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Engineering ToolBox > Total Dissolved Solids (TDS)
When a boiler generates steam, impurities in the feed water will concentrate. When the concentration of impurities becomes higher, foam is generated and the volume of the boiling water increases, resulting in more and more low quality wet steam generated.
The concentration of impurities is often termed TDS - Total Dissolved Solids (ppm) and can be measured using the conductivity or density method.
TDS = 1.1 106 (ρ - 1) (1)
where
TDS = Total Dissolved Solids (ppm)
ρ = relative density of the boiler water at 15.5oC
This is a manual method and requires measuring a cooled down sample with a very sensitive hydrometer.
TDS = 0.7 σ (1)
where
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.
| TDS (ppm) | Conductivity (μs/cm) | Relative Density |
| 0 | 0 | 1.00000 |
| 1000 | 2000 | 1.00091 |
| 2000 | 4000 | 1.00182 |
| 3000 | 6000 | 1.00273 |
| 4000 | 8000 | 1.00364 |
The blow down rate depends on the