Valve Seat Repair
Mechanical Dynamics and Analysis inspected and repaired a KN turbine’s HP and IP MSV and CV seat areas. There is a history of these units having stellite bonding issues. MD&A can perform valve seat repair on-site or in-shop!
The HP valves were the first to be inspected in-shop. It took very little effort to liberate the stellite from these valves. In some cases, the entire inlay popped completely out of the valve seat, which indicated very little, if any, bonding to the parent metal. It is our valves project manager’s expert opinion that a butter pass of either Inco or 309L was not inlayed between the parent metal and the stellite inlay. It has also been his experience that the butter pass is a must to ensure proper bonding takes place.
This is the stellite inlay from one of the HP valves. Obviously there was a bonding issue, specifically a lack thereof. The lack of bonding was found to be worse on the HP valves than it was on the IPs. MD&A has experienced this issue on several occasions, and our preferred method of repair is to remove all of the stellite, along with an amount of parent metal, to excavate any of the material that may have migrated.
We then pre-heat and weld build up using for 410SS filler material. After a PWHT, the valve seats are then machined to the as found contour, and final lapped to achieve 100% blue contact with the associated disc. The method of welding is semi-automatic gas tungsten arc welding (GTAW). This is performed with a machine that was developed by MD&A and built in conjunction with an industry leader in the manufacture of semi-automatic welding equipment. Below is a picture of that machine mounted to the end of a valve body.
All valve seats were overlaid with 410SS, except the LS HP CV. The parent metal was full of porosity/trash and a solid overlay could not be produced. MD&A machined out enough parent metal to allow for the installation of a shrunk in seat insert that was made from 410SS forging.
It was then seal welded both in the bore and in the front face of the valve seat. That was done with a 500°F pre-heat, and no PWHT was performed. After machining, all seats were lapped and 100% blue check was achieved.
Below is a picture of a completely refurbished valve seat, showing the blue contacts from the disc to seat check.
MD&A also performs valve seat repair in-place or on-site, saving you time and money! A fabricated seat can be installed in the valve body and seal welded in lieu of a complete weld repair solution for the seating surface.
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