Page 110 - Reciprocating Compressors Operation Maintenance

Design and Materials for Reciprocating Compressor Components B7

alternates in direction. The net result of this force must be damped and
contained by the mounting. This is in addition to the dead weight load.
There are many compressor arrangements, and the net magnitude of
the moments and forces developed varies a great deal between them. In
some cases, they can be partially or completely balanced out. In others,
the foundation must handle it all.
When balance is possible, this machine also can be mounted on a plat-
form just large and rigid enough to carry the weight and maintain align-
ment. Some units have been mounted on a structural steel platform.
Small units of the "Y" or "W" arrangement can be bolted to a substantial
building floor. However, most reciprocating compressors require consid-
erable attention to foundation design.
FOUNDATION MOVEMENT

A reciprocating piston will tend to move the compressor and its foun-
dation because inertia forces are set up, just as if men standing in a small
boat move suddenly in one direction and then the other. Relative to the
lake bottom, the boat also changes position. The heavier the load or the
broader its bottom and, therefore, the greater the resistance to movement
(the heavier the foundation and the broader its base), the less the move-
ment, but movement still occurs.
Any foundation and the soil upon which it rests has a natural frequency
of vibration. The soil is elastic (as a spring). The degree of elasticity
depends upon its character. The foundation design, mass, and the soil char-
acteristics must be such that this natural frequency of vibration is quite far
removed from the primary and secondary vibration frequencies imposed
by the compressor. Unless this is so, resonance will occur, vibration ampli-
tude will multiply, and the foundation will have to be altered. A good foun-
dation will have a vibratory frequency well above the operating speed.

BASIC FOUNDATION DESIGN

A proper foundation must:

• Maintain the compressor and driver in alignment, level, and at the
proper elevation.
* Minimize vibration and prevent its transmission to adjacent building
structures.