Take It to the Limit: The Dynamics of Rod Load and Rod Reversal in Reciprocating Compressors

Abstract A major factor the engineer must consider when designing a reciprocating compressor package is the structural limit of the compressor. This limit is referred to as the rod load or frame load and is the maximum continuous operating force the compressor can safely withstand. Our discussion will include static rod load, inertia rod load, dynamic rod load, and rod reversal, with guidelines for minimizing rod reversal in the field. Introduction Every compressor has speed and load-carrying limitations. Primarily, the load-carrying capacity of a compressor frame involves horsepower and rod loading. Rod load specifications tell the compressor package operator the limits of static, inertial, and dynamic loads that can be borne by the crankshaft, connecting rod, frame, piston rod, bolting and bearing surfaces. This knowledge and how it is applied can make the difference between downtime and runtime. Mechanics of Principal Moving Parts The forces acting on the principal moving parts of a reciprocating compressor are: - The gas pressure forces - The inertia forces of the reciprocating parts - The dynamic forces of the reciprocating parts - Centrifugal forces of the rotating parts. I will address all except the centrifugal forces in this presentation. The gas pressure forces are the principal forces, but the inertia, dynamic and centrifugal forces become considerable at higher speeds. The gas pressure and inertia forces of the reciprocating parts result in a net force on the piston. The net result is a force along the connecting rod which resolves itself from the turning effort, or torque, on the crankshaft.