|5. Cathodic Protection|
5.9 Groundbed Design [2/3]
2. Sacrifical Anode Systems
In certain situations, for example in reducing stray current effects, a sacrificial system may be specified to protect underground structures. The backfill used with these anodes is different from that described for impressed anodes. A typical backfill contains a mixture of clay and gypsum. The function of this chemical backfill to provide conditions favorable to anode dissolution. It also helps to reduce the groundbed resistance. Groundbed resistances can be calculated using the same procedure adopted for impressed current anodes. Individual galvanic anodes in a horizontal groundbed are generally not used. For this type of groundbed a continuous galvanic anode strip is found to be practical.
Types of Groundbeds
Basically there are two types of groundbeds
Close groundbeds: Limited areas of metal structure are protected by close groundbeds and larger areas are protected by remote groundbeds. A close groundbed is show in the figure below. The structure picks up the current from the soil in which a positive current is introduced and is protected. The area of influence is the area in which the pipe-to-soil potential exceeds -1.2 volts. This is only a small section of the structure which is protected.
Remote groundbeds: They protect large areas of a structure. The change in soil potential decreases with distance from the groundbed. The earth is remote when non-measureable change in potential exists when the current travels through the remote earth and enters a pipeline; it causes the potential of the pipeline to decrease resulting in cathodic protection. This results in a second area of influence: between the pipe and the soil. If there is no overlapping between the two areas, the groundbed is termed as "remote".