Monitoring of the reducing agent and ORP at the deaerator inlet can provide feedback control of the feed rate. The reducing agent concentration in conjunction with the oxygen level at this location should be consistent with the ORP value (Section 3.8.7). ORP is a much more useful measurement than either oxygen or reducing agent.
For all-ferrous feedwater systems, there is evidence linking higher reducing agent concentrations and low oxygen levels at the economizer inlet (<1 ppb) with flow-accelerated corrosion and increased transport of feedwater corrosion products (see Section 2).(26) Thus, it is necessary to advise that for feedwater heater trains constructed in all-ferrous materials, that good operating practice requires that the need for reducing agent addition be confirmed by monitoring of the feedwater corrosion products, ORP and oxygen levels at the economizer inlet. Many operators have found that additions of reducing agent in these all-ferrous systems make very little
difference to the level of oxygen, but produce a major change in the oxidizing/reducing potential of the feedwater. Thus many have been able to eliminate their use, and oxidizing AVT, AVT(O), is the feedwater treatment of choice for all-ferrous systems. This AVT(O) feedwater treatment can be used for units where the boiler is treated with PC or CT.
Cycles with copper alloy tubes in the feedwater system require lower oxygen levels and higher reducing agent levels for AVT(R) than cycles with all-ferrous components. The ORP at the deaerator inlet in these mixed-metallurgy feedwater systems must be determined and coordinated with reducing agent dosage and a low air in-leakage level to ensure reducing conditions exist (ORP of -300 to -350 mV). These levels of reducing ORP are not always achieved because ORP is very dependent on a number of variables (see Appendix B).
3.8.9 pH
Boiler water pH is a core monitoring parameter. Monitoring of pH is necessary for the following reasons:
• corrosion of cycle materials in contact with feedwater and boiler water is a function of pH, and
• alkaline pH values increase the stability of the magnetite film and reduce magnetite solubility in water.
pH Control
It is important to keep in mind that ammonia treatment of the feedwater (alkaline AVT (R) or AVT (O)) is intended to reduce general corrosion and FAC in the pre-boiler system. However, ammonia (or ammonium hydroxide) provides minimal, if any, neutralizing effect in the boiler water. Ammonium hydroxide is essentially fully associated at boiler operating temperatures. It, therefore, does not materially contribute to neutralizing possible acidic feedwater contaminants that may be available to concentrate beneath porous oxide waterwall tube deposits. Alkali solids (trisodium phosphate and/or sodium hydroxide) are added to the boiler water to neutralize any acidic contaminants that may be present. However, at typical sampling conditions, ammonia in boiler water samples is partially dissociated and therefore affects the pH. For this reason extreme care should be taken in the measurement of pH from the standpoint of both sample temperature (25°C, 77°F) and proper calibration of electrodes in an on-line measurement system. The effect of ammonia in the sample on the measured boiler water pH is discussed in the next section.
pH Target Values in Boiler Water
The measured pH in conditioned boiler water samples will be influenced by ammonia. This effect is most pronounced under PC(L) and under CT when levels of sodium hydroxide are less than one ppm. For effective surveillance and control of PC and CT, a correction for the influence of ammonia should be made.
By measuring the boiler water pH and ammonia concentration, it is possible to calculate the corrected pH value, by manipulation of the relationship between the concentration of hydrogen and hydroxide ions in the water; the following approach has been described by Verib that is useful in defining the basic relationships involved(35):
pH = -log[H] (eq. 3-6)
pH + pOH = 14 (eq. 3-7)
Concentrations are expressed in moles per liter. Be rearranging, it is possible to express the OH concentration as a function of the pH:
To this point, all terms relate to initial measurement of the pH in the boiler water. The following equations may be applied to calculate the corrected pH by subtracting the influence of the measured ammonia concentration, also expressed in moles per liter:
[OH]c = [OH]m – [NH3]m (eq. 3-9)
pOHc = -log[OH]c (eq. 3-10)
pHc = 14 – pOHc (eq. 3-11)
In this series of equations, [OH]m is the value calculated in equation 3-8 using the measured pH. Terms with the subscript c denote corrected values, as determined by calculcation.
The corrected pH, if determined by this methodology is, at best, a rough estimate, since not all of the ammonia measured in the boiler water sample will dissociate to ammonium and hydroxide ions. Complete dissociation of ammonia is assumed by direct substitution of the measured ammonia concentration in Equation 3-9. The actual degree of ammonia hydrolysis is incomplete and variable in conditioned samples, influenced by both sample temperature and concentrations of ammonia and alkali treatments present. These influences are considered in the EPRI
thermodynamic model used to develop Figures 4-3 and 4-4 presented in Section 4 for PC and Figure 5-12 in Section 5 for CT.
While Figure 5-12 may be readily used to determine the influence of ammonia on pH when employing CT, the situation is more complicated in the case of PC, where both phosphate and sodium hydroxide may be present, resulting in a spectrum of sodium to phosphate ratios
intermediate to the relationships shown in Figures 4-3 (only trisodium phosphate) and Figure 4-4 (trisodium phosphate plus 1 ppm of caustic.) To facilitate correction of pH for the effects of ammonia, Figure 4-5 has been developed, which can be used for PC(L), PC(H) and CT. The derivation process is described in detail in Appendix G. The corrected pH, that is, the pH that would be produced by the chemical treatment in the absence of the ammonia can be determined as a function of the measured pH value at various measured ammonia concentrations and independent of the actual measured phosphate concentration. However, to use the figure for diagnostic purposes the boiler water phosphate concentrations should be verified to be within the range (0.2–10 ppm) required when operating with PC.
Proper use of Figure 4-5 is addressed in Subsection 4.2.1, Introduction to PC(L). However, the figure is applicable to the entire PC working range and thus may also be used with PC(H) if desired. Comparison of pH correction results obtained with Figure 5-12 for CT with those of Figure 4-5 shows satisfactory agreement. This result was anticipated, since dissociation of trisodium phosphate to disodium phosphate and sodium hydroxide is nearly complete within the pH range of interest at standard sample temperature (25°C (77°F). However, derivation of a corrected pH equation of CT (based only on sodium hydroxide and ammonia) was performed (as discussed further in Appendix G) to confirm this consistency. Thus under normal treatment conditions, and with accurate measurement of boiler water pH and ammonia, the ammonia specific correction curves shown in Figure 4-5 are applicable to operation with CT as well as PC.
When managing boiler water chemistry, it must also be kept in mind that errors in measurement of the boiler water parameters needed to determine corrected pH may be significant and will thus affect the results obtained when making the pH correction. General guidance on chemistry data quality is included in Appendix E.
Immediate shutdown is recommended for low drum boiler water pH of less than 8 (as measured) and descending. Immediate shutdown is recommended since the pH will generally be much lower locally, which could lead to severe localized attack on waterwall tubes depending on the local conditions, particularly if the waterwall tubes are dirty and boiler chemical cleaning is due.
pH Target Values in Feedwater
The corrosion rate of carbon steel reaches minimum values over the pH range of 9.2 to 9.6. The optimal pH for protection of copper has been researched extensively over the last 5 years and is dependent upon temperature and ORP. Thus systems having mixed-metallurgy should now be able to minimize the corrosion rate of both carbon steel and the copper alloy by maintaining the condensate/feedwater pH between 9.0 and 9.3 under reducing conditions. The feedwater pH can be affected by carbon dioxide which enters with air in-leakage (Figure 3-26).
For units with condensate polishers in H-OH form, polisher run lengths decrease significantly at higher pH values. Operation at the lower end of the feedwater pH range may be necessary to maintain reasonable condensate polisher run lengths and acceptable effluent quality when using hydrogen form cation resins. Operation in the ammonia form will maximize run lengths at the expense of somewhat higher sodium leakage (normally within guidelines for sodium).
3.8.10 Ammonia
Monitoring of ammonia can be used to supplement the direct measurement of feedwater pH and/or specific conductivity for the control of the ammonia feed rate. It also influences the pH of samples from boilers operating on PC and CT (Section 3.8.9). Curves displaying these effects, as determined by the thermodynamic model, are presented and discussed in Sections 4 and 5
(Figures 4-3 to 4-5 for PC, and Figure 5-12 for CT).
For plants containing copper alloys in the condensate/feedwater systems, monitoring of ammonia is also important to minimize ammonia attack on copper alloys (particularly when oxygen and carbon dioxide are present), and particularly in the air removal section of the condenser.
Ammonia Target Values at the Economizer Inlet
The ammonia concentration in the final feedwater should be consistent with the pH
requirements. The pH of the feedwater is affected by carbon dioxide as illustrated in Figure 3-26.
Monitoring Ammonia in Boiler Water
As described earlier, ammonia in the boiler water will influence the pH. This effect becomes more pronounced when the levels of trisodium phosphate and/or sodium hydroxide in the boiler water are low. Figures 4-3 to 4-5 for PC and 5-12 for CT illustrate the effect of ammonia on pH as predicted by the thermodynamic model. In reality, the measured pH is also influenced by any other dissolved solids (chlorides, sulfates, organics, etc.) present in the boiler water; however, in the absence of significant contamination (as indicated by chloride and sulfate values in boiler water), this effect does not interfere with routine monitoring and control.
For routine cycle chemistry control purposes, it is quite satisfactory to estimate the corrected pH using relationships shown in Figures 4-3 and 4-4 for PC, and Figure 5-12 for CT. Figure 4-5 allows determination of the required pH correction using the results of analysis for measured (uncorrected) pH and ammonia in the boiler water. The correction will be less precise when excessive levels of contaminants are present but such conditions will result in pH changes and increased conductivity (cation and/or specific) readings and thus require operator response. However, by using the conductivity to detect contamination of the boiler, it is envisioned that better protection of the boiler will be possible.
It should be recognized that ammonia levels in downcomer samples will tend to be somewhat higher than those measured in the blowdown. Thus the former sample point will provide a more realistic indication of bulk water chemistry in generating tubes. However, the latter point is satisfactory for routine monitoring and control of PC and CT.