Page 481 - Practical Machinery Management for Process Plants Major Process Equipment Maintenance and Repair

458 Major Process Equipment Maintenance and Repair

Erosion
Both the axial compressor and the hot gas path parts can be affected by
erosion from hard, abrasive particles, such as sand and mineral dusts. As
they impact upon the compressor blades, they cut away a small amount of
metal. The net rate of erosion, although not precisely quantifiable, de-
pends on the kinetic energy of the particles, the number of particles im-
pinging per unit time, and the mechanical properties of both the particles
and the material being eroded. In general, the experience of one major
gas turbine manufacturer indicates that particles below 10 microns do not
cause erosion, whereas particles 20 microns and above normally cause
erosion when present in sufficient quantities.

Corrosion
Wo types of corrosion are generally to be avoided. These are com-
pressor corrosion and hot section corrosion. Compressor corrosion is
typically caused by moisture in combination with sea salt and in some
cases acids. They cause the compressor blades and vanes to become pit-
ted, adding drag, increasing their susceptibility to fouling and decreasing
their aerodynamic performance. Hot section corrosion is far more seri-
ous and occurs when certain metals combine with sulfur and/or oxygen
during the combustion process, and deposit on the surfaces of the hot gas
path parts. Of primary concern are sodium, potassium, vanadium, and
lead. These metals, either as sulfates or oxides, cause the normally pro-
tective oxide film on hot gas path parts to be disrupted so that the parts
oxidize several times faster than in the presence of exhaust gases free of
these metals.

Fouling
Compressor fouling in the short term is the most noticeable effect of
poor inlet air filtration. The type of performance monitoring outlined
earlier addresses this problem. With increasing cost of fuel the savings to
be realized from operating at maximum efficiency have become signifi-
cant.
The efficiency of an axial compressor is dependent on, among other
considerations, the smoothness of the rotating and stationary blade sur-
faces. These surfaces can be roughened by erosion, but more frequently
roughening is caused by the ingestion of substances which adhere to the
surfaces. These include oil vapors, smoke, and sea salt. The output of a
turbine can be reduced as much as 10 percent by compressor fouling.
There are various methods of removing deposits from blading in use
today. Some can be accomplished with the unit running and others re