Concepto e instrumentación

The following discussion is concerned with PC(L) during non-normal conditions; specifically what modifications to PC(L) are necessary during these contamination episodes and how should an operator react?

When contamination occurs, the boiler blowdown should be opened and the source of

contamination isolated quickly to minimize chemical upset. In parallel with boiler blowdown, sodium hydroxide dosing is required for ensuring the correct pH level in the boiler water. The cation conductivity should be referenced to a unit specific version of Figure 4-14, and if

necessary, the concentrations of chloride and sulfate in the boiler can be determined using an ion chromatograph. If the concentrations exceed limits (such as those given in Figures 4-10 and 4- 11) the pressure of the unit should be dropped to stay within the sliding chloride and sulfate pressure limits adopted for the plant. In order to provide chemical protection during high

contamination episodes, the phosphate concentration can be adequately increased. Using sodium hydroxide, the pH should be maintained within the normal limits of 9.0 to 9.6 adjusted for the presence of ammonia or to the unit specific phosphate/pH diagram corrected for ammonia. With PC(L), a measured pH reading of less than 9.0 requires action, regardless of the ammonia concentration, since the corrected pH (using Figure 4-5) will clearly be less than 9.0 under such circumstances. Boiler water conductivity readings should also be taken to determine if the low pH is due simply to loss of treatment chemical (typically via blowdown and possibly involving malfunction or failure of the chemical feed equipment) or involves contamination. Boiler conductivity and other plant instrumentation readings should also be checked to see if they indicate the possibility of an ongoing contamination event.

In the absence of boiler water contamination, the degree of corrosion protection afforded by lower than desired levels of treatment (and commensurate low corrected pH values) may be completely satisfactory, however, restoration of normal chemistry is required as quickly as possible to ensure that the boiler will be protected in the event contamination is experienced. With PC(L), if the absence of contamination is confirmed and the phosphate residual is within the customized limits of the unit, addition of caustic is needed to raise the pH. On the other hand, addition of trisodium phosphate should be made first if (again, in the confirmed absence of contamination) the phosphate level is less than the minimum allowable concentration; a subsequent addition of caustic would be needed if the trisodium phosphate addition does not restore the corrected pH value to normal limits.

The more serious situation is the one involving the presence of boiler contamination, typically confirmed by increased boiler water conductivity readings. Operation of PC(L) without pH correction, and without surveillance and control of the conductivity, can give a false sense of security with respect to corrosion protection. When the corrected pH and treatment chemical levels are within expected ranges for control, the boiler water can provide corrosion protection as indicated in the impurity and conductivity control curves (Figures 4-10, 4-11, 4-13 and 4-14); corrosion protection is compromised at higher boiler water impurity levels. However, when the corrected pH is less than normal, this means that the levels of treatment chemicals are also lower than needed to provide corrosion protection at the levels indicated in the impurity and

conductivity control curves; in the event of complete loss of chemical treatment, the boiler corrosion protection would be the same as for all-volatile treatment.

If the measured pH is rapidly falling and at risk of reaching the immediate shutdown level of 8, there is clearly no point in making the pH correction until and unless the boiler chemistry can be stabilized quickly and without the need for immediate shutdown. For example, it may be possible to keep the unit in service by reducing pressure and/or load and restoring program treatment levels, in which case correction of the measured pH for ammonia should be resumed. Loss of the ammonia feed system could conceivably lead to a rapid yet relatively minor reduction in the measured boiler water pH. However, the corrected pH should remain at an acceptable level providing the cycle is free of significant contamination and levels of phosphate and/or sodium hydroxide treatments are in expected ranges for control. Under such

circumstances, the boiler corrosion protection is maintained but the ammonia feed system must be returned to service as soon as possible to control the feedwater pH at the optimum level needed to minimize corrosion of metals from the waterside surfaces of preboiler components and piping and subsequent transport of these metals to the boiler.

If at any time the boiler blowdown pH falls to below 8, the unit should be taken off line and drained. In the case of a longterm operation with markedly increased contamination, chemical cleaning should be considered. After the source of contamination has been isolated and the concentration of the contaminants in the boiler blowdown is within limits, the unit can be converted back to standard PC(L) chemistry.