Abstract:
A flame ionization detector burner, includes a housing having a generally cylindrical flame cavity therein extending along a generally longitudinal axis of the housing and a housing exterior with intake passageways communicating between the housing exterior and the flame cavity for passing fuel, air and sample flows into the flame cavity and an exhaust passage communicating between the housing exterior and the flame cavity for passing exhaust gasses out of the flame cavity. A burner carried in the housing cavity receiving the fuel, air, and sample flow generates a flame to ionize the sample. An ion collector plate spaced away from the burner carried in the time cavity collects sample ions and provides an electrical output representative of the sample ions to the housing exterior via an electrical feedthrough between the flame cavity and the housing exterior. The housing includes at least two cylindrical sidewalls removably joined together along the general longitudinal axis of the housing.

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to instruments used to evaluate selected components of a chemical sample. More specifically, the present invention relates to a flame ionization detector. 
     Flame ionization detectors are typically used to analyze hydrocarbon components in a sample taken from a source such as exhaust from an industrial smoke stack, an automobile engine during its testing or adjustment, and high purity gasses during their production and testing. 
     A flame ionization detector burner typically includes a housing with a flame cavity with intake passageways and an exhaust passageway. Inside the flame cavity there is a burner that receives fuel, air, and a sample to create a flame to ionize the samples. An ionization collector plate collects the ions and produces a signal. representative of the concentration of sample ions. Flame ionization detectors burners are often located in hazardous locations where flammable vapors are present in which the flame ionization detector is required to meet hazardous location approvals. Flame ionization detector burners are also often incorporated into other instruments such as hydrocarbon analyzers or gas chromatographs. 
     Flame ionization detector burners are composed of a variety of components that often require seals and can be complex and expensive to manufacture and service. Flame ionization detectors have constraints that limit flame stability resulting in reduced accuracy. 
     Additionally, flame ionization detector burners have thermal constraints that limit the proximity of their installation near heat sensitive components such as instrumentation. 
     For the foregoing reasons there is a need for a flame ionization detector that is simpler to manufacture, has improved flame stability, and has reduced thermal constraints. 
     SUMMARY OF THE INVENTION 
     A flame ionization detector burner, includes a housing having a generally cylindrical flame cavity therein extending along a generally longitudinal axis of the housing and a housing exterior with intake passageways communicating between the housing exterior and the flame cavity for passing fuel, air and sample flows into the flame cavity and an exhaust passage communicating between the housing exterior and the flame cavity for passing exhaust gasses out of the flame cavity. A burner carried in the housing cavity receives the fuel, air, and sample flow and generates a flame to ionize the sample. An ion collector plate spaced away from the burner carried in the flame cavity collects sample ions and provides an electrical output representative of the sample ions to the housing exterior via an electrical feedthrough between the flame cavity and the housing exterior. The housing further comprises at least two cylindrical sidewalls removably joined together along the general longitudinal axis of the housing. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a block diagram of a flame ionization detector in accordance with one aspect of the resent invention. 
     FIG. 2 is an exploded view of a flame ionization detector in accordance with one aspect of the resent invention. 
     FIG. 3 is a more detailed exploded view of a portion of the flame ionization detector shown in FIG.  2 . 
     FIG. 4 is an assembled view of the portion shown in FIG.  3 . 
     FIG. 5 is a cross-sectional view of the components shown in FIG.  4 . 
     FIG. 6 is a more detailed exploded view of a portion of the flame ionization detector shown in FIG.  2 . 
     FIG. 7 is an assembled vies of the component shown in FIG.  3 . 
     FIG. 8 is an exploded cross-sectional view of a burner of FIG.  2 . 
     FIG. 9 is a top view illustrating the flame ionization detector disclosed in FIG. 2 as used in a hydrocarbon analyzer in accordance with another aspect of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIG. 1 is a simplified diagram of analyzer  2  including flame ionization detector  10  shown in cross section in accordance with the present invention. Analyzer  2  includes processing or analysis electronics  24 , gauge  6 , gas sample source  26 , air source  28 , and fuel source  30 . Detector  10  includes ion collector  78 , burner  33  and housing  40 . Housing  40  is joined together along longitudinal axis  4 . Collector  78  is electrically coupled to circuitry  24  by pin  122  which extends through housing  40 . A gas sample is provided from sample source  26  and past a flame at burner  33  and out exhaust passage  126 . The sample may be, for example, methane gas. Assembly  12  also preferably includes an appropriate valve configuration and a controllable flow regulator for tightly regulating the flow of sample and carrier gas therethrough. In one embodiment, the sample inlet assembly  12  implements a sample gas chromatographic technique. A slug of the sample gas is provided in a carrier gas stream and passed through a column containing a material that absorbs gases at rates proportional to the molecular weight of the gas. The slug of sample gas is therefore separated into its constituents, the lighter components coming through the column prior to the heavier components. Air is provided from air source  28  and fuel is provided from fuel source  30 . The fuel is preferably hydrogen or a hydrogen/diluent mixture. Assemblies  14  and  16  include low volume capillaries or conduits for introduction of the air and fuel, respectively. The conduits are coupled through a valve system for selective introduction of the air and fuel to burner  33  for combustion. 
     Ion collector  78  is arranged proximate the flame generated by ignition of the air and fuel mixture at burner  33 . Collector  78  includes polarized electrodes which collect the ions produced as the sample gas passes through the flame. Collection of the ions causes an ionization current to flow through processing or analyzing electronics  24 . The current is proportional to the rate at which carbon atoms enter the flame and is therefore a measure of the concentration of hydrocarbons in the sample. Information related to these concentrations may be stored for further analysis or output on a display device such as gauge  6 . 
     FIG. 2 is an exploded view of flame ionization detector burner  10  in accordance with the present invention. Flame ionization detector  10  includes base  32 , burner  33 , burner seal  34 , flame tip assembly  36 , air baffle  38 , housing or body portion  40  having a cylindrical flame cavity formed therein, burner seal  42 , cap  44 , spark arrestor  46 , and vent adaptor  48 . FIG. 2 also illustrates capillaries  50 ,  52  and  54  which are coupled to air source  28 , fuel source  30 , and sample source  26 , respectively, shown in FIG.  1 . Capillaries  50 ,  52  and  54  provide conduits to conduct the air, fuel and sample to detector  10 . By providing these gases through capillaries, the entire internal volume associated with detector  10  can be kept very small. 
     Capillaries  50 ,  52  and  54  are coupled to receiving apertures in receiving assemblies  56 ,  58  and  60 , respectively. Receiving assemblies  56 ,  58  and  60 , in turn, are coupled to passageways within base  32 . Base  32  directs the transportation of the fuel gas, air and sample gas, through predefined passages which are preferably machined into base  32 . The passages connected to air receiving assembly  56  are coupled to a central region  62  of base  32  where they enter air baffle  38 . Air baffle  38  directs the flow of air therethrough in a generally annular ring from a lower portion thereof out annular exit  64  therein. Flow is preferably highly laminar. Air baffle  38  also provides a plurality of pin holes  66  which act to introduce purge air into the inner body of detector  10 . 
     Fuel introduced through fuel receiving assembly  58  passes base  32  and up through an inner central passage  68 . Flame tip assembly  36  threadably mates with the interior of central passage  68 . Seal  34  is a PTFE seal which is introduced between a shoulder  70  on flame tip assembly  36  and central passage  68 . Fuel enters through a lower aperture in flame tip assembly  36  and exits through upper aperture  72  in the center of air baffle  38 . Flame tip assembly  36  includes sapphire orifice  73  which forms the central passageway in aperture  72 . Sapphire orifice  73  is preferably press fit into the metallic housing of flame tip assembly  36 . Sapphire orifice  73  advantageously provides a very precise and smooth hole therethrough which contributes to flame stability. Further, sapphire is substantially corrosion resistant and will maintain the precision hole even when exposed to caustic gasses or wear over extended use. 
     Flame stability is improved through the use of baffle  38 , sapphire orifice  73 , pin holes  66 , and the relatively small size of the flame cavity (less that l 0 cc). Improved flame stability is beneficial because it improves the sensitivity of the device. Baffle  38  directs air generally inwardly toward the flame tip assembly  36 . Thus, the introduction of the regulated flow of fuel gas through the smooth sapphire orifice  73 , and the regulated flow of air surrounding it, and directed toward it, provide a mixture of fuel and air. Pin holes  66  advantageously purge the chamber from the base  32  and promote laminar air flow. A steady flame is thus generated proximate the tip of fuel tip assembly  36  at a point where the fuel and air meet. 
     Body assembly  40  includes body sections  74  and  76 , seal  42 , flame arrestor  46 , and exhaust adaptor  48 . Sections  74  and  76  are preferably of an electrical and thermal insulator, such as plastic. Electrical resistivity reduces leakage of electrical current from ion collector plates  78 . Thermal insulation allows detector  10  to be placed in a smaller area with reduced insulation requirements. Body assembly  40  includes a air of ion collector plates  78  (only one of which is shown in FIG. 2, the other being oppositely disposed on body section  74 ); ignitor  18  and flame detector  20  shown in FIG.  7 ). 
     Body sections  74  and  76  are each formed as a portion of a cylinder having exterior surfaces  80  and  82 , and mounting surfaces  84  and  86 , respectively. Surfaces  84  and  86  are provided with a plurality of threaded apertures  88 . Screws  90  threadably engage apertures  88  to connect body portions  74  and  76  together at mounting surfaces  84  and  86 , respectively. When assembled, the pair of oppositely disposed recessed portions  90  and  92  form a generally cylindrical flame cavity which encloses flame tip assembly  36 , air baffle assembly  38 , ion collector plates  78 , ignitor  18  and flame detector  20 . The flame cavity is surrounded by grooves  94  and  96 . Seal  42  is preferably made of Viton which is available from Parker Seal, P.O. Box 11751, Lexington, Ky. 40512. Seal  42  has upper generally circular portion  98 , lower generally circular portion  100 , and generally parallel legs  102  and  104  which are connected to circular portions  98  and  100 . All of the portions of seal  42  are integrally formed with one another as a unitary member and seal  42  fits into grooves  94  and  96 . In another embodiment, each of the individual portions of seal  42  are separate, but are connected to one another to form a unitary member. 
     Body sections  74  and  76  are each provided with upper surfaces  106  and  108 , and lower surfaces  110  and  112 , respectively. Surfaces  106 - 112  are provided with threaded holes aligned with corresponding holes in cap  44  and base  32 . Screws  90  are adapted to pass through, and threadably engage, the holes to connect cap  44  and base  32  to body sections  74  and  76 . When body sections  74  and  76  are attached to base  32  and cap  44 , seal  42  seals substantially the entire internal cavity. The design also advantageously provides relatively long flame paths to prevent passage of flame through body assembly  40 . Wings  77 A and  77 B on base  32  and wing  79 A (and an opposed wing which is not shown) on cap  44  cover portions of the seams between sections  74  and  76  to prevent flame passage therepast. This configuration lengthens the flame path in both the radial and longitudinal directions. Further, the internal volume associated with the detector  10  is preferably kept less than 10 cubic centimeters which permits the housing to be formed of a plastic material (such as Thorlon® available from Amoco Polymers, Inc. at 4500 McGinnis Ferry Road, Alpharette, Ga. 30202) pursuant to the CENELEC Standard No. EN 50018. The plastic housing is inexpensive to manufacture through an extrusion process is electrical resistive to reduce leakage of electrical current through the detector and thermally insulating to contain heat within the housing. Therefore, even though detector  10  is in an explosion proof container, it is significantly less expensive than prior art explosion proof containers. 
     Flame arrestor  46  is press fit into an aperture in body section  76 . Flame arrestor  46  is preferably a porous metal made of discrete particles and bonded at their contact points by a sintering process. Flame arrestor  46  provides sufficient mechanical strength to withstand sudden pressure shock, and sufficient heat conduction to assist in extinguishing any flame which approaches flame arrestor  46 . Exhaust fitting  48  transports byproducts of the sample gas combustion to outside of the burner body assembly  40 . Fitting  48  is attached to the burner body by a suitable threaded connection which is assembled over flame arrestor  46 . Exhaust fitting  48  also provides redundant support to flame arrestor  46 . 
     The electrical connection made to ignitor filament  18 , flame detector  20  and polarized plates  78  is by electrically conductive pins which are spot welded to the appropriate leads of those items and which are press fit through corresponding holes (such as holes  114 ,  116  and  118 ) in body sections  74  and  76 . The pins are preferably stainless steel and thus facilitate electrical connections from items in the inner cavity of body assembly  40  to the exterior portion of body sections  74  and  76  as shown in FIG.  5 . 
     FIGS. 3-7 illustrate the connection of these components in greater detail. FIG. 3 is an exploded view illustrating the assembly of one of polarized collector plates  78  onto body section  76 . FIG. 3 illustrates through hole  120  extends from the interior surface to the exterior surface of section  76  Pin  122  has an enlarged diameter portion  124  which is slightly larger than the interior diameter of hole  120 . Thus, press fitting pin  122  into hole  120  causes tight frictional engagement between portion  124  and body section  76  to retain pin  122  in hole  120 . This design provides a highly reliable connection with a long flame path which tightly seals the flame cavity. Plate  78  is spot welded to the end of pin  122  which provides an electrical connection to and provides support of plate  78 . Pin  122  provides electrical connection to plate  78  through body section  76 . FIG. 3 also shows exhaust passageway aperture  126  which receives exhaust fitting  48 . FIG. 4 shows plate  78  assembled to body section  76  via the press fitting process described with respect to FIG.  4 . Pin  122  is shown in phantom in FIG.  4 . FIG. 5 is a cross-sectional view taken along section lines  5 — 5  in FIG.  4 . FIG. 5 illustrates how press fitting pin  122  into hole  120 , and spot welding plate  78  to pin  120 , precisely locates plate  78  through an easy and efficient assembly process. 
     FIGS. 6 and 7 illustrate the connection of flame detector  20  and ignitor filament  18 . Flame detector  20 , in one preferred embodiment, is implemented as a thermistor which has a plurality of leads  128  and  130 . Leads  128  and  130  are spot welded to pins  132  and  134 , respectively. Pins  132  and  134  are press fit into holes  136  and  138  in body section  74 . In a preferred embodiment, the thermistor is placed above the exhaust aperture  126  so that the internal temperature of the detector can be measured and monitored. Ignitor coil  18  is preferably positioned just below vent aperture  126  so that an easy ignition can be made. Coil  18  also has a pair of leads  140  and  142  which are spot welded to pins  144  and  146 . Pins  144  and  146  are, in turn, press fit into apertures in body section  74  to provide electrical connection from the inner cavity defined by body sections  74  and  76  to the exterior thereof. 
     FIG. 8 is an exploded cross-sectional view of burner  33  showing the relationship between baffle  38 . flame tip  36  and sapphire orifice  72 . Sapphire orifice  72  has a diameter of 0.094 inches, a thickness of 0.063 inches and includes passageway  148  having a diameter of 0.015 inches and may be obtained from Bird Precision, of P.O. Box 569, Waltham, Mass. 02254. Orifice  73  is press fit in aperture  72  and tip  36  and baffle  38  are threaded into base  32  of FIG.  2 . In one preferred embodiment, four evenly pin holes  66  are evenly spaced at ninety degree increments around flange  66 . 
     FIG. 9 illustrates the implementation of flame ionization detector  10  in hydrocarbon analyzer  150  which includes housing  152  having three chambers. The design also provides a small profile such that the detector may be used in multiple applications such as analyzer  150 . Electronics chamber  154  houses a vertically mounted printed circuit board  156  containing processing and power supply circuitry. Chamber  154  also includes valve actuator  158  which actuates a valve controlling flow of gas through analyzer  150 . Central isolation chamber  160  is formed by bulkheads  162  and  164  coupled to provide a physical separation between electronics chamber  154  and a pneumatics chamber  166 . Bulkheads  162  and  164  are preferably metal plates connected within housing  152  to form isolation zone  160  and act to prevent any gas leaks in pneumatic chamber  166  from reaching electronics chamber  154 . Isolation chamber  160  provides an. additional buffer between the electronics and the detector  10 . 
     Chamber  166  includes valve  168  which is coupled to valve actuator  158 , heater manifold  170 , flame ionization detector  10  and column configurations  172 . Chamber  166  is preferably a thermally controlled insulated enclosure having a temperature maintained at, for example, 120° C. which allows the instrument to measure certain hydrocarbons without. difficulty. Chamber  166  includes regulator valves  174 , preferably fluistors, which are microprocessor controlled, electrically set valves formed in silicon. The valves are used to control pressure instead of conventional pressure regulators. 
     Electrical connections between electronics chamber  154  and pneumatic chamber  166  are preferably provided by electronic connection assembly  176 . In a preferred embodiment, electronic connection assembly  176  includes a printed circuit board  178  which extends from electronics chamber  154 , through isolation zone  160  and into pneumatic chamber  166 . Printed circuit board  178  preferably contains electrically conductive traces, which are current limited and voltage limited (or power limited) by over-voltage protection devices and current limiting resistors. This prevents power dissipation in pneumatic chamber  166  which could provide an ignition source to any volatile gas which resides in chamber  166 . The traces on printed circuit board  178  are preferably coupled to appropriate circuits on circuit board  156 , as indicated by arrow  180 . Also, the traces are preferably coupled in pneumatic chamber  166  to control fluistors  174 , flame ionization detector  10 , and any other suitable devices which require electronic manipulation or control by the circuitry in chamber  154 . 
     By separating the electronics chamber from the pneumatic chamber, and by providing the flame ionization detector in an explosion proof container, significant problems associated with implementing a flame ionization detector in a volatile environment are overcome. The electrical connections between the electronics chamber and the pneumatic chamber are provided in a power limiting fashion such that the connections cannot dissipate enough power to provide an ignition source. 
     The present invention provides the flame ionization detector of the invention can be disassembled in a highly efficient manner and provides for the placement and assembly of parts in the detector in a highly accurate and efficient manner. The invention may be used in explosion proof, or non-explosion proof designs. 
     Although the present invention has been described with reference to preferred embodiments, workers skilled in the art will recognize that changes may be made in form and detail without departing from the spirit and scope of the invention.