Abstract:
A seal assembly, for an additive pump having a reciprocating piston, is provided. The seal assembly includes a seal carrier having first and second components each having a bore therein to receive the piston. Each component has an end face arranged to abut one another when arranged axially on the piston, and a face seal is interposed between the end faces to inhibit egress of fluid between the end faces. A pair of circumferential seals at axially spaced locations are provided along the seal carrier and operable to engage the piston during reciprocating thereof, and a drain port intersecting one of the bores intermediate the seals is provided to permit egress of fluid from between the seals.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims priority from U.S. Provisional Application No. 60/812,111 filed on Jun. 9, 2006 and is hereby incorporated by reference. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates to injection pumps, in particular to injection pumps for injecting an additive into a pipeline. 
     SUMMARY OF THE INVENTION 
     It is well known to inject an additive into a fluid pipeline, such as a gas pipeline to enhance the serviceability of the pipeline. Typically, such additives are injected to inhibit corrosion or to enhance lubrication of components in the pipeline. The additive is injected in relatively small volumes compared to the volume of fluid carried by the pipeline but the additive&#39;s effect is significant. 
     The additives need to be injected periodically into the fluid and, as such, additive stations are placed at spaced locations along the length of the pipeline. Because of the nature of the pipeline and the terrain through which it must pass, the additive stations are typically located in remote areas and beyond access to normal services. The injection stations must therefore be self contained and capable of working without undue supervision over long periods of time. 
     The siting of additive stations at remote locations also requires the environmental impact of such stations to be minimized. The additives may in some cases be toxic or potentially hazardous and accordingly it is necessary to ensure that any spillage of such additives is minimized. 
     One such an arrangement that addresses these concerns is shown in U.S. patent application Ser. No. 10/742,792 in which the fluid in the pipeline is used as a motive force for an injection pump and the fluid is returned to the pipeline to avoid any egress into the atmosphere. The motive force available from such an arrangement is significant due to the pressure differential that exists in the pipeline and accordingly conventional sealing can be utilized within the plunger to inhibit leakage of additives. 
     In some circumstances the use of the fluid in the pipeline is not desirable or available and in such circumstances an alternative arrangement of pump is required. It has been proposed to utilize a battery powered pump with the battery being recharged from solar cells. With this arrangement however the conventional sealing arrangement used on additive pumps imposes a high load upon the piston of the pump and thereby increases the energy consumption of the additive station beyond that that may typically be available from a solar powered source. Conventional sealing arrangements utilize a packing gland whose sealing capability depends in part on the radial load applied to the shaft on which it is mounted. Such seals are relatively easy to install but impose significant drag on the piston. There is also a need with such additive pumps to provide control of the injection rate of the additive to suit varying conditions and for adjustment of that rate from station to station as circumstances differ. 
     It is therefore an object of the present invention to provide an additive pump in which the above disadvantages are obviated or mitigated. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       An embodiment of the invention will now be described by way of example only with reference to the accompanying drawings in which: 
         FIG. 1  is a general side view showing an additive station. 
         FIG. 2  is an enlarged sectional view of the portion of  FIG. 1  shown within the circle identified as II. 
         FIG. 3  is a schematic representation of the controller shown in  FIG. 1 . 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring therefore to  FIG. 1 , a pipeline indicated at P is supplied with an additive from a reservoir R through a conduit C. The additive is moved through the conduit C by an additive pump assembly generally indicated  10 . Energy for the operation of the pump assembly  10  is obtained from a solar panel  12  that is used to charge a battery  14  and provide a reserve of electrical energy for the assembly  10 . 
     The assembly  10  includes a pump  16  located in a housing  17  and a controller  18  that controls the operation of the pump  16 . The pump  16  includes a stepper motor  20  that is controlled by the controller  18  as will be described in more detail below. The stepper motor is available from Haydon Switch and Instrument, PO Box 3329, 1500 Meridian Road, Waterbury Connecticut 06705, under the Series 57000, size 23 and Series 87000, size 34 motors. The motor  20  includes an armature that cooperates with a drive shaft  24  through a lead screw  25 . Rotation of the drive shaft  24  is inhibited so that rotation of the armature  22  induces a linear axial displacement of the drive shaft  24  through the action of the lead screw  25 . 
     The drive shaft  24  is connected to a transfer shaft  26  that is attached through a coupling  28  to a piston  30 . The coupling  28  is of known construction that permits alignment between the transfer shaft  26  and the piston  30  and inhibits lateral loads being placed upon the piston during reciprocal movement. The piston  30  communicates with a pumping chamber  32  of a positive displacement fluid end  34  that may be of any convenient form known in the industry. The fluid end  34  incorporates an inlet check valve  36  and an outlet check valve  38  to ensure transfer of fluid from the reservoir R to the pipeline P as the piston  30  reciprocates. 
     The connection of the fluid end  34  to the piston  30  is best seen in  FIG. 2 . The piston  30  is slidably supported in a seal assembly  40  that is supported on an end face  41  of pump housing  17 . The seal assembly  40  includes a seal carrier  42  formed from an inner sleeve  44  and an outer nose  46 . The sleeve  44  and nose  46  are axially aligned to define a central bore  60  in which the piston  30  is a close sliding fit. The bore  60  is in fluid communication with the pumping chambers  32  so that reciprocal motion of the piston  30  within the bore  60  induces flow from the reservoir R to the pipeline P. 
     The sleeve  44  has a pair of stepped counter bores  48 ,  49  formed at one end adjacent to the wall  42  to carry circumferential lip seals  50 ,  51 . The seal  50  acts as a wiper to prevent contaminants from entering the central bore  60  and the seal  51  acts as a seal to inhibit egress of fluid from the chamber  60 . The opposite end of the sleeve  44  has a reduced shoulder  52  that is nested within a counter bore  54  of the nose  46 . The shoulder  52  and counter bore  54  define a cavity  56  in which a circumferential lip seal  58  is carried and functions in a manner to the seal  51  to prevent egress of fluid. It will be noted that the lip seals  50 ,  51 ,  58  are located at opposite end faces of the sleeve  44  so that the seals can be readily assembled. 
     The outer surface of the sleeve  44  has an undercut recess  57  in which a face seal  59  is located to but against the radial face of one end of the nose  46 . The face seal therefore provides a static seal between the two components of the carrier, namely the sleeve  44  and nose  46 . Again therefore, the seal may be easily assembled with the seal carrier. 
     The sleeve  44  and nose  46  are supported in a collar  62  having a central boss  64  and a radial flange  66 . The boss  64  is counter bored to receive the sleeve  44  and nose  46  and has a pair of circumferential grooves,  67 ,  68  that locate static seals  70 ,  72  to seal between the nose  44  and the counter bore of the boss  64 . 
     The radial flange  66  is located against the wall  42  by a retaining cap  74  with a seal  76  sealing between the cap  74  and the radial outer face of the flange  66 . A similar seal  78  is provided between the outer surface of the boss  64  and the cap to ensure fluid tight fitting. The outer surface of the flange  66  is bevelled as indicated at  80  to define an annular gallery that is intersected by a drainage port  82 . The drainage port also communicates through cross drillings  84  with the bore  60  at a location between the two seals  51 ,  58 . Any fluid entering between the two seals is therefore drained by the port  82  to the reservoir R as shown in  FIG. 1 . 
     The inner surface of the boss  64  is threaded to receive a threaded male fitting  86  of the fluid end  34 . The fluid end  34  has a elongate cylindrical recess  90  into which the nose  46  is a sliding fit. The distal end of the nose  46  is undercut to provide a notch  92  to form a seat for a high pressure face seal  94 . The notch  92  has a radial face  96  that opposes a complimentary radial face  98  on the fluid end so that the seal  94  is held between a pair of radial faces. Rotation of the fluid end within the boss  64  therefore applies a compressive load to the nose  46  and sleeve  44  to maintain the face seals  59 , 94  in compression. 
     In operation, reciprocation of the piston  30  within the bore  60  causes fluid to be initially drawn into the chamber  32  through a check valve  36  as the piston  30  is retracting and subsequently to expel fluid from the bore  60  through the check valve  38  as the piston  30  advances. During such reciprocal motion, the seals  50 ,  58  bear against the piston but in view of the fact that the piston itself is a close sliding fit within the bore and the seals utilized are preferably a lip seal, the passage of fluid past the seals is minimal. The drag on the piston due to the use of the pair of seals is also minimized and therefore the piston  30  has relatively low resistance to such axial movement. Any fluid that does pass through the seal  58  is drained through the port  82  back to the reservoir and thereby inhibits any loss of the additive during the pumping action. 
     The seal carrier  42  itself provides a sealed environment to inhibit egress of fluid under high pressure by providing a pair of face seals between radially opposed faces of the seal carrier. The seal  94  and seal  59  effectively inhibit the flow of fluid radially outwardly beyond the seal carrier  42  due to the compressive loads that act on the seals. It will also be noted that by forming the seal carrier in two parts namely the sleeve  44  and the nose  46 , the seal  59  is readily located on the seal carrier as is the face seal  94 . Accordingly, the optimum installation and sealing conditions can be provided for the face seals without inhibiting the operation of the piston. The seal  58  is preferably a dynamic spring energized rod seal with a high density, solvent resistant polymer sealing material. Such seals are capable of providing 90% sealing efficiency at pressures greater than 3200 psi. The seals  50 ,  51  are lower pressure lip seals designed to operate at slightly elevated pressures and essentially inhibiting the carriage of fluids on the piston into or from the housing. The face seals  59 ,  94  are static face seals of the O-ring type which provide 100% sealing at pressures over 3200 psi. 
     As noted above, reciprocal motion of the piston  30  is derived from the stepping motor  20 . The controller  18  provides control pulses through the field coils of the motor  20  which in turn produce a defined rotational output. By varying the frequency of the pulses and their polarity, the rate of rotation of the armature and its direction of rotation may be regulated as illustrated in  FIG. 3 . 
     The controller  18  has a program more programmable interface  100  providing keys  102 ,  104  to permit adjustment of the control. The interface  100  communicates with a processor  106  that includes memory  108 . The memory has a pair of registers, one for forward operation  110  and the other for reverse operation  112 . Each of the registers  110 ,  112  includes settings for the torque required, the ramping of the onset of the torque and the acceleration required. The memory  108  also includes a stroke setting  114  that determines the number of pulses that constitute the full stroke of the piston. Each of these setting are manually adjustable through the interface  100 . 
     The current supplied to the field windings of the motor  20  is determined by the current logic module  118 . The rate at which the current is supplied is determined from the ramping and acceleration values in the registers  110 ,  112 . The modules  118 ,  120  are used to drive a pulse generator  122  that outputs pulses of the appropriate amplitude, frequency and polarity to drive the armature in the desired direction of the desired rate. The pulses generated by the pulse generator  122  are monitored by a counter  124  and used to control the selection of the registers  110 ,  112 . Each time the counter  124  attains a value corresponding to that of the stroke register  114 , the register currently in use is terminated and the other register condition is loaded in through the modules  118 ,  120  to reverse the direction of motion. 
     By providing separate adjustment of the forward and reverse motion, different rates of movement can be attained and, with a rapid retraction of the piston, a substantially continuous injection of fluid can be attained if required. 
     The manual interface  100  permits the selection and setting of the conditions implemented by the control logic. The controller may be implemented on a control logic unit available from Trinanic Motion Control GmbH and Co. KG of Hamburg, Germany. 
     It will be see therefore that the use of the controller provides enhanced flexibility over the rate of injection and in particular with a differential rate of advance and retraction to permit enhanced control. The provision of the seal assembly with minimal resistance to motion also ensures that the current available from the solar source and batteries is sufficient for continuous operation. 
     As described above, the reciprocation of the piston  30  is a linear reciprocation with the drive shaft  24  secured to the housing of motor  20 . To enhance the performance and life of the seals, it is also possible to incorporate into the coupling  28  a helical drive such that the linear reciprocation of the transfer shaft  26  is converted to a helical motion of the piston  30  thus, the piston will both rotate and move axially past the seals  50 ,  51 ,  58  to further in prolong the life of the seals. 
     The preferred embodiment of seal assembly has been described in conjunction with a solar powered electrical supply and controller. It will be appreciated, however, that the seal assembly may be used with other forms of drive of plunger and may be used as a retrofit to existing seal assemblies used on additive pumps.