Control And Maintenance Of Field Compressors
Authors: Gilleland R.W. 
Published: December 2007

Abstract
Numerous controls are necessary for the successful operation of field compressors. These controls are similar to those used in other compressor installations but the nature of field operations present some unique operational and maintenance problems. These controls serve to control load, pressures and temperatures of the compressed gas as well as the necessary auxiliary systems and provide safety shut-down to protect personnel and property. Like any piece of personnel and property. Like any piece of mechanical or electrical equipment, they require regular maintenance.

Introduction
Compressors of various types have been used in oil field service for many years. The controls used on early machines were very crude and didn't do much more than prevent an engine from tearing itself up due to low oil pressure or high jacket water temperature. Compressor controls were confined to shut downs on perhaps one pressure and temperature and may be a scrubber liquid level. The early control panels were rarely weather protected. As time went on, operating companies protected. As time went on, operating companies began to make do with fewer field personnel, compression equipment increased in cost, the average size unit became larger and control technology and availability improved. Field compressors are now routinely equipped for completely unattended operation in extremes of climate and operating conditions. Along with these more sophisticated controls comes the need to understand their functions and how to properly maintain them.
This paper deals with the various types of controls used on 60–1000 HP (45–746 KW) field compressors, some of the more common operational and maintenance problems encountered and techniques for preventative maintenance.

TYPES OF CONTROL
Field compressor controls may be divided into three general categories:Control of compressor capacity, engineload and speed.Control of auxiliary systems on the packaged compressor such as oil, water, and gas temperatures; oil and water levelsand scrubber liquid levels; fuel and starting gas pressures and pneumatic control supply gas pressure.Safety shut-down controls to shut down the unit in case of abnormal conditions, of temperature, pressure, speed, liquidlevel, vibration and engine load.

COMPRESSOR CAPACITY CONTROL
There are three primary means of controlling compressor capacity. The most commonly used method is, of course, simply to vary the engine/compressor speed. By means of speed variation it is possible to get perhaps a 50% turn down of capacity.
By use of fixed volume clearance bottles or variable volume clearance pockets, it is possible to more closely match the compressor capacity to that actually required. Variable volume clearance pockets are particularly useful because it enables pockets are particularly useful because it enables the operator to "fine tune" the clearance while the unit is running. Greater variation than are possible by changes of clearance may be achieved possible by changes of clearance may be achieved by single-acting one or more cylinders. This is accomplished by removing suction valves from the cylinder end to be deactivated. By deactivating the crank end of a cylinder, it is possible to utilize fixed volume bottles or variable clearance pockets on the head end of the cylinder.