As a boiler generates steam, any impurities which are in the boiler feedwater and which do not boil off with the steam will concentrate in the boiler water.
As the dissolved solids become more and more concentrated, the steam bubbles tend to become more stable, failing to burst as they reach the water surface of the boiler. There comes a point where a substantial part of the steam space in the boiler becomes filled with bubbles and foam is carried over in the steam main.
This is obviously undesirable not only because the steam is excessively wet as it leaves the boiler, but it contains boiler water with a high level of dissolved and perhaps suspended solids. These solids will contaminate control valves, heat exchangers and steam traps.
Whilst foaming can be caused by high levels of suspended solids, high alkalinity or contamination by oils and fats, the most common cause of carryover (provided these other factors are properly controlled) is a high Total Dissolved Solids (TDS) level. Careful control of boiler water TDS level together with attention to these other factors should ensure that the risks of foaming and carryover are minimized.
The benefits of an automated TDS control system are:
• The labor saving advantages of automation.
• Closer control of boiler TDS level.
• Potential savings for a blowdown recovery system.