Boiler Water Carryover

‘Boiler Carryover’ is the term commonly used to define any contamination - solid, liquid or vapor - that leaves the boiler steam drum with the steam.  This is an undesirable condition because this carryover can lead to damage, either through deposition of the solids or through erosion-corrosion mechanisms or both.

Here are some examples of different types of Carryover:

• Priming – the most common form of carryover and is caused by mechanical conditions.  It is the physical lifting of water by the steam as it breaks the surface of the water and enters the steam release space.  Under certain conditions, the steam separators cannot handle the load placed on them; as a result, carryover occurs.
• Misting or spray carryover is the formation of a mist of fine water droplets generated when bubbles of steam leave the water surface.  These minute droplets are very difficult to separate from the steam and will usually carry over.
• Foaming is carryover caused by the formation of a bubble on the surface of the water.  When the bubbles burst, solids become a part of the steam.  If the foam is stable, it reduces the steam release space available and causes the steam separators to become overloaded.  At its worst, the foam can completely fill the steam space.
• Silica vaporization is the condition whereby silica in the boiler water is volatilized and carries over with the steam.  The degree of carryover is pressure-related and can first be seen at boiler pressures as low as 400 psig.

Carryover due to priming and misting is related to mechanical conditions and is independent of chemical conditions in the boiler.  Foaming and silica vaporization are caused by chemical factors and are independent of mechanical deficiencies in the system.  While the causes and cures of mechanical and chemical carryover are independent of each other, the effects on the system are compounded when the two occur together.

Next week our Watertech blog will deal with mechanical conditions that cause Boiler Carryover and things that can be done to minimize it.