As reported in this month’s news, Siemens is continuing to enjoy a boom in gas turbine orders. But, as yet, turning this into profit has proved elusive. One reason is that introducing advanced gas turbines to the market place always seems to be very much trickier than anyone imagines. This is perhaps not surprising when savage competition is forcing vendors to push their technologies to ever higher ratings and performance, while at the same time keeping NOx low and attempting to achieve unrealistically short development lead times.

Take the Siemens 3A machines, for example. Difficulties with these, resulting in penalties and late payments by customers, have seriously damaged the financial health of Siemens’ power generation activities, and were a major contributor to the losses of some euro 130 million posted in 1998/99, on a turnover of euro 8.9 billion. However, there are now strong indications that the corner has been turned, with the power generation division expected to deliver an overall profit this financial year.

A refreshingly frank account of the 3A saga was presented at the recent Power Gen show in Helsinki, by Bernard Becker, who is responsible for gas turbine development in the Siemens Power Generation Group, Mulheim/Ruhr, Germany.

The 3A gas turbine series was first introduced in 1995. It had been developed in response to market demand for increased efficiency and low generating costs. Some 52 machines of types V64.3A (67 MW, 50/60 Hz version), V84.3A (182 MW, 60 Hz version) and V94.3A (265 MW, 50 Hz version) have been delivered, of which 42 have seen their first firing and 26 have been handed over to the customer.

Some solid operating experience is now being amassed and the 3A can be regarded as “having successfully completed its market introduction phase” according to Dr Becker. But this phase has certainly been more protracted and posed many more problems than expected, demonstrating, once again, that, when it comes to gas turbine design, the devil is indeed in the detail.

The 3A’s with the longest operating experience are the V94.3A machines at the 1390 MWe Didcot B combined cycle station in the UK. It was here that the turbines did their service trials and were tuned for on-line operation. “Our thanks go to the management and staff of Didcot, but also to our other customers, for their co-operation and support during what were not the easiest of times,” said Dr Becker.

The principal problem, and one that has been fairly widely experienced by other vendors, not just Siemens, has been combustion noise, also known as “humming”. “Like when any manufacturer develops a new gas turbine,” said Dr Becker, “the problem of combustion noise in the running turbine had to be brought under control as this cannot be adequately tackled by theoretical considerations alone.” The phenomenon arises from instabilities in the flame, which can cause acoustic resonance. To achieve low NOx despite the high turbine inlet temperatures needed for getting high efficiencies, an annular combustion chamber was introduced in the 3A series. The acoustic resonance in the 3A case was found to stem from vortices forming in the burner outlet region. In these vortices fuel and hot gas products mixed and spontaneously ignited after a very short time, ie ahead of the main flame, with the resulting pressure surges triggering oscillations due to acoustic feedback.

The answer was to add a cylindrical extension to the burner outlet, allowing less fuel to get to the vortices and preventing them from igniting before the main flame.

Several thousand operating hours have now been accumulated with the modified burner, designated HR3 – at Otahuhu, New Zealand, since November 1999, at Seabank, UK, since January 2000, and at Peterhead, UK, since March 2000. So far, the experience has been positive, according to Dr Becker.

A number of other issues arose during the 3A’s “market introduction phase.” These included compressor modifications, faster than expected erosion of the thermal barrier coating on the first blading rows, improvements to the hollow rotor shaft and redesign of the mechanism for securing the ceramic tiles of the combustion chamber.

As Dr Becker concludes from the 3A experience, “pushing the envelope is not without its consequences.” However, the operators and their gas turbine supplier can now reasonably look forward to a pay-back from their efforts. So much for the pain now its time for the gain.