Reliability – the watch word for Sentinel31 March 1998
With some 150 safety-related motor-operated valves (MOVs) at a nuclear power station, valve performance is a critical issue for the US Nuclear Regulatory Commission. Faced with a growing number of performance problems occurring in the industry with MOVs, the NRC has instituted a number of measures that have led to increased outage times and costs for the plants. Taking the results of the NRC’s valve programme as well as the work of EPRI and the MOV Users Group, GE Nuclear Energy has developed its Sentinel gate valve family to address the main performance needs in terms of providing the long-term reliability
The operability problems that had plagued the industry with safety-related valves in the 1980s led to the NRC issuing its now famous Generic Letter 89-10 for MOVs, followed a few years later by GL 95-07 for all power-operated valves. These describe operability problems of valves operating under design basis conditions, including: higher than anticipated stem thrust; unpredictable valve behaviour; damage to the valve internals under blowdown/high flow conditions; significant degradation of performance when cycled under ?P and flow; thermal binding; and pressure locking.
The tests conducted in 1989 by the NRC highlighted the fact that the standard gate valves supplied by the industry required significantly more torque and stem thrust than recommended by the valve manufacturers, and also that damage to the valve internals occurred when attempting to close the valves under design basis, high flow conditions. Tests also revealed that the operating thrust in the opening direction significantly exceeded the valve manufacturers’ recommendations.
While GL 89-10 addresses these issues for all safety-related valves, it did not emphasise the significance of two particular phenomena – pressure locking and thermal binding – which can compromise valve operation by further increasing the opening thrust substantially. It was several years before GL 95-07 was issued to ensure that the industry does systematically identify and eliminate potential pressure locking and thermal binding problems.
These concerns led the NRC to introduce regulatory actions which have resulted in increased surveillance testing, outage time, and increased maintenance and ALARA costs to operators for assuring proper performance of these valves.
GE Nuclear Energy decided to address the industry valve problems with a “clean slate” approach, developing, in alliance with Kaisi Engineering, Inc and Ring-O Valve SpA, an improved gate valve that has predictable and repeatable performance and resolves the main operability issues – eliminating pressure locking and thermal binding concerns, maintaining leak tightness, and exhibiting virtually no degradation when subjected to a large number of cycles even under maximum ?P and blowdown conditions. The comprehensive approach taken to valve development included design optimisation by finite element and computational fluid dynamic analyses, separate effects testing, extensive flow loop testing under blowdown as well as thermal binding conditions and plant in-situ testing.
After careful evaluation of the trade-offs in alternative gate valve design approaches, it was decided to base the new design on the one-piece wedge gate valve construction. The Sentinel design capitalises on the inherent simplicity and superior leak tightness of this type of valve, compared in particular to parallel slide gate valves of the wedge disc principle, while eliminating its shortcomings.
The new design has been demonstrated to provide predictable performance with little or no degradation of valve internals even under repeated severe blowdown conditions. It has been qualification tested under full flow to verify its functionality under high pressure pumped flow water and steam conditions for compatibility with nuclear system applications. Four Sentinel Valves installed at the Pilgrim station (a GE BWR/3 reactor design) are continuing to validate its long-term functionality and leak tightness performance: two 6 inch Class 900 valves in the reactor water clean-up system as inboard and outboard containment isolation valves and two 10 inch Class 900 valves in the high pressure coolant injection system as turbine steam admission and outboard containment isolation valves.
Sentinel valves are scheduled to start operation this year at the Limerick 1 plant (a 12 inch CS HPCI turbine steam admission valve) and at Laguna Verde 1 & 2 (four 20 inch SS recirculation system pump discharge gate valves).
A number of innovative, but simple features were incorporated in the Sentinel design to help assure functional reliability and to address potential weaknesses:
• For valves subject to pressure lock-up applications, an option is offered to provide an integral communication passage between the valve bonnet cavity area and the upstream or downstream side of the valve.
• The patented flexible wedge disc design limits the additional induced unwedging thrust resulting from thermal binding or pressure pinching scenarios. This helps ensure valve opening consistent with the structural capability of the valve and typical actuator designs used.
• The valve incorporates live loaded graphite stem packing and a pressure seal at the body-to-bonnet closure to provide optimum leak tightness with time and minimise maintenance problems associated with bolted closures. A two piece stem with coupling is also offered as an option to further minimise maintenance time on the valve.
• To enhance long-term leak tightness across the valve, the wedge design ensures that the stem thrust is delivered from the stem to the disc centre section, which is axisymmetrically connected to the two disc sealing faces. This in turn results in a uniform sealing stress applied at the disc/seat interface along its entire circumference. Exceptional leak tightness performance was demonstrated during high flow qualification testing on the test unit which was subjected to 86 high pressure closing and opening ?P conditions.
ENDING PRESSURE LOCKING AND THERMAL BINDING
Eliminating pressure locking problems is relatively straight forward, and a number of acceptable approaches are available to suit the application-specific requirements. The new design offers the option of an internal communication passage between the valve body bonnet area and the upstream or downstream side of the valve to eliminate pressure locking. This is the standard approach offered by other valve manufacturers as well.
Thermal binding problems, on the other hand, are more difficult to quantify and mitigate in conventional flexible wedge gate valves. The two classical thermal binding scenarios include: 1) closing a valve that has been open and flowing hot fluid and allowing it to cool prior to reopening; and 2) closing a valve under cold condition and heating the valve up from one side only prior to opening. In both scenarios, the differential expansion characteristics due to temperature and/or material differences between the disc and the body, as well as between the stem and the yoke, can cause an increase in mechanical interference. It is possible for the disc to be bound so tightly due to the increased interference that reopening is either difficult or impossible until the valve is reheated.
The magnitude of increase in force at the seat faces (caused by mechanical interference due to temperature differences), as well as the increase in the unwedging /opening thrust, is directly proportional to the stiffness of the disc. The magnitude of increase in opening thrust can be substantial – in some cases exceeding the structural strength and causing failure of the “weak link”, the stem-to-disc T-slot connection. However, no quantitative stiffness criteria have been developed by the industry to eliminate thermal binding problems in wedge gate valves. In general, the conventional wedge disc design, especially the higher pressure class valves, can be too stiff and susceptible to thermal binding problems.
The disc in the new valve design has been engineered to meet a rigorous disc flexibility criteria that help ensure that the worst-case thermal binding scenarios would not cause the opening thrust to be higher than the closing thrust.
Extensive computational fluid dynamics and finite element analyses were performed to develop the bounding scenarios for thermal binding. The new disc geometry differs from conventional flexible wedge discs in that it has a central section in which a longitudinal slot (perpendicular to the flow axis) of appropriate dimensions can be incorporated to achieve the desirable disc flexibility. Full scale tests have been conducted to simulate postulated worst-case scenarios and to validate the analytical model predictions thermal binding is eliminated.