Eke out more performance from your old gas turbine

It’s not often you get something for nothing, or in this case almost nothing. Pay for an expert review by a couple of top global gas-turbine consultants with man-decades of experience, have them tweak your controls, and you likely can squeeze out a few megawatts and/or efficiency gains from an older gas turbine. No capital costs involved.

According to Bob Johnston, president, Keck Group International, speaking at the 2021 MENA Combined Cycle Conference (virtual), May 25-27, the key is an integrated, system-wide evaluation of the components currently in your machine (Table 1). “Some replacement hot-gas-path (HGP) parts are actually upgraded parts which supersede their earlier versions, and could qualify for higher firing temperature,” Johnston stressed.

This so-named “non-capital-parts uprate program” has been successfully applied to “many dozens” of GE machines and also can be applied to other OEM machines. Johnston explained that OEMs often supply upgraded parts as in-kind replacements but don’t tell the customer that they’ve replaced enough parts to qualify for an uprate. In fact, this is the “likely” situation for older machines, typically those shipped before 2001. For example, most of the later-vintage HGP parts for the MS5002 and MS700EA are directly interchangeable with all prior vintages.

Tweaks identified by Johnston which can arise from the parts evaluation are:

    • Inlet guide vane (IGV) angle change.
    • Isotherm setting increase for power at higher ambient temperature.
    • Exhaust thermocouple (t/c) corrections.
    • Degradation correction to control curve.
    • Tilted control curve.

Exhaust t/cs were biased high on machines between 1980 and 1997, and a straight-forward controls setting change can net up to an 11-deg-F increase in turbine exhaust temperature. Results for 11 gas turbines in the Middle East (Table 2) show that not all the tweaks apply to every unit (Table 3). Output gains ranged from 2.4% to 10% for three separate groups of 56 engines without applying IGV angle-change tweaks.

Standard GE control curves maintain constant firing temperature at all ambient temperatures. A tilted control curve overfires the machine by 16 deg F on hot days, and underfires the machine by 25 deg F on cold days. This can give a 1.6% output boost on hot days, with no net impact on parts life or maintenance cycles. Again, only control setting changes are necessary. Most sites value an output boost during peak summer periods, and can sacrifice small decreases during cooler periods.

Control curves are based on a new machine, but as operating hours mount, non-recoverable performance degradation occurs, which reduces output, but also leads to under-firing the machine because the control curve hasn’t been adjusted. “By applying a suitable correction to the control curve, you can gain 0.8% output,” notes Johnston. This has been done for “hundreds of machines since the 1990s,” he added.

Users are cautioned that the higher turbine exhaust temperatures which lead to the output gains could raise the HRSG’s HP superheater temperature beyond its limit (figure). Two ideas here are to add desuperheater capacity or remove fins from the superheater tubes. Careful analysis could reveal other options as well.

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