Unit (ADU) to distill it into light fractions. The residual oil then goes through a Vacuum
Distillation Unit (VDU) under vacuum (31-38 kPa) and is fractionated into a lightfraction (350°C to 550°C at equivalent atmospheric pressure) and a heavy fraction (550°C
or higher at equivalent atmospheric pressure).
The light fraction sulphur content is reduced through an Indirect Desulphurization Unit,
and then desulphurized Vacuum Gas Oils (VGO) are converted through a Fluid Catalytic
Cracking Unit (FCCU) into higher-value gasoline and middle distillates such as lightcycle oil (LCO) and clarified oil (CLO).

《Blending process of compliant fuel oil》

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For consistent supply and cost effectiveness, residual oil with a sulphur content of 0.50%
or less is commonplace.

Vacuum Heavy Distillate

VGO：Blendstock with good ignition quality, not directly blended with bunker heavy oil,and used as a raw oil for cracking unit (VGO：Vacuum Gas Oil)LCO (Light Cycle oil)sulphur blendstock, low kinematic viscosity"+."K@QHjDC.Hl ) ：Low
Figure 5-3 Manufacturing process at a typical petroleum refinery67

According to current oil refinery technology, there are said to be five production
methods of compliant fuel oil:①

Blending of various low-sulphur blendstocks which have been produced at therefinery,②

Desulphurization of residual oil from high sulphur crude oil,③

Blending of light oil and high-sulphur heavy oil(e.g., crude oil from the Gulf of Mexico, see Figure 5-4),④

Use of MGO ( Marine Gas Oil, lighter distillate oil) or MDO (Marine Diesel Oil, blend of
distillates and heavy oil but with very low content)⑤

Use of low-sulphur crude oil residue (e.g., crude oil from the North Sea oilfields, see
Figure 5-4).

Figure 5-4 Density and sulphur contentof selected crude oils

68

Figure 5-4 shows sulphur content by crude oil production area. Low sulphur oil isessential as a blendstock for compliant fuel oil. Due to financial considerations, it is
not possible to supply a suﬃcient amount of compliant fuel oil using only distillate oiland desulphurised residual oil. The above mentioned production method ① is the mostpromising way to secure a stable supply of compliant fuel oil.
In order to keep the sulphur content of compliant fuel below the new 0.50% sulphur
limit, it is necessary to blend various blendstocks together. This leads to an increase ofvarious fuel oil properties other than sulphur content when compared to conventional
fuel oils for open sea areas outside ECAs (HSHFO: High Sulphur Heavy Fuel Oil withsulphur content of less than 3.5%).

① Use various low-sulphur blendstocks other than light distillate oils whichhave been obtained by repeatedly refining, distilling, fractionating, andcracking from crude oil.
② Since crude oil properties vary depending on the region of production(see Figure 5-4) and each refinery's capabilities, the ratio of blendstocks incompliant fuel oil may vary widely from region to region when compared toconventional fuel oil.

５−２ Precautions and countermeasures for safe
usage of 2020 IMO compliant fuel oil
Safe use of VLSFO requires an essential understanding of the various properties foundin blendstocks (‘VLSFO’ Very Low Sulphur Fuel Oil: fuel oil compliant with 0.50%
sulphur limit required in open sea areas outside of ECAs. It is further sub-classified
into VLSFO-RM (Residual Marine Fuels) and VLSFO-DM (Distillate Marine Fuels)
depending on the production process. Hereinafter, the compliant VLSFO-RM is referred
to as VLSFO)

69

Safe use of VLSFO requires essential awareness of the following 5 properties:① Compatibility② Low viscosity③ Cold ﬂow properties④ Cat-ﬁnes⑤ Ignition and combustion quality

Using the points outlined in ‘Problems of conventional High-Sulphur Heavy Fuel Oil(HSHFO) and Distillate Marine Fuels (MDO: marine diesel oil & MGO: marine gas
oil’, and based on previous experience and knowledge of fuel oil properties, this chapter
aims to identify the various ‘precautions’ and ‘countermeasures’ required for each
property. Please also refer to “Loss Prevention Bulletin Vol.30, Bunkers – Quantity andQuality Disputes” to review fuel quality fundamentals.

Asphaltene micelle

Asphaltene (highest C/H ratio)

Resin (C/H ratio decreases
the further it is locatedfrom the centre)
C/H ratio is equivalent
to maltene
Continuous phase (Maltene)

Figure 5-570Structure of asphaltene micelle5-2-1 ① Compatibility
When two different fuel oils are mixed, the stability of that fuel will decline, andasphaltene sludge and/or other substances contained in either of the original fuel oils
may deposit/precipitate. The capacity of fuel oil to resist this deposition/precipitation is
generally referred to as fuel compatibility.
Dispersed and diffused in whole

Deposition / precipitation
sinking to the bottom
Photograph 5-6

Deposition/precipitation

Photograph: ClassNK Guidance

Depositing and precipitating Sludge
Asphaltene in heavy fuel oil forms stable micelle structures in maltene, as shown in
Figure 5-5. They are dispersed and remain in a suspended state (colloidal) and do not
deposit/precipitate.
However, when the fuel is mixed with other fuel oils, or undergoes thermal shock orwhen the fuel is oxidized, the micelle structure becomes unstable and the colloidal stateis lost. Asphaltene will then start to aggregate, and its particles grow in size. It is finallydeposited/precipitated as asphaltene sludge (See Photograph 5-6).
Generally, when a fuel containing a large amount of aromatic hydrocarbon is mixed withanother fuel containing a large amount of paraﬃnic hydrocarbon, maltene containing theasphaltene reaches a point of discontinuity and asphaltene begins to aggregate.

71

Potential issues and countermeasures for machinery/plant systems affectedby fuel oil sludge
Table 5-7 shows a summary of compatibility issues and countermeasures.

Machinery/plant system issues in the engine room
Please see the Fuel oil piping system (1) in Figure 5-8 and sludge depositionand precipitation in Photographs 5-9 and 10.
1

Asphaltene sludge deposition clogs pipes and prevents the transferalof fuel oil.
Reason: Fuel oils are a mixture of diﬀerent compositions and this leads todeposition of asphaltene in the following areas:① In the hull storage tank when bunkering.
② In either the settling tank/service tank or piping when
the crew switch fuel oil used for onboard machinery to acompatible oil.

2

Low fuel oil supply, or in the worst case scenario, main engine shut
down (loss of propulsion), or generator shut down (loss of power)
Reason: The strainer on the fuel oil supply pipeline clogs with sludge inthe following situations:① When asphaltene sludge deposition generated in the hull tankis transferred.
② When fuel oils of diﬀerent composition are mixed in the piping.

3

During the discharge of sludge, the oil purifier may be damaged orabnormal vibrations may occur in the case of any imbalance in the
puriﬁer's rotation unit.
Reason: Accumulation of sludge on the separating disc in the rotatingbowl of the oil puriﬁer which may occur in the following situation:
When filters in the fuel oil supply pipeline become clogged,
sludge spreads throughout the entire fuel oil supply system. At
the same time, asphaltene sludge deposition increases in the oil
puriﬁer.

72

Countermeasures
The following countermeasures are required:Caution：Do not mix fuel oil in the hull storage tank or pipeline, or at the very
least keep the mixing ratio as low as possible.
1

The following measures should be adopted to avoid any adverse eﬀect
on shipping operations:① Do not change oil in areas of high operational risk such as congested
sea areas.
② Keep the duration of fuel oil mixing to a minimum. Change fuel oil after
a calculated estimation of fuel consumption in the main engine.

2

The following measures should be adopted in the event of sludge
accumulation:① Add sludge dispersants or solubilizers to the hull storage tanks.
② Clean strainers frequently.
③ Regarding oil puriﬁer operating management, reduce the ﬂow rate ofoil puriﬁers, shorten interval of sludge discharge, raise oil treatment
temperature, and reduce overhaul interval of the separating disc for
inspections and maintenance.

3

In case of emergency, it is advisable to keep a reserve stock of sludge
dispersant on board.

Table 5-7 Compatibility issues and countermeasures

73Bunker barge① Sludge precipitation on residual"X: Conventional HSHFO (Aromatic)" when topping
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Figure 5-8 Fuel oil piping system (1)Photograph 5-9 Sludge deposition and precipitation within the rotating bowl of the oil purifier

74

Source: Guide for use of 2020 SOx regulation
compliant fuel oils
(Maritime Bureau of the Ministry of Land,
Infrastructure, Transport and Tourism)

Courtesy of Nippon Kaiji Kentei Kyokai

Photograph 5-10 Sludge deposition and precipitation in fuel oil filters
Photograph: ClassNK Guidance

5-2-2 ② Low viscosity
In order to produce VLSFO compliant fuel oil with a sulphur content of 0.50% or less,the blending ratio of the following low sulphur blendstocks will rise above that ofconventional fuel oil (HSHFO):(1) Desulphurized Vacuum Gas Oils (vacuum residue whichhas passed through an Indirect Desulphurization Unit)(2) LCO (Light Cycle Oil) and CLO (Clariﬁed Oil)Cracking reaction in the desulphurization units and cracking units will cause blendstock
viscosity to be reduced.
Therefore, the viscosity of the blendstocks mentioned above is much lower than thatof conventional blendstocks which have not passed through direct desulphurization(Vacuum Heavy Distillate) and been blended with conventional heavy fuel oil (HSHFO).
The viscosity of VLSFO supplied from 2020 is lower than that of conventional fuel oil(HSHFO).
According to two research reports “Assessment of Fuel Oil Availability - final report,
75

MEPC70/5/3 and MEPC 70/INF.6.” (see right side
Photograph) presented at the 70th Marine Environment
Protection Committee IMO, held in 2016, the futureof marine fuel oils was described as follows: “Low
Sulphur crude oil is available in certain regions such as
the North Sea Oil Fields. However, it is also envisaged
that a VLSFO compliant fuel oil, mainly composedof atmospheric residue or vacuum residue, can beproduced and supplied and which is broadly similar
to conventional HSHFO fuel oil. Sulphur content
of residues exceeding 0.5% can be adjusted whennecessary by blending with an appropriate amount of low sulphur heavy oil or distillateoil. In some regions such as the North Sea oil field where low sulphur crude oil isavailable, it is also predicted that compliant fuel oil VLSFO which mainly is composedof atmospheric residue or vacuum residue can be produced and supplied, similar toconventional fuel oil HSHFO. And if the sulphur content of residue is higher than 0.5%,
the appropriate amount of low sulphur heavy oil or distillate oil will be blended to adjust
it as necessary. This will bring VLSFO viscosity to an equivalent level as that of current
HSHFO.”

Potential issues and countermeasures for machinery/plant systems
affected by low viscosity oil
Table 5-11 shows a summary of issues and countermeasures.

Machinery and plant system issues in the engine room
Refer to images of damaged parts in Photographs 5-12,13 and 14
1

Diesel engine① Sticking of the fuel injection pump (due to decreased lubricity), failure to
start-up, and diﬃculty in increasing rotation speed (load) due to increased76

leakage of the internal sliding parts.
(As reported in the Summary of damage published in the ClassNK
Technical Review (Kaishi) (No312, No316): following enactment in 2015 ofSOx regulations which restrict the sulphur content in fuel oils in overseas
SOx Emission Control Areas, some Class NK registered ships reported
damage to fuel injection devices).
② Diﬃculty in increasing rotation speed (load) because of an insuﬃcient fuelsupply from the fuel supply pump (due to decreased kinematic viscosity).
Internal leakage from the clearance between the rotating body and thecasing occurs, and the fuel supply is insuﬃcient.
③ Low-temperature corrosion of fuel valves and related components.
(This occurs when the fuel injection valve on engines using heated HSHFO
is cooled excessively. This often happens in 4-stroke engines).

2

Fuel supply pump / Fuel transfer pump / Oil puriﬁer supply pump

① Sticking, gear wear, reduction of bearing life (due to decreased lubricity).
② Insufficient fuel oil supply (due to leakage of the sliding parts inside thepump, and decreased kinematic viscosity).
③ Leakage from pump seal (due to decreased kinematic viscosity).

Countermeasures
The following countermeasures are required:
It is essential to comply with the manufacturer's manuals and instructions forsafe use of ECA (Emission Control Areas) compliant fuel oil, even when using
VLSFO.
1

Conﬁrmation of engine and fuel pump speciﬁcations before use ofcompliant fuel.

① Measures to be implemented when replacing or modifying machinery:1）When the minimum viscosity of the fuel in use is lower than 20 cSt@50℃ at the engine inlet, it is necessary to install a fuel oil cooler infront of the inlet to ensure the manufacturer's recommended viscosity.
2）For fuel supply pumps, if the minimum viscosity of the VLSFO in use
is not within the manufacturer's recommended range of viscosity, thefollowing countermeasures should be implemented:

77

(a) Use a fuel supply pump with specifications compatible with low
viscosity.
(b) Replace the pump seal.
(c) Install a fuel oil cooler (viscosity can be adjusted provided it is done
upstream from the supply pump).
② Overhauls and maintenanceSome VLSFOs have a high kinematic viscosity and a large amount ofresidue. The clearance between the sliding parts of the fuel supply pumpand the fuel injection system can increase due to wear.
If the kinematic viscosity of fuel oil is signiﬁcantly lower than that which
has just recently been used, it is necessary to carry out an overhaul
before use.

2 Countermeasures at sea① Kinematic viscosity varies greatly with changes in temperature. Make sure
to maintain thorough control of fuel oil temperature.
When there is no risk of wax precipitation (see next section) at the pourpoint, cease steam tracing and avoid raising the temperature of the fuel oil
as much as possible.
② Monitor fuel oil temperature so that viscosity remains within therecommended range of the manufacturer.
Kinematic viscosity can be controlled if the fuel oil is 100 cSt @50℃ or
higher. However, if kinematic viscosity is low, crew should be aware that it
may be diﬃcult to adjust onboard by steam heating.
③ For fuel oils with low kinematic viscosity 20cSt @50℃ or less that areprone to wax at a high pour point, there is a risk of wax formation when
adjusting kinematic viscosity by cooling. Be aware that there is a narrowtemperature range of acceptable cooling to adjust kinematic viscosity.
④ Where there is concern about the lubricity of VLSFO, a lubricity improver
should be added to the fuel oil.
(Lubricity improvers should be added to the fuel tank in advance.)⑤ It is recommended to keep a reserve of lubricity improvers onboard.
Table 5-11 Low viscosity issues and countermeasures

78

Photograph 5-12

Scratches on the plunger and barrel of the fuel injection pump
Photograph: ClassNK Guidance

Photograph 5-13

Abnormal wear of plunger and barrel in the fuel injection pump
Photograph: ClassNK Guidance

Photograph 5-14

Scratches on the fuel injection valve
Photograph: ClassNK Guidance

79

Importance of viscosity control
Manufacturers provide a recommended kinematic viscosity of fuel oil at the engine
inlet, and to ensure safe and eﬃcient operation of the engine, the fuel oil supply pipeline
is equipped with an onboard viscosity control device (see table 5-11). Precautions forthe handling of fuel oil are also explained in the manufacturer’s instruction manual.
Each manufacturer has a recommended range of approximately between 2 cSt and 20
cSt.
As shown in Figure 5-15, kinematic viscosity is inversely proportional to increases infuel oil temperature. Conventional fuel oil (HSHFO) has a relatively high viscosity of
180cSt@50

or 380cSt@50 , and so a virtually consistent quality could be obtained.

Adjusting it to fall within the recommended viscosity range was therefore just a matterof steam heating. However, after 2020, trade of VLSFO with a wider range of kinematic
viscosity is expected. When kinematic viscosity is lower than recommended levels,
it will be necessary for crew to increase viscosity by cooling. Moreover, regarding
oil purifiers, from the viewpoint of economy and safe operation, the manufacturer
recommends that optimum viscosity be set at, for example, 24cSt for the treatment oil.
So, crewmembers must take care to set the treatment (feeding) temperature of the oil
purifier to its relative (corresponding) temperature.
It is therefore essential for crew members to familiarize themselves with the
manufacturer’s instruction manual and to take appropriate measures to manage fuel oil
viscosity.

80

Q

The role of the Engineer
Consider:M/E inlet viscosity Manufacturer's
recommended viscosity Corresponding
temperature Steam Heating Valve aperture
THEN
Monitor, think, analyse, judge and act!

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81Ｍarine Gas Oil

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5000
4000
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2000
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1

−10

0

10

20

30

40

50

60

70

80

90

100

110

120

130

140

150

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Figure 5-15 Viscosity controller

5-2-3 ③ Cold flow properties
Cold flow properties indicate the lowest temperature atwhich fuel can continue to flow when it is cooled. The
Pour Point (PP) is the temperature below which thefuel loses its flow characteristics.

01 | 2015
CIMAC Guideline
Cold flow properties of marine fuel oils
By CIMAC WG7 Fuels

During the process of wax crystal formation as fuelbegins to cool, Cloud Point (CP) is the temperature atwhich wax crystals start to grow and visibly form in
the fuel. Crystal size is still small, and transparent fuelThis publication is for guidance and gives an overview regarding the assessment of risks associatedwith operating on fuels with poor cold flow properties. The publication and its contents have been
provided for informational purposes only and is not advice on or a recommendation of any of thematters described herein. CIMAC makes no representations or warranties express or implied, regarding
the accuracy, adequacy, reasonableness or completeness of the information, assumptions or analysis
contained herein or in any supplemental materials, and CIMAC accepts no liability in connection
therewith.
The first edition of this CIMAC Guideline was approved by the members of the CIMAC WG7 Fuels
in January 2015.

becomes cloudy. Cold Filter Plugging Point (CFPP)
is the lowest temperature at which these crystals

82

Figure 5-16
01| 2015 CIMAC Guideline Cold flow
properties of marine fuel oils

grow into larger plate shapes and begin to plug the filter. Pour Point (PP) is the lowest
temperature at which the crystals in the fuel grow even larger to the extent that the entire
fuel oil becomes gelled and loses its ability to flow (see Photographs 5-17 and Figure5-18).
According to “01 | 2015 CIMAC Guideline, Cold flow properties of marine fuel oils “
issued by CIMAC (The International Council on Combustion Engines), the temperature
diﬀerence between each of the above stages will be between 2 and 5

for untreatedfuels. In order to ensure eﬃcient management of fuel fluidity at low temperature, thecrew should endeavour to maintain a temperature of at least 10°C higher than the Pour
Point. From a practical point of view, fuel with a low pour point is preferable.
Typical Δtemp（℃）（Untreated fuel）

Cloud Point (CP)2 〜 5℃

Cold Filter Plugging Point (CFPP)
2 〜 5℃

Pour Point (PP)
Photograph 5-17 Wax formation process

Figure 5-18 Definition of cold flow properties

83

Potential issues and countermeasures for machinery/plant systems
affected by cold flow properties
Table 5-19 shows a summary of issues and countermeasures.

Machinery and plant system issues in the engine room
See Fuel oil piping system (2) in Figure 5-21 and wax crystal formation
images in Photographs 5-20, 22, and 23.
1

Inability to transfer fuel oil from hull storage tank by fuel oil transfer
pump.
Reason：Fuel oil temperature has fallen below the pour point (PP) in
the hull storage tank, and the entire fuel oil gels, losing its
ﬂowability. It is impossible to transfer from the hull storage tankto the settling tank in the engine room.

2

Low fuel oil supply, or in the worst case scenario, main engine
shut down (loss of propulsion), or generator shut down (loss of
power).
Reason：Wax crystals can form and clog the strainer in the fuel oil
supply pipeline under the following conditions:
Even if fuel oil temperature exceeds PP, if it falls below CFPP,
wax crystals can form in larger plate-shapes. In this instance,
any wax formed in the hull storage tank will be transferred andlead to potential clogging of filters in the fuel supply pipeline(Cold Filter Plugging Point (CFPP): please see Photographs
5-17 and18).

3

Poor sludge treatment.
When discharging sludge, any imbalance of the oil puriﬁer can lead toabnormal vibrations and may damage the puriﬁer.
Reason：When fuel oil containing wax crystal formations passes
through the oil purifier, the following occurs:① Wax adheres to the separating disc and reduces the separationeﬃciency.
② Wax accumulates on the separating disc in the rotating bowl of
the oil puriﬁer.

84

Countermeasures
The following countermeasures are required:Caution：Maintain, or if necessary, heat the fuel oil temperature to at least
10℃ above the cold ﬁlter plugging point (CFPP+10℃ ).
1When using fuel oil without a heating device, ensure that the operating
environment (*) is PP+10°
C or above, taking into consideration thenavigation route and season.
When fuel oil cannot be used, employ wax suppresants to control wax
formation.
＊ Operating environment: Conditions which aﬀect fuel oil management
include: seawater temperature, surrounding temperature of pipeline,
engine room temperature, and ambient temperature.

2

If wax crystals frequently form on the strainer in front of the puriﬁer,
make the following changes to the oil puriﬁer operating settings:
reduce the flow rate of oil purifiers, shorten interval of sludge
discharge, raise oil treatment temperature, and reduce overhaulinterval of the separating disc for inspections and maintenance.
If necessary, clean the strainer more frequently than usual.

3

In case of emergency, it is advisable to keep a reserve stock ofadditives to mitigate any wax crystal formation on board.

4

When the kinematic viscosity of fuel oil is low(less than 20 cSt @ 50°C), heating is basically unnecessary.
However, if the pour point is high, fuel oil requires appropriate
heating. Be aware though that careful fuel oil management is requiredsince the range of acceptable temperature variation is narrow.
Table 5-19 Cold flow property issues and countermeasures

85

Filter blocked due to wax deposit

Source: Guide for use of 2020 SOx regulation
compliant fuel oils (Maritime Bureau of the Ministryof Land, Infrastructure, Transport and Tourism)

Source:01 2015 CIMAC Guideline

Photograph: ClassNK Guidance

Photograph 5-20 Wax crystal formation in fuel oil

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Figure 5-21 Fuel oil piping system (2)86Photograph 5-22 Wax crystal formation in the fuel oil tankab(a) Clear sample at 28 C

Photograph 5-23(b) Wax crystals formed at 24 C

Wax crystal formation in the diesel oil purifier where the distillate fuel temperature
is below the CFPP of fuel in the oil purifier87

5-2-4 ④ Cat-fines (FCC catalyst particles)
Cat-fines originate from catalyst particles used in the fluid catalytic cracking (FCC)
process. During the FCC process, these catalyst particles break up into smaller particles
and remain in the fuel oil as cat-fines. They are very hard particles composed of alumina(Al2O3) and silica (SiO2).
The catalyst particles are continuously used and recycled during the FCC process.
However, some of the cat-fines catalyst particles still remain in the low sulphur
blendstocks (CLO: Clarified Oil). The presence of cat-fines is evaluated according to thetotal amount of aluminum (Al) + silicon (Si) content.

Potential issues and countermeasures for machinery/plant systems affectedby Cat-fines
Table 5-24 shows a summary of issues and countermeasures

Machinery and plant system issues in the engine roomCat-fines problems (Photograph 5-25) have also occurred on ships using
conventional oil in the past. VLSFO which is rich in CLO has an Al and Si
content approaching the upper limits speciﬁed in the international standard"ISO 8217:2017 Petroleum products ̶ Fuels (class F) ̶ Specificationsof Marine Fuels". When using such VLSFO, any cat-fines which have notbeen properly removed onboard from fuel oil will enter into the engines
and machinery. As a result, if the particle mass of cat fines is large andtheir diameter outstrips oil ﬁlm thickness, engines and equipment may bedamaged, especially sliding parts, such as the fuel injection valve or piston
ring (refer to Figure 5-26, and Photograph 5-27). Potential problems include:（1) Sticking and wear of fuel injection parts / Sticking, wear and nozzle
hole defects of the fuel injection valve(2) Excessive wear and breaking of piston ring / Excessive wear ofpiston ring groove(3) Excessive wear and scuﬃng of cylinder liner88(4) Excessive wear, scuﬃng and excessive leakage of the piston rod andgland packing(5) Wear and blow-by in valve seat on exhaust valve(6) Damage to exhaust gas passage on T/C nozzle and turbine blade

Countermeasures
The oil puriﬁer uses centrifugal force to separate the diﬀerent densities offuel oil and cat-ﬁnes/sludge. If the Al+Si content of fuel oil falls within thevalue specified by ISO 8217 ISO, it is possible to reduce Al+Si content,
before the engine inlet, to the engine manufacturer's recommended value.
To this end, it is necessary for the crew to conduct pre-treatment in thesettling tank, and ensure proper function of the oil puriﬁer by carrying out
the necessary repairs, inspections and maintenance.
The following countermeasures are required:Caution：Ensure the proper operation of the oil purifier and conduct
appropriate pre-treatment of VLSFO in the settling tank asfollows:
1

In order to maintain and improve the separation efficiency of theoil purifier, operation settings should include maintenance of an
appropriate oil treatment temperature, and reduction of the ﬂow rate tojust a little more than expected fuel consumption. Use of two or more oil
puriﬁers in parallel operation can increase puriﬁcation eﬃciency.

2

Whenever bunkering, conﬁrm that oil puriﬁers are functioning correctly
by regularly taking fuel oil samples to a laboratory in order to have Al+ Si content measured. Samples should be taken from (i) ship manifold(inlet of hull storage tank), (ii) before and after the oil puriﬁer, and (iii) atthe engine inlet.

3

It is important to keep the fuel oil level of the settling tank within an
appropriate range.

4

Drain water and sludge from the settling tank should be removed because
cat-ﬁnes deposited in the settling tank are also removed.

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5

When cat-fines are captured by emulsified matter, apparent specificgravity is reduced, and this means a reduction of removal efficiency atthe oil purifier during the pre-treatment process. To prevent this, thewater content of fuel oil should be removed as much as possible andemulsiﬁcation preserved. The following methods are eﬀective:① Prevent the mixing of water with fuel oil.
② Heat the fuel oil settling tank (around 70℃ ), allow suﬃcient settling bygravity, and enhance separation of water content.
③ Be careful not to stir fuel oil upstream of the oil puriﬁer.

6

Backwash oil from the automatic backwash secondary ﬁlter should be
consumed outside the diesel engine and not returned to the settling
tank.

7

Operate the ﬁlter in front of the engine inlet in accordance with theengine manufacturer's operating manual. Pay particular attention to thefollowing:① In principle, use the automatic backwashing ﬁlter side.
② Ensure that the ﬁne ﬁlter in front of the engine inlet and the by-passﬁlter are of the same mesh size. Inspect and maintain the ﬁne ﬁlter
regularly to ensure it is not perforated.
③ Fine ﬁlter cleaning schedules should be logged daily. If cleaning
frequency increases, be sure to improve the fuel separation eﬃciencyof the oil puriﬁer (refer to the countermeasures for compatibility andcold ﬂow properties).

Table 5-24 Cat-fines issues and countermeasures

90Photograph 5-25 Cat-fines particles“Cat-fines particles vary in size and shape”

Excessive wear occurs on sliding surface
between piston rings and cylinder liners.
Cylinder liner
Piston&DWðQHV(AL+Si)Particles

Piston ring

Cylinder liner
Piston ring
/XEULFDWLQJRLOðOP
Figure 5-26

Cylinder abrasion by cat-fines91

(a) Cat-ﬁnes embedded in the cylinder(b) Cat-ﬁnes embedded in thepiston ringand trace of abrasive wear←→

Photograph: ClassNK Guidance

Photograph 5-27 Cat-fines embedded in the piston ring and cylinder liner

5-2-5 ⑤ Ignition and combustion quality
Ignition quality“Ignitability” here is divided into two stages: ignition and combustion quality. Ignition
quality reflects the degree of ease with which a fuel self-ignites. It is generally expressedas the time taken from the initial injection of fuel oil into the combustion chamber toignition (i.e., ignition delay).

FIgure: ClassNK Guidance

Figure 5-28

92

Behavior of diesel sprayFigure 5-28 shows the length of ignition delay for diesel spray. Ignition delay canbe further subdivided into two stages: physical and chemical processes. The processof physical ignition delay begins with fuel oil being injected into a high-pressure
environment from the fine tips of the fuel injection valve whereupon it spreads andatomizes. The atomized oil droplets then draw heat from the surroundings, and as
surface evaporation progresses an air-fuel mixture of fuel vapor and air is formed.
The process of chemical ignition delay begins at the point where the air-fuel ratio (airmass/fuel mass) of combustible air-fuel mixture approaches the stoichiometric air-fuelratio. Chemical ignition delay depends mainly on the ignition quality inherent in the fuelcomponents.
Since both the cetane number and cetane index are not applicable to marine heavy fueloil, CCAI (Calculated Carbon Aromaticity Index) is used as the indicator of ignitability.
CCAI (Calculated Carbon Aromaticity Index) was developed as an indicator for thepractical evaluation of the ignition quality of residual oil in the 1980s. It is an empiricalindex simply calculated from the oil density and viscosity. The higher the CCAI, thehigher the aromaticity is, and consequently the poorer the ignition quality.
The guidelines set forth in “ISO 8217:2017 Petroleum products — Fuels (class F) —Specifications of marine fuels” specify the upper limit value of CCAI for RM (residualoil) grade. The CCAI was developed based on the two following assumptions: “(1) the
ignition delay of fuel correlates to the carbon aromaticity of that fuel” and “(2) there
is a correlation between carbon aromaticity and viscosity/density.” From 2020 various
low sulphur blendstocks will be increasingly blended with VLSFO meaning that thecorrelation between CCAI and actual ignition delay will be less than before.
For the time being, the following formula should be used:

CCAI = ρ 15-81-141・log 10 [ log 10(v+0.85)]-483・log 10
ρ15：density ＠ 15℃ , k g/m3

T+273
323

v ：kinetic viscosity ＠ T ℃, cSt

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Combustion
Whereas the term “ignition” refers to the start of combustion, the term ‘combustion’refers to the latter half of the combustion process. It represents the extended combustion
period, the flame length, and the proportion of unburned components such as blacksmoke and deposits in the combustion chamber.

Photograph 5-29

Ignition and combustion

Potential issues and countermeasures for machinery/plant systems affectedby Ignition and combustion
Table 5-30 shows a summary of issues.

Machinery and plant system issues in the engine room
Compared to conventional fuel oil (HSHFO), VLSFO is blended with higher
levels of LCO and CLO which can inhibit ignitability and ﬂammability.
LCO, mainly composed of 2-ring aromatics, particularly aﬀects the ignition
quality of fuel oil.
CLO, mainly composed of 4 or more ring polycyclic aromatics, particularly
aﬀects the combustion quality of fuel oil.
1 Typical damage on a low speed 2-stroke diesel engine① Excessive wear and breaking of piston ring.
② Excessive wear and scuﬃng of cylinder liner.
③ Excessive wear of piston rod/stuﬃng box packing and excessive

94