Page 49 - Handbook of Materials Failure Analysis

42 CHAPTER 2 Failure analysis in chemical industries

Table 2.8 Cooling Tower Water Design Limits as Suggested by the Designer
Parameters Value
System volume 0.5 m 3
3
Recirculation rate 120 m /h
Temp. drop 06 °C
Water Reverse osmosis
3
Evaporation 0.457 m /h
3
Bleed off 0.015 m /h
3
Make up 0.472 m /h
With permission from Elsevier.

Pitting

(a)
Severe fouling

(b)
FIGURE 2.8
(a) Pitting and (b) fouling of heat exchanger plate from the CT water side.
With permission from Elsevier.

sludge in the close-circuit side. The perforated plates damaged by pitting corrosion
were analyzed metallurgically and electrochemically. Specially, the composition of
raw, R/O, CT feed, and bleed water was critically monitored and chloride levels in
these water samples measured in the laboratory.

3.2.5 Testing Results and Findings
The analysis of plate samples was confirmed as AISI 316L stainless steel and was in
accordance with design specification. There was no heterogeneity in the microstruc-
ture and plate material was metallurgically clean having no inclusions and slag par-
ticles in it. The microstructural analysis and morphology of pit showed twined and
banded structure of austenitic typical to the microstructure of cold rolled stainless
steel as shown in Figure 2.9. The pits geometry was irregular and their approximated
size was in the range of 0.8-0.95 mm.
The water analysis of CT water confirmed the high level of chloride in CT feed
(142 ppm) and bleed water (709 ppm) which was beyond the limits recommended by
the supplier and manufacturer of power engines. Also, the client disclosed that the
need for addition of treatment chemicals was being determined on the basis of total