Sodium Analysis in Power Plant:
1 – Steam
Under the conditions of high pressure and temperature of today's modern power plant, the problems of steam solubility of inorganic compounds, mainly neutral salts, have been paid increased attention.
Of particular significance is the steam solubility (in addition to carry-over) of neutral sodium salts, i.e. NaCl NaOH, because of the possibility of stress corrosion cracking in the superheating section. The measurement of sodium directly in steam immediately before entering the superheater is now recognised as being strongly indicative of possible stress corrosion situation existing in the superheater.
In this connection, it might once again be an opportune time to correct a popular misconception, namely the idea that sodium in steam would a priori be corrosive ! As a matter of fact, corrosion will occur only if sodium is present together with chlorides or hydroxides anions (not, for instance, with S04)
Chlorides and hydroxydes are corrosive, not the sodium; the latter serves only as the carrier and its presence in the steam may or may not indicate corrosive conditions, depending of the anion (which should be monitored simultaneously) !
2 - Condensate
The ubiquitous character of sodium also makes it an effective telltale to reveal possible leak conditions within the circuit.
In the condenser section, the measurement of sodium helps to detect cooling water leaks with a sensitivity unmatched by other techniques. Condenser leakage is normally monitored by the continuous measurement of condensate conductivity. The practical advantage of condensate conductivity measurement lies not only in the actual value but also in increases above the normal running value; it indicates trends or changes. Thus, to the plant operator, conductivity often gives the first indication of condenser leakage.
Although condensate conductivity is a most useful measurement, it is no longer sufficiently sensitive to detect the very small condenser leaks which have assumed greater importance in modern designs of plants. Steam purity can be more accurately assessed by measuring sodium concentration in both steam and condensate and, thus, determining the «sodium balance». The two concentrations should be equal (what goes in goes out). A higher level of sodium in the condensate indicates a condenser leakage. A lower level of sodium in the condensate indicates deposition of sodium in the steam circuit (on heat transfer surfaces, on turbine blading, ...). Sodium hydroxides and chlorides are known to be contributory factors in the stress-corrosion of austenitic steel (cf 1.).
The section most likely to be affected is that part of an austenitic superheater immediately following the point at which the desuperheater spray water is injected. In case of a condenser leak, isolation of the spray water is one the preferred action.
3 - Demineralisation Plant: Cation Exchangers
The performance of cation exchangers is preferably monitored with the sodium as the indicative parameter. Both the resin exhaustion (breakthrough) as well as the acid rinse-out after regeneration may be monitored with high sensitivity and reliability. The benefits of such a practice are considerable:
- Better utilisation of the resin capacity.
- Minimise sodium breakthroughs.
- Optimise acid rinse-out: on-line too soon will result in sodium leakage which in turn causes silica leakage in the anion exchangers; vice versa, on-line too late is a waste of rinse water and energy.
- Optimisation of regeneration cycles.