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Many steam turbine valve manufacturers are using advanced materials for the manufacture of valve components, such as stellite bushings and incoloy stems, which are much stronger and oxidation resistant. Typical valve manufacturers include General Electric (GE), Westinghouse, Toshiba and Allis Chalmers. The control mechanisms for valve movement such as Digital Electro Hydraulic Control (D-EHC) via Electro Hydraulic Control (EHC) oil supply to the valve actuator are now very reliable for steam turbine valve control. The steam turbine valve remains the most important link in preventing overspeed events for the modern turbine generator. In this paper, a real life example will be presented with an in-depth discussion of turbine valve maintenance.
A case study for valve inspection
The TS Power Plant located in Northeastern Nevada began operations in June 2008 and underwent its first valve inspection after two and half years in 2010. The plant has a gross power output of 242 MW at full load and the design inlet pressure to the HP turbine section is 2,415
The reheat stop valve and intercept valves (CRV) also are combined in one body. There is only one MSV/CV assembly on the main steam supply line to the HP turbine section and one RSV/IV assembly on the hot reheat steam line to the IP turbine section. The main valves can be seen in Figures 1a, 1b and 2 for the MSV/CV and RSV/IV.
The preferred mode of operation at the TS Power Plant is the boiler sliding pressure mode with steam turbine valves operating wide open (VWO). The valves are exercised as recommended by the OEM. In the VWO mode of operation, valves are subject to pick up magnetite particles from the boiler during start up, which are deposited on essential valve parts, as will be seen for this case when the valves underwent their first inspection since commissioning. The valve disassembly, inspection and re-assembly were performed according to recommended procedures.
The inspection can be summarized as follows:
- Inspecting for defects and foreign deposits on the valve strainer
- Checking for contact areas between the valve and the valve seat
- Looking for any deposited substances on the valve stem and the bushings
- On sliding parts of the valve stems, checking for wear, seizure and damage
- Inspecting to see if there were any signs of steam leakage through the flange
- Examining if defects were present in the cross head, pin, bearing, etc.
- Checking to see if defects were present in the bolts and nuts of the valves
- Damage to the steam control valve and stem was investigated, as well as the strainers to the MSV and valve seat
- Looking for cracks, bending and wear of the valve stem, as well as contact and damage of the inner bypass valve
- Investigating any sign of cracks and defects in the valve body, especially in the welded portion
Other items inspected on the valves were (i) status of corrosion (ii) galling on bolts, nuts and screws, and cracks and deterioration of washers, (iii) measurement of clearance between the valve stem and the seal head or the bushing, (iv) wear and clearance between the sliding parts, (v) defects in the springs and (vi) defects in the joints of valve stems.