Abstract:
Apparatus for withdrawing a liquid sample from a vessel, the apparatus affixed to the vessel and having (a) a plunger forming a sample-receiving space movable from the vessel interior to and beyond a sample-delivery site, and (b) a valve for closing the vessel when the plunger is beyond the sample-delivery site. In the sampling apparatus, the sample-delivery site is incorporated in the valve.

Description:
FIELD OF THE INVENTION 
   This invention is related generally to apparatus for extracting samples of liquid from vessels, and in particular to positive-displacement sampling apparatus with an inline valve. 
   BACKGROUND OF THE INVENTION 
   There is a need with many manufacturing operations and processes and within liquid transportation systems to monitor the composition or other properties of the liquid material which is either contained in or flowing within a vessel. Often what is required is to capture a sample of the liquid from within the vessel in order to make the necessary measurements. It is most desirable to be able to capture such samples without interference with or contamination to the processes being monitored. Non-interference with a process is often achieved by ensuring that vessel pressure is maintained throughout the sampling-capturing operation. Further, it is often a requirement that samples be taken at regular and frequent intervals so that reliability, operator safety, and sampler maintenance and longevity are important requirements for such samplers. 
   A number of samplers which are used in such manufacturing or process applications are devices which capture small, discrete samples of liquid. An example of such a sampler is disclosed in U.S. Pat. No. 6,792,818 by Ben E. Jaeger. This positive-displacement sampler includes a plunger having a sample receiving recess and a connecting device which couples the sampler to a vessel at an aperture in the vessel. The sampler is attached to the connecting device, and the connecting device is moveable between a first position placing the sampler into, and a second position taking the sampler out of communication with the aperture. With the connecting device in the first position, the plunger is reciprocated to extend the plunger and its recess through the connecting device and vessel aperture into product in the vessel to receive a product sample in the recess. The plunger is then retracted from the vessel and through the connecting device to deliver the product sample to a collection point. During reciprocation of the plunger, a liquid seal is maintained between the vessel interior and sample collection point, and when the connecting device is moved to the second position, the sampler can be detached from the connecting device for service or repair, without escape of liquid from the vessel through the connecting device. 
   Particularly severe requirements exist for samplers used in applications where the liquid being sampled contains a solid phase. An example of this is in the handling of oil sands in which the liquid may contain bituminous sand, oil, hot water, and possibly clay. The solid phase is the source of abrasive material which creates a particularly difficult operational environment for samplers. The abrasive material can become trapped in regions around seals, particularly when elements within the sampler become misaligned due to, for example, frictional forces during movement of sampler elements, thereby allowing the abrasive material to flow by leakage into areas in which it is not intended to be, causing excessive wear. Also, when seals move past entry and exit interfaces within valves and other elements within a sampler, abrasive material can easily cause the seals to wear far too rapidly, necessitating frequent and costly maintenance on the sampler. 
   The sampler disclosed in U.S. Pat. No. 6,792,818 described above, when applied in applications such as oil sands, operates in a manner in which excessive seal/interface traverses and misalignment are both causes of seal wear. As an example, in the sampling of such abrasive liquids, sampling frequencies may be as high as five samples per hour and may result in seals being replaced as often as once each week, creating an extremely high maintenance cost. 
   Another desirable and intended feature for samplers is the ability to be able to achieve what is termed “double block and bleed” capability, the ability to isolate the sampler from vessel pressure and to verify that such isolation has indeed taken place so that the sampler can be disconnected. If seals are worn and misalignment causes further leakage, the sampler cannot be relied on to achieve the required isolation. The invention disclosed in the &#39;818 patent can exhibit such unwanted behavior when the ball valve, which is “floating” within a set of seals, becomes misaligned and causes leakage of unwanted vessel pressure, thereby causing a potentially unsafe condition for an operator performing maintenance on the sampler or simply taking a sample in the course of regular vessel monitoring. 
   U.S. Pat. No. 5,905,213, also by Ben E. Jaeger, discloses a sampler in which the forward end of the sampler housing is coupled to a movable valve for movement with the valve, and the housing and valve have axially-aligned bores. Movement of the valve places the forward end of its bore into and out of communication with an opening in a vessel containing the liquid to be sampled. A plunger in the housing bore has a sample-receiving recess intermediate its ends, and with the forward end of the valve bore placed into communication with the interior of the vessel, the plunger is reciprocated forward in the housing and valve bores to project the recess into the vessel to receive a product sample therein. The plunger is then reciprocated rearward to retract the product sample containing recess from the vessel and through the valve and housing bores to a sample collection point in the housing. When repair or replacement of the sampler is required, the valve is moved to place the forward end of the valve bore out of communication with the vessel interior, whereupon the sampler housing can be disconnected from the valve without outflow of liquid product from the vessel through the valve bore. A disadvantage of the arrangement is that the entirety of the sampler moves conjointly with movement of the valve between its open and closed positions, so a relatively large unobstructed area must be provided around the sampler to accommodate such movement, which limits freedom of location of the sampling apparatus. In addition, to accommodate mounting of the sampler housing on the valve, the valve must be relatively large to accommodate connection of the sampler housing to it, resulting in increased manufacturing costs. 
   Existing samplers currently used as described above fall short of delivering effective, safe and cost-effective sampling. Thus, there is a need for a sampler which satisfies the objectives as set forth in the following section. 
   OBJECTS OF THE INVENTION 
   It is an object of this invention, in the field of process sampling technology, to provide an improved sampler which incorporates the isolation valve within the sampler apparatus. 
   Another object is to provide a sampler which deposits the liquid sample from a location within the valve of the sampler. 
   Another object of the present invention is to provide a sampler which can be used with abrasive liquids. 
   Another object of this invention is to provide a sampler which minimizes wear on seals within the sampler. 
   Another object of this invention is to provide a sampler which minimizes leakage at the seals within the sampler. 
   Another object of this invention is to provide a sampler with a valve in which valve core is completely encapsulated by its trunnion supports and, further, in which the valve core and stem are structurally integral. 
   Another object of this invention is to provide a sampler which reduces the number of times seals traverse across interfaces within the sampler during the sampling process. 
   A further object of this invention is to provide a sampler which protrudes a reduced length from the vessel and thus also has reduced weight. 
   It is an object of this invention to provide a sampler with “double-block-and-bleed” capability with a single valve. 
   It is also an object of this invention to provide a sampler with improved operator safety. 
   Still another object of the invention is to provide a sample which minimizes misalignment within the valve. 
   Yet another object of the invention is to provide a sampler with a valve which is less sensitive to any misalignment that may occur within the valve. 
   These and other objects of the invention will be apparent from the following descriptions and from the drawings. 
   SUMMARY OF THE INVENTION 
   The invention is an improvement in apparatus for withdrawing a liquid sample from a vessel. Such apparatus is affixed to the vessel and has (a) a plunger forming a sample-receiving space movable from the vessel interior to and beyond a sample-delivery site and (b) a valve for closing the vessel when the plunger is beyond the sample-delivery site. The improvement of the instant invention comprises the sample-delivery site being incorporated in the valve. 
   In some preferred embodiments of the invention, the valve is a trunnion valve and the valve has a stem and a central portion, the central portion dividing the stem into a first stem portion and a second stem portion and having a through-hole with an axis substantially perpendicular to the stem axis. In highly preferred embodiments, the first stem portion includes a discharge/bleed port aligned substantially along the axis of the stem for discharge of the sample from the sample-receiving space when such space is at the sample-delivery site. Further, the plunger in some highly preferred embodiments passes through the through-hole to receive a sample from the vessel interior and to deliver the sample to the discharge/port. 
   In some preferred embodiments, the through-hole in the central portion of the valve is a clearance hole for the plunger, and in some preferred embodiments, the second stem portion includes a purge/flush inlet port. 
   In highly preferred embodiments of the inventive sampling apparatus, the valve includes first and second valve seats that form stem-engagement surfaces for the first and second stem portions, respectively, and that together form a central-portion-engagement surface. In such embodiments, the first and second valve seats together surround, support and seal the central portion and the first and second stem portions. 
   In some embodiments, the plunger moves within first and second axially-aligned plunger cylinders, (a) the first on the vessel side of the valve and the second on the opposite side of the valve, (b) the sample-receiving space is an annular space, and (c) the plunger includes first and second plunger seals each mounted beyond a respective end of the sample-receiving space and spaced such that when the sample-receiving space is at the sample-delivery site, the first and second plunger seals are engaging the first and second plunger cylinders, respectively. 
   In highly preferred embodiments, the valve is a ball valve. In other embodiments, the valve is a plug valve. Further, some embodiments include a compound actuator for plunger movement. 
   Highly preferred embodiments of the inventive sampling apparatus include a bleed valve to effect collection of the sample from the discharge/bleed port. 
   The term “liquid” as used herein, in addition to the common usage of the term, also includes liquids which contain a solid phase, such as is the case with a mixture of oil, water and sand. 
   The term “vessel” as used herein refers to any sort of enclosure containing the liquid which is to be sampled. Thus, a vessel as so defined includes a pipe or other conduit through which the liquid flows as well as any container such as a process reactor which is holding the liquid to be sampled. 
   The term “clearance hole” as used herein refers to a hole which is sized such that the object which is referenced thereto is able to pass through or be contained in the hole without touching the walls of the hole. 
   The term “sample withdrawal” or related terminology as used herein refers to the process by which a sample of liquid is removed from a vessel and subsequently delivered to a desired sample location. 
   The term “compound actuator” as used herein refers to a pneumatic or hydraulic actuator (containing a primary piston and a cylinder) which also contains a secondary piston such that the primary and secondary pistons cooperate to effect the movement of the object being moved by the actuator. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The drawings illustrate preferred embodiments which include the above-noted characteristics and features of the invention. The invention will be readily understood from the descriptions and drawings. In the drawings: 
       FIG. 1  is a perspective drawing of one embodiment of the inventive sampling apparatus. 
       FIG. 2  is a cutaway perspective drawing of the sampling apparatus of  FIG. 1  with the sample-receiving space between the vessel interior and the sample-delivery site and within the front sleeve. 
       FIG. 3  is a cutaway perspective drawing of the sampling apparatus of  FIG. 1  with the sample-receiving space in the vessel interior. 
       FIG. 4  is a cutaway perspective drawing of the sampling apparatus of  FIG. 1  with the sample-receiving space at the sample-delivery site. 
       FIG. 5  is a cutaway perspective drawing of the sampling apparatus of  FIG. 1  with the sample-receiving space beyond the sample-receiving site and with the ball valve in a closed position. 
       FIG. 6  is a cutaway perspective drawing of the plunger of the sampling apparatus of  FIG. 1 . 
       FIG. 7  is a cutaway perspective drawing of the sampling apparatus of  FIG. 1  with the valve portion and the actuator portion separated for maintenance. 
       FIG. 8  is a cutaway perspective drawing of the valve of the sampling apparatus of  FIG. 1 , the plane of the cutaway being perpendicular to the cutaway plane of  FIGS. 2-5 . The valve is in a closed position, and the bleed valve is in a closed position. 
       FIG. 9  is a cutaway perspective drawing of the valve of the sampling apparatus of  FIG. 8  with the valve in an open position, and the bleed valve is in an open position. 
       FIG. 10A  is a cross-sectional drawing of the stem, central portion and the seat of a ball valve. 
       FIG. 10B  is a cutaway perspective drawing of a seat for the ball valve of  FIG. 9   a.    
       FIG. 10C  is a cross-sectional drawing of the stem and central portion and the seat of a plug valve. The central portion is a truncated cone. 
   

   DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     FIG. 1  is a perspective drawing of sampling apparatus  10 .  FIGS. 2-5  and  7 - 9  are all cutaway perspective drawings of sampling apparatus  10  with the elements of apparatus  10  in different positions illustrating its operation. (To reduce the crowding of reference numbers, not all elements of sampling apparatus  10  are labeled in every figure.) Referring to  FIGS. 1-5 , sampling apparatus  10  has a valve portion  12  and an actuator portion  14 . Sampling apparatus  10  is affixed by adapter  16  to a vessel  1  (shown in  FIGS. 2-5 ) which contains the liquid (not shown) to be sampled through an adapter opening  18  (shown in  FIGS. 2-5 ). Sampling apparatus  10  also includes a bleed valve  20  attached to valve portion  12  for the purpose of assisting in the collection of a sample taken by sampling apparatus  10 . 
   Referring now to  FIGS. 2-5  and  FIGS. 8-9 , valve portion  12  of sampling apparatus  10  includes valve  11  which has a valve body  22  containing a valve stem  24  and a central valve portion  26 . Stem  24  is divided into two portions, a first stem portion  24   a  and a second stem portion  24   b,  by central valve portion  26 , thus forming the stem and central valve portion of a trunnion valve. Central valve portion  26  is a spherical structure and has a through-hole  26   h  (best seen in  FIGS. 5 ,  8  and  9 ) with an axis substantially perpendicular to the axis of stem  24 ; thus valve  11  in this embodiment is a ball valve. Through-hole  26   h  forms the sample-delivery site of sampling apparatus  10 . 
     FIGS. 10A and 10B  more clearly illustrate some of the details of stem  24  and central portion  26 . Stem  24  and central portion  26  are surrounded, supported and sealed by a first valve seat  28   a  and a second valve seat  28   b.  First and second valve seats  28   a  and  28   b  engage first and second stem portions  28   a  and  28   b  respectively at first and second stem-engagement surfaces  30   a  and  30   b  and together engage central portion  26  at a central-portion-engagement surface  30   c.  First and second valve seats  28   a  and  28   b  thus ensure that stem  24  and central valve portion  26  remain in proper alignment within valve  11 . 
   Referring now to  FIGS. 8 and 9  for further detail with respect to valve portion  12 , first stem portion is partially supported by valve body  22  at a support surface  32  and by a set of gland seals  34   s.  Primary support for stem  24  and central portion is provided by seats  28   a  and  28   b.  First and second stem-engagement surfaces  30   a  and  30   b  thus form trunnion supports for first and second stem portions  24   a  and  24   b  within valve  11 . Gland  34  is held in place by a bevel washer spring  36 , and a packing bolt  38  is tightened into a threaded portion  40  of valve body  22  to complete the internal assembly of valve  11 . Valve handle  42  is attached to stem  24  to effect turning of valve  11 . 
   First stem portion  24   a  includes an axially-aligned sample discharge/bleed port  44 . Port  44  is open to through-hole  26   h,  the sample-delivery site, thus enabling sample liquid to flow through first stem portion  24   a  into bleed valve  20  and, as desired, into a collection container (not shown) which may be connected to bleed valve sample port  54 . 
   Second stem portion  24   b  includes a purge/flush inlet port  46 . Port  46  includes axial passage  46   a  and intersecting passage  46   b.  Passage  46   a  is aligned axially with stem  24 , and passage  46   b  intersects passage  46   a  in order to enable flow in port  46  to reach a pair of annular gland passages  48   a  and  48   b  which are connected together by a connecting passage  50 . Annular gland passage  48   b  is aligned with an external flush port  52  (shown on  FIGS. 1 ,  8  and  9 ). Thus, a flow passage exists in valve  11  between port  52  and through-hole  26   h  to effect purging and flushing of valve  11  regardless of the position of stem  24  of valve  11 . 
     FIG. 6  is a cutaway perspective drawing of a plunger  60  of sampling apparatus  10 . The various elements of plunger  60  can be seen in  FIGS. 2-5  but are most easily seen in  FIG. 6 . Plunger  60  is assembled from a number of plunger elements. Plunger  60  includes a plunger tube  62  welded to plunger adapter  64  at one end of tube  62  and a rear piston  63  welded at the other end of tube  62 . Adapter  64  includes a threaded hole  65  into which a mandrel  66  is inserted. Rear piston  63  includes rear piston seals  63   a  and  63   b.    
   Mandrel  66  includes a mandrel head  66   a  and a mandrel shaft  66   b.  Onto mandrel  66  are assembled, in sequence, a front seal  68   a,  an O-ring  74  (all O-rings in this assembly are labeled with reference number  74  as similar elements), an annular spacer  70 , an O-ring  74 , a middle seal  68   b,  an O-ring  74 , and a rear spacer  72 . Annular spacer  70  forms the sample-receiving space of sampling apparatus  10 . A rear seal  68   c  is placed onto plunger adapter  64 , and mandrel  66  is inserted into threaded hole  65  in adapter  64 , completing the assembly of plunger  60 . Mandrel  66  is thus configured to enable easy disassembly and assembly to facilitate replacement of the seals on plunger  60 . 
   Referring again to  FIGS. 2-5 , plunger  60  is slidably assembled into a rear sleeve  76  which is connected to valve body  22 . Plunger  60  slides within sleeve  76 , through through-hole  26   h  when stem  24  is in position to allow plunger  60  to enter hole through-hole  26   h,  and into and partially through a front sleeve  78  mounted in adapter  16  and body  22 . O-rings  80  provide seals for plunger  60  within front sleeve  78  as shown. Rear sleeve  76  and front sleeve  78  form a pair of first and second axially-aligned plunger cylinders in which plunger  60  moves. The first plunger cylinder, front sleeve  78  is on the vessel side of valve  11 , and the second plunger cylinder, rear sleeve  76 , is on the opposite side of valve  11 . 
   A front piston  82  is slidably assembled onto the outside of rear sleeve  76 , into a front cylinder  84 , and into a rear cylinder  86  such that front piston  82  slides within both front cylinder  84  and rear cylinder  86 . Rear piston  63  of plunger  60  is also slidably assembled into rear cylinder  86 . Front piston  82  includes a forward piston portion  82   a  and an aft piston portion  82   b.  Aft portion  82   b  includes a front piston sleeve bearing  88  in which plunger  60  slides, and rear sleeve  76  includes two rear sleeve seals  76   a  and  76   b  between which is an annular slot  92  connected to an intermediate drain/vent port  94  open to the outside of valve  11 . The purpose of slot  92  and port  94  is that if seal  76   a  were to leak, liquid from sampling apparatus  10  would not find its way into rear cylinder  86  but rather simply flow through port  94  to the outside. Forward piston portion  82   a  includes two seals  85   a  and  85   b  for slidable support of front piston  82  within front cylinder  84 . 
   Front cylinder  84  includes a center head  90  which forms the connection between front cylinder  84  and rear cylinder  86 . Aft piston portion  82   b  includes an air passage  83  to allow air flow through aft piston portion  82   b  when front piston  82  is sliding with respect to rear sleeve  76 . 
   Rear cylinder  86  includes a rear head  95  which closes off rear cylinder  86 . Rear head  95  includes seals  96  which seal between rear head  95  and a stop tube  98  assembled into a threaded hole  100  in rear head  95  with a wing nut  102 . Stop tube  98  functions to provide an air passage for displaced air when plunger  60  moves within rear cylinder  86 . Stop tube  98  with a stop washer  99  mounted thereon also functions as a stop for plunger  60  as described below. 
   Front piston  82 , front cylinder  84 , rear piston  63  and rear cylinder  86 , with their attendant seals, constitute a compound pneumatic actuator for effecting the movement of plunger  60  into the various positions required by sampling apparatus  10 . Plunger  60  could be actuated by numerous other types of actuators including but not limited to simple pneumatic actuators, hydraulic actuators, and various electromechanical actuators. 
     FIGS. 2-5  illustrate four different positions of the plunger within sampling apparatus  60 . These four positions constitute the specific positions which best describe the process of liquid sample withdrawal from vessel  1 . Actuator portion  14  of sampling apparatus  10  moves plunger  60  to and from the various positions in order to effect sample withdrawal.  FIG. 6  illustrates sampling apparatus  10  separated into valve portion  12  and actuator portion  14 . In  FIG. 6 , wing nut  102  has been unscrewed to release stop tube  98 , placing plunger  60  in a maintenance position. In this position, stop tube  98  can be used to push mandrel  66  out of rear sleeve  76 , making it available for removal and replacement of seals  68   a - 68   c  as required or any other maintenance which may be required. 
     FIG. 2  illustrates plunger  60  of sampling apparatus is the “parked” position, a position placing mandrel head  66   a  in a position to protect front sleeve  78  and mandrel head  66   a  from damage caused by abrasive liquid flowing in vessel  1 . In this position, front piston  82  has been moved away from valve  11  to a position stopped by center head  90 . To reach this position, actuator air is applied to a piston-park port  108  causing front piston  82  to move away from valve  11 . Center head  90  provides a stopping position for front piston  82  which serves as a stop for rear piston  63 . 
     FIG. 3  illustrates plunger  60  of sampling apparatus  10  in position to receive a liquid sample from within vessel  1 . Valve stem  24  is in position to align through-hole  26   h  such that plunger  60  may pass through central valve portion  26 . In this sample-receiving position, the sample-receiving site formed within annular spacer  70  is open to capture liquid flowing through or being processed within vessel  1 . In this position, both front piston  82  and rear piston  63  are positioned as close to valve  11  as possible within their respective cylinders. To reach this position, actuator air is applied to a piston-apply port  104  causing pistons  63  and  82  to move toward valve  11 . 
   From the sample-receiving position, the fixed volume of liquid in the sample-receiving space is then moved through adapter opening  18  and through front sleeve  78  into through-hole  26   h  which forms the sample-delivery site. This sample-delivery position of sampling apparatus  10  is illustrated in  FIG. 4 . In the sample-delivery position, rear piston  63  is moved within rear cylinder  86  until it reaches stop washer  99  on stop tube  98 . Stop washer  99  is positioned to place annular spacer  70  in the sample-delivery site within through-hole  26   h  in central valve portion  26 . To reach this sample-delivery position, actuator air is applied to piston-retract port  106  to move rear piston  63  to its stop position created by stop washer  99 . 
   In the sample-delivery position, bleed valve  20  can be used to effect transfer of the liquid sample contained in the sample-receiving space to a sample container (not shown) which may be connected to bleed valve  20  at bleed valve sample port  54 .  FIG. 8  illustrates bleed valve  20 , a ball valve in this embodiment, in the closed position while sampling apparatus also in a closed position, indicating that sampling apparatus  10  is in a position ready for or undergoing maintenance as illustrated in  FIGS. 5 and 6 , respectively.  FIG. 9  illustrates bleed valve  20  in an open position with valve  11  also in an open position, thereby allowing a liquid sample to pass from the sample-delivery site in through-hole  26   h,  through bleed valve  20  and into a sample container (not shown). Bleed valve  20  is attached to first stem portion  24   a  such that when valve  11  is turned using valve handle  42 , the entire bleed valve  20  turns with stem  24 . 
     FIG. 5  illustrates sampling apparatus in a maintenance position, ready for separation of actuator portion  14  from valve portion  12  as illustrated in  FIG. 6 . To reach this position, wing nut  102  is removed from rear head  95  thereby allowing stop tube to move stop washer  99  away from valve  11  and thus allowing plunger  60  to be removed from valve  11  completely. With the stop washer  99  back against rear head  95 , actuator air applied to piston-retract port  106  causes rear piston to move farther back into rear cylinder  86 . 
   A position called a “double-block-and-bleed” position is illustrated in  FIG. 5 . With sampling apparatus in a position ready for maintenance, an operator is able to use external flush port  52  in combination with bleed valve  20  to ensure that valve  11  has been properly closed, thus isolating vessel  1  from sampling apparatus  10 , before sampling apparatus  10  is removed. In this position, it is also possible to ensure that the sample has been properly removed from the sample-delivery site in through-hole  26   h.  This position of sampling apparatus  10  thus provides safety for the operator during operation of sampling apparatus  10 . 
   The selection of materials to be used for the embodiment described herein follows sound engineering practice as known by those skilled in the state-of-the-art of process monitoring, chemical instrument design or mechanical design. In general, parts within the embodiment described herein may be made of stainless steel such as 316SS but are not limited to being made of stainless steel. Seal materials for O-rings can be but are not limited to commercially-available O-ring materials such as Viton® or nitrile, both known to those skilled in the art of mechanical design. 
   Seals such as rear sleeve seals  76   a  and  76   b  may be made of PTFE (polytetrafluoroethylene) well known to those skilled in the art of mechanical design. Valve seats  28   a  and  28   b  may be made of TFM™, a modified PTFE material also well known to those skilled in the art of mechanical design. In some applications, front sleeve  78  may be required to withstand an highly-abrasive environment. In such cases, it may be useful to make front sleeve  78  from a material such as tungsten carbide. This and other materials are well-known to those skilled in the art of mechanical design. Front piston sleeve bearing  88  may be made of an appropriate sleeve bearing material such as silicon bronze or an oil- or PTFE-impregnated bearing material such as is well-known by those skilled in the art of mechanical design. 
   Front piston  82 , front cylinder  84 , rear piston  63 , rear cylinder  86 , center head  90 , and rear head  95  all may be made out of aluminum. 
   None of material suggested herein are meant to be limiting to the scope of the present invention. 
     FIG. 10C  illustrates an alternative embodiment for stem  24  and central valve portion  26 . In  FIG. 10C , central valve portion  26  is a truncated cone; a large end  26   a  of conical central valve portion  26  is adjacent to second stem portion  24   b  and a small end  26   b  of conical central valve portion  26  is adjacent to first stem portion  24   a.    
   Referring to  FIG. 3 , when, for example, front seal  68   a  on mandrel head  66  of plunger  60  enters front sleeve  78  from vessel  1 , seal  68   a  moves across a boundary between being unconstrained and being constrained (fitting tightly) within front sleeve  78 . Such a transition is called a seal/interface encounter. Each such encounter is an opportunity for any abrasive solid phase in the liquid from vessel  1  that is carried by the seal to cause wear of the seals. One significant advantage of placing the sample-delivery site within valve  11  is that there are fewer such encounters during a sample withdrawal cycle than with sampling apparatus of the prior art such as that described in U.S. Pat. No. 6,792,818 mentioned above. If the sample-receiving space was moved to a sample-delivery site on the side of valve opposite to vessel  1  as in Jaeger &#39;818, several additional wear-causing encounters would occur, thereby reducing the life of the seals as compared to those in the present invention. Furthermore, through-hole  26   h  is a clearance hole, i.e., larger in inside diameter compared to the outside diameters of front seal  68   a,  middle seal  68   b,  and rear seal  68   c  on mandrel  66  such that these seals do not touch the walls of through-hole  26   h  and such that any solid phase particles in the liquid being sample will not be pressed into these seals while sampling apparatus  10  is the sample-delivery position. In spite of the fact that first and second valve seats  28   a  and  28   b  ensure that stem  24  and central valve portion  26  remain in proper alignment within valve  11 , through-hole  26   h,  being a clearance hole, also reduces the effect of any misalignment which may occur within valve  11  during the operation of sampling apparatus  10 . 
   While the principles of this invention have been described in connection with specific embodiments, it should be understood clearly that these descriptions are made only by way of example and are not intended to limit the scope of the invention.