Water is the raw material converted in the boiler into the end product—steam. The quality or purity of steam is only as good as the quality of input FW and its conditioning in the boiler. In its passage through the boiler, water

• Is heated

• Undergoes phase modification from liquid to vapor

• Is superheated after becoming steam

The change of phase takes place over several cycles in the evaporator circuit in a sub — critical drum-type boiler only, whereas it happens in one shot in a once-through boiler, which therefore demands water of even greater purity. In power plants, the turbine maker decides the steam purity acceptable based on the following criteria:

• The metallurgy of casing and rotor

• The clearances of internal parts in the turbine

• The planned maintenance program

As the turbine inspection intervals lengthen, the steam purity requirements become more stringent.

Effects of Water on Boilers

Water, although adequately treated, harms the boilers in three ways, unless it is conditioned suitably:

1. Corrosion

2. Scaling

3. Carryover

Corrosion

Corrosion is

1. The dissolution of the metal

2. The formation of an insoluble product (e. g., an oxide)

In both cases, corrosion arising from the interaction of a metal with its environment weakens the metal, leading to a failure in due course. In case of metal oxide, deposited at the site or carried over to another location, heat transfer is impeded or flow restriction is created, which leads to overheating and subsequent failure.

Scaling

Scaling is the deposition of impurities carried with FW inside the tubes. Like the metal oxide layer, a scale also can impede the heat transfer or cause flow restriction, both of which lead to tube failure due to overheating.

Scaling can also lead to corrosion by shielding the underlying metal from the protective chemicals in water and steam.

Carryover

Carryover is the entrainment of minute water particles along with steam during steam separation, carrying solid, liquid, and gaseous impurities of water, thereby reducing the purity of the steam. Carryover arises from increased

1. Concentration of dissolved and suspended solids in drum water

2. Steam-water disengagement speed

The solids, thus carried over, then deposit as thin scales

• In the superheater (SH) tubes, leading to failure due to overheating

• On the turbine blades, leading to a loss of output