You are an expert at summarizing long articles. Proceed to summarize the following text:

You are an expert at summarizing long articles. Proceed to summarize the following text: 
BACKGROUND OF THE INVENTION 
     This invention relates to devices for draining fluids from a tubing string in a hydrocarbon production well. Tubing drains allow fluids to drain from the tubing string of a well. Among other purposes, draining fluid from the tubing string allows the tubing to be removed from a well without pulling the tubing “wet”, which occurs when there is an obstruction in the tubing which prevents the fluid from draining out of the bottom of the tubing. For example, if the well is produced with a rod pump and the rods have parted leaving a pump or plunger at the bottom of the tubing string, the tubing will stand full of fluid unless a drain can be activated to allow the fluid to escape from the tubing into the casing-tubing annulus. 
     Tubing drains may be either activated by manipulation of the tubing, typically by rotation, or by applying pressure to the tubing string to a sufficiently high pressure to burst one or more rupture disks contained within the tubing drain. While each type drain has its application, the hydraulically activated drains have the advantage that rotation of the tubing is not required to activate the drain. There are situations where rotation of the tubing may not be achievable, such as in highly deviated wells or when downhole tubing or tools are stuck from casing collapse or obstructions. However, there are several disadvantages with the commonly used hydraulic drains. 
     One disadvantage is that if the rupture disk is unintentionally ruptured, the production equipment—usually comprising a rod string, downhole pump, and tubing string—becomes inoperable and must be removed to change out the hydraulic drain. Unintentional rupturing of the disk can, of course, be caused by the pressuring up of the tubing pressure by some event, such as the accidental closing of a valve on a surface production line. However, other phenomena may also rupture the disk. For example, the movement of rod couplings within the tubing string presents a potential mechanism for rupturing the disk. Physical contact between the rod coupling and the disk can cause rupturing of the disk by the impact by the coupling upon the disk. In addition, the motion of the coupling in close proximity to the hydraulic drain can cause a localized pressure spike resulting from the piston effect of the coupling inside or adjacent to the drain. The likelihood of such premature rupturing of the disk increases with the decrease in clearance between the rod coupling and the inside diameter of the hydraulic drain. 
     Another disadvantage of hydraulic drains is that many of the drains utilize elastomeric O-ring seals which can degrade over time, particularly in the presence of corrosive wellbore fluid, harsh downhole treatment fluids, high temperatures, and/or high pressures. A seal failure will result in fluid leakage from the tubing which requires the removal of the tubing string to change out the drain. 
     Another disadvantage of some hydraulic drains is that the rupture disks are unrestrained such that the disk remnants end up inside the well, leaving junk/trash which can either interfere with the operation of downhole equipment or which can accumulate with other debris to create a wellbore obstruction. 
     Another disadvantage of the known hydraulic drains is that the replacement of a rupture disk within the hydraulic drain typically requires sending the drain into a shop for replacement of the rupture disk and related elastomeric O-ring seals. If the hydraulic drain is of the type which utilizes threads in the mandrel for retaining the rupture disk, the threads may be damaged and require redressing. The life of the drain may be limited if the threads are damaged through repeated use because satisfactory repair of the threads may not be possible, which means the mandrel can no longer be used. 
     SUMMARY OF THE INVENTION 
     Embodiments of the method and apparatus disclosed herein provide a solution to the problems described above. For purposes of this disclosure, the terms “lower,” “bottom,” “downward,” etc., refer to a direction facing toward the bottom of a well and the terms “upper,” “top,” “up,” etc., refer to a direction facing toward the surface. The terms “inward” and “inwardly” refer to a direction facing toward the central axis of the disclosed hydraulic drain and the terms “outward” and “outwardly” refer to a direction facing towards the inside wall of the casing string. 
     An embodiment of the apparatus is utilized in hydrocarbon producing wells for draining a tubing string which is disposed within a length of well casing. Embodiments of the apparatus have a mandrel which is made up into the tubing string, typically with either a pin-to-pin configuration where the mandrel has threaded male ends on each end which are made up into tubing couplings, or a pin-to-box configuration, where the mandrel has a box with internal threads on one end for receiving a threaded male pin and a pin with external threads on the opposing end. Using the terms defined above, the upward end may have either a pin with external threads or a box within internal threads, while the lower end, in accord with standard oilfield practice, may have a pin with external threads, but may also have box with internal threads if desired. 
     The mandrel has an axially-aligned opening which has an inside diameter which, in accord with oilfield practice, is typically at least as large as the inside diameter of the tubing comprising the tubing string. The mandrel has an interior portion typically, but not necessarily, located in the approximate middle of the length of the mandrel. Penetrating through the mandrel wall from the interior portion of the mandrel to the exterior of the mandrel is an aperture which is generally perpendicular to the long axis of the mandrel. The aperture comprises, in relative position between the inside of the mandrel wall and the outside of the mandrel wall, a first section having a first diameter and a second section having a second diameter. A first circumferential shoulder (hereinafter “first shoulder”) is defined between the first diameter and the second diameter. This first shoulder has an outward face (i.e, facing toward the exterior of the mandrel) and an inward face facing the interior of the mandrel. The inward face may comprise a first sloping sealing surface. 
     A flow diffuser is disposed within the aperture. The flow diffuser has an inside end facing the interior of the mandrel and an outside end which, when installed, faces outwardly toward the well casing. The flow diffuser comprises one or more flow passages which extend from the inside end to the outside end, where the flow passages provide a path for evacuating the fluid within the tubing when the rupture disk has been burst. The flow diffuser has a threaded section which is adjacent to the inside end. 
     The hydraulic drain also has a disk housing which has an exterior end which is placed in facing relationship with the flow diffuser and an interior end which faces the interior portion of the mandrel. The exterior end of the disk housing has a threaded section, where the threaded section of the disk housing is adapted to make up to the threaded section of the flow diffuser. A rupture disk is disposed between the exterior end and the interior end of the disk housing. 
     When the threads of the disk housing are made up to the threads of the flow diffuser, the first shoulder within the aperture is sandwiched between the disk housing and the flow diffuser, with a metal-to-metal seal formed between the diffuser/disk housing combination and the walls of the aperture. This design eliminates the need for threads within the aperture itself, as used in other hydraulic drains. This design also eliminates the need for O-rings for sealing the flow diffuser/disk housing within the aperture. The elimination of a threaded aperture, having threads which are typically redressed following each use, increases the life of the mandrel. Embodiments of the disclosed invention can be used repeatedly by installing a disk housing having a new rupture disk into the mandrel. The disk housing is pushed up against the aperture from the inside of the mandrel while the flow diffuser is screwed into the disk housing from the outside of the mandrel. Separate tools are utilized to make up the flow diffuser to the disk housing, with a tool both inside and outside the mandrel—one tool holding the disk housing on the inside of the mandrel and the other made up to the flow diffuser on the outside. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  schematically depicts a hydrocarbon well depicting an embodiment of the present invention located in the tubing string. 
         FIG. 2  shows a perspective view of an embodiment of the present invention. 
         FIG. 3  shows a front view of the embodiment depicted in  FIG. 2 . 
         FIG. 4  shows a sectional view from  FIG. 3 . 
         FIG. 5A  shows a detailed view of an aperture, disk housing, and flow diffuser from the embodiment depicted in  FIG. 4 . 
         FIG. 5B  shows a detailed view of the aperture with the disk housing and flow diffuser removed. 
         FIG. 6  shows a top view of an embodiment of a disk housing containing a rupture disk which may be utilized with embodiments of the present invention. 
         FIG. 7  shows a sectional view of the disk housing depicted in  FIG. 6   
         FIG. 8  shows a top view of an embodiment of a flow diffuser which may be utilized with embodiments of the present invention. 
         FIG. 9  shows a sectional view of the flow diffuser depicted in  FIG. 8 . 
     
    
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS 
     Referring specifically to the figures,  FIG. 1  schematically shows a hydrocarbon well installation  100 . The well installation may have, among other components, a tubing string  102 , a downhole pump  104 , a rod string  106  which operates the downhole pump, and a hydraulic drain  10  of the present invention. While  FIG. 1  shows a hydraulic drain  10  placed in a hydrocarbon production well, the drain may also be utilized in injection wells, monitoring wells, or other wells where it may be desirable to drain fluid from a string of tubing. When the hydraulic drain is activated by applying pressure to the tubing string, the fluid column above the hydraulic drain will drain out of the tubing string into the tubing-casing annulus through flow passages in the drain  10 . 
       FIG. 2  shows a perspective view of an embodiment of the present hydraulic drain  10 . Embodiments of the hydraulic drain  10  have a mandrel  12  which is made up into the tubing string.  FIGS. 1-2  show one embodiment in which the mandrel  12  has one end which is a threaded pin  14  which is made up into a coupling of the tubing string. The opposite end  16  of the mandrel will typically have internal threads  18  for receiving a threaded pin from a tubing member. 
     The mandrel  12  has an axially-aligned opening  20  which extends between the upper end  22  and the lower end  24  of the mandrel  12  where a central axis L 1  is defined between the upper end and the lower end. It is to be noted that the terms “upper end” and “lower end” are made with respect to the orientation of the drawing figures only, and that the hydraulic drain  10  may be installed with either end facing upward or downward in a well. Axially-aligned opening  20  will typically have an inside diameter which is at least as large as the inside diameter of the tubing. The largest outside diameter of the hydraulic drain  10  is at the lower end  24 . This diameter may be the same diameter as a tubing coupling, which ensures a slim profile for the tool and which mitigates against erosional wear to the hydraulic drain  10  and the inside of the casing as the tubing string and drain are installed in a well. The slim profile also provides more clearance for recovery of the hydraulic drain  10  by a fishing tool, such as an overshot, in the event the apparatus becomes part of a downhole fish. 
     As shown in  FIGS. 4, 5A and 5B , the mandrel  12  has an interior section  26 . Adjacent to the interior section  26  is mandrel wall  28 . Mandrel wall  28  will typically have a thickness greater adjacent to interior section  26  than the wall thickness at upper end  22  and the lower end  24 . Penetrating through mandrel wall  28  into interior section  26  is aperture  30 . Aperture  30  defines a second axis L 2  which is perpendicular to the central axis L 1 . Aperture  30  comprises, in relative position from the inside of the mandrel wall  28  to the outside of the mandrel wall, a first section  32  having a first diameter D 1  and a second section  34  having a second diameter D 2 , wherein a first shoulder  36  is defined between the first diameter and the second diameter. The first shoulder  36  has an outwardly facing peripheral surface  38  which faces outwardly and an inwardly facing peripheral sealing surface  40 . Inwardly facing peripheral sealing surface  40  may, with respect to second axis L 2 , form an angle ranging from between approximately 30 to 60 degrees, with 45 degrees being the approximate angle indicated in the figures. Adjacent to aperture  30 , the inside of mandrel wall  28  may be scooped out to form scalloped opening  31 . The scalloped opening increases the internal volume of the drain  10  directly adjacent to the rupture disk to further reduce the impact of pressure surges which may occur inside the hydraulic drain. 
     A flow diffuser  42  is disposed within the aperture  30 , where the flow diffuser comprises a generally plug-shaped body which is sized to be received within the aperture  30 . The flow diffuser has an inside end  44  which is generally facing the interior section  26  of the mandrel  12 . Flow diffuser  42  has a peripheral shoulder  48  which, when installed within aperture  30 , abuts outwardly facing peripheral surface  38  of first shoulder  36 . The flow diffuser  42  has a first set of threads  50  which mate with threads  60  of disk housing  58  as discussed below. Outside end  49  of flow diffuser  42  is generally flush with the exterior of the mandrel wall  28 , or slightly recessed within the exterior of the mandrel wall, such that outside end  49  of the flow diffuser  42  does not increase the effective diameter of the drain  10 . The flow diffuser  42  has one or more apertures  46  which extend through the flow diffuser  42 , forming a flow passage there through. 
     Disk housing  58  has an exterior end  52  which is in facing relationship with the inside end  44  of the flow diffuser  42  and an interior end  56  which faces the interior of the mandrel  12 . The exterior end  52  has second set of threads  60  which mate with threads  50  of the flow diffuser  42 . Peripheral shoulder  54  has a sealing surface  66  which, when disk housing  58  has been made up to flow diffuser  42 , forms a metal-to-metal seal with face  40  of first shoulder  36 . Sealing surface  66  may be angled to compliment the angle of face  40  which, as discuss above, may have an angle ranging from 30 to 60 degrees, with 45 degrees being the approximate angle indicated in the figures. 
     A rupture disk  62  is disposed between the exterior end  52  and the interior end  56  of the disk housing  58 . Rupture disk  62  is attached to the approximate center of disk housing  58  by a peripheral ring  64  having a reduced wall thickness. When sufficient hydraulic pressure is applied to the rupture disk  62 , the rupture disk will sever from the disk housing  58  along the boundary of peripheral ring  64 . Peripheral ring  64  has diameter D p  which defines the diameter of the severed rupture disk  62 . Diameter D p  is larger than the diameter of the apertures  46  in flow diffuser  42  and the diameter of opening D 3  at interior end  56  of disk housing  58 . Thus, once separated, the rupture disk  62  will be trapped between the flow diffuser  42  on the outside and the interior end  56  of disk housing  58 . This design prevents the rupture disk from moving inwardly and falling down the tubing string or escaping outwardly into the tubing-casing annulus. It is to be appreciated that embodiments of the present invention do not require that aperture  30  have any threads. Instead, the flow diffuser  42  and disk housing  58  are made up to one another, where a shoulder within aperture  30  is sandwiched or captured between the flow diffuser and disk housing. This method of installing the flow diffuser and disk housing reduces the possibility of damage to the mandrel  12 . 
     The mandrel  12  will be manufactured from materials having the mechanical properties and material composition suitable for high tensile loads in a potentially corrosive environment. For example, the mandrel may be manufactured from 3.5 inch round bar complying with AISI 1018 ASTM A108. The flow diffuser  42  and disk housing  58  may be manufactured from 2.0 inch round bar of 17-4 PH (precipitation hardened) H925 to H1025 condition (heat treat condition). The disk housing  58  may be manufactured from 1.75 inch stock round bar of 316 stainless steel, where the rupture disk is rated to shear at a variety of burst pressures, including 3,000 to 7,000 psi. 
     While the above is a description of various embodiments of the present invention, further modifications may be employed without departing from the spirit and scope of the present invention. Thus the scope of the invention should not be limited according to these factors, but according to the following appended claims.

Summary:
A hydraulically actuated tubing drain for service with oil wells, water wells, gas wells and/or thermal wells has a configuration of structural features which, upon hydraulically opening the drain, prevent any debris from the rupture disk from entering either the tubing or the tubing-casing annulus. The disk housing and flow diffuser of the present invention mate directly together, capturing between the disk housing and flow diffuser a shoulder of the mandrel. This design eliminates the need for a threaded aperture through the side wall of the mandrel and the need for elastomeric seals.