Impurities from the feedwater will concentrate in the boiling water.
When a boiler generates steam impurities in the feed water will concentrate in the boiler water. When the concentration of impurities becomes higher, foam is generated and the volume of the boiling water starts to increase. As a result - more and more low quality wet steam will be generated.
The concentration of impurities is often termed TDS - Total Dissolved Solids (ppm) and can be measured using a conductivity or density method.
Measuring TDS with the density method
TDS = 1.1 106 (ρ - 1) (1)
TDS = Total Dissolved Solids (ppm)
ρ = relative density of the boiler water at 15.5oC
This is a manual method requiring measuring a cooled down sample with a very sensitive hydrometer.
Measuring TDS with the conductivity method
TDS = 0.7 σ (2)
σ = conductivity (μs/cm)
This method can be used with an automatic resistance meter controlling the blow down cycles of the boiler. Note that electrolytic polarisation of the probe may be a problem and that an ac (amplitude current) resistance meter normally is used. Be also aware that the conductivity increases with temperature.
Boilers are normally operated in the range 2000 - 3500 TDS.
Blow Down Rate
The boiler blow down rate depends on the
- steam consumption (steam used in the process and not returned as condensate to the boiler)
- concentration of impurities in the feed water
- maximum allowable TDS in the boiler