Page 372 - Practical Machinery Management for Process Plants Major Process Equipment Maintenance and Repair

Steam Turbines and Turboexpanders 351

We already dealt with all of these troubles in other pages of his book
and elsewhere5. In the following we would like to review the problem of
low steam purity because it is unique to steam turbines and particularly
large special purpose steam turbines.
Steam contamination can cause stress-corrosion cracking, corrosion
pitting, general corrosion and erosion, and can leave deposits in the tur-
bine. A good deal of documentation exists describing the problems. One
source6 suggests the following preventive measures against stress-corro-
sion cracking:

Keep contaminants in the steam at the lowest practical achievable
level.
Avoid caustic contamination of the turbine.
Watch condensate and make-up demineralizers carefully.
Maintain feedwater conductivity instrumentation.
Permit only treated condensate in the steam path.
Instrument the feedwater system to control steam chemistry.
Do not use cutting fluids, with high concentrations of chlorine and
sulfur, in machining operations during maintenance.
Do not use cleaning fluids with unacceptable levels of caustic, chlo-
rine, and sulfur.

The same source describes the mechanism of solid-particle erosion and
corrosive pitting in large steam turbines, their effect and the repair meth-
ods used. We are referring our readers to this document.
Deposits on parts in the steam path from boiler carryover may have a
considerable effect on capacity, efficiency, and reliability. The build-up
of deposits can plug or partially plug turbine buckets, thus increasing
thrust bearing load, which could lead to bearing failure with possible ad-
ditional damage.
Deposits and other internal problems may be detected by monitoring of
specific parameters, such as temperature, pressures, flows, and valve
opening. An increase in temperature of thrust shoes as shown by
embedded thermocouples would indicate increased thrust that could be
the result of deposits.
The selection of a fouling detection system will be strongly influenced
by the safety and complexity of a cleaning procedure. If the deposits are
water soluble, internal washing is possible. In the simplest case this may
involve injecting a quart of water into a single stage, mechanical drive
turbine, with 30°F superheated inlet. On the other hand, the cleaning
may involve removing 300" of superheat from 200,000 lbslhour of steam
entering an eight-stage turbine. This is a much more complex case.