Selective back seat valve
BACKGROUND OF THE INVENTION 
     The invention relates to non-rising stem valves in which the stem passes out of the valve housing through a bonnet having a packing gland. Sometimes when such valves are in service, open and under pressure the stem packing must be changed while the valve remains open. 
     Although the invention is not limited to inclusion in gate valves, it was developed for use with a gate valve of the sort shown in these prior U.S. Pat. Nos.: 
     Crain et al., 3,349,789, Oct. 31, 1967 
     McGee, 3,789,875, Feb. 5, 1974 
     Boitnott, 3,990,679, Nov. 9, 1976 
     SUMMARY OF THE INVENTION 
     A non-rising valve stem is provided with a shoulder that, in normal opening and closing of the valve, remains free of a complementary seat formed inside the valve bonnet. Should the valve stem packing in the bonnet begin to leak while the valve is open and under pressure, the bonnet packing gland may be backed out a few turns to effect a seal between the shoulder and seat. Then a disassembly, packing change, reassembly and testing may be effected. 
     The principles of the invention will be further discussed with reference to the drawing wherein a preferred embodiment is shown. The specifics illustrated in the drawing are intended to exemplify, rather than limit, aspects of the invention as defined in the claims. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING 
     In the Drawing: 
     The FIGURE is a longitudinal sectional view of a gate valve equipped with the selective back seat of the invention. At the left, the valve gate is shown fully open; at the right, the valve gate is shown fully closed. 
    
    
     DETAILED DESCRIPTION OF THE PRESENTLY PREFERRED EMBODIMENT 
     In the FIGURE, there is shown a gate valve 10 having a housing 12 which includes a body 14. The body has a longitudinal throughbore 16 to provide a flow passageway. At the ends of the bore 16, the body is provided with flanges 18 wherewith the valve may be connected in a conduit to provide means for controlling fluid flow through the conduit. 
     Intermediate the ends of the bore 16 there is provided a valve chamber 20 which includes a transverse passageway which intersects with the bore 16 and opens outwardly through the body at a tubular boss 22. The boss 22 is provided with a flat outer end surface 24. 
     The housing 12 further includes a valve bonnet 26 that is mated with the surface 24 and secured to the body, e.g. by bolts 28. A sealing ring 30 is interposed between the body and bonnet to provide sealed integrity for this juncture. 
     The bonnet 26 is an annular or tubular element, in the sense that it has a coaxially disposed bore 32. 
     Within the chamber 20, valve seats 34 are provided at the intersections with the throughbore 16, and a valve gate 36 is received for sliding transversally of the throughbore 16. The gate 36 is shown including a pair of gate plates 38, and the valve is shown including an expander 40 which serves to force the plates against the seats 34 when the valve is closed, and to withdraw that force as the gate is opened. 
     In the FIGURE, the gate is shown in its raised, open position at the left, and in its lowered, closed position at the right. When the gate is open, the openings 42 through the respective gate plates are both axially aligned with the valve body throughbore 16. When the gate is closed, the blank regions of the respective gate plates are pressed against the respective valve seats 34. 
     A valve stem 44 is threadably connected to the gate and passes out of the housing 12 through the bore 32 of the bonnet. Outside the housing, an operator 46 is secured on the stem 44. In the instance shown, the operator 46 is a handwheel that is designed to be manually rotated in one angular sense to raise the gate and open the valve, and to be manually rotated in the opposite angular sense to lower the gate and close the valve. Other operators that may be used are shown in the above mentioned prior U.S. patents of Crain et al., McGee and Boitnott. 
     The particular connecting means illustrated in the FIGURE for threadably connecting the gate to the valve stem are shown and described in detail in the copending U.S. Pat. application of McGee, Ser. No. 800,437, filed May 25, 1977 (entitled Drive Nut and Torque Plate Assembly). Suffice it to say here that an upwardly open generally cylindrical socket 48 is formed in the gate. The socket 48 includes a coaxially annular groove 50 near its upper end. A generally semi-cylindrical half of the socket 48 is formed in the inner side of each gate plate so the halves confront one another to constitute the socket 48. 
     An internally threaded, generally tubular drive nut is shown provided at 52. The drive nut 52 has a coaxially annular rib 54 formed on the exterior thereof axially intermediate the ends thereof. A coaxially annular flange 56 is formed on the upper end of the drive nut. The drive nut 52 is received in the socket 48 with the rib 54 projected into the groove 50 and the flange 56 abutting the upper ends 58 of the gate plates 38. The flange 56 is radially surrounded by a torque plate 60 which is pinned at 62 to each of the gate plates distally of the socket 48. The radially inner surface of the plate 60 and the radially outer surface of the flange 56 are provided with splines 64. These interdigitate to prevent relative rotation between the flange 56 and plate 60. Further locking means (not shown) may be provided to secure the flange 56 and plate 60 together and/or these elements may be formed integrally and the splines 64 omitted. 
     Further details of the valve stem 44 and bonnet 26 will now be described so that the structure and operation of the invention will become better understood. 
     The bonnet 26 includes a generally radially extending base flange 66 and a tubular portion 67 extending coaxially upwardly therefrom. The bore 32 passes through both portions. Where the bore 32 emerges into the valve chamber, there is a frusto-conical annular seat 68 coaxially formed at the juncture of the bore 32 with the underside 70 of the flange 66. 
     A short way axially above the seat 68, the generally cylindrical bore 32 undergoes a first abrupt enlargement in radius, providing a first upwardly facing shoulder 72. Axially further up, the bore 32 undergoes a second abrupt enlargement in radius, providing a second upwardly facing shoulder 74, within the tubular portion 67. 
     An axially intermediate portion of the bore 32 within the tubular portion 67 of the bonnet is internally threaded at 76. Above the threaded portion 76, the bore 32 is provided with a radially shallow, axially broad circumferential recess 78, delimited at the top by a radially inwardly projecting annular rib 80. Above the rib 80, an axially short internally threaded band 82 is provided in the bore 32. 
     The valve stem 44 has an externally threaded lower portion 84 that is received within the gate socket and threadably connected with the gate as described above. Just axially above where the stem 44 emerges through the drive nut 52, there is a circumferentially extending boss 86 formed coaxially thereon. The axially upper end surface or shoulder 88 of the boss is frusto-conically tapered, complementarily with the seat 68. 
     In normal use, as shown in the FIGURE, the shoulder 88 remains clear of the seat 68 and there is a significant distance located axially between these two surfaces. 
     Above the boss 86, the stem 44 has an axially long, cylindrical, unthreaded portion 90. Axially beyond its emergence from the bore 32 of the bonnet, the stem undergoes a gradual reduction in radius to provide a tapered band 92. Above the band 92, the stem 44 has an externally threaded shank 94 leading to its upper end. The band 92 mounts the complementarily tapered hub 96 of a handwheel 98. A washer 100 and nut 102 are mounted on the shank 94 to removably secure the handwheel on the stem 44. 
     Within the tubular portion of the bonnet, the cylindrical portion 90 of the stem is coaxially provided with a radially shallow, axially broad recess 104, having opposed, facing shoulders 106, 108. 
     A packing assembly 110 comprising a stack of chevron packing elements is received in the annulus about the stem that is bottomed on the first shoulder 72. As shown, the assembly 110 is of the type that is energized by an injection of plastic material through an injection port 112 after installation. 
     An annular packing gland 114 is received in the annulus about the stem that is bottomed on the second shoulder 74 and projects therebelow, backing the packing assembly 110. 
     A bonnet gland 116 also is provided, generally in the form of an exteriorly threaded tubular element having an external, axially intermediate band of threading 118 and a throughbore 120 which undergoes an abrupt decrease in radius intermediate the axial extent thereof, to provide a downwardly facing shoulder 122. The larger radius portion leading to the shoulder 122 constitutes a cup for receipt of the bearing assembly 124 for the valve stem. As shown, there is provided a bearing bushing 126, a roller bearing 128, a stem split ring 130 radially surrounded by a keeper ring 132, a split back-up ring 134, a retaining ring gland 136 and a roller bearing 138. 
     After the packing assembly and packing gland are installed, the bearing assembly and bonnet gland are installed and the bonnet gland is threaded into place until the packing gland 114 is properly compressed. 
     Below the band of threading 118, the bonnet gland is coaxially provided with a shallow circumferential recess 140. A set screw 142 is installed through a threaded opening 144 in the tubular portion of the bonnet 26 until it projects into the recess 140. When so installed, the set screw 142 prevents the bonnet gland from being rotationally backed out. (In practice several set screws 142 are provided angularly of the bonnet 26.) 
     Above the band of threading 118, the bonnet gland is externally coaxially provided with a shallow recess 144, at least including a lower, generally upwardly facing shoulder 146. 
     A bonnet gland retainer wire 148 is snapped into the pocket formed by the confronting recesses 144 and 78. The pocket 144, 78 is axially longer than the gage of the wire 148. 
     In use, with the valve open and subjected to pressure of fluid being confined or transported in the piping system in which the valve is installed, a leak may occur in the packing assembly. Under such circumstances it may be necessary to change the packing assembly while the valve remains installed in the line and open. 
     The present invention makes such a repair feasible, by making the following procedure possible. 
     The set screws 142 are backed out until they are withdrawn from the recess 140. Then the bonnet gland 116 is backed out a few turns, limited by pinching of the retainer wire 148 between the upper end wall of the recess 78 and the lower end wall 146 of the recess 144. 
     The normally non-rising stem 44 may then be raised and tensioned to form a metal to metal seal between the shoulder 88 and the seat 68. While a positive seal remains established, the retainer wire 148 is snapped out, the gland 116 is backed out, the bearing assembly 124 is removed, the packing gland 114 is removed, the packing assembly 110 is removed and replaced, the packing gland 114 is reinstalled, the bearing assembly 124 is reinstalled, the gland 116 is threaded mostly in and the packing 110 is energized at 112. The packing is tested for leakage by reinstalling the retainer wire 148, tightening the gland 116 and turning the stem 44 which disengages the metal seal that was maintained at 88, 68. 
     It should now be apparent that the selective back seat valve as described hereinabove, possesses each of the attributes set forth in the specification under the heading &#34;Summary of the Invention&#34; hereinbefore. Because it can be modified to some extent without departing from the principles thereof as they have been outlined and explained in this specification, the present invention should be understood as encompassing all such modifications as are within the spirit and scope of the following claims.