Friday, April 27, 2012

Fuel Change-over

Changing fuel from DO to HFO 

A fuel change-over from DO to HFO can be performed when the engine has been put on load and the normal operating temperatures have been reached.
1.  Start up the HFO supply system, if not already in operation.
1.1  Check that the tank heating and the trace heating are activated.
1.2  Open the inlet and outlet valves of the HFO tanks as required.
1.3  Start the fuel separation and transfer system.
1.4  Check the level in the HFO day tank. Also check that the fuel temperature in the tank is correct.
1.5  Start the HFO feeder.

2.  Check that the trace heating in the fuel circulation system is activated.
3. Check that all valves in the fuel circulation system are in the correct positions. Make sure that the fuel heater is not bypassed.
4. Push the HFO selection button on the control panel or give a fuel change-over. The fuel selection valve turns to the HFO position.
5. If no engines are running on DO, the DO feeder may be stopped.

Fuel change over valve
Fuel change over valve
Changing fuel from HFO to DO

A fuel change-over from HFO to DO is normally performed when the engine is to be shut down and the fuel system should be flushed with DO.

1.  Start the LFO supply system, if not already in operation.
1.1  Check the level in the DO tanks.
1.2  Check that all valves are in the correct positions. The outlet valves of the tanks must be open.
1.3  Start the DO transfer pump.
1.4  Start the DO feeder.

2. Push the DO selection button on the control panel or give a fuel change-over order. The fuel selection valve turns to the DO position.

3. Check that the valves by the return fuel cooler are in the correct positions. Also make sure that the cooling water circuit of the heat exchanger is properly filled and vented. During prolonged operation on DO, the fuel cooler must be in operation to prevent overheating.

4. If no engines are running on HFO, the HFO feeder may be stopped. The fuel separators may be kept in operation as required, in order to fill the HFO day tank with treated fuel. Make sure that the trace heating if activated if the circulation is stopped in a system filled with HFO.

Piston rings are to be diagnosed at regular intervals on the basis of the wear pattern. Piston rings have to be replaced after they have been use for time periods given in the maintenance schedule. Replacement will also be necessary if interim checking reveals damage to running surface or excessive clearances.

Different Piston Rings:
The rings being in use are compression rings with chromium-plated, Plasma-coated or (the most recent solution) chromium/ceramic-coated running surfaces, and chromium-plated oil scraper rings.

Piston rings for MAN B&W 9L 58/64
Compressor Ring

Running Surface
Groove 1
Packing ring
Groove 2
Tapper ring
Groove 3
Tapper ring
Oil scraper ring

Groove 4
Oil scraper ring

Wear appearance of plasma-coated, chrome-plated or chromium/ceramic-coated piston rings
Due to its shape, the running face of new compression rings does not have contact across its full width but only the width of the actual contact surface A. With the progress of wear, the actual contact surface width will increase.

Figure 1. Running face of plasma-coated and chrome-plated piston rings (time in use > 1000 hrs)

Figure 2. System of cracks on chromium/ceramic-coated piston rings (time in use approx. > 8000 hrs)
At the end of the useful life, the actual contact surface extends up to the chamfers or radii of the ring sides, please see Figure 3.

Figure 3. Piston ring of good running pattern at the end of the useful life.
The chamfers or radii are still distinctly visible on the piston ring shown in Figure 3. Coated compression rings have to be replaced when the actual contact surface extends over the entire piston ring width, i.e. when the chamfers or radii are hardly visible any more. In this condition, the residual thickness of the coating will not safely endure the subsequent maintenance interval.
Burns (C) produce local discoloration of the piston ring face. Such rings have to be replaced. (See Figure 4).

Figure 4. Piston ring with burns on the face
Coated compression rings and oil scraper rings have also to be replaced if the coating has worn through at one spot.
Figure 5. Piston with worn-through coating
Figure 5 shows a chrome-plated compression ring. Turn chromium layer has completely worn spot wise at (D).
Broken-off particles or cracks in the coating of a piston ring also call for its replace.
Figure 6. Piston ring with broken-off particles in the face
Figure 6 shows a plasma-coated compression ring. Parts of the coating have broken off at (E).
Chromium/ceramic-coated compression rings have to be replaced when the continuous, micro-cracked surface has decayed to a crazed crack appearance. The cracks will then have extended down to the base material, which involves the danger of particles breaking away.

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