Article Authored by SKW Engineers Published in Oil & Gas Journal

September 7, 2010

An article entitled “New Loading Technique Reduces Compressor Interruptions” has been published in the September issue of Oil & Gas Journal. According to their Website, “The Oil & Gas Journal, first published in 1902, is the world's most widely read petroleum industry publication. Each week the Journal delivers the latest international oil and gas news; analysis of issues and events; practical technology for design, operation and maintenance; and important statistics on international markets and activity.”
 
The article describes advanced control algorithms based on numerical analysis that have been implemented by SKW in the field for compressor control. Authors Jake Hansen, Bob Curry, and Mike Bequette are all members of the Instrumentation and Controls Department within SKW’s Energy Services Division.  An Abstract of the paper is reprinted below; you can read the full article HERE.  We congratulate the authors on having their article published by one of the premier publications in the industry.

Abstract

This article describes non-linear control strategies for safe optimization of gas compressor operation.  Given an empirical torque curve, the application of Newton-Raphson’s method enables a multi-stage, multi-service, piston-driven, positive displacement natural gas compressor to optimize its safe operating range  and dynamically respond to process upsets.  A constant speed electric motor is used to drive the reciprocating compressor to reduce complexity and the cost of running the compressor (versus a gas powered engine). Even without speed control, this affords a throughput variation of approximately 30%-100%, depending on pressure conditions.  Volume pockets and crank end deactivation valves provide throughput variability by changing the clearance volume of the compressor. 

All volume changes for the unloading sequence are known, and the control system can then utilize the Newton-Raphson method to calculate the inter-stage pressure change for each step.  Once the inter-stage pressure is calculated for a given step, the control system can use this information in other calculations of interest such as volumetric efficiency, rod loading, etc. which are examined for the step and checked for violations of operating limits.   This allows the control system to leave the standard linear unloading sequence to avoid such violations without being forced to remain at a sub-optimal step. 

This non-linear approach to the unloading sequence lets the compressor to respond quickly to a changing process while avoiding dangerous steps in the unloading sequence by non-linear stepping.  In addition, this control scheme may be used to optimize unloading and loading by allowing steps in the unloading sequence to be skipped during startup or shutdown.