Power plant life extension |
more modern machine achieves improved efficiency, a larger part of the fuel input heat is converted to power – meaning the remaining exhaust heat does not necessarily increase as much as the installed capacity increases. To verify the thermodynamic fit, a “digital twin” of the original design is created. In this performance simulator, the gas turbine model is replaced with one representing the candidate new machine. It is therefore important to find the original heat balances that once formed the design basis for the HRSG and the rest of the balance of plant in the plant archive. From the simulations, you may, for example, find that the new gas turbine needs to be limited somewhat at low ambient temperatures to avoid overloading the steam system. In such cases, air preheating is a useful method to reduce power without loss of efficiency.
With increased electricity generating capacity, the step-up transformer and possibly the switchgear and cabling may need to be upgraded. However, in some cases, a redistribution of the reactive power factor between the gas turbine and the steam turbine may provide the necessary adjustment to avoid replacing of equipment.
A slight increase in exhaust heat from the gas turbine can often be handled. The HRSG may have a certain margin so that the maximum continuous rating (MCR) can be upgraded slightly if the safety valve(s) are modified accordingly. Increased steam production can in many cases be accepted by the steam turbine, provided technical margins allow it. If the steam turbine cannot accept more steam flow without causing an increase in steam system backpressure, then an HRSG safety valve with higher reseating pressure can be used. Alternatively, additional safety valve options exist that force the valve closed until release pressure is reached.
One example of a good fit is replacing an old Westinghouse W251 gas turbine with a Siemens Energy SGT-800. Another example is the replacement of an RB211 with an SGT 700. Both the SGT-800 and SGT-700 are mounted on a base frame that also carries several auxiliary systems, described as a package. This facilitates site installation. Larger gas turbines are assembled on site instead, as the transport size of a frame-mounted machine cannot be much larger than that of the SGT- 800. For frame-mounted gas turbines, it is best to replace the gearbox and generator as well, since they come with the standard GT package. For larger gas turbines, the gearbox (if applicable) and generator may be retained, depending on their condition and fit. The gas turbine foundation can often be reused, but suitability of its condition — free from cracks requiring repair or other issues — can only be confirmed once the original gas turbine has been removed. Confirming that the foundation and below-ground piling are sufficient for the new machine can be challenging if original documentation is lacking. However, in most cases, the structure is more than sufficient — the only issue is proving it. If
the rotor string of the new machine is lighter than the old one, then one may assume that forces resulting from events such as short circuits or blade-loss incidents are lower. The conclusion is that if the foundation was good enough for the old gas turbine, it is likely even better for the new one.
So far, all is fine — but this does not resolve the risk of eigenfrequencies. Even if this risk is low, it must be addressed, and mitigation plans must be in place. After the old gas turbine is removed, the embedded steel is carved out and cavities in the concrete are created to allow new embedment to be cast.
By replacing the gas turbine with a modern one, emissions are brought down to modern standards, complying with permitting requirements equivalent to those for new builds. The risk of a ban on operation in the near or far future is thus mitigated. A new machine may also be supplied as future-ready in terms of fuel flexibility. It may be delivered ready for up to 100% hydrogen for the gas turbine package and auxiliaries, and when the time comes for a fuel change, only the burners need to be replaced – preferably during a routine inspection, when burners are replaced anyway. Converting an existing machine to hydrogen-ready is sometimes possible but is certainly more expensive than the additional cost of including that option in a new-machine delivery.
The plant can also gain improved flexibility features, such as fast starting of the gas turbine and the ability to preserve heat during standstills and to withstand cycling operation better than originally designed. The startup time for the bottoming cycle may be significantly improved by combining better heat retention with warm-keeping measures.
HRSG can be suitable after minimal modifications
The HRSG can reach a lifespan of up to 50 years if water chemistry has been managed well and some restoration is carried out. In some cases, partial tube replacement is required, especially in the low temperature section if sulphur- containing fuels have been used. One issue with changing from an older GT to a more modern one is that the exhaust temperature is typically higher than that of the original gas turbine. In many cases, this is wrongly seen as a project–stopping issue. Unfortunately, some plant owners specify requirements that a new GT must not exceed the old “rated” GT exhaust temperature. This misunderstanding often stems from the HRSG nameplate rating, which states a GT type and maximum temperature. However, such a rating merely defines the heat input used to determine the maximum continuous rating – a steam flow figure. It has little to do with the material limits regarding temperature. When replacing the gas turbine, the HRSG certificate must be revised anyway. In practice, much higher exhaust temperatures can be used if the superheater is adapted, which often requires only cutting out part of it and
22 | April 2026 |
www.modernpowersystems.com
reconnecting piping. In the worst case, a superheater module must be replaced. In other cases, an upgrade of the water-spray capacity between superheater modules is sufficient. Some adjustments to the start sequence and to forced steam venting at steam turbine trips may be needed if the maximum allowable temperature of the superheater material is lower than the new gas turbine’s maximum exhaust temperature. These steps prevent excessive exhaust temperatures when low steam flow passes through the superheater. If the HRSG is equipped with duct-firing burners, normally no modification of the superheater is required. However, changes in exhaust flow and temperature may justify an adjustment of permissible firing rate. The material temperatures of the evaporator and economiser sections of the HRSG mainly follow the temperature of the water and steam on the inside, and not the exhaust temperature. This is due to the very efficient heat transfer via water compared to the transfer from dry exhaust. Thus, material temperatures are not significantly affected by a change in exhaust gas temperature from the gas turbine. This holds even considering the large external surface area created by added fins or serrations. If the steam pressure is reduced slightly (as a result of lower steam production, assuming a sliding- pressure concept), the reduction in steam saturation temperature may lower the material temperature more than any influence from a higher exhaust gas temperature.
If the casing is internally insulated, it is only marginally affected, and no issues arise with a GT replacement. Internal liner plates and insulation systems are normally suited for higher temperatures than originally specified. Duct expansion joints located in areas exposed to high temperature exhaust may need upgraded temperature resistance. However, their replacement is advisable anyway as part of maintenance and life extension measures. For an externally insulated vertical exhaust-gas-flow HRSG, the effects of creep deformation in the casing must be assessed. In such cases, an increase in GT exhaust temperature may require replacement of parts of the duct or reinforcement using internal stays. An alternative method is to modify GT control to allow more air to pass through the machine – diluting the exhaust. Alternatively, dilution air can be introduced using an additional fan.
If a bypass stack and diverter are installed, they are typically of a standard design that tolerates higher temperatures than specified for a particular plant. In other words, there is often hidden margin. This can sometimes be confirmed through scrutiny of drawings or by checking the materials used in the design.
Steam turbine life extension Many steam turbines have been operating for more than 50 years; there are even examples of machines that have survived 100 years of operation. So why should a turbine be at the end of its life after 25 years?
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