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SAN - Surveying Marine Engine Cooling & Salt Water piping systems in ships

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Surveyor Advice Note (SAN) Maritime and Coastguard Agency

SURVEYOR ADVICE NOTE

Document number: SAN 29

Revision: 02

Surveying Marine Engine Cooling & Salt Water piping systems in ships

Date: 22 March 2013

Target document:

Instructions to Surveyors Fishing Vessels MSIS27

Distribution HQ and Marine Offices Class Societies, Recognised Organisations, Red Ensign Group

Expiry date: When included in targeted document Key Changes Original document All amendments are highlighted in yellow. Background The recent total loss of two fishing vessels was attributed to the failure of a seawater pipe in the cooling water system. This subsequently caused the engine room to flood and the vessel to founder. Subsequent recommendations from the MAIB included the revision to the Instructions to Surveyors with respect to the survey of sea water pipework. Tools:

Torch Hammer surveyors type with safety glasses Screw driver flat blade, 150 mm by 6 mm diameter Small magnet

Trace system and determine material of pipework, valves and coolers, check with magnet. - Non-ferrous and stainless steel 316 are not magnetic Determine if vessel has long periods of inactivity or operates from polluted harbours. Cooling Water and Other Seawater Systems All new or replacement installations of sea water piping and fittings for cooling water systems should be of aluminium bronze, cupro-nickel or similar corrosion resistant material. According to MSN 1770 Heavy wall mild steel pipe for cross vessel inlet mains may be used, provided that the internal diameter is 100 millimetres or greater and the pipe is galvanised internally after all fabrication work is complete. To comply with SEAFISH Construction standards the internal diameter is a minimum of 150mm or 50% greater in cross-sectional area than the largest branch pipe


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directly attached to it. The pipe is to be a minimum of schedule 80 and is to be galvanised internally after all fabrication work is complete. Note that 150mm Schedule 80 pipework has a wall thickness of 11mm; 100 mm diameter Schedule 80 has a wall thickness of 8.56mm Non-ferrous pipework Check non-ferrous pipework is secure and supported without being strained. If system has bends of radius less than three diameters remove bends for examination. Check for erosion. If system has been inactive remove sections of pipe for internal examination. Check internal surface for formation of oxide layer. If valves have ferrous body remove from system for examination check for corrosion by graphitic corrosion (sometimes referred to as graphitization. Graphitic corrosion is unique to grey cast iron and is characterised by corrosion of the iron matrix leaving a residue of graphite and iron corrosion have similar dimensions to the original casting. The residue is black and soft and easily cut. As Graphite is a strong cathode it can accelerate the corrosion of copper alloys and stainless steel. Heat exchangers check end plate material and if zinc or iron anodes fitted. When re-fitting valves check that no strain is placed on pipework. Ferrous pipework Check if galvanised or stainless steel. Note:

Black pipe, i.e. uncoated internally, is to be removed as will not last through survey cycle.

Pipes with bandage type repairs are to be replaced. New pipes to be fabricated and once all hot work has been completed and the

pipe shown to fit, it is to be sent for hot dip galvanising.


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Galvanised pipe to be examined externally and rust spots investigated with screw driver, particularly if fitted adjacent to non-ferrous fittings. If system has bends less than three diameters remove bends for examination.

If system two or more survey cycles old, all ferrous pipework to be removed for examination as the internal galvanised coating is expected to have wasted away. If the wall thick is less than 9mm for 150mm diameter pipe or 7.6 for 100mm diameter pipe, the pipe is to be replaced.

wall thickness based on Lloyds register recommendations for heavy wall pipe

6

7

8

9

10

11

12

13

0 50 100 150 200 250

diameter

wal

l thi

ckne

ss

If system has been inactive remove sections of pipe for internal examination Check electrical bonding and anodes fitted to sea chest etc.


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If valves have ferrous body remove from system for examination check for corrosion by graphitic corrosion. Heat exchangers check end plate material and if zinc or iron anodes fitted. Stainless steel pipe check for corrosion at crevices e.g. flange connections. Ship side valves If valves have ferrous body remove from system for examination check for corrosion by graphitic corrosion. Check valve seats for erosion. Check valve are secured by nuts which are not corroded.

In order to make sure the valves operate correctly when needed, as part of a planned maintenance system, the Engineer/Driver should open and close them at regular, say monthly, intervals. Each sea inlet valve should be fitted with a positive means of closure from an accessible position. An accessible position is one that is not rapidly submerged in the event of a leak in the engine room. Such a location is when the main engine and the sea inlets are located in a restricted volume between to wing tanks which are not full height of the engine room space. The valve should be capable of being operated from the uppermost platform in the engine room below the freeboard deck. Report survey on MSF 1324. Revised copy attached. Author Keith Patterson Job Title Principal Marine

Consultant Surveyor Authorised by Paul Coley Job Title Assistant Director

Ship Standards


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MSF 1328 / 13/xx

REPORT OF HULL CONDITION OF A FISHING VESSEL NAME:- OFFICIAL NUMBER:- FISHING LETTERS AND NUMBER:- DATE OF SURVEY:- PLACE OF SURVEY:- HULL CONSTRUCTION: STEEL / WOOD / GRP (delete as appropriate) GENERAL REPORT ON PRESENT CONDITION AND APPEARANCE OF VESSEL AND REPAIRS NOW OR RECENTLY EFFECTED. HULL EXTERNAL:- HULL INTERNAL:- TAILSHAFT:- RUDDER:- PROPELLER:- CONDITION OF SEA WATER PIPEWORK (inlet and discharge):- Materials Repairs Now or Recently Effected CONDITION OF SEA WATER VALVES:- Materials Repairs Now or Recently Effected DRAFT MARKS:- SURVEYORS SIGNATURE:- . DATE:- .


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Background Information. Everyone knows about the effect of corrosion on a ships hull, but few people consider the effect of corrosion on piping. Pipes pose a hidden danger, a danger that is often neglected. Marine Engine Cooling There are three methods employed for water-cooled marine diesel engines: direct, heat exchanger and keel cooling. Direct cooling of the cylinders and heads by seawater is unsatisfactory, because the engine will run too cold and the sea-water will eventually ruin the cylinder block and heads. Keel cooling removes the need for salt water pipework inside the vessel, but the need for pipework external to the hull is a limitation. This can be countered by using box coolers but corrosion problems have been identified with this as per the DNV circular attached at Annex 1 Heat exchanger cooling is the most common method, the seawater being isolated in components which can be designed to withstand its corrosive affect. The closed fresh-water circuit can be thermostatically controlled so that the engine operates at its design temperature. Like corrosion on a ships hull, heat exchanger cooling involves two different metals in contact with seawater. Heat exchanger Engine cooling systems Water carried in pipes is used to cool machinery. The main engine is cooled by two separate but linked systems: an open system in which water is taken from and returned to the sea (sea-to-sea) seawater cooling, and a closed system where freshwater is circulated around an engine casing (freshwater cooling). Freshwater is used to cool machinery directly, whereas seawater is used to cool fresh water passing through a heat exchanger. Many engine room systems also use sea water to cool oil (engine, gear box and hydraulic), and refrigeration systems. The particular feature of any engine room cooling system is continuous fluid flow. Fluid in motion causes abrasive corrosion and erosion. To reduce the effects of turbulent flows, seawater systems incorporate large diameter pipes, the ends of which open to the sea through sea chests where ship side valves are fitted. If a seawater cooling pipe bursts, both suction and discharge valves will have to be closed to prevent engine room flooding. Discharge valves should be of the screw down non-return type or fitted inboard with a non-return valve which should be accessible for maintenance.


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In order to make sure the valves operate correctly when needed, open and close them at regular, say monthly, intervals. The valve should be capable of being operated from the uppermost platform in the engine room. Ensure that all engine room personnel are familiar with the location and isolation of the main sea inlet valves and over-boards. Seawater pipes are manufactured from materials such as galvanised steel, copper, copper alloys and aluminium bronze (Yorcalbro 70% copper, 28% zinc and 2% aluminium). Seawater pipes fabricated from Yorcalbro generally have a sacrificial section made from mild steel to ensure that galvanic corrosion attacks only the sacrificial pipe. Sacrificial sections as well as sacrificial anodes are also designed to limit galvanic corrosion action from metallic material other than Yorcalbro. These sections of pipe should be regularly inspected and renewed. Freshwater cooling pipes are generally made of mild steel. These systems are treated with anti-corrosive chemicals and should be tested regularly using the chemical manufacturers supplied kits to ensure that the water treatment is always at its most effective. Types of Corrosion General Corrosion of a metal is a chemical or electro-chemical reaction between the metal surface and its environments. Sea water corrosion involves reactions where the metal surface is transformed into metal oxides or hydroxides (like rust). The rate of sea water corrosion depends on several factors like environmental and metal impurities, temperature, oxygen access, metallic contact (conductive) with dissimilar metals, sea water flow velocities, type of metals, surface protection, stresses, etc. The most commonly appearing types of corrosion in piping systems exposed to sea water are discussed below: Erosion Corrosion This type of corrosion occurs when there is a simultaneous erosion and corrosion as a result of rapidly flowing liquid. Erosion corrosion is characterized by attack like small pits with bright surfaces free from corrosion products. These pits often have the form of a horse shoe with the nib pointing in the current direction. The following maximum design water speeds are given for guidance: Copper 1.0 m/sec Galvanised steel 3.0 m/sec Aluminium Brass 3.0 m/sec (78% Cu; 20% Zn; 2% Al.) 90/10 Cupro-nickel 3.5 m/sec 70/30 Cupro-nickel 4.0 m/sec


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BS MA 18 recommends reduced velocities for smaller bore pipes, typically for all materials 1.5 to 2.0 m/sec for a 50 mm bore pipe. Erosion corrosion may occur where the velocity of liquid is too high. Most exposed are places where there are effects of turbulence, e.g. joints, bends etc. Bends formed from straight pipes shall be as large as possible with a minimum radius on the centre line which is not less than three times the outside diameter. Bends of less than this radius are to be especially examined. The corrosion rate will accelerate if the liquid contains gas bubbles and/ or solid particles. Sea boxes are to be provided with vents at the highest point. Strainers provided after the ship side valve remove solid particulars and reducing the flow velocity allowing entrained particles to drop out. Galvanic Corrosion Galvanic corrosion occurs when metals of different potential are connected together and simultaneously exposed to an electrolyte thus causing galvanic forces to be set up. The electrolyte may for instance be sea water. If a brass valve is connected to a steel pipe in a sea water circulating system, the steel pipe, which has a lower potential than the brass valve, will corrode more rapidly than without such a connection. This is a typical example of galvanic corrosion. The extent of galvanic corrosion will depend upon the dimension of the surface area of the more noble/cathodic metal in relation to the less noble base/ anodic metal. If the surface area of the less noble metal is smaller than that of the more noble metal surface, the corrosion rate of the less noble metal will be increased. In case of opposite surface area relations the corrosion rate will be reduced.


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Metals are sorted into a series based on which is the most cathodic:

Potential (Millivolts) Noble (Cathodic) - protected Graphite + 270 mV

Titanium + 20 mV

Ag/AgCl reference Cell 0 mV 18 chromium/8 nickel stainless steel (passive);. - 30 mV

Nickel Copper alloy (Monel) - 60 mV

18 chromium/2 nickel stainless steel

70/30 Cupro-nickel Nickel-aluminium-bronze Aluminium-silicon-bronze 90/10 Cupro-nickel (Kunifer10) Gunmetal Phosphor-bronze

- 180 mV

- 200 mV

- 260 mV Aluminium-brass Copper Rolled naval brass

- 300 mV - 310 mV - 330 mV

High tensile brass

Tin Lead Lead/tin packings, solders, etc

- 310 mV

18 chromium/8 nickel stainless steel (active); Cast irons Carbon steel

- 390 mV

- 630 mV - 630 mV

Cadmium

Aluminium - 700 mV

Zinc - 1050 mV

Base (anodic) - corroded Magnesium - 1600 mV

Notes: 1 Alloys listed in the same table cell are equi-potential and may be used together without special precautions. 2 The corrosion resistance of stainless steel arises from a 'passive', chromium-rich, oxide film that forms naturally on the surface of the steel. Although extremely thin, this protective film is strongly adherent, and chemically stable (i.e. passive) under conditions which provide sufficient oxygen to the surface. This 'normal' condition is the passive state. The key to the durability of the corrosion resistance of stainless steels is that if the film is damaged it will normally self repair (provided there is sufficient oxygen available). However, under certain conditions, the passive state can be broken down, resulting in corrosive attack. If damaged, the film will normally repair itself. If the film is destroyed the surface is said to be in the active state.


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Selective Corrosion Typical for this type of corrosion is that the structural elements of an alloy are corroding at different rates. The most typical example of this is dezincification of brass where the zinc content is removed by seawater or hot fresh water leaving behind a porous sponge of copper. The characteristic appearance of a de-zincified brass is the coppery colour of the affected area. Piping System Materials If it is necessary to use pipe or other cooling system components of more than one material, avoid letting the dissimilar metals touch, even by mutual contact with an electrically conductive third material. Corrosion will be much more severe if a flow of electrons is able to pass freely from one of the metals to the other. Bonding of pipes and fittings can be provided so that some protection can be provided by anodes (aluminium, zinc or magnesium) fitted in sea boxes and to the outside of the hull. The material of all the seawater piping should be the same, whenever practical. If parts of the seawater piping, made of different metals, make contact with each other, one of the metals will corrode, sometimes very rapidly. The materials will corrode according to their position in the galvanic or electromotive series. The resistance to corrosion by sea water depends upon the formation of a thin protective on the metal surface which is exposed to the sea water. The mostly consists of oxides. The composition of the layer varies with each alloy and for each environment. Provided the film is maintained intact the corrosion rate will be the minimum for that metal. As the films are very thin they are easily removed by mechanical abrasion and in certain circumstances by erosion of sea water. Turbulent flow can cause impingement attack and corrosion at this spot can be high. Non-ferrous Of the copper- nickel alloys; CuNi 90/10 is recommended for piping carrying seawater. The cost of such piping makes its use unusual for all but the most critical systems. Aluminium brass is acceptable provided the fabrication and installation is especially considered. The seawater corrosion resistance offered by copper-nickel alloys results from the formation of a thin, adherent, protective surface film which forms naturally and quickly upon exposure to clean seawater. The film is complex and predominantly comprises of cuprous oxide, often containing nickel and iron oxide, cuprous hydroxychloride and cupric oxide. The film can be brown, greenish brown or brownish black.


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Initial exposure to clean seawater is crucial to the long-term performance of copper-nickel. The initial film forms fairly quickly over the first couple of days but takes 2-3 months to fully mature. Copper alloys can suffer rapid corrosion if exposed alternately to sulphide polluted seawater and aerated seawater, sulphide films being non-protective. These alloys may not, therefore, be a good choice in polluted harbours. If the seawater velocity/turbulence in a system is excessive 90/10 copper-nickel will suffer impingement attack (corrosion/erosion). Much has been made of this limitation, but in practice it is rarely a problem as the material will normally handle without difficulty, the recommended design velocities. Bends should have a smooth contour and be three times the diameter of the pipe. Welding (MIG or TIG) is the best method of permanent joining. If the composition of the parent metal cannot be matched by available filler wire material, the latter should always be more cathodic than the parent metal. The profile of the weld bead which is in contact with salt water should be smooth and not protrude enough to cause turbulence. The completed lengths of pipework should be in the annealed or stress relieved condition before installation. Installation should be carefully carried out so that no stress is applied to the pipe. In particular length must not be pulled up by flanges nor bent to accommodate badly positioned supports. Supports should be carefully arranged to avoid distortion of the pipe and to damp any fluctuating stresses on the pipe due to vibration. Flanges must be carefully aligned. Pipes should be visually examined and not hit with a hammer as they are thin walled and a dent will lead to rapid surface flow which will cause impingement attack. Ferrous Black iron pipe is not often used in seawater service as replacement should be planned every two or three years. Steel is sensitive to galvanic corrosion by virtue of inclusions in its manufacture, by mill-scale on its surface, or by extraneous substances such as welding slag and copper alloys with which it may be in contact. In these circumstances local corrosion can be rapid. The main advantages of steel pipelines are the low initial cost compared with other materials, the ready availability of pipes and components and the existence of widely used and accepted welding procedures. However, steel corrodes comparatively rapidly in seawater, at reasonably predictable rates (tending to increase as flow rates and oxygen content increase and as the temperature rises). A steel system, though comparatively cheap, will be relatively large and heavy and will have a short life. Failures may occur within a year or two and complete replacement may well be needed within five years. Steel pipes are to be hot dip galvanised are fabrication. The rate of corrosion of zinc in seawater is somewhat less than that of steel, but galvanised coatings have a


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limited life and in almost all applications, a galvanised steel piping system would need to be replaced one or more times during the life of the installation. Painting the pipes after hot work is not acceptable. Such pipes should be examined internally at renewal survey to check that the coating remains intact. Weld areas are to be especially examined. The pipework after heat exchangers are to be examined also as hot sea water is more corrosive and galvanic action is prone to occur down stream of the more noble metal. As an example an ocean-going ship, 6 years old, the original galvanized steel pipe used in the sea water (fire and bilge) circuit started to develop pinhole leaks at the welds. The shipyard used CuNi (70/30) for the main seawater crossover pipe and for the pipes feeding cooling water to all engines, but opted to save money by using galvanised on the bilge/fire system. Ferrous pipe work can be hammer tested as well as visual examination. A pointed implement should be used to investigate any obvious rust spots. Stainless Steel Usually regarded as the Marine grade, 316 Stainless steel has excellent corrosion resistance when exposed to a range of corrosive environments compared to other formulations. It is not resistant to warm sea water as warm chloride environments can cause pitting and crevice corrosion. Above approximately 60 deg C it is also subject to stress corrosion cracking. A passive layer formed on materials that react with oxygen may be attacked by chlorine ions, increasingly so in water with low oxygen for example stagnant water or low velocity flow. Low flow rates also do not prevent marine growth. Cast iron sea valves. Such valves are vulnerable to corrosion by graphitization, graphite being an alloying element and more noble than steel. The damage can be easily overlooked because the hole is usually disguised by the graphite infilling. It is important to check non-ferrous metals for corrosion because graphite is noble e.g. valve seats and discs. The nuts and studs fixing the valve to the shell are to be examined for corrosion. Heat exchangers Heat exchangers should be positioned in the engine in an accessible position for inspection and maintenance. Conventional heat exchangers are of the shell and tube type is normally made from non-ferrous materials. Header ends may be coated steel and fitted with iron anodes to prevent corrosion and aid the formation of the passive oxide layer. Uncoated header ends will have zinc anodes fitted.


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Plate type heat exchangers are fitted with titanium plates which is the most noble/cathodic of all materials. All pipework and valves are to be especially examined, normally when dismantled for cleaning. References: BS MA 18 Salt water piping systems in ships Frederick and Capper Materials for Marine Machinery Todd selecting Materials for Sea water Systems, Marine Engineering practice Volume 1 ,part 10. Copper Development Association Materials for Seawater Pipeline Systems CDA Publication TN38, 1986 DNV Erosion and Corrosion in Piping Systems for sea water; Guidelines No 15 August 2004 MGN 190 (F): Fishing Vessels - The Premature Failure of Copper Pipes in Engine Cooling Water Systems.

Course of eventsExperience feedback in recent years has highlighted a number of

fishing and offshore support vessels, which have reported sea

water leakage at the top of sea chests, located in the engine

room. The sea chest holds a box cooler and the leakage has been

experienced at the bolt flange between the box cooler and the

sea chest. As a result a number of vessels have needed to be

taken into dry dock for unscheduled repairs.

Extent of damageIn a number of cases, severe corrosion damage has been

observed in the carbon-steel mounting flange on top of the sea

chest. In some cases the flange was found to be partially wasted

and in a few cases the bolts connecting the box cooler to the bolt

flange were also heavily corroded. Additionally, the aft shell plat-

ing of a sea chest itself was found to be nearly corroded through,

in an area covering 200 mm by 200 mm. It is of utmost impor-

tance, when mounting a box cooler in a sea chest, that the water-

tight integrity of the vessel is maintained. A considerable sea

water leakage in an undermined sea chest may cause large

ingress of water in the engine room if not noticed in time.

Probable causeA box cooler can be a preferred solution in many cases. The

design eliminates the secondary cooling water circuit of sea water

inside the engine room (pumps, filters, valves, pipes etc.). For

the arrangement with box cooler, the sea chest is provided with

grids and the cooling effect is achieved by natural circulation of

the surrounding sea water.

The design of the box cooler arrangement has to address two

main challenges, galvanic corrosion and marine growth. There

are two main trends of modern box coolers, which give somehow

diverse concerns on the maintenance aspects of the box cooler.

The main trends are:

1.U-tube bundle made of aluminium-brass (CuZn20Al2) and

coated to prevent harmful galvanic effects on the carbon-steel

sea chest.

2.U-tube bundle made of copper-nickel (CuNi10) and uncoated

to prevent marine growth on the tubes.

A box cooler made of coated aluminium-brass tubes is exposed

to marine growth. The coating itself give some reduction of the

thermal efficiency but this should have been compensated for in

the design (size). However marine growth may reduce the ther-

mal efficiency severely and an active anti-fouling system is often

needed. One such common system utilises anodes of pure cop-

per mounted under the box cooler, releasing Cu-ions when an

impressed current is applied between the copper and the sea

chest (actually between unpainted steel strips acting as cathodes

and with electrical continuity to the

sea chest, being installed next to

the copper bars).

A box cooler made of copper-

nickel tubes, which are directly

exposed to the sea water in the sea

chest, must be electrically isolated

from the hull in order not to intro-

duce galvanic corrosion of the sea

chest (naked steel becomes the sac-

rificing material). Isolation must be

made and maintained of all bolts

and flanges connecting the box

cooler to the sea chest and to sys-

tems in the engine room. A natural

anti-fouling of the bundle is

obtained when Cu-ions are released

from the tubes due to a slow spon-

taneous corrosion. >>

Information from DNV to the maritime industry No. 2-10 March 2010

Casualty Information

Sea chest corrosion with boxcooler arrangementShip type: Fishing & Offshore Support Size (GT): any Year built: any

Fig.1: Example of a box cooler retracted for maintenance

of sea chest and its bolt flange.

Fig.2: Illustration of the principle box

cooler arrangement.

kpattersTypewritten TextAnnex 1SAN - Surveying Marine Engine Cooling & Salt Water piping systems in ships

www.dnv.com/maritime

Casualty Information is published by Det Norske Veritas,

Classification Support.

Det Norske Veritas

NO-1322 Hvik, Norway

Tel: +47 67 57 99 00

Fax: +47 67 57 99 11

The purpose of Casualty Information is to provide the maritime industry

with lessons to be learned from incidents of ship damage and more serious

accidents. In this way, Det Norske Veritas AS hopes to contribute to the

prevention of similar occurrences in the future. The information included is

not necessarily restricted to cover ships classed with DNV and is presented,

without obligation, for information purposes only.

Queries may be directed to

Det Norske Veritas, Classification Support, NO-1322 Hvik, Norway.

Fax: +47 67 57 99 11, e-mail: [email protected]

Det Norske Veritas AS. This publication may be reproduced freely on

condition that Det Norske Veritas AS (DNV) is always stated as the source.

DNV accepts no responsibility for any errors or misinterpretations.

We welcome your thoughts!

7,500/ 4-2010 Coor Design 1004-015 Printing: 07 O

slo AS

Casualty Information No. 2-10 March 2010

A general reference is made to the Casualty Information published on the Internet:

http://exchange.dnv.com/ServiceExperience/CasualtyInformation/CasualtyInfoTable.asp

The probable causes to the corrosion of the sea chests reported

are three-fold.

The natural circulation around the cooling elements causeswarm sea water to rise towards the top of the sea chest

(top plating) arranging for a more corrosion friendly environ-

ment and marine growth (barnacles, shells, etc.).

Exposed noble material (corrosion resistant) in the box coolertubes may cause galvanic currents between the box cooler and

the sea chest causing galvanic corrosion (of the sea chest if the

steel is exposed) if proper insulation is not maintained.

The corrosion protection systems applied (coating, sacrificialanodes and/or impressed current cathodic protection) are not

able to suppress the corrosion ratio of the sea chest. This can

be due to poor design and installation, or lack of maintenance

of the system as intended.

Lessons to be learned Box coolers are sometimes assumed to be maintenance free.

However, the ships crew should pay regular attention for signs

of corrosion and leakage in the mounting frame and the corre-

sponding sea chest.

The ships crew should be most aware of the corrosion protec-tion arrangement of the specific box cooler and sea chest. Con-

sult the relevant manufacturers instruction manual for guid-

ance.

Rapid galvanic corrosion may appear if the components make aclosed loop (tube-steel-saltwater) and appears strongest in the

sea chest at the closest distance between the tubes and the sea

chest (which normally is the top plate). A smaller area of dam-

aged coating in the sea chest will concentrate galvanic corro-

sion to such area of exposed steel (paradoxically, the smaller

area, the more aggressive corrosion).

Introducing additional sacrificial anodes (most often zinc)inside the sea chest to protect the hull, may under some cir-

cumstances counteract the effect of the tube bundle anti-foul-

ing and thermal behaviour (Zn deposits on the tubes). Consult

the relevant box cooler manufacturer for guidance on the posi-

tioning of sacrificial anodes.

It is recommended that box coolers are taken out for inspec-tion periodically (about every 5 years). Such inspection should

in general focus on (consult the manufacturer for more specific

guidance):

Inspection of the mounting flange, bolts and gasket(s)

Inspection of the isolation parts (if applicable)

Marine growth (barnacles, shells etc) may cover local severe

corrosion, especially in upper part of the sea chest.

Inspection of the sea chest inlet- and outlet grids (to ensure

sufficient free flow area)

Cleaning of the tube bundle (do not harm the coating or

take away the copper-oxide layer)

Inspection of anodes and cathodes as applicable

Assessment of the sea chest coating condition (and if new

coating is applied, do not coat tube bundle, anodes or

cathodes, as applicable)

Fig.3: Example of severely corroded mounting flange in

the top plate of the sea chest.

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