Explain the operation of equipment described in Q.(a)
Discuss the single fuel concept.
a.
The sketch shows the system for automatic monitoring and regulation of fuel viscosity. The main components of the system are:
Fuel Oil Heater: Uses steam as the heating medium although electrical models are in existence. Tubular design.
Viscotherm: Basically consists of a small gear pump that pumps some of the oil through a capillary tube. The pressure in the capillary increases as the viscosity of the oil increases.
DP Transmitter: Converts and transmits the differential pressure measured at the viscotherm to an air signal.
P+I Controller: Amplifies the air signal from the DP cell and feeds it to the steam control valve via the Hand/Auto station.
Hand/Auto Station: Can be used to directly control the air signal to the steam control valve.
Control Valve: Controls the amount of steam flowing to the heater.
b.
Consider a rise in the viscosity of the fuel being delivered to the engine. The pressure in the capillary of the viscotherm increases, leading to a rise in the differential pressure between the beginning and end of the capillary. This increased differential pressure is converted into an air signal which is proportional to the rise in differential pressure. The amplified output signal is fed via a hand/auto station to a control valve which opens the steam control valve. The valve position may be fed back to the transmitter to provide more accurate control of the valve and prevent hunting. The viscotherm is fitted with a bypass, and the steam control valve can be operated by hand from the hand/auto station.
c.
When engines started burning the lower grades of residual fuel, the systems incorporated diesel change over valves, because prior to manoeuvring (i.e. stand by arrival, departure, canal and river transits etc.), the engine was changed over to run on the distillate marine diesel oil which required no heating. Older engine fuel pump and injector systems did not incorporate recirculation valves, and so if the engine was at standstill any heated fuel in the long HP fuel pipes would cool, and would not be at the correct viscosity for injection when the engine was restarted. This would lead to poor combustion, and associated pollution until the fuel at the correct viscosity reached the injectors. In extreme cases, the injectors would have to be bled before the engine could be started.
Modern 2 stroke injectors incorporate recirculation valves, so that there is no need to change over to MDO for manoeuvring purposes, as the oil at the injectors is always at the correct temperature to give it the correct viscosity. This represents a considerable saving to the ship operator as the difference in cost between residual and distillate fuel is considerable.
An exception to the above is when the engine is to be shut down for maintenance to the injection pumps. MDO is often used for the last 30 minutes or so before shutdown, to avoid the mess made by heavy oil when opening up fuel pumps etc.
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