TECHNICAL FIELD OF THE INVENTION 
     The present invention relates to the field of down-hole back pressure containing valves as used in the oil field in assembly with hydraulic workover-snubbing or coil tubing operations to work under pressure, preventing well fluids and gases from flowing up the workstring, thus allowing pipe or coil tubing to be run or pulled from the well bore without killing the well and possibly damaging formations with kill fluids. The back pressure valve also serves as a safety valve when the pipe or the coil tubing is being pulled from the well bores having formation pressure in the tubing. For example, if a pinhole develops in the workstring or coil tubing the pinhole exposes well gases and fluids to the atmosphere when the pinhole is pulled above the stripper rubbers or the blow-out preventors. The back pressure valve then seals the workstring on the bottom and pressures are successfully bled off, allowing the pipe or coil tubing to continue safely out of the well bore without pollution from gases and well bore fluids blowing through the pin hole. The invention thus saves expensive time and labor which would normally be required to run the pipe with the pinhole back down to the bottom of the oil well bore and pump killing fluids into the well in order to remove the tubing. The back pressure flapper valve also operates to protect pumps and auxiliary equipment from damage by down-hole pressure surges when drilling or treating wells with pressurized fluids or mixtures. 
     BACKGROUND OF THE INVENTION 
     Oil well drilling requires the pumping of specially compounded drilling muds at high flow rates in order to bail cuttings from the hole and control formation pressures. Penetration of a formation at times may result in pressure surges known as kicks, which can damage the pump or related equipment, and are undesirable in any case. Ball type check valves can be included in the drilling pipe string for the purpose of controlling down-hole back pressure, and such devices are effective in that regard. The necessity of fitting such a valve within the confines of the drilling pipe diameter compromises its fluid flow capacity so that increased mud pump pressure is required to overcome the restriction. This penalty tends to discourage the use of such valves. 
     Wells are also pressure treated with various special purpose fluids and mixtures on occasion, for remedial purposes such as stimulating production or extending the life of the well. The pressurized treatment medium may be a mud compound, cement, resin coated sand, gravel or some other material, depending upon the nature of the operation. Back pressure control requirements and the equipment used are much the same as for drilling practice, with similar compromises and limitations. 
     Flapper type valves such as the OTIS ENGINEERING CORPORATION &#34;Series 10&#34; and &#34;Type Q&#34; safety valves, are known to be used in production pipe strings where they are remotely controlled from the surface by hydraulic pressure. Other applications have been made of flapper type valves, used facing upwardly in the production pipe string, so as to allow free upward flow only. Prior to the instant invention however, no down hole pressure actuated flapper-type safety valve has been known to the industry. 
     The ball type check valves known to the art are capable of rapid closure during a pressure kick. It is an object of the present invention to retain this rapid response while providing an improved flow path compared to that of a ball type valve and thus a relatively low pressure drop. This reduced pressure resistance allows pumping of stiffer, more viscous mixtures by reducing the necessary pumping pressure and can also improve pump life. It is also an object to provide the valve of the present invention in a form adaptable to the various tubing and pipe diameters and to the different joint thread standards used in the industry. 
    
    
     DESCRIPTION OF THE DRAWINGS 
     The aforementioned and other objects and features of the invention will be apparent from the following detailed description of specific embodiments thereof, when read in conjunction with the accompanying drawings, in which: 
     FIG. 1 shows an exploded view of a first preferred embodiment of the invention; 
     FIG. 2 shows a cross section view of the assembly of FIG. 1; 
     FIG. 3 shows a cross section view of an alternate form of the embodiment of FIG. 1 with concentric diameters; 
     FIG. 4 shows a detail section view of the embodiment of FIG. 1 during passage of downward flow; 
     FIG. 5 shows a detail section view of a gasketed sealing surface; 
     FIG. 6 shows an alternate offset form in a valve sub fitting the embodiment of FIG. 3; 
     FIG. 7 shows a detail view of an alternate embodiment having a torsional closing spring; 
     FIG. 8 shows a detail view of a second preferred embodiment having a reduced closing force when fully open; and 
     FIG. 9 shows a view of a third preferred embodiment. having a reduced closing force when fully open. 
     FIG. 9A shows a detail view of the third preferred embodiment 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring now to FIGS. 1 and 2 of the drawings, there are shown exploded and assembled views of the valve assembly 10 of the present invention as interposed in string 11. In this case, spring housing 12 has a HYDRIL 2-step threaded pin 14 to match the connections used in string 11, and a standard straight thread 16F for assembly with mating thread 16M of valve sub 18. Valve sub 18 has flapper mounting lug 26 attached at the periphery of sealing surface 23 for the pivotal connection of flapper 20. An assembly pin 28 provides this connection, allowing the flapper to pivot from the closed position indicated by arrow 22 to an open position as indicated by arrow 24. Valve sub 18 has a standard straight thread 32F at its upper end for assembly to an adaptor sub 30, which has mating thread 32M and a HYDRIL 2-step threaded box 34 to match the connections used in string 11. Sealing of connection 32F-32M is augmented by seal packing 33. 
     Upon assembly, coil spring 31 fits freely inside of flapper bore 38, seating on shoulder 39 and bearing against flapper 20 in a lightly compressed state. Flapper 20 is thus urged to the closed position indicated by arrow 22. The length of assembly pin 28 fits closely within the wall of flapper bore 38 for positive retention. In this embodiment, the reduced internal diameter valve inlet 35 is offset from the concentric internal bore 37 of adaptor sub 30, and from the similarly concentric flapper bore 38, in order to provide more clearance outside of sealing surface 23 for the location of flapper mounting lug 26 and more swing clearance for opening of flapper 20. 
     In FIG. 3 is shown an alternate assembly 50, of the embodiment of FIG. 1, which illustrates its adaptability. Here are furnished the standard tapered thread pin 42 and tapered thread box 43 connections required for installation in pipe string 45. Also in assembly 50, the reduced diameter valve inlet 48 of valve sub 44 is seen to be coaxial with the concentric flapper bore 46 and pipe string 45. The selection of coaxial or offset design is optional, but it should be considered that if offset, the reduced diameter valve inlet 48 could be larger and, in the manner of following FIG. 8, flapper 47 would be cleared to open more fully and both would reduce back pressure. 
     In FIG. 4 is shown a detailed view of the circled portion 4 of FIG. 2, illustrating the opening operation of flapper 20 previously indicated by arrow 24. Downward flow 52, through valve inlet 35, pushes under face 21 of flapper 20 against coil spring 31 which is compressed. The sealing face 19 is lifted from sealing surface 23 and flow 53 passes on through flapper bore 38. It is clearly seen that a reverse flow 54 will impinge upon lower face 21 and, along with the force of spring 31, will quickly return flapper 20 to the closed position indicated by arrow 22 in FIG. 1. In this position, sealing face 19 fits tightly against sealing surface 23, stopping flow 53 along with any associated pressure surge. 
     An improved sealing surface 60 is shown in FIG. 5, achieved by the addition of resilient sealing ring 62, which may be a standard &#34;0&#34; rings, in seal retaining groove 64. The outer wall 63 of retaining groove 64 is undercut, and sealing ring 62 must be deformed upon installation, which holds it in place in service. Retaining groove 64 is sized so that sealing ring 62 protrudes slightly beyond sealing surface 60, affording more perfect contact with sealing face 61 upon closure. In addition to better sealing, with more latitude for manufacturing tolerances, the sealing ring 62 provides a degree of cushioning for the rapid closure of flapper 65. 
     FIG. 5 also shows the preferred construction of mounting lug 66 as a screw-in attachment with a flattened head 68. The flattened head 68 receives assembly pin 69 while the threaded body 67 is installed in receiving threads 71, which are tapped outside of seal retaining groove 64. 
     In FIG. 6 is shown an alternate valve sub 54 wherein valve inlet 56 is inclined, being concentrically placed at upper end 57 with respect to pipe string 55, but offset at lower end 59 with respect to flapper bore 58. The functional result is effectively that provided by the offset inlet 35 of FIG. 2, but with slightly less flow disturbance and hence somewhat less pressure drop. 
     In FIG. 7 is shown an alternate flapper assembly 70 with the flapper 72 connected to mounting lug 73 by assembly pin 74. Torsional spring 80 is a symmetrical part with two active arms 78A and 78B, on either side of mounting lug 73, and &#34;U&#34;-shaped anchor arm 79 which bears against flapper bore 76. Anchor arm 79 transitions into coils 75A and 75B which encircle the extended ends of assembly pin 74 on either side of mounting lug 73 and continue to form active arms 78A and 78B. Torsional spring 80 is deflected slightly on assembly so as to create a force for holding flapper 72 in the closed position shown, and as flapper 72 pivots toward the open position indicated by arrow 77 the closing force is increased. 
     In FIG. 8 is shown a second preferred embodiment 90 which includes flapper 92 connected for pivotal movement on assembly pin 94. Flapper bore 95 is enlarged to depth 93, providing clearance to accept movement of flapper 92 to the open position as shown, and the lower adjacent internal diameter 96 is reduced somewhat. Compression spring 98 is fitted to telescoping spring guide assembly 100, comprising telescoping member 99 and tubular housing 101. Telescoping member 99 has a clevis end fitting 102 for connection to flapper 92 by means of clevis pin 103. Pad eye 104, at the opposite end of tubular housing 101, is fitted into recess 97 in the wall of lower diameter 96 by means of through pin 105 which may be retained by means not shown such as sealant or pipe plugs. 
     Through pin 105 and clevis pin 103 are located so as to approach, but not come into alignment with, assembly pin 94. In this manner, even though spring 98 exerts a greater force when compressed by the opening of flapper 92, the net closing force thereon is reduced, but still adequate for closure. 
     FIGS. 9 and 9A show a third preferred embodiment 110 which includes flapper 112 connected for pivotal movement on assembly pin 114. Flapper bore 115 is enlarged to depth 113, providing clearance to accept movement of flapper 112 to the open position as shown, and the lower adjacent internal diameter 116 is reduced somewhat. Compression spring 118 is fitted to telescoping spring guide assembly 120, comprising telescoping member 119 and tubular housing 121. Telescoping member 119 has a sliding end fitting 122 for connection to flapper 112 by means of retaining groove 123. End fitting 122 fits closely in retaining groove 123, but can move pivotally and slide longitudinally therein. The longitudinal movement is limited by stop pins 127 at the open end 125 of retaining groove 123 and by groove end 126 in the opposite direction. Pad eye 124, at the opposite end of tubular housing 121, is fitted into recess 117 in the wall of lower diameter 116 by means of through pin 128 which may be retained by means not shown such as sealant or pipe plugs. 
     When flapper 112 is in the closed position as shown, sliding end fitting 122 is moved away from assembly pin 114 to bear against stop pins 127. In this manner, spring 118 is favorably positioned to hold flapper 112 so that only a light force is needed. As flapper 112 moves to the open position indicated as 112&#39;, sliding end fitting 122 moves toward assembly pin 114 to bear against closed groove end 126. Through pin 128 and groove end 126 are located so as to allow through pin 128 and sliding end fitting 122 to approach, but not come into alignment with, assembly pin 114. In this manner, spring 118 is not so severely compressed by the full opening of flapper 112 so that the length thereof may be relatively short. The force of spring 118 is applied with reduced leverage so that the net closing force on flapper 112 is reduced, but again adequate for closure. 
     The valve opening of the first embodiment, although less full than shown to be achieved with the alternate embodiments, represents an improved flow capacity relative to the existing ball-type valves. The full opening second and third preferred embodiments, and the torsional spring alternative, provide a yet greater improved flow capacity. In any case, the valve opening increases with flow until back pressure induced by the flapper balances the spring force urging it to close. As the flapper approaches a fully open position, this induced back pressure falls to a minimum. The coil spring of the first embodiment, as well as the torsional spring alternative, give rise to progressively increasing closing force as the valve opening increases, thus the valve opening is always less than ideal in actual practice. The reduced valve closing force, as disclosed in the second and third embodiments permits the valve to achieve a virtually full opening without the need for significant added back pressure to overcome spring force. 
     It will be understood that the invention is not limited to the disclosed embodiments, but is capable of rearrangement, modification and substitution of parts and elements without departing from the spirit of the invention.
In an oil field tubular assembly, a flapper valve permitting downward flow and blocking upward flow is closed by down-hole pressure. A pivotally mounted flapper is urged to the closed position by a spring force, which may be moderated approaching the open position to reduce flow resistance, and an offset valve inlet passage provides clearance for flapper opening and room for the pivotal mounting.