Novel high temperature steam transport pipes

Research area  Materials in Harsh Environments
Dates 01 Oct 2017 - 30 April May 2019
Funder EPSRC
Contact person Martyn Pavier

There is a requirement to achieve higher efficiencies in steam turbine electrical power generating plant. For this the temperature of the steam entering the turbine should be as high as possible and in response to this, designers are seeking to raise the operating temperatures up to about 700°C.

Designs for advanced ultra super critical plant (A-USC) operating at these high temperatures may achieve cycle efficiencies as high as 55%, about 15% greater than typical conventional plants. Similar aspirations are under consideration for the next generation of nuclear electrical power generating plant, referred to as GEN IV. In this case, in addition to higher temperatures for increased efficiency there is the potential for producing hydrogen.

For existing power plant, components are fabricated from a range of steels, including ferritic alloys such as P91 and austenitic stainless steels like Type 316L. Unfortunately, alloys suitable for the required greater temperature applications are limited to the higher cost austenitic stainless steels and nickel base alloys, such as Inconel 617, to ensure that the required mechanical properties, creep strength and corrosion resistance are sufficient to provide practical design service lives.

Hence it is important to explore alternative engineering design options that would allow existing ferritic and austenitic stainless steels to be selected. These steels are known to have good service performance at lower operating temperatures, below 600°C, enabling lives of approximately 40 years to be achieved with confidence.

In this project we consider a design for steam transport pipes that would allow the temperature experienced by these components to be reduced by use of a Thermal Barrier Coating (TBC), whilst achieving the design temperatures necessary for the thermal efficiency. Thermal barrier coatings are thermally insulating layers made of low thermal conductivity ceramics. They are used to increase the operating temperature in gas-turbines and diesel engines. There are a range of different ceramics used for TBCs, with yttrium stabilised zirconia being widely used. There are many different methods for applying TBCs and a coating thickness of about 4mm can be achieved using solution-precursor plasma spraying.

The concept is shown in the figure on the right. High temperature steam (i) flows through the inner transport pipe (iv). A thermal barrier coating (iv) is applied to the inner surface of this pipe. The necessary cooling of the external surface of the transport pipe is achieved using turbine exhaust steam (ii), flowing through the outer transport pipe (v). The external surface of the outer transport pipe is lagged (vi).

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