Mitigation of Interference Effects from Impressed Current Cathodic Protection Systems

Summary of Equations

2.2 Detecting Stray Current

2.2.1 Mitigation of Interference Effects from Impressed Current Cathodic Protection Systems

There are a number of methods that can be used to lessen the deleterious effects of CP system stray currents, as listed below:

• remove the source or reduce its output

• install electrical isolating fittings in the interfered-with structure

• bury a metallic shield parallel to the interfered-with structure at the stray current pick-up zone

• install additional CP at current discharge locations on the interfered-with structure

• install a bond between the interfered-with and interfering structures

• apply a coating to the interfered-with structure in the area of stray current pick-up or to the interfering structure where it picks up the returning stray current.

CP Interference

Before any mitigation activity can commence, it is necessary to conduct mutual interference tests where the output of the suspected interference source is cyclically interrupted and field measurements taken in the presence of representatives of the interfering and interfered-with companies. Interference cases are often reported through local electrolysis committees, especially where there may be more than one interfered-with party.

Presuming that a need for mitigation is determined, the mutually acceptable mitigation technique(s) will depend on the location and severity of the interference, on the CP operational preferences of each party, and on the relative capital and maintenance costs of the mitigation options.

2.2.1(a) Source Removal or Output Reduction

It is a difficult proposition to have a source removed if the interfering system was present before the interfered-with structure was installed. However, in the opposite situation, where the interfering source is newly installed, this method has greater appeal.

If the interference is caused primarily by the proximity of the interfered-with structure to the interfering groundbed, it may not be necessary to remove the transformer-rectifier but simply relocate the groundbed location or reduce the current output.

Equation 2-5 or similar equations¹ can be used to estimate how remote a particular groundbed needs to be from a foreign structure in order to minimize the interference effects.

It should be noted, however, that the voltage rise at any point distance “x” from the groundbed is a percentage of the total voltage drop to remote earth (Vx,re/Vgb,re × 100).

The voltage rise is a function only of the geometry of the groundbed (i.e., its length

“L”) because the groundbed current output and soil resistivity would not change.

Therefore, only the length parameter in the equation significantly affects the percentage.

1Von Baekmann, Schwenk, and Prinz, Cathodic Corrosion Protection, 3^rd Edition Gulf Publishing, 1997, pp.538-539.

CP Interference

Reducing the current output of the source is also a viable option as long as there are safeguards to prevent the output from being raised inadvertently.

2.2.1(b) Installation of Isolating Fittings

Installing isolating fittings as a stray current mitigation measure is an attempt to increase the path resistance (Rs) of the interfered-with structure, thus decreasing the stray current (Is). This is seldom adequate as a stand-alone method.

The stray current will certainly be reduced, but the lesser amount of stray current will bypass each isolating fitting in the soil path. Hence, several points of interference will be created (as previously shown in Figure 1-15a). Consequently, additional CP may be needed at each isolating joint to compensate for the residual stray current.

The installation of isolating fittings to electrically sectionalize piping systems, as illustrated in Figure 2-22, is a common practice.

T/R

I'''_s

I'_s

I''_s

isolating fitting

Figure 2-22: Stray Current Arising from Installation of Isolating Fittings

Unfortunately, inserting electrical isolation often produces a stray current condition at the isolating fitting. Therefore on piping networks protected with impressed current systems, electrical isolation should be used sparingly. When electrical isolation is used, facilities to mitigate the expected interference should be provided at each point of electrical isolation.

CP Interference

2.2.1(c) Burying a Metallic Shield Next to the Interfered-with Structure

The intent of a buried metallic conductor is to intercept the stray current and thus provide an alternative low-resistance path for the stray current compared to the metallic structure path. Connecting the metallic shield, which could be a bare cable or pipe, directly to the negative terminal of the offending transformer-rectifier—as shown in Figure 2-23 and modeled in Figure 2-24—would be more effective than connecting it to the interfered-with structure.

Interfered-with Structure

I_cp

I_s

Bare Shield

Cathodically Protected Structure T/R

Figure 2-23: Using a Buried Metallic Cable or Pipe as a Shield to Reduce Stray Current Interference

CP Interference

= shield resistance to earth R_sh,e

R_sh R_sh,e

= shield cable longitudinal resistance R_sh

Icp = Icp' + ^I's + I''s

Is' ''

= stray current in shield wire path

= residual stray current in foreign pipeline

I''s

I's

Figure 2-24: Cathodic Protection Current Model for a Buried Metallic Shield Connected to the Negative Terminal of the Transformer-Rectifier

The alternative approach which would be to connect the buried metallic shield to the interfered-with structure, increasing the stray current discharge at point “B.”

This buried metallic shield method has most merit either where the interfered-with structure is made of an amphoteric material or there is a concern about coating blistering or cathodic disbondment.

For the interfering system, there is considerable disadvantage to this technique because it could seriously disrupt the current distribution pattern to the cathodically protected structure, perhaps even necessitating the installation of additional CP units to make up for the poorer current distribution.

2.2.1(d) Installation of Galvanic Anodes on the Interfered-with Structure at Point of Stray Current Discharge

When the area of stray current discharge is very localized—such as at a crossing with the interfering structure—and where the total stray current (Is) is typically less than an ampere, the installation of galvanic anodes (Figure 2-25) has considerable benefit.

CP Interference

Figure 2-25: Interference Mitigation using Galvanic Anodes at Stray Current Discharge Location

If the interfered-with structure is coated at the crossing, then the path resistance (Rap) through the galvanic anodes will be substantially lower than the interfered-with structure resistance (Rs1,p). The electrical circuit model in Figure 2-26 depicts the structure resistance. Although there can still be a residual stray current (Is′′), it is expected that the total CP current (∑Icp) will be greater—thus assuring total remediation of the interference.

Test Station

Interfering Structure

Interfered-with Structure

I_cp Is,t

Icp

I_cp Icp

Icp

I's'

I's

I'_s ^I'^s ^I'^s

I's ^I'^s ^I'^s

CP Interference

Ra,p = anode(s) resistance to the interfering pipe

Icp

Eg = galvanic anode driving voltage

Icp,g = galvanic anode current

I_cp = Icp' + ^I's + I''s

= stray current through galvanic anodes

= residual stray current discharging from foreign pipeline

I's

I''_s

Figure 2-26: Electrical Circuit Model for Mitigating Stray Current Interference at a Stray Current Discharge Site Using Galvanic Anodes

Ideally, the galvanic anodes are distributed alongside the interfering structure in order to minimize the path resistance (Ra,p); therefore, the stray current (I′_s) is a large percentage of the total stray current (Is). The design life of the galvanic anodes must take into account the additional consumption by the stray current (I′s) component of its total output.

Several advantages of this method are as follows:

• the interfered-with structure can maintain CP independence

• the galvanic anode CP current output boosts the level of protection at the crossing as an added buffer should the interference current (Is) increase

• low maintenance requirements compared to a direct bond.

The disadvantages are that it is relatively expensive compared to a direct bond and the interference current mitigation capacity is somewhat limited. To mitigate large interference currents, an impressed current system can be utilized having the drain point at the crossing but the groundbed remote from both piping systems.

CP Interference

Example: Design Calculations for a Galvanic Anode Interference Mitigation System At a crossing of two coated and cathodically protected pipelines, a temporary resistance bond of 3 Ω passing 350 mA was required to mitigate the interference on Pipeline A caused by Pipeline B’s impressed current systems. It has been decided to mitigate that interference problem using magnesium anodes because the soil resistivity is low (3100 Ω-cm) and Pipeline A wishes to maintain its cathodic protection independence.

Step 1: Choose a #17D2 magnesium anode from Table 1 in Appendix C.

Step 2: Calculate the resistance of a single vertical anode from Dwight’s Equation:

⎭⎬

where: Ra = resistance of vertical anode to remote earth ρ = soil resistivity (Ω-m) = 31 Ω-m

Step 3: Calculate minimum number of anodes (N) to achieve a 3-Ω resistance.

Assuming no mutual resistance effects between anodes then

anodes

Step 4: Calculate anode CP current output assuming a pipeline polarized potential of –850 Vcse

CP Interference

= 75.9 mA + 4 350 mA It = 163.4 mA

Step 6: Calculate anode life:

r U = utilization factor

E = efficiency

Ia = current output (A)

Cr = theoretical consumption rate (kg/A-y) Note: Consumption rate for magnesium @ 50% efficiency is

approximately 8 kg/A-y. An utilization factor of 0.85 is assumed.

y y

Step 7: Calculate minimum anode weight to achieve a 20-year life using equation [2-19].

therefore: Total anode weight for the mitigation system would need to be 4 × 30.7 kg = 122.7 kg

Step 8: Chose a larger weight anode from Table 1 in Appendix C.

In document NACE CP Interference January 2008 (Page 67-75)