Uni means one.
Uni fuel means one fuel system for both generators and for main engine
Fuel Oil System
- the ‘Unifuel’ system
MAN B&W Diesel’s two-stroke low speed diesel
engines and MAN B&W Holeby four-stroke diesel
GenSets are designed to operate in accordance with
the unifuel principle, i.e. with the same fuel for both
main and auxiliary diesels.
For guidance on purchase, reference is made to ISO
8217, BS6843 and to CIMAC recommendations
regarding requirements for heavy fuel for diesel
engines, edition 1990. From these, the maximum
accepted grades are RMH 55 and K55. The
mentioned ISO and BS standards supersede BS MA
100 in which the limit is M9.
Based on our general service experience, and as a
supplement to the above-mentioned standards, we
have prepared a guiding fuel oil specification, shown
in Fig. 8. Fig. 9. Heavy fuel oil treatment concept
Density 15°C kg/m³ 991 *
Kinematic viscosity
at 100°C cSt 55
at 50°C cSt 700
Flash point °C ³60
Pour point °C 30
Carbon residue %(m/m) 22
Ash %(m/m) 0.15
Total sediment after ageing %(m/m) 0.10
Water %(v/v) 1.0
Sulphur %(m/m) 5.0
Vanadium mg/kg 600
Aluminium+ silicon mg/kg 80
Equal to ISO 8217/CIMAC - H55
* 1010 provided automatic modern clarifiers are
installed
Fig. 8. Guiding fuel oil specification
On heavy fuel oil research we have, in Copenhagen
and on board ship, run several tests with modified
injection equipment to establish a basis for experience
and confirm development within injection
equipment, fuel treatment before injection, and
emission. In 1995, a representative from MAN B&W
Diesel has been elected chairman of the CIMAC
Heavy Fuel Oil working group.
The common system covers the entire fuel oil flow
from storage tank to injection into the engine cylinders.
With regard to centrifuge recommendations, fuel oils
should always be considered as contaminated upon
delivery and should therefore be thoroughly cleaned
to remove solid as well as liquid contaminants before
use. The solid contaminants in the fuel are mainly
rust, sand, dust and refinery catalysts. Liquid contaminants
are mainly water, i.e. either fresh water or
salt water.
Impurities in the fuel can cause damage to fuel
pumps and fuel valves, and can result in increased
cylinder liner wear and deterioration of the exhaust
valve seats. Also increased fouling of gasways and
turbocharger blades may result from the use of
inadequately cleaned fuel oil.
Effective cleaning can only be ensured by using a
centrifuge.
Results from experimental work on the centrifuge
treatment of today’s residual fuel qualities have
shown that the best cleaning effect, particularly in
regard to the removal of catalytic fines, is achieved
when the centrifuges are operated in series, i.e. in
purifier/clarifier mode.
This recommendation is valid for conventional centrifuges.
For more modern types, suitable for treating
fuels with densities higher than 991 kg/m3 at 15°C, it
is recommended to follow the maker’s specific
instructions.
In view of the fact that some fuel oil standards
incorporate fuel grades without a density limit, and
also the fact that the traditional limit of 991 kg/m3 at
15°C is occasionally exceeded on actual deliveries,
some improvements in the centrifuging treatment
have been introduced to enable the treatment of
fuels with higher density.
With such equipment, adequate separation of water
and fuel can be carried out in the centrifuge, for fuels
up to a density of 1010 kg/m3 at 15°C. Therefore, this
has been selected as the density limit for new high
density fuel grades.
Thus high density fuels are fully acceptable for our
engines provided that appropriate centrifuges are
installed. They should be operated in parallel or in
series according to the centrifuge maker’s instructions
