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02.1.1.1. Fuel separation

Most fuels, except good quality distillate fuels (ISO-F-DMX, DMA and DMB) must to be separated by a centrifugal separator before entering the engine. However, also separation of distillate fuels shall be con‐ sidered, since fuels can e.g. be contaminated in storage tanks. The separator removes solid particles down to 5 microns and additionally free water from the fuel very efficiently. Even smaller particles are separated, but with reduced efficiency. There are several things to be taken into consideration, when designing the separation system. The following fuel parameters are the most important:

Viscosity

The viscosity is very critical when dimensioning the separator. Higher fuel viscosity means lower separation capacity for a certain separator model and will require a bigger separator size (or more separators) for the same amount of fuel/hour to be separated. The separator manufactures have tables with flow capacities for standard fuel vis‐ cosities. Standard separators can handle fuels up to 700 cSt at 50 °C. The viscosity is lowered as much as possible by heating the fuel to maximum temperature, which in standard HFO separators is 98 °C. For liquid biofuels the separation temperature is typically 50-55 °C. Also higher separation temperatures than 98 °C are possible to utilize, but then the question is not about standard separators and those need to be designed case by case. Higher separation temperatures are used with extremely high viscosity fuels. Separation temperature for distillate fuel is typically 40 °C.

Density

The separation of water from fuel is based on the difference in density between those two fluids. The water should always be heavier than the fuel to make the separation process possible. This limits the max‐ imum density of the fuel to 1010 kg/m³ at 15 °C for standard separa‐ tors and the separation capacity is de-rated when the fuel densities are between 990 and 1010 kg/m³ at 15 °C. It is also possible to sep‐ arate fuels with higher density than 1010 kg/m³ at 15 °C, but this re‐ quires a special separator including a separate water treatment sys‐ tem to increase the density of the operating water. In that case it’s not possible to separate water from fuel, only solid particles.

Flash point

Flash point of heavy fuels varies a lot and some heavy fuel qualities are at a temperature above their flash point and some respectively below their flash point when separated. Distillate fuels are normally separated at a temperature being below their flash point. For special fuels, like many crude oils, having low flash point and containing light,

with explosion proof electrical motors and other electrical components are needed. With these fuels also a system avoiding explosion inside the separator bowl by adding an inert gas is required. These systems are expensive and are used only in special cases.

Water content

If the water content is > 0.3% and the fuel density is > 990 kg/m³ at 15 °C the separator is de-rated (see diagram below).

Dimensioning of the separator

Requirement for the fuel separator unit flow is calculated according to the following formula:

V

n

b

c

f

ρ

1

100

⎛_⎜

+

⎞_⎟

⎝

⎠

=

VHFOS = Required separator capacity [m3/h]

n_ENG = Amount of engines [pcs]

ρFUEL = Density at actual temperature [kg/m3]

bE = Fuel consumption / Engine [kg/h]

c_S = Separator safety factor [min. 15% is Alfa Laval’s recommenda‐ tion]

De-rating factor depending on the fuel density and water content 990 992 994 996 998 1000 1002 1004 1006 1008 1010 1 0,9 0,8 0,7 0,6 0,5 0,4 Density kg/m3 at 15 C De-rating factor Water content >0.5% Water content 0.3-0.5% Water content < 0.3% Fig 02-1 V1 Separator modules

The suppliers are making complete modules today including heaters and feeder pumps, which means that the equipment are automatically calculated and ready for the customer demand. The modules include a number of separators needed to meet the capacity of the installation along with an extra stand-by separator. This is made to ensure fuel supply also during the service of one separator.

02.1.1.2. Heating

See diagram, Fig 02-2. Keep the fuel temperature about 10 °C above the minimum storage temperature indicated in the diagram in order to minimize the risk of wax formation, and the temperature after the final heater 5 - 10 °C above the recommended temperature before injection pumps to compensate for heat losses between heater and engine.

Fuel oil viscosity-temperature diagram C -10 0 10 20 30 40 50 60 70 80 90 100 110 120 130 140 150 3 4 5 6 7 8 9 10 12 14 16 20 25 30 40 50 60 80 100 200 300 400 600 1000 2000 5000 Centistokes F D E C K B G H A 14 cSt at 40 C GAS OIL

MARINE DIESEL OIL

11 cSt at 40 C 5.5 cSt at 40 C

APPROX. PUMPING LIMIT

VISCOSITY BEFORE 80 cSt at 50 C 180 cSt at 50 C 380 cSt at 50 C 700 cSt at 50 C 40 cSt at 50 C FUEL PUMPS CENTRIFUGING TEMPERATURE TEMPERATURE MINIMUM STORAGE MAX. TEMP RECOMMENDED RANGE Fig 02-2 320261 V1

Example:A fuel oil with a viscosity of 380 cSt (A) at 50 °C (B) or 80 cSt at 80 °C (C) must be preheated to 112 - 126 °C (D-E) before the fuel injection pumps, to 97 °C (F) at the centrifuge and to minimum 40 °C (G) in storage tanks. The fuel oil may not be pumpable below 36 °C (H).

To obtain temperatures for intermediate viscosities, draw a line from the known viscosity/temperature point in parallel to the nearest vis‐ cosity/temperature line in diagram.

Example:Known viscosity 60 cSt at 50 °C (K). The following can be read along the dotted line: Viscosity at 80 °C = 20 cSt, temperature at fuel injection pumps 74 - 86°C, centrifuging temperature 86 °C, minimum storage tank temperature 28 °C.

Conversion from various current and obsolete viscosity units to cen‐ tistokes can be made in the diagram, Fig 02-3. The diagram should be used only for conversion of viscosities at the same temperature. The same temperatures should then be used when entering the vis‐ cosity/temperature point into the diagram, Fig 02-2.

Viscosity conversion diagram Centistokes 3 4 5 6 7 8 9 10 12 14 16 20 25 30 40 50 60 80 100 200 300 400 600 1000 2000 5000 10 20 50 100 200 500 1000 2000 5000 10000 Sec. Saybolt Furol

1 2 5 10 20 50 100 200 500 1000

¡ Engler

10 20 50 100 200 500 1000 2000 5000 10000 Sec.Redwood I

10 20 50 100 200 500 1000 2000 5000 10000 Sec. Saybolt Universal

Fig 02-3 320253 V1

When converting viscosities from one of the units on the abscissa to centistokes or vice-versa, keep in mind that the result obtained is valid only at one and the same temperature. When converting the viscosity in any unit at a given temperature to a viscosity at another tempera‐ ture, a viscosity-temperature diagram or conversion rule must be used.

02.1.1.3. Viscosity control

An automatic viscosity controller, or a viscosimeter, at least, should be installed in order to keep the correct viscosity of the fuel before the fuel enters the engine fuel system.