Insulating Nossener-Brücke from problems5 July 2002
During the first five years of operation a number of difficulties arose with the gas turbine insulation at Drewag's Nossener-Brücke CHP plant in Dresden. A radically new approach was needed.
There are three 70 MW Siemens V64.3 silo combustion chamber machines at Nossener-Brücke. The original insulation materials did not resist the constant high temperatures. The rock wool was not able to provide its full insulating effect because binding materials evaporated. The loss of the binding materials caused the insulation material to clot and the insulation assemblies slumped. The clotting of the rock wool led to excess temperature on the mat surface, damaging the outer aluminium cover.
Another problem was the fixing of the insulation mats. The steel pins, fixed to the turbine body, caused damage to the glass cloth where they were pinned to the mats. Further, the steel pins themselves posed an unacceptably high risk of injury.
Also, the combination of clotting insulation material with inadequate fixing led to serious problems in the lower parts of the turbine. The insulation tended to sag downwards. The trapping of hot air between the insulation and the turbine casing led to different temperatures in the upper and lower casings, resulting in damage to the rotor, with enormous financial consequences for the plant.
Last but not least, there was no insulation documentation, so at every refitting there were immense problems installing the insulation the right way.
Meanwhile, in response to just these kinds of problem, Siemens and Arnold Insulation had developed a new jacket type insulation system for the newer, 3A, annular combustion chamber turbines.
The new insulation promises longer life, short dismantling and installation times, and easy handling due to the use of Velcro.
On the inner side of the jackets a stainless-steel strengthened glass cloth is used with a maximum working temperature of 750°C. The surface of the jackets consists of a silicon coated glass cloth to protect the insulation from water and oil splashes.
Needled glass fibre and silicate fibre mats are used as the insulation material. These needled mats have the advantage of resisting permanent high temperatures, because there is no binding agent.
For fixing, stainless-steel supports are attached to the turbine body and the jackets are fixed to these with high temperature Velcro. Velcro has the advantage of easy installation without special tools.
Also, the new insulation is designed with 3D-CAD and therefore adjusted to the shape of the turbine body, even including complex cast iron casings.
To protect the insulation from physical damage, perforated sheet metal is fixed on the upper casings and the cross flanges. This means the insulation can be walked on without damage.
Adaptation of the insulation design to the V64.3 was done in co-operation with Drewag, with whom first contact was made in May 2000. The main focus was on ease of O&M, for example, enabling an operator to change a thermoelectric couple at the exhaust diffuser without help or special insulation tools. It is just a matter of loosening the Velcro fasteners, opening the jacket, changing the couple and then reclosing the jacket.
To design the insulation, the V64.3 turbine body was digitised with a 3D scanning system and a CAD model generated.
Installation of the new insulation began on the first turbine immediately after closure of the casings following the 2000 maintenance outage. Areas where continuing access for completion of the outage work was necessary (eg combustion chamber manhole) were kept free. Once the work was completed, the jackets for remaining areas were subsequently installed by the Siemens site manager - in just a few minutes. Also, during commissioning, the ability to dismantle and refit individual jackets saved a lot of time.
Overall, the new insulation contributed to the success of the outage.
Final acceptance was achieved following temperature measurements after four weeks of operation.
In May 2002 the second turbine at Nossener-Brücke was fitted with the new insulation, while the third will be re-insulated during an outage in August 2002.