The Influence of System Design, Station Operations and Cycle Chemistry on Corrosion Product Reprint R-983E Generation and Flow-Accelerated Corrosion (FAC) at Coryton Combined Cycle Gas Turbine (CCGT) Power Station By: Richard Herbert, Science Officer, Coryton Power Station Peter de Graaf, Industry Development Manager, Nalco Europe BV Peter Blokker, Staff Scientist, Nalco Europe BV Charles A. Campbell, Key Account Manager, Nalco Ltd. ABSTRACT INTRODUCTION A systematic study of the influence of system The practical measurement of oxidation-reduction design, station operations and cycle chemistry on potential (ORP) of condensate and feedwater at corrosion product generation and flow-accelerated operating temperature and pressure can provide corrosion (FAC) was carried out at Intergen’s visibility to unfavorable conditions or events. Coryton CCGT in Essex, UK. Coryton is a modern These conditions or events impact plant reliabil- 779 MW two-by-one combined-cycle natural gas ity, thermal efficiency and plant emissions, and plant equipped with an air-cooled condenser (ACC). require both identification and development of The plant was designed and constructed for opera- strategies to prevent damage. It is usual to monitor tional flexibility and functional efficiency in a mer- and control pH, dissolved oxygen, acid conductivity chant power environment, which in practice means and, when used, reducing agent residual, and to a cycling operation with occasional two-shifting. consider these parameters as an indirect measure The study was instigated by the frequent occurrence of the corrosion potential of the water. Corrosion of FAC in combined cycle power plants and the ret- products are released every time there is a ther- rospective nature of non-destructive testing (NDT). mal, chemical, or hydraulic shock to the system. The station wanted to invest in advanced monitor- Intermittent corrosion product monitoring often ing techniques with the objective of reducing corro- leads to questions on the validity and interpreta- sion in the air-cooled condenser and the potential tion of the data produced. Grab sampling only for FAC in the heat recovery steam generator provides a snapshot, but if the full extent of the (HRSG) during normal and cycling operations, and events could be made visible, responses can be to determine the impact of cycling operation on unit developed to minimise corrosion and iron trans- life expectancy and reliability. Of special interest port. The results from particle monitoring can be was the evaluation of the use of all-volatile treat- related to particulate iron levels with sensitivity ment oxidising (AVT-O) versus all-volatile treat- in the ppb range. Moreover, the results from ment reducing (AVT-R) chemistry control in the particle monitoring can be correlated to the ORP all-steel system as a means to reduce FAC in the at operating temperature and pressure. HRSG and iron pick-up in the air-cooled condenser. On-line “at-temperature” Oxidation-Reduction Potential and Particle Monitor instruments were FLOW-ACCELERATED CORROSION applied to supplement the standard array of chem- Consensus has formed about the causes of Flow- istry and operational monitoring. The study should Accelerated Corrosion (FAC): these include mate- allow Coryton Power to identify the best cycle chem- rials of construction and geometry, low pH, low istry control specifications to minimise FAC and oxygen concentration, rapid or turbulent flow, and ACC corrosion, leading to more reliable, efficient temperatures of ca. 150°C ± 50°C. The economizer and lower cost generation for the life of the plant. outlet (typically single-phase) and the LP evapo- rator section (two-phase) are particularly susceptible to FAC, especially the upper bends and the upper header of the evaporator tubes. The metal Presented at PowerGen Europe, Netherlands, 8-11 June, 2010 wastage mechanism is accelerated by flow hydro- temperature oxidation-reduction potential (RT dynamics, but the basic process is the dissolution ORP), provides a much better technical solution. of magnetite, which is directly related to the oxi- AT ORP is a more sensitive and realistic indicator dation-reduction potential at system temperature of the actual condition at the system’s metal sur- and pressure. If the rate of dissolution of magne- faces. Research by Nalco on AT ORP measurement tite exceeds magnetite formation, then the metal and control resulted in the development of a reli- surface becomes less protective resulting in tube able, sensitive, high-temperature probe and control thinning, which impacts station reliability and strategy that allow power stations to operate at a integrity. constant AT ORP value that facilitates the forma- 5,6 The Electric Power Research Institute (EPRI) sug- tion of protective metal surfaces . gests that iron measurements can be used as an The ideal monitoring programme would correlate indicator for FAC. As the dissolution of magnetite the interplay of the oxidising environment with results in the formation of iron hydroxide, the spe- all of the relevant variables contributing to sys- cies of iron present can also be relevant. Particles tem corrosion and FAC across the AT ORP “space” of iron oxide may be dislodged when the passive and relate that to corrosion product generation and film is disrupted by magnetite dissolution, contrib- transport (Figure 1). The diagnostic and preven- uting to the particulate levels and iron balance. tative value of this approach will be discussed in a EPRI suggests that high iron levels, under severe case history format that describes the initial out- FAC conditions, always consist of over 95% come of a study which is currently in progress. This particulate iron1. All of the above considerations study was prompted by the frequent occurrence of contribute to make the measurement of the oxida- FAC in combined cycle power plants, the retrospec- tion-reduction potential at operating temperature tive nature of non-destructive testing (NDT), the and pressure and particle monitoring a vital part need for advanced diagnostic information, and the of a FAC monitoring programme. evaluation of cycle chemistry improvements. Specific mechanical and operational factors can- not be changed in most existing systems. The ques- tion of using reducing agents for metal surface Removal passivation complicates matters. Long considered Particle ofsurface monitoring a best practice, institutes like EPRI now recom- Iron mend against the use of reducing agents in all- steel systems, citing evidence that they increase M l s a e s u e c y t r ta et a t a f FAC by increasing the dissolution rate of magne- f li b a l M r i i c u b li e s a t t y tite in the more reducing environment. However, s Ions when station operations, such as cycling or Temperature Plant design Flow two-shifting, results in high variability in the Metallurgy Plantoperation Water pH at-temperature ORP environment, then the com- Constructionmaterials quality DO position and properties of the passivation layer will Watertreatment … ATͲORP Reductant also be in constant flux and under stress. Variable … at-temperature ORP values lead to variable oxides with variable density and protective properties. Additionally, there are many systems where Figure 1 — Holistic representation of how mechanical oxygen removal in the condenser is so design, operating conditions and cycle chemistry influence the metal surface stability. effective that the feedwater system runs in a reduced state even with no reducing agent present. The opposite can also be observed in stations that experience highly variable oxygen levels, in excess of 10 ppb, during cycling and two-shifting. Although EPRI has recommended against the use CASE STUDY – CORYTON POWER of a reducing agent in all-steel systems, they sug- STATION gest using ORP measured in-situ as a means of Coryton Power is a modern 779 MW two-by-one 2 controlling feedwater chemistry . For all- steel sys- combined-cycle natural gas power station equipped tems they suggest a room temperature ORP value with an air-cooled condenser (ACC) and a pre-coat between 0 and +100 mV (Ag/AgCl , saturated KCl 2 filter for particulate matter removal from the reference electrode). Research by Nalco has dem- steam condensate. The plant was designed and onstrated that oxidation-reduction potential constructed for operational flexibility and func- should be measured at operating temperatures and tional efficiency in a merchant power environment, ® 3 pressures (AT ORP ) . Using the Nalco AT ORP which in practice means a cycling operation with 4 technologies for high-temperature and pressure occasional two-shifting. evaluation of corrosion stress, as compared to room 2 Picture 1 – Coryton Energy Company Ltd (Source: Google Maps). Figure 2 – Correlation between particle index and total iron with 99% confidence interval for the Coryton station. For the Coryton Power Station case study, we will examine the impact of station design, operation and chemistry on AT ORP and corrosion product on grab samples. These data can be augmented generation from the air-cooled condenser and the with online AT ORP measurements, particle moni- low pressure (LP) section of the heat recovery toring, online pH and online dissolved oxygen mea- steam generator. The LP section is part of the surements. The latter two are located after the feedwater system and is currently operated in an condensate pump discharge. AVT-R cycle chemistry environment (pH 9.4-9.6, Oxygen ingress at the seals of the condensate pump 10-30 ppb reductant). The station operates at dis- was discovered during the trial, which hindered solved oxygen levels in the range of 30-70 ppb, the establishment of a direct correlation between peaking to levels above 100 ppb during low load the on-line pH and oxygen measurements and cor- conditions when cycling. For corrosion control in rosion of the air-cooled condenser (Figure 3). the air-cooled condenser, the water chemistry parameters, most noticeably dissolved oxygen and Daily variability of ca.