Abstract:
A method for determining the maximum allowable working pressure of a microchannel device, particularly a diffusion-bonded, shim-based microchannel device operating at a temperature greater to or equal to a base material threshold temperature where significant creep may predominate, and when employing non-traditional materials of construction, when non-traditional fabrication or joining methods are used, or when spurious artifacts arise.

Description:
CROSS-REFERENCES TO RELATED APPLICATIONS 
   (Not Applicable) 
   STATEMENT REGARDING FEDERALLY-SPONSORED RESEARCH AND DEVELOPMENT 
   (Not Applicable) 
   BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   This invention relates generally to microchannel pressure vessels. Specifically, this invention relates to maximum allowable working pressure determination and pressure vessel certification of diffusion-bonded microchannel heat exchangers and microchannel heat exchanger/reactor combinations operating at higher temperatures. 
   2. Description of Related Art 
   Pressure vessel certification organizations, such as the American Society of Mechanical Engineers (ASME), establish rules of safety governing the design, fabrication, and inspection of boilers and pressure vessels. Other pressure vessel certification organizations include the European Commission through its Pressure Equipment Directive (PED), the Japanese Industrial Standards Committee which coordinates the standardization process of creating the Japanese Industrial Standards (JIS) which are then published by the Japanese Standards Association, and the International Organization for Standardization (ISO) which is developing ISO 16528. For example, the International Boiler and Pressure Vessel Code, published by the ASME, provides the procedures to follow to become accredited to certify products comply with the Code. Accreditation packages include the use of, for example, ASME Code Symbol Stamps such as the so-called “U” stamp for pressure vessels. To meet the requirements of the Code, the strength of the vessel may be computed based upon established formulas or hydrostatic tests which determine the maximum allowable working pressure (MAWP). 
   Many factors must be considered in MAWP determinations, such as the basic materials of construction (e.g., 10xx carbon steel, type 316L stainless as well as other steels, nickel alloy 617 as well as other nickel alloys, aluminum, titanium, platinum, rhodium, copper, chromium, brass, alloys of the foregoing materials, polymers; such as thermoset resins, ceramics, glass, polymer/fiberglass composites, quartz, silicon, or combinations thereof), methods of preparing construction components (e.g., stamping, photochemical etching or machining, electrodischarge machining, laser cutting, drilling and milling), methods of joining (e.g., welding, brazing, diffusion bonding, soldering, and adhesives), the design and various cross-sections of the vessel, the pressure and temperature regimes experienced by the vessel, not only during normal operating conditions, but during startup and shutdown, and the presence or absence of spurious artifacts. 
   When the MAWP cannot be satisfactorily determined using established methods, however, especially when employing non-traditional materials of construction, when higher temperatures are expected, particularly when those higher temperatures may cause significant creep, when non-traditional fabrication or joining methods are used, and, more particularly, when spurious artifacts arise, other methods must be employed. There exists then, a need for a method to determine MAWP and meet certification standards when employing higher temperatures, particularly when creep may be significant, when using non-traditional materials of construction, non-traditional component fabrication and joining methods, when fabrication-related artifacts arise, or combinations thereof. 
   BRIEF DESCRIPTION OF THE INVENTION 
   It is an object of an exemplary embodiment to provide a method of determining a maximum allowable working pressure (MAWP) of a pressure device. 
   It is a further object of an exemplary embodiment to provide a method of meeting certification requirements for pressure vessels. 
   It is a further object of an exemplary embodiment to provide a method for meeting certification requirements for pressure vessels fabricated with non-traditional fabrication methods. 
   It is a further object of an exemplary embodiment to provide a method of determining an MAWP of a microchannel device comprising a plurality of shims formed by stamping. 
   It is a further object of an exemplary embodiment to provide a method for meeting certification requirements for pressure vessels joined with non-traditional joining methods. 
   It is a further object of an exemplary embodiment to provide a method of determining an MAWP of a microchannel device joined by diffusion bonding. 
   It is a further object of an exemplary embodiment to provide a method of determining an MAWP of a microchannel device. 
   It is a further object of an exemplary embodiment to provide a method of determining an MAWP of a microchannel device comprising a plurality of shims. 
   It is a further object of an exemplary embodiment to provide a method of determining an MAWP of a microchannel device operating at or above a creep threshold temperature. 
   It is a further object of an exemplary embodiment to provide a method of determining an MAWP of a microchannel device where a property for a joined material at a room temperature is not superior or equal to that property for the base material at that room temperature. 
   It is a further object of an exemplary embodiment to provide a method of determining an MAWP of a microchannel device where a property for a joined material at a design temperature is not superior or equal to that property for the base material at that design temperature. 
   It is a further object of an exemplary embodiment to provide a method of determining an MAWP of a microchannel device containing spurious artifacts whose affects on MAWP are not calculable. 
   It is a further object of an exemplary embodiment to provide a method of burst testing a device. 
   It is a further object of an exemplary embodiment to provide a method of burst testing a device by independently increasing temperature and pressure from a first state to a second state, where the second state comprises a temperature greater than or equal to a base material creep threshold temperature. 
   Further objects of exemplary embodiments will be made apparent in the following Description of the Invention and the appended claims. 
   A method is disclosed for determining the maximum allowable working pressure (MAWP) for microchannel pressure vessels and particularly for microchannel pressure vessels fabricated from stamped Inconel® (Special Metals Corporation, New Hartford, N.Y.) alloy 617 shims with diffusion bonding, for which established calculations or certification procedures may be inapplicable. In addition, special considerations may be necessary when operating temperatures are high, for example, where creep becomes significant, such as at temperatures at or above about one-half the absolute melting point of the material, or when fabrication-related artifacts may be present in the finished vessel. 
   If properties and calculations for a base material of construction itself are not known, one may first test the base material alone considering various operating conditions, including startup, normal operation, and both normal and emergency shutdown. Although the base material of construction may be tested, in practice, the vessels themselves are constructed, or joined together, using various methods. To account for this, selected joining methods are covered in the published code materials. Thus, if the properties of the base material are known or established, and the joining method is prescribed and approved, it may be possible to use published computational methods to meet the applicable requirements for certification. 
   In most cases, the base material properties are, in fact, known. Even so, when the joined material properties are known, however, prescribed and approved certification procedures may not be applicable. In such cases, it may be possible to obtain joined material properties such as tensile strengths, toughness, creep, fatigue life, and fatigue strength using traditional testing methods. Such tests generally involve preparing specimens which are then subjected to the appropriate tests. With complex pressure vessels such as microchannel devices fabricated from, for example, alloy 617, with non-traditional joining methods such as, for example, diffusion bonding, even the resultant joined material properties may not be representative of the final finished device due, at least in part, to operating temperatures above the creep threshold temperature and artifacts inherent in the overall fabrication process. When this happens, complex, but representative, burst tests must be designed and performed. In the instant case, such tests include variations of temperature, pressure, rates of increase, and time. 
   If the temperatures of interest are above the creep threshold temperature and the joined material properties at either low temperatures (below significant creep range) or high temperatures (above significant creep range) are inferior to those properties for the conventional base material, however, neither prescribed certification calculations nor prescribed burst test procedures may be satisfactory. Particularized burst test procedures may burst test procedures may be satisfactory. Particularized burst test procedures may be required to receive certification approval. 
   The presence or absence of artifacts which arise from fabrication of vessel elements, for example, shims or plates for microchannel devices, or which arise from joining of vessel elements, for example, diffusion bonding shims to create a microchannel device, may also affect the MAWP and the certification procedure. For example, the presence of such artifacts may mandate a burst test certification procedure instead of utilizing prescribed certification calculations. 
   In the event the fabricated material properties are not comparable to those of the base material and the joining method is not included within published certification methodologies, or, the tested joined material properties are not believed to be representative, further testing may be required, including, for example, actual burst testing of representative devices. 
   In one exemplary embodiment, a method is provided for determining the MAWP of a microchannel device comprising a plurality of shims comprising a base material and joined with at least one microchannel fabrication technique. In an exemplary embodiment, a determination is made whether a condition of a device operating temperature is greater than or equal to a base material threshold temperature (T Threshold ) is true or false. If the condition of the device operating temperature is greater than or equal to T Threshold  is true, then a determination is made of whether a condition of an at least first material property at a low temperature for a specimen of joined material is superior or equal to the at least first material property at the low temperature for a specimen of base material is true or false. If the condition of the at least first material property at a low temperature for a specimen of joined material is superior or equal to the at least first material property at the low temperature for a specimen of base material is false, then at least one burst test of at least one representative burst test device is conducted. If the condition of the at least first material property at a low temperature for a specimen of joined material is superior or equal to the at least first material property at the low temperature for a specimen of base material is true, then a determination is made of whether a condition of an at least first material property at a design temperature for a specimen of joined material is superior or equal to the at least first material property at the design temperature for a specimen of base material is true or false. If the condition of an at least first material property at a design temperature for a specimen of joined material is superior or equal to the at least first material property at the design temperature for a specimen of base material is false, then at least one burst test of at least one representative burst test device is conducted. If the condition of an at least first material property at a design temperature for a specimen of joined material is superior or equal to the at least first material property at the design temperature for a specimen of base material is true, then a determination is made of whether a condition of the presence of at least one spurious artifact is true or false. If the condition of the presence of at least one spurious artifact is true, then a determination is made whether a condition of at least one effect of at least one spurious artifact on MAWP at the design temperature is calculable is true or false. If the condition of at least one effect of at least one spurious artifact on MAWP at the design temperature is calculable is false, then at least one burst test of at least one representative burst test device is conducted. 
   In a further exemplary embodiment, the method further includes determining the device operating temperature. 
   In a further exemplary embodiment, the method further includes selecting the device operating temperature as one of: the normal operating temperature, the maximum temperature caused by random operational perturbations, the maximum temperatures caused by operational changes, the maximum startup temperature, and the maximum shutdown temperature. 
   In a further exemplary embodiment, wherein the device comprises a steam methane reformer, the method further includes selecting the device operating temperature from between about 800 deg. C. and about 1200 deg. C. 
   In a further exemplary embodiment, the method further includes selecting the device temperature from between about 800 deg. C. and about 950 deg. C. 
   In a further exemplary embodiment, the base material is a nickel alloy containing at least 35 percent nickel. 
   In a further exemplary embodiment, the nickel alloy contains at least 60 percent nickel. 
   In a further exemplary embodiment, the base material is alloy 617. 
   In a further exemplary embodiment, the method further includes determining T Threshold . 
   In a further exemplary embodiment, the method further includes selecting T Threshold  as 0.5*T MP , where T MP =the absolute melting point of the base material. 
   In a further exemplary embodiment, the method further includes calculating T Threshold  as between about 530 deg. C. and about 552 deg. C. 
   In a further exemplary embodiment, the method further includes selecting T Threshold  as 0.3*T MP , where T MP =the absolute melting point of the base material. 
   In a further exemplary embodiment, wherein the base material is alloy 617, the method further includes calculating T Threshold  as between about 209 deg. C. and about 220 deg. C. 
   In a further exemplary embodiment, the method further includes selecting T Threshold  as the temperature at which a base material creep predominates. 
   In a further exemplary embodiment, the method further includes selecting T Threshold  as the temperature at which a base material creep stress limit becomes less than a base material tensile limit. 
   In a further exemplary embodiment, the base material creep stress limit is 80 percent of the minimum stress which causes rupture at the end of about 100,000 hours. 
   In a further exemplary embodiment, the base material creep rate stress limit is 100 percent of the average stress which causes a creep rate of about 0.01 percent per 1,000 hours. 
   In a further exemplary embodiment, the base material tensile limit is about the tensile strength divided by 3.5. 
   In a further exemplary embodiment, wherein the base material is alloy 617, the method further includes selecting T Threshold  as between about 625 deg. C. and about 710 deg. C. 
   In a further exemplary embodiment, the method further includes selecting T Threshold  as the temperature at which a base material creep stress limit becomes less than a base material yield limit. 
   In a further exemplary embodiment, the base material yield limit is about two-thirds the yield stress. 
   In a further exemplary embodiment, the method further includes selecting the at least first material property from the group consisting of ultimate tensile strength, yield strength, yield tensile strength, percent elongation at failure, or combinations thereof. 
   In a further exemplary embodiment, the method further includes selecting the low temperature as less than T Threshold  and about room temperature. 
   In a further exemplary embodiment, the method further includes selecting the design temperature as greater than or equal to the device operating temperature. 
   In a further exemplary embodiment, the method further includes selecting the design temperature as the operating temperature in deg. C. plus about less than 50 deg. C. 
   In a further exemplary embodiment, the device is a microchannel reactor and the at least one representative burst test device is representative of the device with respect to channel dimensions including, but not limited to, height, width, length, or combinations thereof; fabrication methods, including, but not limited to, stamping, bonding, including, but not limited to diffusion bonding, considering, but not limited to, method, time, temperature, pressure, or combinations thereof; surface preparation, including, but not limited to, finish, passivation, etching, cleaning, coating, flatness, lay, waviness, or combinations thereof; wall thicknesses; base material, including, but not limited to, alloy 617; rib dimensions; heat treat cycles; heating cycles during manufacture; shim thickness; symmetry; size scale; or combinations thereof. 
   In a further exemplary embodiment, the method further includes determining the presence of a stamp rollover, carbide precipitates, misalignment or offset of shim ribs, bowing of channel walls, grain size growth, or combinations thereof. 
   In a further exemplary embodiment, the method further includes comparing the size of a stamp rollover, carbide precipitates, misalignment or offset of shim ribs, shim thickness, bowing of channel walls, or grain size growth to channel size. 
   In a further exemplary embodiment, the method further includes determining the presence of grain size growth relative to shim thickness. 
   In a further exemplary embodiment, a method is provided for burst testing a representative device. In an exemplary embodiment, the device is hearted at a substantially constant rate from a first state temperature to a second state temperature, the second state temperature greater than or equal to a base material threshold temperature and allowed to thermally equilibrate. The device is then held at the second state temperature while being pressurized at a substantially constant rate from a first state pressure to a second state pressure. Finally the device is held at substantially the second state temperature and substantially the second state pressure for a fixed period of time. 
   In a further exemplary embodiment, the method further includes pressurizing the device with preheated gas. 
   In a further exemplary embodiment, the method includes the second state temperature greater than about a design temperature. 
   In a further exemplary embodiment, the method includes the second state pressure greater than about a threshold temperature. 
   In a further exemplary embodiment, the second state temperature is greater than about 30 bar. 
   In a further exemplary embodiment, the constant rate of heating is selected to avoid significant creep. 
   In a further exemplary embodiment, the constant rate of heating is between about one deg. C. per minute and about ten deg. C. per minute. 
   In a further exemplary embodiment, the second state temperature is greater than about 900 deg. C. 
   In a further exemplary embodiment, the constant rate of pressurizing is between about one bar per minute and about ten bar per minute. 
   In a further exemplary embodiment, a method is provided for burst testing a representative device. In an exemplary embodiment, the device is pressurized at a substantially constant rate from a first state pressure to a second state pressure and held for a fixed period of time. The device is then held at the second state pressure while being heated at a substantially constant rate from a first state temperature to failure. 
   In a further exemplary embodiment, the second state pressure is greater than about 30 bar. 
   In a further exemplary embodiment, the constant rate of pressurizing is between about one bar and about ten bar per minute. 
   In a further exemplary embodiment, the constant rate of heating is between about one deg. C. and about ten deg. C. per minute. 
   In a further exemplary embodiment, a method is provided for burst testing a representative device. In an exemplary embodiment, the device is heated at a substantially constant rate from a first state temperature to a second state temperature and allowed to thermally equilibrate. The device is then held at the second state temperature while being pressurized at a substantially constant rate from a first state pressure to an excess pressure. 
   In a further exemplary embodiment, the method includes a second state temperature about a design temperature. 
   In a further exemplary embodiment, the method includes the design temperature greater than about a threshold temperature. 
   In a further exemplary embodiment, the constant rate of heating is between about one deg. C. per minute and about ten deg. C. per minute. 
   In a further exemplary embodiment, the second state temperature is greater than about 900 deg. C. 
   In a further exemplary embodiment, the constant rate of pressurizing is between about one bar per minute and about ten bar per minute. 
   In a further exemplary embodiment, the method further includes pressurizing the representative burst test device to failure. 
   In a further exemplary embodiment, a method is provided for determining the MAWP of a microchannel device operating at a temperature greater to or equal to a base material threshold temperature (T Threshold ). In the exemplary embodiment, the microchannel device comprises a plurality of shims, the shims comprising a base material and the shims joined with at least one microchannel fabrication technique. The method comprises determining whether a first condition of an at least first material property at a low temperature for a specimen of joined material superior or equal to the at least first material property at the low temperature for a specimen of base material is true or false; conducting at least one burst test of at least one representative burst test device when the first condition is false, the burst test comprising independently increasing temperature and pressure of the at least one representative burst test device from a first state to a second state, the second state comprising a temperature greater than or equal to the base material threshold temperature; determining whether a second condition of an at least first material property at a design temperature for a specimen of joined material is superior or equal to the at least first material property at the design temperature for a specimen of base material is true or false when the first condition is true; conducting at least one burst test of at least one representative burst test device when the second condition is false, the burst test comprising independently increasing temperature and pressure of the at least one burst test device from a first state to a second state, the second state comprising a temperature greater than or equal to the base material threshold temperature; determining whether a third condition of the presence of at least one spurious artifact is true or false when the second condition; determining whether a fourth condition of at least one effect of at least one spurious artifact on MAWP at the design temperature is calculable is true or false when the third condition; and conducting at least one burst test of at least one representative burst test device when the fourth condition is false, the burst test comprising independently increasing temperature and pressure of the at least one burst test device from a first state to a second state, the second state comprising a temperature greater than or equal to the base material threshold temperature. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is an exploded axonometric projection view of an exemplary microchannel device. 
       FIG. 2  is an axonometric projection view of the exemplary microchannel device illustrated in  FIG. 1 . 
       FIG. 3   a  is an axonometric projection view of a cross-section of the exemplary microchannel device along the line  3 - 3  in  FIG. 2   
       FIG. 3   b  is an elevation view of the cross-section of  FIG. 3   a.    
       FIG. 4   a  is an axonometric projection view of a cross-section of the exemplary microchannel device along the line  4 - 4  in  FIG. 2 . 
       FIG. 4   b  is an elevation view of the cutaway cross-section of  FIG. 4   a.    
       FIG. 5   a  is an axonometric projection view of a cross-section of the exemplary microchannel device along the line  5 - 5  in  FIG. 2   
       FIG. 5   b  is an elevation view of the cutaway cross-section of  FIG. 5   a.    
       FIG. 6  is an illustration of a base material test specimen. 
       FIG. 7  is an illustration of a fabricated material test specimen. 
       FIG. 8  is an enlarged view of a cross-section of a portion of the fabricated material test specimen indicated at  8  in  FIG. 7  and illustrating grain artifacts. 
       FIG. 9  is an enlarged view of a cross-section of a portion of an exemplary microchannel device and illustrating rib offset artifacts. 
       FIG. 10  is an enlarged view of a cross-section of a portion of an exemplary microchannel device and illustrating “rollover” artifacts. 
       FIG. 11  is a flowchart illustrating an exemplary embodiment of the present invention comprising creep considerations. 
       FIG. 12  is a flowchart illustrating an exemplary embodiment of the present invention comprising burst testing at constant temperature and pressure. 
       FIG. 13  is a flowchart illustrating an exemplary embodiment of the present invention comprising burst testing at constant pressure with increasing temperature. 
       FIG. 14  is a flowchart illustrating an exemplary embodiment of the present invention comprising burst testing at constant temperature with increasing pressure. 
       FIG. 15  is a graph illustrating various measures of stress versus temperature for an exemplary base material, alloy 617. 
       FIG. 16  is a graph illustrating the results of an exemplary burst test with temperature and pressure held constant. 
       FIG. 17  is a graph illustrating the results of a further exemplary burst test with temperature and pressure held constant. 
       FIG. 18  is a graph illustrating the results of a further exemplary burst test with temperature and pressure held constant. 
       FIG. 19  is a graph illustrating the results of an exemplary burst test with constant pressure and increasing temperature. 
       FIG. 20  is a graph illustrating the results of a further exemplary burst test with constant pressure and increasing temperature. 
       FIG. 21  is a graph illustrating the results of a further exemplary burst test with constant pressure and increasing temperature. 
       FIG. 22  is a graph illustrating the results of an exemplary burst test with constant temperature and increasing pressure. 
       FIG. 23  is a graph illustrating the results of a further exemplary burst test with constant temperature and increasing pressure. 
       FIG. 24  is a graph illustrating the results of a further exemplary burst test with constant temperature and increasing pressure. 
   

   In describing the preferred embodiment of the invention which is illustrated in the drawings, specific terminology will be resorted to for the sake of clarity. However, it is not intended that the invention be limited to the specific term so selected and it is to be understood that each specific term includes all technical equivalents which operate in a similar manner to accomplish a similar purpose. 
   DETAILED DESCRIPTION OF THE INVENTION 
   An exemplary microchannel device  10  is shown in  FIGS. 1-5   b . Turning first to  FIG. 1 , the exemplary microchannel device  10  is shown in an exploded axonometric projection view. The microchannel device  10  comprises a plurality of shims (e.g.,  12 ,  14 ,  16 ) which cooperate to form a plurality of various features of the microchannel device  10 . Shims generally refer to substantially planar plates or sheets that can have any width and height and preferably have a thickness (smallest dimension) of ten millimeters (mm) or less, and, in some preferred embodiments, between 50 and 1,000 microns (1 mm). The microchannel device  10  comprises a first end shim  12  which is a solid plate to partially enclose and define the microchannel device  10 . Next is a first manifold shim  14  which includes a first manifold slot  30 . Next is one or more first channel shims  16  which comprise a plurality of first ribs  33 , which first ribs  33  at least partially define a plurality of first channel slots  32 . As will be appreciated by those skilled in the relevant art, multiple first channel shims  16  may be provided to at least partially define the dimensions of a plurality of first channels  132  (e.g.,  FIGS. 3   a - 5   b ). The dimensions of the first manifold slot  30  may at least partially define the dimensions of a first manifold  130  (e.g.,  FIGS. 2-3   b ,  5   a , and  5   b ). As will also be appreciated by those skilled in the relevant art, multiple first manifold shims  14  may be provided to at least partially define the dimensions of the first manifold  130 . As will also be appreciated by those skilled in the relevant art, the first manifold  130  may, for example, enable the distribution of fluids entering the microchannel device  10  to the first channels  132 . Next is a second manifold shim  18  which includes a second manifold slot  34  and the dimensions of the second manifold slot  34  at least partially define the dimensions of a second manifold  134  (e.g.,  FIGS. 2-3   b ,  5   a , and  5   b ). As will be appreciated by those skilled in the relevant art, multiple second manifold shims  18  may be provided to at least partially define the dimensions of the second manifold  134 . As will also be appreciated by those skilled in the art, the second manifold  134  may, for example, enable the collection of fluids exiting the microchannel device  10  from the first channels  132 . Next is a third manifold shim  22  which includes a third manifold slot  36  and the dimensions of the third manifold slot  36  at least partially define the dimensions of a third manifold  136  (e.g.,  FIGS. 2-3   b ,  5   a , and  5   b ). As will be appreciated by those skilled in the relevant art, multiple third manifold shims  22  may be provided to at least partially define the dimensions of the third manifold  136 . Next is one or more second channel shims  24  which comprise a plurality of second ribs  39 , which second ribs  39  at least partially define a plurality of second channel slots  38 . As will be appreciated by those skilled in the relevant art, multiple second channel shims  24  may be provided to at least partially define the dimensions of a plurality of second channels  138  (e.g.,  FIGS. 3   a - 5   b ). As will also be appreciated by those skilled in the relevant art, the third manifold  136  may, for example, enable the discharge of fluids from the second channels  138 . Next is a fourth manifold shim  26  which includes a fourth manifold slot  40  and the dimensions of the fourth manifold slot  40  at least partially define the dimensions of a fourth manifold  140  (e.g.,  FIGS. 5   a  and  5   b ). As will be appreciated by those skilled in the relevant art, multiple fourth manifold shims  26  may be provided to at least partially define the dimensions of the fourth manifold  140 . As will be appreciated by those skilled in the art, the fourth manifold  140  may, for example, enable the distribution of fluids to the second channels  138 . Finally, a second end shim  28  is provided to partially enclose and further define the microchannel device  10 . 
   The microchannel device  10  may be a reactor and heat exchanger in combination. The microchannel device  10  may be designed or operated to conduct one or more chemical unit operations, including mixing, chemical reaction, heating, cooling, heat exchange, vaporization, condensation, distillation, absorption, adsorption, or solvent exchange. The shims (e.g.,  12 ,  14 ,  16 ) comprise a base material which may comprise any material that provides sufficient strength, dimensional stability, and heat transfer characteristics to permit operation. These materials include steel, stainless steel (e.g.,  304 ,  316 ) aluminum, titanium, nickel, platinum, rhodium, copper, chromium, brass, alloys of any of the foregoing metals (e.g., Inconel 617® (Special Metals), Haynes HR-120® (Haynes, Int&#39;l., Inc., Kokomo, Ind.), Haynes HR-230® (Haynes Int&#39;l.), Hastelloy® (Haynes Int&#39;l.), Monel® (Special Metals), or oxidative dispersion-strengthened alloys), polymers (e.g., thermoset resins), ceramics, glass, composites comprising one or more polymers (e.g., thermoset resins) and fiberglass, quartz, silicon, or a combination of two or more thereof. These materials may be supplied in rolled form, cat, forged, or extruded. The components of the microchannel device  10  may be fabricated using known techniques including wire electrodischarge machining, conventional machining, laser cutting, photochemical machining, electrochemical machining, molding, water jet, stamping, etching (e.g., chemical, photochemical, or plasma etching), and combinations thereof. A stack of shims (e.g.,  12 ,  14 ,  16 ) may be joined via diffusion bonding, laser welding, diffusion brazing, and similar methods to form an integrated device. 
   As will be appreciated by one skilled in the relevant art, a virtually limitless variety of microchannel devices are possible, most extremely complex, but which embody the basic features described herein. 
   Turning now to  FIG. 11 , a method is shown for determining the MAWP of a pressure vessel, and particularly for a microchannel device. When entering at node  200 , a determination is made as to whether an operating temperature (T Operating ) is greater than or equal to a threshold temperature (T Threshold ). T Operating  is that temperature at which the device will operate during those operations of interest and which will present the conditions for which the MAWP must be determined. For example, T Operating  might normally be selected at the normal operating temperature of the device under normal, sustained operation. Other considerations would include, for example, the maximum temperatures expected during normal startup or shutdown. Additionally, the maximum temperature expected from random operational perturbations or normal operational changes may be considered. As a selection, T Operating  may be selected as the maximum of one of the abovementioned temperatures. If, for example, the pressure vessel is a microchannel device and the microchannel device comprises a steam methane reformer, T Operating  may be selected from between about 800 deg. C. to about 1200 deg. C. Additionally, T Operating  may be selected from between about 800 deg. C. and about 950 deg. C. As will be appreciated by those skilled in the relevant art, T Operating  will vary but will be easily and eminently determinable from the process being considered. T Threshold  may be chosen as generally the lowest temperature at which the base material creep properties limit allowable stress over such base material properties as tensile stress or yield stress. T Threshold  may be selected by comparing various limits for allowable stress set by considering engineering safety factors. One limit, for example, may be one-half the absolute melting point temperature of the base material. For example, when the base material is alloy 617, the base material melting point may vary somewhat (approximately 1,333 deg. C. to about 1,377 deg. C.). T Threshold  may fall between about 530 deg. C. and about 552 deg. C. using this criterion. Alternatively, for example, T Threshold  may be selected as about 0.3 times the absolute melting point of the base material. Thus, for a base material of alloy 617, T Threshold  may fall between about 209 deg. C. and about 222 deg. C. T Threshold  may, for example, be selected as the temperature at which a base material creep stress limit becomes less than a base material tensile stress limit or a base material yield stress limit. For example, the limit may be based upon the lowest temperature associated with one of the following criteria: (a) a base material creep stress limit of 80 percent of the minimum stress which causes rupture at the end of about 100,000 hours; (b) a base material creep rate stress limit about 100 percent of the average stress which causes a creep rate of about 0.01 percent per 1,000 hours, or (c) a base material tensile limit of about the tensile strength divided by 3.5. Turning to  FIG. 15 , for example, a base material of alloy 617 would show T Threshold  of between about 625 deg. C. and about 710 deg. C. T Threshold  may, for example, be selected at the temperature at which a base material creep stress limit becomes less than a base material yield limit. For example, when the base material yield limit is about two-thirds the yield stress limit. Turning to  FIG. 15 , for example, a base material of alloy 617 would show T Threshold  of about 625 deg. C. As will be appreciated by those skilled in the relevant art, the base material may comprise, for example, a nickel alloy containing at least 35 percent nickel or at least 60 percent nickel. 
   Returning now to  FIG. 11 , node  202 , if T Operating  is not greater than or equal to T Threshold  (node  200  is false), an at least one material property at a design temperature for joined material (JMP Design ) is compared with the at least one material property at a design temperature for the base material (BMP Design ). A typical base material test specimen  50  is shown in  FIG. 6  while a typical joined material test specimen  52  is shown in  FIG. 7 . Also shown in  FIG. 7 , as an example, are representative shims  56  joined together with a diffusion bond  58 . In practice a plurality of representative shims  56  comprising base material are bonded together and a test specimen  52  prepared. Material properties may include, but not limited to, ultimate tensile strength, yield strength, yield tensile strength, percent elongation at failure, creep rate, creep rupture, crack propagation rate, or combinations thereof. Design temperature (T Design ) is selected to take into account safety factors and unknowns in the device and its operation. For example, T Design  will invariably be greater than or equal to T Operating  and may be selected as T Operating  plus, for example, 50 deg. C. If the evaluation represented by node  202  does not produce a JMP Design  superior or equal to BMP Design  (node  202  is false), a conventional burst test prescribed by a pressure vessel certification organization, such as the ASME, will be satisfactory to determine the MAWP (node  208 ). 
   If node  202  of  FIG. 11  produces a true determination (JMP Design  is superior or equal to BMP Design ) the presence or absence of spurious artifacts is determined (node  204 ). Such spurious artifacts are those generally unavoidable elements which may occur during manufacture and fabrication of the device. Turning to  FIG. 8 , they include, in a shim-based microchannel device joined with diffusion bonding, for example, metal carbide precipitates  60 . Grain growth (not shown) may also exist, for example, when the grain size grows to at or near shim thickness. 
   Turning now to  FIG. 9 , four representative shims  70 ,  72 ,  74 ,  76  are bonded together. Representative ribs  73 ,  75  of two of the shims  72 ,  74  are misaligned, producing offsets  78 ,  80  of the ribs  73 ,  75 . Such offsets  78 ,  80 , may produce additional stress concentration points as well as reduced areas of bonding and changes to the dimensions of the channels  82 ,  84 . 
   Turning now to  FIG. 10 , in representative fashion, a top shim  186 , a middle shim,  190 , and a bottom shim  194  sandwich a first shim portion or rib  188  and a second shim portion  192 . Formed therein are two channels  92 ,  292 . As is illustrated in  FIG. 10 , notches, or rollover artifacts  94 ,  96  and a bowed portion  296  are present. Stamp rollover artifacts, for example, may be produced during the stamping process and may contribute to stress concentration points  94 ,  96 . Stresses in manufacturing may also contribute to bowing artifacts  296  which can be exacerbated by rollover artifacts. 
   The determination of whether an artifact is, in fact, spurious, depends upon, for example, comparing the size of the stamp rollover  94 ,  96 , the metal carbide precipitates  60 , the misalignment or offset of shim ribs ( FIG. 9 ), shim thickness (not shown), bowing of channel walls ( FIG. 10 ), comparative channel-to-channel or layer-to-layer pressure drop, or grain size (not shown) to channel size or shim thickness. 
   Returning again to  FIG. 11 , if the evaluation represented by node  204  is true, a conventional burst test prescribed by a pressure vessel certification organization, such as the ASME, will be satisfactory to determine the MAWP (node  208 ). If the evaluation represented by node  204  is false, calculations promulgated by a pressure certification organization, such as the ASME, will be satisfactory to determine the MAWP (node  206 ). 
   If the determination at node  200  produces a true result (T Operating  is greater than or equal to T Threshold ), node  210  is entered and a determination made whether a condition of an at least first material property at a low temperature for a specimen of joined material (JMP Low ) is superior or equal to the at least first material property at the low temperature for a specimen of base material (BMP LOW ) is true or false. Material properties may include, but not limited to, ultimate tensile strength, yield strength, yield tensile strength, percent elongation at failure, creep rate, creep rupture, crack propagation rate, or combinations thereof. The low temperature may be selected as a less than T Threshold  and is often selected as about room temperature or between about 20 deg. C. and about 23 deg. C. 
   If the determination at node  210  is false (JMP Low  is not superior or equal to BMP Low ), at least one burst test is performed on a representative burst test device (node  214 ). Such tests comprise independently increasing temperature and pressure of the representative burst test device from a first state to a second state, the second state comprising a temperature greater than or equal to T Threshold . 
   If the determination at node  210  is true (JMP Low  is superior or equal to BMP Low ), node  212  is entered and a determination made whether JMP Design  is superior or equal to BMP Design . As will be appreciated by those skilled in the relevant art, node  212  comprises the same determination as node  202  and the description stated above will apply. If the determination at node  212  is false (JMP Design  is not superior or equal to BMP Design ), at least one burst test is performed on a representative burst test device. Such tests comprises independently increasing temperature and pressure of the representative burst test device from a first state to a second state, the second state comprising a temperature greater than or equal to T Threshold . 
   If the determination at node  212  is true (JMP Design  is superior or equal to BMP Design ), node  216  is entered and a determination made about the presence or absence of spurious artifacts. As will be appreciated by those skilled in the relevant art, node  216  comprises the same determination as node  204  and the description stated above will apply. 
   If the determination at node  216  is false (no spurious artifacts), calculations promulgated by a pressure certification organization, such as the ASME, will be satisfactory to determine the MAWP (node  206 ). 
   If the determination at node  216  is true (spurious artifacts present), node  218  is entered and a determination made if the effects at T Design  of spurious artifacts are calculable. If so, node  208  is entered and the burst test described earlier is performed. The effects of artifacts may not be calculable, for example, depending upon the size of stamp rollover, carbide precipitates, misalignment or offset of shim ribs, shim thickness, bowing of channel walls or grain size growth relative to shim thickness. 
   If the determination at node  218  is false (T Design  effects of spurious artifacts not calculable), node  214  is entered and at least one burst test performed on a representative burst test device. Such tests comprise independently increasing temperature and pressure of the representative burst test device from a first state to a second state, the second state comprising a temperature greater than or equal to T Threshold . 
   As noted above, a microchannel device may be extremely complex. The representative burst test device must, however, be representative of the device with respect to channel dimensions including, but not limited to, height, width, length, or combinations thereof; fabrication methods, including, but not limited to, stamping, bonding, including, but not limited to diffusion bonding, considering, but not limited to, method, time, temperature, pressure, or combinations thereof; surface preparation, including, but not limited to, finish, passivation, etching, cleaning, coating, flatness, lay, waviness, or combinations thereof; wall thicknesses; base material, including, but not limited to, alloy 617; rib dimensions; heat treat cycles; heating cycles during manufacture; shim thickness; symmetry; size scale; or combinations thereof. 
   Returning now to  FIG. 11 , node  214  comprises at least one burst test of at least one representative burst test device, the burst test comprising independently increasing the temperature and pressure of the burst test device from a first state to a second state, the second state comprising a temperature greater than or equal to the T Threshold . 
   Turning now to FIGS.  12  and  16 - 18 , a method is shown for burst testing a representative burst test device when the conditions shown in  FIG. 11  apply as described above. The method comprises first heating the representative burst test device at a substantially constant rate from a first state temperature to a second state temperature, the second state temperature being greater than or equal to a T Threshold  and allowing the device to thermally equilibrate by holding the device at substantially the second state temperature for a fixed period of time. Subsequently, the device is held at the second state temperature while being pressurized at a substantially constant rate from a first state pressure to a second state pressure and held for a fixed period of time. As shown in exemplary fashion in  FIGS. 16-17  creep eventually causes the device to fail. The step of pressurizing the device may comprise introducing a pressurizing gas into the representative burst test device. Alternatively, the pressurizing gas may be preheated prior to being introduced into the device. The second state pressure may be greater than about 30 bar. The constant rate of pressurizing may be selected as being between about one bar per minute and about ten bar per minute. The constant rate of pressurizing may be below a pressure shock limit, that is, the increase in pressure does not contribute to an impact load on the material being tested. The second state temperature may be greater than about 900 deg. C. The constant rate of heating may be selected to avoid significant creep. The constant rate of heating may be selected as being between about one deg. C. per minute and about ten deg. C. per minute. 
   Turning now to FIGS.  13  and  19 - 21 , a further method is shown for burst testing a representative burst test device when the conditions shown in  FIG. 11  apply as described above. The method comprises first pressurizing the representative burst test device at a substantially constant rate from a first state pressure to a second state pressure and holding the device at substantially the second state pressure for a fixed period of time. Subsequently, while holding the device a substantially the second state pressure, the device is heated at a substantially constant rate from a first state temperature to failure. The second state pressure may be greater than about 30 bar. The constant rate of pressurizing may be between about one bar per minute and about ten bar per minute. The constant rate of pressurizing may be below a pressure shock limit. The constant rate of temperature increase may be selected to avoid significant creep. The constant rate of temperature increase may be between about one deg. C. per minute and about ten deg. C. per minute. 
   Turning now to FIGS.  14  and  22 - 24 , a further method is shown for burst testing a representative burst test device when the conditions shown in  FIG. 11  apply as described above. The method comprises first heating the representative burst test device at a substantially constant rate from a first state temperature to a second state temperature and allowing the device to thermally equilibrate. Subsequently, and while holding the device at the second state temperature, pressurizing the device at a substantially constant rate from a first state pressure to a second state pressure. The second state temperature may be selected as about T Design  and it may be greater than about T Threshold . The constant rate of heating may be selected to avoid significant creep. The constant rate of heating may be selected as being between about one deg. C. per minute and about ten deg. C. per minute. The second state temperature may be greater than about 900 deg. C. The constant rate of pressurizing may be selected to avoid a pressure shock limit. The constant rate of pressurizing may be selected as between about one bar per minute and about ten bar per minute. The step of pressurizing the device may further comprise pressurizing the device to failure. 
   This detailed description in connection with the drawings is intended principally as a description of the presently preferred embodiments of the invention, and is not intended to represent the only form in which the present invention may be configured or utilized. The description sets forth the designs, functions, means, and methods of implementing the invention in connection with the illustrated embodiments. It is to be understood, however, that the same or equivalent functions and features may be accomplished by different embodiments that are also intended to be encompassed within the spirit and scope of the invention and that various modifications may be adopted without departing from the invention or scope of the following claims.