| Power plant life extension


If the turbine has been exposed to poor quality steam, blades may be damaged even after very few years of operation. Likewise, erosion damage may occur quickly if the exhaust is operated too wet due to low condenser pressure. Naturally, the number of start–stop cycles and the extent of load cycling have significant influence, as does continued operation under vibration issues. Even if an old turbine is still functional, it may have severely degraded performance due to worn blades, increased internal leakages, etc. Turbine degradation results in increased temperature at extractions and at the outlet, at worst resulting in risk of trips disturbing operations. But even if the turbine requires a complete re-blading or even shaft replacement, the cost is often lower than replacing the turbine entirely. When replacing a turbine, the foundation is not always reusable, and interfaces to the condenser and piping differ, resulting in many additional costs that can be avoided by revamping the existing machine. Naturally, some auxiliary systems and instrumentation – such as hydraulics, oil pumps, steam valves, and the evacuation system – should be replaced or refurbished to ensure they last through the extended life cycle. If the steam cycle concept is dual- or triple- pressure, the GT change may affect both HP steam production capacity and the balance between HP, IP, and LP steam flow. If the steam turbine is re-bladed anyway, this presents an opportunity to rematch the new blading to the new operating conditions to optimise plant performance. A rematch mainly affects the last- stage blading, which is also the most exposed to erosion and fatigue – meaning a rematch typically aligns well with refurbishment needs. In many cases, the steam turbine power is reduced somewhat when replacing the GT, even when GT capacity is increased. This means the gearbox (if any), generator, and electric gear can be kept. However, the gearbox and generator should be inspected and serviced to ensure a lifespan consistent with the extended plant life. This may require pulling the rotor and possibly rewinding. If suitable, the old generator from the replaced gas turbine can be kept as a strategic spare for the steam turbine.


Balance of plant equipment Naturally, other plant components need to be replaced or renovated to extend plant life, but most costly infrastructure can remain. Whether pumps and valves should be replaced regardless of condition – to achieve a full refurbishment – or maintained on demand depends on financial considerations. Adding variable frequency drives (VFDs) is a way to improve plant efficiency and achieve capacity upgrades if needed. However, typically many pumps are severely oversized in the original design, which actually argues for efficiency gains from VFDs through speed reduction rather than for capacity increases. A wet surface condenser may require replacement of protective plates, the most erosion-exposed tubes, and possibly re-lining


Location of barring vent and stack damper


of the water chamber. However, condensers generally have a long lifespan unless exposed to severe corrosion or erosion. If the plant uses open cooling water, piping should be properly cleaned, as fouling significantly reduces flow. Pipes should also be inspected and, if required, repaired or lined. It is often desirable to replace the plant control system in conjunction with a life extension project.


When replacing a control system, a large number of logics and signal interfaces must be translated to the new system architecture. It is not always possible to find identical logic building blocks in the new system, so similar logic functions may need to be constructed differently. Since a new system may integrate and automate the plant more than before, additional commissioning tasks arise – not just re tuning of controls. To allow comparison and minimise risk, it is recommended that plant DCS is switched only when all plant systems are in a steady state. In short: do not combine DCS replacement with other plant structural changes, run it as separate projects, separated in time to minimise risk of disturbances and delays. Small PLC controllers may be used as patching temporarily if the DCS exchange is to be made after the plant re-commissioning. For a lifetime extension project where the gas turbine is replaced and DCS replaced later, the integration between the old DCS and the new GT should be kept to a minimum. In conjunction with a control system update it is recommended that instrument redundancies and alarm philosophies are reviewed. Plant reliability and safety can be improved by modern 2oo3 voting and addition of new automated shift to emergency back-up control mode for critical controls can be programmed (initiated, eg, when valve positioning failure is detected). Suppression of consequential alarms or “first- out indication” helps operators to understand and swiftly act. Categorisation of alarms and


change where possible from alarm to event- indication reduces the risk of alarm-flooding. An opportunity to clean up!


Flex operation adaptations Re purposing and adapting an old plant for more flexible operation is both possible and advisable. Adding a stack damper plus a small barring vent (see diagram) to reduce HRSG stress during gas turbine stops – stress caused by flushing with cold air – is relatively simple, and there are other measures that reduce cycle stress and improve startup time. Most such measures are relatively easy to implement, for example warm-keeping using steam from a nearby unit or a small auxiliary boiler. Replacement of drain systems for the HRSG and steam lines with designs that prevent loss of pressure during standstill is advisable. Keeping an HRSG warm, slightly above 100°C, significantly reduces startup stress, allowing the heat-soak period during startup to be skipped. Steam production begins rapidly and helps limit the superheater material-temperature peak. The addition of a purge-credit system saves time in the gas turbine start sequence and enables fast start. Purge credit stores a validated purge sequence in a safety-classified memory and tracks valve positions, ensuring no fuel ingress occurs during the waiting period. Traditional thermodynamic drain traps automatically open when steam temperature has dropped only slightly; these traps should be replaced with other types to avoid energy loss during standstill.


To conclude


Your old plant can be reborn to a new, high performing life through a renovation in which gas turbine replacement is the key to achieving a new lifecycle and, at the same time, bringing the plant up to modern performance, modern emission standards, flexible operation capability suitable for the future energy system, and – if desired – readiness for zero-emission fuels.


www.modernpowersystems.com | April 2026 | 23


Page 1  |  Page 2  |  Page 3  |  Page 4  |  Page 5  |  Page 6  |  Page 7  |  Page 8  |  Page 9  |  Page 10  |  Page 11  |  Page 12  |  Page 13  |  Page 14  |  Page 15  |  Page 16  |  Page 17  |  Page 18  |  Page 19  |  Page 20  |  Page 21  |  Page 22  |  Page 23  |  Page 24  |  Page 25  |  Page 26  |  Page 27  |  Page 28  |  Page 29  |  Page 30  |  Page 31  |  Page 32  |  Page 33  |  Page 34  |  Page 35  |  Page 36  |  Page 37  |  Page 38  |  Page 39  |  Page 40  |  Page 41  |  Page 42  |  Page 43  |  Page 44  |  Page 45