During stoppages, thermal power stations have to tackle very severe corrosion risks. If appropriate prevention measures are not taken, corrosion can cause a lot of damage to many different system components, and can result in major disruptions to the restart.
Dry air method
The dry air method has superseded previous methods, because it is simpler, cheaper and quicker.
The following comments should be taken into account when employing this conservation method:
The only necessity here is the prevention of condensation, so maintaining relative humidity at 50% is enough. It works in open or closed (recirculating all the air) systems.
Relative humidity must be kept at 30%. Sizing depends on variations in absolute humidity with respect to the exterior temperature difference. It works in an open system (all exterior air). Pumping air to the areas most sensitive to humidity. Air conditions must be measured in the outlets.
Corrosion is the result of the presence of sulphuric acid. This is less aggressive when its concentration increases: and the concentration increases when the environment is dehumidified. According to the graphs, the relative humidity required is 5%. We have however learned from experience that 20% is enough.
When the dry air method is used, excellent results are achieved:
- It is easy to check whether there are corrosion risks, by simply evaluating the humidity in the air outlets.
- It enables access to the parts conserved, while they are being protected.
- Less energy is required than in the heating method, and no dangerous and expensive chemical compounds are required, as for humid conservation.
All the above-mentioned points of a technical and economic nature mean the dry air conservation method is preferable to all other possible methods.