EVALUATION OF STAINLESS STEEL PLATES OF HEAT EXCHANGER … · 2015. 5. 31. · of cooler plates had led to the failure due to Stress Corrosion Cracking (SCC) failure. Keywords: Plate

Proceedings of the 6th International Conference on Mechanics and Materials in Design,

Editors: J.F. Silva Gomes & S.A. Meguid, P.Delgada/Azores, 26-30 July 2015

-637-

PAPER REF: 5667

EVALUATION OF STAINLESS STEEL PLATES OF HEAT

EXCHANGER DAMAGE

H. Abdel-Aleem1(*)

, B. Zaghloul2, S. A. Khodir

1

1Reasecher at Welding Technology and inspection Lab., Central Metallurgical research & Development

Institute (CMRDI), Egypt 2Prof. Emeritus atWelding Technology and inspection Lab., Central Metallurgical research & Development

Institute (CMRDI), Egypt (*)Email: [email protected]

ABSTRACT

An attempted has been made to unravel the exact reasons for premature damage of stainless

steel plates of plate cooler used as plate heat exchanger (PHE) to cooling down the

temperature of process water using normal cooling water. Analysis suggested that as a result

of presence of some factors such as, harmful corroded element such as chloride ions from

cleaning solution and probably water, in addition to accumulated residual stresses caused by

from water pressure, heavy deformation at protuberance shape and probably over tightening

of cooler plates had led to the failure due to Stress Corrosion Cracking (SCC) failure.

Keywords: Plate heat exchanger, Damage, stainless steel 316, stress crack corrosion.

INTRODUCTION

Plate heat exchangers (PHE) have many uses in different industries such as, fertilizing plants,

milk revival, pre-heater, cooler, heater, regeneration and etc. The Plates Heat Exchanger

(PHE), as shown in Fig.1, is composed of corrugated thin alloy plates, which are hung

between top and bottom guide bars. The plates are compressed by tightening bolts in between

the frame plate and pressure plate until metal - to - metal plate is reached and a channel is

formed. The manner in which the gaskets are fitted enables alternative flow channels to be

created and heat transfer to pass from one side of the plate to the other. The alternative

channels maximize the heat transfer surface in compact manner. Therefore, it can produce the

most effective performance from the compact size (Sunders AED, 1988 and Pearce N, 2001)

The heat exchanger is thermally designed to the required operation and service supplied. The

product, flow rates and required temperature are used to find the required plate arrangement

based on the surface area and allowed pressure drop. Usually, the PHE total heat co-efficiency

is 3 to 10 times greater than the shell and tube heat exchanger (Pearce N, 2001)

316 Stainless steel is a candidate material for manufacturing of PHE's plates due to having its

corrosion resistance properties in cooling water but the parameter of normal cooling water

determine the overall corrosion stability of plate heat exchanger such as temperature, pH,

conductivity, hardness, alkalinity as well as sulphate, nitrate and most importantly Chloride

ion concentration ( Georgiadis MC, 2000 and K.M Dean, 2010).

This paper details the study carried out on twelve stainless plates of PHE having some cracks

after three years in service and recommends certain measures to be taken in term of cleaning

material, operation practice to prolong the life of these plates.

Track_H

Modes of Failure

-638-

Cooler plate heat exchanger

Fig.1 - Illustrating diagram for construction of cooler plate heat exchanger.

BACKGROUND INFORMATION

In this study the PHE has set of 132 plates made of 316 stainless steel. These plates are used

as heat exchanger to cooling down the temperature of process water using normal cooling

water. The plates cooler are separated from each other by a gasket. It is reported that, after

three years in service, twelve of total 132 plates have some cracks. During the operation and

as the result of improper cooling due to scale formation in the plate, it was necessary to

perform chemical cleaning with materials containing acid and zinc chloride several times and

the last time was done by injection of Nitric acid to remove the scale. During the plant shut

down all the plates were changed and during cleaning process cracks were visually observed

in some plates.

Two pieces from two different failed plates were investigated to study the cause of the failure.

Table 1 shows the actual operating condition for cooler plate. It is also reported that, during

the operation, the maximum temperature for process side outlet side was raised until 113Co.

Main feature of a heat transfer plate



-639-

Table 1 - Operation condition

Condition

Process side Cooling side

inlet outlet inlet outlet

Normal

Operating

Temp.(c)

95 80 30 40

Actual Temp.(co) 104 80 30 35

Normal

Operating

Press.(bar)

10.0 9 3.6 2.8

Actual

Press.(bar) 15 3.6

EPERIMENTAL PROCEDURE AND RESULTS

Visual inspection

The general view of the plates cooler is presented in Fig.2. It is clearly obvious that there was

no mechanical damage. It is also noticed that the cracks are located near to circumference of

the opening holes at each plate and around the areas which were subjected to high

deformation stresses during manufacturing using die pressing process as shown in Fig.3.

Moreover, it is also noticed that there is corrosion losses.

Fig. 2 - General view of the cooler plates after chemical cleaning

Dye penetrant testing (PT) and chemical analysis

The damaged plates showed also crack, pitting and corrosion losses areas as detected by dye

penetrant testing (PT) as shown in Fig.4. The chemical composition done on the received

specimen of the damaged plates is confirmed with the standard 316 stainless steel as given in

the data sheet received.

Track_H

Modes of Failure

-640-

Damaged area

Corrosion losses

Fig. 3 - Location of damaged area.

Fig. 4 - Result of dye penetrant test (PT)

Metallographic examinations

Metallographic examinations were conducted on cut pieces from the damaged cooler plates.

Figure 5 shows the macro-photographic appearance of the damaged area using the

stereoscope. It can be noticed that there are some pitting corrosions around the cracks and at

other places as well. The crack started its initiation at the pitting area. Moreover, it is also

Crack area

Proceedings of the 6th International Conference on

Editors: J.F. Silva Gomes & S.A. Meguid, P.Delgada/Azores, 26

noticed that there are some small corrosion spots on the plate near the crack and petting areas.

Figure 6 shows the nature of the crack propagation of

of cooler plate. There is a distinguished main crack propagated in the matrix with small hair

cracks are branched like a tree. The figure indicates also that there are several other cracks

had initiated at the same area.

The cracked area was investigated in the as polish surface condition at high magnification

using optical microscope. Figure 7 shows branching of cracks, the main crack is not

uniformly propagated and it has small branches. The microstructure of the etched surface of

the specimen taken near the crack area is shown in Figs.

morphology of the crack resembles the intergranular

nature ( Metals Handbook ASM Vol. 11 and 13)



-641-


Figure 6 shows the nature of the crack propagation of a specimen taken at the damaged area



.

Fig. 5 - Pitting and Corrosion losses

Fig.6 - Cracked area as polish



pagated and it has small branches. The microstructure of the etched surface of

the specimen taken near the crack area is shown in Figs. 7a and 7b, which reflects that the

morphology of the crack resembles the intergranular stress corrosion cracking (IGSCC)

( Metals Handbook ASM Vol. 11 and 13).

5 mm

(50 X)


a specimen taken at the damaged area





pagated and it has small branches. The microstructure of the etched surface of

b, which reflects that the

stress corrosion cracking (IGSCC)

5 mm

Track_H

Modes of Failure

-642-

(a) Microstructure near the cracked area (50 X)

The micrograph near the cracked area shows partly recrystallized austenite grains, carbide

particles, and remnants of twinning (slip bands) while the matrix is austenite as shown in Fig.

8a. This structure is typically cold worked and annealed structure (Metals handbook ASM

Vol. 17).

In order to investigate the effect of cold working process during the manufacturing of the

plates on the mechanical characteristics at each part of the deformed plate according to the

degree of deformation which was applied on the plate, microhardness measurements were

done on several locations at the plate. The hardness measurements away from the cracked

area (which were exposed to heavy deformation during manufacturing to create curved shapes

on the plate) is in the range of 187-205 Hv, and the hardness values at similar location but

near the tip of the crack is decreased to 157 Hv as the result of stress release due to the

occurrence of the crack at this area as shown in Fig. 8a. While the hardness measurement at

flat areas far from cracked area is 164 Hv, this area was exposed to much less deformation

stresses during the manufacturing process of the plate as compare with the curved cracked

area. The microstructure at the flat area shows normal annealed structure with less twinning

i.e. less residual stresses, as shown in Fig. 8b (Metals handbook ASM Vol. 17)

Fig. 7 - Microstructure near the damaged and cracked areas

(b) Microstructure near the crack area

at higher magnification (100X)

Proceedings of the 6th International Conference on

Editors: J.F. Silva Gomes & S.A. Meguid, P.Delgada/Azores, 26

(a) Microstructure at the cracked area

(sever deformed area) (200X)

Fig. 8

Scanning electron microscopy and scale characterization

Scanning Electron Microscope (SEM) was used to examine the cracked area from the

damaged cooler plate. It has shown a typical intergranular

as shown in Fig. 9 ((Metals handbook ASM Vol. 17)

Due to the chemical cleaning of the cooler plate before receiving the investigated specimens,

no corrosion product could be obtained for chemical or x

precipitates from the cooling water of other cooler plates were removed, in order to be

examined by X-Ray Diffraction (XRD). The results showed the presence of Calcite (Ca CO

and silica SiO2. No indications for any other corrosion products cou

DISCUSSION

It was reported that the cooler plates were cleaned several times using chemical solution

which contained chloride compound

of the cleaning solution and the duration of the cleaning time were increased. Therefore, it can

Hv = 157

Hv = 187



-643-

(a) Microstructure at the cracked area (b) Microstructure far from the crack

(sever deformed area) (200X) area (less severe deformed area) ( 200X)

Fig. 8 - Microstructure variation in the plate surface

Scanning electron microscopy and scale characterization


damaged cooler plate. It has shown a typical intergranular stress corrosion cracking behavior

(Metals handbook ASM Vol. 17).


no corrosion product could be obtained for chemical or x-ray analysis. How


Ray Diffraction (XRD). The results showed the presence of Calcite (Ca CO

. No indications for any other corrosion products could clearly be detected.

Fig. 9 - SEM near the cracked area


which contained chloride compound. In last few cleaning processes, both of the concentration


Hv = 157

Hv = 187-205

(b) Microstructure far from the crack

area (less severe deformed area) ( 200X)


stress corrosion cracking behavior


ray analysis. However, some


Ray Diffraction (XRD). The results showed the presence of Calcite (Ca CO3)

ld clearly be detected.


In last few cleaning processes, both of the concentration


Track_H

Modes of Failure

-644-

be deduced that the austenitic stainless steel cooler plate has failure due to exposure to

chloride containing solution during chemical cleaning for several times and each time the

concentration of the harmful element, chloride ions was increased. The cracks were developed

at the protuberance shape which reflected the highest hardness values as a result of heavy

deformation during manufacturing process causing relatively high residual stresses. The

Chlorine ions replaced oxygen/hydroxide ions in the protective layer. Usually the cooler

plates are tied together; sometimes they are over-tightened to eliminate a leakage situation,

then cooler plates would have a permanent stress. Moreover, the damaged area is located at

protuberance place at the sheet. This area is expected to have residual stress as indicated from

both microstructure and hardness measurements at damaged area. This condition creates good

atmosphere to cause the intergarnullar stress corrosion cracking. Also, results of XRF for

water cooling precipitates showed presence of silica and calcite which promoted localized

pitting at high flow rate and water pressure.

CONCLUSION

It can be concluded that as a result of presence of some factors such as, harmful corroded

element such as chloride ions from cleaning solution and probably water, in addition to

accumulated residual stresses caused by from water pressure, heavy deformation at

protuberance shape and probably over tightening of cooler plates had lead to the failure due to

Stress Corrosion Cracking (SCC) failure.

ACKNOWLEDGMENTS

The authors thank ALEXFERT Co. for supporting this work.

REFERENCES

[1]-Sunders AED. Heat exchangers, Selection, Design and Construction. London: Longman;

1988.

[2]-Pearce N. Plate Exchanger Defeats Industry Conservatism. Eur Power News 2001 (10) :7-

16.

[3]-Georgiadis MC, Macchietto S: Dynamic Modeling and Simulation of Plate Heat

Exchanger Under Milk Fouling, CHEM ENG SCI, Vol 55, (2000), 1605-1619.

[4]-K.M. Dean, M.A. Virk, C.I. Haque, R. Ahmed, I.H. Khan: Failure Investigation of Heat

Exchanger Plates Due to Pitting Corrosion, Engineering Failure Analysis, Vol 17 (2010),

886-893.

[5]-Metals hand book, 9th ed., vol. 13, Corrosion. Metals Park (OH):ASM Pub.

[6]-Fontana MG. Corrosion engineering. New York; McGraw-Hill; 1987.

[7]-El-Batahgy AM: Failure Avoidance - Stress Corrosion Cracking of a Plate-Type Heat

Exchanger, Materials Performance, Vol 38, (1999).

[8]-Metals handbook, 9th ed., Vol 11, Failure analysis and prevention. Metals Park (OH):

ASM Pub.; 1988.

[9]-Metals handbook, 8th Edition, Vol 7, Atlas of Microstructures of Industrial Alloys. Metals

Park (OH): ASM Pub.; 1973.

Documents

EVALUATION OF STAINLESS STEEL PLATES OF HEAT EXCHANGER … · 2015. 5. 31. · of cooler plates had led to the failure due to Stress Corrosion Cracking (SCC) failure. Keywords: Plate