text: the present inventions will be described by referring to the drawings of apparatus and methods showing an example of how the inventions can be made and used . in these drawings reference characters are used throughout the several views to indicate like or corresponding parts . in fig1 one example installation of how the present invention can be used in the downhole portion of a subterranean well is shown . well 10 is shown having a casing 1 2 extending to a producing formation 14 . although not shown , it is to be understood that the casing 12 extends to the surface . mounted within the interior of the well casing 12 is a production tubing string 16 which , likewise , extends from the surface to a point adjacent to formation 14 . a packer 18 is typically utilized to close the annulus ( annular space ) 20 formed between the exterior of the production string 16 and the interior of the casing . for purpose of illustration , the production tubing string 16 is shown including a tubing retrievable safety valve 22 and a landing nipple 24 . since valve 22 is illustrated as being of a tubing retrievable type , it is made up or connected in the production tubing string 16 as it is lowered into the well . a typical tubing retrievable valve can consist of a safety valve designed to close off the flow through tubing string 16 upon the occurrence of a preselected event as is well known in the industry . valve 22 is of the type which utilizes a hydraulic control line 26 to , likewise , extend to the well surface . landing nipple 24 is also made up in the tubing string 16 as it is assembled and lowered into the well . landing nipple 24 is of the type which can receive a downhole tool , such as a valve therein . landing nipple is a generic term used in the oil industry to describe a hollow receptacle which is made up in the tubing string and in which the well tool can be installed and / or removed as desired . for purposes of description the inventions have been described by referring to a landing nipple ; however , the inventions could be used in any device containing a control line receptacle such as , landing nipple , safety valve , subseal test tree , or any other hydraulically actuated downhole device . landing nipple 24 is shown having a hydraulic control line 28 connected thereto . hydraulic control line 28 extends to the surface and is present for use in operating a tool set or mounted within the landing nipple 24 . although not shown , as is well understood in the industry , the well surface will typically have a series of valves and control mechanisms for receiving products produced through the well string and for supplying hydraulic pressure to the control lines 26 and 28 . a typical example is shown as described in u . s . pat . no . 5 , 323 , 859 which is incorporated herein by reference for all purposes . in accordance with the present invention , a flow control device 30 is connected between the landing nipple 24 and control line 28 . as will be described in detail , this flow control device 30 is of the type which is normally closed and prevents flow between the landing nipple 24 and control line 28 . the flow control device 30 is designed to withstand internal pressures within the production tubing string 16 and is designed to fail or rupture or open the fluid pathway between the landing nipple 24 and hydraulic control line 28 when pressure is applied through the hydraulic control line 28 above a specified limit . in accordance with the present invention , once a well 10 is assembled and the casing is set , production string is made up and installed in the well . in the example shown in fig1 the production string includes a downhole tubing retrievable safety valve 22 which is connected through hydraulic lines 26 . hydraulic line 26 is assembled at the same time and is lowered into the well with the tubing string . in the fig1 embodiment , the tubing string 16 is also made up or assembled at the surface to include the landing nipple 24 having the device 30 of the present invention connected thereto and hydraulic control line 28 extending from the device 30 to the surface . once the tubing string is installed , production from the well is through the production tubing string 16 to the surface . valve 22 remains in the open position unless and until it is closed via the hydraulic control line 26 . should the safety valve 22 lose its integrity , a second safety valve can be placed in the well through the tubing string and landed or mounted within landing nipple 24 . prior to installing the valve in the landing nipple 24 , the pressure in hydraulic line 28 is increased to a sufficient value to rupture and open device 30 flushing the passageway of debris prior to setting the valve . after the valve is set , hydraulic control line 28 is used to operate the valve or the device seated therein . turning to fig2 cross sectional schematic view of a landing nipple 24 installed in casing 12 is shown . landing nipple 24 has a hollow body 32 connected in the tubing string 16 by threaded collars 34 . body 32 has a chamber 33 formed therein which includes a landing or locking groove 36 formed in the interior wall of chamber 33 . it is this locking groove 36 that is used to engage a locking mandrel ( not shown ) to mount a safety valve or other device within the landing nipple 24 . a port or passageway 38 in the wall of nipple 24 is in fluid communication with chamber 33 and is connected to device 30 . device 30 is , in turn , connected to hydraulic control line 28 as shown . as can be seen in this figure , well fluids moving through the tubing string 16 and chamber 33 are in direct communication with the port 38 . without the presence of device 30 , these fluids can enter the hydraulic control line and corrode , plug , or otherwise deteriorate the same . in a well which has a long productive life , a well designed and manufactured subsurface safety valve can function for years or even decades before its integrity falls into question . thus , in nipples without device 30 , the hydraulic control line 28 could be exposed to well fluids for years or even decades . in fig3 flow control device 30 is shown in cross section . in this embodiment device 30 has a body 39 with male threads 40 of the size to connect with female threads ( not shown ) formed in port 38 of landing nipple 24 . in this embodiment it is preferable that the threads 40 be pipe threads sealing with the threads formed in the nipple 24 . even though a threaded engagement is shown , it is to be appreciated that device 30 could be welded or otherwise connected by use of o - rings , packing , or the like to form a fluid tight connection to the landing nipple . male threads 42 are also formed in the opposite end of body 39 of device 30 . these male threads are for connection to a hydraulic fitting on the hydraulic control line 28 . a central passageway 46 extends through the body 39 of the device 30 . a rupturable valve disc assembly 50 is installed in the body 39 adjacent the threaded end 40 to close the central passageway 46 . in fig4 the details of this rupturable valve assembly 50 is shown . the assembly 50 consists of a curved disc 52 and backup mounting ring 54 . in the embodiment shown the disc 52 forms a valve element and is concaved in shape in the direction of the landing nipple . the body 39 has a corresponding concaved annular surface 56 therein of the size and shape to mate with disc 52 . although the disc is shown in an exploded view in fig4 when properly assembled the disc is welded in place along the maximum diameter edge 58 of the surface 56 . in the embodiment shown ring 54 has an outer cylindrical surface 60 of the size and shape to be press fit in counterbore 62 in body 39 . ring 54 has an axial depth corresponding to the depth of the counterbore 62 and is installed in a press fit therein . ring 54 has an interior wall defining a cylindrical axially extending central passageway . in this embodiment the chamber or passageway formed by the interior wall 64 is slightly smaller than the disc 52 to assist in retaining the disc in the assembly 50 . in the embodiment shown the diameter of the surface forming wall 64 is less than the diameter of the edge 58 . when a positive pressure differential is applied across the disc 52 in the direction of arrow a , surface 56 contacts and supports the disc surface giving it added strength in the direction of arrow a . in addition , the concave shape gives strength to the disc 52 . if the positive pressure differential is reversed and applied from the passageway 46 across disc 52 , surface 56 does not provide support and burst strength of the disc 52 in this direction is less . in this manner , the flow control device is eccentric in its performance . it is preferred that the geometry and thickness of the disc be selected such that ordinary positive well pressures in the direction of the arrow 80 will not cause the disc to rupture or fail . the geometry and thickness of the same disc can also be designed such that when the positive pressure in passageway 46 is increased sufficiently above well pressure but within attainable hydraulic control pressures disc 52 will rupture and the flow of hydraulic fluid will flush the disc through the chamber formed by wall 64 and , in turn , through the port 38 and into the production tubing string 16 . in this manner the flow control device 30 performs the function of sealing the hydraulic control line 28 from well fluids flowing through the landing nipple 24 but , at a later date , a flow control device 30 can be opened to allow hydraulic line 28 to be used to control a device set in the landing nipple 24 . the flow control device 30 is attached to the landing nipple 24 at the well surface prior to its installation . hydraulic line 28 is , likewise , connected to the flow control device as the tubing string 16 is installed in the well . when it becomes necessary to bring the landing nipple to life and open the hydraulic control line connected to the landing nipple , the pressure in hydraulic control line 28 is increased to exceed the burst point of the flow control device 30 thus establishing hydraulic communication through the line 28 and port 38 . thereafter , the downhole tool , such as a safety valve , can be set in the landing nipple 24 and operated by use of the hydraulic control line 28 . it is to be appreciated that the device 30 could be formed in the landing nipple within the port 38 without departing from the spirit and scope of the invention . in addition , the flow control device 30 could be placed in the hydraulic line 28 in a point adjacent to the landing nipple . it is to be appreciated that it is desirable to have the device 30 as close as possible to port 38 , thus , providing as much protection as possible from the well fluids . however , the flow control device 30 and its assembly in the system could still have substantial advantages if placed adjacent to the landing nipple in the hydraulic line a short distance away . it is also to be appreciated that the well control device of the present invention could be utilized at ports in devices other than landing nipples even though no hydraulic line is connected on the annulus side of the flow control device . in these installations , the pressure in the annulus could be increased above that of the well string to rupture the flow control device and establish communication between the annulus and the tubing string if desired . the embodiments shown and described above are only exemplary . many details are found in the art , such as , the construction of the various well tools , landing nipples , safety valve , lock mandrels , sliding side door nipples , sliding side door tools , and the like and are incorporated herein by reference to the patents listed herein . therefore , many such details are neither shown nor described . it is not claimed that all of the details , parts , elements , or steps described and shown were invented herein . even though numerous characteristics and advantages of the present inventions have been set forth in the foregoing description , together with the details of the structure and function of the inventions , the disclosure is illustrative only , and changes may be made in the detail especially in matters of shape , size , and arrangement of the parts within principles of the invention to the full extent indicated by the broad general meaning of the terms used in the attached claims . the restrictive description and drawings of the specific examples described above do not point out what an infringement of this patent would be but are to provide at least one explanation of how to make and use the inventions . the limits of the inventions and the bounds of the patent protection are measured by and defined in the following claims :