Page 373 - Practical Machinery Management for Process Plants Major Process Equipment Maintenance and Repair

352 Major Process Equipment Maintenance and Repair

The potential problems of water washing steam turbines are:

1. Misalignment due to piping stress as the temperature is reduced.
2. Water slugging.
3. Loss of clearance due to differential contraction between rotor and
stator.
4. Vibration due to nonuniform deposit removal.
5. Thrust failure due to almost complete plugging of one stage.
6. Damage to blading if hit by water retained in the exhaust casing.

On most machines, the misalignment due to pipe stress will not be sig-
nificant. After all, we are only disposing of the superheat, whereas dur-
ing run up, the machine is exposed to a temperature change at least two
times as large. However, if piping strains are a problem on startup, one
must make sure all sliding supports are free before attempting to wash.
W have had no problems other than an increase (doubling) of axial vi-
bration due to misalignment.
We try to avoid water slugging by two measures. We always use a ven-
turi nozzle for desuperheating. Also, we insist that the piping fall contin-
uously between the desuperheater and the machine inlet. Even if the wa-
ter is not broken up into droplets in the desuperheater, it will pass
inoccuously through the turbine as a constant stream.
To prevent loss of clearance, we always limit the rate of temperature
change to 180°F per hour. The greatest hazard would be failure of the
injection pumps when at maximum injection rate. Such a failure would
produce a very high rate of change of temperature and would most likely
result in an axial rub. To guard against this, we try to use boiler feedwa-
ter, since these pumps are the most reliable in the plant.
We attempt to reduce the chances of nonuniform deposit removal by
halting the increase of injection rate whenever a deposit is actually being
removed. This condition is detected by measuring the conductivity of the
exhaust condensate.
It is alleged that thrust bearing failures have occurred because of stage
plugging when an upstream wheel has shedded its deposit before a down-
stream one. We have not found this to be the case.
One of the criteria we use to check a turbine design before purchase is
“Can all the condensate be removed from the exhaust?” Some turbine
designs are such that the blading is within about 1 in. of the bottom of the
casing. Others don’t have a casing drain at the lowest point. Others have
a 1/2 in. or 3/4 in. casing drain. All of these designs are suspect unless the
condensate can drain freely out of the exhaust.
We consider that a stage is washed adequately when the condensate
conductivity falls to half its peak level. When this point is reached, the