All-ferrous Feedwater Systems Optimization

All of the three feedwater treatments mentioned above in Section 2.2 are possible for use in all- ferrous feedwater systems. The basis of either of the AVT treatments is an elevated pH in all cycle streams. The common alkalizing agent is ammonia. Originally, and up to the late 1980s, the ammonia dosing was always combined with a reducing agent feed, such as hydrazine. This treatment is now termed AVT(R), which indicates that the oxygen level at the condensate pump discharge (CPD) is low enough (< 10 ppb) (minimum air in-leakage) that a reducing agent can be added to the cycle to produce a reducing environment with ORP < 0 mV. Figure 2-2 illustrates the oxide formation (magnetite, Fe3O4) which will be formed on all the ferrous surfaces throughout the feedwater system. The dissolution of Fe3O4 into the feedwater flow is dependent on the ORP. The more reducing is the feedwater the greater is the dissolution and thus the higher is the amount of iron corrosion products measured at the economizer inlet. Flow- accelerated corrosion (FAC) occurs by exactly the same mechanism, which is accelerated at locations where the flow hydrodynamics are elevated.(5-7) Under reducing conditions that produce FAC or normal corrosion, organizations are not able to meet the guideline requirements of less than 2 ppb iron in the final feedwater at the economizer inlet.

Figure 2-2

Schematic Representation of Oxide Formed on Ferrous Feedwater Surfaces During Operation with Reducing AVT

Investigations performed since the late 1980s, and hundreds of unit operating experiences have indicated that eliminating the reducing agent feed minimizes the corrosion product generation.(8) This treatment is now termed AVT(O). It also requires the air in-leakage be minimized to produce oxygen levels at the CPD of less than 10 ppb. Figure 2-3 shows a very typical example in a 600 MW drum unit with an all-ferrous feedwater system. As the reducing agent (hydrazine) was reduced over a period of 90 days, the ORP increased from about –350mV to around

+100mV in the oxidizing range, and the iron levels reduced markedly. There was no change in the feedwater oxygen level.

Figure 2-3

Change in Oxidizing Reducing Potential (ORP) and Feedwater Iron Levels (Fe) at the Economizer Inlet when Hydrazine (N₂H₄) is Gradually Reduced on a 600MW Drum Unit with an All-Ferrous Feedwater System(8)

The feedwater key parameters for AVT(R) and AVT(O) are summarized in Table 2-3.

Table 2-3

Feedwater Limits for All-Ferrous Systems

Parameter AVT(O) AVT(R) OT

pH 9.2–9.6 9.2–9.6 D 9–9.6 O 8–8.5 Cation Conductivity (µS/cm) < 0.2 < 0.2 < 0.15 Fe (ppb) < 2 (<1) < 2 < 2 (0.5) Cu (ppb) < 2 <2 Oxygen (ppb) at EI < 10 < 5 (< 2) D 30–50 O 30–150 Oxygen (ppb) at CPD < 10 < 10 < 10

Reducing Agent No Yes No

ORP (mV) at DAI Not needed –300 to –350 Not needed

Notes: EI - economizer inlet, CPD - condensate pump discharge, DAI - deaerator inlet, D - drum unit, O - Once-through unit

While an elevated pH is the basis of the two AVT treatments, oxygenated treatment (OT) uses oxygenated high purity feedwater to minimize the corrosion and FAC in the feedwater systems. The addition of oxygen is the preferred choice. The feedwater key parameters for OT are shown in Table 2-3.

For the application of oxygenated treatment in units with once-through and drum boilers, there is one indispensable prerequisite that the cation conductivity must be less than 0.15 µS/cm (at 25°C).

The basis for the success of OT depends on the formation of a layer of ferric oxide hydrate (FeOOH) on the surface of the magnetite oxide layer and within the pores. This is illustrated in Figure 2-4. In this case the Fe(OOH) has a much lower solubility than Fe₃O₄ in feedwater, so the dissolution of surface oxide layers and the measurement of iron corrosion products will be minimal. FAC should also be eliminated on feedwater surfaces that have the red FeOOH appearance.

It should be also noted that the use of AVT(O) also produces red surface layers of FeOOH throughout the feedwater systems. However, the lower levels of oxygen, maintained by good air in-leakage control, are usually not high enough to passivate 100% of the feedwater (particularly the deaerator) surfaces. Thus the lowest levels of iron transport cannot be expected; typically iron levels for optimized AVT(O) feedwater treatment will be around 1 ppb.

Figure 2-4

Schematic Representation of Oxide Formed on Iron-Based Feedwater Surfaces During Operation with Oxidizing AVT and OT

Optimization of All-Ferrous Feedwater Chemistry

Figure 2-5 shows a road map for optimizing the feedwater treatment in all-ferrous systems. The primary purposes of this important activity are to minimize corrosion product transport,

eliminate any possibility for FAC, and thus to reduce the accumulation of corrosion product deposition on the boiler water walls. The methodology described here is equally applicable for both drum and once-through units with all-ferrous feedwater systems.

Step 1—Review Normal or Current Feedwater Treatment

This step involves a review of the current feedwater treatment, which is probably AVT(R) with a reducing agent. If there are no problems such as indicated in Table 2-2 then continue to use the current treatment. Such a review would indicate that the operating experience has been good, that minimal chemical control problems have been experienced, that no BTF in the waterwalls relating to waterside problems have occurred in the last five years, that no turbine deposition or blade failure problems have occurred, and that the feedwater is operating in the optimum fashion with minimum levels of feedwater corrosion products (less than 2 ppb at the economizer inlet). In such cases of good experience, no changes need to be made. However, it is suggested that the road map is reviewed as there may be considerable economic savings to be gained by converting to AVT(O) or OT, and it should be remembered that FAC is always possible with reducing feedwater chemistry.

Figure 2-5

Step 2—Monitoring Baseline on Current Feedwater Treatment

This step involves a complete base-line monitoring to quantify the current chemical parameters and, in Step 3, to determine whether continued use of a reducing agent and AVT(R) or a change to AVT(O) should be contemplated.

This program would utilize the installed chemistry monitoring system, supplemented by monitoring those parameters in the cycle chemistry diagrams under “troubleshooting and commissioning.”

The monitoring program should pay particular attention to the adequacy of the makeup and chemical feed systems, condenser tightness, air in-leakage, and corrosion product transport. This monitoring involves taking a “thumb-print” of the unit under “typical operating conditions” to identify under controlled conditions exactly how the unit chemistry is behaving. It may involve a review of the operating chemistry logs, but this often is not satisfactory and it is

preferable to undertake a monitoring campaign. Before this campaign is initiated, it is important to review the utility’s chemistry monitoring capability and reliability. This should include Quality Assurance (Q/A) and Quality Control (Q/C) of existing and normally utilized analytical chemistry monitoring and analysis methodology and equipment (see Appendix E).

The monitoring campaign should include:

• Varying Operating Conditions—base load, startup and shutdown

• Steam Chemistry—cation conductivity, sodium, chloride, silica and sulfate

• Feedwater Chemistry—cation conductivity, chloride, corrosion products (Fe, Cu), oxygen, pH, oxidizing-reducing potential (ORP)

• Operation of Condensate Polishers (if included)

If this step indicates a low level of feedwater corrosion product transport (such as Fe <2 ppb) and acceptable feedwater purity from a dissolved solids standpoint with the control chemistry

meeting the guideline values provided in Sections 4 and 5, then an organization might consider to continue with the current AVT(R) feedwater chemistry. Further confirmation of optimum feedwater chemistry will be that waterwall deposition rates have historically been much less than 1 mg/cm2/1000 hours (1 g/ft2/1000 hours), that the interval between chemical cleans has

historically been better than every 10 years, and that there has been no indication of FAC in the feedwater, feedwater steamside, deaerator, or feedwater heater drain lines.

Step 3—Evaluate Reducing Agent Requirements

This step is a subset of Step 2 and should involve a series of tests to minimize the generation and transport of feedwater corrosion products.

Many utilities with all-ferrous systems have found that, with proper air in-leakage control

(dissolved oxygen at the CPD of less than 10 ppb), the reducing agent (such as hydrazine) can be eliminated without jeopardizing chemistry control on the unit(8) and moving to AVT(O). Thus in

Step 3, a series of tests could be performed to evaluate the need for a reducing agent and, if needed, to determine the proper reducing agent level. The tests should utilize the monitoring system instrumentation (used in Step 2) while reducing or eliminating the reducing agent dosage. Particular note should be made of dissolved oxygen levels and the level of corrosion products generated during each test. Reference can be made to Figure 2-3, which illustrates the results from a typical test. Table 2-3 should be referenced for typical parameter ranges. Particular attention should be paid to the fact that there can be a long time between changes in the

feedwater chemistry and corrosion product transport, so careful planning is needed for accurate extended tests. For instance, two months after the test outlined in Figure 2-3, the iron levels were below 2 ppb and generally approaching 1 ppb.

Step 4—Monitoring with New Feedwater Treatment

Step 4 involves a period of normal operation with the new feedwater treatment, which

occasionally requires repetition of monitoring in Step 2 to confirm that running with reduced or zero reducing agent provides the optimum feedwater treatment. This might involve a reduced monitoring effort which just looks at feedwater oxygen and corrosion products at the economizer inlet, in parallel with the cycle “core” parameters (Table 3-1), which should be normally

continuously recorded and alarmed.

In units with all-ferrous feedwater systems, but with copper based condenser tubing, there is a possibility of copper corrosion particularly in the air removal section of the condenser. If increased copper transport is measured at the CPD, then considerations should be given to a reduction in cycle pH from 9.2–9.6 to 9.0–9.3 as needed to reduce copper levels to those present during baseline monitoring.

Steps 5 and 6—Consider Converting to OT

Once the baseline monitoring (Step 2) and a period of normal operation has been undertaken then the question can be raised about whether the unit could run on oxygenated treatment (Step 5). The reader is referenced to the OT Guideline,(9) which provides step-by-step guidance on whether a unit is suitable for OT and how to convert units to OT (Step 6) in a sequential fashion. The transition check list in the OT Guidelines(9) should be referenced. It should be noted that conversion to OT will take a number of weeks (more for drum units) and must include the heater drains.

Step 7—Continue to Optimize the Feedwater Treatment

This step continues the efforts described in Steps 3 and 4.

Step 8—Operation and Continuing Monitoring

There are now three treatments that can be used for the feedwater: • AVT(R),

• AVT(O), and • OT

The feedwater treatment used on each unit should be continually checked to ensure it is always the optimum treatment. As well as the core parameters at the key cycle points, it is now necessary to ensure that the operation of the deaerator vents and heater vents are operated in a manner which provides minimum levels of corrosion products in all parts of the cycle.