Welding Challenges Of 9-Chrome-Moly Steel

With care taken in the welding process, more efficient power-generating system can be operated today with minimum down time for repairs and maintenance. This is due to the development of alloys and knowledge of their performance and behaviors in varying stress and temperature ranges.

The key is in the understanding of these materials by designers and maintenance managers. I have excluded welding engineers from this group because most of the time their knowledge is not in question and often welding engineers are not the final decision makers in these situations.

A “combined cycle power plant” is described as a combination of gas turbine generator (Brayton cycle) with turbine exhaust waste directed to heat boiler and steam turbine generator (Rankine cycle) for the production of electric power. If the steam from the waste heat boiler is used for process or space heating, the term "cogeneration” is the more appropriate term to use meaning simultaneous production of electric and heat energy. Most of the newer plants are designed as or upgraded to co-gen.

The simple cycle gas turbine generator operates as an independent electric power producing unit. The system is a relatively inefficient process because its net heat efficiency at full load is more than 15,000 Btu per kilowatt-hour. Consequently, simple cycle gas turbine generators are used only as standby units or in some applications as a supplementary power unit. The fuel economy of such standby units is of small importance.

A gas turbine releases large quantities of gas through its exhaust system, the temperature of these released gases is more than 480oC (900oF). In a combined-cycle system, these gases are piped to operate a waste heat boiler. These boilers generate high-pressure steam. This superheated steam is then piped to a steam turbine generator. The resulting “combined-cycle” heat rate is in the range of 8,500-10,500 Btu per net kilowatt-hour which is less than a simple cycle gas turbine generator.

A typical combined-cycle system may have either a single-shaft or multi-shaft configuration. A single-shaft system consists of one each of gas turbine, steam turbine, generator and a heat recovery steam generator. The gas and steam turbines are coupled to a single generator on a single shaft in a tandem arrangement. The single-shaft arrangement gives the operational simplicity, smaller footprint, and lower startup cost, but they are less flexible and have lower functional reliability. Typical heat-recovery steam generator types and layout configurations can be listed as follows: 1) D-frame evaporator, 2) O-frame evaporator, 3) A-frame evaporator. 4) I-frame evaporator, 5) horizontal tube evaporator, and 6) economizer configuration.

On the multi-shaft systems, there is more than one gas turbine-generator and heat-recovery steam generator. They supply steam through a common header to a separate single steam turbine-generator.