FIELD OF INVENTION
The present invention relates to a method of manufacturing hard faced seat assembly in blow down (pressure drop) valves for high pressure steam generators. More particularly the invention relates to the development of a hard faced stellite seat ring that eliminates critical dissimilar welding inside the blow down valves.
BACKGROUND OF INVENTION
Blow down valves (BDV) for high pressure steam generators are manufactured as per boiler and pressure vessel section codes and power piping codes and are used us boilers for steam boiler blow down, water column drain, shut of and stop functions.
Figure 1 illustrates a class 1500 blow down valve comprising micrometer adjustment (1) for hand wheel setting, stem disc assembly (2) and a seat ring
(3).
Referring to figure 2, this seat ring (3) has two components - cylindrical
grooved seat (4) and stellite material filling (5) by melting stellite electrodes,
which helps in cushioning the stem-disc assembly. Hard faced (stellited) seat ring

provides high resistance to galling, wear and erosion. In practice, the seat ring (3) is fabricated by filling the cylindrical groove (4) of approximately 40mm diameter and 40mm depth with stellite material (5) using Oxyacetylene Welding (OAW) process. This method was highly prone to defects like cracks and porosity due to the high deposition of stellite and the required skill level. Hence there is a need to find out an alternate method to overcome the said drawbacks.
OBJECTS OF THE INVENTION
Therefore, it is an object of the invention to propose a method of manufacturing hard faced seat assembly in blow down (pressure drop) valves for high pressure steam generators which is capable of reducing the repair / rejection level of the components to negligible range.
Another objection of the invention is to propose a method of manufacturing hard faced seat assembly in blow down (pressure drop) valves for high pressure steam generators which is able to eliminate critical dissimilar welding inside the blow down valves.
A still another object of the invention is to propose a method of manufacturing hard faced seat assembly in blow down (pressure drop) valves for high pressure steam generators which is capable of eliminating the high volume of stellite material welding.

Further object of the invention is to propose a method of manufacturing hard faced seat assembly in blow down (pressure drop) valves for high pressure steam generators which is able to reduce the welder fatigue and conversion of the welding technique from very high skill oriented to a merely skill oriented.
SUMMARY OF THE INVENTION
The hard facing (stellite welding) in blow down valve seat is replaced by manufacturing a new seat assembly with two components containing outer sleeve having the same material specification as that of the material used in the blow down valve and stellite bush machined from a bar stock.
The stellite bush is welded with outer sleeve using conventional Gas Tungsten Arc Welding (GTAW) process. Thus the hard facing which was performed manually (described in the background of the invention) has been replaced by the above process and installed inside the blow down valve, thereby substantial saving in the volume of stellite welding and hence reduction in the cycle time of fabrication and reduction of operator fatigue. By this new method of manufacturing of seat assembly, rejection is eliminated as well as dissimilar weld inside the valve is eliminated.

BRIEF DESCRIPTION OF THE ACCOMPAYING DRAWIG
Fig.l - Shows a cross-sectional view of pressure drop (blow down) valve
with hard faced seat assembly.
Fig.2 - Shows a cross-sectional view of seat ring with cylindrical grooved
seat and stellite material filling according to prior art.
Fig.3 - Shows a cross sectional view of new seat assembly having a stellite
bush inserted and welded in a sleeve made of similar material as that of the valve according to the invention.
DEATILED DESCRIPTION OF A PREFEINVENTION
in the proposed invention, referring to the figure 3, a stellite bar stock (7) is taken and machined to suit inside a sleeve (8) of the material having same specification as that of the blow down valve. The machined stellite bush (7) is welded (6) with the sleeve (8) by conventional Gas Tungsten Arc Welding (GTAW) process to form a seat assembly, which can then be welded inside the blow down valve body by conventional Shielded Metal Arc Welding (SMAW)

process after a non destructive test (NDT). By the selection of the same material specification of the sleeve (8) of that of the blow down valve body, eliminates the dissimilar welding in a critical position where GTAW is not feasible. Thus by this new method of inserting bar stock (7) into the sleeve (8) of required shape, the drawbacks said in the background of invention are eliminated. The weldment connecting the sleeve (8) and stellite bush (7) is shown as item number 6. The improved welding technique gives optimum cycle time reduction and enhances
product quality.
Therefore the method of manufacturing seat assembly comprising the
steps of:
1. The machined stellite bush (7) is inserted inside the outer sleeve (8).
The sleeve material is selected in such a way that the same material specification of that of blow down valve body.
2. The machined stellite bush (7) is welded with sleeve (8) using
conventional GTAW process.
3. The above said seat assembly is placed inside the blow down valve
body and welded by conventional SMAW process, after performing non
destructive test (NDT).

WE CLAIM
1. A method of manufacturing hard faced seat assembly in blow down (pressure drop) valves for high pressure steam generators comprising the steps of: machining a stellite bar stock (7) to suit inside of a sleeve (8); selecting the material of the sleeve (8) same as that of blow down valve
body;
inserting the machined stellite bush (7) inside the outer sleeve (8);
welding (6) the stellite bush (7) with the sleeve (8) by conventional gas tungsten arc welding (GTAW) process;
wherein the above seat assembly is then placed inside the blow down valve (V) body and welded by conventional shielded metal arc welding (SMAW) after performing a non destructive test (NDT) so that a hard faced stellite seat ring is provided in the valve body.