BOILER STEAM PURITY
- It is important that the steam leaving the boiler drum should be free from impurities which could be deposited in the superheater or the turbine.
- Such impurities may arise from two distinct causes, one being the property which steam at high temperature and pressure has of dissolving significant amounts of certain substances, notably SILICA & CAUSTIC SODA.
- the other cause is more familiar, namely priming or carry-over, where there is incomplete separation of the two phases in the drum and droplets of boiler water are entrained in the steam leaving the drum.
- Impurities caused by steam solubility can only be controlled by careful regulation of the boiler water analysis, particularly with regard to alkalinity and silica, and it is accepted that if the silica in steam doesn’t exceed 0.02ppm , it will cause no trouble by deposition in the superheater or the turbine.
- The concentration of silica in the boiler water which is in equilibrium with 0.02ppm in the saturated steam varies with the operating pressure as shown in the following figure.
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