Volume 6 Preprint 5
An Investigation of the Corrosion Behaviour of Transfer Lining Exchanger (TLE) of the Olefin Plant in the Arak Petrochemical Complex
A.A.Riahi M.H.Shariat and M.S.Aboutorabi
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Volume 6 Paper H069
An Investigation on the Corrosion Behavior of
Transfer lining Exchanger (TLE) in Olefin Plant
of Arak Petrochemical Complex
M.H. Shariat1, A.A. Riahi2, M.S. Aboutorabi3
of Materials Science & Engineering, Shiraz University,
Shiraz, Iran, firstname.lastname@example.org
2 Department of Materials Science and Engineering, Tarbaiat Modarres
University, P.O. BOX 14115, Tehran, Iran, email@example.com
3Corrosion Centre, Arak Petrochemical Complex, Arak, Iran
In Olefin plant of Arak Petrochemical Complex a Transfer Lining
Exchanger (TLE) (A Quench Boiler) which is a vertical heat exchanger is
used to quench the gasses coming from the cracking unit. The gasses
at a temperature around 870 C coming out of cracking plant are
passed through a diffuser to be distributed uniformly into the tubes of
heat exchanger. D.M. water passes through the shell side which cools
the gas. Relatively large holes were observed on the surface of water
side of tube sheet. As the work of complex was very dependent on the
performance of (TLE) a through investigation started with the help of
Shiraz and Tarbiat Moddares universities. The fouling on the tubes
were analyzed and the design of the TLE was altered. Examinations
showed that the main reason of corrosion and generation of ho les on
tube sheet is perhaps due to non-uniform distribution of heat and the
presence of precipitate on the tube sheet, formed from the additives to
the water, thus causing under deposit corrosion. Design alternation
was performed and the TLE with considering the corrosion behavior
was constructed. After two years in service, no corrosion on the unit
has been reported and the unit has a good efficiency. In this paper the
result of laboratory experiments and the techniques used to prevent
corrosion is presented.
Keywords: TLE, under deposit corrosion.
Naphta is cracked in 860°C after being passed through the coils of the
olefin furnace and the produced gases enter the tube side of the TLE.
These gases leave the exchanger at a pressure of 0.5 bar and a
temperature of 390°C to the other parts of the process. Boiler feed
water with the specification shown in Table 1 is pumped with a
pressure of 135 bar into the convection section and from there to
steam drum at 350°C and 115 bar. From the lower part of the steam
drum, water is fed into the tube sheet which leaves the TLE after
passing through the shell. To gain a better understanding a PFD of the
above process is shown in figure 1.
A Brief Description of TLE
Transfer Lining Exchanger is used for the purpose of cooling the gases
produced from the cracking furnace and is usually known in the
abbreviated form as TLE. A general view of it is shown in Figure 2.
Tube sheet of the exchanger is a thin plate (15 mm) which is hold in
place using a stiffening plate. Tube sheet is connected to this plate
using some fingers to transfer the load from the tube sheet to the
stiffening plate and then to the shell itself. Center of each tube is
surrounded by four fingers to take the advantage of exerting no load
on the tube sheet. The gap between tube sheet and stiffening plate is
designed to allow for the upward flow of water into shell. There also
exists in this section a number of nozzles for blow-down and
inspection purposes. As a result of heat transfer from the tubes, vapor
bubbles are formed near tube walls which tend to move upward in the
shell. As the gases leaving are cooler than those entering it, the upper
tube sheet has a lower thickness. In this section tubes are welded to
tube sheet after being expanded. Water flow velocity in the surface of
tube sheet is around 1 m/s.
1. There is a considerable reduction in thickness observed in the gas
side of the tube sheet. In some places thickness is reduced to
around 5 mm. This could be due to impact of high velocity vapor
2. In the welded joint between the tube and tube sheet a high degree
of erosion due to gases are observed which makes the tubes loose
in their place.
3. In the water side of the tube sheet, large pits with dimensions in
the range 15 to 20 mm and depth of 1 to 15 mm are observed.
(Fig. 3 ). These have all led to the perforation of the part.
Some are experiments were performed on deposits and boiler feed
water. In Table 2 the results of a 1-month investigation on the water
and in Table 3 the result of chemical analysis of deposits with
cooperation of GE Betz are presented.
Results and Discussion
In high pressure boilers, mineral free water is usually used for the
related purposes. These equipments contain a great amount of
condensed water and points of very high temperature are also
observed in them. These factors make the high pressure boilers
susceptible to damage and failure against caustic soda. Hydroxide ion
is the dominant anion in such boilers. So the boiler feed water is
readily able to dissolve the Fe3O4 protective layer. It is said that great
amounts of caustic soda is mixed by magnetite with some mechanisms
and a sodium ferro-ferrate is produced. This compound is then moved
to regions near damaged areas and iron oxide is produced in these
places. The liberated hydroxide ion could then damage the protective
pH/phosphate ratio could be useful to control this type of damage.
accumulation of the hydroxide ions. The mechanism could be best
explained by the hydrolysis of sodium tri-phosphate according to the
Na3 PO4 + H 2O = NaOH + Na2 HPO4
NaH 2 PO4 + H 2O = H 3 PO4 + NaOH
In the early investigations cavitation was considered as the main
damage mechanism. But regarding this fact that surface was covered
with deposits in some parts, this theory was disapproved.
1. To avoid overheating of the cone connected to TLE, it’s been coated
with refractory material from inside. The purpose of this work is to
help make uniform the distribution of gases and maintaining their
flow in the laminar range. Sometimes as a result of thermal stresses
at shut-down times and in general gas erosion, the refractory must
be repaired. It’s obvious that the designed shape cannot be exactly
rebuilt, so we will face the problem of non-uniformity of the flow
on the surface of tube sheet and overheating is the result on some
points on the tube sheet.
2. Presence of oxygen due to inefficiency of deairator and hydrazine
injection and also improper preservation could be blamed for the
observed damages. As a result, places with higher temperatures
could be easily attacked by oxygen.
3. Thermal differential cell in the inlet water nozzle is another cause
4. Under deposit corrosion in the form of caustic gauging is the other
major cause of damage.
1. A basket of the material Incolloy was installed in the connection to
tube sheet to make the flow distribution uniform. A schematic of
this is shown in figure 4.
2. Using flush-nozzle at shut-down times may help improve the
cleanliness of the tube sheet surface.
3. Minute control of water treatment including phosphates, hydrazine
and pH control.
4. Hard facing heat treatment to improve resistance to corrosion and
5. Using a continuous blow-down to prevent the increase of mineral
concentration in the water.
1- Corrosion in the Petrochemical Industry, ASM, edited by: Linda
2- Metals Handbook, Vol.13, 9th Edition, 1990
3- Uhlig’s Corrosion Handbook, 2nd Edition, by R.Winston, 2000.
4- Materials Science and Technology, R.W.Cahn, 2000.
Table 1. Boiler Feed Water Specification
Clear and colorless
Conductivity (at 25°C)
Less than 0.2
Greater than 9
Less than 0.020
Less than 0.020
Less than 0.003
Less than 0.02
Total carbon dioxide
Less than 5
ppm (as C)
Less than 0.5
Table 2. Results of water analysis in a 1-month period
Table 3. Deposits Chemical Analysis
Primary Composition (%)
Figure1. PFD of the Transfer Lining Exchanger
Figure2. A general view of TLE
Fig3. Large Pits on Tube Sheet
Figure4. Basket used to make flow distribution uniform