Carryover in Boiler Steam

Carryover is the escape of moisture and impurities such as silica, copper, sodium, etc. along with the steam.  These impurities which are carried away by the steam can affect the piping and other equipment in the system.

Carryover occurs in two ways.  The first is due to mechanical action wherein the high velocity steam carries with itself small droplets of water which carry the impurities .  This is called mechanical carryover.  The second way in which carryover occurs is called vaporous carryover. 
In vaporous carryover, the impurities are carried over by steam. 

The sum of the mechanical and the vaporous carryover gives the total carryover. 
Causes of Carryover

The causes of carryover in boilers  are
  • Boiler Design
  • Fluctuating load on the boiler.  This causes the water level to rise and fall owing to the change in steam pressure.  This results in small droplets of water being ejected from the water surface into the steam above.
  • High quantity of impurities and dissolved substances
  • Overloading of the boilers
  • High water level
Use of efficient steam separation devices can minimize carryover.  A well designed blowdown schedule will help drain impurities and greatly reduce carryover.  Antifoam agents can also reduce carryover.