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Viscosity
Knowledge of bunker fuel viscosity is important for several reasons, as it determines the temperature for handling, the size of the centrifuges and the temperature at which the fuel is injected into the engine. It is well known that as the temperature of the fuel is increased, the viscosity is reduced.
Some oil suppliers publish temperature/viscosity charts that are based on the average data of a large number of samples. However, estimations from the charts cannot be regarded as precise, as the exact relationship depends on the source and composition of the fuel.
The viscosity usually quoted for a residual fuel oil is the kinematic viscosity expressed in centiStokes (cSt) at a reference temperature. ISO 8217:2010 uses 50°C, a value widely employed throughout the industry.
Although fuel may have been ordered to one of the grades in ISO 8217, frequently on delivery only the viscosity grade is stated. For example IF 180 - this means that the viscosity is a maximum of 180 cSt at 50°C. The majority of engine installations are fitted with fuel viscosity controllers so it is not normally necessary to estimate the injection temperature. If it is necessary, the chart shown can be used. Whilst a satisfactory injection temperature may be attained, it must be appreciated that the performance of the centrifuge may fall below design conditions.
Typically, a fuel with a viscosity up to 15 - 20% above that ordered can still be successfully used in the fuel treatment plant and engines. If the centrifuge is only marginally sized, the slight increase in viscosity will reduce its performance. This may be overcome, if the piping configuration permits, by operating two purifiers in parallel with a clarifier in series. There are also various ways a small increase in temperature may be achieved. One is by applying more heat to the fuel oil heater, but heater element surface temperatures should not go above 180°C and temperature at inlet to the centrifuge above 98°C. This may not be possible, especially if fouling has occurred in the heater. Alternatively, the temperature of the heavy fuel service tanks can be raised.
| Injection temperatures for range of viscosities | |||||
|---|---|---|---|---|---|
| Injection Viscosity | Injection Viscosity | ||||
| Fuel | 13 cSt | 17 cSt | |||
| IF 180 | 119°c | 109°c | |||
| IF 200 | 121°c | 111°c | |||
| IF 220 | 123°c | 113°c | |||
| IF 240 | 125°c | 115°c | |||
| IF 380 | 134°c | 124°c | |||
| IF 400 | 135°c | 125°c | |||
| IF 240 | 136°c | 126°c | |||
| IF 460 | 138°c | 127°c | |||
Comment
Because of the viscosity & temperature relationship, a few extra degrees of pre-heating can often make a fuel suitable for injection equipment. In practical terms, this means that if the actual fuel viscosity is greater than that ordered, it is likely that the fuel oil heater can accommodate this. The table shows a typical temperature / viscosity chart of residual fuel. From this it may be seen that if the delivered viscosity is marginally above IF 180, the practical effect on injection temperature is only a few degrees. Tank heating capacity must also be considered before accepting a high viscosity fuel.
The operational effect of a bunker fuel viscosity lower than ordered is discussed separately under the combustion section.
Click to view the Kittiwake Heated Viscometer, one of a range of Kittiwake tests for bunker fuel viscosity.
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