By Andre de Bache, Kurita Europe; Bill Smith, Uniper UK Ltd.; and Paul McCann, Uniper UK Ltd.

Film Forming Products (FFP) based on Film Forming Substances (FFS) or Film Forming Amines (FFA) are increasingly being used as an alternative to conventional treatment programs for water/steam cycle protection against corrosion in industrial steam generators, as well as in fossil, combined cycle and biomass power plants.

If applied correctly, FFPs can provide the following advantages if plant design or operation means that effective protection is not achieved with optimized traditional treatment programs:

The number of units treated with FFP has significantly increased over recent years. The International Association for the Properties of Water and Steam (IAPWS) has released two Technical Guidance Documents (TGD) dealing with the Application of Film Forming Substances (FFS) in Industrial Steam Generators (IAPWS TGD11-19), and in Fossil, Combined Cycle and Biomass Power Plants (IAPWS TGD8-16, 2019). In these TGDs three substances, among them, Oleyl Propylenediamine (OLDA) are listed. These have been the subject of intensive research and where significant application experience is available [1], [2].

Kurita’s Cetamine® technology, based on OLDA, provides an excellent treatment option for water/steam cycles, especially for plants operating in cycling mode where preservation is required during shutdowns, but unit availability must be maintained [3]. Successful applications of Film Forming Products have been reported for plants which are both continuously operated, [4 – 8] and under wet or dry-lay-up [9, 10].

The Film Forming Amine (FFA) molecule (OLDA) adsorbs onto metal/metal oxide surfaces to form a hydrophobic film or barrier, which prevents corrosion by stopping water and other corrosive agents from contacting the metal/metal oxide surface. Furthermore, the thin film fosters the formation of a smooth and compact iron oxide layer [11].

Once formed, the protective film remains intact in both wet and dry conditions. This offers significant potential benefits for plants under a cycling mode of operation, and for the preservation of both drained and (partially) filled plants during a shutdown. In particular, it is especially important to demonstrate the presence of a protective Cetamine®-film on system surfaces to assure long-term protection during shutdown conditions. To demonstrate this, Kurita has developed the patented Cetamine® Wipe Test to be applied on accessible system surfaces during routine plant inspections.

In this article, the successful application of Cetamine® technology in a combined cycle power plant under cycling mode conditions is described.

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Connah’s Quay Power Station consists of four 345 MW single-shaft combined cycle units. The heat recovery steam generators (HRSGs) are vertical gas path drum boilers, with three pressure stages (6, 36 and 120 bar) and reheat (Figures 1 and 2). The final superheated steam temperature is 540 °C [3].

The station operating regime varies, with between one and four units running with daily start-ups and shutdowns, and longer standby periods where unit availability must be maintained. The implementation of conventional preservation methods was difficult without compromising start-up times and it was not possible to protect all plant areas due to plant design [3].

Film Forming Technology was identified as a flexible preservation option and the application of Cetamine® Technology was implemented in December 2013. The conventional cycle chemistry was kept on ammonia dosing to the feedwater to pH 9.4 to 9.6, and sodium hydroxide to the drums to achieve a pH between 9.2 and 9.4 in the high pressure (HP) drum and, between 9.5 and 9.8 in the intermediate pressure (IP) and low pressure (LP) drums.

The Cetamine® product was dosed into the condensate line. After initial dosing, the target residual FFA in the return condensate before the injection point was measured in Unit 4 after 420 hours of operation and in Unit 1 after 380 hours of operation [3]. The measurement was realized by the Cetamine® Photometric Method.

During inspections of Unit 1 and Unit 4, systematic investigations have been done to answer one of the key questions: Had the protective Cetamine®-film been established in the entire system. For the determination of the FFA molecule OLDA on the internal surfaces of components in the water/steam cycles, three different test methods were applied: the hydrophobicity test (droplet test) and the Cetamine® Wipe Test were used on-site; X-ray photoelectron spectroscopy (XPS) was carried out on small tubing samples by a specialist external laboratory.

The hydrophobicity test is very simple to apply and qualitatively, the presence of a film can be demonstrated by the lack of wettability of the metal surface. This is the most common and easiest method to qualitatively detect the FFA on a metal/oxide surface. However,  hydrophobicity sometimes cannot be observed, e.g. in the case of a rough surface with porous iron oxide deposits, even though FFA is present on the surface [1], [2].

Consequently, Kurita has developed the Cetamine® Wipe Test, an easy, non-destructive method to semi-quantitatively determine FFA on the inner surfaces of water-steam cycles immediately on-site during plant inspections (Figure 3). XPS was applied as an additional technique to analyze the presence of the FFA nitrogen atoms on a total of four HP evaporator tubes and two reheater tubes.

During the inspection of the LP turbines from Unit 1 and Unit 4, the surfaces in Stage 5 (outlet) were submitted to extensive evaluation with the wipe test due to ready in-situ access. Each wipe test sample showed a clear pink coloration and a significantly higher absorbance value compared to the blank. Furthermore, wipe tests from all stages of the Unit 1 LP turbine were taken from both the trailing face and from the front face of the turbine blades. The results are summarized in Table 1 [3].

Table 1 Cetamine® Wipe Test short procedure

A total of four evaporator HP evaporator tubes and two reheater tubes were analyzed by XPS. The FFA nitrogen was found in all of the samples. As a comparison, nitrogen could not be detected on the HP evaporator tube from 2012 before the Cetamine® treatment, which had been taken as a blank. The results are summarized in Table 2. The positive results of the Cetamine® Wipe Test on the Unit 1 HP evaporator tube confirmed qualitatively the findings of the XPS study [3].

Table 2 Cetamine® Wipe Test short procedure

After more than seven years of use of Kurita’s Cetamine® technology in all four units of Uniper’s Connah Quay power station in the UK, the following plant monitoring, inspections, and operating experiences have been observed:

During plant inspections, the presence of the FFA OLDA has been verified on HP evaporator boiler tube surfaces both with the Cetamine® Wipe Test and by XPS. OLDA was also present on the surfaces of reheater tubes and LP turbine blades. The investigations strongly indicate that the Cetamine®-film has been established in the entire system both water and dry steam stages. This demonstrates that this technology can be used to protect all components in water/steam cycles, including areas that could not be preserved by conventional lay-up methods.

The Cetamine® Wipe Test is an easy to apply tool which enables operators to verify film formation on the water/steam cycle surfaces during inspections [3].  Outage inspections showed that the HRSG and LP steam turbine internal surfaces remained clean and free of corrosion. Cetamine® technology has enabled plant preservation to be significantly improved without affecting unit availability or start-up times.

[1] Technical Guidance Document: Application of Film Forming Amines in Fossil, Combined Cycle, and Biomass Power Plants, 2019. International Association for the Properties of Water and Steam, IAPWS TGD8-16, available from http://www.iapws.org

[2] Technical Guidance Document: Application of Film Forming Substances in Industrial Steam Generators, 2019. International Association for the Properties of Water and Steam, IAPWS TGD11-19, available from http://www.iapws.org

[3] Hater, W., Smith. B., McCann, P., de Bache, A., PowerPlant Chemistry 2017, 19(3), 129-140

[4] Allard, B., Chakraborti, S., Svensk Papperstidning 1983, 86(18), R 186

[5] Hater, W., Rudschützky, N., Olivet, D., PowerPlant Chemistry 2009, 11(2), 90

[6] Kolander, B., de Bache, A., Hater, W., VGB PowerTech 2012 92(8), 69

[7] van Lier, R., Gerards, M., Savelokoul, J., VGB PowerTech 2012, 92(8), 84

[8] Hook, B., Hater, W., de Bache, A., PowerPlant Chemistry 2015, 17(5), 283

[9] Hater, W., de Bache, A., Petrick, T., PowerPlant Chemistry 2014, 16(5), 284

[10] Wagner, R., Czempik, E., VGB PowerTech 2014, 94(3), 48

[11] Topp, H., Hater, W., de Bache, A., zum Kolk, C., PowerPlant Chemistry 2012, 14(1), 38