Page 365 - Practical Machinery Management for Process Plants Major Process Equipment Maintenance and Repair

344 Major Process Equipment Maintenance and Repair

Carbon ring seals Garter spring

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Figure 8-3. Carbon rlng gland assembly.

bons fit rather loosely in the stuffig box and in the cold condition have a
large clearance from the rotor shaft. In this condition, the seals are rela-
tively immune to the sudden thermal changes that a turbine goes through
during its normal duty cycle.
The differential coefficient of expansion between steel and carbon is
.OOOOO4 in. per in. of diameter for each degree Fahrenheit increase in
temperature. Since the thermal expansion of carbon is less than that of
steel, too rapid of a wear-in often will result in broken rings, or excessive
carbon ring to shaft clearances. Large clearances produce poor sealing
and destructive steam “wire drawing” across the carbon ring faces. High
levels of vibration, high gland box surface temperatures, noise and a big
cloud of steam occurring shortly after turbine startup to full speed are
sure signs that the carbon rings were inadequately broken in and are
grabbing the shaft. If this happens, don’t even ask if the carbon seal rings
are “broken”-they are.

Warm up. Of the utmost importance for any turbine operation, including
carbon ring break in, is proper warmup. The entire rotor case assembly
must be allowed to heat up to its equilibrium temperature prior to starting
slow roll. Even heating is required to avoid rotor rubs, high thermal
stresses and unequal expansion of the seal rings. During heatup, if the
steam plume starts at the case drain pipe outlet and is noisy, this means
that water is flashing even if no liquid appears. Dry steam travels