Page 364 - Practical Machinery Management for Process Plants Major Process Equipment Maintenance and Repair

Steam Turbines and linrhoexpnnders 343

Proper Break-In of Carbon Rings’

Incentives are:

Long runs
Higher turbine efficiency
Protection of bearings and journals by keeping water out of the oil
due to blowing steam past the seals into the bearing housing.
In the winter the machine results in happy operators and a safer unit.
Lower vibration levels

A common method of breaking in carbon rings involves mounting dial
thermometers on the gland housing and observing its temperature rise at
incremental speeds for about three hours. Stuffing box temperature rise
is a function of carbon ring wear rate, heat transfer rate from the carbon
rings through the gland housing, and steam conditions. Surface tempera-
ture monitoring procedures are highly questionable due to their poor time
response to events happening at the sealing zone between the carbon
rings and turbine shaft. Directly observing shaft vibration gives real time
knowledge of the condition of the seals.

Factors affecting break-in. Figure 8-3 is a typical carbon ring gland housing
assembly for a small steam turbine. The carbon rings that actually do
the steam sealing are made of a special form of graphite that is self-lubri-
cating. The seal is usually constructed of three or more segments bound
together and against the rotor shaft by a garter spring. The carbon rings
are prevented from rotating by a tang.

Mechanism of break-In. Assuming that the carbon ring packing clearances
are within design specifications, the carbon rings are “broken in” when
they acquire a slick glaze due to controlled rubbing action. Time required
for the packings to wear in varies as a function of: steam temperature and
pressure, clearances, pressure drop across the seals, sealing steam flow,
shaft surface smoothness, shaft surface speed, seal casing configuration
and carbon ring composition and design.
Break-in may take from 3 to 12 hours and occurs at about 2,500-3,500
rpm for 3-4 in. internal diameter carbon rings. Cold carbon ring to shaft
clearance for 3-4 in. internal diameter rings is about 15-16 mils. Hot
running clearance should be about 1-2 mils. Following wear-in, the car-

* Adapted from “Avoid Problem with Steam Turbine Carbon Ring Seals,” by s. W.
Mazlack, Amoco Oil Co., Hydrocarbon Processing, August 1981. By permission.