Variable capacity natural gas compressor

Abstract

The invention relates to a method and apparatus for gas compression. More particularly the invention is directed to a variable capacity screw compressor. The screw compressor has an engine, an intake slide valve, and a programmable logic controller for controlling the transmission speed of the engine and for controlling the position of the slide valve. The compressor also has means for monitoring the engine load. The transmission speed and the slide valve position are adjusted in accordance with the engine load. The invention attempts to match available horsepower of the engine with available gas volume by adjusting the compressor to match gas throughput with horsepower. This automated process assists the user in shipping greater volumes of gas.

1 . A variable capacity screw compressor comprising:
 (a) an engine coupled to a variable speed hydrostatic transmission;   (b) an intake slide valve, and actuation means for varying the intake slide valve;   (c) means for monitoring engine load;   (d) means for varying transmission speed; and
 (e) control means comprising an engine load sensor input operatively connected to the engine load monitoring means, a transmission speed output operatively connected to the transmission speed varying means, an intake slide valve output operatively connected to the intake slide valve actuation means, wherein said control means automatically adjusts transmission speed, and intake slide valve position, according to engine load. 
   
   
   
       2 . The compressor of  claim 1  further comprising a Vi control slide valve, and actuation means for varying the Vi control slide valve operatively connected to the control means, which automatically adjusts Vi control slide valve position according to engine load. 
   
   
       3 . The compressor of  claim 1  wherein the variable speed hydrostatic transmission comprises a variable speed motor, and a variable speed pump. 
   
   
       4 . The compressor of  claim 1  wherein the control means is operatively connected to a speed control device on the variable speed motor, or to a speed control device on the variable speed pump, or speed control devices on both the motor and the pump. 
   
   
       5 . The compressor of  claim 1  wherein the control means comprises a programmable logic controller. 
   
   
       6 . The compressor of  claim 1  wherein the control means responds to a change in engine load during operation by first adjusting compressor speed, then by adjusting intake slide valve position, and lastly by adjusting Vi control slide valve position. 
   
   
       7 . The compressor of  claim 6  further comprising a gas recycle line connecting a gas discharge end of the compressor to a gas suction end, a recycle valve controlling flow through the gas recycle line, and means for actuating the recycle valve, said actuation means operatively connected to the control means. 
   
   
       8 . The compressor of  claim 1  further comprising a suction pressure control system comprising a PID loop operatively connected to a suction pressure control valve and a suction pressure transmitter, wherein said PID loop operates independently of the control system. 
   
   
       9 . The compressor of  claim 8  further comprising a discharge pressure control system comprising a PID loop operatively connected to a discharge pressure control valve and a discharge pressure transmitter, wherein said PID loop operates independently of the control system. 
   
   
       10 . A method of efficiently compressing gas from a gas well using a variable capacity compressor having an intake slide valve, and driven by an engine coupled to a hydraulic transmission including a variable speed pump and variable speed motor, said method comprising the steps of:
 (a) sensing engine load;   (b) attempting to maintain engine load within a desired range by:
 i. adjusting compressor speed by varying the hydraulic transmission in response to changes in engine load; or 
 ii. adjusting the intake slide valve; or 
 iii. both. 
   
   
   
       11 . The method of  claim 10  wherein compressor speed is first adjusted, and then intake slide valve is adjusted, if engine load is not brought within the desired range by adjusting compressor speed. 
   
   
       12 . The method of  claim 11  wherein the compressor further comprises a Vi control slide valve, and the Vi control slide valve is adjusted in order to vary engine load. 
   
   
       13 . The method of  claim 10  further comprising the step of controlling suction pressure independently of controlling engine load. 
   
   
       14 . The method of  claim 13  further comprising the step of controlling discharge pressure independently of controlling engine load.