Hydraulic nuts have revolutionized the make up and disassembly of critical flanges. Some of their advantages are as follows:
- Typically reduce, by 90 percent or more, the time it presently takes to install or remove existing nuts. A large flange can be completely tightened in less than one hour;
- Quickly pay for themselves in time and manpower, taking only minutes instead of hours or days;
- Totally eliminate the need for any conventional tools, the only tightening tool needed is a small tommy bar;
- All studs are tightened in a single operation to accurately achieve identical clamping pressure on each and every bolt;
- Deliver exceptionally powerful bolt loads to ensure superior gasket sealing; and
- Very compact, ideal for constricted areas.
Dominion Energy Surry Nuclear Plant focus on improving reliability on critical assets led to the review of the assembly and disassembly problems experienced with governor valves on Units 1 and 2.
Surry’s reliability-centered maintenance practice puts the governor valves on a 39-month inspection frequency. These inspections form part of the over speed analysis requested by the turbine vendor. On average, two valves are opened during each refueling outage.
Old Method of Assembly and Disassembly
Previous to the installation of hydraulic nuts, the governor valves were disassembled, first using slugging wrenches and hammers and then later using a hydraulic torque wrench. The time to disassemble one valve bonnet consisting of 32, 1.75” nuts was approximately one shift for two millwrights. Problems regularly resulted with each disassembly with the galling of several bonnet nuts and bonnet stud holes.
The bonnet studs would need to be cut to enable the removal of the valve. Removal of the bonnet studs would usually result in damage to the bolt holes. The holes were first re-tapped and if the threads were still damaged, a helicoil was installed or the bolt hole machined and threaded oversized to accommodate a step stud. Repaired holes were sometimes not perpendicular to the valve flange, contributing to difficulty in installing the valve bonnet.
On reassembly, hydraulic torquing methods were employed using two millwrights for one shift (8 to 12 hours) for each valve bonnet. A multi-pass (up to eight passes) method of torquing was used to bring the bonnet in parallel with the valve body. Gap measurements were taken to control the gap between the mating faces to within .002”. A final torque of 2200 ft-lbs. was applied.
Disassembly and Reassembly using Hydraulic Nuts
The hydraulic nuts were designed to replace existing cap nuts and fit to the valve bonnet. Details of the governor valve hydraulic nuts are shown in Figure 1. 1.75” x 8UN Hydraulic Nut.
The nuts were assembled and tightened following the procedure outlined above in Hydraulic Nut Operation. The nuts were simultaneously tightened, bringing the bonnet down to the valve body uniformly. The highly accurate hydraulic nuts maintained the gap specification of .002”. After completion of the tensioning operation, an air line was connected to the nipple and the small volume of oil found in the hydraulic nut was blown out. The hydraulic nipples were removed, and replaced with plugs to prevent contamination from entering the nut. A final assembled view of the governor valve with the hydraulic nuts is shown in Figure 9. Final Assembly.
The total time to reassemble 32, 1.75” bonnet bolts was 30 to 45 minutes per valve.
The first sets of nuts for two governor valves were installed in October 2000. One valve was disassembled in March 2002. The total disassembly time was also 30 to 45 minutes with no galling or other damage to the studs, nuts or bonnet stud holes.
Significant savings in time, manpower, repair and materials cost were realized using hydraulic nuts. These are summarized in Table 1. Governor Valve Installation Comparison.