RATIONALE FOR SAMPLE POINTS

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 for drum and once-through units in Sections 4 and 5 respectively: 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 3-1 for a listing of core instruments for units operating on AVT.

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 AVT 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.

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. 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.

Using the approach in Section 3.3, contaminant control curves were developed for drum units. 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. These are presented in Section 4.

Table 3-1

EPRI’s Core Monitoring Parameters and/or Minimum Level of Continuous Instruments for All Units Operating on AVT

Oxidizing AVT (AVT(O))

Drum or once – through units without a reducing agent and with an all-ferrous feedwater system. Cation conductivity CPD, CPO or EI, RH (or MS), Downcomer (or blowdown)

Specific conductivity Makeup

pH (drum units) Downcomer (or blowdown) Dissolved oxygen CPD, EI

Sodium CPD, CPO or EI, RH (or MS) Air in-leakage

Carryover (drum units)

Reducing AVT (AVT(R))

Drum or once – through 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 (or downcomer) Specific conductivity Makeup

pH (drum units) Blowdown (or downcomer) Dissolved oxygen CPD, EI

Sodium CPD, CPO or EI, RH (or MS)

ORP DAI

Air in-leakage

Reheat Steam/Superheated Steam

Factors affecting steam chemistry, other than the mechanical and vaporous carryover from the boiler, include the following:

• contamination by attemperation water of the superheater and reheater, and

• precipitation of impurities as deposits in the superheater, reheater, and in the turbine (mostly in the low pressure 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 impurity ingress with the superheater and reheater attemperating water (feedwater).

Steam sampling requires special techniques, which are described in an EPRI Report(21) and in Appendix E.

Saturated Steam (Drum Boilers Only)

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 diagnostic tool to monitor the total carryover of impurities into the high pressure turbine.

The saturated steam chemistry data can also be related to the performance of the steam drum moisture separator by measuring the carryover from the boiler water of impurities into the steam. Excessive carryover may indicate poor moisture separator performance. Mechanical carryover has the greatest impact upon steam chemistry at boiler drum operating pressures below 2500 psi, where mechanical carryover is the major component of the total carryover. For SiO2 and copper, however, the volatile component is the most important. As indicated, previously, a knowledge of carryover is considered so important, that it is now a core parameter (Table 3-1 and Figures 4-1 and 4-2) which should be routinely checked (every six months).

Boiler Water (Drum Boilers Only)

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 steam purity. Blowdown or downcomer boiler drum samples 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 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.

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 and 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 also monitors oxygen and pre-boiler corrosion, 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.

Deaerator Inlet

Much of the reduction of 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 atmosphere (AVT(R) or AVT(O)). As explained, 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 hydrazine, in addition to minimizing

oxygen ingress. 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.

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) results in superior results relative to iron transport while, at the same time, minimizing chemical feed costs.

This sample point also monitors the deaerator performance by comparison with the sample at the deaerator outlet.

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 level is excessive usually during startup and periods of high makeup with air-saturated water, or when the air in-leakage is high.

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.

Condensate Pump Discharge

This sample point monitors the following:

• magnitude of contaminants introduced by condenser leakage,

• magnitude of contaminants introduced by the makeup treatment system, • amount of oxygen entering the feedwater train, and

• corrosion products from heater drains returned to the condenser. • carryover of contaminants and treatment chemicals in steam,

For plants without condensate polishers, this point monitors the chemistry at the start of the feedwater. The major change in these new AVT Guidelines is a tightening of the dissolved oxygen normal target to 10 ppb. This is important for both mixed-metallurgy and all-ferrous feedwater systems.

Condenser Leak Detection Trays and/or Hotwell Zones (if Applicable)

This sample point 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 corrective action. 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 condenser hotwell section may also be monitored.

Makeup Treatment System Effluent

This sample point monitors the performance of the makeup treating system. The makeup water purity is dictated by the design of the makeup water treatment equipment and also depends 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.

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. Refer to Appendix A for further discussion of oxygen control.

Air Removal System Exhaust

Condensate oxygen as well as carbon dioxide levels are a direct function of air in-leakage. Air in-leakage in excess of that 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. Minimization of air in-leakage will also prevent corrosion of the condenser shell. The

monitoring of air in-leakage is considered so important that it has been designated a core parameter. A further discussion on measurement of air in-leakage can be found in Appendix C.