Page 264 - Reciprocating Compressors Operation Maintenance

Overhaul and Repair of Reciprocating Compressors 249

The most serious type of failure is foundation cracking at a location in a
plane parallel to the crankshaft. These cracks may be caused by inade-
quate design, or by operating conditions that exert excessive forces on
the foundation. Unless these foundation cracks are repaired at the time of
regrouting, grout life will be greatly reduced (usually to about 10% of its
normal life).
Lateral dynamic forces are generated by compressor pistons. Theoreti-
cally, if a machine were perfectly balanced, only dead weight forces
would be exerted on the foundation. In such a condition, anchor bolts
wouldn't be needed. In reality, a perfectly balanced reciprocating
machine has never been built.
After establishing the fact that unbalanced forces do exist on well-
designed and maintained equipment, consider what happens when main-
tenance is postponed. Suppose there are lubricating oil leaks that puddle
on the foundation shoulder. If any movement exists between the machine
and grout, oil will penetrate voids caused by the movement, and
hydraulically fracture any remaining bond between the machine base and
grout. As movement between the machine and grout increases, forces
exerted on the foundation increase at an exponential rate because of
change in direction and impact.

REPAIRING FOUNDATION CRACKS

The notch provided in the top of a foundation for the oil pan creates a
perfect location for stress risers. A moment is created by lateral dynamic
forces multiplied by the distance between the machine base and trans-
verse reinforcing steel in the foundation below. The possibility of foun-
dation cracking at this location increases as the depth of the notch
increases. The farther the distance between the horizontal forces and
transverse reinforcing steel, the greater the moment.
Figure 4-11 illustrates a method of repairing such cracks by drilling
horizontal holes spaced from one end of the foundation to the other end.
A series of holes is placed at an elevation just below the oil pan trough. A
high tensile strength alloy steel bolt is inserted into each hole and
anchored at the bottom of the hole.
Alternatively, the long bolt can extend all the way through.
Next, a small diameter copper injection tube is placed in the annular
space around the bolt. The end of the hole is then sealed and the nut
tightened to draw the two segments of the block back together.