Gas Turbine Handbook : Principles and Practices

Detectable Problems 195
COMPRESSOR FOULING
Indications And Corrective Action
Compressor fouling normally occurs due to foreign deposits on
the airfoils. In petro-chemical operations, this is the result of the oil
and other hydrocarbons in the atmosphere caused by neighboring
process plant flares, etc. In coastal areas, this could be the deposit of
salt in the compressor. In any event, compressor fouling is indicated
by a drop in compressor efficiency, which is more readily seen as a
drop in compressor discharge pressure at a constant speed and load.
This will also manifest itself as a reduction in load capacity at a constant
compressor inlet temperature. A compressor efficiency drop of 2
percent is indicative of compressor fouling. This can be calculated as
shown.
η c =
CDP
k–1
k
P in
–1
CDT
T in
–1
(12-4)
For the field engineer or operator, compressor fouling is best
indicated by a 2 percent drop in compressor discharge pressure at
constant speed and load. Another indication of fouling is a 3 percent
to 5 percent reduction in load capacity at constant compressor inlet
temperature or ambient air temperature. The 1 psig decrease in compressor
discharge pressure, which will accompany the load reduction
is not easily detectable with field instrumentation.
As shown in Figure 12-7 for the single spool gas turbine, a decrease
in compressor efficiency (at constant load) results in a decrease
in rotor speed, compressor discharge pressure, and airflow, and a
increase in fuel flow, exhaust gas temperature, and compressor discharge
temperature.
Similarly, for the dual spool gas turbine, a decrease in low pressure
compressor efficiency results in decreases in N 1
rotor speed, low
pressure compressor discharge pressure (P 3
), and airflow, and increases
in fuel flow, exhaust gas temperature (T 7
), N 2
rotor speed, and
low and high pressure compressor discharge temperatures (T 3
and T 4
)
as shown in Figure 12-8. The decrease in high pressure compressor
discharge pressure (P 4
) is small and not easily detected.