Abstract:
A snap pilot used to pneumatically energize and de-energize a control device, such as a valve. The pilot has two valves—one to admit pilot pressure, and one to exhaust pressure. The pilot incorporates valves having an elastomeric seal but maintaining the precision of metal to metal contact. In the preferred embodiment, the elastomeric seal is an O-ring placed in a groove, without adhesive, having only a small amount of deflection when the seat is closed, whereby simultaneous metal-to-metal and elastomer-to-metal contact is achieved.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    1. Field of the Invention  
           [0002]    This invention relates generally to pneumatically actuated pilots (also called relays) used to energize or de-energize a control device, and, more specifically, to a snap pilot for use, for example, in a liquid level controller.  
           [0003]    2. Background  
           [0004]    A snap pilot is typically used to pneumatically energize and de-energize a control device, such as a valve. A conventional snap pilot is comprised of a body having an upper chamber, a middle chamber and a lower chamber; an inlet for receiving a supply gas into the upper chamber; a first valve fluidly connecting the upper chamber with the middle chamber; a gas outlet for passing the supply gas from the middle chamber to the control device; a second valve fluidly connecting the middle chamber with the lower chamber; and an exhaust port for venting gas from the lower chamber. The vented gas is from the control device as it is de-energized. The first and second valves are normally actuated by a thrust pin.  
           [0005]    The first valve generally comprises a ball located in the upper chamber that mates with a ball seat machined into the pilot body. The ball controls the flow of gas into the pilot and is held closed with force exerted by supply pressure on the seating area of the ball when the thrust pin is in a non-contacting first position. In a second, ball-contacting position, the thrust pin is used to overcome the force holding the ball seated in order to open flow between the upper and middle chambers.  
           [0006]    The second valve typically comprises a spherical segment of the thrust pin which, when in the second position, sealingly engages a second valve seat machined into the pilot body between the middle and lower chambers. Thus, a simultaneous action occurs as force is applied and removed from the thrust pin whereby in the first position the first valve is closed while the second valve is open and whereby in the second position the first valve is open while the second valve is closed.  
           [0007]    The pilot ordinarily passes natural gas or other gases including air. The loss of natural gas to the atmosphere has two consequences, degradation of environmental air quality and the loss of value of the exhausted gas. Thus, there is a need to minimize gas leakage from the pilot.  
           [0008]    The usual practice has been for the first and second valves to be metal-to-metal contacts. Zero leakage would then occur when the two mating metal surface have perfect continuous contact, which, of course, is difficult to achieve. Past efforts at reducing leakage have included the use of an elastomeric seat seal to improve sealing of the ball against the ball seat. In such pilots, the ball is solely supported by an elastomer with its vertical position affected by the load from the supply gas pressure. The relatively large resiliency of the elastomeric seals usually detracts from the desired crisp snap action. Moreover, the elastomer is typically attached to the pilot body with adhesive that often adversely affects the resiliency of the elastomer. Accordingly, there remains a need to minimize gas leakage from a snap pilot without reducing the performance thereof.  
         SUMMARY OF THE INVENTION  
         [0009]    According to the present invention there is provided an improvement in a snap pilot wherein the pilot allows no leakage while preserving crisp snap action. The inventive snap pilot incorporates a valve having an elastomeric seal therein but maintaining the precision of metal-to-metal contact. In the preferred embodiment, the elastomeric seal is an O-ring placed in a groove within the valve seat, without adhesive, having only a small amount of deflection when the seat is closed, whereby simultaneous metal-to-metal and elastomer-to-metal contact is achieved.  
           [0010]    In one aspect, there is thus provided a pneumatically actuated pilot comprised of a body having an upper chamber, a middle chamber and a lower chamber; an inlet for receiving a supply gas into the upper chamber; a first valve fluidly connecting the upper chamber with the middle chamber; a gas outlet for passing the supply gas from the middle chamber to the control device; a second valve fluidly connecting the middle chamber with the lower chamber; an exhaust port for venting gas from the lower chamber; and a thrust pin for actuating the first and second valves; wherein the first valve includes a ball located in the upper chamber that, when in the closed position, simultaneously engages both a valve seat machined into the pilot body and an elastomeric sealing element, preferably an O-ring. The O-ring is strategically positioned in a recess within the valve seat, as aforesaid, such that it is self-restrained and exposed only a small degree. Consequently, near simultaneous contact is achieved between the ball, the O-ring and the valve seat upon closing of the valve and, when closed, the ball sealingly engages both the O-ring and the metallic valve seat.  
           [0011]    Although beneficial results may be obtained through the use of the invention, as described above, even more beneficial results may be obtained by providing a like second valve connecting the middle chamber with the lower chamber. In connection with this aspect of the invention, an elastomeric seal, preferably an O-ring, is provided in a recess on the thrust pin shaft at a point immediately adjacent the seat-contacting portion of the thrust pin. Thus, the thrust pin&#39;s O-ring contacts the second valve seat immediately prior to lifting the ball. Again, the O-ring is strategically positioned in the recess such that it is self-restrained and exposed only a small degree. Consequently, near simultaneous contact is achieved between the O-ring, the seat-contacting portion of the thrust pin and second valve seat upon closing of the valve and, when closed, both the O-ring and seat-contacting portion of the thrust pin sealingly engage the metallic valve seat.  
           [0012]    A better understanding of the present invention, its several aspects, and its advantages will become apparent to those skilled in the art from the following detailed description, taken in conjunction with the attached drawings, wherein there is shown and described the preferred embodiment of the invention, simply by way of illustration of the best mode contemplated for carrying out the invention. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0013]    FIGS.  1 A-C are sectional views of a snap pilot according to the preferred embodiment of the present invention shown in its first, intermediate, and second positions.  
         [0014]    [0014]FIG. 2 is an enlarged, partial sectional view taken from FIG. 1A of a ball valve fluidly connecting the upper chamber with the middle chamber when the pilot is in its first position.  
         [0015]    [0015]FIG. 3 is an enlarged, partial sectional view taken from FIG. 1B of a valve fluidly connecting the middle chamber with the lower chamber when the pilot is in its intermediate position.  
         [0016]    [0016]FIG. 4 is an enlarged, partial sectional view taken from FIG. 1C of both valves of the pilot when the pilot is in its second position. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0017]    Before explaining the present invention in detail, it is important to understand that the invention is not limited in its application to the details of the embodiments and steps described herein. The invention is capable of other embodiments and of being practiced or carried out in a variety of ways. It is to be understood that the phraseology and terminology employed herein is for the purpose of description and not of limitation.  
         [0018]    Referring now to the drawings, wherein like reference numerals designate identical or corresponding parts throughout the several views, the snap pilot or relay is generally indicated by the reference numeral  10 . It comprises a body assembly  12  which, in the illustrated embodiment, is composed of a top cap  14  and a body  16  separated by a gasket  18 . A bottom cap  20  is affixed to the bottom surface of the body  16 . Within the body assembly  12  there is provided an upper chamber  22  which receives a supply gas through a supply gas inlet  24 . A first valve, generally indicated by the reference numeral  26 , separates the upper chamber  22  from a middle chamber or passageway  28 . A gas outlet port  30  fluidly connects with the middle chamber  28  for passing the supply gas from the middle chamber  28  to a control device, such as a control valve. A second valve  32  separates the middle chamber  28  from the lower chamber  34 , which lower chamber  34  fluidly communicates with an exhaust port  36 . A thrust pin  38  guided by the bottom cap  20  serves to actuate the first and second valves  26  and  32  as further described below. A retaining ring  40  secures the thrust pin  38  within the body assembly  12  of the snap pilot  10 .  
         [0019]    FIGS.  1 A-C illustrate the inventive snap pilot  10  in its various operating positions. FIG. 1A corresponds to a first position (of both the pilot  10  and the thrust pin  38 ) wherein the first valve  26  is closed while the second valve  32  is open. FIG. 1B, on the other hand, illustrates an intermediate position wherein the thrust pin  38  has been forced upwardly in order to contact the ball  42  while simultaneously closing the second valve  32 . Lastly, FIG. 1C exemplifies the invention upon the thrust pin  38  having reached a fully extended second position whereby the first valve is fully open while the second valve is closed.  
         [0020]    As best shown in FIG. 2, the first valve  26  includes a ball  42  located in the upper chamber  22  that, when in the closed position as illustrated, simultaneously sealingly engages a first valve seat  44  machined into the body  16  and an elastomeric sealing element  46 , preferably an O-ring. As is evident in FIG. 2, when the first valve  26  is in its closed position there is a space  48  created between an upper shaft portion  50  of the thrust pin  38  and the ball  42 . Thus, the supply pressure from the supply gas maintains the ball  42  in its engaged position.  
         [0021]    The improvement provided by the present invention encompasses the use of an elastomeric seal within the first valve  26  while maintaining metal-to-metal contact between the ball  42  and the valve seat  44 . In this regard, the valve seat  44  is provided with a recess  52  of the depth sufficient so that the O-ring  46  may be self-restrained therein and exposed only a small degree such that when the first valve  26  is in its closed position, the ball  42  simultaneously sealingly engages both a surface portion of the valve seat  44  and the O-ring  46 . There is thus provided a contact point  54  between the ball  42  and valve seat  44  and a contact point  56  between the ball  42  and O-ring  46 . Thus, it can be appreciated that the O-ring  46  is exposed outside the plane of the valve seat  44  an amount that minimizes deflection of the O-ring while maintaining an adequate sealing surface and while allowing the ball  42  to achieve sealing engagement with a surface portion of the valve seat  44 .  
         [0022]    In the intermediate position illustrated in FIG. 1B the thrust pin  38  is forced upward as indicated by the arrow. (If, for example, the inventive pilot  10  is utilized in a liquid level controller, the thrust pin  38  will ordinarily be actuated by the translated movement of a displacer or float.) At this point, as shown more particularly in FIG. 3, there begins a sealing engagement in the second valve  32 . The thrust pin  38  includes a seat-contacting portion  58  that moves into abutting relationship with a surface  60  of a second valve seat  62  machined into the body assembly  12 . In a second aspect of the preferred embodiment of the present invention, a circumferential recess  64  is provided around the thrust pin  38  at a point immediately adjacent the seat-contacting portion  58  of the thrust pin  38 . Within the recess  64  there is seated a second elastomeric sealing element  66 , preferably in the form of an O-ring. As the thrust pin  38  is directed upward a near simultaneous contact is achieved between the O-ring  66  and the surface  60  of the second valve seat  62 . In a manner similar to that described above in connection with the first valve  26 , the recess  64  is of a depth sufficient so that the O-ring  46  may be self-restrained therein and exposed only a small degree such that the outer surface of the O-ring  66  contacts the surface  60  of the valve seat  62  only a fraction of an instant prior to sealing engagement occurring between the seat-contacting portion  58  of the thrust pin  38  and the surface  60 .  
         [0023]    Turning now to FIG. 4, wherein the thrust pin  38  has achieved its second position, it can be seen that with respect to the second valve  32  there is accordingly provided a contact point  68  between the self-contacting portion  58  of the thrust pin  38  and the second valve seat  62  and a contact point  70  between the O-ring  66  and the valve seat  62 . Thus, it can be appreciated that the O-ring  66  is exposed outside the plane of valve seat  62  an amount that minimizes deflection of the O-ring while maintaining an adequate sealing surface and while allowing the seat-contacting portion  58  of the thrust pin  38  to sealingly engage the surface portion  60  of the valve seat  62 .  
         [0024]    Still with respect to FIG. 4, when thrust pin  38  is fully upwardly extended to its second position it can be seen that the upper shaft portion  50  of the thrust pin  38  displaces the ball  42  from the first valve seat  44 , including the O-ring  46  such that a fluid passageway  72  is created between the upper chamber  22  and the middle chamber  28 .  
         [0025]    Consequently, in operation the inventive snap pilot provides leak resistant first and second valves  26  and  32  while maintaining crisp snap action. The thrust pin  38  alternatingly and oppositely opens and closes the first and second valves to admit and exhaust pilot pressure in order to affect control of a control device, such as a valve.  
         [0026]    While the inventive pilot may be employed in any number of products directed to the petroleum, marine, steel, and industrial markets, the pilot is especially useful in connection with a liquid level controller, an understanding of which is well known to one of ordinary skill in the art.  
         [0027]    While the invention has been described with a certain degree of particularity, it is understood that the invention is not limited to the embodiment(s) set for herein for purposes of exemplification, but is to be limited only by the scope of the attached claim or claims, including the full range of equivalency to which each element thereof is entitled.