calculations to assure backfill availability and supply
Since an adequate amount of carbonaceous backfill is so vital to the success and life of a deep groundbed, several calculations will be made to illustrate how to arrive at items "a" and "b" listed in 6.5.

Item "a" may be illustrated by the following example. Assume a design life of 15 years is required using a groundbed delivering 30 amperes. Hole diameter is 20.3 cm (8 in.). Determine the minimum amount of carbonaceous backfill that should be installed above the top anode in the string.

Then from Table VI at a diameter of 20.3 cm (8 in.) one sees that 0.026 meters (1 in.) of backfill is used for every ampere year. With a current of 30 amperes for 15 years, the backfill usage will be:

Utilization = (0.026

m Ay

)(15yr)(30A)=11.7 meters = 38.4 ft.  {8}

or 11.7 meters (38.4 ft.) of backfill will be used over a 15-year period @ 30 amps.

This amount of backfill at: 1187 kg/m3 (74 lb./ft.³)

Wt = 11.7m x 38.5 kg/m = 451 kg (992 lb.)  ^{9}

Item "b" will be illustrated by another example calculation. What is the amount of backfill in pounds to add to a cased 20.3 cm (8 in.) diameter. hole each year with a current delivery of 40 amperes?

Again from Table VI, it is seen that 0.026 m (1.0 in.) of backfill are used per ampere-year. Since 40 amperes is delivered to the bed: (Refer to equations {8} and {9}).

From Table VI:

B.F Utilization year

= (0.026

m   Ay

)(40A)(1 yr)

Utilization = 1.0 m (3.28 ft.) per year of backfill.

The weight to add each year to the groundbed would be:

wt = 1.0 m x 38.5 kg/m which is approximately 39 kg (86 lb.)

7.0 Vent pipes
At this point, the user has decided to design the deep groundbed to meet an acceptable total resistance. A LIDA^® anode type has been chosen to deliver a required current and life. Hole dimensions have been selected based upon a workable backfill current density. Also, a high quality backfill has been chosen with a design allowance to keep the anodes covered with backfill over the bed life.

The applicator must now select a vent pipe. The purpose of the vent pipe is to assist with the removal of gases generated downhole, allow for heat dissipation and provide means to water the hole in the future if necessary. Venting of gases also assists the replenishment of water into the backfill column since gas blocking is largely prevented. Vent pipes can aid in lowering anodes into the hole.

In order to do its job, the vent pipe must be constructed in such a manner to allow for the movement of water and gases, but not the carbonaceous backfill or other silty products in the deep anode bed.(7) The vent pipe therefore must have holes or slit dimensions smaller than the smallest particle size of backfill.

ELTECH recommends use of the LORESCO® ALLVENT^TM deep anode vent pipe. This is a 2.5 cm. diameter (1 in.) PVC pipe rated at 3100 kPa (450 psi). The portion of the vent pipe above the backfill column may be non-perforated pipe.

8.0 Ventralizers
The purpose of the ventralizers is to ensure that anodes will be installed in the center of the backfill column. Once the backfill settles in place and the bed is energized, the steel ventralizers will quickly oxidize.

The beneficial effects of carbonaceous backfill are decreased if an anode is located off center or at the sidewall of the coke column. The portion of the anode surface in contact with the soil will operate at a much lower current density than the anode surface in contact with the coke.
Some LIDA^® users install two ventralizers per anode when using the longer units (2.5/100, 1.6/100 and the LIDA^® ONE anodes). Ventralizer/spacers can also be used to keep the vent pipe from lying next to the anode. Also, in the case of a LIDA^® double tail string (where a return cable is run out of the bottom anode and back to the top of the hole), the ventralizer can be used to help separate the return cable from the primary cable by attaching the return cable to the outside of the ventralizer.(8) The LIDA^® groundbed designer should specify the number and size (hole diameter is the size) of ventralizers when ordering the anodes and cable.

9.0 Anode bottom weight
A bottom or clump weight is generally specified to aid in lowering the lightweight LIDA^® anode strings into vertical groundbeds. This subject will also be discussed further in Section 13 on site installation.

10.0 Casing
Deep groundbed casing provides support and access to the hole. Casing also prevents current discharge at or near ground level. Casing material, depth of casing, use of down hole casing screens, extension above the earth are all dependent upon well type, soil conditions, etc. The user of impressed current must initially determine company practices, etc. Area regulatory practices also apply. Generally 5-6 meters (16-10 ft.) of permanent non-metallic casing is used at the top of the hole.

11.0 Non-conductive backfill
The non-conductive backfill has the technical purpose of minimizing current discharge near the earth surface of the bed. Physically, this backfill is installed to fill unused, top portions of the backfill column. Since the resistivity of the carbonaceous backfill in the active groundbed length is reduced with increased pressure (which enhances bed performance), the non-conductive fill above the active length should consist of high-density material. This fill may consist of gravel, porous sand or similar dense aggregate.

12.0 Power Supply
Rectifiers are by far the most commonly used types of power supply for all impressed current cathodic protection including LIDA^® deep groundbeds. LIDA^® applicators employing the ELTECH software program have the benefit of close approximation rectifier sizing presented as part of the data summary. Thus, if the LIDA^® bed is designed on the basis of total groundbed resistance, the program results will indicate the attendant voltage requirement. The program user must recall however, that the total bed resistance is assumed to be equal to 85% of the circuit resistance. Knowing the estimated total circuit resistance and bed current requirements, the corrected rectifier voltage can be determined by ohm's law:

(The 2.0 volts are added to overcome back voltage of the anode and cathode.)
Thus, for a current of 50 amperes and a circuit resistance of 1.25 ohm, the voltage would be:

V_total = 50 A x 1.25 ohm + 2V = 62.5 + 2 = 64.5V

It is advisable to obtain a rectifier with 10-15% excess capacity to handle changing soil conditions, pipe coating conditions or future expansion.

Where multiple anodes are individually connected to the same rectifier through a junction box, it is recommended that a variable resistor be installed in each circuit for individual adjustment.

Virtually all rectifier manufacturers provide excellent instructional literature on their products.

13.0 Ordering Anodes and Site Installation

13.1 Ordering LIDA^® tubular string anodes
LIDA^® anodes are available from selected full service cathodic protection firms. When ordering tubular string anodes, the user should specify the following:

a. Anode size and type; for example size 2.5/100 for soil service (Section 1.1).

b. Number of anodes per string and string number.

c. Cable types (Section 1.2).

d. End-to-end or center-to-center spacing of anodes along the cable.

e. Cable tail length above the top anode.

f. Cable tail length from the bottom anode back to the top of the hole.

g. Total cable length of the string.

h. Number of ventralizers required and bore hole diameter.

i. Required shipping date and destination information

13.2 Ordering LIDA^® ONE Anodes
The user should specify the following when ordering LIDA^® ONE anodes:

a. Number of anodes

b. Weighted or non-weighted version

c. Cable type (Section 1.2)

d. Length of cable tail for each anode

e. Number of ventralizers and bore hole diameter

f. Required shipping date and destination information

13.3 Shipment and receipt of anodes
LIDA^® tubular anodes are packed in crates in a manner to minimize mechanical damage to the cable and anodes. Each anode will be covered with a foam tube for mechanical protection. This covering must be removed just before the anode is installed in the groundbed. The string will be packed in the crate with the bottom of the string placed at the top of the crate so that the string can pass directly from the crate to the groundbed if the user so desires.

Note: The anode ventralizers are also packaged in the anode-cable crate.

13.4 Casing the hole
Once the groundbed hole has been drilled, it is recommended that 6 meters (20 ft.) of non-metallic casing be placed in that top of the hole. The diameter of the casing should be several inches greater that the bore hole diameter. This is indicated in Figure 1. ELTECH recommends that the casing be left in place and not be pulled after anode installation.

13.5 Bentonitic plug material
Installation of a bentonitic plug material or fill around the outer casing shell is carried out after the casing has been set in place. (Figure 1)

13.6 Downhole tests
Especially at new site locations, it is recommended that a depth versus resistance profile test be made to determine the most conductive earth strata to assure optimum current output with minimum circuit resistance. A simple profile test consists of measuring the resistance between an auxiliary anode in the water-filled bore hole and the structure to be protected.

The data gained above may be used to better define spacing between LIDA^® tubular anodes on a string and the length of cable on each LIDA^® ONE anode.

13.7 Hole cleaning
Once the groundbed components and accessories are on site, the installer must again assure that the bore hole is as clean as possible. Drilling mud remaining in the hole should be displaced with clear water before coke is added to the hole. Holes, which will not support a water level, should be air cleaned as much as possible. Contamination of carbonaceous backfill with drilling mud increases bed resistance.

13.8 Anode placement: LIDA^® tubular strings

13.8.1 Initially the vent pipe system is strung out on the ground leading away from the hole and cemented together as a continuous unit. The bottom of the vent pipe is permanently capped. The top of the vent pipe is temporarily capped to prevent dirt or backfill entry.

13.8.2 Next, the LIDA^® string is removed from the shipping crate and laid alongside the vent pipe. The lower capped end of the vent pipe should be 0.5 m (19.7 in.) beyond the terminal anode on the string. The double tail of the string, if present, should also be laid along the vent pipe and primary string.

13.8.3 A 7-kg (15.4 lb.) bottom weight should be tied to the LIDA^® terminal anode by means of a small plastic line or steel wire. The bottom of the weight should be approximately 3 m (9.8 ft.) below the end of the terminal anode. When used, the steel wire will quickly oxidize upon energizing the groundbed thus freeing the weight if desired. (Figure 1). The double tail LIDA^® string should have the bottom weight affixed to the cable loop 3.5 m (11.5ft.) below the bottom anode. The weight should be attached with a plastic tie rope. The bottom of the weight should be 3-4 m (9.8-13.1 ft.) below the lower anode in the string.

13.8.4 The vent pipe, the primary anode string and double tailed cable are now attached together. During this operation, the ventralizers are attached to the anodes. The anode cable(s) are attached to the vent pipe at approximately 2 m (6.6 ft.) intervals with electrical tape. Care must be taken not to cover the slits in the vent pipe with tape. Attachment of the vent pipe and double tail cable to the anode ventralizers takes place during this step. The anodes should be fixed approximately 5-7 cm. (2-3 in.) from the vent pipe and secondary tail if present. The protective foam covers are removed from the anodes at this time.

13.8.5 The next step in the anode placement procedure is that of running the backfill slurry fill pipe to the bottom of the bore hole. This procedure varies with applicators and hole depth. Some installers will introduce the fill pipe after the anodes and vent pipe have been located in the bore hole. Some installations simultaneously introduce the anodes, vent pipe and fill pipe into the hole. All the above are satisfactory.

13.8.6 The anode string and vent pipe bundle is now fed into the bore hole. Rapid lowering of the vent pipe may not be possible if a high water level exists in the bore hole since it will have a tendency to float. However, water will soon enter the vent pipe and allow submersion. Once the anode string and vent pipe have been located in the hole, the electrical cable(s) should be securely tied off at the top of the hole.

13.8.7 The carbonaceous backfill (Section 6.2) is now slurried and pumped through the fill pipe into the bore hole. Once pumping has commenced it must not be interrupted until all the backfill is in place. By pumping the backfill from the bottom up in a deep anode system, the problem of voids, contamination, poor compaction and poor settling are avoided.(9) This practice tend to push any remaining drilling debris and fluids above the more dense coke slurry.

13.8.8 As the slurried backfill is introduced and the fill pipe is raised, the change in anode resistance to ground may be measured to assess if the backfill has been placed around the anodes. The resistance will decrease as the backfill column is lengthened.

13.8.9 The backfill bed should be allowed to settle for 24 hours before the anodes are energized.

13.8.10 The hole length from the top of the carbonaceous backfill column to the bottom of the casing level may be filled with additional coke or non-conductive, non-abrasive permeable backfill material such as sand or fine gravel. (Figure 1)

13.8.11 Native soil backfill may be used within the casing length of approximately 6 meters. (Figure 1)

13.8.12 The casing above grade must be provided with a cap to protect the installation and provide security from entry. The casing beneath the cap must be ventilated with 6.35-mm (0.25 in.) diameter drilled holes to prevent accumulation of gases. The vent pipe is also terminated below the casing cap. A one-inch diameter bushing with nylon screen should be filled to the top of the vent pipe to prevent insect entry. A plastic screen should also be fitted over the drilled holes in the casing periphery to further guard against insect and material entry. Figure 1, Detail "A" shows a sketch of a recommended hole completion at ground level.(10)

13.9 Anode placement: ELTECH LIDA^® ONE anodes

13.9.1 Where applicable, identified sections of 13.8 will be indicated for LIDA^® ONE anode placement.

13.9.2 Vent pipe layout; see item 13.8.1

13.9.3 The individual LIDA^® ONE anodes, numbered and designated are laid along the vent pipe.

13.9.4 Bottom weight placement does not apply.

13.9.5 Attachment of vent pipe to LIDA^® ONE anodes and cables; see item 13.8.4. Comments on string and double tail attachments do not apply. Protective foam covers are removed at this time.

13.9.6 Carbonaceous backfill slurry fill pipe entry into the hole; 13.8.5.

13.9.7 Introduction of LIDA^® ONE anodes, cables and vent pipe into the hole; see 13.8.6.

13.9.8 Backfill slurry addition; see 13.8.7.

13.9.9 Determination of change in resistance to ground with backfill addition; see 13.8.8.

13.9.10 Settling; see 13.8.9.

13.9.11 Completion of hole; see 13.8.10, 13.8.11 and 13.8.12.

USER NOTE:
In an effort to promote good design practices in the CP industry, Eltech Corporation provides a free Groundbed Design Software package to any and all qualified CP users. This software allows the user to design deep, horizontal and shallow vertical groundbeds based either on the desired groundbed resistance or the operating voltage of the rectifier.

An added feature of the LIDA^® Software is the economic comparison module, which allows the user to compare the performance values, and installed costs of LIDA^® anodes against silicon iron and graphite.

REFERENCES

1. Eltech Systems Corporation, LIDA^® Tubular Anodes; Technical Brochure, 1997.
2. Eltech Systems Corporation, LIDA^® ONE Anode, Technical Brochure, 1997.
3. A.W. Peabody, Control of Pipeline Corrosion, NACE, Houston, Texas, 1967, pp. 88-93.
4. H.B. Dwight, "Calculations of Resistance to Ground", Electrical Eng. Vol. 55, 1319 (1936) Dec.
5. Eltech Systems Corporation, LIDA^® Version 4.0 Software "Designing Groundbeds Using ELGARD LIDA^® Anodes", 1993.
6. Draft T-10A-10, NACE Proposed Tech. Committee Report "Impressed Current Anodes for Underground Cathodic Protection Systems", May 1993.
7. Technical Bulletin No. 110791, "Current Densities and Venting - Deep Anode Systems," Cathodic Engineering Equipment Co., Inc., Hattiesburg, MS.
8. John Reding "The LIDA^® Letter", Oronzio DeNora Technologies, Second Issue, April 1990.
9. Technical Bulletin No. 1091, "Carbon Backfill Placement - Deep Anode Systems" Cathodic Engineering Equipment Co., Inc., Hattiesburg, MS.
10. Ralph Stephens, Cathodic Protection Service Company, Houston, TX, Technical Correspondence, December 2, 1991.