Reciprocating Compressors – Rod Loading

Rod Loading is a general term having many interpretations. It is also called “pin loads,” “frame loads,” and “combined rod loads.” Excessive rod loading can result in rod or crosshead failure and possible destruction of the machine.

API 618 gives definitions for the terms “combined rod loading,” “gas load,” “inertia force,” and “rod reversal.” Combined rod loading is the important term as it is the net effect of gas and inertia loading. Rod reversal occurs when the force on the rod changes from tension to compression, or vice versa. It is a critical lubrication factor for the crosshead pin and bushing. If there is no reversal, the pin and bushing surfaces do not separate; hence, oil is prevented from lubricating these surfaces. This can result in catastrophic damage to the crosshead, connecting rod, bushing, and crosshead pin.

Frame loading is the result of reaction forces from the gas-pressure resultant forces. Frame loading stresses static items such as crosshead guides, distance pieces, the frame itself, and the bolting between these items.

Rod loading is somewhat of a misnomer in some cases. A manufacturer’s advertised maximum allowable continuous rod load (MACRL) rating is based on the weakest link in the running gear/frame system. The piston rod is not always the weakest link. It may be the crosshead pin or bushing, for example.

Years ago, quite often only gas loads were considered in evaluating proposed compressors. In most cases, a quick comparison of gas loading with the MACRL was adequate. But occasionally, inertia forces augmented the gas loading at certain crank angles so that the MACRL was exceeded.

Rod reversal becomes a problem at low pressure ratios. It can also be a problem at high-pressure ratios for single-acting cylinders, half loaded cylinders, or when the bore of double-acting cylinders is not much larger than the piston rod. Tail rods can be used to alleviate the latter situation.

Figure 300-43 shows gas, inertia, and combined loading for pressure ratios of 4.00 and 1.05. The 4.00 ratio examples show the maximum combined loading is less than the gas loading (the inertia force helped the situation). Such is not always the case, however. It depends on the differential pressures throughout the stroke, and the weights of the reciprocating parts. It is impossible to provide a “rule of thumb” for this phenomenon.

Rod Loading vs. Crank Angle

Figure 300-43 also illustrates a cylinder with a very low pressure ratio (1.05). In this case, the maximum combined load is greater (negative) than either the gas or inertia loads. Also the combined load only reversed for 15-20 degrees of crankshaft rotation. (Note that API 618 suggests a minimum of 15 degrees of reversal). Figure 300-44 shows the ideal reversal (see dark areas labeled “A”) at the crosshead pin.

Load Reversal at Crosshead Pin

A subtlety often overlooked is that the actual differential pressure that exists at a certain crank angle inside the cylinder is higher than the theoretical differential pressure. Refer to Figure 300-15 and note that the vertical distance between points C and F is significantly greater than discharge minus suction pressure. Pressure pulsations can make matters worse. It should be noted that, at any one instant, the upper part of the diagram in Figure 300-15 would correspond to the head end, while the lower part would correspond to the crank end.

When purchasing new compressors, you should insist that the vendor make a study of combined rod loading versus crank angle using internal cylinder pressures with: (1) normal operating conditions, and (2) with any broken valve, with relief valve setting as discharge pressure.

Calculated gas loads can be very misleading, but sometimes they can be used as a preliminary index to estimate the frame size or number of cylinders per stage. These calculations can be made using pressures at the cylinder flanges and the full area of the piston on the head end and the net area (piston area minus rod area) for the crank end. Loading should be calculated for tension and compression. When one end of a cylinder is unloaded, that end will have suction pressure in it at any crank angle.

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