Carryover

Carryover is a general term used that describes all types of water leaving the boiler with the steam.
It can be caused by foaming, priming or mechanical inefficiencies.
The effect of impurities in the boiler water is to increase the surface tension of the water and so inhibit the separation of steam from the water, this tends to cause foam to form.
Good TDS control is required to keep the solids levels below that at which this can occur.


Foaming

In a boiler steam bubbles are continually bursting at the steam water interface and ejecting boiler water drops into the steam space.
As the rate of steaming increases a point is reached where the steam bubbles are arriving at the surface faster than they are being removed, they accumulate as foam.
Steam released from the bursting bubbles move towards the steam outlet carrying with it smaller water droplets and taking with it any foam.
The steam also drags the surface of the boiler water towards the steam outlet so that the water level at that point can be higher than elsewhere, depending on the positions of the level controls this may cause the feed pumps to deliver water and thus raising the water level further.

Priming

As the water level is raised the volume of the steam space is decreased, the speed of the steam across the surface is increased drawing foam and water droplets towards the steam outlet. A sudden increase in steam demand can cause a slug of boiler water to enter the steam outlet.
This is known as priming.
Operation of the boiler below its design pressure will also produce a similar effect.

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