The cycle chemistry treatments and control on fossil and combined cycle plants influence a high percentage of the availability, reliability and safety issues experienced on these plants worldwide. This article – the first in our series — describes chemistry treatments that can be used to help keep corrosion at bay in these plants.
This first article introduces the equipment and materials of construction and how reliability depends on various protective oxides, the formation of which relates directly to the cycle chemistry treatments that are used in the condensate, feedwater, boiler/evaporator water, and steam.
Fossil and Combined Cycle /HRSG Plants
Fossil and combined cycle/HRSG plants operate across a wide range of temperatures and pressures. Both once-through and drum boilers coupled to high pressure (HP), intermediate pressure (IP) and low pressure (LP) steam turbines are employed in traditional fossil-fired plants. Multi-pressure drum-type heat recovery steam generators (HRSGs) are normally used in combined cycle plants, but there are also a number of HRSGs with once-through HP or HP/IP circuits.
Mainly mild and low alloy steels are used in the construction of boilers and feedwater heaters, although copper alloys are also used for some condensers and feedwater heaters. High alloy steels and austenitic stainless materials are used in superheaters, reheaters and steam turbines. The protective and passive oxides that grow on the surfaces of this equipment and materials provide protection from corrosion.
The feedwater system is the major source of corrosion products, which can be transported into the fossil boiler or HRSG evaporator and deposited on the heat transfer surfaces of the water/steam cycle. Impurities in condensate, feedwater and cooling water increase corrosion, and corrosion products can also be generated by flow-accelerated corrosion (FAC).
Corrosion of copper alloys, if present in the feedwater heaters of fossil plants, can lead to the transport of copper into the boiler and deposits on the waterwalls, evaporators and the high pressure turbine. Some early combined cycle/HRSG plants also had feedwater heaters fed by extraction steam. The build-up of deposits in the steam generating tubes of the boiler or HP evaporators in HRSGs, in combination with the presence of impurities, can lead to under-deposit corrosion (UDC) during operation, and pitting in those sites during non-protected shutdowns.
The carryover of impurities into the steam can lead to deposits in the steam turbine, stress corrosion cracking in the superheaters and steam turbines, and pitting (particularly in reheaters) during non-protected or inadequate shutdown conditions.
Leaks in water-cooled condensers are a common source of impurities, such as chloride and sulfate, entering the water/steam circuit, whereas air-cooled condensers are subject to low temperature flow-accelerated corrosion and can be a source of high levels of corrosion products and air ingress.
One of the main purposes of good cycle chemistry is to provide protection through oxide formation on the internal steam/water touched surfaces and to prevent or reduce corrosion and deposits in the steam/water circuit of these power plants. A combination of chemical techniques is required to achieve this, and chemical conditioning can be applied to the condensate, feedwater and boiler water. Guidance limits have to be developed to control the corrosion processes. Failure to use optimal cycle chemistry and control will lead to major availability and reliability problems and can result in safety issues for plant staff.
Optimum Cycle Chemistry Treatments
Optimum cycle chemistry requires owners to consider all the cycles of fossil and combined cycle plants. Most often, the cause of cycle chemistry influenced failure and damage mechanisms in a particular section or circuit does not originate at that location. For instance, feedwater corrosion products can be transported and deposited into the boiler/evaporator. Also, contaminants in the boiler/evaporator originating in the condensate can be carried over into the steam turbine.
A quick “tour” of the chemistry for fossil and combined cycle plants follows. This overview provides an introduction to key features required for the cycle chemistry control of power plants. The first requirement is for high purity feedwater recycled from the condenser or added as makeup. The purity is monitored by measuring the conductivity after cation exchange (CACE) (previously known as cation conductivity) of the condensate, feedwater, boiler and evaporator water, and steam. These measurements include contributions from impurities and corrosive species such as chloride, sulfate, carbon dioxide, and organic anions. The higher the temperature and pressure of operation, the higher the purity of water required to prevent corrosion and, thus, the lower the CACE allowed.
The chemistry of the condensate and feedwater is critical to the overall reliability of fossil and HRSG plants. Corrosion takes place in fossil plant feedwater systems (heaters, drains, and interconnecting pipework) and in the feedwater of HRSG plants (preheaters and economizers) and the resulting corrosion products flow into the boiler or HRSG evaporators where they deposit on heat transfer areas. In the boiler/HRSG evaporator, these deposits can act as initiating centers for many tube failure mechanisms and as a source of efficiency losses or blade/disk failures in the steam turbine. The choice of feedwater chemistry depends primarily on the materials of construction and secondly on the feasibility of maintaining purity around the water/steam cycle.
Most often, a volatile alkalizing agent, usual ammonia, is added to the condensate/feedwater to increase the pH. Alternatively, a neutralizing amine or film-forming product (FFP) can be added in lieu of ammonia. FFP include film forming amines (FFA) and film forming compounds which don’t contain an amine.
Condensate and Feedwater Cycle Chemistry Treatments
Three main variations of volatile conditioning can be applied to the condensate and feedwater:
a. AVT(R) – All-volatile Treatment (Reducing)
This treatment involves adding ammonia or an amine, FFP, blend of amines of lower volatility than ammonia and a reducing agent (usually hydrazine or one of the acceptable substitutes such as carbohydrazide) to the condensate or feedwater of the plant. In combination with a relatively low oxygen level (from air in-leakage) of about 10 ppb (μg/kg) or less in the condensate (usually measured at the condensate pump discharge, CPD), the resulting feedwater will have a reducing redox potential (usually measured as Oxidation-Reduction Potential, ORP). Higher levels of oxygen (>20 ppb (μg/kg), (due to high air in-leakage) will usually preclude generation of the reducing environment, but are often incorrectly accompanied by excessive dosing of the reducing agent. AVT(R) provides protection to copper-based alloys in mixed-metallurgy feedwater systems in fossil plants. Under optimum conditions, a fossil plant should be able to operate with feedwater corrosion products which are Fe < 2 ppb (μg/kg) and Cu < 2 ppb (μg/kg). Here the Fe and Cu values refer to the total concentrations of particulate metal oxides plus dissolved metal ions. In multi-pressure HRSG systems, AVT(R) should not be used in these cycles due to concerns for single-phase FAC and because the corrosion product levels in the feedwater would be most likely to exceed 2 ppb (μg/kg). Thus, a key basic international rule is that reducing agents should not be used in combined cycle / HRSG plants.
b. AVT(O) – All-volatile Treatment (Oxidizing)
This all-volatile treatment has emerged as the treatment of choice for multi-pressure combined cycle/HRSG plants with no copper alloys in the feedwater. In these cases, a reducing agent should not be used during any operating or shutdown/layup period. Ammonia or an amine, FFP, blend of amines of lower volatility than ammonia is added at the CPD or condensate polisher outlet (CPO) (if a polisher is included within the cycle). In combined cycle/HRSG plants with relatively good control of air in-leakage (oxygen levels in the range 10-20 ppb (μg/kg)), the resulting feedwater will yield a mildly oxidizing ORP. Under optimum conditions, a fossil plant should be able to operate with corrosion product levels of total Fe < 2 ppb (μg/kg) in the feedwater; for multiple pressure combined cycle plants, the total Fe should be < 2 ppb (μg/kg) in the feedwater and < 5 ppb (μg/kg) in the drums.
c. OT – Oxygenated Treatment
For conventional fossil plants, optimized OT involves one oxygen injection location at the CPO, operating with the vents on the feedwater heaters and deaerator closed, and with knowledge of the total iron levels at the economizer inlet and in the feedwater heater cascading drain lines. Ammonia is added at the condensate polisher outlet. Often, a minimum level of oxygen is required to provide full passivation of the single-phase flow locations in the main feedwater line and the drain lines, and to maintain this protection. For drum units, this is usually between 30 and 50 ppb (μg/kg) at the economizer inlet (with the actual level being set in accord with the boiler recirculation ratio), and for once-through/supercritical units this is usually 30-150 ppb (μg/kg) at the economizer inlet. Application of OT in combined cycle/HRSG plants is much rarer; in these plants, it is often found that the use of AVT(O) with low levels of oxygen (< 10 ppb (μg/kg), does not provide sufficient oxidizing power to passivate the very large internal surface areas associated with preheaters, LP, IP and HP economizers, and LP evaporators, especially if a deaerator is included in the LP circuit. In these cases, oxygen can be added at the same level as for conventional recirculating cycles. This is the treatment of choice for fossil units with all-ferrous feedwater heaters, a condensate polisher, and the ability to maintain a CACE of < 0.15 μS/cm under all operating conditions. Under optimum conditions, a fossil plant should be able to operate with corrosion product levels of total Fe < 1 ppb (μg/kg) in the feedwater; for multiple pressure combined cycle plants the total Fe should be < 1 ppb (μg/kg) in the feedwater and < 5 ppb (μg/kg) in the drums.
FFP – Film Forming Products
The application and use of FFP in fossil and combined cycle/HRSG plants is increasing worldwide. Unlike conventional treatments, FFP are adsorbed onto metal oxide/deposit surfaces, providing a physical barrier (hydrophobic film) between the water/steam and the surface. Three main chemical substances have been used historically: Octadecylamine (ODA), Oleylamine (OLA) and Olyeylpropylendiamine (OLDA). Along with these compounds, the commercial products also contain other substances such as: alkalizing amines, emulsifiers, reducing agents, and dispersants. There is currently much confusion about their application for both normal operation and shutdown/layup, and there is no international guidance on deciding whether to use an FFP and whether it will provide a benefit to the plant.
Some basic international rules are in place for the application of these condensate/feedwater treatments. The all-volatile treatments — AVT(R), AVT(O), or OT — must be used for once-through boilers without any further addition of chemicals in the boiler or HRSG evaporators. AVT(R), AVT(O) or OT can also be used for drum boilers of fossil plants or combined cycle/HRSGs without any further addition of chemicals to the boiler/HRSG drum. However, impurities can accumulate in the boiler water of drum-type boilers and it is necessary to impose restrictive limits on these contaminants as a function of drum pressure, both to protect the boiler from corrosion and to limit the number of impurities possibly carried over into the steam, which could put the superheaters, reheaters, and steam turbines at risk. AVT has essentially no capability to neutralize or buffer feedwater/boiler water dissolved solids contamination. Ammonia is a rather poor alkalizing agent at high temperatures, offering very limited protection against corrosive impurities.
Fossil Boiler Water and HRSG Evaporator Cycle Chemistry Treatments
For some drum-type boilers, the addition of solid alkalizing agents to the boiler/HRSG water may be necessary in order to improve the tolerance to impurities and reduce the risk of corrosion. The alkalizing agents which can be used for this are tri-sodium phosphate [phosphate treatment (PT)] or sodium hydroxide [caustic treatment (CT)] used alone, or in combination. The amounts of sodium hydroxide added have to be strictly limited to avoid excessively alkaline conditions, which can result in a UDC mechanism (caustic gouging), which destroys the protective oxide layer in the boiler or HRSG evaporator. The amounts of both sodium hydroxide and tri-sodium phosphate added to the cycle also have to be controlled to avoid an increase of carryover of these conditioning chemicals into the steam, possibly putting the superheaters and turbines at risk.
Boiler and HRSG evaporator treatments are critical to the overall reliability of fossil and HRSG plants as they control and influence not only the major tube failure mechanisms but also a number of damage mechanisms in the steam turbine.
PT – Phosphate Treatment Phosphates of various types have been the bases of the most common boiler/HRSG evaporator treatments worldwide. However, historically a multitude of phosphate compounds and mixtures blended with other treatment philosophies have resulted in a wide range of control limits for the key parameters (pH, phosphate level, and sodium-to-phosphate molar ratio) and a number of reliability issues. Some of the traditional phosphate treatments such as congruent phosphate treatment (CPT), coordinated phosphate treatment, and equilibrium phosphate treatment (EPT) have been used over the last 50 years across the fleet of fossil boilers and HRSG evaporators, sometimes successfully, sometimes resulting in tube failures and other problems. For instance, the use of CPT, where mono- and/or di-sodium phosphate are used to develop operating control ranges below sodium-to-phosphate molar ratios of 2.6:1, has resulted in serious acid phosphate corrosion (APC) in many boiler waterwalls and HRSG HP evaporators which have heavy deposits and have experienced phosphate hideout.
More recently, 20 years of collective global operating experiences have shown that tri-sodium phosphate (TSP) should be the only phosphate chemical added to a boiler/HRSG and that the operating range should be bounded by sodium-to-phosphate molar ratios of 3:1 and TSP + 1 ppm (mg/kg) NaOH with a pH above 9.0 and a minimum phosphate limit above 0.3 ppm (mg/kg). The 0.3 ppm (mg/kg) level is considered a minimum; better protection is afforded by operating at the maximum level of phosphate possible without experiencing hideout or exceeding the steam sodium limits.
PT can be used in a wide range of drum units, up to high pressures (19 MPa, 2800 psi), so it is often the only alkali treatment available because CT is not suggested to be used above 16.5 MPa (2400 psi). However, hideout and hideout return become more prevalent with increasing pressure. Hideout and hideout return are always associated with large swings of pH causing control problems, but if only TSP is used, then no harmful corrosion reactions can be initiated as was experienced with CPT using sodium-to-phosphate molar ratios below 2.6:1.
For multi-pressure HRSGs, PT can also be used in each of the pressure cycles; however, use of PT in these cases is for different reasons depending on the pressure of the circuit. At high pressure (HP drums >10.3 MPa (1500 psi)), TSP is added to address contamination as it is for conventional fossil plants. In the lower pressure circuits, with temperatures below 250 °C, PT is used to help control two-phase FAC much as CT is used in these circuits. Of course neither solid alkali is used in the LP evaporator in units where the LP drum feeds the IP and HP feedpumps and attemperation.
CT – Caustic Treatment Caustic treatment (CT) can be used in “conventional” fossil and HRSG drum-type boilers to reduce the risk of under-deposit corrosion and in HRSGs for controlling flow-accelerated corrosion, where all-volatile treatment has proved ineffective, or where PT has been unsatisfactory due to hideout or has experienced difficulties of monitoring and control.
The addition of sodium hydroxide to the boiler/evaporator water has to be carefully controlled to reduce the risk of caustic gouging in the boiler and carryover into the steam, which could lead to damage of steam circuits and turbine due to stress corrosion cracking. Of primary risk are austenitic materials, stellite, and all steels with residual stresses (e.g., welds without heat treatment) in superheaters, steam piping and headers, turbine control and check valves, as well as components in the steam turbine. CT is easy to monitor, and the absence of the complications due to the presence of phosphate allows on-line conductivity and CACE measurements to be used for control purposes.
This high-level overview of optimum cycle chemistry treatments for fossil and combined cycle/HRSG plants is the starting point of our discussion. Future issues of News & Views will describe cycle chemistry failure/damage mechanisms and how they are typically dealt with retroactively. We will also explore how SI’s advanced analytical tools can help plant owners identify the risk and root cause of cycle chemistry-related damage and failure.
There are a plethora of international guidelines and guidance available in many countries of the world for the reader: IAPWS (International), EPRI (US), VGB (Germany), JIS (Japan), Russian, Chinese, Manufacturers of major fossil and combined cycle/HRSG equipment (International), Chemical Supply Companies (International). Structural Integrity Associates uses the Technical Guidance Documents (TGD) of the International Association for the Properties of Water and Steam (IAPWS) in all the cycle chemistry related plant assessments and root cause analyses conducted. These are freely downloadable on the IAPWS website (www.IAPWS.org).