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TRANSFER LINE AND VALVE ASSEMBLY COMBINATION FOR HANDLING MOLTEN LIQUIDS Disclosed herein is an electrically-heated transfer line and valve assembly combination which is particularly adapted for transferring molten metals from one point to another. The valve assembly is basically a cylindrical valve body having a slidable stem therein, with the stem being moved between open and closed positions by mechanical power, such as an air cylinder or hydraulic cylinder. In the apparatus described herein the transfer line and valve assembly form a part of the same electrical circuit, the objective being to achieve uniform heating throughout the entire transfer system. Specific applications of the valve assembly include its use as an end valve on a transfer line in die casting or permanent mold casting, and as a metering valve in cold chamber die casting of magnesium alloys.
What is claimed is: 1. In an apparatus for transferring a molten liquid from one point to another, the combination which includes: a. a transfer line for conducting the molten liquid, including a valve assembly connected to the outlet end of the transfer line, b. the valve assembly being defined by a cylindrical valve body having a neck portion at the upper part of the valve body, a cone-shaped valve seat at the lower end of the valve body, and a cylindrical stem slidable within the valve body, whereby c. the central portion of the valve stem is in sliding contact with the neck portion of the valve body and the upper and lower portions of the valve body are spaced from the valve stem to define an upper annular chamber above the neck portion and a lower annular chamber below the neck portion, d. the lower chamber is connected into the outlet end of the transfer line and is adapted to receive a molten liquid from the transfer line, e. a packing material is positioned in the upper chamber to provide a liquid-tight seal between the valve stem and valve body, f. the valve stem further includes a hemispherical lower end adapted to sealingly engage the cone-shaped valve seat, a central bore which extends lengthwise through the valve stem, an inlet channel in communication with the upper end of the stem bore, for directing a fluid material into the bore, and several outlet channels which connect the lower end of the stem bore with the lower end of the valve stem, for discharging the fluid material into the valve seat, g. the upper end of the valve stem is connected by a coupling member to a power means adapted to move the valve stem up and down in the valve body, and h. the transfer line and valve body are connected by an electrical circuit into a source of electrical power, to thereby heat the transfer line and valve assembly. 2. The apparatus of claim 1 which includes a second electrical circuit connecting the valve stem with a source of electrical power. 3. The apparatus of claim 1 in which the molten liquid being transferred is a magnesium alloy or a zinc alloy. 4. The apparatus of claim 1 in which the fluid material directed into the valve stem bore is a protective gas. 5. The apparatus of claim 1 in which the packing material is graphite. BACKGROUND OF THE INVENTION In a typical cold chamber process for die casting light metals, such as magnesium alloys, the molten metal to be die cast is held in a suitable vessel, such as a crucible or melting pot. In one of the conventional magnesium alloy die casting procedures, the molten metal is transferred from the holding vessel to the shot well of the die casting machine through a heated transfer line, which includes a metering valve at the outlet end of the transfer line. The metering valve commonly employed in this process is comprised basically of a cylindrical valve body having a slidable stem therein. The stem is moved between open and closed positions (i.e., up and down) usually by mechanical power, such as an air cylinder or hydraulic cylinder. With the valve in open position, the "shot" is made by allowing the desired quantity of the molten metal in the valve body to flow out through a cone-shaped valve seat at the lower end of the valve body and into the shot well. The apparatus utilized in the die casting procedure described above is not entirely satisfactory, however, because of various problems encountered in the operation of the metering valve. For example, in order to successfully transfer the molten metal from the holding vessel to the shot well, the transfer line and metering valve must be kept heated to a temperature above the melting point of the metal. Usually, the transfer line and valve are heated by enclosing these components in a gas-fired heater shroud. A primary disadvantage of the heater shroud, however, is that it is very difficult to heat the valve assembly uniformly with such a heating system. Using the heater shroud method, for example, it is difficult to heat the stem seal area, i.e., the point of sliding contact of the upper valve stem with the valve body. This is particularly true in a valve assembly in which a packing material is used to provide the desired liquid-tight seal at the stem seal point. In this situation, therefore, if the molten metal in the valve body comes in contact with a "cold" valve stem, it will freeze and deposit a layer of solid metal on the stem. On the upstroke of the stem, therefore, the frozen metal film will tear out the softer packing material and destroy the stem seal. One solution to the problem of stem seal damage is to allow the molten metal to only partially fill the valve body, i.e., so that the level of the molten metal is below the stem seal area. This procedure is also undesirable, however, in that a protective gas atmosphere must be maintained in the unfilled portion of the valve body to prevent surface oxidation of the liquefied metal. Another disadvantage in this procedure is that the maximum pressure that can be exerted against the metal must be controlled to prevent the molten metal from rising too high in the valve. SUMMARY OF THE INVENTION Accordingly, a broad object of the invention is to provide an apparatus that is useful for transferring molten liquids from one point to another which does not have the drawbacks of the prior apparatus. A more specific object is to provide an apparatus particularly adapted for transferring molten metal which comprises the combination of an electrically heated transfer line and valve assembly, wherein the valve body functions as a resistance element in the heating circuit. Another object is to provide an apparatus as described in which the valve assembly is suitable as a metering valve for a cold chamber die casting machine. Another object is to provide an apparatus as described in which the valve assembly is suitable as an end valve on a line for transferring molten liquids. Broadly, the invention provides an apparatus for transferring molten liquids from one point to another. The basic apparatus comprises a transfer line for conducting the molten liquid, which includes a valve assembly connected to the outlet end of the transfer line. The valve assembly is defined generally by a valve body which includes a neck portion at the upper part of the valve body, a cone-shaped valve seat at the lower end of the valve body and a cylindrical stem within the valve body. The central portion of the stem is in sliding contact with the neck portion of the valve body and the upper and lower portions of the valve body are spaced from the stem to define annular chambers above and below the neck portion. The lower chamber, which connects into the transfer line, provides a reservoir for receiving molten liquid from the transfer line. A packing material is positioned in the upper chamber to provide a liquid-tight seal between the valve stem and valve body. The valve stem further includes a hemispherical lower end adapted to sealingly engage the cone-shaped valve seat and a central bore which extends lengthwise through the stem. Communicating with the upper end of the stem bore is an inlet channel for directing a fluid material into the bore. The fluid material is discharged into the valve seat through several outlet channels which connect the lower end of the stem bore with the lower end of the valve stem. The upper end of the valve stem is connected by a coupling member to a power means for moving the stem up and down in the valve body. To keep the liquid being transferred in a molten state, the transfer line and valve body are heated by a common electrical circuit which receives current from a source of electrical power. DESCRIPTION OF THE DRAWINGS FIG. 1 is a side elevation view, partly in section and partly schematic, of a conventional die casting apparatus which includes a transfer line and valve assembly combination according to an embodiment of this invention. FIG. 2 is an enlarged detail view of the valve assembly illustrated in FIG. 1. DESCRIPTION OF A PREFERRED EMBODIMENT Referring to the drawing, the numeral 10 designates generally a conventional cold chamber apparatus of the type employed in die casting of a magnesium alloy. The apparatus 10 includes a holding pot 11 for containing a molten metal 12 to be die cast. From the holding pot 11 the liquid metal 12 is pumped into a transfer line 13 by means of a pumping assembly in the holding pot. In general, the pumping assembly comprises a pump 14 including an impeller unit 15. The impeller unit is positioned in a metal-conducting passageway 16 which is defined in a housing enclosing the unit. The metal inlet end of transfer line 13 is connected into the metal outlet end of passageway 16 by a two-part coupling member 17. A valve assembly, as indicated generally by numeral 18, is connected to the metal outlet end of transfer line 13. In a metal die casting operation, the valve assembly 18 may provide a means for metering a "shot" of molten metal from transfer line 13 into the shot well 19 of the die casting machine. The transfer line and valve assembly are electrically heated to keep the metal 12 in a molten condition during transfer from the holding pot 11 to shot well 19. One side of the electrical circuit comprises a lead 20, which connects at one end to electrode 21 mounted on transfer line 13 and at the opposite end to a transformer. The other side of the circuit comprises a lead 22, which connects at one end to an electrode 23 mounted on valve assembly 18, and at the opposite end to the transformer. The transformer, in turn, is connected into a source of power by a lead 24. Referring particularly to FIG. 2, the valve assembly is illustrated in enlarged detail. In general, the valve assembly 18 is defined by a cylindrical valve body 25 and a cylindrical valve stem 26, which is slidable within the valve body. In the operating position of stem 26, the stem is in sliding contact with the inner wall surface of a neck portion 27, which is generally defined at the upper part of valve body 25. Above neck portion 27 the inner wall surface of valve body 25 is spaced from valve stem 26 to define an upper annular chamber within the valve body. Also, below neck portion 27, the inner wall surface of valve body 25 is spaced from valve stem 26 to define a lower annular chamber in the valve body. At the metal outlet end of transfer line 13, the line connects directly into the lower annular chamber of valve body 25. The lower chamber of the valve body, therefore, provides a reservoir for receiving the molten liquid 12 from transfer line 13. The upper annular chamber of valve body 25 is filled with a packing material 28, which is compressed and held in place by a packing gland 29. The packing material 28 provides a liquid-tight seal between the valve stem 26 and the inner wall surface of the valve body. The valve body 25 and transfer line 13 are covered by a conventional high temperature heat insulating material 30. Suitable packing materials include graphite and asbestos. The preferred packing is a flexible graphite material sold under the name Graphoil. A preferred insulating material is a stable, high temperature alumina-silica ceramic fiber, which is sold under the name Kaowool. The lower end of valve body 25 is defined by a cone-shaped valve seat 31. The lower end of valve stem 26 defines a hemispherical tip which will wedgingly engage the valve seat 31 when the valve is in shut off position. Extending lengthwise through valve stem 26 is a central bore 32. A nipple fitting 33 is attached to the upper end of valve stem 26. The nipple fitting is in direct communication with a small crosswise bore 33a, which intersects with the upper end of lengthwise bore 32. The fitting 33 and bore 33a, therefore, provide an inlet channel for directing a fluid material into bore 32. At the lower end of bore 32, several outlet channels 34 extend downwardly and outwardly from bore 32 and extend on through the hemispherical tip of valve stem 26. One example of the use of bore 32 is in a magnesium alloy die casting operation. In this type of operation a gas, such as argon, is directed into bore 32 through the fitting 33 and bore 33a and is discharged through the outlet channels 34 into the lower end of valve seat 31. The purpose of the gas is to provide a protective atmosphere which will prevent oxidation of the small amount of molten metal which remains in the valve seat after each "shot." The upper end 26b of valve stem 26 is connected to the lower end of a piston rod 35 by a split coupling 36. Piston 35, which moves the valve stem 26 up and down in valve body 25, is operated by a power cylinder 37. Cylinder 37 is preferably a fluid power cylinder, such as an air cylinder, hydraulic cylinder, or the like. A mounting assembly for cylinder 37 is provided by a lower mounting plate 38, which is attached to valve body 25, and an upper mounting plate 39. The mounting plates are tied together by rod supports 40 and 41. The upper part of the left half of split coupling 36 includes an ear member 42 on which is mounted an upstanding threaded stud member 43. Similarly, the right half of split coupling 36 includes a corresponding ear member 44, on which is mounted an upstanding threaded stud member 45. Stud members 43 and 45 are in direct alignment with the underside of mounting plate 39, so that the top of each stud member can bump against plate 39 on the upstroke of valve stem 26. The stud members, therefore, function as adjustable stop members for adjusting the length of stroke of the valve stem 26. An optional feature of the valve assembly 18 is a helical compression spring 46, which is carried on piston stem 35 and is fitted between the underside of mounting plate 39 and the upper face of split coupling 36. Spring 46 provides a return means for urging the valve stem 26 downwardly into the seating position in valve seat 31 (shut off position) after the upstroke is completed. An additional function of the spring is to act as a safety device. For example, in case of a sudden loss of power to cylinder 37, the spring 46 will automatically return valve stem 26 back to the seating position, so that the valve is immediately shut off. In practice, it is contemplated that the valve assembly of this invention may be employed in any of various applications requiring transfer of a molten liquid from one point to another. Representative of such applications are the use of the valve assembly as an end valve on a pipe line for transferring molten metals, such as magnesium or zinc alloys, molten salt compositions, and the like. With regard to molten metals, the valve assemblY is particularly adapted for handling molten magnesium alloys in die casting and in permanent mold casting operations. Specific applications include use as a metering valve in cold chamber die casting, as an end valve on an electrically heated transfer pipe, as an end valve on a pipe line used for casting ingots, and as a control valve for filling a permanent mold in a permanent mold casting process. To illustrate the practice of the invention, the use of the valve assembly 18 as a "shot" metering valve for a cold chamber machine for die casting a magnesium alloy, will now be described. As mentioned previously, it is essential that certain internal areas of the valve body be kept uniformly hot, i.e., at a temperature above the melting point of the metal. In the present valve assembly there are two critical areas in the valve body 25 which require uniform heat. One of these areas is defined by the upper chamber around the valve stem 26, which includes the packing material 28. The other critical area is that part defined by the lower chamber which contains the molten metal 12. To keep the critical areas of the valve body heated, the apparatus 10 is constructed such that the valve body 25 is connected into a common electrical circuit which heats the transfer line and the valve assembly. For example, as shown in the drawing (note particularly FIG. 1) the valve body 25 is connected into one side of the circuit through electrode 23 and lead 22, which, in turn, connects into a transformer and power source. The other side of the circuit is formed by transfer line 13, which connects into the transformer and power source through electrode 21 and lead 20. By placing the valve body 25 directly in the current path, therefore, the valve body actually functions as a resistance element in the circuit. Since the valve body does act as a resistance element, several factors must be considered in designing the valve body. For example, the amount of electrical power required to heat the valve body to the desired operating temperature will depend on variables such as the physical properties of the molten liquid being handled, the amount and type of insulation used to cover the valve body, the material used to construct the valve body, and the cross-sectional area of the valve body. Since electrical resistance heating is proportional to the effective resistance offered by the current conductor, it is of prime importance to design the valve body with a cross-sectional area which will provide the desired amount of effective resistance. The amount of alternating current power required, therefore, can be determined according to the formula P = I.sup.2 R, in which P = power, I = current and R = effective resistance. From this formula, therefore, it can be concluded that the amount of heat or power dissipated can be controlled by changing either the current input or by changing the amount of effective resistance offered by the conductor. The usual way to regulate the current input is to control the source voltage. The direct current resistance offered by the conductor, however, is an inherent property of the conductor, which depends on factors such as the resistivity of the material which comprises the conductor, the length of the conductor, and the cross-sectional area of the conductor (the area which lies perpendicular to the direction of current flow). Resistivity is determined according to the formula R = K ((L/A), in which R = resistance, K = resistivity, L = length and A = cross-sectional area. The effective resistance encountered with alternating current is greater due to the skin effect, eddy currents, and hysteresis losses in the metal pipe. Based on the foregoing principles, therefore, it will be apparent that the resistance characteristic of the valve body can be altered in several ways. One way is to vary the cross-sectional area. Another way is to change the type of material used to construct the valve body. Understandably, the choice of materials of construction is limited to those materials which are compatible with the molten liquid being handled by the valve assembly. To successfully handle molten magnesium alloys, for example, materials of construction are limited almost exclusively to the various chrome steel compositions. The reason for this is the highly corrosive nature of molten magnesium. Since the K factor (resistivity) of most steel compositions is very similar, however, it is generally considered impractical to attempt to alter the resistance characteristic of the conductor (valve body) by a change of material. Since the area defined by the upper part of valve stem 26 and packing material 28 is difficult to heat, several solutions have been proposed to alleviate the problem. One solution is to vary the cross-sectional area of the upper part of the valve body 25, which is achieved by decreasing the thickness of the upper valve body wall. Another solution is to connect the valve stem 26 into a second electrical circuit. In this arrangement, therefore, the valve stem, like the valve body, will function as a resistance element to add additional heat to the valve stem-packing area. As partly shown in FIG. 2, the second circuit is provided by an electrical lead 47, which connects the valve stem 26 with the power source through an electrode 48 mounted on the valve stem. If the resistance of the valve stem is not sufficient to provide the desired amount of heat, the resistance can be increased by altering the cross-sectional area of the stem. Another factor which must be considered in the electrically-heated transfer system of this invention is the overall resistivity offered by the transfer line-valve assembly combination. In other words, the relative sizes of the transfer line and valve assembly must conform to the extent that each part of the combination has essentially the same amount of electrical resistance per unit of length. The reason for the conformance requirement in this system is that both the transfer line and valve assembly form a part of the same electrical circuit. In a magnesium alloy die casting operation, small particles of extremely hard materials, such as magnesium oxides, nitrides and inter-metallic compounds, may form inside the valve body. These hard particles have a tendency to wedge between the upper valve stem and valve body and the valve stem tip and valve body seat, and thereby cause excessive wear at these points. In practice, therefore, it is preferred to provide a covering layer 26a on the upper part of valve stem 26, a covering layer 26c on the valve stem tip, and an inner liner 31a in valve seat 31, which comprises an abrasion-resistant material. Suitable materials are high temperature grade, wear-resistant metal compositions, which are relatively inert to reaction with the aluminum component present in some magnesium alloys. Preferred materials include alloys comprising a mixture of cobalt, chromium and tungsten. In a working embodiment of the invention, the valve body 25 was fabricated of type 410 stainless steel and the transfer line 13 of type 430 stainless steel. The length of the valve body was about 7.5 inches. The average outside diameter of the valve body above and below the neck portion 27 was about 2.67 inches. At the neck portion the average outside diameter was about 2.375 inches. The average inside diameter of the valve body was about 2.0 inches above and below the neck portion 27 and about 1.25 inches at the neck portion. The length of the transfer line was about 3.0 feet and the inside diameter was 2.0 feet. Both the valve body and transfer line were covered with an insulation material 30 (Kaowool) having a thickness of about 2.0 inches. In die casting a magnesium alloy, the valve body and transfer line were maintained at an operating temperature of about 1,200.degree.F. The current required to maintain the operating temperature was about 1,450 amperes and the voltage was about 3.2 volts, as measured across the electrical leads 20 and 22. The power input was maintained at about 4,640 volt amperes. From past experience with the use of electrically-heated pipe lines in a magnesium alloy die casting operation, it has been found that the power factor is usually about 0.75 in a set up similar to that described above. For this reason the power input was estimated to be about 4,640 .times. 0.75, or 3,480 watts. For U.S. patent law, rules, and procedures see MPEP. Disclaimer. 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