Abstract: A well pumping system includes a pivotally mounted walking beam and "horsehead" connected to a downhole pump by a pump rod in the conventional manner. A hydraulic lift piston and cylinder and a pneumatic balance piston and cylinder are connected to the walking beam. A process control computer controls input signals to a hydraulic control valve for controlling the hydraulic cylinder rate and direction of travel to provide corresponding control over the motion of the walking beam. The computer receives input information from a position sensor indicating the displacement of the beam in its range of travel. The computer program also is responsive to a timer for determining actual stroke rate and acceleration of the beam. The computer monitors and controls operation of the hydraulics and pneumatics as the pumping unit produces the lift necessary to extract fluid from the well.

Abstract: A rotary encoder comprises a rotary plate having a plurality of primary slits defined in the rotary plate in a circular row coaxial with the axis of rotation of the rotary plate. The rotary plate also has at least one reference slit defined therein at a position radially of the circular row of the primary slits. A primary photoelectric detector including a source of light and a photo-sensor positioned one above the other with the circular row of the primary slits intervening therebetween is used to detect the passage of the primary slits therethrough while an auxiliary photoelectric detector including a source of light and a photo-sensor positioned one above the other with the path of travel of the reference slit intervening therebetween is used to detect the passage of the reference slit therethrough.

Abstract: The installation serves to adjust the flow rate of air in a network of ducts such as a mine. It comprises a fan (10) provided with an electric motor (16) powered at a variable frequency. The installation also includes a converter having an adjustable output frequency and measuring means (12, 22) for measuring the flow rate and the total pressure supplied by the fan. It also includes a processor (20), a memory (26) for storing in digital form at least one total-pressure/flow-rate characteristic curve, and optionally a pressure/flow-rate characteristic curve of the fan at at least one predetermined speed; means (30) for displaying said curves; and data input means enabling a reference value (Q.sub.

Abstract: A multichannel pump system for HPLC and other uses has a plurality of fluid pumps whose input is fed separately to at least one high pressure mixing unit downstream of the pumps and any associated items such as pulse dampeners and pressure transducers. A pump controller is connected to control each of the pumps separately in a coordinated manner to provide a plurality of operational modes. The operational modes include an isocratic mode, in which optionally the output of solvent of like composition from two or more pumps is pooled to provide increased output capacity, as well as binary, ternary, etc., gradient modes in which different solvents are selectively mixed before being applied to an analytical unit such as an HPLC column. The pump system further includes a novel linear drive pump having interchangeable piston modules providing different ranges of flow rates. The linear drive pump is capable of providing precise solvent delivery at high pressure for flow rates from 20 ml/minute down to 0.

Abstract: A method for monitoring a rod pumped well to detect various problems, i.e., fluid pound, rod parts, pump problems, high fluid levels, imbalance of the pumping unit, rate of production and others. The method utilizes measurements made at the surface to calculate a downhole pump card. The method utilizes the downhole pump card to detect the various pump problems and control the pumping unit.

Abstract: A variable-speed drive controller and associated methods are disclosed for reciprocated lifting systems, particularly long-stroke, fluid-pumping systems such as certain oil-producing equipment. A variable-speed drive is controlled through an electronic feedback loop by a programmable controller, which varies the speed of the drive when sensors detect the passage of the reciprocating mechanism. The controller can be set to slow the drive when the reciprocating mechanism reaches the end of the stroke, to permit faster production of fluid without increased mechanical wear on the reciprocating mechanism or increased stress on the lifting string. The controller also can be set to slow the drive on the downstroke, such as to permit faster production of fluid when pumping viscous liquid.

Abstract: The present invention is a liquid level control system for selectively activating and deactivating a pump according to the liquid level indicated by capacitive sensors. Activating circuitry starts the pump motor when both upper and lower sensors indicate the presence of liquid. The pump motor is latched on by the activating circuitry and continues operation until the lower capacitive sensor indicates the absence of liquid, in which case the pump is shut off. A high frequency oscillator operates reliably over a wide range of operating conditions to maintain relatively constant charging time for the capacitive sensors.

Abstract: An electric circuit for operating a toilet room soap dispenser includes a sensor and an electric operator, with the sensor providing an operating signal in response to the presence of a person positioned to use the soap dispenser The electric operator, which is responsive to a signal from the sensor, provides a measured amount of soap from the soap dispenser. The improvement is specifically directed to a prime circuit for causing a predetermined number of operations of the electric operator to prime the soap dispenser for normal use, after refilling the soap supply.

Abstract: A method and apparatus for vertically adjusting the stroke of the polished rod of a pumpjack during its operation in a producing well, while detecting abnormal conditions such as gas-off, pump seizure, sucker and separation, gas-lock pounding or fluid pounding. The method and apparatus involves a hydraulic or pneumatic cylinder interposed between the polished rod clamp and the carrier bar of the pumpjack. Abnormal conditions are detected by an electronic device incorporating a pressure transducer which senses fluid pressure changes in the hydraulic or pneumatic cylinder, converts the pressure changes into electrical voltages, and then determines if an abnormal condition is present. If an abnormal condition is sensed, the pumpjack can be stopped, and restarted automatically after a momentary interval in the event of gas-lock or fluid pounding, or manually after inspection by an operator in the event of a more serious condition.

Abstract: New apparatus and methods are disclosed for controlling the operation of a rod-pumped oil well. The load on and position of the rod string are measured, an integration calculation related to the work done during a portion of the downstroke is performed, and the result is compared to a reference value, to detect a pumped-off condition in the well.

Abstract: A method and apparatus for controlling a fire pump motor for use in a building having a water supply for a fire protection system, a power line, and a pressure sensor. The pressure sensor detects a high or low system water pressure with respect to a reference water pressure. A voltage controller is coupled between the power line and the fire pump motor for controlling the voltage from the power line to the fire pump motor, responsive to indications from the pressure sensor. The voltage controller increases the voltage to the fire pump motor to full voltage and runs the fire pump motor at full voltage for a minimum run time and until high pressure is indicated. The voltage controller then decreases the voltage to the fire pump motor from full voltage to a predetermined voltage level. The voltage controller holds the voltage to the fire pump at the predetermined voltage level for a first preset period of time.

Abstract: A system and method for analyzing and collecting fluids for recycling or other disposal, such as used motor oil, by determining whether the liquid has a specific gravity falling within a predetermined range, selectively activating a pump cycle to transfer motor oil exhibiting a specific gravity within the predetermined range to a holding tank for environmentally safe storage until transportation to an appropriate recycling or other disposal center.

Abstract: The current invention detects malfunctions in a variable flow delivery pump. These malfunctions prevent the pump from sustaining inherently high accuracy and/or cause damages to the pump. The invention detects the malfunctions without comparing sampled values to a preset absolute value. A malfunction is determined by a pattern of force exerted on a displacement pump in relation to the displacement positions. The pattern is compared to an empirically established range of pattern values. Once a malfunction is determined, the invention responds in a preselected manner.

Abstract: A system adapted for use to transfer desired volumes of liquid chemicals from supplies of the liquid chemicals to a container. The system includes a reversible D.C. motor driven pump for moving liquid in either of two opposite directions, a counting system for counting part revolutions of the pump, and a calibrated chamber. The pump is operated for a predetermined number of part revolutions, the volume of liquid delivered by those part revolutions is determined using the calibrated chamber, and entered into a computer which then determines the amount of liquid delivered by each part revolution of the pump. In response to appropriate inputs, the computer then determines the number of pump part revolutions needed to deliver a needed amount of liquid to a container, and activates the pump for that number of part revolutions to deliver the needed amount of liquid to the container.

Abstract: A syringe pump comprises a housing with a syringe retainer. The syringe includes a barrel for liquid medication and a plunger movably positioned in the barrel for expelling the medication. A driver is movably mounted on the housing for engagement with the plunger. A control mechanism within the housing regulates the rate of movement of the driver and plunger during delivery of medication. A monitoring circuit connected to receive signals from a potentiometer and a movable contact associated with the driver indicates driver motion. A microprocessor with memory includes a preset standard to which the signal of motion from the driver is compared. An additional input signal, of motion from a rotary indicator associated with a part of the driver movement motor assembly, is provided to the memory and is compared to plunger travel into the barrel.

Abstract: A control system and method are provided for controlling the timing of a dispenser including a valve which moves alternately between a product-loading position and a product-dispensing position in a repeated cycle. The dispenser also includes a mechanism for dispensing a product from the dispenser after each time the valve moves to its product-dispensing position. The control system includes a detector which detects the time of which the valve reaches its product-dispensing position and a mechanism for generating a control signal at a predetermined time to actuate the dispensing mechanism when the valve reaches its product-dispensing position. The control system also includes a sensor for sensing the time that the dispensing mechanism actually begins dispensing product. The control system further determines the time difference between the time the valve reaches its product-dispensing position and the time the dispensing mechanism actually begins dispensing the product.

Abstract: A compressor with a flexible diaphragm used as the pumping element such as for Stirling coolers or pulse-tube refrigerators, said compressor further including a sensor to monitor the position of the diaphragm and a control circuit for maintaining the diaphragm position in a preselected range to avoid contact between the diaphragm and the rest of the compressor. The compressor also includes an expansion member for compensating for the thermal expansion of the working fluid of the compressor.

Abstract: A method of calibrating a well pumpoff controller by calibrating the controller for determining the average load during a pumping stroke, measuring the actual load and averaging the measured maximum and minimum load during each operational stroke, comparing the average measured load with the calibrated average load and providing an offset to the load measurement to correct the measured load towards the calibrated load.

Abstract: An air sampling device comprises a pump assembly including a pump and a motor operably associated with the pump, the air flow rate of the assembly being substantially characterized by a function having a plurality of measurable variables; sensors for periodically automatically monitoring the variables; a programmable computer operably associated with the sensors for periodically estimating the instantaneous air flow rate of the pump from the function and the monitored variables and for camparing the calculated instantaneous air flow rate with a previously programmed desired air flow rate; and a DC-DC converter responsive to the computer and operably associated with the motor for modulating the motor speed in response to the computer to thereby cause the instantaneous air flow rate of the pump to approach the desired air flow rate.

Abstract: A liquid pump, particularly though not exclusively suitable for use in flow injection analysis, and comprising a reservoir 3 an outlet tube 6 for conducting liquid 2 from the reservoir and a device for introducing gas into the liquid in the reservoir to displace liquid therefrom through the tube. The gas may be introduced in the form of a series of rapid pulses of controlled duration and frequency and is introduced at a fixed level relative to the entrance to the outlet tube.

Abstract: The device for the compression of a gas comprises a motor (1), a piston (2) adapted to be displaced linearly in a pump body, transmission means (T) between the motor (1) and the piston (2) adapted to convert the rotary motion generated by the motor (1) into a reciprocating translatory motion by the piston (2). Speed varying means comprising a cam (12) are provided to impart a different speed to the piston (2) of the compressor according to the resistance opposing the motor (1) during one complete admission/compression cycle, so that the resisting torque is substantially constant and so that the motor (1) operates at its maximum torque and therefore at maximum efficiency. The section of the cam (12) is determined so as to ensure substantially isothermal compression of the gas.

Abstract: The present invention is a liquid level control system for selectively activating and deactivating a pump according to the liquid level indicated by capacitive sensors. Activating circuitry starts the pump motor when both upper and lower sensors indicate the presence of liquid. The pump motor continues operation until both of the capacitive sensors indicate the absence of liquid, in which case the pump is shut off. The circuitry of the present invention operates reliably over a wide range of operating conditions.

Abstract: A cartridge fed infusion pump containing a plurality of linear peristaltic pumps. Each pump is powered by a direct current motor. The motor has a shaft which rotates to perform a pump cycle. A position encoder mounted on the shaft is used to determine when the shaft has reached the stop position in the pump cycle. A reverse pulse is used to quickly stop the motor shaft.

Abstract: A protector device connected in series with a pool pump motor. When the automated pool timer contacts close, supplying AC power to pool pump motor, the protector apparatus logic permits instantaneous motor turn on and logic reset of apparatus clock timer. After a nominal period of time, pump motor will be shut off, unless water is flowing through pump motor. In the event water is flowing through pump motor, a water flow sensing switch in the water line to pump will override clock logic, and hold pump motor on, until the automated pool timer reaches a preset trip turn off time.

Abstract: A control system for the working piston of a valveless metering pump for the precision delivery of fluids to a plurality of sequentially spaced output ports is adapted to control and alter the speed of rotation of the pump piston depending on predetermined angles of rotation of the piston as it cycles.

Abstract: A system for and a method of establishing and maintaining fluid at desired viscosity and temperature levels in a flow system. The system includes a driven pump that circulates fluid from a fluid source through a hydraulic loading system that includes an unloading valve. As fluid flows through the valve, a pressure drop is created across the valve operator, which restricts flow through the valve. This pressure drop across the valve operator results in the dissipation of heat, increasing the temperature of the fluid flowing through the valve. During operation of the system, if a low temperature condition is sensed in the system, power will be supplied to the pump, which pumps fluid through the valve at a controlled low pressure drop to result in a controlled heating of the fluid. When the fluid is restored to a desired temperature level, power to the pump is discontinued to terminate flow through the valve.

Abstract: The motor protector for a motor is located so as to be responsive to the discharge temperature of a compressor. The protector is thereby responsive to motor current and compressor discharge temperature. Additionally, by conduction, the protector is responsive to the motor temperature. For scroll compressors, specifically, this provides protection from excess heating of the scroll wraps without requiring an additional sensor.

Abstract: An apparatus for the generation of compressed air is disclosed, where the supply of air from the compressor (1) to the user (6) is interrupted in case of preset user-side pressure. This apparatus prevents a freezing in case of low ambient temperatures in the pressure lines in which condensed water has formed. A temperature sensitive switching device (14, 29) responds to the actual ambient temperature. The switching device (14) does not stop the compressor (1) in case of a preset user-side pressure and in case of a falling below of a preset ambient temperature. The compressor (1) carries then the warmed-up air through a region of the device before discharging it into the surrounding atmosphere. The device is particularly suited for compressed-air generation plants in motor vehicles which are adapted to be used in cases of low ambient temperatures.

Abstract: A reciprocating type fluid delivery pump has a driving motor, plungers for driving two pump heads, respectively, and a converting mechanism for converting the rotational motion of the driving motor into a reciprocating motion of each plunger. The converting mechanism includes a cam having such a configuration that, when the driving motor is rotated at constant velocity, the delivery flow rate during the delivery starting period of each of the plungers is in excess of that during the other periods of the cycle. The speed of rotation of the driving motor is reduced during the excess delivery period according to need. Thus, the load on the driving motor is reduced and it is therefore possible to employ a driving motor having a relatively low output. The delivery pump also includes a rinsing chamber for each plunger for allowing pressurized fluid to pass therethrough. The rinse chamber includes a rinse pump which uses the reciprocal movement of the pump piston for the rinse mechanism.

Abstract: A variable speed turbo vacuum pump system (10) utilizes a motor driven compressor (12) to evacuate a gaseous medium from an enclosed space (22). The compressor (12) is a rotational type having at least one compressor wheel (40), the rotational speed of which is increased in inverse proportion to the decreasing inlet density of the gaseous medium from within the enclosed space (22).

Abstract: The method and system for detecting occlusion in a fluid line upstream of a pump concerns a fluid pump with a pump cycle in which the upstream pressure of the fluid line is communicated to the downstream portion of the fluid line. A pressure sensor located downstream of the pump is capable of measuring a relatively large negative-going pressure in the fluid line. A signal is generated to indicate occlusion based upon detection of negative-going pressure.

Abstract: There is disclosed an electronic control means for a pump, the control means comprising: input means, for accepting input signals, the input signals being indicative of the values of variable quantities which determine a desired flow rate of a fluid which, in use, is to be supplied by a pump; an electronic memory, comprising a plurality of memory locations, each memory location corresponding to a particular combination of values of the variable quantities and containing a stored control signal corresponding to the desired fluid flow rate determined by those values; and output means, for supplying the control signal, in use, to the pump. More specifically, there is disclosed equipment suitable for distributing a fluid over an area of land, the equipment comprising distribution means, to which fluid is supplied by a pump driven by an electric motor, the delivery rate of the fluid from the pump being a function of the operating speed of the pump, the equipment further comprising such control means.

Abstract: Automatic treatment of a drain system is provided by the controlled injection of a bio-active liquid at established intervals into the drain system. A container of the treatment liquid is provided with a pump coupled between the container and the drain. Fluid level sensing means is located in the container and fluid temperature sensing means is located in the drain system. When the temperature is within range and the fluid level is sensed, the pump is periodically operated to inject a quantity of fluid directly into the drain system without operator control.

Abstract: Precise gradient control is accomplished using a high flow three-way valve, appropriate mixing, and precise metering control. Gradient elution profiles are generated for flow rates above 100 ml/min. Control involves precise selection of two solvents, and correction factors for valve lag times and pump/valve synchronizations. The gradient is controlled simultaneously with other non-timed operations through a single microprocessor controller.

Abstract: A DC motor drive system for mechanically linking to a reciprocable pump, having safety circuits for disconnecting power from the system upon detection of excessive line voltage variations, excessive line current variations, excessive pressure demands, and electrical and mechanical faults which may occur in system operation.

Abstract: A compressor with a flexible diaphragm used as the pumping element, said compressor further including a sensor to monitor the position of the diaphragm and a control circuit for maintaining the diaphragm position in a preselected range to avoid contact between the diaphragm and the rest of the compressor. The compressor also includes an expansion member for compensating for the thermal expansion and compressibility of the working fluid of the compressor.

Abstract: A protection system for a rotatable element (20) which is rotatably supported by hydrodynamic bearings (22, 24) pressurized with a lubricating fluid provided from a pressure source (26 and/or 34) with the fluid being subject to a change in state from liquid to vapor when heat from the bearings during rotation of the rotatable element is absorbed by the pressurized fluid during supporting of the rotatable element in accordance with the invention includes a pressure sensor (302) in fluid communication with fluid flowing from the hydrodynamic bearings for producing a pressure signal representative of pressure of the fluid flowing from the bearings; a temperature sensor (304) in fluid communication with the fluid flowing from the hydrodynamic bearings from producing a temperature signal representative of temperature of the fluid flowing from the bearings; a prime mover (17) for driving the rotatable element with a variable rotational velocity; and a controller (400), responsive to the pressure and temperature sig

Abstract: In a liquid well pumping system which is provided with a downtime between pumpoff cycles, the method of determining the maximum downtime includes running a plurality of tests to determine the relationship between runtime and downtime and determining when the relationship becomes non-linear for at least two consecutive tests. The downtime is selected adjacent the last linear relationship between runtime and downtime for providing a downtime which allows adequate fluid buildup in the well, but not so long a period as to loose well production.

Abstract: Pumping apparatus which compresses or decompresses an actuating fluid alternately, which is in turn used to pump an artificial heart. To reduce the size of the apparatus, a diaphragm which contains a non-compressible actuating liquid is driven reciprocatingly by an electric linear motor. To prevent the occurrence of an excessive suction pressure which may be caused by the non-compressibility of the actuating fluid, at the commencement of the suction interval, the current to energize the linear motor is increased gradually and the pressure of the actuating liquid is detected. When the liquid pressure goes out of a selected safety range, the gradual increase ceases. A discharge stroke is detected from an integrated value of the motor current during the discharge interval, and when it reaches a given value, a reversal of the direction of energization of the motor is initiated.

Abstract: A pump supply system for furnishing well water or the like which is sensitive to and operates within a range defined by a pressure switch to supply water when pressure in the system falls below a preset minimum pressure and shut off the pump when the pressure reaches a predetermined value. Timing means is employed in the system and preset to time out after a predesignated interval of operation of the pump. The system will retime the next pumping cycle, if that interval of operation is not exceeded. If the pump seeks to operate beyond the predesingated time period, the timing circuit is disabled, the pump is shut down, and an alarm is activated.

Abstract: A microprocessor controlled bellows-type pump system for the precise and accurate pumping and dispensing of fluids. The pump comprises a longitudinally contracting bellows which is supported from warping under fluid pressure by a support bushing in contact with the inside folds of the bellows. A stepper motor drives the bellows position via a lead screw. A system of four valves: a dispense valve, shut-off valve, vent valve, and suck-back valve, control fluid routing within a manifold. One or two pressure transducers allow the microprocessor to control valve positions and stepper motor turning for well behaved dispensing operations.

Abstract: An infusion pump system comprising means for measuring internal diameter of an associated pump supply tube formed from a flexible material, such as plastic or rubber. The means for measuring internal diameter may comprise an ultrasound system including at least one transmiter and one receiver, as well as an evaluation unit. Alternatively, a mechanical system may be employed in the form of a counterpressure unit arranged perpendicular to the axis of the supply tube and including a movable plunger and an abutment surface for compression of the supply tube to be measured. A measurement and evaluation unit associated with the counterpressure unit determines the distance traversed by the movable plunger. The internal diameter of the tube may be determined by generating a force-displacement diagram. In accordance with one embodiment, the movable plunger is a pumping rod of a linear peristaltic pump.

Abstract: A fluid governor including an electric motor for driving a fluid compressor and causing a build-up of pressure in a storage reservoir, pressure switch responsive to the pressure in the storage reservoir, electromagnetic solenoid valve connected to the pressure switch, a pneumatically operated electrical contact controller pneumatically connected to the magnetic solenoid valve for causing the pneumatically operated electrical contact controller to be connected to the storage reservoir when the pressure in the storage reservoir exceeds a first predetermined value to deenergize the electric motor and for causing the pneumatically operated electrical contact controller to be connected to atmosphere when the pressure in the storage reservoir drops to a second predetermined value to energize the electric motor.

Abstract: Differential motor load drops are employed to indicate gas lock or pump off of an electrical submersible pump.

Abstract: The invention provides a cell having a variable volume chamber and a fluid supply circuit for an ink jet printing head which is equipped therewith. The cell includes a variable volume chamber (1) connected to a pressure sensor (5) and to at least a pair of valves (7, 9) each associated with a restriction (8, 10). The variation of volume is obtained by means of a piston actuated by an eccentric (3) secured to the rotor of a stepping motor (4). The maximum pressure difference generated at the ends of the restrictions (8, 10) is used to measure the viscosity of the fluid which flows through the corresponding valves (7, 9).

Abstract: The compressor controller includes a timing circuit for timing-out a detected fault in a compressor. Upon the fault existing for longer than a predetermined time limit, the controller removes power from the compressor. If the fault is corrected within the predetermined time limit, even momentarily, the timing process is discontinued and the controller continues to monitor the status of the compressor. The controller also includes a delay timer for delaying the re-energizing of the compressor after an interruption or loss of power occurs. The delay prevents short cycling of the compressor. The controller further includes a status display for indicating compressor functions, connections to an alarm which is initiated in the event of compressor failure, a switch for initiating a self test of the controller, and circuitry for a field applied current sensing relay.

Abstract: A pumping apparatus comprises a fluid end cylinder carrying a double-acting piston which is reciprocated by a hydraulic servo actuator to provide an expanded flow range ability for metering fluids. The hydraulic servo actuator comprises: a hydraulic cylinder carrying a drive piston connected to the fluid end piston; a servo valve; a position transducer; and a microprocessor-based controller. The controller is programmed to accommodate accleration and deceleration of the extend and retract phases of a pump cycle to maintain constant average flow rate out of the fluid end. A control apparatus for controlling a pump and a related method are also disclosed.

Abstract: To provide smooth constant flow from a pump, a chromatographic system comprises: a chromatographic column having an inlet; a pump for supplying fluid to the inlet of the chromatographic column; a power means for the pump motor; positive and negative feedback loop means for controlling said power means; means for energizing and de-energizing said positive and negative feedback control means; said negative feedback control means receiving a signal from said means for measuring flow rate and including means for comparing said signal with said corrected flow rate reference signal while said second feedback loop is energized to generate an error signal controlling said power means; and said positive feedback control means applying an acceleration voltage to said motor from a time a preset period after the initiation of a return stroke of said piston until after a timed duration.

Abstract: A method and apparatus for regulating the operation of a compressor system including a compressor moving a gas from an intake conduit through a discharge conduit to a gas storage reservoir are disclosed. In accordance with the method, an unload valve in an unload conduit connected to the discharge conduit is initially fully closed and an inlet valve in the intake conduit is initially fully opened. Then the inlet valve is closed by an amount necessary to maintain the discharge pressure at a design pressure level and at a gas flow rate between the design flow level and a minimum flow level. When the gas flow rate drops below the minimum flow level, the inlet valve is maintained in its last position and the unload valve is opened by an amount necessary to maintain the discharge pressure below a first pressure level set higher than the design pressure level.

Abstract: A method and apparatus for regulating the operation of a compressor system including a compressor moving a gas from an intake conduit through a discharge conduit to a gas storage reservoir are disclosed. In accordance with the method, an unload valve in an unload conduit connected to the discharge conduit is initially fully closed and an inlet valve in the intake conduit is initially fully opened. Then the inlet valve is closed by an amount necessary to maintain the discharge pressure at a design pressure level and at a gas flow rate between the design flow level and a minimum flow level. When the gas flow rate drops below the minimum flow level, the inlet valve is maintained in its last position and the unload valve is opened by an amount necessary to maintain the discharge pressure below a first pressure level set higher than the design pressure level.