SL2005-460.pdf

Wear of the overlayer changes the

geometry of the bearing surface and,

thereby, the intended “embedded arch”

geometry. A change in the bearing geo-

metry will obstruct the oil film formation,

which is critical for the correct functioning

of the crosshead bearing.

Inspection of crosshead bearings

As described in earlier service letters

(SL87-219, SL90-272 and SL90-273),

AlSn40-based crosshead bearings must

urgently be replaced as soon as possible

if an open-up inspection shows more

than 5% of the nickel layer exposed. If

one bearing unit has more than 5% of

the Ni-layer exposed, it is very likely that

all the other units suffer from a similar

extraordinary wear, and they should

therefore be replaced as well.

Furthermore, excessive wear of the

overlayer in AlSn40 bearings could,

eventually, expose the interlayer to

nickel-to-steel contact with the cross-

head pin, and result in scuffing action

between the pin and the bearing shell.

To decide which bearings to inspect

first, all top clearances should be

measured and compared with the shop

test/sea trial records. If large deviations

are found, and/or if bearing “dust” is

found in the surrounding area in the

frame box, those bearings should be

the first to be inspected.

The photos below show the stages of

corrosive wear of the overlayer.

Countermeasures against corrosion –

Lube oil system

The water content in the lube oil system

is normally not measured on a regular

basis. However, to prevent water from

accumulating in the lube oil and, thereby,

causing damage to the bearings, the oil

should be monitored manually or, alter-

natively, automatically by means of an oil

condition monitoring system.

Initial scuffing of Ni-layer

Manual oil monitoring can be carried out,

e.g. visually, as the oil changes in ap-

pearance when mixed with water.

Portable equipment can be used as well,

but to ensure efficient and constant

monitoring, use of a continuous meas-

uring sensor can be used.

Bearing metal worn out,

steel-steel contact

In addition to damaging the compon-

ents, there is, in extreme cases, a risk of

a crankcase explosion.

A water monitoring system should

trigger an alarm when the water content

exceeds 0.2%, and preferably again

when exceeding 0.5%.

If 0.2% water (0.5% for short periods)

contaminates the lube oil, the water

leakage should be found, and the source

of the water contamination (e.g.

separators, heat exchangers, cooling

water leakage, etc.) should be inspected

and rectified if defect.

results, it is possible to monitor and

alarm for an evolving corrosion condition

of the overlayer, by evaluating the lead

content in the oil system.

The following values for the lead

content in the oil system can be used

as a guideline:

For online monitoring of the water-in-oil

content, we are currently testing a large

number of different types of equipment.

We have found that some of the equip-

ment in use in the industry today does

not work as expected with the prevalent

system oils. At this relatively early stage,

we recommend four products which are

suitable for all system oil conditions, and

another two seem suitable for most

situations, see the enclosed table of on-

line water monitoring equipment.

•

0-4 ppm

Normal

•

5-10 ppm

Inspect filters and crankcase

for bearing debris, and prepare

inspection of crosshead bearings

when convenient

•

>10 ppm

Inspect filters and crankcase

for bearing debris, and prepare

inspection of crosshead bearings

as soon as possible.

Today, the water content can be

measured manually with an on-board

analysing kit from the available equip-

ment makers on the market, such as

Kittiwake or Unitor . The oil companies

will normally be of assistance and

provide similar equipment. However,

we recommend, as a minimum, that

manual equipment be acquired at first

opportunity.

It is important to note that in case of a

corroded overlayer, the lead, once

corroded and thereby contained in the

system oil, is un-removable by non-

chemical means, and the lead content

will remain high until new clean oil is

added.

The laboratory analyses performed

by the oil suppliers at regular intervals

provide important information on an on-

going or previously encountered over-

layer corrosion, because the lead

content will build up as lead compound

dissolved in the oil.

Note :

The above values should be considered

as relative values compared with the

values that were recorded when the

condition of the bearings was well

known, e.g. when newly built. The lead

content of a new oil may vary as much

as 4 ppm (0-4 ppm), which should also

be considered when observing the

relative lead values as a guideline for

any active corrosion of the overlayer.

These elements will not be removed by

filters or purifiers, and they will there-fore

appear in the elementary analysis

normally provided in the analysis results.

When observing the regular analysis

MAN B&W Diesel

Enclosure 1 for

SL05-460/NHN

Water-in-oil Monitoring Systems

Suitable for MAN B&W Two-stroke Engines

Manufacturer

ppm or %

indication

Water

in-rush

sensitiv

Suitability

for all oils

Scale 1-3

(3 is best)

Alarm

relay

output

Price US$

(approx.)

Product

Lubrizol

Fluipak

Yes

330

Dr. E. Horn

FRG

No ***

Yes

1-2 **

Yes

490

Parker

MS 100

Yes

475

E+E Elektronik

E+E 36

Yes*

Yes

1,610

Vaisala

HMP 228

Yes*

Yes

1,940

Gertsen & Olufsen

Survey Model 2

Yes

5,400

* Only if calibrated at regular intervals

** Under development

*** Possible if used together with a PC

As an alternative to the above systems, the relative humidity in the crankcase /

crankcase breathing pipe close to the engine can be monitored using a suitable

humidity sensor. Tests have shown that if the relative humidity is below 70-80% , the

water in oil can be expected to be below the 0.2% , which is our stated maximum value.

However, such a system will not react quickly on water in-rush.

NHN/JCB

2005-11-20

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