Deposition on the inside surfaces of heat recovery steam generator (HRSG) high pressure (HP) evaporator tubing is always a precursor to any of the under-deposit corrosion (UDC) HRSG tube failure (HTF) mechanisms, which continue to rank as the third most important around the world for combined cycle/HRSG units. However, prior to this current work very little was known about the process or morphology of deposition as a function of the HRSG cycle chemistry. Very few HP evaporator tubes had been removed (estimated at < 5 % of the total number of HRSGs) and about the same number of units had been chemically cleaned. About 45 HP evaporator tubes have now been analyzed in this study to characterize the effects of operating time and HRSG cycle chemistry. Some important relationships have become clear which are starting to provide guidance and confirmation of the optimum cycle chemistry that should be used not only to control the UDC mechanisms but which also have a major effect on flow-accelerated corrosion in the lower pressure parts of the cycle. There are also important implications for chemical cleaning of HRSG HP evaporators.

This paper, the first in a series of overviews of the most recognized international fossil cycle chemistry guidelines, introduces the Electric Power Research Institute (EPRI) and its current and planned fossil cycle chemistry guidelines. These guidelines cover, among other topics, all-volatile treatment, the phosphate continuum and caustic treatment, oxygenated treatment, the control of flow-accelerated corrosion, cycle chemistry in combined cycle/heat recovery steam generators, and the shutdown, layup, and startup of combined cycle units with heat recovery steam generators.

We report the results from a trial at the National Physical Laboratory Stack Simulator Facility of a new alternative method (TGN M21) drafted by the England and Wales Environment Agency for monitoring SO₂ emissions from flues and stacks. The alternative method has been written to address the desire of test houses to move from wet chemistry to instrumental techniques. The method is not instrument specific and can be used with any technique meeting the required performance characteristics, which are consistent with the England and Wales Environment Agency's Monitoring Certification Scheme and EN 15267. The equivalence of the alternative method to the standard reference method (SRM) with regard to the in-field tests was carried out in accordance with CEN TS 14793 using instrumental techniques (non-dispersive infrared, Fourier transform infrared, infrared gas-filter correlation, electrochemical cell and ultra-violet absorption). Using trial mixtures from 0–715 mg · m^–3 SO₂ (STP) it was demonstrated that the alternative method conformed to the acceptance criteria for repeatability and correlation to the analogous SRM measurements.

With few changes, the cycle chemistry guidelines have advised using the same cycle chemistry parameters with the same or similar numerical values for the continuous plant cycle monitoring of fossil plant streams for several decades. One parameter, however, is conspicuously absent: degassed cation conductivity of steam. The brief appearance of this parameter in one of the guidelines and its subsequent non-inclusion in the revision do not seem appropriate considering the advantages of using this monitoring technique, in particular for cycling and peaking units. This contribution discusses what role degassed cation conductivity should play in the plant cycle chemistry monitoring, and argues for its inclusion in future cycle chemistry guidelines.

Varnish deposits on internal surfaces in turbine lube systems result in a number of adverse operational issues, especially the restriction and sticking of the moving parts of servo- or directional control valves, resulting in their malfunction. The lubrication fluid has limited solvency for the varnish-forming material, hence a typical turbine will have the majority of this material as deposits and a relatively small portion as suspension in the fluid phase, in quasi-equilibrium with the deposits. The lube system needs to be cleaned by removing the suspended varnish-forming material from the fluid phase, which allows the deposits to re-entrain into the fluid phase, until the majority of the transferable deposits are removed and the fluid carries no significant amount of the material to have any adverse effect. The methods used for the removal of varnish from turbine lube systems include chemical cleaning/flushing, electrostatic charge induced agglomeration/retention, and the adsorption of the varnish suspended in the oil on an adsorbent medium. The paper discusses an absorption-based removal method that utilizes a fibrous medium that has pronounced affinity for the removal and retention of the varnish-forming material from the fluid as well as the deposits from surfaces that are in quasi-equilibrium with the varnish precursors in the fluid. The filtration medium is a composite, made with cellulose bonded by specially formulated, temperature-cured resins. The absorptive medium exhibits high structural and chemical integrity and has been thoroughly tested on operating turbines, showing reduction in varnish levels from the critical range to below normal range in a relatively short time. The experience with the utilization of the absorptive medium in laboratory tests and in two operating turbines is presented.

Many power stations dose feedwater with oxygen scavengers such as carbohydrazide; these compounds remove the dissolved oxygen but release inorganic carbon dioxide into the water. The effect of carbon dioxide upon corrosion levels is a controversial subject and as such is not within the scope of the work discussed in this paper. The effect of carbon dioxide upon conductivity measurements is the major consideration.

Degassed cation conductivity (DGCC) is a widely used technique to remove dissolved gases from high purity water. A typical DGCC instrument consists of a reboiler which raises the temperature of the sample water above its saturation temperature, thus reducing the solubility of gases, such as carbon dioxide, effectively boiling the gas out of the water sample stream.

The present method used for measuring water or steam purity is cation (or acid) conductivity, often denominated as after-cation-exchange conductivity. This technique should indirectly assess levels of anions such as chloride, sulphate, formate and acetate for corrosion avoidance purposes. However, due to the presence of carbon dioxide dissolved in the sample, the monitoring results are not appropriate for this purpose. The degassed cation conductivity technique can be applied to power station start-ups when the steam conditions have to be monitored closely. By removing the dissolved carbon dioxide from the sample stream, more accurate information about the actual purity of the water or steam is given. This paper will give the results and economic benefits when this monitoring technique is applied to a cold start on a combined cycle gas turbine (CCGT) power station.