Page 46 - Handbook of Materials Failure Analysis

3 Case Studies 39

3. During potentiostatic scan, the line pipe specimen did not attain passivity due to
high chloride level in spent caustic solution compared to water.
4. The rate of penetration due to pitting was increased with time as perforation.

3.1.8 Recommendations
On the basis of visual, metallurgical, design consideration, and electrochemical test-
ing results it was suggested to replace 1.5-in. CFL400 pipe with 4-in. diameter pipe to
eliminate the velocity-reduction effect in the horizontal line pipe which could also
prevent the formation of stagnant layer and hence the deposition of suspended solids.
In a continuous flow situation, the formation of local cells will be restricted by the
elimination of differential aeration cells.

3.2 FAILURE OF PLATE HEAT EXCHANGER DUE TO PITTING
Reprinted from Deen et al. [15]. Copyright 2010, with permission from Elsevier.

3.2.1 Background
The heat-exchanger plates failed due to pitting and perforation. This plate-type heat
exchanger was the part of a power generation system and it was designed to transport
heat of close-circuit jacket, primary cooling system auxiliary water by cooling with
open-circuit cooling tower (CT) water (secondary cooling system) to decrease the
temperature by 20 °C. Almost 32 plates were perforated by the secondary cooling
water in the open-circuit side of CT system.
In the jacket water side, the plates were arranged sequentially in two pairs, each
having two plates one for jacket water and the other two plates for CT water; thus, 24
plates were tightened in a bundle. There was similar arrangement in the auxiliary
water side of heat exchanger. The hot and cooling water flow arrangement is shown
schematically in Figure 2.7.
Internal lube oil and cylinder head jackets of the power engine were cooled by
auxiliary water and jacket water heat exchanger in a close circuit, respectively.
For efficient release of heat from the four power engines and maintaining the
appropriate temperature, each was connected with an individual CT having capacity
50 tons/h (Table 2.5).

3.2.2 Analytical Approach
To investigate the reason of perforation and pitting of plate heat exchanger, the
following procedure was adopted:
1. Collection of historical, design, and operational data
2. Visual inspection
3. Metallurgical analysis
4. Chemical analysis of every water system (raw water, reverse osmosis (R/O)
water, CT feed, and CT bleed water)
5. Investigating the electrochemical behavior of heat exchanger plates in water
systems