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Control of fuel oil viscosity explained
Fuel is the lifeblood on marine engines and hence proper characteristics of fuel need to be maintained on ships. Viscosity is one of the most important factors which directly affects combustion quality.
Controlling the viscosity of fuel oil is an important aspect of an efficient combustion. A high viscosity fuel oil leads to improper atomisation which in turn leads to incomplete combustion.High viscosity fuel prevents correct atomisation,which takes place in the fuel injectors. An efficient atomisation is the basic need for the healthy mixing of fuel and heated air, without which, no ignition or combustion can be derived. Thus, it is absolutely important to set the viscosity value in the right range.
Viscosity of high fuel oils can be reduced by raising its temperature with the help of a fuel oil heater. This can be done by using either, automatically regulated heaters in order to maintain a constant temperature or by using a manual control which can be adjusted according to the requirement.
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What leads to high viscosity of fuels?
It is a usual practise on ship, to store fuels of different grades and properties into the same tank. Even if they are stored in different tanks, they finally get mix with each other when brought to the service tank. This practise of deriving fuels from different sources is the main reason for the high viscosity of fuel oil.It is for this reason that the viscosity of oil is checked and maintained within close limits, before sending it to the engine's cylinder. Generally, fuel oil viscosity is regulated between 10-15 centistrokes at 50 degree Celsius.
Viscosity regulators are used to regulate and maintain the fuel oil viscosity.
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Viscosity Regulator or Viscotherm, is a device used to measure viscosity of the fuel oil before sending it to the engine. It is generally fitted at the outlet of the fuel oil heater. The regulator is connected with the heater so that it can measure the viscosity of the oil and regulate the temperature at the same time.
Viscosity regulator is an "L" shaped instrument which consists of a small gear pump that rotates at a constant slow speed of 40 rpm. The pump takes suction from the heater discharge and is generally fitted close to it. A regulated flow of fuel is sent by the pump to a capillary tube. The capillary tube is so designed that the form of flow between the inlet and outlet of the tube generates a pressure difference, which is equal to the viscosity of oil flowing through it.
Bourdon tubes are used to monitor the viscosity at different points within the capillary tube. Thermometer is fitted on the regulator to measure the fuel temperature and maintain the final temperature accordingly. Differential pressure transmitters are also fitted to directly control the heater temperature and set the right viscosity limit.
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Diesel Engines 3rd Edition, by A.J Wharton