Surface Blowdown in Boiler

Surface Blowdown in boiler is carried out to remove dissolved substances at the surface of the water.  That is, it is used to remove impurities which are in the liquid or dissolved phase.  Impurities which are in solid phase precipitate to the bottom where they are removed by the bottom blowdown.

The impurities and dissolved substances tend to form a layer of foam on the surface of the water.  This layer of foam needs to be removed to reduce the level of the dissolved substances. 

Since the blowdown involves removing water from the surface, it is called surface blowdown.
Surface blowdown is done by a pipe which is made to float a few inches below the water surface.  The pipe is connected to the outlet by means of a swivel joint.  The pipe can thus freely float in the water.  The pipe is held afloat by means of a float.

The pipe has a needle valve at its end.  The size of the valve opening can be adjusted based on the frequency and amount of blowdown required during each session.  Today, Automatic blowdown controllers which can control the rate and volume of the blowdown are also available.