There are three methods of mounting reciprocating compressors on foundations:
1. Direct grouting
2. Mounting on sole plate (rails)
3. Mounting on a base plate (skid)
Direct grouting of the frame to the foundation is used on smaller machines, such as air compressors. These machines are short and not likely to experience frame distortion due to differential foundation displacement.
If longer machines with several crank throws are directly grouted, the heat from the frame will cause thermal distortion of the foundation. The frame then conforms to the foundation, causing misalignment of the crankshaft. Misalignment can lead to crankshaft failure; therefore, sole plate mounting is used. With this method there is air space between the frame and the foundation. Chock blocks and/or shims are used between the sole plate and the frame. If foundation distortion occurs over the years, the frame can be re-chocked or re-shimmed to restore alignment.
Figure 300-62 shows a typical sole plate and chock arrangement for a crosshead pedestal. A similar arrangement would be used under the frame, although the foundation bolt is usually one-piece. Note that the foundation bolt sleeve is packed to prevent entry of grout. This feature is important.
Many Class B units are typically skid-mounted. In this case, the frame is mounted on a base plate, and the base plate is grouted to the foundation. When the base plate becomes quite large, sole plates are sometimes used under the base plate.
Except for very small units, epoxy grout is recommended for reciprocating machines. Although more expensive than cementatious grout, epoxy grout has superior strength, is oil-resistant, and has negligible shrinkage. Some cementatious grouts include an additive that expands during the curing process to reduce shrinkage. However, this process has been unreliable in many cases, and the grout deteriorates with constant exposure to lube-oil. One recommended rule-of-thumb is to use epoxy grout on all reciprocating machines rated at 500 HP or more, or that weigh 5000 pounds or more.