Poster PO-25

PO-25.1

IMPACT OF SEA COOLING WATER (SCW) MANAGEMENT AND HEAT EXCHANGER DESIGN ON TUBE FAILURE IN ONCE-

THROUGH SEA COOLING WATER SYSTEM

L’IMPACT DE LA GESTION DE L’EAU DE REFROIDISSEMENT DE MER (SCW) ET LE DESSEIN DE L’ECHANGEUR DE

CHALEUR SUR UNE PANNE DE TUBE DANS UNE SYSTEME OU L'EAU DE REFROIDISSEMENT DE MER PASSE

QU’UNE FOIS A TRAVERS

Kua Siong Yan Wan Badrul Hisham B Wan Abdullah

Baharom B Che Isa Technical Service Department

Malaysia LNG Snd. Bhd.

ABSTRACT

The Petronas LNG Complex in Bintulu, Malaysia comprises the MLNG, MLNG Dua and MLNG Tiga plants. MLNG (1/2/3) has a once-through sea cooling water (SCW) system that uses about 160,000 m3/hr of seawater and all the exchangers are fed from the bottom. MLNG Dua (4/5/6) is a partly seawater cooled. The seawater cooling system for train 4,5 and 6 were connected to the existing sea water system of train 1,2 and 3 respectively. Due to head limitations of the sea cooling water system, all the MLNG Dua exchangers are designed to be fed from the top with the total sea cooling water consumption of about 50,000 m3/hr. MLNG Tiga (7/8) is fully air-cooled plant.

Since start up of MLNG Dua in 1996, the propane condensers and sub-coolers (aluminum brass low finned tubes) has experienced a number of tubes failures in these condensers. Tube leaks was considered to be due to air ingress through the sea cooling water standpipes on the 112” Sea cooling water supply lines and in combination with insufficient water pressure levels in the system and varying internal tube diameters. Combination of these factors leads to erosion-corrosion damage as observed in particular at tube inlet and at the internal diameter transitions at the unfinned sections of the tubes under the tube support plate. Severe fouling leading to under-deposit corrosion was also experienced in some of the tubes.

Measures taken to improve the SCW system includes, throttling the main outlet valves of the condensers to prevent air ingress into the SCW standpipe, installation of 36” crossover line between the three 112” SCW line and improvement in chlorination of the sea cooling water to prevent biofouling. Other measures taken to improve the reliability of the condensers are, re-tubing with equal diameter tubes, installation of mesh screen at the tube sheet, passivation of aluminum brass tubes and installation of soft anode. A detail Computational Fluid Dynamic (CFD) study was also carried out to see the impact of the hydraulics due to SCW fed from the top of the exchanger. This paper discusses the impact of sea cooling water management and heat exchanger design on tube failure in the once-through sea cooling water system.

Poster PO-25

PO-25.2

RESUME

Le complexe de LNG de Petronas à Bintulu, Malaisie contient le MLNG DUA, le MLNG et les usines de MLNG Tiga. MLNG (1/2/3) a une système ou l'eau de refroidissement de mer passé qu’une fois-à travers (SCW) qui utilise environ 160.000 m3/hr d'eau de mer et tout les échangeurs sont alimenté du fond. Le MLNG DUA (4/5/6) est en partie refroidi avec l'eau de mer. Le système de refroidissement d'eau de mer pour le train 4.5 et 6 était reliés au système existant d'eau de mer du train 1.2 et 3 respectivement. En raison des limitations principales de l'eau de mer, tous les échangeurs de DUA de MLNG sont conçus pour être alimentés du dessus avec une consommation totale de l'eau de refroidissement de mer d'environ 50.000 m3/hr. MLNG Tiga (7/8) est entièrement refroidie par air.

Depuis le mis en train de MLNG DUA en 1996, les condensateurs de propane et les secondaire-refroidisseurs (des tubes à ailettes en laiton-aluminium) ont souffert un certain nombre de panne des tubes dans ces condensateurs. Des fuites de tube ont été considérées d’etre le résultat de l'entrée d'air par les colonnes de l'eau de refroidissement de mer sur les 112” Lignes d'approvisionnement de l’eau de refroidissement de mer en combination avec les niveaux de pression insuffisants et une difference du diamètre interne de la tube La combinaison de ces facteurs mène aux dommages de érosion-corrosion comme observé en particulier à l'admission de tube et aux transitions internes de diamètre de tube sous la plaque de maintien aux sections ou les tubes ont pas des ailettes Encrassement grave qui mène a corrosion sous depot a été également constaté dans certains tubes.

Les mesures prises pour améliorer le système de SCW incluent: étranglement des soupapes d'échappement principales des condensateurs pour empêcher l'entrée d'air dans la colonne de SCW, installation d’une ligne de croisement de 36” entre les trois 112” SCW lignes et amélioration de de la chloruration de l'eau de refroidissement de mer pour empêcher l’encrassement biologique. D'autres mesures prises pour améliorer la fiabilité des condensateurs sont, retubage avec les tubes égaux de diameter, installation d’une tamis à mailles sur la feuille de la tube, la passivation des tubes laiton-aluminium et l'installation d’une anode molle .Une étude en detail de “Computational Fluid Dynamic (CFD)” a été également effectuée pour voir l'impact des hydrauliques sur SCW alimenté du dessus de l’échangeur. Cet article discute l'impact de la gestion de l'eau de refroidissement de mer (SCW) et le dessein de l'échangeur de chaleur sur une panne de tube dans une système ou l'eau de refroidissement de mer passe qu’une fois à travers.

INTRODUCTION

The Petronas LNG Complex in Bintulu, Malaysia comprises the MLNG, MLNG Dua and MLNG Tiga plants. The total capacity of the three plants is able to produce 23 million tonnes per annum of LNG. MLNG (1/2/3) has a once-through sea cooling water (SCW) system that uses about 160,000 m3/hr of seawater and all the exchangers are fed from the bottom. In the early ‘90’s, MLNG was extended with trains 4,5 and 6 (MLNG Dua). These trains are partly seawater cooled and air cooled plant. The seawater cooling system for train 4,5 and 6 were connected via the existing sea water system of train 1,2 and 3 respectively. Crossover piping had been made as well to ensure supply of sea-water to all the new trains in case an old train would be shut down. No additional pumps may have been made. Due to head limitations of the sea cooling water system, all the MLNG

Poster PO-25

PO-25.3

Dua exchangers are designed to be fed from the top with the total sea cooling water consumption of about 50,000 m3/hr. MLNG Tiga (7/8) is fully air-cooled plant.

The seawater intake station, is about 3.6 km away from the process area, contains a series of screens to remove debris and jellyfish (the final screen having 1.7 mm opening) and 4 submerged pumps for each module of MLNG. Normally, 11 pumps are in operation with total SCW consumption of about 200,000-210,000 m3/hr. Approximately 70 % of the sea cooling water is supplied to trains 1,2 and 3, 20% to train 4, 5 and 6 with the balance to the condensers of the steam turbine and the Common Facilities.

The seawater is chlorinated before it is pumped via three 112” main cooling water lines to the MLNG and MLNG Dua. Sea cooling water are routed to MLNG Dua via three 72” cooling water lines to Module 4/5/6. Each Module 4,5 and 6 has six seawater coolers:

• 3 propane condensers, each with 2 passess, 12.5 m long with 2645 low finned 1” tubes with total heat duty removal of 122 MW each

• 1 propane subcooler with 2 passes, 7.5 m long with 2636 low finned ¾” tubes with total heat duty removal of 9.4 MW each

• 2 smaller coolers with total heat duty removal of about 1-2 MW each

The tubes are of Al/Brass (C68700), tube sheets and header boxes are of Al-bronze D (C61400) clad. Steel anodes are installed inside the header boxes.

The line-up of the 4 large exchangers was compromised for available pressure with the water flow from the top down rather than from the bottom up. The cooling water flows are controlled using butterfly valves in the outlets of the heat exchangers. The combined outlet flow of each train is discharges via a common butterfly valve into the train’s outlet channel via a concrete weir. The weir is designed to prevent any point in the cooling water system from falling below atmospheric pressure under normal flow conditions

HISTORY OF TUBE FAILURE IN MLNG DUA’S PROPANE CONDENSERS

After the start-up of trains 4, 5 and 6 a chain of leakage events has occurred in the Propane condensers for the three modules. Investigations has revealed the key source of problem leading to the tubes leak of the propane condensers are:

Leaks related to high water tube velocity, air ingress via the SCW standpipe and varying internal tube diameter leading to erosion corrosion

Leaks related to severe fouling caused by barnacles, foreign material and anode material leading to under-deposit corrosion

Leaks related to inherent design that is, the top-down design leading to erosion corrosion and under-deposit corrosion

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CORRECTIVE MEASURES TAKEN

Improve water tube velocity

Operating within the design tube velocities of 1.5 m/s, by throttling the common outlet valve (MOV 111) of the three condensers to avoid erosion corrosion. The flow distribution over the three Propane Condensers has been measured by using portable ultrasonic flow meter to ensure that the flow through the heat exchangers are within the acceptable tube velocity limits.

Figure 1

Air ingress

Maintaining good discharge pressure at the SCW pumps to avoid possibility of air ingress (photograph 1&2) via the 112” SCW standpipe. One 36” common crossover header (refer to photograph 4) was installed at the discharge of the SCW pumps to balance the SCW between the three 112” SCW lines.

Sketch on the piping configuration of propane condensers, E-2415 A/B/C

E-2415 A

2.219 m

3.2 m

6.5 m0.3 m

0.3 m

3.3 m

8.819 m

Pressure : 1.55 - 1.65 barg

2.6 m ID

38 "

E-2415 BE-2415 A 3.2 m

6.5 m0.3 m

3.3 m

4.362 m

8.819 m

Pressure : 1.55 - 1.65 barg

Pressure : 1- 1.2 barg

2.6 m ID

38 "

32 "

E-2415 C

OUTFALL

60"38"

60" x 48" x 38"

153 m

52"

5.4 m38"

Remark:1. Piping for E-2415 B/C has the same dimension as per E-2415 A2. Seacooling water flow to each E-2415 A/B/C : 3000 m3/h - 4000 m3/h

5 m 5 m

5 m 5 m

38" x 34" 38" x 34" 38" x 34"

34" x 32" 34" x 32" 34" x 32"

MOV111

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Photograph 1 Photograph 2

Photograph 3 Photograph 4

Unequal tube diameter

Generally, the internal surfaces of the tubes were covered with the protective brown iron oxide layer, but it was observed that the erosion-corrosion damage occurred, in particular at tube inlet and at the internal diameter transitions a the un-finned sections of the tubes under the tube support plates (refer to photograph 5). It was decided to re-tube the condensers with equal diameter tube in order to resolve this erosion corrosion problem.

Photograph 5

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Fouling by barnacle

Chlorination control was also improved to reduce marine fouling, which have caused partial blockages of the tubes. Due to aging and under performance of the existing old chlorination unit, the existing chlorination unit was revamped to a new bipolar cell. Close monitoring and tight control of the free residual chlorine at the intake station of the SCW as well as at the outfall of the SCW (0.2 –0.4ppm) to reduce marine fouling.

Photograph 6 Photograph 7 Fouling by foreign material

Tube blockages by debris such as anode material and other extraneous material, which can cause low or no flow are avoided by installation of screens (refer to photograph 9) at the inlet tube sheets or individual heat exchangers.

Photograph 8

Five (5) blocked tubes by anode debris. Entry into bottom five (5) tubes caused by openingin top bottom left m esh screen.

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Photograph 9

Fouling by anode material

Convert steel anode to soft iron anode. Steel anode (Photograph 10) shows the typical exfoliation type of rust that is often seen on iron anodes. This corrosion product is a cause of fouling, but also prevent the ferrous ion to dissolve in the water. By using soft iron (Armco iron) anodes to obtain even dissolution of the anodes without rust built up leading to under-deposit corrosion.

Photograph 10 Mitigate inherent design problem

Beside all the corrective measures, MLNG also engaged Cheng Fluid System to do a Computational Fluid Dynamic (CFD) calculation, to sees possibility in improving the system by having a better understanding on the impact of the hydraulic leading to mal-distribution inside the propane condenser channel box. The computer analysis studies of the flow through the inlet piping and water-box into the condenser shows that there are fluid flow turbulence problems based on the current design. These CFD results showed that the problem with the current piping design is that the fluid must flow through two closely coupled elbows; through a reducer; through a sudden expansion; and then having to make a 90 degree turn in order to find its way to the tube sheets. Below shows the

Clean condition of mesh screen viewed from north of 4E-2415A

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erosion corrosion problem encountered at the inlet elbows as well as the bottom section of the tube sheets matches the CFD results.

Resulting from this finding it was decided to mitigate the problem by installing the bottom section of the tubes in the first pass with Copper Nickel tubes 70/30 instead of Al/Br which are more resistance to high velocity and erosion effect. The first module to incorporate this installation will be done in Module 4 at September 2005.

Figure 3

Photograph 11

Figure 4

Photograph 12

Figure 2

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Figure 5 Photograph 13

FACTORS TO CONSIDER IN REDUCING SCW HEAT EXCHANGER FAILURES

(1) Water Quality

(a) Cleanliness – Debris and sediment (such as sand and mud) that are pssed through or around the screens and filters can abrade the protective film on copper-alloy tubes and creates downstream turbulence, which can cause pinholes in copper alloy tubes.

(b) Dissolved Oxygen and Sulfides – Copper-alloy tubes do not stand up well in severely polluted water which dissolved oxygen has been consumed in the decay process and sulfides are present.

(c) Residual Chlorine- Both copper-alloy and stainless steel tubes have performed well in water containing up to 2 ppm residual chlorine, and have failed in heavily chlorinated water. Although it is usual to keep the residual chlorine at about 0.3-0.4 ppm at the inlet tubesheet.

(d) Acidity – In aerated water of pH less than 5, a protective film does not easily form on copper-alloy tubes, so they corrode and thin rapidly. For Al/Br tubes, which tend to corrode under highly alkaline conditions.

(e) Temperature - A protective film readily forms on copper-alloys in warm water, but forms very slowly in cold water.

(f) Scaling Tendency – Copper-alloy and stainless steel tubes perform well in both hard(scaling) and soft (nonscaling) water.

All the above factors needs to be considered for the selection of tube metallurgy of the exchangers.

(2) Operation and Maintenance

(a) Passivation of new tube - To protect the copper-alloy tubes surfaces by ensuring the formation of the protective iron oxides layer, it is recommended that all new tubes should undergo initial passivation treatment with Ferrous Sulphate (FeSO4.7H 2O).

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(b) Preventive maintenance - Preventive maintenance is also carried out every year when all the exchangers in one module are hydro-jet cleaned. Eddy current tests are carried out on selected exchangers and tube which show > 30 % loss in wall thickness are plugged. Also, if an exchanger is to be left full for more than 2 or 3 days, water should be pumped through it once a day to displace the stagnant water. If an exchanger will be down for at least a week, it should be drained and blown dry.

(3) Exchanger Design

The principal design factors that influence tube performance are:

(a) Water Velocity – At velocities of less than 1 m/s, sediment deposit, debris buildup and biological fouling in and on tubes can be excessive, which can cause copper-alloy and stainless-tube tubes to fail prematurely due to underdeposit corrosion.

(b) Tube Diameter – Tubes of large diameter are preferred for heat exchangers because any solids that pass through the screens will also flow through the tubes.

(c) Shape(i.e once-through or U-bend) – Because U-tubes bundles are difficult to clean. U-tubes are particularly prone to such corrosion if sediment and debris are not removed from their bends.

(d) Orientation – Although the TEMA standards recommend that exchangers be installed level, exchangers should really be slope slightly so that they will drain completely when shut down, avoiding corrosion where tubes sag between support plates and retain water even after being drained.

(e) Venting – Exchangers are normally fitted with vent cocks so they can be purged to clear gas pockets. Condensers, particularly when chlorine used as a biocide, tend to suffer corrosion when gases are not vented.

(f) Tubesheet Material – Correct matching of tubesheet material with tube material is necessary in order to avoid possibility of galvanic corrosion.

(g) Channel(waterbox) Material- Corrossion product from waterbox due to wrong choice of material matching between the waterbox and the tube and tubesheet can cause adverse galvanic corrosion.

(h) Channel Inlet Arrangement – Uneven flow, restricted water passages and poor inlet-piping entry arrangements have caused numerous failures of copper-alloy tubes. Basically, the entry and flow pattern in both large and small channels should distribute the water uniformly to all tubes with as little swirling as possible.

CONCLUSION

Most unexpected failures of heat exchangers can be traced to some of the above factors that had not been fully taken into account during the design and operation and maintenance phase.

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REFERENCES

1. Arthur H.Tuthill “The right metal for Heat Exchanger Tubes” Chemical Engineering/January 1990.

2. Cheng Fluid Systems, Inc, USA”CFD study report for MLNG Dua Propane Condenser” 2002.