Cation Conductivity

Cation conductivity is a core monitoring parameter in steam, boiler water (blowdown or downcomer) condensate and feedwater, and can be used to indirectly assess levels of chloride and sulfate for corrosion avoidance purposes. It serves as an excellent diagnostic tool as the cations are removed via cation exchange and the H+ ion has a markedly higher equivalent conductance than all other cations. Target values of cation conductivity at the condensate pump discharge can be more relaxed for units with polishers than for those units without polishers. The reason for this relaxation is the removal of possible condensate contaminants in condensate polishers. Cation conductivity measurements do not detect contamination by alkali hydroxides (NaOH and ammonia) and only imperfectly of contamination by alkaline reacting contaminants. In high pressure units, if mixtures of amines or organic reducing agents are used for feedwater treatment, the thermal decomposition products will increase the cation conductivity.

Cation Conductivity Target Values at Economizer Inlet

The economizer inlet cation conductivity target values reflect the sum of cation conductivities contributed by individual anions and some carbon dioxide that may exist from air in-leakage, or as a result of the thermal decomposition of chemical additions such as alternate amines or organic reducing agents.

Measurement of cation conductivity at the condensate polisher effluent (if applicable) can substitute for cation conductivity measurements at the economizer inlet.

Cation Conductivity in Boiler Water

In order to help operating personnel avoid boiler damage caused by impurities, it is recognized that controls based on monitoring of boiler water cation and specific conductivity are needed for routine control of PC and CT. This need was also recognized and followed during development of interim chemistry guidelines for heat recovery steam generators (HRSGs) in combined cycle plants(22). The derivations need to consider the thermodynamic model predictions (to prevent carryover) and the experience of fossil plants that monitor cation conductivity (to avoid boiler corrosion damage by chloride and sulfate compounds).

AVT limits on maximum allowable boiler water impurity concentrations to minimize corrosion in the boiler are based on longstanding fossil plant experience with boiler water cation

conductivity around the world(6). It is possible to calculate the concentrations of chloride and sulfate from cation conductivity, provided the proportions of these and the other anions present are known. However, these proportions vary from plant to plant and operating conditions, thereby requiring that the proportion be assumed. As it is known that chloride represents a greater risk than sulfate in initiating corrosion, the concentration of sulfate has been taken as twice that of chloride when calculating the limits from cation conductivity. Note that the 1:2 concentration ratio for chloride:sulfate corresponds to a 40/60 stoichiometric ratio. Taking less than 50% ratio for chloride also makes some allowance for the fact that the cation conductivity includes other impurities. This approach was used in EPRI’s new AVT Guidelines(6).

In order to develop cation conductivity curves for the high and low phosphate extremes of PC, it is necessary to consider the effect of the phosphate on the measured cation conductivity value; hydroxides have no effect on this parameter. It is also necessary to assess the level of

contamination that the alkali solid in the boiler water is capable of neutralizing and/or buffering. The cation conductivity control curves for PC(L) and PC(H), and CT determine these effects and then overlay the cation conductivity associated with the allowable (un-neutralized and/or un- buffered) contaminants (based on and as used with AVT) to define the boiler water limit values. The sodium boiler water control curves presented in Sections 4 and 5 are based on maximum permissible levels of chloride and sulfate in the boiler water assuming they are present as sodium salts. They also account for additional sodium associated with allowable levels of any phosphate and free hydroxide in the boiler water. For PC this allowable concentration is 1 ppm sodium hydroxide as NaOH while for caustic treatment it ranges from 1-3 ppm sodium hydroxide as NaOH depending on the pressure. Effects of other contaminants that may be present (such as carbon dioxide and organics) on boiler water cation conductivity are also considered.

In units where boiler water sodium is not routinely monitored, there is risk of caustic corrosion if free sodium hydroxide is present in the boiler water at higher than intended levels. This is one reason why sodium is a core parameter (Table 3-1). Boiler water pH limits are set to avoid excessive free hydroxide conditions but this parameter is affected by ammonium hydroxide for all of the alkali solid treatments. Like ammonium hydroxide, sodium hydroxide affects the specific conductivity of the boiler water but not the cation conductivity. Thus, for indirect, conductivity-based monitoring and control of free sodium hydroxide in boiler water in units that do not monitor sodium directly, it may desirable to monitor the specific conductivity as

discussed in Section 3.8.11.

Thus, the measurement of cation conductivity in the blowdown or downcomer provides a reliable on-line indication of the level of contamination entering the boiler and concentrating by

recirculation. For this reason, plant operators can rely on this measurement to initiate control measures to mitigate the undesirable effects associated with increasing levels of contamination. Corrective action, typically blowing down and/or pressure reduction, is needed when increased contaminant levels are observed even if there is no apparent change in boiler water pH or chemical treatment levels.

It should be kept in mind that samples from the downcomers will be diluted by feedwater to a level dependent on the boiler recirculation ratio. Care should be taken to ensure optimum sample flow rate and temperature control.

The boiler cation conductivity control curves, which are discussed in Sections 4 and 5, and are designed to protect the boiler from corrosion. They are currently based on experience of limiting the concentrations of impurities in the boiler water to minimize corrosion as well as the effects of the alkali solid treatments in use. They indirectly consider the boiler water chloride and sulfate levels as a function of pressure together with a number of other contributors such as organic acids, carbonates, etc. For PC, phosphate and contaminant anions all affect the cation

conductivity, making it difficult to assess the individual contributions without further data (pH and phosphate). With CT, cation conductivity provides a direct indication of anionic

contamination.

Cation conductivity control curves have been constructed for CT (Figure 5-4). It is much more difficult to construct similar curves for PC since the measured cation conductivity is strongly influenced by the phosphate concentration, which increases by a factor of 50 over the entire PC operating range. Thus it becomes very difficult for EPRI to produce generic control curves that are applicable to all or even most fossil units. Figures 3-22 and 3-23 show the results of estimates for boiler water cation conductivity under PC(L) and PC(H) conditions. Each figure includes two curves; the lower curves indicate the cation conductivity attributable only to the phosphate, while the upper curves reflect the conductivity associated with the same level of phosphate and an additional maximum allowable level of chloride and sulfate in the boiler water. It should be noted that the curves presented in these figures correspond to a selected phosphate concentration at sodium to phosphate molar ratios of 3.0; the allowable cation conductivity limits suggested by these figures would change with variations in treatment chemical inventories (although cation conductivity is not affected by sodium hydroxide or ammonia) and be influenced by contaminant inventories actually present in the boiler water. Also, steam purity could become an issue in boilers at the higher operating pressures unless the mechanical carryover rate is lower than that used to establish the guidelines.

At present, cation conductivity is often not measured in the boiler water of fossil units that utilize phosphate treatments. As was indicated earlier for specific conductivity, personnel responsible for operation of fossil units on PC(L) and PC(H) are encouraged to monitor boiler water cation conductivity and examine the correlation this parameter displays with respect to pH, phosphate and sodium levels. Over time, this should lead to a better understanding of “normal” readings for each individual boiler. Once this is known, samples of boiler water should be tested for chloride and sulfate to verify that the accepted normal cation conductivity readings also ensure

Cation conductivity will increase as a result of addition of phosphate to the boiler water. Increasing cation conductivity that is not related to chemical feed activity is most likely due to contamination entering the boiler with the feedwater; phosphate concentrations will begin to fall if the contamination includes any hardness. In most instances, the contamination can be readily identified and responded to if the cycle chemistry program includes the appropriate

instrumentation and operator response procedures. However, in many fossil units, extremely low level contamination is a serious problem that can lead to concentration of impurities in the boiler water to levels capable of initiating corrosion damage. A longterm trend of increase in the

conductivity readings may be the only indication that a problem exists until the damage produces failures. Figures 3-24 and 3-25 show the cation conductivity values versus pressure for PC(L) and PC(H) only taking into account the maximum (normal) levels of chloride and sulfate (1:2 ratio). No contribution of phosphate is included.

Organizations seeking to establish unit limits for boiler cation conductivity are encouraged to dose the boiler to the maximum allowable phosphate concentration; this should be done carefully and with the blowdown closed or at the normal minimum setting since overfeed will require opening the blowdown, which will impact subsequent sampling and analysis activities. Once this treatment level is established the cation conductivity should be checked and samples collected for analysis of chlorides and sulfates. The effect of treatment on steam purity will also need to be checked at this time. By using this data and the information presented in this section it will then be possible to establish site specific limits that will require blowing down the boiler and/or reduction of pressure before contaminants increase to levels that put the boiler at risk. As continual reliance on blowdown is costly and limits flexibility in response to more serious chemistry excursions, personnel responsible for the unit are advised to identify and correct sources of chronic contamination at the earliest possible opportunity. Examples of boiler water cation conductivity control curves for PC are provided in Section 4 (Figure 4-14 for PC(L) and Figure 4-24 for PC(H)).

Calculation of boiler water cation conductivity limits for PC(L) and PC(H) at a drum pressure of 2500 psi (17.2 MPa) and 0.18% carryover has been performed and reported to be around 10 and 25 µS/cm, respectively(28)

; similar values are indicated in Figures 4-14 and 4-24. It will be noted that these values reflect considerably different capacities to cope with boiler contamination. The cation conductivity value for PC(L) (10 µS/cm) at a normal chloride limit of 150 ppb is about the same as for CT (see Figure 5-4) (about 9 µS/cm) at a normal chloride limit of 300 ppb, while the value for PC(H) is more than double that permissible with CT. The relative levels of boiler protection available with these treatments is reflected in both the continuum of treatments (Figure 2-1) and the boiler water treatment selection road map (Figure 2-8) presented and discussed in Section 2.

As with other parameters, it is very important to establish customized cation conductivity limits for each boiler as just outlined. This is included in the road map to optimize PC and CT in Section 2.

Figure 3-22

Estimates of Cation Conductivity Against Pressure for PC(L). The lower curve reflects the conductivity if only phosphate is present in the boiler water. It is based on the phosphate concentration corresponding to the sodium concentration given in Figure 3-18 to ensure less than 2 ppb sodium in steam. The upper curve reflects the situation with additional chloride and sulfate present in the boiler water. Note: these curves must not be used for boiler control; they are provided here to assist the reader to develop a set of cation

Figure 3-23

Estimates of Cation Conductivity Against Pressure for PC(H). The lower curve reflects the conductivity if only phosphate is present in the boiler water. It is based on the phosphate concentration corresponding to the sodium concentration given in Figure 3-17 to ensure less than 3 ppb sodium in steam. The upper curve reflects the situation with additional chloride and sulfate present in the boiler water. Note: these curves must not be used for boiler control; they are only provided here to assist the reader to develop a set of cation conductivity control curves in Section 4 for PC(H).

Figure 3-24

Cation Conductivity Versus Pressure for PC(L) only taking into account the maximum (normal) levels of chloride and sulfate (1:2 ratio). It must be noted that any contribution from phosphate has been excluded.

Note: These curves must not be used for boiler control; they are provided here only to assist the reader to develop a set of cation conductivity control curves in Section 4 for

Figure 3-25

Cation Conductivity Versus Pressure for PC(H) only taking into account the maximum (normal) levels of chloride and sulfate (1:2 ratio). It must be noted that any contribution from phosphate has been excluded.

Note: These curves must not be used for boiler control; they are provided here only to assist the reader to develop a set of cation conductivity control curves in Section 4 for PC(H).

Cation Conductivity Limit in Steam

Continuous measurement of cation conductivity at this location will provide a reliable indication of the presence of harmful salts and acids that are known to cause turbine corrosion. Elevated steam cation conductivities can be caused by an increased carryover and/or decomposition of alternate amines and organic reducing agents.