Recommended sample points were developed based on the optimal cycle chemistry
requirements, and the rationale for each sample point is discussed in this section. The parameters to be monitored fit into two categories, as shown on the cycle chemistry diagrams in Sections 4 (PC) and 5 (CT): a) those parameters which all fossil plants should have for optimum chemistry control (core parameters) and b) those parameters which are regarded as diagnostic for
troubleshooting or commissioning. See Table 3-2 for a listing of core instruments for units operating on alkali solid treatments.
The core parameters are considered the minimum level of surveillance needed for all units. In general, use of on-line analyzers for continuous analysis of chemistry is preferred. However, it is recognized that certain parameters such as iron, copper, sulfate, total organic carbon and
chlorides (usually) will require the procurement and analysis of grab samples. Primary chemistry control for these units is through the use of cation and specific conductivity. Conductivity
instrumentation is reliable, relatively maintenance free and provides an early indication of cycle contamination and increased carryover. Specific conductivity also provides an accurate
indication of ammonia levels in the feedwater. For units on PC, conductivity needs to be used in conjunction with pH and phosphate measurements.
Target levels have been established for sodium, chloride, silica, sulfate and organics in steam. Limits for these parameters were set to minimize the risk of deposition and corrosion in turbines, while ensuring that the limits provide adequate corrosion protection to the boiler. In drum units, it is important to avoid levels of mechanical and vaporous carryover, which will exceed these target values. The routine measurement of carryover (every six months) is considered of such importance that it is included as a core parameter.
Table 3-2
EPRI’s Core Monitoring Parameters and/or Minimum Level of Continuous Instruments for All Units Operating on Phosphate Continuum and Caustic Treatments
Oxidizing AVT (AVT(O)) Feedwater and PC or CT in the Boiler Water
(Drum units without a reducing agent and with an all-ferrous feedwater system) Cation conductivity CPD, CPO or EI, RH (or MS), Blowdown
Specific conductivity Makeup, Blowdown
pH Blowdown
Dissolved oxygen CPD, EI
Sodium CPD, CPO or EI, Blowdown, RH (or MS) Phosphate Blowdown (PC only)
Air in-leakage Total Carryover
Reducing AVT (AVT(R)) Feedwater and PC or CT in the Boiler Water
(Drum units with a reducing agent and with a mixed-metallurgy or all-ferrous feedwater system) Cation conductivity CPD, CPO or EI, RH (or MS), Blowdown
Specific conductivity Makeup, Blowdown
pH Blowdown
Dissolved oxygen CPD, EI
Sodium CPD, CPO or EI, Blowdown, RH (or MS) Phosphate Blowdown (PC only)
ORP DAI
Air in-leakage Total Carryover
Using the approach in Section 3.3, contaminant control curves were developed for drum units. These curves are presented in Sections 4 and 5. By confirming these values with mechanical and vaporous carryover (partitioning) modeling it has been possible to stress the importance of steam carryover and boiler drum pressure to provide the concentration of the contaminant of interest.
3.4.1 Reheat Steam/Superheated Steam
Factors affecting steam chemistry, other than the mechanical and vaporous carryover from the boiler, include the following:
• contamination of steam in superheaters and reheaters by attemperation water, and
• precipitation of impurities as deposits in the superheater, reheater, and in the turbine due to changes in steam temperature and pressure.
Monitoring of key contaminants at this sample point indicates the actual impurity levels in the steam and indicates whether the turbine blades are protected against deposition and/or corrosion. This monitoring also verifies compliance with the turbine manufacturer’s guarantee condition.
Should this sample point not be available, the steam chemistry may be calculated from the chemistries of the saturated steam and feedwater, accounting for any impurity ingress associated with the superheater and reheater attemperating water (feedwater).
Steam sampling requires special techniques, which are described in an EPRI Report(23) and in Appendix E.
3.4.2 Saturated Steam
Monitoring of this sample point provides verification of compliance with the boiler
manufacturer’s performance guarantee for steam purity, which may apply only to the saturated steam. The sample source may be from one steam offtake or all steam offtake tubes from the drum as long as a representative sample is taken. This sample point also serves as a vital diagnostic tool to monitor the total carryover (a core parameter) of impurities into the steam. The saturated steam chemistry data and resulting carryover figures can also be related to the performance of the steam drum moisture separator devices. Excessive carryover generally indicates either poor moisture separator performance or operation with a higher than intended water level in the steam drum. Mechanical carryover has the greatest impact upon steam chemistry at boiler drum operating pressures below 2500 psi (17.2 MPa); for most species, particularly for sodium and sulfate, mechanical carryover is the major component of the total carryover. For silica and copper, however, volatile carryover is very significant, even at lower pressures. As indicated previously, a knowledge of carryover is considered so important, that it is now a core parameter (Table 3-2), which should be routinely checked (every six months).
As discussed in Section 3.3.1, EPRI research on volatility has shown that, for sodium, chloride and sulfate, carryover of these constituents is almost entirely attributable to mechanical
carryover. Volatile carryover only becomes significant at the higher operating pressures. Thus, it becomes clear that the steam purity with respect to these constituents is dependent on factors such as the integrity of steam separator devices and drum level control. Also, at higher pressures, the boiler water chemistry must be carefully controlled to prevent contamination of the steam. The control curves presented in Sections 4 and 5 define boiler water conditions that will protect both the boiler and turbine. However, the turbine protection aspect assumes that boiler carryover is effectively controlled. As in earlier chemistry guidelines, industry mechanical carryover criteria were used and a safety factor was applied; see Figure 3-2. Vaporous carryover was determined with the EPRI thermodynamic model developed as a result of earlier volatility studies (see Section 3.3.1).
To ensure that the control curves of Sections 4 and 5 are providing turbine protection it is
imperative to periodically assess boiler carryover in every drum-type unit. This involves making a routine check of total carryover at six month intervals. Total carryover is defined as the fraction of a given constituent in the saturated steam, expressed as a percentage of the same constituent in the boiler water. For routine chemistry surveillance of total carryover, the determination should be made using sodium as the selected constituent. Total carryover typically varies with boiler load and pressure. Thus it is necessary to evaluate the total carryover under a range of operating conditions, preferably with the boiler chemistry in normal operating limits.
Sodium monitoring in boiler water and steam may be performed with either on-line analyzers or by laboratory analysis of grab samples. For units that meet the instrumentation criteria of Table 3-2, it will be possible to make total carryover assessments with the installed analyzers,
providing that the saturated steam sample can be directed to the sodium analyzer normally used to monitor reheat steam or main steam. (Increased sodium readings at these sample points are a possible indication of increased carryover from the boiler water and this may be evaluated by checking sodium in the saturated steam.)
Once the carryover has been calculated, then the value of the sodium in boiler water for PC(L) or PC(H) should be compared with Figure 3-9 (i.e., should be less than the values shown in Figure 3-9 to ensure that the sodium value in steam is not exceeded). The value of sodium in steam for CT should be compared with Figure 3-14. These curves provide the overriding criteria to protect the turbine. The boiler water sodium curves for PC(L) (Figure 4-8), PC(H) (Figure 4-17), and CT (Figure 5-7) in Sections 4 and 5 have been designed to be the same as or lower than the sodium values in Figures 3-9 and 3-14, respectively.
3.4.3 Boiler Water
This sample point monitors drum boiler water chemistry to minimize deposition and corrosion in the boiler tubes. This sample point allows control of boiler water chemistry through blowdown and chemical feed, and is a primary control point for saturated steam purity. Boiler drum samples (either blowdown or downcomer) can be used for boiler water analysis. With some boiler
designs, the blowdown may not provide a representative boiler water sample, and the
downcomer sample should be used. The downcomer samples should also be used for cycling units and for layup chemistry control.
Samples from the downcomer will be diluted with the feedwater, and this will reflect a lower concentration of the various chemical species when compared to blowdown samples. This effect may be considerable, depending on boiler design. Therefore, limits derived from downcomer samples must reflect this dilution effect when compared to limits given in these guidelines, which are derived for blowdown samples.
Once a satisfactory data base on total carryover is established, it may be possible to customize the boiler limits from what is indicated in the generic control curves presented in Sections 4 and 5. If warranted, relaxation of the boiler water sodium and phosphate (for PC) limits may provide added boiler protection, while still protecting the turbine, so long as the sodium values in Figure 3-9 for PC(L) and PC(H), and Figure 3-14 for CT, are not exceeded. Thus the need to
periodically appraise total carryover remains, regardless of whether or not the boiler limits are customized.
On the other hand, tightening of the boiler water limits due to steam purity concerns (exceeding the maximum allowable boiler water sodium values predicted to ensure satisfactory steam purity as indicated in Figures 3-9 and 3-14), will compromise the boiler protection aspect of the Section 4 and 5 control curves. If total carryover testing indicates that the limits for boiler water are impacting steam purity, it is imperative that actions be initiated as soon as possible to identify and correct the root cause responsible for the high carryover rate.
3.4.4 Economizer Inlet and Attemperation Water
This sample point allows the direct measurement of the total contaminant ingress to the boiler and to the steam via the attemperation water. In new units, it also permits the determination of whether the feedwater entering the boiler meets the feedwater chemistry limitations required by the boiler manufacturers. This sample point is also used to monitor oxygen and feedwater corrosion product transport, and serves as a sampling point for control of ammonia feed. However, it does not permit the evaluation of flow-accelerated corrosion (FAC) in the economizer header or tubes.
3.4.5 Deaerator Outlet
This sample point (in conjunction with the deaerator inlet point) permits an evaluation of how the deaerator performs in removing dissolved oxygen from the feedwater, especially during periods when the condensate oxygen levels are excessive, such as during startups and periods of high makeup with air-saturated water, or when the air in-leakage is high and occurring at
locations that do not allow the oxygen to be removed in the condenser.
3.4.6 Deaerator Inlet
In most fossil units, the significant reduction of feedwater dissolved oxygen occurs in the
deaerator. In these guidelines a distinction is made as to whether the feedwater is being operated in a reducing or oxidizing mode (AVT(R) or AVT(O)). As explained in the guideline document on copper(24), it is considered mandatory that mixed-metallurgy cycles be operated in a reducing regime to minimize copper alloy corrosion, transport and subsequent deposition. This requires the feed of a reducing agent such as hydrazine, in addition to minimizing oxygen ingress to the cycle. To properly determine that a reducing atmosphere is being provided, the measurement of oxidizing-reducing potential (ORP) at the deaerator inlet has been designated a core parameter. Although the reaction between reducing agents and oxygen increases at high temperatures, the reactivity of dissolved oxygen with metal is greater and predominates in the high pressure heater trains. Thus the sample point should be used as a control point for reducing agent (hydrazine) feed at the condensate polisher outlet or condensate pump discharge.
All-ferrous systems can be operated in either a reducing or oxidizing mode. As previously noted, an increasing number of power plants with all-ferrous systems are finding that the elimination of the reducing agent (hydrazine) leads to superior results relative to iron transport while, at the same time, minimizing chemical feed costs.
This sample point also monitors deaerator oxygen removal performance by comparison to results for samples collected at the deaerator outlet.
3.4.7 Condensate Polisher Effluent (if Applicable)
This sample point is required to determine the effectiveness of the condensate polishers and to determine their need for regeneration. This sample point also permits the evaluation of resin particle “throw” from the condensate polishers. Measurements of sodium and cation conductivity at this point can substitute for the measurement of these parameters at the economizer inlet.
3.4.8 Condensate Pump Discharge
This sample point monitors the following:
• Presence and magnitude of contaminants introduced by condenser leakage.
• Presence and magnitude of contaminants introduced by the makeup treatment system. • Amount of oxygen entering the feedwater train and thus, indirectly, the air in-leakage. • Corrosion products from heater drains returned to the condenser.
• In extreme cases, carryover of contaminants and treatment chemicals in steam.
For plants without condensate polishers, this point monitors the chemistry at the start of the feedwater system. A major change was made in the new AVT Guidelines(6), which involved a tightening of the dissolved oxygen normal target to 10 ppb. This is important for both mixed- metallurgy and all-ferrous feedwater systems and is included in these PC/CT Guidelines.
3.4.9 Condenser Leak Detection Trays and/or Hotwell Zones (if Applicable)
This sample point, if provided and available, may allow condenser tube leaks to be detected earlier than by the sample point located at the condensate pump discharge. This aids in timely determination of needed corrective actions. Monitoring at these leak detection tray sample points allows determination of the tube sheet on which the leak is occurring. Quick detection and repair of leaks minimizes the ingress of impurities into the heat cycle. Each individual condenser hotwell section may also be monitored.
3.4.10 Air Removal System Exhaust
Condensate oxygen as well as carbon dioxide levels in condensate and low pressure feedwater are a direct function of cycle air in-leakage activity. Air in-leakage in excess of that which can be removed by the air removal system design will result in increased oxygen and carbon dioxide levels, which may cause an increase in corrosion-product generation. The monitoring of air in- leakage is considered so important that it has been designated a core parameter. Those
organizations that have active air in-leakage programs can easily maintain the 10 ppb oxygen limit at the condensate pump discharge. Minimization of air in-leakage will also prevent corrosion of the condenser shell. A further discussion on measurement of air in-leakage can be found in Appendix C.
3.4.11 Condensate Storage Tank Effluent
Monitoring the effluent from the storage tanks indicates the quality of the available makeup to the condensate. Also, certain water-quality conditions in aluminum condensate storage tanks can lead to aluminum corrosion, resulting in difficult-to-remove aluminum corrosion-product
deposits in the boiler and turbine. Serious consideration should be given to protecting the condensate storage tank from oxygen ingress; this is especially important for units that are subject to frequent startups. Refer to Appendix A for further discussion of oxygen control.
3.4.12 Makeup Treatment System Effluent
This sample point monitors the performance of the makeup treating system. The achievable makeup water purity is dictated by the design of the makeup water treatment equipment and, may also depend on the makeup rate. Target values have been recommended for the makeup treatment system based on current state-of-the-art equipment capabilities. These guidelines should be used as a performance guarantee for any new makeup treatment systems.
Serious consideration should be given to providing oxygen removal equipment and procedures for the makeup effluent and/or the condensate storage tank. Additionally the condensate storage tank containing deoxygenated water should be provided with protection against air ingress. Air ingress not only increases oxygen levels but also introduces carbon dioxide to the cycle. A full discussion on the subject of oxygen removal is given in Appendix A. Oxygen removal for the makeup effluent is of particular importance for mixed-metallurgy systems where a reducing atmosphere is essential for the control of copper corrosion. Use of deoxygenated water to fill equipment prior to startup is also recognized as good practice.