La organización de la educación: entre autonomía y dependencia

1 General

The following chapters of the instruction manual describe the operation and mainte-nance of the pressure part. As this is only a part of the complete boiler plant, it is important to study the remaining chapters in this manual very thoroughly. It is espe-cially important that the operator of the boiler plant becomes familiar with the operation instructions of the burner and the control panel.

Important: To ensure a safe and reliable operation of the boiler plant, all oper-ation and/or maintenance of the boiler should be carried out only by skilled personnel.

2 Boiler operation modes

The MISSION OC (-TCi) boiler is a combined oil fired and exhaust gas fired boiler.

The combination between an oil fired section and an exhaust gas fired section makes it possible to operate the boiler in different modes. The different modes are:

- Operation only on the oil fired section

- Operation only on the exhaust gas fired section

- Operation on both the oil fired and exhaust gas fired section

2.1 Boiler operation modes

2.1.1 Combined operation mode

In normal operation, the boiler operates as a combined oil fired and exhaust gas fired boiler. The steam production is controlled by the amount of exhaust gas from the engine and the gauge board which controls the oil burner. If the engine produces an insufficient amount of exhaust gas to maintain the steam pressure, the gauge board gives a signal to initiate start of the oil burner. The oil burner operates as long as the steam pressure does not exceed the set point for burner stop. If the steam pressure increases above this set point, the gauge board gives a signal to initiate stop of the oil burner.

The combined operation mode always requires that the water level is within the normal limits of the boiler.

2.1.2 Oil fired operation mode

If the engine is stopped, the only heat source is the oil burner. When the steam pressure is below the set point for start of the burner, it will be in operation. By means of signals from the gauge board, the control system aims to maintain the desired steam pressure regardless of the steam output. The oil burner operates as long as the steam pressure does not exceed the set point for burner stop.

The oil fired operation mode always requires that the water level is within the normal limits of the boiler.

2.1.3 Exhaust gas fired operation mode

When the exhaust gas flow through the boiler, produced by the main engine, is suffi-cient to maintain the steam pressure, the burner will not be in operation. If the exhaust gas flow is above the required flow to maintain the steam pressure at a given steam output, the pressure increases. At a pre-selected set point the steam dump equipment, located elsewhere in the steam system, opens the dump valve and regulates the steam line pressure. If, however, the steam dump equipment is not operational the boiler pressure may rise above the set point for high steam pressure shut down and even up to the maximum allowable working pressure. At this point, the safety valves open and ensure any further increase of the steam pressure. Please note that some classification societies require a separate switch which reduces the engine load before the maximum allowable working pressure is reached.

Note: If the high steam pressure shut down has been activated, the oil burner will not be operational until the shut down function can be and is reset in the control system.

The construction of the MISSION OC (-TCi) boiler permits operation with low water level and even without water in exhaust gas mode with the full exhaust gas flow through the smoke tubes. In the latter case the boiler must be operated depressurised.

Please note that operation without water should be considered as an emergency oper-ation mode and should only be used if no other operoper-ation alternatives are present. The most effectual operation of the boiler is with the water level within the normal limits as this creates the most stable operating conditions

Warning: Before start of the boiler it must be drained and the manhole cover should be dismounted to ensure that it is operated completely depres-surised. Please note that the temperature inside the boiler must not exceed 400°C.

If the boiler operates without water, it will gradually be heated to the same temperature as the exhaust gas. It is therefore important to allow the boiler to cool before refilling it with feed water. If this is not possible, the feed water should be pre-heated, and the refilling should be carried out very carefully.

Note: If the too low water level shut down has been activated, the oil burner will not be operational until the shut down function can be and is reset in the control system.

Furthermore, operation of the boiler without water eventually dries out the gaskets, and a replacement of the gaskets is necessary.

3 Start-up

When the boiler is commissioned or started during normal operation, it is preferable to lighten-up the boiler with the oil burner as the heat source. This is because the oil burner load is much easier to control than the exhaust gas amount from the engine.

It is possible, however, to start the boiler when exhaust gas is used as heat source. This requires more attention with regard to water level control, pressurising pipes, etc.

When the boiler is started and lightened-up by means of the exhaust gas flow, it is important to reduce the heating-up rate in order not to cause stresses in the boiler.

This can be done by reducing the engine load to fit the appropriate heating-up rate or by by-passing some of the exhaust gas flow if a by-pass system is provided.

3.1 Before start-up

When the boiler is started, the lighten-up rate of the boiler must not be accelerated too much as this might cause an unnecessary overstrain of the boiler material by quick and uneven temperature rises. It might be necessary to perform a number of start/stop sequences to reduce the lighten-up rate.

Attention: At the commissioning start-up of the boiler and after any repair work of the refractory, it is very important to further reduce the lighting-up rate. This is because the new refractory still contains a small amount of water. When heated the water vaporises and expands which might cause fissures and cracks in the refractory. The burner must therefore only be operated at minimum load and in intervals of 1-2 minutes for the first hours. Between each operation interval the burner should remain stopped for approximately 8-10 minutes.

Before start-up of the boiler plant, some general work and check procedures must be considered.

Step A: Check that all pipelines are ready for operation and that possible blind flanges used during pressure tests have been removed.

Step B: Check that the main steam valve, scum valve, and blow-down valves are closed.

Step C: Open the feed water valves and the air valve. Fill the boiler with feed water to approximately 50 mm below normal water level. The water level rises due to expansion when the boiler is heated. If the temperature difference between the boiler and feed water exceeds approximately 50ºC, the boiler must be filled very slowly.

Note: When filling a pressure less boiler, the shut-off valve after the feed water pump must be throttled. Otherwise the pump motor will be overloaded.

Step D: Check the water level in the water level gauges. Check frequently during the complete start-up. The water level gauges should be blown down several times to ensure a correct indication.

Step E: Check that the water level control system is connected and operational.

Step F: Check the oil system and start the fuel oil supply pump. Pre-heat the fuel oil if the burner should operate on heavy fuel oil.

Step G: Check the burner and the safety functions according to the separate instruc-tion.

3.2 Start and pressure rise

The following work procedures must be followed during start-up of the boiler.

3.2.1 Start and pressure rise by means of the oil burner:

Step A: Check that the gauge board valve and pressure gauge valves are opened.

Step B: Check that the air valve is open if the boiler pressure is below 1.0 barg.

Step C: Start the burner on manual control and on low load. Check that the water level does not rise too high during the pressure rising period.

Step D: Drain via the blow down valves if the water level is too high.

Step E: If the air valve was opened close it when only steam blows out. A pressure reading should be indicated on the boiler pressure gauge before the air valve is closed.

Step F: Check that any connected remote indicators are working correctly.

Step G: Re-tighten all covers such as manholes, hand holes, inspection doors, flang-es, etc. during the pressure rising period. If required, check all flange joints on the plant.

Step H: Change to automatic control of the burner when the boiler pressure is 0.5 barg lower than the working pressure of the boiler.

Step I: Open the main steam valve slowly to heat-up and pressurise the steam