This article will discuss shipboard boiler operation from starting up to lay-up. All boiler operators should make sure that correct operating procedures are followed as for the safety of the crew as well as for the boiler.
Starting up of boiler should not be speeded up, and it normally takes about four hours to complete. Strict adherence to the manufacturer’s recommended procedures will make the boiler operation more efficient and safer.
- Check that the boiler is properly closed up (especially after repairs).
- Check physically that all appropriate valves are shut or open for safe starting of the boiler.
- Boiler filled to slightly below normal level.
- Boiler treatment chemicals may now be added to the boiler water.
- Check and clear the furnace of any flammable materials.
- Ensure that the boiler uptakes passage is clear.
- Pre-purging of furnace for a specified amount of time is necessary to clear the gas-side of flammable gases to avoid a staring explosion.
- On a cold boiler, firing-up must not be speeded up too much in order not to overstrain the boiler material unnecessarily by quick, uneven temperature raises.
- Keep the boiler vents open until a heavy steam jet is flowing out (until a boiler pressure of about 1 bar is reached).
- Before the boiler is put on load, blow through the gauge glasses, testing the safety valves using easing gear and try out the safety cutouts.
Starting of Water Tube Boiler Fitted with an Integral Superheater:
Integral superheaters fitted to boilers are prone to overheating as they are normally situated in a high temperature path. Superheaters depend on adequate steam-flow through them to keep their tube metal temperature within limits. Overheating of superheater tube element result in sagging and failure of tubes and may even trigger a hydrogen fire in the boiler. To protect the superheater from overheating during start-ups, the superheater is drained of any accumulated condensate and a vent provided in its outlet header is left partially open until the boiler is put on load.
Points to be Noted while the Boiler is On Load:
- Operate the boiler at a load where its efficiency is the highest.
- Maintain correct air/ fuel ratio, under perfect condition a brownish hazy color of exhaust smoke is noticed from uptake.
- CO, CO2, O2 contents as monitored in the exhaust gas will indicate the combustion condition inside the boiler. Every morning mud is blown from the boiler through the bottom blow-off valves and float chambers. (Bottom blow is not recommended for drum type of water tube boiler while the boiler is on load).
- Ensure that all safety cut-outs are operational.
- Maintain the fed water quality and quantity as observed from the water level indicator.
Boiler care while running on load/ Monitoring of perfect combustion:
- Flame should fill the furnace without any impingement.
- Color of flame should be moderately bright orange.
- Flue gas analysis should give the following readings:
- 3-4 % by volume (for fire tube boiler).
- 0.5-1.5 % by volume (for water tube boiler with high efficiency burners).
2. Carbon Dioxide:
- 12 to 14 % by volume, the higher the readings, the better the combustion efficiency. It must be remembered that the CO2 reading changes with the fuel type/ quality and the level of excess air supplied to the burners.
3. Carbon Monoxide:
- A reading range of 100-200 ppm indicates good combustion efficiency. This reading is very reliable unlike in the case of CO2 reading of flue gases.
- Black color indicates improper combustion due to insufficient air, poor atomization etc. whereas white color of smoke indicates too much air (or profuse leakages of steam or water from damaged boiler parts).
Boiler Flue Gas Analyzer
1. Wet Storage:
Used for short lay up of less than a month and the boiler are maintained in a stand-by condition. Not suitable for boiler exposed to freezing conditions. The boiler is completely filled with hot distilled de-areated alkaline water. The water should overflow through the vent during filling-up. Daily checks are necessary to ensure fullness and that alkalinity is maintained.
2. Dry Storage:
Used for longer lay-up of more than a month. The boiler is completely dried out using heaters or on light fire or passing hot air through the boiler parts. When dry completely, all the boiler outlets are sealed tight after placing dehydrant (such as a silica gel at the rate of 2.7 kg/ cu.meter) inside the boiler.