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Boiler Water Carry-Over

Boiler Water Carry-Over

Sometimes boiler water can get mixed with steam leaving the boiler. This makes the steam wet and can result in large slugs of water in the steam distribution system.

The terms “carry-over” or “boiler water carry-over” are used to describe the situation when water rather than steam accidentally leaves the boiler. Following are the descriptions of the three main causes of carry-over.


The steam load on the boiler may suddenly increase, if for example, a large piece of equipment is started. The large increase in load causes the pressure in the steam pipework to drop, As the pressure in the distribution main drops, the contents of the higher pressure boiler surge into the pipework to equalize the pressure. This leads to a reduction of pressure in the boiler. As the pressure in the boiler suddenly drops, the boiler water boils very quickly and violently. This makes the boiler contents surge even more into the steam distribution system. This is similar to shaking a bottle of champagne surges out from the high pressure bottle to the lower air pressure (Figure 3.12)


Natural substances in the raw water, improper boiler feed water treatment, or lubrication oils picked up by returned condensate can cause bubbles to form in the boiler. These bubbles can fill the boiler and the foam can get blown by the steam in to the distribution system. The bubble films contain water which is released as the bubbles burst. This is similar to blowing through a straw in a bottle filled with water and liquid soap. Another example of foaming, is the common case of a pan of milk boiler over. (Figure 3.13)