Page 185 - CW E-Magazine (8-10-2024)

Special Report

process is then repeated with the result- fatigue resistance; however, corrosion solid contaminants, the flow rate and
ing loss of metal and accumulation of fatigue resistance is markedly affected velocity become critical. At lower flow
oxide residue. Seizing and galling often by the stress cycle frequency. Oxygen rates, solids may settle down, thus re-
occur, together with loss of tolerances content, temperature, pH, and solution sulting in crevice corrosion at some
and loosening of mating parts. Further it composition influence corrosion fatigue. locations. If the flow velocity of water
causes fatigue fracture since the loosen- In seawater, aluminum bronzes and is designed for a higher value, then
ing of components permits exces- austenitic SS retain only70-80% of erosion and cavitation may occur for
sive strain, and the pits formed act as their normal fatigue resistance and which suitable protective coatings must
stress raisers (Figure-11). The failure is high-chromium alloys retain only about be specified.
caused due to insufficient interference 30-40% of their normal fatigue resis-
fit between outer race and housing with tance. Changes in the chemistry of the
the formation of patchy discoloration water being pumped may influence the
of outer race and generation of reddish- The most common areas of corro- corrosion rate. A temperature rise can
brown debris. This type of failure has sion fatigue crack initiation are at the increase the rate of corrosion. When
been observed in a series of service stress concentrations occurring at the a pump is out of service for a long
water pumps mounted on a common keyway root radius and sharp changes period, there is the risk of increased cor-
base plate in a power plant. The failure in cross-sectional area of the pump rosion due to trapped water and air. In
took place in idle/stand-by pumps as shaft. Bearing failures in pumps due to such cases, adding a corrosion inhibitor
vibration got transmitted to the shafts ingress of water or dirt are common and to the water inside the pump during a
of the stand-by pump from the running will result in severe shaft vibration. Un- prolonged outage may be considered.
pumps through the common base plate. balanced impeller and shaft misalign- Draining the entrapped water can also
This vibration (slippage) of the station- ment also induces high shaft vibration. be attempted.
ary shaft on the bearing housing/races High levels of vibration can exaggerate
causes fretting corrosion in the outer fatigue. By preventing corrosion of the Case studies on centrifugal water
race of the bearing as shown in the shaft, the stress concentration factor pump shaft failure due to corrosion
figure. can be reduced and corrosion-fatigue One of the most catastrophic forms
failure of pump shaft can be avoided. of corrosion encountered on shafts is
pitting attack. This localised form of
Microbiologically influenced corrosion corrosion results in the formation of
(MIC) holes or cavities in the metal and gene-
MIC occasionally occurs in pumps rally results in the shaft having to be
that handle bore-well water and in wa- scrapped due to the depth of penetra-
ter pumps used by the oil and gas indus- tion of the pits. The relatively stagnant
try. While some microorganisms induce liquid conditions, which exist between
corrosion directly, others actively assist the shafts and sleeve during pump ope-
pre-existing corrosion reactions. Dif- ration as well as periods of shutdown,
Fig. 11: Fretting corrosion of centrifugal ferent types of organisms that influence create ideal conditions for pit initiation.
pump shaft bearing outer race due to corrosion include sulphate-reducing Figure-12 shows a portion of water
inadequate fitting of the bearing outer race bacteria (SRB), sulphur oxidizers and pump shaft subjected to corrosion failure
with the bearing housing
iron oxidizers. which had been in service for approxi-
Corrosion fatigue mately 17 years. The bronze shaft
Corrosion fatigue is defined as Iron ions present in the water enable
the reduction of fatigue resistance of microbes to produce iron hydroxide,
a component due to the presence of a oxidize manganese and create metal
corrosive medium. Accelerated crack precipitate on the surface. Steel compo-
propagation with deposition of corro- nents, for example, are also susceptible
sion products takes place due to cor- to acidic corrosion, which are caused
rosion fatigue. Environmental factors by certain organisms assisted by SRB.
strongly influence corrosion-fatigue.
In ordinary fatigue the stress-cycle fre- Damage prevention Fig. 12: Steel pump shaft severely pitted
quency has only a negligible effect on If the water being pumped carries under the bronze sleeve

Chemical Weekly October 8, 2024 185

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