Abstract
Rolls-Royce has many years of experience of satisfactory performance of cobalt based valve seats used in Pressurised Water Reactor (PWR) environments.
During design development testing of a pilot operated pressure relief valve for a PWR application, rapid degradation of the main seat was observed. This paper reflects on the issues encountered during testing and the investigation into potential degradation mechanisms and causes of the failure.
Excessive leakage of the valve seat was experienced after a very low number of operating cycles and short test duration. Visual inspection of the valve identified damage to the weld deposited, cobalt-based alloy (Co – 25Cr – 5W – 1C) contact faces of the main seat. A metallurgical examination of the damaged surfaces subsequently concluded the most likely cause to be flow erosion-corrosion. This was supported by a literature review of valve seat failures and laboratory studies on wear and degradation of cobalt-based alloys.
An unrelated failure had occurred earlier in the development test programme, which is believed to have introduced a minor surface defect onto the valve seat face. It is believed that this defect created the conditions required for flow erosion-corrosion to occur.
Furthermore, the investigation concluded that given the right environmental conditions and surface defect, flow erosion-corrosion of cobalt-based alloy (Co – 25Cr – 5W – 1C) can occur rapidly. This can cause extensive surface damage over a very short duration, resulting in excessive valve seat leakage. This highlights the importance of pre-service Non-Destructive Examination (NDE) of Pressure Relief Valve seating faces, specifically the allowable acceptance criteria and method applied.
The results of the metallurgical examination and literature review are presented, along with the investigation into the cause of the observed damage to the valve seating faces.
