Many years prior to the upgrade project, the original tie-down system was modified. Two-inch "Vibratherm" chocks with high strength anchor bolts were installed on all of the engines, which performed successfully for many years at the original rated engine speeds. When the upgrade project was implemented, the use of precision "Super Nut" fasteners and stud tensioners were used to carefully control the engine anchor bolt preload. The preload values were also increased.
The modifications were completed and the unit was re-commissioned. After a short operating period at 330 RPM, several problems developed. The vibration levels of the cylinder and frame were of concern. Severe oil leaks were experienced. The composite chocks slipped at certain locations. Ultimately, both main bearing and crankshaft problems occurred.
The gas pipeline company carefully disassembled and inspected the engine. Weights of all reciprocating and rotating parts were obtained using a precision scale. The resultant inertial unbalance calculated based on the actual weights is listed in Table 4. Note that the forces are not zero due to the variation of component weights; however, they were still very low. The moments were significant, but less than the recommended values.
The problems resulted from the frame/foundation system not being able to withstand the applied forces. The question is, "Are the forces too high, or is the system under designed?" For large integral compressors such as this, the frame alone is not usually capable of restraining the forces. That is, the frame is not infinitely rigid and the foundation must supply additional stiffness. The effectiveness of the foundation is also very dependent on the tie-down system (anchor bolts, grout, and chocks).
The forces transmitted to the foundation are a function of the frame stiffness as shown in Figure 8. If the frame were infinitely rigid, the foundation would see the net inertial unbalance loads given in Table 4. However, at the other extreme, if the frame is very flexible, the foundation would see loads on the order of the individual main bearing loads. These forces can be significantly higher than the inertial unbalanced forces. It therefore becomes important to consider the main bearing loads when evaluating reciprocating machinery dynamics.
Calculation of the main bearing loads is typically done as a function of crank angle as the engine progresses through a single revolution. All of the mechanical forces from the slider crank mechanisms and the pressure forces from the compressor and power cylinders are considered. Computer programs exist which utilize input from the engine/compressor design (geometry, mass properties, pressure data, and gas properties) and generate all of the necessary information pertinent to engine dynamics such as main bearing loads, torque-effort diagrams, crankpin loads, gas forces, rigid body shaking forces and moments, etc. The main bearing loads are often presented in the form of a force hodograph.
The main bearing loads nearly doubled as a result of the upgrade. It has been EDI's experience that large integral compressors behave closer to the flexible frame region of Figure 8 than the rigid frame region. Experience also indicates that it is difficult to adequately restrain horizontal dynamic forces larger than about 100,000 lbs (0-p) with typical engine tie-down systems. Therefore, the approach taken to solve this problem was to re-balance the engine to minimize the main bearing loads instead of the net forces and moments.
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