DUAL-METAL MOLECULARLY BONDED PLUG ASSEMBLY FOR NON-LUBRICATED DOUBLE BLOCK AND BLEED PLUG VALVES AND METHOD OF FABRICATION THEREOF

Abstract: A plug assembly for plug valves is made of two distinct metals, one for the plug and another for the stem or trunnion. In a preferred embodiment, the plug is made of ductile iron and the stem or trunnion is made of high grade stainless steel. The two metals are molecularly bonded at their respective surfaces when molten iron is cast into a plug mold containing the steel stem. A mechanical bond is also provided in the form of a iron pin through an aperture in the stainless steel stem.

Description: DUAL-METAL MOLECULARLY BONDED PLUG ASSEMBLY FOR NON-LUBRICATED DOUBLE BLOCK AND BLEED PLUG VALVES AND METHOD OF FABRICATION THEREOF FIELD OF THE INVENTION The present invention relates generally to valves having a stemmed plug and more particularly to double block and bleed plug valves wherein the plug and stem are made of different materials. BACKGROUND ART Today's valve plugs are cast of one uniform material. Usually the trunnions (upper and lower) are cast integrally with the body of the plug. Sometimes, depending on structural requirements or producibility choices, the trunnions are welded on and generally a structured superior metal is used, but generally, not a more corrosion resistant material. The upper trunnion is the only portion of the plug that penetrates the pressure vessel and is therefore generally considered (but not code required) as qualifying as pressure containing. Some extremely large plugs are fabricated rather than cast. In this case the welded on trunnions are generally chosen for their weldability and structural integrity, not necessarily for corrosion resistant properties. With regard to the ste /trunnion's penetration of the pressure vessel, (at stuffing box and gland), code does not require the material to be an approved pressure vessel material. However, some valve manufacturing companies have chosen the cast plug material to be of pressure vessel quality just to obtain trunnion material of pressure vessel quality. Other companies less concerned about the risk of catastrophic failure, have not chosen pressure vessel quality material, but instead have selected low cost iron which indeed is inexpensive and exhibits good bearing properties, but provides cast iron trunnions which are not suitable for hazardous application due to their propensity to catastrophic failure. Many of the welded on trunnions are chosen for their material properties as a result of heat treatment and therefore, the heat affected zone of the weld, modifies these properties undesirably. Further, the stress riser caused by the geometric transition of the comparatively small cross-sectibn trunnion to large mass plug body in combination with and sometimes enhanced by the weld, further intensifies possible mechanical problems with all joint designs. SUMMARY OF THE INVENTION The optimum material combination in a plug having stems would be American Iron and Steel Institute 410 SS trunnion/stem which is commonly used for a variety of valve stems in the industry for its high strength, toughness, pressure vessel quality, corrosion resistance, availability, wear resistance, etc. and a plug body of ductile iron (Dl) since it is inexpensive (about 40% less than WCB grade carbon steel) and displays good strength and excellent wear and anti-galling properties important for slip support and movement. However, how does one attach a 410 SS bar to a cast Dl plug body? They can't be welded because they melt at 600° F difference in temperature. They can't be press fit because the Dl might crack and the parts would rotate within one another since iron has such good anti-galling properties. They could be attached mechanically, but then must withstand push, pull, CCW rotate and CW rotate motions. In the present invention we employ a bar of 410 SS and lay it in the mold and pour molten iron around it at 2950° Fahrenheit. One embodiment of the invention was constructed by pouring molten Dl around a variety of stem configurations, i.e., all of which passed the mechanical requirements test even with no mechanical joint. The manner in which a perfectly cylindrical stainless steel stem attached itself so voraciously to a ductile iron plug, was through molecular bonding at the surface. The bond does not seem to be uniform or predictable in a commercial foundry. Therefore, the preferred embodiment incorporates a hole drilled through the stem which allows molten iron to mechanically lock the stem and plug together in addition to the molecular bond. The invention provides the following advantages: 1) A plug having a) corrosion resistant quality; b) tough stem; c) industry standard stem metal; d) good stem metal availability; e) meets Nace MRO 175 requirements; 2) The joint between stem and plug is continuous and displays less stress risers; 3) No welding is performed; 4) Joining of parts is complete when received; 5) Iron plug body displays good wear and anti-galling characteristics; 6) No heat treatment required; 7) Minimum machine stock on trunnion required since shrink, gates, risers and warping are not an issue; 8) Bar stock is homogeneous so casting porosity and weld repair is eliminated on the only critical plug surfaces; 9) Cost is reduced by about 40%; 10) Back seating on a stainless surface is feasible; 11 ) New and old plugs may be interchangeable; 12) For balanced plug designs, a) both upper and lower stems may be installed; b) stem does not need plating, surface treatment or coating to inhibit corrosion; c) stem pin hole, which is difficult for corrosion inhibitor access, will not rust to stem pin; d) bar stock does not have sand pockets so the stem finish will be superior; 13) Ductile iron and 410 SS both machine easily; 14) Trunnion intersection with plug body when cast is frequently filled with casting solidification shrinkage resulting in porosity. With twin metal plug this does not happen. OBJECTS OF THE INVENTION It is therefore a principal object of the present invention to provide a twin metal plug assembly for a double block and bleed plug valve, the plug stems being made of a different metal than the plug to which the stem is made integral. It is another object of the invention to provide a twin metal plug assembly for a plug valve, the assembly having a ductile iron plug and at least one stem/trunnion made of stainless steel. It is yet another object of the invention to provide a plug assembly for a plug valve wherein a plug made of one metal is molecularly bonded to at least one stem made of another metal to provide benefits of the respective metals in regard to the diverse functions of the respective valve parts. It is still another object of the invention to provide a plug assembly for a plug valve, the assembly comprising a ductile iron plug and an integral stainless steel stem, the iron being melted around the steel to form a molecular bond therebetween. BRIEF DESCRIPTION OF THE DRAWINGS The aforementioned objects and advantages of the present invention, as well as additional objects and advantages thereof, will be more fully understood hereinafter as a result of a detailed description of a preferred embodiment when taken in conjunction with the following drawings in which: FIG. 1 is a partially phantom, partially cross-sectional view of a plug valve having a plug assembly shown in a finished condition in accordance with a preferred embodiment of the invention; and FIG. 2 is a partially cross-sectioned view of the plug assembly of FIG. 1 with the trunnion shown in the raw or "as cast" condition. DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT Referring to the accompanying figures, it will be seen that a double block and bleed plug valve 10 comprises an inlet 12, an outlet 14 and a body 15. Within body 15 there is a rotatably and vertically translatable plug 16 having opposing slips 18 and 20. Plug 16 provides a flow passage 22 which when aligned with inlet 12 and outlet 14, permits flow through valve 10. The position of plug 16 and thus passage 22 is determined by control of a stem 24. It is the material of which the plug 16 and stem 24 are made and their means of interconnection, that constitute the inventive features of the present invention. In the preferred embodiment shown herein, plug 16 is made of ductile iron and stem 24 is made of 410 stainless steel. The manner of interconnection is by surface molecular bonding which occurs when the stainless steel stem is placed in the casting mold for the plug. The mold is then filled with molten ductile iron such as at a temperature of 2,950 ° F. The molten iron forms the plug and surrounds the stem. A hole 25 in the stem is also filled with iron forming a pin 26. This assures a strong mechanical bond between iron and steel. However, the principal form of interconnection to form an integral plug assembly having an iron plug and a steel stem, is surface molecular bonding between the two components. The resultant assembly of a ductile iron plug and stainless steel stem, provides superior characteristics in the plug operation as well as in the stem quality as outline above. Those having skill in the valve art will now understand the structure and benefits of the invention. Moreover, it will also now be understood that other embodiments of the invention are readily constructed by using other metal materials for the plug, the stem or both. Accordingly, the scope of the invention is not limited by the specific embodiment illustrated and described herein, but only by the appended claims and their equivalents.

Claims: CLAIMS 1. A plug assembly for a plug valve, the assembly having a plug with a flow through passage for selective opening and closing of the valve, the plug also having at least one integral trunnion for controlling the orientation of the plug; the assembly comprising: an iron plug and a steel trunnion, the plug and trunnion being molecularly bonded together at their common surfaces. 2. The plug assembly recited in claim 1 further comprising a mechanical bond between said plug and said trunnion. 3. The plug assembly recited in claim 2 wherein said mechanical bond comprises an aperture in said trunnion and a pin formed in said plug and extending through said aperture. 4. A plug assembly for a plug valve, the assembly having a plug with a flow through passage for selective opening and closing of the valve, the plug also having at least one integral trunnion for controlling the orientation of the plug; the assembly comprising: a plug made of a first metal and a trunnion made of a second metal, the first metal having a lower melting point than said second metal, said first and second metals being molecularly bonded at the interface between said plug and said trunnion. 5. The plug assembly recited in claim 4 further comprising a mechanical bond between said plug and said trunnion. 6. The plug assembly recited in claim 5 wherein said mechanical bond comprises an aperture in said trunnion and a pin formed in said plug and extending through said aperture. 7. The plug assembly recited in claim 4 wherein said first metal is iron and said second metal is stainless steel. 8. A method for fabricating a plug assembly for a plug valve having a plug with a flow through passage for selective opening and closing the valve, the plug having at least one integral trunnion for controlling the orientation of the plug; the method comprising the steps of: a) placing a trunnion made of steel in a plug-shaped casting mold; b) heating iron to a temperature above its melting point to form molten iron; c) pouring said molten iron into said casting mold and around said steel trunnion; d) allowing said iron to cool and harden into a plug assembly; and e) removing said plug assembly from said mold. 9. The method recited in claim 8 further comprising the steps of: providing an aperture in said trunnion prior to step a); and permitting said molten iron to form a pin through said aperture during step c).

Inventors: MERCHANT HASHIMALI, SMITH GORDON M, MERCHANT, HASHIMALI, SMITH, GORDON, M.
Applicants: GEN VALVE INC, GENERAL VALVE, INC.
Application year: 2001
Main classification: F16K
IPC codes: B22D19, F16B5, F16D1, F16K5, F16K27