Abstract:
A reciprocating drive system that utilizes energy available from pressure changes in flowing fluid systems, said drive used for the injection of additives into pipelines. The reciprocating drive includes a hydraulic accumulator having a gas chamber connected to a gas pipeline to contain said gas, whereby variation in the elevated pressure of the pipeline is reflected in the pressure applied to an incompressible fluid in a hydraulic conduit. The drive is capable of recycling gas used to drive the system back into pipelines.

Description:
BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to linear drive systems that utilize the energy available due to pressure changes in flowing fluid systems; particularly but not exclusively to linear drive systems that may be used for the injection of additives into pipelines. 
   2. Description of the Prior Art 
   It is frequently necessary to inject an additive into a well or pipeline. These installations are often located in remote locations so the systems must be self-contained. Due to road conditions in some remote locations, chemicals which are injected into pipelines cannot be transported to the site for months at a time, and standard sources of power to run the pipeline system may not exist. Examples of additives that might be injected into pipelines include; chemicals for the prevention of line freezing due to hydration, chemicals that disperse waxes or asphaltene, and chemicals that prevent corrosion of pipelines. Therefore, there is a need for pipeline injection systems that offer both dependable and accurate metering, as well as having the capability to operate without traditional sources of power. 
   A number of different types of systems are available on the market for the injection of chemicals into remote pipelines or wellheads. Many of these systems utilize the natural gas carried by a pipeline as a prime mover. Use of natural gas for this purpose, however, is fraught with numerous problems. 
   A first problem with this type of system is that the natural gas used to drive the system is exhausted into the atmosphere, as the majority of these systems are unable to recover the gas. Pipeline natural gas often contains high levels of hydrogen sulphide, which is toxic and harmful to the environment. As a result, a number of governmental regulations have recently been put in place to restrict the release of natural gas into the environment. Further, the loss of natural gas to the environment represents a substantial, cumulative economic loss to operators. 
   An additional problem of using gas driven systems is a difficulty in controlling the mass of additive injected per unit of time. Gas driven systems suffer in performance due to the high compressibility of gas. Specifically, such systems are often typified by erratic piston motion, and as a result valve damage can also occur. Further, injection systems are required to operate efficiently at as low a pressure as possible so as not to restrict movement of gas within pipelines any more than necessary. 
   An alternative form of injection uses air/oil hybrid systems, but these are also characterized by specific deficiencies. Such systems often experience a loss of oil caused by the reciprocating motion of a piston rod. As a result of the oil loss, gas can replace oil in the system. Mixing of gas and oil in this manner causes a frothing of the oil component of the system, which can lead to erratic and uncontrolled movement of the piston rod used to inject the additive. 
   It is therefore an object of the present invention to obviate or mitigate the above disadvantages. 
   SUMMARY OF THE INVENTION 
   A reciprocating drive for use with a gas pipe line carrying gas at an elevated pressure, said drive comprising a drive rod, a pair of fluid motors each having a reactive surface acting on said rod to move said rod in opposite directions upon application of fluid pressure thereto, a valve connected between said pipeline and said motors and operable to direct gas from said pipeline to one or other of said motors, reversing a mechanism acting on said valve to change periodically the setting of said valve and reverse direction of movement of said rod, and a speed control device to control the rate of movement of said drive rod, said speed control device comprising a body of incompressible fluid disposed in a pair of chambers interconnected by a hydraulic conduit, each of said chambers including a cylinder and a piston moveable within the cylinder upon movement of said rod to vary the volume of said chamber, said chambers being arranged relative to one another such that a decrease in the volume of said chambers causes a corresponding increase in the volume of the other of said chambers, said speed control valve further comprising a flow control valve located in said conduit and a hydraulic accumulator connected to said conduit, said accumulator having a first chamber in communication with said conduit to contain said uncompressible fluid and a second chamber connected to said gas pipeline to contain said gas, whereby variation in said elevated pressure of said pipeline are reflected in the pressure applied to said incompressible fluid in said conduit. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Preferred embodiments of the invention will now be described by way of example only with reference to the accompanying drawings wherein: 
       FIG. 1  is a schematic representation of a pipeline additive installation. 
       FIG. 2  is a schematic representation of the components used in the system of FIG.  1 . 
       FIG. 3  is a representation of an alternative embodiment of the components used in FIG.  2 . 
       FIG. 4  is a further embodiment of the component shown in FIG.  3 . 
       FIG. 5  is a representation of the alternative configuration of cylinders shown in FIG.  2 . 
       FIG. 6  is a sectional view of a further embodiment. 
   

   DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   Referring therefore to  FIG. 1 , a pipeline additive system generally indicated  10  is connected to a pipeline  12  at a location where there is a pressure drop in the pipeline such as that provided by a restriction such as an elbow indicated at  14 . The elbow  14  provides a pair of spaced locations along the pipeline such that there is a small but discernible difference in pressure of gas in the line. The system  10  includes a reservoir  16  containing a supply of additives and connected to a pump  18  through a supply line  20 . A check valve  22  controls the direction of flow from the reservoir  16  to the pump  18 . The pump  18  discharges the additive through a supply line  24  and check valve  26  to the pipeline  12 . The pump  18  is driven by a drive assembly  28  that utilises the pressure of the gas or fluid in the pipeline  12  as its motive force. A supply line  30  is connected between the pipeline  12  and drive assembly  28  and an exhaust line  32  is similarly connected between the drive assembly and the pipeline  12 . The connection of the supply line  30  and exhaust line  32  is at respective ones of the spaced locations along the pipeline such that there is a discernible pressure difference between the two locations. 
   Further details of the drive assembly and pump may be seen from FIG.  2 . The pump  18  is a reciprocating pump having a cylinder  34  with an elongate internal chamber  36 . A piston rod  38  is slideable within the chamber  36  to induce fluid through the supply line  20  and expel it from the discharge line  24 , which are in communication with the chamber  36 . 
   The piston rod  38  extends from the pump  18  through a pair of actuators  40 ,  42 . The actuator  40  has a cylinder  44  within which the rod  38  slides and a piston  46  secured to the rod  38 . The piston  46  divides the cylinder  44  into a pair of chambers  48 ,  50 . The actuator  42  is similarly includes a piston  52  secured to the rod  38  and defining a pair of chambers  54 ,  56 . 
   The piston rod  38  carries a pair of adjustable stops  58 ,  60  that co-operate with a toggle mechanism  62  to actuate a valve  64 . The valve  64  is a two position four way valve that controls the supply of gas from the inlet  30  to respective ones of the chambers  50 ,  54  and similarly connects the chambers  50 ,  54  to the exhaust line  32 . 
   The chambers  48 ,  56  are connected to one another through branch conduits  66 ,  68  that are each connected to an accumulator  70 . Adjustable flow restrictors  72 ,  64  are included in the branch conduits  66 ,  68  respectively to control the flow of fluid between the chambers  48 ,  56  through the accumulator  70 . However, as described further below, in an additional embodiment of fluid does not have to flow through the accumulator  70 . The accumulator  70  has a gas chamber  76  that is connected through a branch conduit  80  to the inlet  30  an a hydraulic chamber  78 . The pressure in the gas chamber  76  thus corresponds to the pressure supplied to the inlet of the valve  64 . The chambers  48 ,  56  and the hydraulic chamber  78  of the accumulator  70  are filled with an incompressible hydraulic fluid, typically an oil, so that movement of the rod  38  causes displacement of fluid between the chambers  48 ,  56 , and  78 . 
   The supply line  30  includes a filter  82  and pressure regulator  84  to control fluctuations in the pressure supplied to the valve  64 . A backpressure valve  86  is connected in the exhaust line  32  to inhibit reverse flow of gas through the valve assembly. 
   In operation, with the components in the relative position shown in  FIG. 2 , the piston rod  38  is fully retracted from the chamber  36  which is filled with the additive drawn from the reservoir  16 . Pressure from the inlet  30  is supplied through the valve  64  to the chamber  54  and the chamber  50  is connected through the exhaust line  32  to the lower pressure zone of the pipeline. The pressure difference between the chamber  54  and chamber  50  induces movement of the piston rod  38  to expel fluid from the chamber  36 . The rate of movement of the rod  38  is controlled by the flow rate through the restrictor  72 ,  74  which is proportional to the pressure differential applied across the restrictors. Any variation in volume between the chambers  48  and  56  is accommodated by compression of the gas in the chamber  76 . As the piston rod moves to expel fluid from the chamber  36  through the discharge  24 , the abutment  58  contacts the toggle  62  and moves the valve  64  into its alternative position. In that position, the higher gas pressure is applied to the chamber  50  and the chamber  54  connected to the exhaust  32 . The direction of movement of the rod  38  is thus reversed causing the chamber to again expand and draw additive into the chamber  36 . The rate of movement of the piston rod  38  again is controlled by the flow of fluid through the branch conduit  66 ,  68  to maintain the speed at the desired rate. The reciprocal motion will continue to dispense the additive from the chamber  36  at each reversal utilising the gas supplied in the pipeline in a closed system. The stroke length of the piston rod may be adjusted by positioning the abutments  58 ,  60  at different locations along the piston rod  38  between the two actuators  40 ,  42  to co-operate with the toggle  62  at different points during the stroke. 
   Because the rate of movement of the rod is determined in part by the pressure difference across the restrictor  72 ,  74  it is necessary to prevent variation in the rate of movement due to fluctuations of the gas pressure within the line, which are in turn supplied to the chambers  50 ,  54 . Variations in the gas pressure are transmitted through the branch conduit  80  to the gas chamber  76  and thereby cause a corresponding increase in the pressure in the fluid chamber  78 . Thus, an increased pressure in the drive chambers  50 ,  54  due to an increase of pressure in the supply line  30  will cause a corresponding increase in the chamber  78  and maintain the pressure differential across the restrictor  72 ,  74  constant. The rate of movement of the piston rod  30  therefore remains constant and the volume of additive dispensed per unit of time can be maintained. 
   In the above embodiment, each of the branch conduits contains a restrictor  72 ,  74 . However, as shown in  FIG. 3  in which like components will be denoted with like reference numerals with a suffix “a” added for clarity, a single variable restrictor  74   a  is included in the branch conduit  68   a . The single restrictor  74   a  may be used to control the flow of fluid through the accumulator  70   a  and branch conduit  66   a . Again the pressure in the chamber  76   a  is adjusted with variations of the inlet  30  to maintain the pressure differential across the restrictor  74   a , substantially constant. 
   A further embodiment is shown in  FIG. 4  which permits control of the speed at different rates in opposite directions. In the embodiment of  FIG. 4 , the branch lines  66   b ,  68   b  are interconnected by a pair of cross flow lines  100 ,  102 . Each of the cross flow lines  100 ,  102  includes a check valve  104  and a variable flow restrictor  74   b . The check valves  104  are oppositely facing and that inhibits flow in opposite directions through each of the lines  100 ,  102 . The accumulator  70   b  is similarly protected by a pair of check valves  106 . In this embodiment the accumulator  70   b  acts as a pressurized reservoir. The accumulator  70   b  provides fluid to chambers  48   b  and  56   b  as these chambers lose fluid during operation. The accumulator  70   b  ensures that fluid that is lost during operation is replaced in order to ensure that gas and fluid are not mixed. The flow through each of the restrictors  74   b  may be adjusted independently and therefore the rate of movement of the piston rod  38  in each direction may be different. 
   In the embodiments shown in  FIGS. 2 ,  3  and  4  the actuators  40 ,  42  have been shown in spaced relationship with the toggle mechanism  62  located between. Other arrangements of the actuator may be utilised as shown in FIG.  5 . In  FIG. 5   a , the rod  38  extends through both sides of the actuator  42  to provide for double acting power transfer on both advance and retraction. 
   In the arrangement shown in  FIG. 5   b , the actuators  40 ,  42  abut each other on opposite sides of a partition  90  and the toggle mechanism  58 ,  60  may be moved externally of the actuators  40 ,  42 . The partition  90  separates the oil chambers  48 ,  56  with the gas chamber  50 ,  44  outboard of the partition. The rod  38  may extend through the cylinder  42  similar to  5   a  as shown in ghosted outline. In a further arrangement shown in  5   c , a pair of actuators  40 ,  42  are supplemented by an additional actuator  110  to provide additional surface area to move the piston rod  38  in each direction, thereby providing more power (Force×Distance). The actuators  40 ,  42  are arranged as in  FIG. 5   a  with oil chambers  48 ,  56  and gas chambers  68 ,  66  respectively. If preferred, the gas chambers may be incorporated in a single actuator with oppositely acting oil chambers paired on the other actuators. 
   In a further embodiment shown in  FIG. 6 , the gas chambers  48 ,  54 ,  56  actuators  40 ,  42  are combined in a by a diaphragm device  200 . The device  200  has an external housing  201  and an internal diaphragm  203  to which the rod  38  is secured. The oil chambers  48   56  are similarly combined in separate hydraulic dashpot  202 . The dashpot  202  includes a piston  204  sliding in a cylinder  206  and connected through ports  208  to the accumulator  210 . Pipeline pressure is applied to the accumulator through conduit  80   c . The restrictors  72 ,  74  are incorporated in valve block  212  located in the body of the accumulator  210 . 
   Flow of gas to the opposite sides of the diaphragm  200  is controlled by a valve block operated through stops  58   c ,  60   c  to reverse the porting of the valve. The function of the device is similar to that described above, with the diaphragm  200  providing the reciprocal motive force and the pipeline pressure acting through the conduit  80   c  to maintain flow through the restrictors in valve block  212  at the required rate. 
   In a further embodiment shown in ghosted outline in FIG.  6 . Additional diaphragm devices are attached to the drive system in order to provide a greater surface area. Addition of one or more diaphragms to the disclosed embodiment is preferred if: (1) a greater force is required to operate the drive, for example if a large volume chemical injector is driven or (2) if a lower supply differential exists, such lower supply differential requiring a greater surface area to obtain a desired force.