SL2008-498.pdf

Service Letter SL08-498/AAB

Action code: Immediately

Spark Erosion Update

August 2008

SL08-498/AAB

Concerns

Owners and operators of all MAN B&W

two-stroke marine diesel engines.

Dear Sirs

Contents

About Spark Erosion page 2

Shaftline Earthing Device,

installation & maintenance page 2

Discover Spark Erosion page 3

Incorrect Installation page 6

Insuficient Maintenance page 7

This is an update on spark erosion, and how to discover and prevent it.

Spark erosion can occur if electrical potential in the crankshaft discharg-

es through the main or thrust bearings. If undiscovered, spark erosion

can cause critical wear on the main bearings, and consequently severe

crank train damage.

Spec. No. 0792182-1 Appendix

From practical experience and testing, we know that the best protection

against spark erosion is to use a correctly installed and well-maintained

shaftline earthing device.

Therefore, MAN Diesel now recommends all MAN B&W two-stroke en-

gine operators to:

• Equip all newly ordered engines with a shaftline earthing device with

speciications according to MAN Diesel speciication No. 0792182-1.

• Equip all engines in operation without a shaft monitoring system with

such a shaftline earthing device.

• Inspect and evaluate all existing shaft monitoring systems on engines

in operation. Update and overhaul if necessary.

Shaftline

Earthing Device

Please direct any questions about this service letter to our Operation

Department: leo@mandiesel.com

Yours faithfully

Shaft mounted

alternator

Current

Thrust

bearing

Main bearings

Mikael C. Jensen

Vice President, engineering

Stig B. Jakobsen

Senior Manager, operation

Head ofice (& postal address)

MAN Diesel A/S

Teglholmsgade 41

2450 Copenhagen SV

Denmark

Phone: +45 33 85 11 00

Fax: +45 33 85 10 30

mandiesel-cph@mandiesel.com

www.mandiesel.com

PrimeServ

Teglholmsgade 41

2450 Copenhagen SV

Denmark

Phone: +45 33 85 11 00

Fax: +45 33 85 10 49

PrimeServ-cph@mandiesel.com

Production

Teglholmsgade 35

2450 Copenhagen SV

Denmark

Phone: +45 33 85 11 00

Fax: +45 33 85 10 17

manufacturing-dk@mandiesel.com

Forwarding & Receiving

Teglholmsgade 35

2450 Copenhagen SV

Denmark

Phone: +45 33 85 11 00

Fax: +45 33 85 10 16

MAN Diesel A/S

Denmark

Reg. No. 39661314

MAN Diesel – a member of the MAN Group

Service Letter SL08-498/AAB

About Spark Erosion

Unfortunately, spark erosion is still a problem on some ves-

sels with two-stroke marine diesel engines. However; engine

design and our recommendations for service and mainte-

nance procedures have changed since our last service letter

on spark erosion.

Shaftline Earthing Device

The best spark erosion cure is to prevent it. So, MAN Diesel

still strongly recommends these precautionary measures

against spark erosion:

• Correct installation of a shaftline earthing device.

• Follow maintenance instructions carefully.

Former MAN Diesel service letters on spark erosion:

SL 83-193, SL 86-213, and SL 08-495

A full and updated shaftline earthing device speciication,

including installation and maintenance instructions, is en-

closed (see appendix).

Engine Developments

To meet new market demands we continuously develop and

improve our main bearing design, lining material, and service

recommendations. As a consequence, particularly three

recent developments have, unfortunately, also affected our

engines’ resistibility to spark erosion adversely:

Installation

We recommend installing the shaftline earthing device in as

dry, warm, well-ventilated, and easy-to-access environ-

ment as possible, preferably close to the engine. If a shaft

generator is installed, install the device forward of the gen-

erator, i.e. between the generator and the engine lywheel.

Lining thickness

To improve lining fatigue strength, white metal bearing lining

thickness has been reduced from about 2.5 mm to 1.5 mm.

When lining thickness is reduced, the time from spark ero-

sion onset until the situation becomes critical will also be

reduced.

When installing and maintaining the device, remember:

• Install slip rings, brushes, brush holders, and cables that

meet MAN Diesel’s speciications.

• Install brushes to run on a slip ring, never directly on the

shaft.

• Fit slip rings on a clean, polished shaft.

• Apply rust protection between shaft and slip ring.

• Adjust brush holders precisely.

• Install a monitoring voltmeter and connect output to the

alarm system. Always use a separate and dedicated slip

ring and brush system for the voltmeter.

• Follow installation and maintenance instructions.

Tin/aluminium lining

A tin/aluminium lining is used on small and medium bore

engines to improve lining fatigue strength.

A few case studies suggest that in case of spark erosion

a tin/aluminium lining may wear through to the steel-back

faster than a white metal lining.

Oil ilm thickness

Over the years, higher engine rating has led to a reduced

minimum oil ilm thickness in our main bearings.

A reduced oil ilm thickness can make an engine more sensi-

tive to spark erosion.

Maintenance

Be particularly cautious with maintenance if the shaftline

earthing device is located in the far aft end of the shaft or

other exposed place.

Brush holder

arrangement

Cable connected

to the hull

Always overhaul and replace worn parts in due time:

• Replace the slip ring before the silver layer is worn

through.

• Replace worn out brushes.

• Keep the slip ring clean and dry.

Monitoring

equipment

with mV-meter

Cable connected

to the hull

Corrosion between the slip ring and the shaft can be dificult

to detect. We recommend these procedures:

• No monitoring voltmeter installed:

Measure electrical contact between slip ring and shaft at

least monthly to reveal any concealed corrosion.

If resistance exceeds 5 m Ω : Overhaul the slip ring.

• Monitoring voltmeter installed:

Check shaftline earthing device monitoring monthly.

Slip ring

Cable to

alarm system

Slip ring for

monitoring

equipment

Brush holder

arrangement

Shaftline earthing device parts

Spark Erosion Update. Page 2 of 7

Service Letter SL08-498/AAB

Discover Spark Erosion

A few spark erosion situations have developed dramatically

because severe main bearing wear was not discovered in

time. The wear was discovered too late, because the main

bearing top clearance and crankshaft delection measures

were compared to the maximum values in the manual, in-

stead of the reference values for the particular engine.

Measurement procedures and intervals are speciied in the

instruction manual. More frequent measurements will im-

prove condition monitoring considerably.

Signs of spark erosion can be dificult to discover and recog-

nise, even during open-up inspections of main bearings and

thrust bearing cam. To help you discover and recognise

signs of spark erosion and spark erosion risk situations in

time, we have prepared a series of photos showing where to

look and what to look for.

Our minimum recommendation is to perform yearly main

bearing top clearance measurements and crankshaft

delection readings. Store the values for future reference.

AlSn40 Lining

This is a journal, main bearings and thrust collar with clear

signs of spark erosion. Spark erosion signs are obvious,

even in the upper main bearing shell.

The bearing damage is irreparable: This bearing cannot be

re-used, it has to be replaced.

Upper main bearing shell

This journal MUST be polished to meet surface roughness speciications.

Spark Erosion Update. Page 3 of 7

Service Letter SL08-498/AAB

White Metal, Severe

Main bearing journal with signs of severe spark erosion.

The condition was not discovered in time, so this main bear-

ing cannot be saved; is has to be replaced. The journal can

be-used, but has to be polished to meet surface roughness

speciications.

The consequences of undiscovered spark erosion are shown

below. The best protection against severe spark erosion in-

cidents like this, is to use a correctly installed and well-main-

tained shaftline earthing device.

This journal MUST be polished to meet surface roughness speciications

Corresponding main bearing lower shell with wide spread journal imprint.

0.5 mm of the shell is worn off between the two distinct, visible lines.

Spark Erosion Update. Page 4 of 7

Service Letter SL08-498/AAB

White Metal, Moderate

Main bearing with moderate spark erosion, discovered in

time to save the bearing shell.

Thrust Collar Face

This is a thrust collar face with clear signs of spark erosion.

Spark erosion may not be visible on the entire surface.

This bearing can be re-used after dressing up. The journal

must be polished to meet surface roughness speciications.

When inspecting the thrust collar surface or the main bear-

ing journal, remember:

Slowly turn the engine 360˚/1 revolution, and inspect the en-

tire surface very carefully.

Main bearing journal

The greyish stripes on this thrust collar face

indicate spark erosion

Corresponding lower bearing shell. Visible spark erosion,

but no or limited wear can be measured.

Spark Erosion Update. Page 5 of 7

Plik z chomika:

Inne pliki z tego folderu:

Inne foldery tego chomika: