Low Water Level
When low water level situation is encountered, following procedure to be followed:
- Check the water gauges; if they blows steam, this is a very dangerous situation
- Shut down the boiler immediately once low water level is reached
- Switch off the feed pumps. Turn off the feed water valve and fuel supply valve. Wait for slow cool down
- Check to see if there is any damage to furnace, internal structure and other parts of the boiler
- Check the tripping of fuel supply valve as it should cut the supply of fuel oil when water level reaches low low (LL) level
- After the problem is fixed, boiler could be put back to service
Immediate remote action
- Start reserve feed pump if needed
- Change to manual feed water control if required
- Never by-pass the very low drum level trip
- Trip the unit manually if the water level is not visible and if auto mode does not work
Immediate local action
- Check the tripped pump, rectify the cause and inform the boiler operator who is ready for restart
- Check for possible tube leak first in furnace and then other areas if needed
- Check drum drain and low point drain for possible opening
- Check the feed water auto controller at local position and arrange for rectification
- Check feed controller for any link failure
Blowback
Blowback from boiler furnace is a very dangerous situation where the hydrocarbon mixture in the furnace catches fire which may lead to explosion. If the hydrocarbon mixture is in rich quantity, it can lead to devastating effects and can even cause injuries/ causalities. Following points to consider during boiler blow back condition:
- Never stand behind the burner when starting the boiler
- Stop the boiler and start purging the furnace
- Open the burner door and check for fuel accumulation within the furnace to check nozzle dripping
- Check the condition of nozzle
- If all checks are satisfactory, shut the burner door
- Start the burner sequence with pre purging
- If the burner backfires again, do the troubleshooting before starting the boiler
Boiler Fire
There are three main stages of boiler fire:
Stage 1: Normal Soot fire
Stage 2: Hydrogen fire
Stage 3: Iron fire
Stage 1- Normal soot fire: Soot is deposited in the water tube of the exhaust boiler. When the ship is at slow speed, the exhaust temperature of main engine may vary from 100 to 200 °C . This temperature is enough to ignite “wet soot” whose ignition temperature is around 150 °C .
If the soot is “dry”, it will not get ignited at such low temperature (150 °C) but when the engine is running at higher speed and the temperature of gases reaches to above 300 °C , then in the presence of excess oxygen the deposits of combustible materials will liberate sufficient vapor which can be ignited by a spark or a flame.
The above type soot fire is called small or normal soot fire because the heat energy is conducted away by the circulating boiler water and steam. Also the sparks remain inside the funnel or diminish while passing through the flame arrestor in the funnel top.
Stage 2: Hydrogen fire
Hydrogen fire in an EGB occurs when the chemical reaction of dissociation of water takes place at temperature above 1000 °C. This leads to formation of Hydrogen (H2) and Carbon mono-oxide (CO) which are both combustible in nature.
2H2O= 2H2 + O2 (Dissociation of water leading to formation of hydrogen H2) H 2 O + C =H 2 + C O (Reaction of water with carbon deposits leads to formation of carbon monoxide-CO)
Stage 3: Iron fire
At this stage, the chain reaction of oxidation of iron metal starts at a high temperature of 1100 °C. This means at such high temperature the tube will start burning itself, leading to complete meltdown of tube stacks. 2Fe + O2 = 2FeO+ heat
It is strictly advisable not to use water or steam at this stage to fight the fire as the overheated iron will react with water to continue this reaction. Fe + H2O =FeO+ H2 + heat
Steps for prevention of fire
- Avoid slow steaming of main engine
- Ensure good fuel combustion in the main engine
- Ensure fuel is treated and is of good quality while supplying to the engine
- Do regular soot blow of boiler tubes
- Do water washing in ports at regular intervals of time
- Ensure design of exhaust trunk is such that it provides uniform heat to complete tube stack
- Pre-heated circulating water to be supplied to boiler mainly at the time of start up
- Do not turn off the circulating pump at any time while main engine is running
- Do not stop the circulating pump for at least two hours after the main engine is stopped
- Start circulating pump 2 hours before starting the main engine
- Pre-heated circulating water to be supplied to boiler mainly at the time of start up
How to Tackle Exhaust Gas Boiler Fire?
The response for tacking EGB fire will be different for different stages. In stage 1 fire, i.e. normal soot fire:
- Stop the main engine and thereby the oxygen supply to the fire
- Continue operating the water-circulating pump. Never stop the pump
- Never use soot blowers (steam or air) for fire fighting as it will accelerate the effect of fire
- Ensure all the exhaust valves in the stopped main engine are in closed position so as to cut any chance of air supply to the soot fire
- Cover the filter of turbocharger
- Water washing system, if fitted, may be used to extinguish the fire. This is normally connected to the ship’s fire fighting water system
- External boundary cooling can be done
For major fire:
- Stop the main engine, if it is not stopped already
- Stop the circulating water pump
- Shut all the inlet and outlet valves on the water circulation line
- Discharge the (remaining) water from the exhaust gas boiler by draining
- Cool down with plenty of splash water directly on the heart of the fire (Take care not to splash water in other parts as water can accelerate the reaction)
Oil in the Boiler water
Oily matter gets into the boiler from damaged heating coils of fuel oil heaters, lubricating oil heaters, fuel oil tanks, or waste oil tanks. When oily matter is detected in boiler, it is important to specify and repair the place where oil leaks.
Oil inside the boiler can lead to complete shutdown of the steam system. Taking action in the initial stage of the problem will save a lot of man-hours and money. Following action to be taken when oil is detected inside the boiler:
- Keep a track on the hot-well sight glass for oil on the water surface
- Ensure hot-well sight glass light is working
- If oil is seen in boiler water gauge glass, inform the chief engineer
- Stop burner immediately if oily matter is detected in boiler, regardless of the quantity
- Completely discharge water from boiler and cascade tank, then wash their insides with fresh water thoroughly
- If no abnormality is found in boiler and cascade tank by visual inspection, close manholes of boiler and inspection holes of the cascade tank
- Feed water to the boiler up to normal water level
- Put appropriate chemical compound in the boiler, and inspect leakage under ambient temperature and atmospheric pressure. Check that boiler body, valves and others parts do not have any leakage before restarting the boiler operation
- Normal pH value is controlled at 10.5-11.5 when using distilled water, but gradually increase boiler compound quantity and maintain pH 11.5-11.8. Operate the boiler for 7 or 10 day periods until the water quality test shows good result. To discharge residual oil and saponified oil from boiler, perform surface blow-off and bottom blow-off once per day without fail
- It is recommended to use sludge dispersant for reducing sludge precipitation
The above steps are taken when oily matter is detected relatively early.
Emergency running of auxiliary boiler during automation failure
Emergency running of composite boiler during automation failure
General procedure for emergency running of boiler during automation failure:
- Turn the key placed inside the power panel into emergency operation
- Start one of the oil pumps from the operating switch located inside the power panel
- Adjust the oil pressure on the pressure-regulating valve to a level of 20-25 bar at the burner unit inlet flange (or as recommended by the manufacturer)
- The adjustment is carried out by means of the manually operated hand wheel on the actuator
- If the burner is meant for operation in heavy oil, ensure the oil temperature is maintained within prescribed limit
- Adjust the steam-regulating valve for the pre heater. It is important to maintain the viscosity of oil in the range of 15- 20 cst at the point of injection
- Refer the viscosity temperature chart for temperature adjustment to achieve the required viscosity rang
In most of the boilers, the water level regulator will not be operational in manual mode. The water level in the boiler to be maintained manually by controlling the operation of feed water pump, which supplies water to marine boiler.
Manual operation:
- Go to the local control panel located near the boiler
- Turn the knob switch / key to emergency mode operation side
- Start the combustion air fan switch
- Purge the furnace by opening the inlet vanes to maximum position with combustion air
- The air-flow to be kept maximum for at least 2 minutes
- Adjust the steam-regulating valve for atomizing steam by means of hand wheel or actuator
- The atomizing steam pressure to be maintained in the range of 2-3 bar
- Set the air flow to ignition load position at approx. 15 -20% load by reducing the inlet vanes angle for combustion air fan
- Set the oil flow to ignition load position to approximately 15-20% load by controlling the oil flow regulating valve
- Press the pilot burner ignition button for oil valves inside the control panel
- Once the pilot burner is ignited, push the power button for main burner and as soon as the main burner ignites, release the pilot burner button
- The burner is now in operation, and adjustments of the burner load must be carried out as described in the following section:
- Adjust the air flow and oil flow to the desired load by means of potentiometers inside the local control panel
- Check combustion air fan and the oil pressure indicated on the pressure gauge just before the burner can be used for first-hand settings
- The final settings must be determined by visual checks of the flame and the flue gas
- Adjust steam-atomizing pressure through steam regulating valve
- If the steam atomizing pressure is too low at a given burner load, the flame has a tendency to sparkle. Furthermore, the flame becomes unstable at low loads due to poor atomization of the oil
- If the steam atomizing pressure is too high, the combustion zone is cooled, and the flame becomes unstable at low loads
- To ensure that the burner receives sufficient amount of oil, it might be necessary to adjust the oil pressure on the pressure-regulating valve
- If the burner is operating on heavy fuel oil, adjustment of the oil temperature might be necessary at large burner load variations
- The oil temperature should be adjusted on the steam-regulating valve for the pre-heaters by means of manually operated hand wheel on the actuator
- The water level must be carefully and continuously supervised during operation of the boiler plant
- When one of the feed water pumps is in operation, the water level can be regulated on the feed water control valve by means of manually operated hand wheel on the actuator
Emergency Dry Running of EGB
The only situation when the exhaust gas boiler should be allowed to run dry is in emergency. The following normal dry running procedures must be followed:
- Inform the shore management office regarding the situation
- Read instruction manual section before operating the equipment in a dry condition
- The engine exhaust gas temperature must be maintained within limit
- The period of dry running must be as short as possible. Prior to dry running, the tube banks must be cleaned to remove all traces of soot and other accumulations
- Cleaning by soot blowing must continue throughout the dry running period with at least the same frequency as for normal operation
- The tube bank must be drained and vented
Monitoring of inlet and outlet gas temperatures is important to ensure no sudden rise occurs. Temperature excursions may indicate the presence of soot fire.
It is commonly found that boilers are often overlooked onboard ships until any problem related to automation or boiler structure occur. Failure of boiler affects several machinery systems on board, most importantly fuel oil heating and cargo operations.
Efficient operation of marine boilers greatly depends on the ship's maintenance system and regular inspections involving all safety aspects. Professionals working on ships must be aware of the common operational problems related to boiler, especially those concerning change in fuel type and controlling systems.
The hazards involved in operating a ship's boiler plant under various conditions should also be mapped carefully, specifying all recommended inspections and tests that are to be carried out during routine and class surveys.
References
A Guide to Boiler Operation and Maintenance. Anish Wankhede [2015]
