A Closer Look At Rod Load Definitions And Rod Load Limitations Of Reciprocating Compressors
One of the important design parameters of reciprocating compressors is the rod load. At the same time, it is probably one of the most misunderstood quantities. Specifications usually define the rod load by only a single number, the maximum rod load, which makes the strength evaluation very convenient and simple and, therefore, attractive to use. However, as most possible failure mechanisms of reciprocating compressor parts are related to fatigue the essential load is more complex. Moreover, the decisive load kind is not the same at every compressor part and location which makes it even more incorrect to define a single rod load level valid for the whole compressor. In this situation, the compressor OEMs may find themselves in a conflict between keeping it simple or doing it right. There is a second and even more important aspect. Usually, a lot of effort is put in the quantification of the rod load. Apart from the basic crank mechanism kinetics, additional influences like valve pressure drops, pulsation and vibration effects as well as gas inertia phenomena inside the cylinder are considered. This makes the load aspect very complete. However, the definition of the load limits often misses such a comprehensive approach and restricts on merely using material strength properties which are only of secondary importance at the critical locations. So, there might be a discrepancy between the effort taken for load quantification and load limit quantification. However, one quantity cannot really live without the other. This paper tries to describe appropriate rod load definitions and on which properties and effects load limits really depend.