Valve design hots up4 July 2013
Some kinds of Gen-IV reactors planned for development over the next 20 years propose raising primary- and secondary-circuit temperatures and pressures beyond current PWR levels.
?Raising steam parameters to temperatures up to 750°C and pressures up to 500 bar at the same time requires the qualification and optimisation of suitable materials and power plant components to meet these demands.
One research and development project was COMTES700 (July 2005-December 2011) at E.ON's Scholven coal-fired power plant in Gelsenkirchen, Germany. The insights gained shall be used in the development of components such as safety, control and needle-type globe valves, seals or coating systems, which can be employed even at temperatures above 700°C in a reliable and economic manner.
For that project, in June 2004 AS-Schneider received an order from ALSTOM Power Boiler GmbH to develop an isolation valve which is able to withstand temperatures up to 750°C at 400 bar.
The first issue is selecting the right materials. Normally steels with 9 to 12% chromium are used for temperatures up to 630°C (example: material 1.4901 or ASTM A182 grade F92). Only nickel-based alloys such as 2.4663 (Alloy 617) are used for temperatures above 700°C. These alloys are very expensive, and their difficult machining requires extremely great production knowledge.
When a power plant is started up and shut down the piping components as well as the fittings are subjected to large temperature variations, which strain the materials. Also, components' different degrees of thermal expansion may cause undesirable tensions or leaks. To prevent the valve tip from lifting off during temperature changes, the initiation of the closing force of the valve tip via a spring assembly, which compensates the thermal expansion differences, was included.
However, not only the metallic materials of a valve must withstand the extremely high pressures and temperatures but also the sealing elements, especially the packing, which seals the valve stem dynamically toward the atmosphere.
Graphite packings are used here as a rule. However, since graphite begins to oxidise in the presence of oxygen in the air from 550°C, the stem seal was moved to an area where the temperatures are considerably lower and no more risk of oxidation is present. This was achieved by extending the bonnet and using additional cooling ribs. In addition, the valve was equipped with a metal back seat which supports the packing when the valve is fully opened. Also, a vent hole was installed above the packing through which the hot steam is directed away from the handwheel in the event of a packing leak.
Finally, the welding joints needed to be closely scrutinized. Especially high-temperature materials such as 1.4901 are very sensitive to the formation of heat cracks during welding. Particularly stringent requirements apply for so-called black-white connections which occur frequently at the process-to-instrument interface. To avoid having 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.