Blow down in Boilers - An Overview

Blow down in Boiler is a very important procedure.  The Blow down helps flush the boiler of impurities which may accumulate as the water evaporates.  If the blow down is not carried out, the impurities can reach dangerous levels which can result in the formation of scales in the pipelines and the formation of sediments due to precipitation.  Scaling and deposit formation reduces the heat transfer between the boiler and the water and affects the efficiency. 

Impurities can also cause foaming which results in the loss of water which gets carried away in the steam. 

There are two ways in which blowdowns can be carried out in Boilers.  The Bottom blowdown and the Surface blowdown. 

The Bottom blowdown is done by opening a drain at the bottom of the boiler.  The boiler pressure pushes the impurities and deposits out. 

Surface blowdown is done to remove the impurities which have formed a foam on the surface of water.  The foam needs to be removed for optimum heat transfer.  A pipe placed at the water level in the steam drum is used for this purpose.  Opening the pipe causes the water on the surface to be vented. 

The duration and frequency of blowdown depends on the boiler design and the conditions of operation.  It also depends on the levels of the contaminants in the feedwater and thus on overall water quality.  The boiler blowdown rate should be determined uniquely for each boiler installation.