Steam Turbines - Steam Purity
|Publication Date:||1 June 2002|
|ICS Code (Gas and steam turbines. Steam engines):||27.040|
This technical specification describes the importance of the chemical characteristics of steam supplied to steam turbines and the need to prevent corrosion and deposition in steam space, in order to minimize the risk of turbine corrosion failures or loss of efficiency or output. The significance of relevant chemical parameters, mostly related to impurity levels, is discussed in terms of types of plant and chemistry operating regimes.
This specification is applicable to turbines of all electrical output rating and any exhaust condition, i.e. condensing or back pressure. However, the range of application in terms of output rating or steam pressure is dictated by economic factors such as the relative cost of monitoring equipment and turbine plant. It is designed for new plant, but may be adapted for use on existing plant.
The limits described in this specification are specifically designed to protect the steam turbine. The user should also be aware of steam purity requirements imposed by other considerations such as components of the boiler or steam generator.
This specification is applicable to steam turbines driven from any source, except geothermal plants in which the turbine is fed direct from the geothermal sources.
After the initial start of the turbine it may take some months for best achievable chemistry to be reached. This tends to be achieved sooner for some parameters (sodium, cation conductivity) than for others (silica, iron, copper). It is therefore recognized that during commissioning, it may be difficult to meet normal targets, but a steady improvement should be observed as the steam cleans up.
Annex A provides recommended guidelines for this specification and verification of the chemical purity of steam entering a steam turbine, in order to maintain the integrity and efficiency of the turbine. The limits stated are not intended to be mandatory, but to be used as the basis for local instructions for individual plant. Verification is addressed as the selection of appropriate sampling locations and continuously or intermittently measured parameters.