Patent Publication Number: US-6212989-B1

Title: High pressure, high temperature window assembly and method of making the same

Description:
The invention described herein may be manufactured, used, and/or licensed by or for the United States Government. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates generally to a window assembly for transmitting light into and from a vessel containing a high pressure, high temperature, highly reactive environment. The invention relates more specifically to a window assembly which can be mounted in a pressure vessel, such as the breech of a large caliber artillery cannon, in a manner that allows the high pressure surface of the window to be substantially flush with the high pressure surface of the surrounding metal case. 
     2. Description of the Related Art 
     When employed in a weapon such as a large caliber artillery cannon, a window can facilitate the introduction of a pulse of laser light to ignite the propellant charge of the gun. In such service, however, the environment inside the gun to which the window is exposed during firing typically includes a rapid transition from ambient conditions at a pressure of one atmosphere to pressures of 400 MPa (60,000 psi) and flame temperatures near 3000 Kelvin, followed by the return to one atmosphere, in a time of about 20 milliseconds. In addition, high flow velocities of gas are present in the vicinity of the window. Because of the high pressure and gas flow rates, heat transfer rates to the gun wall, the window, and the surrounding mount in which the window is located are very high. Therefore, a window in such service must be very robust in order to survive many hundreds of gun firings without loss of either optical transmission or damage to the pressure seal. 
     Furthermore, the high-pressure surface of the window must be nearly flush with the surrounding mount in order to allow for rapid automatic cleaning of residue from the window between firings. The window and its seal must survive numerous gun firings without attention beyond automatic cleaning of propellant residue. 
     A window which employs a 20° full angle conical sapphire window seated in pyrophillite and operating at pressures of up to 12 Kbars (175,000 psi) is described in Stromberg, H. D. and Schock, R. N., “A Window Configuration for High Pressure Optical Cells,”  The Review of Scientific Instruments , Vol. 41 (1970), pp. 1880-1881. Pyrophillite (AlSi 2 O 5 (OH)) is a soft, natural, mineral-like talc that is related to agalmatolite. The ductile seat distributes stress over the entire crystal sealing surface, the curved surface of the window does not need to be polished when so seated, and the compressive strength of the window is aided by placing the window material under compression. Testing of this design during the development of the present invention, however, revealed that this design is unacceptable for the performance requirements in a gun, primarily because the soft material erodes rapidly when exposed to the temperature, chemical corrosiveness, and high gas flow rates of the gun environment. Once the sealing material is eroded from the edge of the window, the window is unsupported in that region and fractures with subsequent pressure pulses. 
     The use of an 18° full angle conical diamond window with 64 facets seated in a 0.010 inch thick gold foil pre-formed sleeve at approximately 500° C. and 100 MPa (15,000 psi) is described in Marley et al., N. A., “High Temperature and Pressure System for Laser Raman 
     Spectroscopy of Aqueous Solutions,”  The Review of Scientific Instruments , Vol. 59 (1988), pp. 2247-2253. The cost of diamond windows, however, is prohibitive for production applications. When the gold seal concept was tested with sapphire windows in a gun environment, it was found to be satisfactory for short-term application. After a number of gun cycles, however, the gold was eroded by the gun gases, and the windows failed. 
     A method of soldering cylindrical sapphire windows into a device used at 850K and 200 MPa (30,000 psi) is described in Gorbaty, Y. E. and Bandarenko, G. V., “Soldered High-Pressure High-Temperature Sapphire Window,”  The Review of Scientific Instruments , Vol. 65 (1994), pp. 2739-2740. Testing of this design during the development of the present invention, however, revealed that the rapid pressure and temperature cycling to which the window is exposed in a gun application causes failure in sapphire windows mounted in accordance with this technique. 
     The use of sapphire cylinders sealed on the cylinder surface at conditions of up to 450° C. and 360 MPa (52,000 psi) is described in Lentz, H., “A Method of Studying the Behavior of Fluid Phases at High Pressures and Temperatures,”  The Review of Scientific Instruments , Vol. 40 (1969), pp. 371-372. Because this design uses a deeply recessed window which is held in place by a mechanical retainer, however, it is not applicable for flush mounting. 
     A device employing a conical window shape and the aforementioned sealing techniques is described in U.S. Pat. No. 5,124,555 to Hartl. The disclosed device is not suitable for the present application, however, because the window is mounted substantially below the metal case surface, and the technique cannot be adapted for flush, or even near-flush, window mounting. In a gun application the recessed region on the high-pressure side of a window mounted according to this design would fill with residue after a small number of cycles and become unusable. The service life of a window with this design is also expected to be much shorter due to loading by the retaining forces on the high-pressure face of the window. 
     Thus, each of the aforementioned conventional designs have inadequacies which render them unsuitable for use in the service encountered by the present invention. For example, where the seal requires the window to be substantially recessed below the surface of the surrounding material or main body of the chamber, a large amount of solid residue (resulting from the gun propellant combustion) tends to accumulate on the window and in the recessed area. No reasonable rapid and automatic method of cleaning this recessed area is available once the recessed depth is below a few millimeters. An additional design weakness of windows that are recessed with a retainer ring on the high-pressure face is the uneven loading of the window that results. This additional loading may result in a much shorter service life when the window is repeatedly pressure cycled. 
     Second, hard seals such as soldered windows do not withstand the cyclic loading of repeated gun firings. This is because the window and mount interface must exhibit plastic behavior. Due to the thermal mismatch and the difference in material stiffness (as measured by Young&#39;s Modulus) between the steel mount (or similar high-strength, ductile material) and the sapphire window (high-strength, transparent, brittle material), movement must be allowed at the interface while intimate contact and support are maintained. 
     Finally, while affixing the window to the seat with an adhesive may be possible, ceramic sealants were found to be too inflexible to withstand the repeated temperature and pressure cycles encountered in a gun environment. Softer, i.e., polymeric, sealants erode from the bond area between the window and seat as a result of the high temperature, high-pressure environment, thus leaving the window partially unsupported. The unsupported portion of the window fails on subsequent cycles. 
     SUMMARY OF THE INVENTION 
     It is an object of the present invention to provide a window assembly which facilitates the transmission of light into and from a chamber containing a high pressure, high temperature, highly reactive environment. It is a further object of the present invention to provide a window assembly which can withstand repeated exposure to a very rapid transition from ambient conditions to elevated pressures and temperatures, followed by a return to ambient conditions. It is an even further object of the present invention to provide a window assembly which is seated substantially flush with the surrounding case so as to facilitate the automatic cleaning of residue from the high-pressure surface of the window. 
     Accordingly, the present invention advantageously relates to a window assembly which satisfies each of the aforementioned criteria. The window assembly comprises a truncated conical window which is substantially transparent to radiation, with the window comprising a first end surface having a first diameter, a second end surface parallel to the first end surface and having a second diameter which is smaller than the first diameter, and a truncated conical surface connecting the first end surface and the second end surface. The window assembly further comprises a truncated conical seal of a high ductility metal having a geometry capable of circumferentially engaging the truncated conical surface, and a case to which the window and the seal are secured. The case comprises a surface exposed to the high pressure environment and a truncated conical aperture having a geometry capable of circumferentially engaging the truncated conical seal, with the truncated conical aperture terminating in a through bore contiguous with the second end surface. To facilitate automatic cleaning of the window, the high-pressure first end surface of the window is substantially flush with the high-pressure surface of the case. 
     The invention further relates to a method of forming the window assembly which includes initially seating the window assembly by means of mechanical pressing, then subjecting the window assembly to a series of successively higher transient combustion pressures, which results in a window assembly which can withstand prolonged service in a harsh environment such as that encountered in a large caliber artillery cannon. 
     The advantages associated with the present window assembly are numerous. First, from a structural standpoint, the invention provides a sealing surface that does not degrade from the high pressure and temperature of the interior of a gun during multiple firings. The invention provides a soft, resilient, sealing surface which allows slight motion of the window material relative to the surrounding metal case, evenly distributes reactive loads on the window due to thermal or pressure effects, and thus allows use of the assembly for a large number of gun firings. In fact, the invention provides an active sealing system between the window and the case which becomes stronger with the application of the pressure against which it is sealing. 
     Additionally, use of an optional configuration is possible which substantially increases the force required to remove the window from the case, but which does not add loading to the hot, high pressure, face of the window. 
     From a maintenance standpoint, the invention provides for the mounting of the high-pressure window in a manner that has no retainer or other surface of the optical window. Thus, the window can be mounted such that its high-pressure surface is flush or nearly flush with the surrounding metal surfaces to provide ready access for cleaning. Finally, the overall cased design of the window assembly facilitates rapid replacement of the unit by relatively unskilled personnel. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Other objects, features, and advantages of the present invention will become more fully apparent from the following detailed description of the preferred embodiments, the appended claims, and the accompanying drawings. As depicted in the attached drawings: 
     FIG. 1A is a sectional view of a window assembly constructed in accordance with the teachings of a first preferred embodiment of the present invention. 
     FIG. 1B is a front view of the window assembly of FIG. 1A constructed in accordance with the teachings of the first preferred embodiment of the present invention. 
     FIG. 2A is a sectional view of a window assembly constructed in accordance with the teachings of the first preferred embodiment of the present invention having a machined case shown in operative cooperation with a pressure vessel. 
     FIG. 2B is a front view of the window assembly of FIG. 2A constructed in accordance with the teachings of the first preferred embodiment of the present invention shown in operative cooperation with a pressure vessel. 
     FIG. 3A is a sectional view of a window assembly constructed in accordance with the teachings of a second preferred embodiment of the present invention having an enhanced retention feature. 
     FIG. 3B is a front view of the window assembly of FIG. 3A constructed in accordance with the teachings of the second preferred embodiment of the present invention. 
     FIG. 3C is an exploded sectional view of the enhanced retention feature of FIG. 3A before engagement of the seal with the case. 
     FIG. 3D is an exploded sectional view of the enhanced retention feature of FIG. 3A after engagement of the seal with the case. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The present invention will be disclosed in terms of the currently perceived preferred embodiments thereof 
     Referring to FIGS. 1A and 1B, a window assembly  5  constructed in accordance with a first preferred embodiment of the present invention is shown. The window assembly  5  comprises a window  10  which has a truncated conical geometry and which is substantially transparent to radiation, such as, for example, light of near infrared wavelength. The window  10  comprises a high-pressure first end surface  11  having a first diameter, a second end surface  12  substantially parallel to the first end surface and having a second diameter which is smaller than the first diameter, and a truncated conical surface  13  connecting the first end surface and the second end surface. The material of window  10  is sapphire or another suitable material that can be fabricated in the truncated conical geometry and which is substantially transparent to radiation. 
     The window assembly  5  further comprises a truncated conical seal  20  of a high ductility metal having a geometry capable of circumferentially engaging the truncated conical surface  13 , and a case  30  having a machined cavity to which window  10  and seal  20  are secured. The case  30  includes a truncated conical aperture having a geometry capable of circumferentially engaging the truncated conical seal  20 , and a high pressure surface  32 . The truncated conical aperture terminates in a through bore  35  contiguous with the second end surface  12 . Through bore  35  has a diameter substantially equal to the diameter of second end surface  12 , and serves as a conduit for the transmission of light from the light source to window  10 . 
     Though first end surface  11  and second end surface  12  of window  10  are both depicted as being flat, other surface configurations are possible. Depending upon the particular application, both first end surface  11  and second end surface  12  can instead be curved so as to function as a lens. Alternatively, first end surface  11  is flat and second end surface  12  is curved, or first end surface  11  is curved and second end surface  12  is flat. An optical advantage can be realized with some configurations having a convex surface on first end surface  11 . 
     First end surface  11  and second end surface  12  are optically polished. To facilitate seating of window  10 , truncated conical surface  13  can have either a highly polished or a fine ground surface. 
     In the embodiment depicted in FIG. 1A, window  10  has a first end surface  11  having a diameter of 12.8 mm, a second end surface  12  having a diameter of 7.5 mm, and a truncated conical surface  13  which has a distance, or thickness, from first end surface  11  to second end surface  12  of 15.0 mm. The interior angle defined by the extension of truncated conical surface  13  to terminate in an apex beyond the truncation at second end surface  12  is 20 degrees. Other diameters, thicknesses, and angles, however, can also be used successfully. For example, while the truncated conical shape is essential to distributing the load forces to which the window assembly will be subjected when in service, the specific angle is not critical. The aforementioned dimensions provide a design which has been found to be readily constructed and assembled. The window  10  is fabricated with the sapphire crystalline “c” axis parallel to the axis of through bore  35 . 
     Since metal seal  20  must have a geometry capable of circumferentially engaging truncated conical surface  13  of window  10 , seal  20  must be fabricated from a material having specific properties. Specifically, the material must be relatively soft and pliant at the time of assembly, i.e., a high ductility metal; the material must work harden during the seating operations in order to minimize long-term extrusion losses and to maintain full support of the window after the assembly operation; the material must be chemically inert under the conditions encountered inside the pressure vessel; and the material must be impervious to hot oxidizing and reducing atmospheres. Copper, aluminum, niobium, nickel, and tantalum are all possible materials for metal seal  20 , with tantalum being the most preferred. 
     The method of assembling window assembly  5  is as follows. Seal  20  is placed in case  30 , and window  10  is then placed in seal  20 . Window  10  is then seated in seal  20 , and window  10  and seal  20  seated in case  30  by mechanically pressing window assembly  5  together with a laboratory press with a force of at least several hundred pounds up to about 9,000 N (2,000 pounds). Next, the initially seated window assembly  5  is mounted in a semi-closed seating vessel, i.e., one having very limited venting to the atmosphere. A quantity of gun propellants sufficient to achieve transient pressure pulses of 100 MPa (15,000 psi) for about 20 to 50 milliseconds is then burned in this vessel. During this pressure seating step, the pressure in the vessel is increased by the gas generated by the burning propellant; the pressure then rapidly decreases from heat loss to the chamber walls and venting to the atmosphere through a small orifice. This pressure cycle enables window  10  to seat itself securely into seal  20  and obtain full support. Advantageously, the pressure cycle also pre-compresses window  10  to a limited extent, which can enhance the window strength. This process is repeated at increasing pressures until the transparent window material has been seated at the operational pressure at which it is intended to be used, typically up to a pressure in the range of 400 MPa (60,000 psi). The finally seated window assembly  5  is then removed from the semi-closed seating vessel and is ready for installation and service in a pressure vessel. 
     Referring to FIGS. 2A and 2B, a window assembly  5  constructed in accordance with a first preferred embodiment of the present invention is shown in operative cooperation with a pressure vessel  70  which has been machined to receive the window assembly. In this embodiment, case  30  is fabricated from high-strength steel, and comprises M24×1 threads  40  to facilitate engagement to a machined cavity in pressure vessel  70 , a 25 mm hexagonal end  33  for wrench grip when attaching the case to the pressure vessel, and a flat sealing surface and high-pressure metal case seal  50  which be made of any suitable sealing material. Vessel  70  includes a countersunk through bore  72  to facilitate optical communication with through bore  35  of window assembly  5 . Window seal  20  is pre-formed from 99.95% pure tantalum with a thickness of 0.200 mm (0.008 inch) and with a geometry to match the machined cavity of case  30  and truncated conical surface  13  of window  10 . The method of assembling window assembly  5  is the same as that described above with respect to FIGS. 1A and 1B. 
     In the aforementioned embodiments depicted in FIGS. 1 and 2, window  10  is retained securely in case  20  by the frictional forces between the window  10 , seal  20 , and case  30 . For most applications this combination of frictional forces provides sufficient force to hold the window securely in the case. 
     For safety reasons or in those environments where excess forces may be present as a result of shock or vibration, a more positive retention of the window  10  is desirable. Referring to FIGS.  3 A and  3 B, a window assembly  5  constructed in accordance with a second preferred embodiment of the present invention is shown. FIG. 3D illustrates a positive retention lip  80  which increases the force required to displace the window by a factor of greater than  25 . As depicted in FIG. 3C, the window  10  is recessed slightly into the case  30 . Seal  20  is sized to extend slightly beyond the end of the window, but can be trimmed as required after assembly. A small circumferential undercut  75  having a substantially “U” shape is first made in the surface of case  30  which circumferentially engages the truncated conical seal  20 . Circumferential undercut  75  is approximately 0.2 mm deep, and is located approximately 3.5 mm from the high-pressure surface  32  of case  30 . Sufficient clearance is present to assemble the components, which appear as depicted in FIG. 3C when first assembled i.e., after mechanical pressing. During the final assembly phase, which constitutes the aforementioned pressure seating step, the soft metal seal  20  extrudes to fill the undercut  75  and form seal retention lip  80 , as depicted in FIG.  3 D. Seal retention lip  80  provides positive retention of the window  10  in the case  30  by mechanical interference when the soft metal seal  20  has extruded into the undercut  75 . This embodiment therefore advantageously provides an enhanced retention force, yet with no diminution of window lifetime. Furthermore, the slight amount of recess, approximately 4 mm, of the high pressure surface  11  of window  10  from the high pressure surface  32  of case  30  in no way hinders automatic cleaning of the window. 
     The present invention, therefore, provides a window assembly having a sealing surface that does not degrade during prolonged service in a cyclical high pressure, high temperature, environment, and which in fact provides an active sealing system between the window and the case which becomes stronger with the application of the pressure against which it is sealing. By virtue of its substantially flush-mounted window, the assembly also advantageously provides for rapid automatic cleaning of residue from the window. 
     While only certain preferred embodiments of this invention have been shown and described by way of illustration, many modifications will occur to those skilled in the art. For example, the use of the term “vessel” herein is meant to denote not only embodiments such as large caliber guns, but also includes all other means for containment, e.g., devices employed in scientific and industrial applications which accommodate a high pressure, high temperature, highly reactive environment, and equivalent containers. It is, therefore, desired that it be understood that it is intended herein to cover all such modifications that fall within the true spirit and scope of this invention as defined in the appended claims.