VISCOTHERM

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The sketch shows the operating principle of a viscotherm, a device to measure and control the viscosity of heavy fuel oil. The viscosity of a fuel is dependant on the temperature; The higher the temperature, the lower the viscosity.

Made of stainless steel, the housing contains the measuring element which is a capillary tube connected to the discharge of a gear pump driven by an electric motor through a reduction gear and magnetic coupling.

Mounted on the engine fuel supply line after the pump and heaters, the gear pump supplies the capillary with a continuous and constant flow. Because the flow through the capillary is laminar, the difference in pressure between each end of the tube is directly proportional  to the viscosity of the oil (the higher the viscosity, the higher the pressure differential).

A tapping from just before the capillary and a tapping in the main body is led to a transmitter which measures the difference in the pressure using a dp cell and converts it to an output signal. This output signal is compared to the desired value signal and the difference outputted as a proportional air signal between 0.2 and 1 bar or as  an electrical signal between 4 - 20 mA DC which is then amplified to operate the heater temperature control valve, which opens or closes to allow more or less steam to the fuel oil heater.

As the viscosity of the fuel increases so the differential pressure increases. The output signal from the dp transmitter increases and the heater temperature control valve opens further. If the viscosity falls then the differential pressure decreases and the heater temperature control valve closes