Abstract:
A stream selector for a process analyzer prevents contamination of one sample from previous samples without requiring a common outlet header. The stream selector includes a valve manifold having a base plate and a series of valve modules individually removeably attached to the base plate. The base plate includes all the process connections for the fluid lines. Each valve module includes a module body with a pair of valve assemblies. The valve assemblies in each module are operated simultaneously through a common actuation passage. A poppet valve in each valve assembly has a valve head with a double-seated seal. The double seated seal controls the flow of fluid through a vent passage in the valve body; and between inlet and outlet passages in the valve body. The outlet passage from one valve assembly is fluidly-connected to the inlet passage of an adjacent valve assembly to create a double-block and bleed configuration. The downstream valve assembly is then connected to the process analyzer. If additional valve module(s) are used, the outlet of the downstream valve assembly in the first module can be routed through the vent passage of the downstream valve assembly of the second module, and so on. A cover encloses the valve assembly in each valve module. The cover is attached with a series of threaded bolts to the module body. At least some of the bolts fix the module body to the base plate. Removing the bolts allows individual modules to be removed from the base plate without removing adjacent modules.

Description:
CROSS-REFERENCE TO RELATED CASES  
       [0001]    The present application claims the benefit of the filing date of U.S. Provisional Application Ser. No. 60/254,335; filed Dec. 8, 2000 and U.S. Provisional Application Ser. No. 60/226,216; filed Aug. 18, 2000. 
     
    
     
       FIELD OF THE INVENTION  
         [0002]    The present invention relates generally to stream selectors for process analyzers.  
         BACKGROUND OF THE INVENTION  
         [0003]    Stream selectors for process analyzers control the flow of fluid into the analyzer. The selector selects a single sample stream from multiple flow streams to pass on for analysis. This reduces the cost of analyzing multiple gas and liquid process streams in, e.g., a manufacturing or laboratory facility, as each analyzer is relatively expensive. The stream selector includes a series of valves which are typically electrically controlled. It is conventional practice to use a common outlet header connected to each valve to route the selected sample stream to the analyzer. It is important that only one sample is routed through the header to the analyzer at one time.  
           [0004]    Important factors in analyzing process streams are i) cross-contamination of samples; ii) the size of the analyzer and associated components; and iii) the ease of installation, maintenance and repair of the analyzer. Cross-contamination of samples can be caused by leaking valves and/or dead volume (e.g., irregular passageways, large internal volumes, etc.) allowing contamination from previous samples. To overcome this problem, longer sample purge times and/or stream analysis have been used. However, this increases the time and cost associated with process analyzing, as well as requires the disposal of the greater purge volume. The size of the analyzer and related components is also an issue as large and bulky analyzers take up valuable panel space. The ease of installation, maintenance and repair of the analyzers is also an important consideration as there is a continuing demand to reduce the cost of the analyzers, and hence minimize the cost of the entire process system.  
           [0005]    Certain stream selectors have been developed in an attempt to address some of these issues. For example, one stream selector is known which has a double block and bleed structure, where a set of three O-rings are carried on the head of a poppet valve. The poppet valve is normally biased into a closed (non-actuated) position by a compression spring, and can be moved into an open (actuated) position by pressurized gas or other means. The O-rings are designed to seal against opposed flat surfaces in the selector body to control the flow of fluid through the body. A first and second of the O-ring seals are located in grooves formed in one surface of the poppet valve head to seal against the inlet and outlet passages, respectively, when the selector is not actuated; while a third of the O-rings is loosely located around the poppet stem against the opposing surface of the poppet valve head and is designed to seal against a vent passage when the selector is actuated. An intermediate position is also provided, where the poppet valve head is in a position where all passages are open to completely purge any fluid in the selector system.  
           [0006]    The double block and bleed structure is located in a valve module, and a series of such modules can be arranged adjacent one another to form the stream selector. A common outlet passageway (loop) is typically required between each valve module and the analyzer. The common outlet passageway allows purging of the passageway between different samples.  
           [0007]    It is believed the O-ring seals in the selector described above can dry and crack, and/or swell and dislodge during repeated cycling, particularly in liquid applications. This can allow leak paths to occur between the inlet and outlet passages, and also through the vent passage. The selector also does not fully minimize the volume between the valve modules and the analyzer, as it requires an outlet loop. Providing an outlet loop increases the overall size of the selector, as well as adds additional cost to the system. The selector described above also requires the adjacent valve modules to be firmly pressed together and connected with long bolts extending horizontally through the modules. The bolts must be removed and all the modules disturbed if one of the modules is to be replaced. Since this can be quite an involved operation, the selector typically must be taken to a repair shop remote from the application. Process connections are also required for each module, so that the process connections for typically all modules must be disconnected when the one module is to be replaced. Leak paths can be introduced into the selector during all these disconnections and reconnections. Further, the stream selector of this design tends to be complex and include small C-clips and hidden roll pins, which makes assembly, maintenance, and repair, time-consuming and expensive.  
           [0008]    It is also known to provide ball valves in stream switching, however, such ball valves are not known for their robust design during repeated cycling, and typically must be inspected and repaired or replaced at regular intervals. Selectors using such ball valves also tend to be quite large and require considerable space.  
           [0009]    In light of the above, it is believed there is a demand for an improved stream selector for a process analyzer which prevents cross-contamination of samples, has a relatively small size, and has a simple valve structure which facilitates installation, maintenance and repair of the selector to minimize costs.  
         SUMMARY OF THE PRESENT INVENTION  
         [0010]    The present invention provides a novel and unique stream selector for a process analyzer which prevents contamination of one sample from previous samples. The stream selector does not require a common outlet loop to connect the valves to the process analyzer, and has robust, long-lasting seals to prevent leakage. The stream selector also has modules of a compact size, which are relatively simple in construction and are individually and separately removeable. This reduces installation, maintenance and repair of the selector and minimizes costs of the entire system.  
           [0011]    According to the present invention, the stream selector includes a series of base plates, and a series of valve modules which are individually removeably attached to the base plates. The base plates are connectable to each other via interengaging bolts, and include all the process connections for the fluid lines. This facilitates the assembly, maintenance and repair of the stream selector, as well as reduces the complexity of the valve modules.  
           [0012]    Each valve module for the selector includes a module body with a pair of valve cavities. A valve assembly is received in each valve cavity. Each valve assembly includes a valve body enclosing a moveable piston, a valve bonnet, and a poppet valve. The valve assemblies are preferably operated simultaneously through a common actuation passage.  
           [0013]    The poppet valve in each assembly extends through the bonnet and is connected to the piston, and moves in conjunction therewith. The poppet valve has a valve head with a double-seated seal. The double seated seal has on one side an annular plug seal received around the stem of the poppet valve for sealing to a first valve seat to control the flow of fluid through the vent passage in the valve body; and on the other side includes a solid cylindrical plug seal which seals to a second valve seat to control the flow of fluid between the inlet and outlet passages in the valve body. The first and second plug seals are preferably received in respective seal holders on the opposite surfaces of the head, and provide long-lasting, fluid-tight sealing over repeated cycling.  
           [0014]    The outlet passage from one valve assembly in the module is routed to the inlet passage of the adjacent valve assembly in the module to create a double-block and bleed configuration. The downstream valve assembly in the module is then connected to the process analyzer. This configuration prevents downstream contamination of samples, and purges the valve module during each stream selection. No outlet loop is necessary to purge the modules.  
           [0015]    If more than one valve module is used for the stream selector, the outlet of the downstream valve assembly in the first module is routed through the vent passage of the downstream valve assembly in the next module to purge residual fluid in the second module when the first module is actuated. This configuration is replicated for all the modules in the stream selector, and completely purges the selector of previous samples to prevent cross-contamination.  
           [0016]    A cover encloses each valve assembly in the valve cavity of the module. The cover is attached by a series of threaded bolts to the module body. At least some of the bolts pass through the module body and fix the module body to the base plate. Loosening the bolts allows individual modules to be removed from the base plate without removing adjacent modules. This also allows the valve assemblies to be easily removed from each module.  
           [0017]    As indicated above, the stream selector does not require a common outlet header (loop) to connect to the process analyzer. Rather, the outlet from the last module in the string of modules is connected directly to the analyzer. The previous process streams are fully vented before the introduction of a new process stream into the last module. The stream selector thereby prevents contamination of one sample with residual fluid from a previous sample, and has little dead volume. Still further, the stream selector has a compact size, and is relatively simple in construction. This also reduces installation, maintenance and repair costs. The seal plugs of the poppet valve are robust, long-lasting components that withstand repeated cycling without leaking. The modules each easily connect into and disconnect from the base plates, which carry all the process connections. This also simplifies installation, maintenance and repair of the stream selector valve.  
           [0018]    Further features of the present invention will become apparent to those skilled in the art upon reviewing the following specification and attached drawings. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0019]    [0019]FIG. 1 is a side view of a stream selector constructed according to the principles of the present invention;  
         [0020]    [0020]FIG. 2 is a top view of the stream selector of FIG. 1;  
         [0021]    [0021]FIG. 3 is an end view taken from the left side of the stream selector of FIG. 2;  
         [0022]    [0022]FIG. 4 is a multi-plate cross-sectional side view of the valve module of the stream selector taken substantially along the plane described by the lines  4 - 4  in FIG. 2;  
         [0023]    [0023]FIG. 5 is a top view of the base plate for the stream selector taken substantially along the plane described by the lines  5 - 5  of FIG. 1;  
         [0024]    [0024]FIG. 6 is a cross-sectional end view of the valve module taken substantially along the plane described by the lines  6 - 6  in FIG. 2;  
         [0025]    [0025]FIG. 7 is an exploded view of the stream selector of FIG. 4;  
         [0026]    [0026]FIG. 8 is a cross-sectional side view of the poppet valve for the valve module;  
         [0027]    [0027]FIG. 9 is a cross-sectional side view of the valve module similar to FIG. 4, but showing the module in an actuated condition;  
         [0028]    [0028]FIG. 10 is a side view of a series of modules for the stream selector mounted adjacent one another;  
         [0029]    [0029]FIG. 11 is a schematic representation of the flow paths through multiple valve modules in a stream selector system;  
         [0030]    [0030]FIG. 12 is a schematic representation of the flow through a three module stream selector when neither of the valve modules are actuated;  
         [0031]    [0031]FIG. 13 is a schematic representation of the flow through a three module stream selector when one of the valve modules and the header module are actuated;  
         [0032]    [0032]FIG. 14 is a side view of a valve module in partial cross-section, showing a filter assembly attached to the stream selector;  
         [0033]    [0033]FIG. 15 is a top view of one of the base plates for the stream selector, illustrating a bolt assembled with the base plate;  
         [0034]    [0034]FIG. 16 is a cross-sectional side view of one of the bolts for the stream selector;  
         [0035]    [0035]FIG. 17 is an end view of the bolt; and  
         [0036]    [0036]FIG. 18 is a cross-sectional side view of another of the bolts for the stream selector. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0037]    Referring to the drawings, and initially to FIGS.  1 - 3 , a stream selector constructed according to the principles of the present invention is indicated generally at  20 . The stream selector  20  includes a valve module  22  removably attachable to a base plate  24 . Referring also to FIG. 4, the base plate  24  includes an inlet passage  25 , an outlet passage  26 , a first vent passage  27 , an actuation passage  28 , a second vent passage  29 , and a common connection passage or channel  30 , as will be described herein in more detail. The passages  25 - 29  all have one end that opens to the top flat surface  31  of the base plate adjacent the valve module, as shown in FIG. 5. Common passage  30  is open along the length of the passage. Appropriate O-ring seals are provided in grooves in the upper surface  31  of the base plate surrounding the openings to provide a fluid-tight seal with the flat lower surface  32  (FIG. 7) of the valve module.  
         [0038]    The passages ( 25 - 29 ) all have other ends that open along the end and side surfaces of the base plate. For example, referring to FIGS. 4 and 5, the inlet passage  25  opens to one end of the base plate; the actuation passage  28  opens to the opposite end of the base plate; the first vent passage  27  opens along both sides of the base plate, the second vent passage  29  opens to one side of the base plate; and the outlet passage  26  opens to the other side of the base plate, preferably at approximately the same location (but on the opposite side) as second vent passage  29 . Of course, this is only one example of the flow paths through the base plate, and the passages could have other configurations depending upon the particular application.  
         [0039]    Each module includes a module body  33  having a pair of valve receiving cavities  34 ,  35 . A valve assembly is received in each valve cavity. That is, valve assembly  36  is received in valve cavity  34 , while valve assembly  37  is received in valve cavity  35 . Covers  38 ,  39  are provided to enclose each valve assembly  36 ,  37 , respectively, in a valve cavity. Referring also to FIGS. 6 and 7, threaded bolts  40  extend partway into the module body  33  to connect the covers  38 ,  39  to the module body; while threaded bolts  41  are longer and extend entirely through the module body  33  to the base plate  24  to removably connect the module to the base plate. With the module so connected, the lower flat surface  32  of the module in adjacent, surface-to-surface relation to the upper flat surface  31  of the base plate. While not shown, other appropriate means (clamps, etc.) could alternatively or additionally be used to removably attach the module to the base plate, as should be appreciated by those of ordinary skill in the art.  
         [0040]    As shown in FIGS.  4 - 7 , each valve receiving cavity  34 ,  35  is preferably identical, and includes an enlarged actuation cavity portion  48 , a slightly (radially) smaller vent cavity portion  49 , and a still slightly (radially) smaller control cavity portion  50 , with the vent cavity portion  49  located between the actuation cavity portion  48  and the control cavity portion  50 . Inlet passages  51 ,  52  are provided through module body  33  from the lower surface  32  directly into the central portion of the control chamber portion  50  for each valve receiving cavity. Inlet passage  51  for valve assembly  36  is fluidly aligned and connected to inlet passage  25  in base plate  24 ; while inlet passage  52  for valve assembly  37  is fluidly aligned with one end of common passage  30 . Outlet passages  53 ,  54  are similarly provided from the lower surface  32  into the side of each control chamber portion of each valve receiving cavity. Outlet passage  53  for valve assembly  36  is fluidly aligned with the other end of common passage  30 ; while outlet passage  54  for valve assembly  37  is fluidly aligned with outlet passage  26  in base plate  24 . Common passage  30  directly fluidly connects the outlet passage  53  of valve assembly  36  with the inlet passage  52  of valve assembly  37 .  
         [0041]    Vent passages  55 ,  56  are similarly provided into the side of the vent cavity portion  49  of each valve receiving cavity. Vent passage  55  for valve assembly  36  is fluidly aligned with first vent passage  27  in the base plate  24 ; while vent passage  56  for valve assembly  37  is fluidly aligned with second vent passage  29  in base plate  24 .  
         [0042]    A common actuation passage  57  is provided for both valve assemblies, and extends from the actuation chamber  48  of each valve receiving cavity  34 ,  35 , where it is fluidly aligned with the actuation passage  28  in the base plate.  
         [0043]    The valve assemblies are also preferably identical for ease of manufacture and assembly, although, of course, this could also be different. Each includes a valve body  58 , a piston  59  moveably disposed in the valve body, a valve bonnet  60  and a valve poppet  61 . The valve body  58  includes a cylindrical wall portion  64  defining a central piston receiving cavity  65 . The cylindrical wall portion  64  is closely received in the actuation cavity portion  48 . The valve body  58  further includes a cylindrical base portion  66  which is closely received in the vent cavity portion  49 . The base portion  66  includes a central through-bore  67 , and one or more radial passages  68  at the junction between the wall portion  64  and the base portion  66 , all opening into piston receiving cavity  65 . Appropriate O-ring seals  69  are provided in grooves in valve body  58  to provide a fluid-tight seal between valve body  58  and the module body  33 .  
         [0044]    Valve bonnet  60  has a cylindrical body portion  70  closely received in vent cavity portion  49 , and a cylindrical base portion  71  closely received in control cavity portion  50 . Covers  38  and  39  retain the valve bonnet  60  and valve body  58  securely in each valve receiving cavity, such that these components are fixed relative to the module body  33 . An O-ring seal  72  is received in a groove in the valve bonnet  60  and provides a fluid-tight seal between the bonnet  60  and the module body  33 . Bonnet  60  also includes a central through-bore  74 , and one or more radial passages  76  which fluidly connect with the through-bore.  
         [0045]    The piston  59  has a cylindrical base  80  closely received in the piston receiving cavity  65  of the valve body  58 . Piston  59  is allowed to move axially within cavity  65 , and includes a groove receiving an O-ring seal  81  for providing a fluid-tight seal between the piston and the valve body  58 .  
         [0046]    The upper surface of the base  80  provides a spring stop for a compression spring  82 . As best shown in FIG. 4, the compression spring  82  is located between the base  80  and a respective cover  38 ,  39  and normally urges the piston  59  downwardly toward the base  66  of the valve body  58 .  
         [0047]    Referring also to FIGS. 7 and 8, the poppet valve  61  has an elongated cylindrical stem  86  which extends through the passage  74  in the valve bonnet  60  and through the bore  67  in the valve body  58 . The stem extends upwardly to a threaded connection with the piston  59 , and as such, the piston and poppet valve move axially in conjunction with one another. A series of O-rings  89  or other packing is provided in a counterbore  90  in the bore  67  of the valve body  58  to provide a fluid-tight seal between the stem  86  of the poppet valve and the valve body  58 .  
         [0048]    The poppet valve  61  further has a cylindrical valve head  93  at the lower end of the valve stem. The valve head is located in the control cavity portion  50  of the valve receiving cavity. As shown particularly in FIG. 8, an annular sleeve  95  extends outwardly away from one (upper or rear) surface of the valve head  93  toward the valve bonnet  60 ; while a similar annular sleeve  96  extends outwardly (downwardly) away from the opposite (lower or front) surface of the valve head  93  toward the inlet passages  51 ,  52 , respectively. Sleeves  95 ,  96  define seal holders.  
         [0049]    An upper or rear plug seal  98  is received in upper/rear seal holder  95 , while a lower or front plug seal  99  is received in lower/front seal holder  96 . Each plug seal is preferably formed from PCTFE, or other appropriate resilient polymer. Plug seal  98  has an annular configuration and is closely received around stem  86  and is received, preferably with a friction fit, in seal holder  95 . Plug seal  99  has a solid cylindrical configuration and is closely received, preferably with a friction fit, in seal holder  96 . Seal holders  95 ,  96  can be crimped to facilitate retaining the plug seals  98 ,  99 , respectively. Each plug seal  98 ,  99  has an outwardly-facing tapered surface  100 ,  101 , respectively, and is designed to sit squarely against a valve seat. Specifically, when the module is in a non-actuated condition (as shown in, e.g., FIGS. 4 and 6), the tapered surface  101  of lower seal  99  sits squarely against a valve seat  104  defined at the upper/inner end of inlet passage  51 ,  52 , respectively, and provides a fluid-tight seal therewith. Likewise, when the module is in an actuated condition (as shown in, e.g., FIG. 9), the tapered surface  100  of the upper seal  98  sits squarely against a valve seat  105  defined at the lower/inner end of through-bore  74  in valve bonnet  60 , and provides a fluid-tight seal therewith.  
         [0050]    The operation of the module will now be described. It is typical and preferred that both valve assemblies in the module operate simultaneously, that is, in conjunction with one another. When the module is in its non-actuated condition (e.g., FIGS. 4 and 6), spring  82  forces the piston  59  in each valve assembly downwardly toward the valve body  58 . The poppet valves are likewise pushed downwardly such that they seal the respective inlet passages  51  and  52 . In this condition, fluid is prevented from flowing through the module. As the poppet valve is in a position where the upper seal  98  is spaced-apart from the valve seat  105 , any fluid in the control cavity portion  50  vents through bore  74  in valve bonnet  60  (in the area surrounding the poppet valve stem  86 ), through radial passage  76  in the bonnet, to vent cavity  49  and then through vent passage  55  (for valve assembly  36 ) or vent passage  56  (for valve assembly  37 ). The vented fluid then flows through the respective vent passages  27 ,  29  in the base plate, which exhaust the fluid (typically to atmosphere).  
         [0051]    When the module is actuated, actuation fluid (e.g., compressed gas) is provided through actuation passage  28  in base plate  24  to common actuation passage  57  in the module body  33 . The actuation fluid is directed into actuation cavity  48  of both valve assemblies, where it flows through ports  68  in valve body  58 , and into the space between each valve body and the piston. The fluid is applied against the lower surface of the piston to force the piston upwardly against spring  82 . When the pressure of the actuation fluid exceeds the pressure of the spring, the piston, and hence the valve poppet  61 , moves upwardly in the module, such that the lower seal  99  moves away from the valve seat  104 , and the upper valve seal  98  moves into sealing relation with the valve seat  105  for each assembly, as shown in FIG. 9. Fluid can pass from the inlet passages, into control cavity  50 , and then out through outlet passage  53  (for valve assembly  36 ) or outlet passage  54  (for valve assembly  37 ). As should be appreciated, fluid applied through outlet passage  53  from valve assembly  36  flows through common passage  30  (FIG. 5) and directly into inlet passage  52  to valve assembly  37 , where the fluid passes to outlet passage  54 .  
         [0052]    An intermediate mode can also be provided, where the seals  98 ,  99  are each spaced a short distance from their respective valve seats  104 ,  105 . A complete purge of fluid occurs from the module in this condition.  
         [0053]    While it is described above that passage  51  is an inlet passage and passage  53  is an outlet passage for valve assembly  36 ; and passage  52  is an inlet passage and passage  54  is an outlet passage for valve assembly  37 , it should be appreciated that the inlet and outlet passages could be reversed, such that sample fluid is provided into passage  54  and out of passage  52  of valve assembly  37  and then into passage  53  and out of passage  51  of valve assembly  36 . This can be accomplished by switching the fluid lines into base plate  24 , or alternatively reconfiguring the passages  26 ,  27  and  30  in the base plate.  
         [0054]    One of the advantages of the present invention is that it is relatively simple to service the modules. Since the modules are individually attachable to the base plates, only one module need be removed at a time—without disturbing the other modules. With all the process connections for the liquid lines provided on the base plate, this makes it quick and easy to replace any particular module. In addition, horizontal through-bores  107  (FIG. 4) are provided through each base plate  24  to facilitate the securing of the modules adjacent one another. As shown in FIG. 11, appropriate bolts  108  can be inserted through the through-bores. The module/base plate combinations can also be easily removed and serviced and/or replaced simply by removing bolts  108 , the air line ( 28  or  123 ) and only one liquid process connection ( 25  or  127 ). The side-by-side arrangement of the modules also allows the number of modules to be easily scaled (increased or decreased) depending upon the particular application.  
         [0055]    As shown in FIGS. 15 and 16, bolts  108  include an elongated shank  209 , with a threaded portion  211  at one (distal) end of the bolt, and an enlarged head  212  at the other end of the bolt. An annular shoulder  213  radially connects the enlarged head  212  and the shank  209 . A central threaded blind end bore  214  extends axially inward from the head end of the bolt, as can be seen in FIG. 17. Bore  214  is sized so as to receive a threaded shank  209  from an adjacent bolt.  
         [0056]    Both the head  212  and shank  209  are generally smooth and cylindrical, and the end of the head  212  includes an external tool engaging portion  215  having a non-cylindrical geometry allowing a tool to grasp and rotate the bolt. As illustrated in FIG. 17, the tool engaging portion  215  can have a hex shape to fit a hex tool, although of course, other shapes are possible. While not shown, the tool engaging portion could also be provided internal to bore  214 , rather than external.  
         [0057]    Head  212  and shank  209  are sized so as to be closely received in throughbores  107  in the base plates. As can be seen in FIG. 15, throughbores  107  each include a main portion  216 ; a first enlarged end  217  with an inner cylindrical surface; and a second enlarged end  219 , also with an inner cylindrical surface. An annular shoulder  220  radially interconnects first enlarged end  217  and main portion  216 . Head  212  of bolt  108  is closely received in first enlarged end  217 , with shoulder  213  engaging corresponding shoulder  220 ; while threaded portion  211  is received in second enlarged end  219 . The head  212  and first enlarged end  217  are sized so that the tool engaging portion  215  and a portion of the cylindrical head extend outwardly from the throughbore  107 , from one edge of the base plate. Likewise, the bolt is sized such that a portion of the end of shank  209  projects outwardly from the other side edge of the base plate. The second enlarged end  219  of the throughbore  107  is further sized so as to closely receive the tool engaging portion  215  and cylindrical head portion of an adjacent base plate, with the threaded shank  209  of one bolt received in the threaded bore  214  of the adjacent bolt. The cylindrical head  212  and second enlarged end  219  locate and align the adjacent base plates together for a proper fit. The outwardly-projecting tool engaging portion  215  enables the assembler to engage the opposite end of the bolt with an appropriate tool, and tighten the one bolt to the other, and hence fix the one base plate to the other.  
         [0058]    Once the bolt is properly tightened down within the throughbore to the adjacent bolt, a still further base plate can be assembled, by repeating the above assembly steps.  
         [0059]    [0059]FIG. 18 illustrates a bolt  222  useful for the final base plate, where the bolt does not include a central bore. The bolt  222  in FIG. 18 is also illustrated as being somewhat smaller than the bolt  108  in FIGS. 15 and 16, but this is because the bolt is illustrated as being used with an end plate (see FIG. 11), which tends to be thinner than a base plate, however, this is not always the case, and it should be apparent that the length of the bolt will depend upon the dimensions of the plate it is used in. Bolt  222  in FIG. 18 is otherwise the same as bolt  108  illustrated in FIG. 16, and includes and enlarged head  224  with a tool engaging portion  225 , and an elongated shank  226 .  
         [0060]    It is noted that a pair of bolts are illustrated as being used with the base plate  24  of FIG. 15, and while this is preferred, it is noted that only a single bolt, or more than two bolts, may be appropriate in certain situations.  
         [0061]    As should be apparent from the above, the use of the bolts  108 ,  224  provides an easy and rapid method of assembling the base plates together, to further facilitate assembling the stream selector of the present invention.  
         [0062]    If necessary or desirable, additional support can be provided to the valve modules by inserting bolts (not shown) through threaded bores  109  (FIGS. 3, 6,  10 ) on base  24 , and attaching the bolts to a support surface.  
         [0063]    As shown in FIG. 11, each additional module is preferably connected to a previous module by fluidly connecting the common outlet passage  26  from the base plate in the first module to the normally-open vent passage  29  of the base plate in the adjacent module. This flow path addresses the issue of dead volume in the outlet header without adding an outlet loop. The vent passages  27  between adjacent modules are also aligned and fluidly connected. As indicated previously, passages  27  preferably open to the opposite side surfaces of the base plate at the same location. A first end plate  112  can be provided adjacent the upstream module “A” at one end of the module stack with a common vent passage  113  to atmosphere for both vent passages  29  and  27 . Similarly, a second end plate  114  can be provided at the other end of the module stack with a passage  115  connecting with passage  26  in the downstream module B to pass gas to the analyzer. A passage  116  connects the gas flow from the analyzer back through the module stack. End plates  112 ,  114  facilitate mounting the modules at an appropriate location on the panel.  
         [0064]    A low pressure header  117  can also be provided between the downstream module “B” and end plate  114 . Header  117  includes a valve module having a pair of valve assemblies, preferably with the same structure and function as the assemblies described above. Header  117  also has a base plate  118 , such as described previously, however, the passages through the base plate are different than as described above.  
         [0065]    Specifically, the base plate  118  includes an inlet passage  119  which fluidly connects with outlet passage  26  of the adjacent, upstream module “B”. Passage  119  is fluidly connected with the passage  52  of valve assembly  37  in the header module body. Passage  54  in the header module body is fluidly connected to outlet passage  120  in base plate  118 , which is itself fluidly connected to passage  115  in the adjacent plate  114 . The vent passage  56  in the module body is fluidly connected to one end of common passage  121 . A further passage  122  has one end which is fluidly connected to the other end of common passage  121 , and an opposite end which is fluidly connected to vent passage  27  in the adjacent module “B”. Actuation passage  123  provides actuation gas to common actuation passage  57  in the module body. Actuation of the valve assembly  37  controls the flow of gas from inlet passage  119  to outlet passage  120 . When the valve assembly  37  is closed, gas is vented back through the module stack via passages  27 .  
         [0066]    Likewise, passage  125  in base plate  118  is fluidly connected to passage  116  in end plate  114  and receives gas exiting from the analyzer. Passage  53  in valve assembly  36  is fluidly connected with passage  125 . Passage  51  in valve assembly  36  is fluidly connected to passage  127 , which leads to atmosphere or preferably, to a collection tank (not shown). Passage  55  in valve assembly  36  is fluidly connected to passage  121  in plate  118 , which leads to passage  122 . When the valve assembly  36  is activated, fluid flows from passage  125  to passage  127  and then to atmosphere or collection. When the valve is not activated, fluid flows from passage  125  to passage  122  to vent.  
         [0067]    The flow paths through multiple modules “A” and “B” and the header  117  is shown schematically in FIGS. 12 and 13. When the modules and header are not actuated (FIG. 12), the seals in each module prevent fluid from passing through the respective modules. Any fluid leaking through the poppet valve seals is vented, and therefore does not contaminate the sample being analyzed. Any fluid from the analyzer is purged to vent. When one module (for example module B in FIG. 13), and the header  117  are actuated, the outlet of the module is applied through the header to the analyzer, while module A remains purged. The gas returning from the analyzer is collected or vented to atmosphere.  
         [0068]    As such, the stream selector does not require a common outlet header to connect to the process analyzer. Rather, the outlet from the last module in the string of modules is connected to the analyzer. The previous process streams are fully vented before the introduction of a new process stream into the last module. The stream selector thereby prevents contamination of one sample with residual fluid from a previous sample, and has little dead volume. Still further, the stream selector has a compact size, and is relatively simple in construction. This also reduces installation, maintenance and repair costs. The seal plugs of the poppet valve are robust, long-lasting components that withstand repeated cycling without leaking. The modules each easily connect into and disconnect from the base plate, which itself carries all the process connections. This also simplifies installation, maintenance and repair of the stream selector valve.  
         [0069]    An additional feature that can be easily provided with one or more of the valve modules is a filter assembly, indicated generally at  140  in FIG. 14. The filter assembly  140  provides a filtration function for fluid received in inlet passage  25 . Filter assembly  140  includes a body  142  having an inlet port  144  and an outlet port  146 . Inlet port  144  has appropriate connections (e.g., screw threads) to allow the filter assembly to be easily connected within the fluid system. The outlet passage  146  receives a fitting/regulator  148 , which is connected, such as with cooperating threads, to the inlet passage  25 , and is received with a close fit in passage  146 . An O-ring seal  149  is received around the inner end of fitting  146  and provides a fluid-tight seal within passage  146 . The filter body  142  includes an elongated removable filter canister  150  which receives a filter element or cartridge, indicated generally at  153 . Filter cartridge  153  includes an annular filter media  155  formed of an appropriate material, and an upper seal or end cap  157 . End cap  157  receives a central rod  160 , which removably attaches the end cap to the body  142 , and allows the filter element to be replaced when spent. To this end, canister  150  is removably connected such as by cooperating screw threads, to body  142 . When canister  150  is removed, end cap  157  can be screwed off from rod  160 , and the element inspected and replaced if necessary. Fluid flows from inlet  144 , upwardly in the peripheral chamber between the canister  150  and the media  155 , radially inward through the media, and then downwardly and outwardly through fitting/regulator  148  to the module. In this manner, the sample can be filtered prior to passing through he module and passing onto the analyzer.  
         [0070]    The principles, preferred embodiments and modes of operation of the present invention have been described in the foregoing specification. The invention which is intended to be protected herein should not, however, be construed as limited to the particular form described as it is to be regarded as illustrative rather than restrictive. Variations and changes may be made by those skilled in the art without departing from the scope and spirit of the invention as set forth in the appended claims.