PROFILE
EDF Energy is one of the UK’s largest energy companies and the largest producer of low-carbon electricity in the UK. A wholly-owned subsidiary of EDF Group, one of Europe’s largest energy groups, EDF Energy generates around one fifth of the UK’s electricity and employs around 15,000 people.
EDF Energy owns and operates eight nuclear power stations with a combined capacity of almost 9,000 megawatts providing the UK with 20% of its power.
Hinkley Point B nuclear power station in Somerset has two advanced gascooled reactors and was commissioned in 1976. It has two 660 MW turbo alternator sets and is capable of supplying electricity for more than 1.5 million homes.
CHALLENGE
With many more years of operation planned for Hinkley Point B, the station continues to invest in the plant and materials to ensure that safety, reliability and efficiency is maintained and improved further where possible.
Each of the boiler feed pump turbines are controlled by a pair of governor valves on a common steam chest. The two valves control the steam entering the 15MW turbines. These valves are modulated to control the flow of steam to the turbine from the 160 bar boiler pressure.
Wear and degradation to the boiler feed pump governor valve actuator covers was affecting reliability of the units between the planned major overhauls. This was primarily due to the increased throttling of valves for lower turbine loads resulting in larger forces being transmitted to the valve components.
Removal of the valve covers revealed wear rings where the harder piston rings had fretted into the bore. The bore has a traditional coating of Stellite 6 and the piston rings a harder cobalt alloy.
SOLUTION
Hardide Coatings’ principle hardface coating was identified as the most appropriate solution to extend the operational life between overhauls of the boiler feed steam turbine valves. It is cost effective and provides a tribological improvement to the original design.
The Hardide-A coating, which had suitable toughness, crack and impact resistance, and surface finish was employed to reduce the piston ring and bore friction coefficient. The only material change was the application of the coating to the valve piston sleeve liner to reduce system wear from abrasion, adhesion and fretting. This would transfer wear to the piston rings which are easily exchanged during planned overhaul outages.
The Hardide-A coating consists of low temperature CVD Tungsten Carbide nano-particles (1-2 nano-meters) dispersed in a tungsten matrix. It has a hardness of 800 to 1200Hv and can be applied up to 100 microns. The low temperature CVD, being both binder and cobalt-free lends itself to the nuclear industry given it will not produce radioactive and hazardous Co60 isotopes if irradiated.
The Hardide coating was particularly attractive because it could be applied in a uniform 50 micron layer which requires no post machining and could be accommodated by the existing piston ring tolerances.
Graham Young, steam & rotating plant engineer at EDF Energy, said: “The benefits of using Hardide-A were its anti-irradiation properties, its ability to uniformaly coat complex surfaces and its excellent tribological properties; which enabled us to continue using the existing and proven design of valves thus avoiding an expensive replant. These qualities ensured simplicity of the Engineering Change and minimum risk to operation”
“Hardide Coatings has provided excellent troubleshooting support to the technical challenges in applying the coating process. They also appreciate the importance of timely delivery so as not to affect the availability of the power plant equipment.”
The coating’s ability to easily coat internal surfaces evenly, its performance in testing and the ease of implementation and maintenance made it the solution of choice for EDF Energy. It enabled the company to continue to use the existing equipment which would be very expensive to replace as well as require validation and safety case development, and will allow EDF Energy to operate smoothly between planned outages.
EDF Energy has now been operating with coated valves on both turbine generator units for six years at Hinkley Point B.
