Total Dissolved Solids (TDS)

In a boiler generating steam, impurities in the feedwater concentrates in the boiling water

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.

Measuring TDS with the density method

TDS = 1.1 10⁶ (ρ - 1)  (1)

where

TDS = Total Dissolved Solids (ppm)

ρ = relative density of the boiler water at 15.5^oC

This is a manual method and requires measuring a cooled down sample with a very sensitive hydrometer.

Measuring TDS with the conductivity method

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. 

TDS Range

Boilers are normally operated in the range 2000 - 3500 TDS.

Blow Down Rate

The 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

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