Woodyard D. (ed.) Pounders Marine diesel engines and Gas Turbines

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138 Fuels and Lubes: Chemistry and Treatment

Water accumulates in tanks when there is no drain or water scavenge sys-

tem, where the drain is not at the lowest point in the tank, and where draining

procedures are not enforced. Tanks in the engineroom or other warm locations

and tanks receiving recirculated distillate fuel from injectors are ideal for incu-

bating microbes. Double bottom tanks, due to their lower temperatures, are less

prone to microbial proliferation.

The following visual symptoms of fuel microbial contamination are given

as a guide:

l Aggregation of microbes into a biomass is observed as discolouration,

turbidity and fouling.

l Biosurfactants produced by bacteria promote stable water hazes and

encourage particulate dispersion.

l Puriﬁers and coalescers, which rely on a clean fuel/water interface, may

malfunction.

l Tank pitting.

Operational indications of fuel microbial contamination are as follows:

l Bacterial polymers may completely plug ﬁlters and oriﬁces within a few

hours.

l Filters, pumps and injectors foul and fail.

l Non-uniform fuel ﬂow and variations in combustion may accelerate

piston ring and cylinder liner wear rates, and affect camshaft torque.

When heavily infected fuel is used, all or some of these phenomena will

confront the ship’s engineers within a few hours. Initially, they will probably

be faced with ﬁlter plugging, fuel starvation, injector fouling and malfunction-

ing of any coalescer in use.

The extent of the contamination must ﬁrst be established by the following

procedure:

1. Clear glass sample bottles rinsed with boiling water should be used to

take drain or bottom samples from service, header and storage tanks.

2. Microbial contamination in a sample will be apparent as a haze in the

fuel from the presence of sludge. This sludge readily disperses in the

fuel when the sample is swirled; at this time sticky ‘cling ﬁlm’ ﬂakes

may be seen adhering to the wall of the bottle. The water will be turbid

and there may be some bottom sludge; if this is black, SRB are present

and they will be an added corrosion hazard.

3. The engineer can now evaluate the various fuel locations and instigate

an emergency strategy by using the cleanest fuel. If only heavily con-

taminated fuel is available, it should ﬁrst be allowed to settle for as long

as possible and then drawn off from the top to a clean tank, preferably

via a ﬁlter, puriﬁer, centrifuge or coalescer. Using a biocide at this stage

(see the next section) is usually not advisable due to a tendency to block

ﬁlters with the dislodged bioﬁlms.

4. At the earliest opportunity, drain or bottom samples should be taken and

the supplier’s retained sample forwarded for microbiological examina-

tion. This will identify, by sophisticated ‘ﬁngerprinting’, whether or not

the bunkers supplied were contaminated.

The standard methods for curing heavily contaminated fuels involve the use

of biocides, which may be toxic and incur disposal problems. Careful handling

is necessary. Once a biocide has been added, the dead biomass must be allowed

to settle before drainage and ﬁltration. The careful monitoring of ﬁlters is also

required to prevent blockage by the dead microbes.

In expert hands, biocides eliminate the microbial menace, but using an

incorrect biocide can exacerbate the problem rather than effect a remedy. The

choice of biocide is therefore important. Prevention is better than cure, and the

source of the infections should be identiﬁed to prevent their recurrence.

Lube oiL ConTaminaTion

Lubricant operating temperatures of crankcase oils in wet engines are normally

sufﬁcient to control and prevent infestation. Any problems are usually associ-

ated with ships which have shut down their lubricant system, allowing tem-

peratures to drop and water to accumulate.

There are many thousands of types of microbes, but only a few are able

to grow in lubricating oils at the elevated operational temperatures in use.

Provided the plant treatment equipment is functional, the puriﬁer heater

temperature is maintained and the water content kept at a minimum, the lubri-

cating oil system will be both environmentally and nutrient deﬁcient. Such

conditions will prevent microbes from establishing themselves. Should heavy

contamination of the system occur, however, this self-regulating mechanism

will be unable to prevent microbial proliferation (Figure 4.11). Fortunately, the

symptoms of microbial contamination will not occur immediately, allowing

engineers to implement remedial physical and/or chemical decontamination

programmes.

The visual symptoms of microbial contamination of lubricants are as

follows:

l Slimy appearance of the oil, the slime tending to cling to the crankcase

doors

l Rust ﬁlms

l Honey-coloured ﬁlms on the journals, later associated with corrosion

pitting

l Black stains on white-metal bearings, pins and journals

l Brown or grey/black deposits on metallic parts

l Corrosion of the puriﬁer bowl and newly machined surfaces

l Sludge accumulation in the crankcase and excessive sludge at the

puriﬁer discharge

l Paint stripping in the crankcase.

Lube oil contamination 139

140 Fuels and Lubes: Chemistry and Treatment

Operational symptoms of microbial contamination in lubricants are as

follows:

l Additive depletion

l Rancid or sulphitic smells

l Increase in oil acidity or sudden loss of alkalinity

l Stable water content in the oil, which is not resolved by the puriﬁer

l Filter plugging in heavy weather

l Persistent demulsiﬁcation problems

l Reduction of heat transfer in coolers.

Microbial growth occurs in the water associated with the lubricant, and the

phenomenon is therefore characteristic of crankcase oils in wet engines, partic-

ularly those with water-cooled pistons. Lubricant infection can be identiﬁed by

slimy ﬁlm formation on the crankcase doors. There may also be a rancid odour

and white-metal parts may be stained black. As the problem progresses, ﬁlters

choke up, organic acids are formed and the oils tend to emulsify.

If SRB are present—a problem particularly associated with laid-up tonnage—

copious pitting of ferrous and non-ferrous metals may occur (Figure 4.12).

As microorganisms feed upon the additives within the oil, the lubricity of the

oil may be impaired and its viscosity altered; and there is a greater resultant acid-

ity and increased potential for emulsiﬁcation and corrosion. Hydrogen sulphide

may also be produced as a by-product. Serious corrosion problems will result

within weeks of the initial contamination if these factors occur at the same time.

FiguRe 4.11 interactive problems associated with microbial corrosion (Lloyd’s

Corrosion

Nutrients

Oil

Increased wear

Poor lubrication

Aggressive

species

Microbial

growth

Water

Conductive salts

Lowering of surface

tension of oils

Emulsification

Filter

pipe blockage

Microbial

infection

Unpleasant

odours

Health and

safety risks

Sludge

formation

Differential

operation

Particles

Sources of contamination within the lubricant are the fuel, cooling water

and sea water. Cooling water in particular has featured as a common contami-

nant of crankcase oil at engine operating temperatures, since the use of chro-

mates as corrosion inhibitors is banned. The use of chromates also acted as an

effective anti-microbial biocide.

Prevention is better than cure and it is known that microbial growth is

retarded by extreme alkaline conditions. To date, there have been no microbial

problems reported with medium-speed engines during operation when using

highly alkaline lubricants of BN 12 to 40. At elevated temperatures lubricating

oils tend to be self-sterilizing. Unfortunately, however, there still remains the

possibility of infestation from contaminated bilges and tanktops, which may

inadvertently leak into the oil system.

Since the majority of operational microbial problems in lubricating oils arise

due to infection from cooling water, sea water from cooler leaks and overﬂows

of contaminated bilges, the effectiveness and efﬁciency of plant treatment are

important remedial measures.

The following procedures are recommended to avoid microbiological

problems:

l Ensure that the water content of crankcase oil does not rise above

0.5 per cent by weight.

l Check that puriﬁer suction is as near to the bottom of the sump as

possible.

l Maintain a minimum oil temperature after the puriﬁer heater of 70°C or

higher for at least 20 s and/or 80°C for 10 s.

l Circulate the volume of oil in the sump via the puriﬁer at least once

every 8–10 h.

Lube oil contamination 141

3SO

+6H

SRB

xCO

Organic

substrates

Water

Phase

2−

Cathode

4FeS

4Fe

+2H

SRB

Iron

Anode

–

FiguRe 4.12 anaerobic corrosion by sRb

142 Fuels and Lubes: Chemistry and Treatment

l Regularly check that coolant corrosion inhibitor concentrations are at

the manufacturer’s recommended values.

l Monitor the microbial population of the cooling water and prevent water

leaks into the oil system.

l Test for microbial contamination of the oil system and monitor whether

the oil after the puriﬁer is sterile.

l Prevent ingress of contaminated bilge water.

l Inspect storage tanks and regularly check for water.

The above information is based on advice from Lloyd’s Register’s Fluid

Analytical Consultancy Services (FACS) department, which has considerable

theoretical and practical expertise in assessing and resolving shipboard micro-

bial contamination problems with fuel and lube oils.

143

C h a p t e r | f i v e

Performance

An important parameter for a marine diesel engine is the rating ﬁgure, usually

stated as brake horsepower (bhp) or kilowatts per cylinder at a given revolu-

tions per minute.

Although enginebuilders talk of continuous service rating (csr) and max-

imum continuous rating (mcr), as well as overload ratings, the rating which

concerns a ship owner most is the maximum output guaranteed by the engine-

builder at which the engine will operate continuously day in and day out. It is

most important that an engine be sold for operation at its true maximum rating

and that a correctly sized engine be installed in the ship in the ﬁrst place; an

underrated main engine, or more particularly an auxiliary engine, will inevit-

ably be operated at its limits most of the time. It is wrong for a ship to be at

the mercy of two or three undersized and thus overrated auxiliary engines, or a

main engine that needs to operate at its maximum continuous output to main-

tain the desired service speed.

Prudent ship owners usually insist that the engines be capable of main-

taining the desired service speed fully loaded, when developing not more

than 80 per cent (or some other percentage) of their rated bhp. However, such

a stipulation can leave the full-rated power undeﬁned and therefore does not

necessarily ensure a satisfactory moderate continuous rating, hence the appear-

ance of continuous service rating and maximum continuous rating. The

former is the moderate in-service ﬁgure; the latter is the enginebuilder’s set

point of mean pressures and revolutions which can be carried by the engines

continuously.

Normally a ship will run sea trials to meet the contract trials speed (at a

sufﬁcient margin above the required service speed), and the continuous service

rating should be applied when the vessel is in service. It is not unknown for

ship owners to then stipulate that the upper power level of the engines in serv-

ice should be somewhere between 85 per cent and 100 per cent of the service

speed output, which could be as much as 20 per cent less than the engine mak-

er’s guaranteed maximum continuous rating (Figure 5.1).

144 Performance

MaxiMuM rating

The practical maximum output of a diesel engine may be said to have been

reached when one or more of the following factors operate:

1. The maximum percentage of fuel possible is being burned effectively in

the cylinder volume available (combustion must be completed fully at

the earliest possible moment during the working stroke) (Figure 5.2).

2. The stresses in the component parts of the engine generally, for the

mechanical and thermal conditions prevailing, have attained the highest

safe level for continuous working.

110

100

bar

mep, mip

Comp

, P

max

100

bar (abs)

3.0

2.0

1.0

Scav.

T-Exhaust Gas

450

400

350

300

250

4.0

3.5

3.0

195

190

185

g/kWh

145

140

135

g/BHPh

1450

1973

2175

2959

2900

3945

kW/cyl.

BHP/cyl.

50% 75% 100%

Specific fuel oil

consumption

Total air excess

ratio

Exhaust gas temp.

receiver

Exhaust gas temp.

after valves

Exhaust gas temp.

after turbines

Scav. air pressure

Comp. pressure

Max. pressure

mip

mep

Engine r/min

bar

r/min

-Total

Figure 5.1 typical performance curves for a two-stroke engine

3. The piston speed and thus revolutions per minute cannot safely be

increased.

For a given cylinder volume, it is possible for one design of engine effec-

tively to burn considerably more fuel than one of the other designs. This may

be the result of more effective scavenging and higher pressure turbocharging,

by a more suitable combustion chamber space and design and a more satisfac-

tory method of fuel injection. Similarly, the endurance limit of the materials of

Maximum rating 145

Figure 5.2 Layout diagrams showing maximum and economy ratings and corre-

sponding fuel consumptions

146 Performance

cylinders, pistons and other parts may be much higher for one engine than for

another; this may be achieved by the adoption of more suitable materials, by

better design of shapes, thicknesses, etc., more satisfactory cooling and so on.

A good example of the latter is the bore cooling arrangements, which is now

commonly adopted for piston crowns, cylinder liner collars and cylinder cov-

ers in way of the combustion chamber.

The piston speed is limited by the acceleration stresses in the materials,

the speed of combustion and the scavenging efﬁciency: that is, the ability of

the cylinder to become completely free of its exhaust gases in the short time of

one part cycle. Within limits, so far as combustion is concerned, it is possible

sometimes to increase the speed of an engine if the mean pressure is reduced.

This may be of importance for auxiliary engines built to match a given alterna-

tor speed.

For each type of engine, therefore, there is a top limit beyond which the

engine should not be run continuously. It is not easy to determine this maxi-

mum continuous rating; in fact, it can only be satisfactorily established by

exhaustive tests for each size and type of engine, depending on the state of

development of the engine at the time.

If a cylinder is overloaded by attempting to burn too much fuel, combus-

tion may continue to the end of the working stroke and perhaps also until after

exhaust has begun. Besides suffering an efﬁciency loss, the engine will become

overheated and piston seizures or cracking of engine parts may result; or, at

least, sticking piston rings, as well as dirty and sticking fuel valves, will be

experienced.

exhaust teMPeratures

The temperature of the engine exhaust gases can be a limiting factor for the

maximum output of an engine. An exhaust temperature graph plotted with

mean indicated pressures as abscissae and exhaust temperatures as ordinates

will generally indicate when the economical combustion limit, and sometimes

when the safe working limit, of an engine has been attained. The economical

limit is reached shortly after the exhaust temperature begins to curve upwards

from what was, previously, almost a straight line.

Very often the safe continuous working load is also reached at the same

time, as the designer naturally strives to make all the parts of an engine equally

suitable for withstanding the respective thermal and mechanical stresses to

which they are subjected.

When comparing different engine types, however, exhaust temperature

cannot be taken as proportionate to mean indicated pressure. Sometimes it is

said and generally thought that engine power is limited by exhaust tempera-

ture. What is really meant is that torque is so limited and exhaust temperature

is a function of torque and not of power. The exhaust temperature is inﬂuenced

by the lead and dimensions of the exhaust piping. The more easily the exhaust

gases can ﬂow away, the lower their temperature, and vice versa.

Derating

An option available to reduce the speciﬁc fuel consumption of diesel engines

is derated or so-called ‘economy’ ratings. This means operation of an engine at

its normal maximum cylinder pressure for the design continuous service rating,

but at lower mean effective pressure and shaft speed.

By altering the fuel injection timing to adjust the mean pressure/maximum

pressure relationship, the result is worthwhile saving in fuel consumption. The

horsepower required for a particular speed by a given ship is calculated by the

naval architect, and, once the chosen engine is coupled to a ﬁxed pitch pro-

peller, the relationship between engine horsepower, propeller revolutions and

ship speed is set according to the ﬁxed propeller curve. A move from one point

on the curve to another is simply a matter of giving more or less fuel to the

engine.

Derating is the setting of engine performance to maximum cylinder pres-

sures at lower than normal shaft speeds, at a point lower down the propeller

curve. For an existing ship and without changing the propeller, this will result

in a lower ship speed, but in practice when it is applied to newbuildings, the

derated engine horsepower is that which will drive the ship at a given speed

with the propeller optimized to absorb this horsepower at a lower than normal

shaft speed.

Savings in speciﬁc fuel consumption by ﬁtting a derated engine can be sig-

niﬁcant. However, should it be required at some later date to operate the engine

at its full output potential (normally about 15–20 per cent above the derated

value), the ship would need a new propeller to suit both higher revolutions per

minute and greater absorbed horsepower. The injection timing would also have

to be reset.

Mean eFFeCtive Pressures

The term brake mean effective pressure (bmep) is widely quoted by engine-

builders and is useful for industrial and marine auxiliary diesel engines that

are not ﬁtted with a mechanical indicator gear. However, the term has no useful

meaning for shipboard propulsion engines. It is artiﬁcial and superﬂuous as it

is derived from measurements taken by a dynamometer (or brake), which are

then used in the calculation of mechanical efﬁciency. Aboard ship, where for-

merly the indicator and now electronic pressure transducers producing PV dia-

grams, are the means of recording cylinder pressures, mean indicated pressure

(mip) is the term used, particularly in the calculation of indicated horsepower.

Many ships now have permanently mounted torsionmeters. By using the

indicator to calculate mean indicated pressure and thus indicated horsepower,

and the torsionmeter to calculate shaft horsepower (shp) from torque readings

and shaft revolutions, the performance of the engine both mechanically and

thermally in the cylinders can be readily determined.

Instruments such as pressure transducers, indicators, tachometers and pres-

sure gauges (many of which are of the electronic digital or analogue type of

Mean eective pressures 147