Page 258 - Steam Turbines Design, Applications, and Rerating

Reaction vs. Impulse Type Steam Turbines 237

3. Water washing, the on-line (running) removal of deposits, when
deposits are water soluble
Since deposits tend to accumulate to a greater extent during steady
high-load conditions (e.g., base load generator drive, ethylene process
drive), the application and plant operating conditions will dictate
which method will best serve to restore the turbine to optimum perfor-
mance. In a generator-drive service, shutting down the unit or water
washing at low speed and reduced load may create minimum plant
upset. The size of process drivers and plant-operating conditions
present different circumstances.
Turbine washing at full speed (on-stream cleaning) can be and has
been successfully accomplished on many mechanical drive steam tur-
bines. Considerable hazard attends any method of water washing and
full-speed washing is more hazardous than washing at reduced speed.
But this can be accomplished provided great care and judgment is
exercised. While we know of no steam turbine manufacturer who
would guarantee the safety of turbines for any washing cycle, capable
manufacturers recognize that deposits do occur. They will, therefore,
help operators as much as possible in dealing with the problems until
effective prevention is established.
Saturated steam washing by water injection is the conventional and
well-tried method of removing water soluble deposits from turbines.
The amount and rate of superheat to be removed and the amount of
steam flow required for operation determine the water injection rate. It
is the injection of large quantities of liquid (such as may be required on
process drivers) that create potential problems.
The nature of a typical impulse turbine lends itself to full-speed
water washing. Axial clearances between first-stage buckets and noz-
zles and between moving buckets and diaphragms will range from
0.050 to 0.090 in. The NiResist labyrinth packing radial clearance
when the unit is cold will be approximately 0.007 in. The labyrinth will
seal on the shaft only; the moving blades will not require seals. With
impulse turbines, these liberal clearances help minimize the hazards
associated with water washing. Nevertheless, numerous reaction tur-
bines have also been successfully water washed.
Water injection is accomplished by a piping arrangement for the
atomization and injection of water into the steam supply to ensure a
gradual and uniform reduction in the temperature of the turbine inlet
steam until it reaches 10 to 15°F superheat. It is probably a safe rule
that the temperature should not be reduced faster than 25°F in 15 min
or 100°F/h. Figures 12.16 through 12.18 show suggested piping
arrangements for the admission of water and steam as well as a simple
assembly of fabricated pipes to form a desuperheater. Failure of water