Raising the temperature in power plants

28 October 2013

Control Engineering Europe looks at the design requirements for valves used at high temperatures, which are needed to run power plants at increased efficiencies - currently the subject of a variety of research projects, including the European Commission's COMTES700.

The continuous technological development of power plants has resulted in efficiencies being raised from around 30% to between 45 and 50%, helping plants to meet increasingly stringent requirements surrounding environmental legislation and the scarcity of raw materials.

Increasing efficiency by between 15 and 20% for one power plant has resulted in a cut in CO2 emissions of around 50%, with no reduction in the amount of power being produced. This has been achieved by operating the power plant at higher pressures and temperatures.

However, raising the steam parameters to temperatures of up to 750°C and pressures up to 500 bar at the same time required the qualification and optimisation of materials and components to ensure they were able to meet these more arduous conditions.

Valve requirements
To ensure the reliable operation of the valves in this higher pressure and temperature scenario firstly requires the right choice of materials. Normally, steels with 9 to 12% of chromium are used for temperatures up to 630°C (For example: material 1.4901 or ASTM A182 grade F92). However, only nickel-based alloys, such as 2.4663 (Alloy 617), are used for temperatures above 700°C. The drawback is that these alloys are very expensive, as the material is difficult to machine.

To ensure the safety of power plant components the material requirements must meet specific power plant specifications in addition to those of the EN and VdTÜV standards.

During start up and shut down of a power plant, the piping components and the fittings, will be subjected to high temperature differences. These differences strain the materials and because of the different thermal expansion of the components, can cause undesirable tensions.

Another risk comes from the fact that the valve stem can expand less during the heating phase than the valve body, which can result in the valve tip slightly lifting off the valve seat, causing internal leaks. For this reason, special attention needs to be given when selecting the materials for the valve components, to ensure that the materials used have the same thermal expansion coefficients. To prevent the valve tip from lifting during temperature changes the initiation of the closing force of the valve tip, via a spring assembly, which compensates the thermal expansion differences, is recommended.

The sealing elements, especially the packing that seals the valve stem dynamically toward the atmosphere, also need to be able to withstand high pressures and temperatures. Graphite packings are generally used or this purpose. However, graphite begins to oxidise in the presence of oxygen in the air from 550°C; moving the stem seal to an area where the temperatures are considerably lower and no more risk of oxidation is present at all is therefore advised. This can be achieved by extending the bonnet and using additional cooling ribs.

The welding joints need to be closely scrutinised. Especially high-temperature materials, such as 1.4901, are very sensitive to the formation of heat cracks during welding. Even if the valve itself does not have any welds, it must be remembered that during fitting, when welding the valve in the pipe assembly the weld must be subjected to subsequent heat treatment; this takes place over a period of at least half an hour (depending on the size of the seam) at approximately 750°C.

Particularly high demands on the weld will call for so-called ‘black-white’ connections which occur frequently at the process to instrument interface. To avoid the need to make the welding joint on-site, this special connection is already made on the primary isolation valve, for example, by welding a short piece of pipe with the material 1.4952 to the valve body from 1.4901.

Research and development projects
To observe and test innovative high strength boiler and piping materials under extreme conditions several research and development projects have been established and it is hoped that the insights gained from these will be used to help develop components such as safety, control and needle type globe valves, seals or coating systems, which can be employed at temperatures above 700°C in a reliable and economic manner.

One project, set up by the European Commission, is COMTES700 (Component Test Facility for a 700°C Power Plant). This project was started in July of 2004 and was made possible through a collaboration of European power plant operators and manufacturers.

After the successful manufacturing of high temperature components by AS-Schn eider and their installation into E.ON Kraftwerke’s Scholven coal-fired power plant in Germany the test plant was operated from July 2005 to December 2011. The operating conditions of the plant were a steam temperature of 535°C and a pressure of 230 bar. The steam flow of 2,200 t/h produced a net output capacity of 676 MW.

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