A Discussion of the Various Loads Used to Rate Reciprocating Compressors
Reciprocating compressors are usually rated in terms of horsepower, speed, and rod load. Horsepower and speed are easily understood; however, the term rod load is interpreted differently by various users, analysts, OEMs, etc. This paper discusses the various definitions of rod load, including historical and current API-618 definitions. Examples are provided of what rod loads ratings applied prior to the current edition of API618 meant when compared to the current definition. Also, in general most performance programs make assumptions of uniform pressure inside the cylinder, ideal valve motion, constant pressure at the cylinder flange and neglect gas inertia effects. This paper will discuss the effects of real world compression including pressure pulsation at the outside of the valve, non-uniform piston-face pressure distributions and torsional pulsation effects on rod load. Examples will be provided along with guidance on when these effects will be significant. The user will find this paper helpful for understanding rod load ratings on existing machinery. It will help in interpreting real world PV card analysis compared to compressor performance data. It will also provide guidance for when compressor performance assumptions regarding rod load may result in significant deviation from real rod load.