Patent ID: 6862972

Claim:
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 fluid chambers interconnected by a hydraulic conduit, each of said fluid chambers including a cylinder and a piston moveable within the cylinder upon movement of said rod to vary the volume of fluid chamber, said fluid chambers being arranged relative to one another such that a increase in the volume of one said fluid chambers causes a corresponding increase in the volume of the other of said fluid chambers, said speed control device further comprising a flow control valve located in maid 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.