Designing a reciprocating compressor installation for minimum vibrations & failures requires a complete knowledge of the forces imposed on the system. The crank forces are a function of the combined reciprocating and rotating inertial forces of the power and compressor pistons and rods, plus the pressure loads from the power and compressor pistons. The design philosophy for most units is to minimize the unbalanced inertial forces and to absorb the remainder of the energy with the large mass of the concrete foundation. The unbalanced inertial forces can be minimized by the use of counterweights or dummy crosshead weights.
As an aid in designing the foundation, the engine manufacturers provide information on the unbalanced forces and moments at the running speed and second harmonic. These are a summation of forces and moments about the center of gravity of the frame. Since these forces are the resultant based on the assumption that the frame is rigid, this information can be misleading. Since the frame is not entirely rigid, individual forces at each bearing cause local frame deflections even though the summation of forces are small.
The rotating unbalance can be corrected by the addition of a counterweight; however, the reciprocating parts produce a variable force which is dependent on the crank angular position which cannot be eliminated by balancing. EDI can perform balance calculations to properly size and locate counter weights.
By changing the counterweights, the amount of primary horizontal and vertical forces can be changed. It is preferable to reduce the horizontal forces even if the vertical forces are increased because the horizontal forces a resisted by bolt flexing and friction. The anchor bolts will withstand more force in the vertical direction under tensile loading. Secondary forces cannot be balanced with counterweights because they are caused by the connecting rod motion and vary twice per revolution. Equal weight pistons and rods will reduce the secondary forces, however, multistage integral compressors will have different piston weights.
To see how EDI has applied their extensive knowledge to this type of problem, please view the following Case History.
