Page 27 - Boiler plant and distribution system optimization manual

12 Boiler Plant and Distribution System Optimization Manual

For boilers with superheaters, this carryover ter. Conditions in the drum provides a path for
of water droplets presents an additional problem. introducing solid material into the steam and is
Each water droplet contains solids at the same con- referred to as mechanical carryover which may
centration as the boiler water in the steam drum. be classified under four headings above. This can
When the steam passes through the superheater, result in troublesome deposits in the superheater
these water droplets will evaporate, leaving the or on turbine blades.
solids to form deposits’ on the inside of the super-
heater tubes. These solids reduce the transfer of
heat from the combustion gases through the tube MIST CARRYOVER
wall and into the steam. This layer of insulation
between the hot tube wall and the cooler steam. A fine mist is developed as water boils. A bub-
With this layer of “insulation,” can result in over- ble of water vapor (steam) reaches the water sur-
heating and tube failures. face and bursts, leaving a dent in the water. Liquid
Solids from carry-over may possibly pass collapses in on the dent, with the center rising at
completely through the superheater arid into a faster rate than the edges. This results in a small
downstream equipment. Solids entering a steam droplets being tossed free of the boiler water sur-
turbine can damage the turbine blades. Turbine face. These droplets form a fine mist. This mist is
repairs are not only expensive, but can result in removed to a great extent in various types of sep-
long outages. arators in the steam drum. However, any mist that
High drum level conditions can usually be remains entrained in the steam will have the same
reduced by decreasing the firing rate. Steam bub- level of contamination as the boiler water.
bles below the water level will tend to reduce
causing the water level to fall.
FOAMING CARRYOVER

CARRYOVER The alkalinity, TDS and suspended solids,
can interact to create a foam in the boiler. A heavy
There are two types of carryover; mechanical foam layer is another source of liquid carryover
and vaporous. into the steam. The level of foaming can normally
Mechanical carryover includes: be controlled to a reasonable level by maintaining
the total alkalinity at a level less than 20 percent
of the TDS and maintaining the total suspended
CARRyOvER

Pure steam becomes contaminated FOAMINg
with boiler water
• Spray & mist Surface layer obstructs steam
• Foaming Caused by
• Priming
• Alkalinity
• Poor steam separation
• Organic material

• Oil
The water evaporated to produce steam
should not contain any contaminating materials, • Grease
however there may be water droplets carried into
• Suspended Solids
steam due to mechanical carryover of boiler wa-