Page 366 - Practical Machinery Management for Process Plants Major Process Equipment Maintenance and Repair

Steam Turbines and Turboexpanders 345

through a foot or more of clear space before a wet plume develops and
there is much less noise. Again: do not start rolling until all case drains
are blowing dry, without puffing. This heat-up may take several hours.

Sealing steam. For a condensing turbine, if possible, have the condenser
vented and the sealing steam initially turned off. Sealing steam should
not be turned on unless the rotor is turning. Cold air sucked across seals
into a hot rotating or nonrotating turbine can distort the shaft as severely
as hat sealing steam entering the seal area of a cold nonrotating turbine.
Shaft distortion will cause a rotor bend or “bow” to form which can re-
sult in destructively high vibration levels. If the shaft develops or has a
thermally induced rotor bow, a 1-hour 300-600 revolutions per minute
slow roll usually will allow most of it to relax out.
The normal sealing steam pressure of a condensing turbine is about 3-
4 psig. A higher sealing steam pressure is recommended at the outset to
assist the outboard seals to begin break-in. This is important, for if the
low pressure end is primarily sealed with the high pressure end seal leak-
ing off steam, exhaust end packings may not get much steam until the
unit actually is coupled up and running at normal speed. If this is the
case, the low pressure seal area suddenly may get its first dose of hot
steam preceded by a slug of water at full speed. The result may be a sud-
den seal “grab,” carbon shattering, and violent failure. This is the cause
of the mysterious severe turbine vibration that occurs shortly after the
operator walks away from a machine that was just put on line.

Surface condenser use. The turbine is heated up and brought to minimum
speed as a reduced back pressure machine. This maximizes heat input
into the seal areas. Caution must be exercised to avoid overpressurizing
the surface condenser expansion joint and the steam turbine exhaust cas-
ing. Steam flow to the condenser is minimal during an uncoupled slow
speed run. As such, the exhaust pressure of the turbine, either positive or
negative, can be controlled by balanced use of the surface condenser vent
valve, cooling water flow to the condenser exchanger, proper hogging jet
operation, and turbine case drain valve positions.

Use of vibration probes. For monitoring carbon ring break in, one tempu-
rary probe holder bracket mounted on the inboard face of each bearing
housing, with a reasonably clean and nick-free shaft surface for the probe
to monitor, will work. If the machine is to be permanently monitored
with vibration probes, see API-670, “Non-Contacting Vibration and Ax-
ial Position Monitoring Systems” for additional details.
Normally, carbon ring break-in is performed with the turbine uncou-
pled from the driven unit. Having the turbine uncoupled eliminates most