High TDS levels in the feedwater tank lead to water carry over increasing the moisture content of steam. Wet steam reduces the process efficiency increasing fuel consumption.
As 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 into the steam main.
Increased moisture content in the steam is wastage of fuel as only steam participates in heat exchange in the process.
Eg: The incremental fuel consumed due to high level of TDS maintained in the boiler is illustrated below.
| Boiler design pressure: 10 bar |
Operating pressure: 8 bar |
| Boiler efficiency: 70% |
Fuel (Coal) GCV: 3000Kcal/kg |
| Feedwater (softned) TDS: 800 ppm |
Blowdown TDS: 8000ppm |
| Steam demand: 2000kg/hr |
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| At a TDS level of 8000ppm, the steam would be 85% dry i.e. there would be 15% moisture carry over, while at 3500ppm steam is 98% dry. |
| Under given conditions, the plant would loose 83tons of coal or 24kL of FO annually! |
Increased moisture is undesirable not only because the steam is excessively wet as it leaves the boiler, but it contains water with a high level of dissolved and perhaps suspended solids. These solids contaminate control valves, heat exchangers and steam traps.