A note from Robert X. Perez:
Welcome back to Compressor University!
We are constantly asked to push our machinery a little harder. The days of underloaded, overdesigned machines have gone by way of the dinosaurs, eight-track tapes and slide rules. After numerous process nudges, prods and rerates, we are finding some machines are operating against the proverbial wall. When we go too far, our machinery begins talking to us by failing prematurely or, in extreme cases, failing catastrophically.
Judiciously determining the safe and reliable operating limits of process machinery is one of the most critical responsibilities of machinery professionals. Ultimately, this function requires us to weigh process throughput against machine life or welfare. Many years of working in production environments have taught me that it is always better to operate at lower reliable rates than to operate at higher rates that can lead to upsets and outages in order to maximize process profit. In other words, slow and steady is better than fast and reckless. Finding the operating point that satisfies the process folks without adversely affecting machinery life is the key to profitable, carefree production.
In the next three installments of Compressor University, Atkins, Hinchliff and McCain discuss compressor load ratings that can limit processes that use reciprocating gas compressors. They will walk you through the definitions developed to protect compressors from various types of overload conditions. By the end of their articles, the reader should understand the history of rod loads and how they are computed. Remember that it is ultimately the user/owner's responsibility to select the proper loading limit criteria for his situation. Those who work with reciprocating compressors on a regular basis should keep all three parts of this article for future reference.
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. Rod load is one of the most widely used, but least understood, reciprocating compressor descriptors in industry. Typical end users know that rod load is a factor used to "rate" a compressor, but they do not generally have a good understanding of how this rating is developed and how to utilize it for machinery protection.
These three articles discuss the various definitions of rod load, including historical and current API-618 definitions, manufacturer's ratings and various user interpretations. It also explains that there are load limits based on the running gear (moving parts such as pistons, rods, crosshead, crankthrow, etc.) as well as load limits based on the stationary components (frame, crosshead guide, etc.).
The basic kinematics and forces acting on a slider-crank mechanism will be reviewed to provide a better understanding of the various definitions used. Analytical results and field rod load measurements will be compared to illustrate the various factors that influence rod load on typical compressor installations.
Consider the typical double-acting compressor cylinder geometry illustrated in Figure 1. The loads (forces) that are generally of concern include the piston rod loads, the connecting rod loads the crosshead pin loads, the crankpin loads and the frame loads. As the crankshaft undergoes one revolution, all of these loads vary from minimum to maximum values. The loads are generated by both gas and inertia forces.
As the compressor piston moves to compress gas, the differential pressures acting on the piston and stationary components result in gas forces illustrated in Figure 2. An ideal pressure versus time diagram for a typical double acting compressor cylinder is shown in Figure 3. The pressures acting on the piston faces (head end and crank end) result in forces on the piston rod. The force acting on the piston rod due to the cylinder pressures alone alternates from tension to compression during the course of each crankshaft revolution.
It is straightforward to compute the net force on the piston rod due to pressure. A plot of this force versus crank angle for the ideal P-T diagram is shown in Figure 4. The forces due to pressure also act (equal and opposite) on the stationary components.