Some Aspects of Steam Turbine Valves: Materials, Operations and Maintenance

Abstract

Steam turbine valves are the most essential components of modern steam turbines from an operation, performance, reliability and safety aspects of a modern power plant. Current designs are pushing the operational envelope and it is not uncommon for large ultracritical plants to run on pressures exceeding 4500 psi and 1200 °F. These conditions are not only challenging for materials of construction for turbines and boilers but also for main steam turbine valves. The tendency of materials to oxidize at these temperatures is all too common causing problems for valve heads, stems, discs, bushings and seats. OEMs around the world are pushing to develop valve components with 9–12% Cr martensitic steels and nickel based alloys which offer better creep strength at elevated temperatures. For existing power plants at temperatures of a 1000 to 1050 °F range there is a push to retrofit valve components with Incolloy 901 type, Inconel 718 and Stellite alloys. Scale build up in traditional alloys happens too quickly for the usual two year maintenance cycle. The application of better alloys for steam turbine valves makes it possible to increase the maintenance cycle from two to four or even six years, while increasing the operational reliability of the valve. Elimination of main steam valve failures removes risks of turbine overspeed events and increases plant availability. Solid particle erosion is not forgiving on valve parts such as stems, discs and valve seats and over a period of time, excessive wear causes the valve to be rendered unsafe to continued service. Nitrided materials and chrome-carbide-coated materials are much harder than the stem base material; and to slow down wear, a nitriding process is used to develop a thin, hard, wear-resistant surface. Some of the material often used for Stellite liners are Nitralloy 135M, 410 SS, 422 SS Nitrided, Incolloy 901 Nitrided, 347 SS, 13Cr-13Ni-10Co-3Nb-2.5W-2Mo. Different OEMs use a variety of alloys for valve seats, discs and stems. Antigalling characteristics are particularly favorable. Valve casings are cast materials and usually specifications include the ASTM A217 and ASTM A356. The ASTM A217 cast steels are typically, 1.25Cr-0.5Mo Grade WC6 and the 2.25Cr-1Mo Grade WC9 materials. Some of the problems experienced with steam turbine valves, are sticking to the valve seat requiring excessive pull-out force, wear of the seat surface, valves not closing properly due to oxidation build up, Stellite weld cracking, cutting or gouging due to solid particle erosion. The material presented in this paper is of interest to fossil power plant personnel experiencing challenges on valve performance and maintenance. The paper looks at all aspects of steam turbine valves as far as current trends in valve material, operation and maintenance and lastly, looks at recent occurrences of valve failures leading to steam turbine overspeed catastrophic failures around the world.