| Power plant life extension
Replacement of a V84.2 gas turbine with the newer STG6-2000 at PowerSouth’s McWilliams power plant in Covington County, Alabama, USA. Photo: Siemens Energy
Some gas turbine models designed in the 1980s and 1990s and supplied around 2000 were taken out of production long ago. As a result, access to spare parts and qualified service personnel is becoming increasingly difficult, costly, and subject to longer lead times – especially when components must be fabricated as single-order items rather than taken from stock.
Gas turbines that have been taken out of production will also no longer receive service upgrades, since development of technology for a machine is generally halted some years prior to the end of sales. Replacement with a new gas turbine model reduces service costs and shortens downtime during each service interval. Typically, any machine approaching the end of its design life also shows an increasing rate of ageing-related faults and outages, and the risk of a serious fault rises. Even if the design lifetime of a gas turbine can be 30 years, it may be economic to replace the gas turbine earlier unless upgrades have been implemented. In some cases, simply evaluating the performance gain by replacing an existing turbine with a new machine reveals a payback time of as little as two years, since running costs have a decisive impact. When considering replacement of an old GT with a new one, you first search for a machine with similar capacity, but layout considerations are also vital. For example, if the old machine has the exhaust directed sideways, then a new machine with axial exhaust would require positioning far from the old foundations to leave room for a duct elbow. You would be better off finding an alternative machine with a side exhaust. The position of the generator
A 3 x SGT-400 installation integrated into an industrial energy and process environment. Image: Siemens Energy
– at the hot or cold end of the gas turbine – is another important factor.
If the old machine is an aeroderivative, then a new industrial machine may be used instead. A modification of the exhaust duct is usually required, even S shaped ducts have been used to enable the fit.
Improvement of gas turbine inlet air filtration may be wise. The existing GT inlet air structure can be kept, but the GT interface typically needs to be modified. However, it is often found that the filtration quality of old installations is rather poor, and modern HEPA
filtration should replace the old system, even if the old housing structure is kept. Better air filtration improves gas turbine operating performance due to less fouling, reduces need for compressor washing and reduces gas turbine degradation thanks to less wear caused by particles in the air.
If a gas turbine of a more modern vintage replaces an older machine of the same type, performance upgrades often go hand in hand with increased generating capacity. Thus, the replacement machine typically has higher capacity as well as better efficiency. Since the
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