The engine or compressor frame is not normally designed to withstand the bearing loads as an independent structure. For a successful installation, this design philosophy requires that the frame and foundation work together as a composite beam to withstand the forces. Consequently, the attachment of the compressor to the foundation is critical for controlling vibration of the unit. Each element of the structural support system down through the foundation and including the soil must be capable of restraining the unbalanced forces. EDI has experience in evaluating skids and foundations from a dynamic standpoint. Whenever a reciprocating compressor unit has high vibrations and repeated grout and foundation failures, there is a possibility that the anchor bolts do not have sufficient holding force to withstand the shaking forces generated by the rotating and reciprocating masses.
It is desirable to use a conservative approach when sizing engine anchor bolts and to assume that the grout will not be effective, since in most compressor installations, the grout deteriorates with time and eventually cannot resist the shaking forces. By designing the anchor bolts to withstand all of the shaking forces, there is less stress on the grout and it will last longer.
The first step in sizing anchor bolts should be to calculate the shaking forces on the engine/compressor unit. This involves calculating the bearing forces for each of the main journals. This can be accomplished through the use of an EDI computer program. This analysis uses the weights and WR2 of the rotating and reciprocating masses, the physical dimensions of the cylinders and clearances and the gas loading to develop the bearing force versus crank angle. These bearing forces are analyzed to determine the horizontal and vertical bearing loads and the engine unbalanced forces and moments.
A conservative estimate of the anchor bolt forces can be obtained by assuming that the anchor bolts nearest the web of the main journals carry the horizontal and vertical forces at that journal. The anchor bolt loads can be calculated by simulating the engine frame structural stiffness or by adjusting its lateral bending stiffness (EI) to match measured field vibration data. This also requires the simulation of the crankshaft and the unbalanced
