Marine Engineering Study Materials

When the boiler is started, the lighten-up rate of the boiler must not be accelerated too much as this might cause an unnecessary over strain 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.

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. The article explains starting and stopping boiler during normal and emergency working conditions.

Precautions on Starting Boiler

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

1. Check that the main steam valve, by-pass valve and circulation valves if provided, scum valve, and blow-down valves are closed.
2. 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.
3. 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.
4. 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.
5. Check that the water level control system is connected and operational.
6. 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.
7. Check the burner and the safety functions according to the manufacturer instruction.

Starting Boiler and Pressure Rise

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

1. Check that the gauge board valve and pressure gauge valves are opened.
2. Check that the air valve is open if the boiler pressure is below 1.0 bar.
3. 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.
4. Drain via the blow down valves if the water level is too high.
5. 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.
6. Tighten all covers such as manholes, hand holes, inspection doors, etc. during the pressure rising period. If required, check all flange joints on the plant.
7. Change to automatic control of the burner when the boiler pressure is 0.5 bar lower than the working pressure of the boiler.
8. Open the by-pass valve slowly to heat-up and pressurise the steam system. If the boiler is not provided with a by-pass valve, the main steam valve should be used to heat-up and pressurise the steam system.
9. Open the main steam valve and close the by-pass valve.
10. Open the valves to the steam consumers carefully in order to avoid water chocks.
11. When the boiler is in normal operation, check that the water level control system and the gauge board functions are fully operational.

After 3-4 weeks in operation, mud and deposits in the piping system may have accumulated in the boiler water. This may cause level variations which disturb the steam generation, and it is therefore recommended to blow down the boiler. It should then be inspected, cleaned, and refilled with boiler water.

Stopping Boiler

Normal boiler shut down

If necessary, the boiler can be shut down at any load without special preparations.

When the boiler is stopped, sudden temperature and pressure drops should be avoided as they might expose mountings, pipe lines, and the boiler plant to inadmissible temperature gradients.

1. When minimum load is obtained, stop the burner.
2. Keep the water level at normal level until the boiler stops producing steam.
3. Stop the feed water pump and close the feed water valves.
4. Close the main steam valve.

Emergency shut down

The boiler must be taken out of service immediately if parts of the heating surface have been glowing or the boiler shows recognisable deformations.

The supervising authorities must be informed, and the boiler must not be used until approval from these authorities is available a substantial loss of water is noted the feed water system is unable to provide the necessary amount of feed water, e.g. due to failure of parts the safety valve cannot function sudden cracks or damage are noted in the refractory, and if steam or moisture is coming out of the refractory oil in the feed water is detected too high salinity level is detected.

If an emergency shut down must be carried out, the fuel supply should be stopped. The main steam valve should be closed gradually, and the boiler must be cooled. The safety valves must not be operated. Parallel working boilers should be disconnected at once.

Stop for repair or inspection

The following describes the measures to be taken when the boiler is shut down for repair or inspection.

1. Clean the boiler from soot with water.
2. Operate the burner for at least 15 minutes after the soot removal to dry out the remaining water.
3. Stop the boiler as mentioned previously.
4. Check the furnace and the pin tubes with regard to cleanliness.
5. Empty the boiler from water and clean it. Check if lime stone appears.
6. Check and clean the outer fittings. Change gaskets where required.
7. Clean the feed water tank and feed water pipes.
8. Clean and grease the bearings of motor, pump, and fan.
9. Check and align the burner, if necessary.
10. If the boiler is shut down for a long period of time, the pin tubes must be thoroughly cleaned.
11. Check that the necessary spare parts are available. Order complementary parts in time.

It is of extreme importance that the boiler is NOT operated without water when the oil burner is in operation, e.g. due to disconnection of the water level safety devices. This will immediately cause complete break down of the boiler.

Reference:

MISSION™ OL Boiler Manual by Aalborg Industries