CDU OverHead Corossion MAB

Under Deposit Corrosion of Fractionator OVHD Exchangers in Crude Distillation Unit and its Remedial Measures

Musaed M. Al-Arada, Bader Al-Otaibi, Faisal H. Al-Refai, Anil Kumar Ray, Ali R. Al-Azemi

Kuwait National Petroleum Company

Mina Abdulla Refinery P.O. Box-69, Safat, Kuwait 13001

Fax: +965-23280402

E-mail: [email protected]

ABSTRACT

The Fractionator Overhead (OVHD) System of a Crude Distillation Unit (CDU) consists of shell and

tube type Exchangers. Carbon Steel metallurgy is used in both shell and tube with Fractionator Tower

OVHD vapor on shell side and crude feed on the tube side. Since commissioning, frequent failures of

these exchangers have been reported causing loss of production. Failures were in the form of erosion-

corrosion, pitting and general wall thickness loss on the tube Outer Diameter (OD) surface. These

failures were as a result of under deposit corrosion due to presence of moderate to heavy fouling and

formation of Iron Sulfide scales. Initially there were three exchangers and one more was added

afterwards to combat erosion-corrosion problem of tubes. Tubes with different higher metallurgy (UNS

# K41545, K90941) were tried from time to time however no improvement was noticed in the frequency

of failure of tubes. Poor supply of wash water to the fractionator OVHD system resulted in accumulation

of fouling on the tube OD surfaces leading to under deposit corrosion.

Keywords: under deposit corrosion, fouling, tube OD, carbon steel, pitting, general wall loss, CDU,

OVHD.

1. INTRODUCTION

The Crude Distillation Unit of Mina Abdullah Refinery(2)

was originally designed for a throughput of

156 Kilo Barrels Per Day (KBPD); however it is presently being operated at 190 KBPD after the revamp

carried out in September 2001. Main products of the unit are Liquefied Petroleum Gas (LPG), Naphtha,

Kerosene,(3)

Diesel and Atmospheric Residue. A brief process flow diagram of crude unit is shown in

fig. 1. (1)

Unified Numbering System for Metals and Alloys (UNS). UNS numbers are listed in Metal & Alloys in the Unified

Numbering System, 10th ed. (Warrendale, PA: SAE International and West Conshohocken, PA: ASTM International, 2004).

(2) Trade name and

(3) Trade name.

Government work published by NACE International with permission of the author(s).The material presented and the views expressed in this paper are solely those of the author(s) and are not necessarily endorsed by the Association.

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Paper No.

2225

2. DESCRIPTION OF OVHD CONDENSERS

The Crude Distillation Unit of a refinery has four fractionators OVHD exchangers (E-11-101A/B/C/D).

These OVHD exchangers are used to recover heat from fractionator overhead vapours. Crude is on tube

side and overhead vapour is on shell side. To provide more free area for incoming vapour, a vapour belt

is employed on shell inlets of all four bundles.

Design/operating conditions of OVHD exchangers and material of construction are given in Table 1 and

Table 2 respectively.

Table 1

Design and Operating Conditions of OVHD Condensers

Parameters Shell Side Tube Side

Design Operating Design Operating

Pressure 5.27 Kg/cm2 (75 psig) Inlet - 1.4 Kg/cm

2 (20 psig) 28.12 Kg/cm

2 (400 psig)

Inlet - 21.30 Kg/cm2

(303psig)

Temperature 154.4° C (310° F) Inlet - 96.6° C (206° F),

Exit - 83.3° C (183° F) 93.3° C (200° F)

Inlet - 28.8° C (84° F),

Exit - 61.1° C (142° F)

Table 2

Material of Construction of OVHD Condensers

Components Material of Construction

Shell Carbon Steel (CS)

Tubes Carbon Steel (CS)

As shown in fig. 2, OVHD exchangers are equipped with wash water connection to remove fouling on

shell side.

3. HISTORY OF OVHD EXCHANGERS (E-11-101A/B/C/D)

Three OVHD exchangers (E-11-101A/B/C) were commissioned in 1988. One more OVHD exchanger

(E-11-101D) was introduced in the OVHD system in 2007 to combat erosion-corrosion problem on the

tubes Outer Diameter (OD) surface which was due to increase in throughput of the unit from 156 KBPD

to 190 KBPD. Tubes with different higher metallurgy (UNS # K41545, K90941) were tried from time to

time however no improvement was noticed in the frequency of failure of tubes.

The exchangers failed and were retubed from time to time; the history of retubing of the same is

tabulated below:

Table 3

E-11-101A

Nov. 1991 Retubed the bundle with CS tubes

Aug. 1993 Retubed the bundle with UNS # K41545 tubes


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Jun. 1997 Retubed the bundle with CS. Two UNS # S 32100 tubes were installed as test samples

Oct. 1998 51 periphery tubes replaced with CS

Jul. 2001 255 periphery tubes replaced with CS

Sep. 2001 Retubed the bundle with UNS # K90941 tubes

Jul. 2007 Replaced the bundle with fully retubed spare bundle (CS)

Dec. 2007 Replaced the bundle with fully retubed spare bundle (CS)

May 2008 Retubed the bundle with UNS # K41545 tubes

Table 4

E-11-101B

Dec. 1992 Retubed the bundle with CS tubes

Feb. 1993 Retubed the bundle with UNS # K90941 tubes

Jan. 1997 Retubed the bundle with UNS # K41545 tubes

Dec. 1998 Retubed the bundle with CS tubes

May 2002 Retubed the bundle with CS tubes

May 2003 Retubed the bundle with CS tubes

Jan. 2007 Retubed the bundle with UNS # K41545 tubes

Nov. 2008 Replaced the bundle with fully retubed spare bundle (CS)

Table 5

E-11-101C

Nov. 1991 Retubed the bundle with CS tubes

Apr. 1995 Retubed the bundle with UNS # K90941 tubes

Oct. 1998 Retubed the bundle with CS tubes

Mar. 2003 Retubed the bundle with UNS # K41545 tubes

May 2004 27 tubes replaced (CS)

Jan. 2007 Replaced the bundle with fully retubed spare bundle (CS)

Dec. 2007 Retubed the bundle with UNS # K41545 tubes

Table 6

E-11-101D

Sep. 2008 Replaced the bundle with fully retubed spare bundle (CS)

4. DISCUSSION

Since commissioning the life cycle time of the bundles of the OVHD exchangers E-11-101A/B/C/D

ranges from 1 – 2 years with exception of the period from 1993 – 1996 and 1998 - 2001.


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Failure mechanisms observed were as follows:

a. Under deposit corrosion – 1991 to 1998

b. Erosion-corrosion – 2001 to 2006

c. Under deposit corrosion – 2006 to 2008

4.1. Failure of OVHD Exchangers from 1991 to 1998 - Under Deposit Corrosion

Moderate to heavy fouling was noticed on the tubes OD surface of the tube bundle predominantly on the

shell outlet side. Hard and adherent scales were also noticed on the shell inlet side.

The accumulation/deposition of this fouling product on the tube OD surface shields the area underneath

those locations from the local environment, creating a difference in oxygen concentration i.e. the

shielded area will have less or no oxygen. The exposed surface where ready access to oxygen is there,

becomes cathode relative to the area which is shielded by deposit/fouling (i.e. anode area). The anode

area will get corroded because of oxygen differential cell and this phenomenon is called Under Deposit

Corrosion.

4.2. Failure of OVHD Exchangers from 2001 to 2006 - Erosion Corrosion

Increase in unit capacity from 180 KBPD to 190 KBPD in 2001 has increased the vapour load/vapour

velocity entering into the OVHD condensers and resulted in metal loss due to erosion corrosion

phenomenon. From the preferential metal loss observed on peripheral tubes OD surface of the tube

bundle (limited to vapour entry zone only) it was believed that vapour impingement in the above zones

was able to destroy the protective corrosion inhibitor film. Thus corrosive constituent H2S present in the

OVHD vapours reacts with unprotected tubes OD surface mentioned above and forms FeS. As a result,

continuous loss in the above mentioned zone may be due to alternatively formation and destruction of

FeS corrosion product layer by vapour impingement/turbulence at its entry zone.

When all three bundles were in operation, design velocity at the vapour belt opening was 25-27 ft/sec.

Against this, at a reflux rate of 11 KBPD, actual velocities were in the range of 32-35 ft/sec. Though

these numbers are higher than design, they were not alarming. When two bundles were online and other

one is out of service for preventive maintenance or emergency, the shell side velocities increases in the

range of 45-50 ft/sec. These velocities were almost twice the design numbers and this represents an area

of concern. In addition of this, shell inlet velocity also increased from 52 ft/sec to 102 ft/sec when two

bundles were on line.

4.2.1. Prevention of Erosion-Corrosion - Addition of Exchanger: In view of the higher vapour rate

and keeping maintenance requirement in mind, provision of an additional bundle (E-11-101D) was made

during General Refinery Turn Around (GRTA) 2006. Erosion problems of tubes OD surface facing inlet

vapours was found appreciably reduced compared to previous observations.

4.3. Failure of OVHD Exchangers from 2006 to 2008 - Under Deposit Corrosion

Lab analysis of the scales and fouling products on the tube OD surface indicated the presence of Iron,

sulphur and Carbonaceous matter (refer to Table 7).


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Table 7

Scale Sample analysis report

Component Result Units

Ammonia as NH3 192.0 ppm

Chloride as Cl- 57.3 ppm

Sulphur total 31.4 % mass

Loss On Ignition (LOI) @ 700° C 25.2 % mass

C 0.69 % wt.

Cr 0.3 % mass

Cu 0.0 ppm

Fe 51.2 ppm

Ni 0.0 ppm

The addition of a fourth OVHD condenser had lead to low flow of OVHD vapour through each

condenser (reduced by approx. 25%). This low flow reduced the velocity but increased the

fouling/deposit settlement on the shell side. Although the wash water facility has been provided in each

exchanger (as shown in fig. 3) on the shell side to minimize fouling by salts and deposit, the amount of

wash water injected on the condenser was 2.7% of the OVHD flow as provided in the design basis

without considering the additional condenser. Therefore, the wash water quantity injected on each

condenser has been substantially reduced (approx. 25%). This insufficient amount of wash water

available for each exchanger was not able to remove/flush the fouling/deposition products.

4.3.1. Prevention of Under Deposit Corrosion - Water Wash of Individual Exchanger: The

fouling observed on the tube OD surface were mainly due to low flow of OVHD vapours and

insufficient amount of wash water passing through shell side of each condenser as the total quantity of

OVHD flow and the amount of wash water has been divided into four exchangers instead of three. Wash

water quantity was increased from 2.7% to 5% of OVHD capacity and carried out on stream

cleaning/flushing for OVHD exchangers (E-11-101A/B/C/D) once in a week for 2 hours by allowing all

the wash water to flow through one of the OVHD exchanger shell side which is to be cleaned, closing

the flow of wash water on the other three OVHD exchangers.

This online cleaning helped to remove scales/deposits from OVHD exchangers shell side and same was

reflected on the Iron analysis in tail water of fractionator reflux accumulator (V-11-103). After

completion of first online cleaning, a significant reduction in Iron level in tail water was noticed. Second

online cleaning of these OVHD exchangers was carried out after two months. During the second time

online cleaning, tail water (V-11-103) Iron level was as high as observed in the first time online

cleaning. After the online cleaning, the iron level came down to acceptable limits (See Appendix).

From June 2008, it was decided and agreed to make online wash water cleaning for E-11-101A/B/C/D

frequently on weekly basis as good results were noticed during first and second on-stream wash water

cleaning. In October 2008, OVHD exchanger E-11-101B was taken out of the service for preventive

maintenance after working on-stream for about 10 months. Visual inspection of the tube bundle before

hydro-blast cleaning was carried out to evaluate the performance of online cleaning. This time, fouling

results were compared with fouling observed last time i.e. Nov’07 (Fig. 4).


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The comparison of fouling showed that on-stream cleaning/flushing have drastically reduced the fouling

this time as compared to the last inspection. The readable stencil marks on the tubes indicated the extent

of reduction in fouling (Fig. 5).

5. COST EFFECTIVE SOLUTION

Since 1991, every year at least one tube bundle was failed and retubed except during 1993 – 1996 and

1998 – 2001. The approximate cost for retubing one bundle is 35,000 USD. However, since on-stream

wash water cleaning/flushing was implemented (from June’2008) none of the OVHD exchanger has

been leaked and retubed except E-11-101B/D wherein old tube bundle was in use.

Thus, the on-stream washing of the OVHD exchanger has averted on-stream leak of the OVHD

condensers and enhanced the integrity of the crude unit. It has also contributed a substantial saving of

the company in monitory terms by avoiding frequent retubing.

6. CONCLUSIONS

After implementation of on-stream cleaning/flushing for OVHD exchangers (E-11-101A/B/C/D) it was

found that the amount of fouling observed on the tubes OD have been drastically reduced thereby

reducing under deposit corrosion phenomena.

7. RECOMMENDATIONS

a. Continue the on-stream wash water cleaning of OVHD exchangers on weekly basis for 2 hours.

b. Three condensers should be in service and the fourth condenser is considered to be idle and kept

under positive nitrogen pressure to have sufficient flow of Naphtha through shell side of each

condenser which will help in minimizing deposits/scales accumulation.

8. ACKNOWLEDGEMENTS

The authors wish to thank the management of Kuwait National Petroleum Company(4)

for their

permission to present this paper.

(4)

Trade name


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9. APPENDIX

Iron level (ppm) in OVHD reflux accumulator during first on-stream cleaning of

E-11-101 A/B/D (E-11-101C was idle)

Date

(Year’08)

Iron

(ppm)

Iron level during on-stream cleaning of E-11-101D

Iron level during on-stream cleaning of E-11-101A

Iron level during on-stream cleaning of E-11-101B

18 Jan 4.56

19 Jan 1.64

20 Jan 10.3

20 Jan 3.3

21 Jan 3.75

21 Jan 1.64

21 Jan 14.8

22 Jan 12.5

22 Jan 13.3

22 Jan 23.4

22 Jan 17.5

23 Jan 9.6

24 Jan 8.82

25 Jan 7.7

26 Jan 6.7

26 Jan 9.9

27 Jan 6.3

27 Jan 10

27 Jan 119

27 Jan 125

27 Jan 10.6

28 Jan 11.02

28 Jan 20.3

28 Jan 27.1

28 Jan 33.5

29 Jan 1.16

30 Jan 2.08

31 Jan 2.16

01 Feb 1.16

02 Feb 0.34

02 Feb 3.96

02 Feb 1.96

03 Feb 0.6


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Iron level (ppm) in OVHD reflux accumulator during second on-stream cleaning of E-11-101

A/B/D (E-11-101C was idle)

Date

(Year’08)

Iron

(ppm)

Iron level during on-stream cleaning of E-11-101A

Iron level during on-stream cleaning of E-11-101B

Iron level during on-stream cleaning of E-11-101D

23 Mar 2.08

24 Mar 0.56

25 Mar 0.26

26 Mar 1.5

27 Mar 0.94

28 Mar 1.24

29 Mar 1.67

29 Mar 1.68

30 Mar 2.8

30 Mar 36.2

30 Mar 26.5

30 Mar 17.5

31 Mar 2.3

31 Mar 2.77

01 Apr 31

01 Apr 19.9

01 Apr 18.3

01 Apr 16.2

02 Apr 6.9

02 Apr 5.35

02 Apr 6.51

03 Apr 2

03 Apr 71.2

03 Apr 16.8

03 Apr 19.5

04 Apr 1.8

05 Apr 0.73

06 Apr 1.3

07 Apr 0.6

07 Apr 0.52

08 Apr 1.7

08 Apr 1.5

09 Apr 0.84

09 Apr 0.66

10 Apr 0.44

10 Apr 0.41


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Flash

Drum

Preheat

Exchangers

Fired

Heater

Overhead

Accumulator

Sour Gas

Compressor

Accumulator

Sour Gas to Gas Handling

Naphtha to Storage

ARDS/RCD/Storage

LPG to Gas Handling

Atm. Resid. to

Crude

Feed

Preheat

Exchangers

Preheat

Exchangers

Stabilizer

Feed Drum

Cru

de

Fracti

on

ato

r

Cold Naphtha RefluxKerosene

Stripper

Kerosene to

HTU/Storage

Diesel

Stripper

Diesel to

HTU/Storage

Steam

Two Stages Desalters

OVHD CrudeExchangers

Fig. 1: Process Flow Diagram of Crude Unit

Kerosene (3)

(3)


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Fig. 2: OVHD Exchangers Wash Water Cleaning System

Fig. 3: Wash Water Facility for Each Exchanger

OVHD vapor

from T -11-101

Wash

Water

Wash

Water

Wash

Water

Wash

Water

E-11-101D

E-11-101C

E-11-101B

E-11-101A


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Before Implementation of On-Stream Wash

Water Cleaning (Mar’07 – Nov’07) After Implementation of On-Stream Wash

Water Cleaning (Jan’08 – Oct’08)

Fig. 4: Comparison of Fouling Condition When It Was Opened in Nov’07 With Oct’08

FOULING

FOULING LOOSE SCALES

FOULING

FOULING


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Fig. 5: Tubes Showing Stencil Marks Before Hydro Blast Cleaning In Oct’08 Which Indicates

That Online Cleaning Is Found To Be Effective In Removing Scales/Deposits


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Documents

CDU OverHead Corossion MAB