Galvanic Anode - Design Considerations

Design Considerations

The design of a galvanic anode cathodic protection system should consider many factors, including the type of structure, the resistivity of the electrolyte (soil or water) it will operate in, the type of coating and the service life.

The primary calculation is how much anode material will be required to protect the structure for the required time. Too little material may provide protection for a while, but need to be replaced regularly. Too much material would provide protection at an unnecessary cost. The mass in kg is given by equation (5).

Mass = (Current Required x Design Life x 8760) ÷ (Utilisation Factor x Anode Capacity)

(5)

• The design life is in years (1 year = 8760 hours).
• The utilisation factor (UF) of the anode is a constant value, depending on the shape of the anode and how it is attached, which signifies how much of the anode can be consumed before it ceases to be effective. A value of 0.8 indicates that 80% of the anode can be consumed, before it should be replaced. A long slender stand off anode (installed on legs to keep the anode away from the structure) has a UF value of 0.9, whereas the UF of a short, flush mounted anode is 0.8.
• Anode capacity is an indication of how much material is consumed as current flows over time. The value for zinc in seawater is 780 Ah/kg but aluminium is 2000 Ah/kg, which means that, in theory, aluminium can produce much more current than zinc before being depleted and this is one of the factors to consider when choosing a particular material.

The amount of current required corresponds directly to the surface area of the metal exposed to the soil or water, so the application of a coating drastically reduces the mass of anode material required. The better the coating, the less anode material is needed.

Once the mass of material is known, the particular type of anode is chosen. Differently shaped anodes will have a different resistance to earth, which governs how much current can be produced, so the resistance of the anode is calculated to ensure that sufficient current will be available. If the resistance of the anode is too high, either a differently shaped or sized anode is chosen, or a greater quantity of anodes must be used.

The arrangement of the anodes is then planned so as to provide an even distribution of current over the whole structure. For example, if a particular design shows that a pipeline 10 kilometres (6.0 mi) long needs 10 anodes, then approximately one anode per kilometere would be more effective than putting all 10 anodes at one end or in the centre.