| Combined cycle
Ageing CCGT infrastructure: why water quality is the hidden maintenance priority
Around 70% of the UK’s combined cycle gas turbine fleet has exceeded its expected operational life. As these plants shift from baseload to flexible peaking duty, the demands on water chemistry control are increasing at precisely the point where many operators are still running decades-old treatment systems. This article examines why water quality has become one of the most critical maintenance priorities for ageing gas-fired plant, and what operators can do about it
Chris Payne Co-founder & Director, PureTec Separations Ltd, Ledbury, UK (email
cpayne@puretecseparations.com)
The UK’s gas-fired generation fleet is older than most people in the industry care to admit. The majority of CCGT stations were built in the 1990s during the dash for gas. Many are now well past their original design life. And yet they remain essential. When wind drops and solar output falls, these stations fill the gap. Some sit idle for weeks, then fire up at short notice to meet grid demand. A typical CCGT station uses between 30 and 80 cubic metres of water per hour across its HP and LP boiler feed, steam turbine feed and condensate systems. Water quality at every stage directly affects the life of high- value assets: HRSG tube bundles; condensers; feedwater heaters; and turbine blades. In a new plant, materials and protective coatings can absorb minor chemistry drift without immediate consequences. In a plant that has been running for 25 years, the tolerance is far narrower, with thinner tube walls, degraded welds, and worn protective oxides. Small deviations in water chemistry now accelerate damage that would have
been negligible a decade ago. Most of these stations still rely on ion exchange (IX) demineralisation systems installed when the plant was built. These systems were well suited to baseload operation. They are less well suited to the demands of flexible running, and many are now reaching the end of their serviceable life. Replacement parts are increasingly difficult to source. Service intervals are growing shorter. And the water quality these ageing systems produce is becoming harder to keep within specification.
Three chemistry risks that worsen with age There are three water chemistry failure modes that cause the most damage in ageing CCGT plant. All three become more severe as
infrastructure deteriorates. ● Corrosion is the most familiar. Older boilers, economisers and condenser tubes have thinner walls. When pH drifts low, oxygen control falters or chloride contamination occurs, corrosion rates
climb sharply. A tube that might have lasted another decade under tight chemistry control can fail within months if water quality slips.
● Deposition is the second. Scale, silica and iron oxide deposits insulate heat transfer surfaces. In ageing units where heat transfer margins are already reduced, deposits push metal temperatures higher, leading to creep, cracking and tube failures.
● Flow-accelerated corrosion (FAC) is the third, and arguably the most dangerous. FAC risk increases sharply with age. Slight changes in oxygenation, pH or iron transport can trigger rapid metal loss in feedwater piping and economisers. Older piping geometries and welds are particularly vulnerable.
And cycling operations compound the problem. Most UK CCGTs now operate in a flexible, start–stop regime rather than the steady baseload running they were designed for. Frequent starts and shutdowns break protective oxide layers, cause thermal
Figure 1. Simplified process flow schemes comparing a traditional IX demineralisation train with a modern RO-CEDI configuration. The RO-CEDI route eliminates regeneration chemicals, reduces the number of process steps and delivers more consistent output quality
www.modernpowersystems.com | July/August 2026 | 21
All images: PureTec
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