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Guidelines on Operation on Distillate Fuels - Low-viscosity fuels
Service Letter SL09-515/CXR
Action code:
WHEN CONVENIENT
Guidelines on Operation on
Distillate Fuels
Low-viscosity fuels
SL09-515/CXR
September 2009
Dear Sirs
Concerns
Owners and operators of MAN B&W
two-stroke diesel engines.
With the introduction of new and tighter fuel sulphur content regula-
tions by the EU and the California Air Resources Board, MAN B&W
two-stroke engines will increasingly operate on distillate fuels, i.e.
marine gas oil (MGO) and marine diesel oil (MDO) in such areas where
required.
Summary
Guidelines on low-viscosity fuel.
MAN Diesel recommends fuels
with a viscosity of 2 cSt at engine inlet.
MAN B&W two-stroke engines are optimised to operate on heavy fuel.
However, MGO/MDO fuels can be used when appropriate consider-
ations are made to the following to ensure a safe and reliable perfor-
mance, i.e.:
• the viscosity and lubricity of the fuel
• keeping a high fuel pump pressure
• the use of a cooler or chiller in an external fuel supply system
• other considerations, references for more information.
This service letter gives guidelines and recommendations on how to
ensure a safe and reliable operation on low-viscosity distillate fuels.
For any questions or inquiries regarding the recommendations in this
service letter, please contact our Operation Department at
the e-mail
address:
leo@mandiesel.com
Yours faithfully
Ole Grøne
Senior Vice President
Promotion and Sales
Kjeld Aabo
Director
Promotion and Customer Support
DIESELswitch for automatic change-over
between HFO and MGO/MDO
Head ofice (& postal address)
MAN Diesel
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
Branch of MAN Diesel SE, Germany
CVR No.: 31611792
Head ofice: Teglholmsgade 41
2450 Copenhagen SV, Denmark
German Reg.No.: HRB 22056
Amtsgericht Augsburg
MAN Diesel – a member of the MAN Group
Service Letter SL09-515/CXR
Contents
Introduction
page 2
ISO 8217
According to ISO 8217, distillate grades DMX/DMA can be
sold with a viscosity down to 1.4/1.5 cSt at 40°C. This will
especially be the case if the DMX/DMA provided origins from
automotive gas oil. The 1.4/1.5 cSt can only be applied if the
distillate is cooled/chilled down correspondingly to reach the
2 cSt minimum viscosity at engine inlet.
ISO 8217
page 2
Inluence of lubricity and viscosity
page 2
Fuel oil pump pressure
page 3
Installation of cooler or chiller & cooler
page 4
Other considerations
page 5
Introduction
The lowest viscosity suitable for two-stroke diesel engines
is 2 cSt at engine inlet. However, this viscosity limit cannot
necessarily be used as a fuel speciication for purchasing the
fuel, as the viscosity in a purchase speciication is tied to a
reference temperature. This is due to the fact that the exter-
nal fuel systems have an individual effect on the heating of
the fuel and, thereby, the viscosity of the fuel when it reaches
the engine inlet.
Inluence of lubricity and viscosity
Lubricity
The reinery processes intended to remove, e.g., sulphur
from the oil result not only in low viscosity, but also impacts
the lubricity enhancing components of the fuel. Too little lu-
bricity may result in fuel pump seizures.
Although most reiners add lubricity-enhancing additives to
distillates, MAN Diesel recommends testing the lubricity be-
fore using fuels with less than 0.05% sulphur. Independent
fuel laboratories can test lubricity according to ISO12156-1
(High-Frequency Reciprocating Rig, HFRR). The HFRR wear
scar limit is max 460
µ
m.
The external fuel oil systems on board today have been de-
signed to keep a high temperature for HFO operation. This
can make it dificult to keep the fuel system temperature as
low as possible, and thereby as high a viscosity as possible,
when changing to DO and GO operation. The crew must
therefore make an individual test.
Viscosity
A low viscosity fuel oil challenge the function of the pump in
three ways: 1. Breakdown of hydrodynamic oil ilm (result-
ing in seizures), 2. Insuficient injection pressure (resulting in
dificulties during start and low-load operation), and 3. Insuf-
icient fuel index margin resulting in limitation in acceleration.
Many other factors inluence viscosity and its inluence on
the engine, such as engine condition and maintenance, fuel
pump wear, engine adjustment, actual fuel temperature in
the fuel system, human factors, etc. Although achievable, it is
dificult to optimise all of these factors at the same time. This
complicates operation on viscosities in the lowest end of the
viscosity range.
Due to the design of conventional pumps versus the pres-
sure booster, ME/ME-C/ME-B engines are more tolerant to-
wards a low viscosity compared with the MC/MC-C engines.
Many factors inluence the viscosity tolerance during start
and low-load operation:
To build in some margin for safe and reliable operation, MAN
Diesel recommends operators to test the engine’s and ex-
ternal systems’ sensitivity to low viscosity. Furthermore, the
necessity for installation of a cooler or cooler & chiller should
be evaluated before purchasing fuels with the minimum level
of viscosity necessary.
• Engine condition and maintenance
• Fuel pump wear
• Engine adjustment
• Actual fuel temperature in the fuel system
• Human factors, etc.
In principle, fuels according to the speciied grades DMX/
DMA can be purchased, if the engine and external system
are designed to keep a minimum viscosity of 2 cSt at engine
inlet. If 3 cSt can be obtained, this is preferred to ensure a
higher safety margin.
Although achievable, it is dificult to optimise all of these fac-
tors at the same time. This complicates operation on viscosi-
ties in the lowest end of the viscosity range. To build in some
margin for safe and reliable operation, and availability of high-
viscosity distillate fuels, it is expected that installation of cool-
ers or cooler & chiller will be necessary for many operators.
Guidelines on Operation on Distillate Fuels. Page 2 of 5 pages
Service Letter SL09-515/CXR
Fuel oil pump pressure
Worn fuel pumps increase the risk of starting dificulties, as
the fuel oil pump pressure needed for injection cannot be
achieved. An indication of fuel pump wear can be achieved
by reading the actual fuel pump index for comparison with
the test bed measurements. As a rough guideline, we con-
sider the pump worn out when the index increase is 10 or
more. Such fuel pumps should be replaced for better engine
performance.
• At different operating conditions, e.g. start, idle, astern
and steady low rpm, gradually change the temperature
of the fuel at engine inlet, corresponding to respectively
2, 2.5 and 3 cSt, see Fig. 1 for the typical viscosity and
temperature relationship.
• Test start ahead/astern from the control room. If the
engine does not start at the irst attempt, cancel and re-
peat the start attempt. If the start ahead/astern functions
properly with cancelled limiter, this solution can be used
temporarily until either new fuel pumps are installed or a
higher viscosity fuel becomes available.
It is always advisable to make start checks at regular intervals.
However, as distillates of required minimum viscosity may not
be available in all ports, it is an imperative necessity to per-
form start checks prior to entering high-risk areas (e.g. ports
and other congested areas). By such action, the individual low
viscosity limit can be found for each engine. It is recommend-
ed to perform such a check twice a year, in the following way:
An outcome of the test might be that the speciic engine
requires a viscosity that cannot be kept due to the inluence
from the many factors. If the fuel pumps are worn, they must
be replaced and the start check repeated.
• In an area for safe operation, change fuel to an available
distillate.
Fuel Temperatur vs Viscosity
140
1.5 cSt
120
NOT GOOD
Fuel below 2cSt
MAN Diesel does not recommend to operate the
engine on fuel with viscosities lower than 2 cSt
2.0 cSt
100
DEPENDING ON INSTALLATION
Fuel viscosity 2-3 cSt
MAN Diesel strongly recommends to make start checks
prior to port operation
3.0 cSt
80
4.0 cSt
60
5.0 cSt
40
Viscosity at reference condition (40°C) according to ISO8217 DMA/X
20
GOOD
Fuel above 3 cSt
MAN Diesel recommends to operate the engine on
fuels with viscosities above 3 cSt
0
1
2
3
4
5
6
7
8
Viscosity (cSt)
The horizontal axis shows the bunkered fuel viscosity in cSt, which should be informed in the bunker analysis report. If the temperature of the
MGO is below the lower red curve at engine inlet, the viscosity is above 3 cSt.
The black thick line shows the viscosity at reference condition (40°C) according to ISO8217, marine distillates.
Example: MGO with viscosity of 4 cSt at 40°C must have a temperature below 55°C at engine inlet to ensure a viscosity above 3 cSt.
Example: MGO with a viscosity of 5 cSt at 40°C is entering the engine at 50°C. The green curves show that the fuel enters the engine at
approximately 4.0 cSt.
Example: MGO with a viscosity of 2 cSt at 40°C needs cooling to 18°C to reach 3 cSt.
Fig. 1: Fuel temperature vs viscosity
Guidelines on Operation on Distillate Fuels. Page 3 of 5 pages
Service Letter SL09-515/CXR
Installation of cooler or cooler & chiller
To be able to maintain the required viscosity at the engine
inlet, it is necessary to install a cooler in the system. Fig. 2
shows the recommended location to install a cooler.
External pumps
Not only will the engine fuel pumps be inluenced by the fuel
viscosity. Also most pumps in the external system (supply
pumps, circulating pumps, transfer pumps and feed pumps
for the centrifuge) need viscosities above 2 cSt to function
properly. We recommend contacting the actual pump maker
for advice.
For the lowest viscosity distillates, a cooler may not be
enough to cool the fuel suficiently due to the cooling water
available onboard. In such a case, it is recommended to in-
stall a so-called ‘chiller’. The chiller principle is shown in Fig. 3.
Aut. de-aerating valve
Deck
Venting tank
Heavy fuel oil
service tank
Diesel
oil
service
tank
Temperature
f
eeler
Cooling
medium
inlet
Overflow valve
Adjusted
to
4 bar
Main
engine
Visco.
HFO
Diesel oil
c
ool
e
r
Pre-heater
Circulating
pumps
Supply pumps
Full flow filter
Steam
inlet
Condensate
outlet
Fig. 2: Fuel system (cooler installed after the circulating pumps)
Chilling unit
Compressor
Water pump
u
n
it
Pressurised
expansion
tank
Diesel oil cool
er
u
nit
Condenser
Water
coole
r
Diesel oil
coole
r
Water
tank
Pump
Central
cooling
water
in-/outlet
Diesel Oil
Water
Refrigerant Liquid
Central Cooling Water
Fig. 3: Chiller principle
Guidelines on Operation on Distillate Fuels. Page 4 of 5 pages
Service Letter SL09-515/CXR
Other considerations when operating
on distillates
The following items are described in more detail in a separate
MAN Diesel paper “Operating on distillate fuels”:
• external pumps
• pour point restrictions (distillates should not be cooled
below pour point)
• change-over between HFO and MGO/MDO (to ensure
optimal conditions for the fuel pumps)
Also described in the instruction book, Operation
• DIESEL
switch
for automatic change-over between HFO
and MGO/MDO
• correlation between fuel sulphur level and cylinder condi-
tion (choice of cylinder lube oil).
Contact PrimeServ for more information on installation of
coolers/chillers and DIESEL
switch
(e-mail:
Primeserv-cph@
mandiesel.com
)
.
For operational assistance, please contact our Operation De-
partment at e-mail:
leo@mandiesel.com
.
Guidelines on Operation on Distillate Fuels. Page 5 of 5 pages
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