What is Boiler ?
Boiler is a pressure vessel used on ships/industry in which the water is heated to evaporate and generate the steam and the unit is so arranged that the generated steam accumulated in it.
The two main types are water tube and fire tube.
What is the boiler mounting ?
It is directly fitted to the boiler shell. They are:
- Safety valve
- Main steam stop valve
- Air vent cock
- Two gauge glass level indicators
- Feed check valve
- Scum blow down valve
- Bottom blow down valve
- Salinity cock or test cock
- Man hole doors.
Where are air vent cock located and its purpose ?
The air vent cock is fitted at the top of the steam space of the boiler.
Its purpose are-
- To release air from the boiler either filling the boiler water or raising the steam.
- To allow air to enter in the event of boiler cooling down or blowing down. ( to prevent vacuum effect)
What are the safety arrangements in marine boiler ?
- Safety valve
- Low / high water level alarm
- Too low water level alarm and shut down
- Water level indicators
- Pressure gauge
- Low fuel oil pressure alarm
- Low / high fuel oil temperature alarm
- Flame failure alarm
- Smoke density alarm
- Easy gear arrangement
- Air vent
- Force draught fan stop alarm
- Low / high steam pressure alarm
Why need to blow down the gauge glass ?
Gauge glass blow down is made to know the boiler and gauge glass connection is clear or not and to give the exact water level in the boiler.
Boiler gauge glass blow through procedure ?
- Shut steam and water cocks than open the drain cock.
- After draining water from the sight glass, there is nothing come out, both steam and water cocks are good in order.
- Open the steam cock side, blow out the steam.
- Close the steam cock side, the steam connection is clear.
- Open the water cock side, blow out the water.
- Close the water cock side, the water connection is clear.
- Close the drain cock.
- Open the water cock, water should then gradually come rise up to the top of the gauge glass.
- Open the steam cock, the water in the glass should fall to the level of the water in the boiler.
Why need to blow down the boiler water ?
- Boiler blow down is made to reduce the density of salt and to remove the dissolved and suspended solids, also the floating solid impurities in the boiler system.
- If there are not removed from the boiler water system, foaming, priming, corrosion will occur in the boiler steam space and feed water system.
Why scum blow down or surface blow down is carried out ?
- To remove accumulated, suspended and floating solids, impurities and also remove dissolved solid concentration, they hinder the formation of steam.
Why bottom blow down is carried out ?
- To remove suspended solids and residual sludge that have settled at the bottom.
- If these contaminants are not removed regularly they will build up until they hinder the circulation patterns.
What is the purpose of boiler water test ?
- To ensure that proper chemical treatment are maintained at all time.
- To detect the present of contaminants in the water that may be injurious to boiler and system.
Enumerate the boiler water test ?
- Alkalinity test [Phenolphthalein ( p) alkalinity test, Total ( T ) alkalinity test (2 x P)]
- Chloride test
- Condensate PH test
- Amerzine test ( Hydrazine test )
- Excess phosphate test
- Conductivity test (Total dissolve solids test)
- Hardness test
- This is to ensure that the boiler water prevent corrosion by neutralization of acidic gases.
(a) Phenolphthalein or ‘p’ alkalinity
- This test is carried out to prevent acidic corrosion
- To test for presence of all of the hydroxide, one half of the carbonate and one third of phosphate present in a water sample.
(b) Total ( T ) alkalinity test
- To determine the amount all of hydroxide, all of the carbonate, and two thirds of the phosphates
Note: Hydroxides and carbonates can co-exist together in a solution but hydroxides and bi-carbonates cannot.
- To know the amount of salt in boiler water.
- To minimize chloride level and to adjust the blow down.
Condensate PH test
- To control condensate PH value within a limit.
- To minimize corrosion in steam and condensate system.
Amerzine test ( Hydrazine test )
- To test for dissolved Oxygen content
- To know reserve hydrazine (N2H4 ) ppm and to prevent corrosion and aeration.
- To minimize oxygen pitting and corrosion in boiler, steam and condensate system.
- To control the scale formation due to hardness (presence of Phosphate in sample means no hardness salts)
- A reserve of phosphate should be maintained in the boiler water ready to neutralize any hardness salts which may enter.
- Measure of the total amount of dissolved solids (T.D.S) including the treatment chemicals. (Excessive density leads to priming and or deposits)
- To remove dissolved and suspended solid by blowing down.
- To check for salt causing “hardness”
Note: Hardness test of boiler water are not necessary when the phosphate is above the lower limit of the control range.
What are the advantages of boiler water treatment ?
- To prevent the scale formation in the boiler and feed system
- To prevent corrosion in boiler and feed system.
- To control the sludge formation and prevention of carry over with the steam.
- To maintain the boiler water in alkaline condition and free from dissolved gases.
- To prevent of entry into the boiler of foreign matter such as oil, waste,mill scale, FeO, Cu, Sand, etc.
Exhaust Gas Boiler on Ships
Exhaust gas boiler (EGB or EGE) or economiser described in this article is a smoke tube boiler with steam space, used for heat recovery from main engine exhaust gas. An illustration of the boiler is shown in figure below.
EGB is designed as a vertical boiler with a cylindrical shell surrounding the boiler tubes and water drum as well as the steam space. The tubes consist of a large number of smoke tubes and a small number of stay tubes. The stay tubes with an increased diameter act as support for the boiler. Both types are welded onto the lower and upper tube plates. The steam space is formed by the shell plate and internal cone. At the top it is closed by means of the end plate.
What is foaming in auxiliary boilers and how to prevent it ?
It is the formation of thick layer of steam bubbles on the top of the water surface inside the boiler due to:
- High concentration of impurities.
- By the animal or vegetable fats in feed water carry over from the oil heaters.
- Increase in level of dissolved & suspended solid TDS level.
- Increase in water level.
To prevent foaming, surface or scum blow down should done frequently to expel any floating impurities for the boiler and no lube oil should be allowed to enter the boiler.
What is boiler priming and how to prevent it ?
It is condition in which large amount of water are carried along with the steam into the steam line. It is caused by:
- Excessive foaming
- Improper amount of steam space
- By a sudden rush of steam such as is produced when steam stop valve is suddenly opened.
To prevent priming, never keep the water level too high. Open steam stop valve slowly.
What action do you made take in case of foaming and priming ?
- Scum blow down
- Reduce boiler fire rate
- Check whether boiler chemical added are in excess
- Detect the source of contamination
- In case of bad priming the boiler may have to be taken out of service, shut down.
Note: For contamination due to oil, auxiliary boiler have to be chemically cleaned.
What is the boiler cool down procedure ?
- Change the fuel oil burning system from HO to DO and then shut down the burning system.
- Stop feed p/p close feed check valve.
- Drain down the boiler after allowing it to cool down.
- If no sufficient time to do this, lower the boiler pressure to 3 to 4 bar.
- Shut the main steam stop valve.
- Open the ship side valve then open the blow down valve.
- Banging noise will appear when boiler is empty.
- Close the boiler blow down valve and ship side valve.
- Then release the steam pressure through safety v/v by means of easing gear.
- When pressure is off, open the air vent and the boiler to cool down.
Boiler Opening up and Inspection Procedure
Follow as procedure above, after making sure no vacuum in it, first:
- To remove top man hole door, slacken the dog holding nuts but do not remove them until first broken the joint
- Remove nuts and dogs and take out the door.
- The bottom door can be removed after warning personnel to keep clear of the top door.
- Make ventilation before entering. Do not allow naked light near the boiler.
- Preliminary internal inspection carried out before cleaning, to check scale deposits and any special points.
- Plug orifice to blow down valve to prevent choke, place guards over the manholes landings to prevent damage.
- Carried out cleaning and internal works.
- When all works completed, a full internal examination must be carried out
- Cleanliness, all openings are clear, water level gauge connection clears from deposits
- All internal pipes and fittings have been replaced correctly and securely attached,
- Remove plug from the blow down valve orifice
- The face of manhole doors and landings inspect to clean and undamaged.
- Replace manhole doors by using new joints.
- Operate all boiler mountings. Open air vent cock and fill the boiler with water to sufficient level.
Describe the boiler start up procedure from cool condition ?
The boiler is carried out firing from cold condition to normal working pressure condition very slowly to avoid thermal stress.
- Check the boiler blow down valve in close position.
- Shut the main steam stop v/v.
- Open the air vent cocks.
- Open the feed check valve and pumping up to ¼ of gauge glass level.
- Start the force draft fan with dampers, open correctly to purge the furnace and combustion space of any foul gas.
- Light the burner after closing the recirculating valve.
- Normally allow the fire to burn for 5-minutes and stop for 15 mins. This step continues until steam come out.
- When steam coming out from the air vent close the air vent (at 1 to 2 bar pressure).
- Rise up the working pressure step by step slowly. (Allow the fire to burn for 30 mins and stop the fire for 10 min.)
- When the steam pressure reached is working pressure drain the steam line. (to avoid water hammer)
- Main steam stop valve open slowly (crack opening)
How to take action if gauge glass showing low water level ?
- If water level has not yet dropped completely out the sight glass, water may be put into the boiler.
- If water drops completely out of sight glass, check another sight glass, if both disappear water; do not add water until the boiler is cool enough to prevent any possible damage due to rapid cooling of over heated plate.
- The fire must be immediately stopped.
- The main steam stop valve must be closed.
- Blow down and cool down the boiler.
- Check leakage, drum outside, Located the cause of trouble and make necessary repair.
- Enter the boiler after it has cooled and examined any possible damage.
- After repair, water fill up slowly and restore to normal operation.
- If no damage occurred, inject the water slowly into the boiler and restore it to operation.
What are the causes if gauge glass shows low water level ?
- One gauge glass defective
- Boiler tube crack and leaking
- Feed water regulator jammed
- Failure of feed water pump (Air in feed water line or pump)
- Level controller malfunction
- Steam consumption is too much
What is caustic embrittlement? How to prevent it ?
- It is inter crystalline fracture. It is cause by high concentration of caustic soda and the material under stress. The stress corrosion cracks follow the grain or crystal boundaries of the material and failure.
- Sodium sulphate or sodium nitrate is used for the prevention of caustic embrittlement.
- It can be found in highly stress area in boiler. Such as tube and tube plate connection, riveted head, seam and boiler mountings.
Starting and Stopping Boiler
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 , following general work and check procedures must be considered.
- Check that the main steam valve, by-pass valve and circulation valves if provided, scum valve, and blow-down valves are closed.
- 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.
- 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.
- 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.
- Check that the water level control system is connected and operational.
- 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.
- 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.
- Check that the gauge board valve and pressure gauge valves are opened.
- Check that the air valve is open if the boiler pressure is below 1.0 bar.
- 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.
- Drain via the blow down valves if the water level is too high.
- 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.
- 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.
- Change to automatic control of the burner when the boiler pressure is 0.5 bar lower than the working pressure of the boiler.
- 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.
- Open the main steam valve and close the by-pass valve.
- Open the valves to the steam consumers carefully in order to avoid water chocks.
- 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.
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.
Normal 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.
- When minimum load is obtained, stop the burner.
- Keep the water level at normal level until the boiler stops producing steam.
- Stop the feed water pump and close the feed water valves.
- 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.
- Clean the boiler from soot with water.
- Operate the burner for at least 15 minutes after the soot removal to dry out the remaining water.
- Stop the boiler as mentioned previously.
- Check the furnace and the pin tubes with regard to cleanliness.
- Empty the boiler from water and clean it. Check if lime stone appears.
- Check and clean the outer fittings. Change gaskets where required.
- Clean the feed water tank and feed water pipes.
- Clean and grease the bearings of motor, pump, and fan.
- Check and align the burner, if necessary.
- If the boiler is shut down for a long period of time, the pin tubes must be thoroughly cleaned.
- 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.
Properties of Boiler Water – Marine Engineering
There is a number of ways to produce good quality feed water for boiler plants. Methods such as e.g. reverse osmosis plants or ion exchange plants produce good quality distillate. Also evaporators generally produce good distillate. The important thing is that the distillate used should be clean and without foreign salt contamination.
In practice most distillates used contain minor parts of various salt combinations which can and must be chemically treated away. Furthermore, the distillate may contain dissolved gases like for example oxygen (O2) and carbon dioxide (CO2) which may lead to corrosion in the boiler, steam, and condensate system.
Boiler and feed water must be chemically treated in order to avoid corrosion and scaling in the boiler.
The following text regarding feed and boiler water treatment is the normal recommendations for boilers with working pressure below 20 bars. These recommendations should be followed strictly in order to have the best working conditions for the boiler plant and to extend the working life of the plant.
Feed and Boiler Water Maintenance
The following are recommended water maintenance instructions. More exact details concerning analyses and blow downs should be set up together with the supplier of chemicals for water treatment.
- Analyses of feed and boiler water.
- Skimming (surface blow down) according to analyses, but at least once per week (2 minutes with fully open valve).
- Blow down (bottom blow down) according to analyses, but at least once per week (each blow down valve 1 minute in low load condition).
- Check the functions for salinity and oil detection systems.
Every six months
- The boiler water side (interior) must be carefully inspected at least twice a year.
- Check of the water side of the boiler and hotwell/deaerator for corrosion and scaling.
- Check the chemical pump unit.
Auxiliary Boiler Mountings
The following is a brief description of the most important items of the boiler mountings. The components mentioned in this section are referring to the general arrangement drawing of the boiler.
Two safety valves are fitted to the pressure vessel of the boiler. They are installed for security reasons, and designed to prevent the boiler pressure from rising above the design value.
The safety valves must be supplied with waste steam pipes and either expansion devices, or bellows.
Main steam valve
The main steam valve is a shut off valve. When closed, it isolates the boiler from the main steam line. A check valve is mounted after the main steam valve which prevents steam from flowing backwards into the boiler.
The steam piping up to the main steam valve must be flexible in order to minimise external loads on the main steam valve and associated branch. The flexibility can be achieved with loops build into the main steam line to take up heat expansion from the boiler and steam piping or with an expansion joint next to or close to the main steam valve.
close to the main steam valve.
The by-pass valve is a shut off valve. The purpose of the valve is to equalise the pressure between the boiler and the steam system when the main steam valve is closed. Please note that the by-pass valve is only supplied for larger boilers.
Feed water valve / feed valve (SDNR)
Two groups of feed water valves are provided in the boiler. Each group comprises a shut-off valve and a non-return valve. The shut-off valve in the ordinary group must be open when the boiler is in operation, or if the boiler is used as a steam drum. The shut-off valves should be closed when the boiler is not in use.
Water level gauges
Two local water level gauges are connected to the front of the boiler, each gauge being provided with two shut-off valves and a drain valve. The shut-off valves, fitted at the top and bottom of the sight glass, have a quick-closing mechanism to be used in case of broken glass. The pipes from the drain cocks on the water level gauge must lead to an open drain, visible for inspection.
Two blow-down valves are mounted at the bottom of the boiler body. If connected to a separate drainage system the valves are of the ball type. When connected to a common drainage system two valves are provided in each group, one shut-off valve and one shut-off/non-return valve. The shut-off function is for security and the non-return function prevents steam/water from flowing into an empty boiler by mistake.
The air/ventilation valve located on top of the boiler is a shut-off valve. It is normally closed except when the boiler is being filled or completely drained. The end of the drain pipe from the air valve must be visible in order to determine when water or steam is coming out.
The scum valve is mounted at the top of the boiler body. In the event of scum in the boiler, this scum can be blown off from the water surface by opening this valve. If connected to a separate drainage system the valve is of the ball type. When connected to a common drainage system two valves are provided, one shut-off valve and one shut-off/non-return valve.
A sample valve is installed enabling connection to a sample cooler for taking test samples to perform boiler water analyses.
Two small inspection holes are provided in the furnace wall to enable inspection of the burner flame. A proportion of the air supply is bled off from the burner fan to cool the window of the inspection hole, and prevent soot deposits.
Two manholes placed at the boiler top and boiler bottom allow inside inspection of the steam/water drum.
Access to the furnace and smoke outlet box are possible through the access doors placed at the bottom of the furnace and at the smoke outlet box respectively. Both access doors enable inspection of the generating tube bank.
Drain for furnace
The furnace bottom is provided with a socket for drain of the washing water.
Auxiliary Boiler on Ships
The vertical two drum water tube boiler used normally on ships is top-fired and equipped with a steam atomising burner. As the burner, the local control panel and all relevant boiler mountings are mounted on top of the boiler, this can easily be operated and monitored from the burner platform.
The principal drawing of a typical auxiliary boiler is shown in figure below. The steam drum is cylindrical with two flat plates of equal thickness. Because of the internal pressure, the flat plates are mutually connected by vertical solid stays. The steam drum is furnished with the necessary internal fittings to ensure an even distribution of the feed water, of the circulation water from the exhaust boiler and to ensure a sufficient dryness of steam.
The burner cone in the furnace opening are an integrated part and accordingly no refractory are provided in the top of the furnace room. Manholes are conveniently placed in both drums and inside the drums enough space is available for inspection and maintenance. The water drum is designed similar to the steam drum. The drum size gives optimal space for the heating coil and easy access for inspection.
As standard the auxiliary boiler is prepared for tack welding along the foundation of the boiler shell. Some boilers may, however, be provided with four support feet instead, one as fixed foot and the others with the possibility of thermal expansion.
Both the furnace and the generating tube bank are located asymmetrically and are separated by the screen wall. Besides the screen wall the furnace consists of gas tight polygon shaped membrane walls. The generating tube bank consists of vertical pin-tubes arranged in a staggered configuration. To avoid any risk of vibration problems supports are arranged.
The flue gas passes through the deflected tubes at the bottom of the screen wall, up through the generating tube bank and out through the smoke outlet box. An effective circulation in the boiler is achieved bye means of down comers. The bottom tube plate is first covered with coat of insulation refractory and above with castable refractory. Access to the furnace is possible through the access door at the bottom of the furnace. Inspection of the generating tubes is also possible through the inspection door at the bottom of the furnace.
Inspection of the burner flame is possible through the two inspection holes arranged in two heights on the furnace panel wall.
What are the types of boiler by pressure ?
- Low pressure boiler – up to 10Kg / Cm2
- Medium pressure boiler – 10-25 Kg /Cm2
- High pressure boiler – Over 25 Kg / Cm2
What is water hammering and how will you prevent it ?
- It is the impact of moving water in steam line when the steam is allowed to enter a line with condensed water.
- The steam will condenses and partial vacuum occurred and move back the water the along the pipe with very high velocity, and the water will strike at the vent or valves.
To prevent the water hummer –
- Install steam trap in the line
- Open the drain first before allowing the steam into the line.
- Crack open of steam valve at first.
How to make boiler water test on your ship ?
The boiler test is carried out as follows.
- Firstly take the boiler water sample from the salinometer cock or test cock and cool down.
- The test is carried out by the using maker’s supply test kit, chemicals, instruments and instructions.
How do you take boiler water sample ?
- Slowly open the salinometer cock until clean hot water coming out.
- Then collect the boiler sample with copper jar.
How to know scum blow down valve open or not ?
- It can be check by opening of ship side blow down valve. Banging noise will appear, scum blow down valve is in open position.
- It can also detect, over heating of scum blow down pipe.
Where is manhole doors fitted ?
- They are fitted at one at steam side and other one for water side.
Effect of foaming and priming ?
- Can cause water hummer
- Can cause contamination and scaling.
- Can cause fluctuation of working water level.
What is boiler blow back ?
- It occurs when lighting up with explosive gas and oil droplet (Oil residue) inside the furnace without pre-purging sufficiently.
- The ignition results in a large flue gas inside the furnace and these gases blow out with increase high pressure through the furnace opening.
Procedure for hydraulic pressure test on marine boilers ?
Hydraulic pressure test is 1.25 times working pressure, ( 10 minutes maintain )
- Close all openings.
- Open air vent cock.
- Fill up boiler water fully
- Close the air vent cock
- Place hydraulic jack to feed water line.
- Fit standard pressure gauge.
- Applied hydraulic pressure 1.25 times of working pressure and maintain 10minutes.
What is liquid coagulant in marine boilers ?
- It has more molecular weight, colorless solution, containing sodium aluminates, starch, terming, and resin.
- Once being dose into the boiler water floating solid particles and suspended solid are settled to the bottom of the boiler and easily remove by blowing down.
How to renew the boiler gauge glass ?
- Shut steam and water cocks and open drain cock.
- Check any leakage from the respective cock.
- Slacken the gland nuts.
- Open the cap and remove gauge glass.
- Fit new gauge glass, correct size and length into its space.
- Give the expansion allowance 1/8 in vertical clearance.
- Use new packing.
- Close the cap.
- After fitting, warm the glass by steam.
- Tighten the gland nut.
- Then steam and water pressure should be tested whether they are fair and clear.
- Close the drain and open the water and steam side cocks.
- Check the leakage; all are satisfactory put back into operation.
Routine boiler maintenance carried out on ships ?
- Clean the rotating cup.
- Check and adjust the belt tension between motor and rotating shaft.
- Clear the pilot burner nozzle and fitter.
- Clean carbon deposit on electrodes ( igniter ) and adjust the gap.
- Check the fuel valve and air register linkages and joints.
- Check and clean the flame eye cover glass.
- Check and clean inspection peep hole glass cover.
- Clean fuel oil fitter.
- Check the fuel oil pressure.
When boiler safety valve setting made ?
- Every boiler survey.
- After safety valve overhaul.
Why ship side blow down valve is open first and close last ?
- To prevent pressure build up in blow down line.
- In pipe burst, it may be injurious to watch keeper.
What if no test and treatments is done to boiler ?
- Reduction in boiler efficiency due to poor heat transfer
- Reduction in tensile strength
- Reduction in factor of safety
- Overheating of metal resulting distortion and eventual failure
- Increase in fuel consumption
- Excess concentration of NaOH (caustic soda) may cause caustic embrittlement on boiler metal and tend to failure of boiler metal
- Scale formation
- Foaming, Priming, carry over (due to increase T.D.S level, foaming is present and tend to carried over and priming)
How to control steam pressure in exhaust gas boilers ?
- By regulating the amount of gas flowing over the extended surface, i.e. by damper regulation.
- By dividing the unit into sections so that each section is controlled by an inlet valve.
- By passing steam through an automatic pressure controlled surplus valve to a dump condenser.
- By a system designed for a higher pressure than that it is to operate.
- to operate.
What is meant by soot blow ?
- Purpose is to remove soot, to prevent EGE fire or soot fire.
- To get steam generating efficiency good
- To increase heat transfer efficiency
- Time – Day time at sea 2 -3 times/day
What is the procedure to soot blow ?
- Inform to bridge
- Check wind direction, good if transverse direction
- Raise boiler pressure
- Open drain cock until soot blow pipe drain clear
- Open soot blower valve
- Increase the air to boiler to more than 50 %
- Open steam valve & carried out by turning wheel 20 -30 sec
- Then close steam valve, soot blower valve
- Open drain valve
Boiler safety valve examination and attention during overhauling ?
- Checked its valve and valve seat for wear, cavity corrosion and any fault.
- They should be grounded properly but maker’s limit must be maintained such as width of seating, clearance between valve lip and seat.
- Valve chest must be cleaned condition and drain line clear.
- The spring should be hammer tested for any fracture and check for corrosion. Free length is limited to 0.5 % of original free length.
- The spindle should be hammer tested for any crack and its straightness.
- The guide plates and bushes are checked for uneven wear and have sufficient clearance to allow free movement of spindle.
- The compression nut and cover bush’s threads are carefully checked it for any sign of wear and tear.
- The easing gear should be checked in good order including bearings, cable pulley, and connecting links.
- Connecting pin should be a free fit in the lid and pin should not bent or pinhole gone out of shape.
- All safety valves are to be set to operate under steam a little above working pressure not greater than 3% above the approve working pressure of the boiler.
Why safety valve is fitted in two numbers ?
- To make sure that the positive discharging of higher steam pressure from boiler efficiency.
Why fitted in one valve chest ?
- To reduce stress by hole on pressure vessel (boiler)
- To minimize the numbers of cutting hole on boiler to save it’s strength.
Accumulation Pressure Test, Corrosion of Boilers on Ships
What is meant by erosion in boilers ?
- It is a mechanical attack on the metal surface which may be due to a disturbance in the flow of the fluid over the metal surfaces, resulting in a loss of metal. Suspended abrasive matters in the fluid can increase the rate of metal losses.
What is meant by corrosion in boilers ?
- It is the deterioration of metals due to oxidation. The present of water in an acidic condition provides the electrolyte required for corrosion action.
Two forms of corrosion
Direct chemical attack
- It occurs when metal at high temperature comes into contact with air or other gases, resulting in oxidation or sulphidation of the metal.
- Galvanic action, this being set up when two dissimilar metals are placed in an electrolyte. The noble of the two metals form a cathode to the base metal which, forming the anode, is wasted away.
What is meant by caustic embrittlement in boiler ?
- Caustic embattlement is a form of inter crystalline cracking, which results from a solution of sodium hydroxide or caustic soda, becoming more and more concentrated at the bottom of a crack or fissure (narrow opening) which may be the result of fatigue, in the boiler plate or furnace.
- The plate must be stressed, so that wastage take place at the bottom of crack, the plate weakens, the crack extends to expose new metal to the caustic action and thus it proceed.
- Caustic embattlement will only occur when there is a high caustic alkalinity that is when the ratio of NaOH to the alkalinity is high. To keep this ratio at the safe level the sodium sulfate to sodium hydroxide should be maintained above 2:5.
- The sodium sulfate comes out of solution in high sodium hydroxide concentration and by doing forms a protective layer on the surface of the plate.
- Also keep the lowest T.D.S level. (Total dissolved suspended & solid)
What is pH value for water ?
- It is the logarithm of reciprocal of hydrogen iron concentration.
What do you do for preventing of oxygen in boiler ?
- The oxygen-scavenging chemical which is used for deaeration the water are usually sodium sulphate or hydrazine.
- 2Na2SO3 + O2 = 2Na2SO4
- N2H4 + O2 = 2H2O +N2
- Excessive dosage of hydrazine could lead to steam and condensate line corrosion due to ammonia being produced as the excess hydrazine decomposed.
- In high pressure boiler, by using the sodium sulphite, the sulphite can break down to give hydrogen sulphide which can attack steel brass and copper.
If chloride is found too much in your boiler water what will you do ?
- Blow down frequently
- Reduce the boiler load to minimum.
- If highly contaminant shut down completely and wash out.
- Trace and find out the fault and remedies.
What do you understand by composite boiler ?
- If exhaust gas and oil fired can be used the same time, it is termed the composite boiler.
- In this type a separate tube nest of tube for exhaust gas is provided, situated above the return tubes from the fire furnace.
- Uptake from the tube nest are separated.
What do you understand by alternative boiler ?
- If the exhaust gas and the oil fire are arranged to be used only at a time, the exhaust gas boiler term as the alternative boiler.
- This unit can be oil fire heated by the exhaust gases alternatively.
- It required only one uptake.
What do you understand by economizer boiler ?
- In this system a separate exhaust gas economizer is connected to an oil fired auxiliary boiler or an accumulator by means of piping and a set of circulation pump.
- Exhaust gas is used as heating medium and it has no steam space.
- It required only one uptake.
What is meant of accumulation pressure in marine boilers?
- Accumulation pressure is the rise in boiler pressure which take place when the spring loaded safety valve lift due to the increase loading caused by further compression of the spring.
What is the purpose of accumulation pressure test ?
- To detect the safety valve is suitable for this boiler or not.
- To limit the rise in boiler pressure under full fire condition.
What is the procedure of accumulation pressure test ?
This test is carried out a new boiler or new safety valve.
- Shut off feed water
- Closed main steam stop valve.
- Increase cut off pressure of boiler.
- Bypass high pressure cut off of the boiler.
- Arrange the boiler fire rate to a maximum.
- Safety valve will be lift during the test.
- The test is carried out as long as the water permits in the boiler.
- Accumulation pressure should not exceed 10% of working pressure in the specified time.
- Specified time is 15 mins for a smoke tube boiler and 7 mins for water tube boiler.
Procedure of safety valve setting under steam pressure ?
Setting of safety valves:
- Take standard pressure gauge (approved by surveyor) for accuracy.
- Fill up water up to ¼ of gauge glass level, and shut main steam stop valve, feed check valve.
- Without compression rings, hoods and easing gears, reassembled the safety valves with spring compression less than previous setting.
- Raise the boiler pressure to desired blow off pressure.
- Screw-down spring compression nuts of any lifting valves, until all are quite.
- Arrange to have the desired steam pressure
- Adjust each valve in turn: Slacken compression nut until the valve lifts. Screw-down compression nut sufficiently enough, so that when the valve spindle is lightly tapped, valve return to its seat and remain seated. Measure gap between compression nut and spring casing. Make a compression ring equal to this gap, and insert under compression nut. Gag the spindle of this safety valve, to prevent opening, while remaining valve is being set.
- Remaining valve is again set and insert compression ring.
- Remove gag and retest both valve to lift and close together.
- Cap, cotter and easing gear to be refitted
- Caps and cotter pins padlocked to prevent accidentally altering the setting.
- When the surveyor satisfied the setting pressure, easing gear should be tested.
- All safety valves set to lift at not greater than 3% above approved working pressure (design pressure).
How do you take action if one of the passages of gauge glass is chocked ?
- Steam and water cocks and passages in the gauge can be cleared while the boiler is still steaming.
- To do this, shut the steam and water cocks and open the drain cock.
- Remove the check plug opposite the obstruction.
- Insert the cleaning plug. Screw in the plug with small hole about 5 mm diameter, drilled through it in place of the cleaning plug.
- Insert into this hole a rod of such a size that held by a gloved hand, it can be easily moved without being stuck.
- Then open the chocked and push the rod through to clear the blockage.
- When clear, open drain to prevent a build up of pressure and only a small amount of steam will blow past the rod, the glove protecting the operator from injury.
- operator from injury.
- Then close the cock and replace the normal cleaning plug. The gauge glass can be tested now and if satisfactory return to service.
- Do not carry out this operation on a plate type glass on a high pressure boiler.
What are the causes of boiler furnace blow back ?
- Insufficient purging time
- Accumulation of oil in furnace from the leaky burners
- Boiler tubes and uptake have full of soot deposits.
- Air registers control not operating for the high flame mode.
- Too little air
- Insufficient oil temperature
How to keep boiler not in service ?
For fire tube boiler out of service for short period
- The boiler must be completely filled with alkaline water.
- The boiler must be topped up periodically and any air in the system must be got rid off.
- Regularly test the boiler water and keep the alkalinity in the range of recommended value.
If the boiler is to be taken out of service for long period
- It should be drained completely and open up.
- Dried out by means of heater units.
- Then the trays of quick lime (moisture absorbent material) should be place internally in suitable positions.
- Blanks should be fitted to the pipe connections in the event of steam being maintained in other boiler and blow down line.
- The quick lime should be renewed at least once every two months.
What is the purposes of boiler water treatment ?
- To prevent scale formation ( Trisodium phosphates is used)
- To remove trace of oxygen (sodium sulphide or hydrazine is used )
- To give alkalinity and minimize corrosion.(sodium hydroxide is used )
- To reduce risk of caustic cracking ( sodium sulphate or sodium nitrate is used)
What is the difference between safety valve and relief valve ?
- Directly open the design lift at set pressure.
- Can open manually by easing gear.
- Setting pressure is just above the working and not more than 3% above the approved working pressure.
- Setting pressure is 10% above working pressure .
- Opening is proportion to the increase in pressure.
- Can not be open manually.
What are the markings on safety valve ?
- Manufacturer’s name
- Serial number
- Inlet diameter
- Operating pressure
- Discharge capacity
- Safe working pressure
- Blow off pressure
Boiler Uptake Fire and Exhaust Gas Economiser Fire
Auxiliary boiler uptake or flue gases normally pass through economisers or air pre-heaters or feed water heaters. All these types of heat exchangers are fin type. Hence large amount of carbon or soot deposits on the fins. Along with the flue gases some un-burnt carbon particles also get deposited on the economisers.
Normally dry soot deposits have a very high ignition temperature. But when the soot gets wet with hydrocarbon vapours, their ignition temperature comes down to around 150 degree Celsius. This may result in boiler soot fire or boiler uptake fire. When an uptake fire is suspected, never soot blow the economiser. Soot blow with steam causes steam to convert as hydrogen gas and result in metal fire.
- Regularly inspect boiler flame to ensure correct air fuel ratio is maintained. This makes complete combustion with clean flue gas.
- Carry out boiler burner and swirler routines as per PMS.
- Regularly soot blow economisers and water wash periodically to remove soot deposits.
- Use proper grade of fuel oil for boiler.
- Fuel oil used for boiler to be treated well.
- Avoid low load operation of boiler for long period.
- Running boiler near full load periodically, helps in expelling out carbon deposits adhered in uptake passages and reduces chances of boiler uptake fire considerably.
What is exhaust gas economiser (EGE) fire and how it occur ?
- A flame appear at the economizer coil during running of main engine at sea while the EGE is put into service with circulation water passing through the coils, it is called economizer fire.
- It is actually cause by soot fire at the economizer coils.
- For the economizer coils fire, heat is already presented due to passing the gases of main engine.
- Air is available due to excess supply of scavenging air into the unit combustion chamber.
- Thick deposit of unburnt fuel, carbon residue (soots) are sticking at the economizer coils .
- Soots are formed from incomplete combustion and use of low grade fuel and high carbon content fuel.
- At the manoeuvring time, the more incomplete combustion may occur and at that time leaving flue gas velocity is very low, thus unburnt fuel can be adhering on the economizer coils.
- A flame is produced when air and fuel are proportionally mixed in the sufficient heating temperature
What are the indications of EGE fire ?
- Smoke smell will get from the economizer.
- Over heat at economizer body ( external casing of uptake.)
- Heavy smoke and sparks will emit from the funnel.
- Sudden unexpected increase in uptake gas temperature ( Abnormally high stack pyrometer reading)
- Flame visible in the smoke indicator.
How to prevent EGE fire on ships ?
- To get complete combustion of fuel, maintain the main engine (ME) in optimum combustion condition.( governor, fuel injection pump, fuel injection valve, fuel injection timing, fuel condition, air cooler, turbocharger, do not run too long ME with slow speed)
- Regular open up & cleaning smoke side of economiser depending upon soot accumulation. (water washing)
- Regular soot blowing operation. Check & maintain soot blowing equipment.
- Regular overhaul boiler burner, correct air fuel ratio & damper.
What are the actions to be taken during economizer fire ?
- Inform to the bridge and chief engineer. Take the permission to stop main engine.
- Gradually slowdown the ME and then stop.
- If fire fighting equipment is fitted, it should be brought into operation.
- Cool down the economizer by means of boundary cooling.
- Circulation pump must be run but, if large amount of water lost it must be stopped.
- There is no heat source and fire is out.
- When economizer fire occurs never do soot blow to economizer.
- It can cause hydrogen fire and melt down the coils stack. It can cause deformation and tube bulking because intensive heat and high thermal stress formed at economizer coils.
What are the actions to be taken after economizer fire ?
- Cool down to ambient temperature.
- Open the economizer cover.
- Clean inside parts of the economizer.
- Check the any defective coil and repair for temporary used by welding or plugging the tubes.
How to effectively fight the economiser fire ?
- Slow down the engine.
- Shut off oil burners, draught fans, dampers & air register.
- Raise water level full and blow down continuously so as to maintain good flow of water.
- Reduce boiler pressure by easing gear.
- Spray water on the external casing of the uptake to cool the affected area.
- A few times starting and stopping of main engine should be done to blow out collected soots at the uptake.
What is self perpetuating fire or hydrogen fire on ships ?
- When the fire causes the metal itself burning at about 700 °C, if steam smothering soot blowing system or water jetting system have been attempted, the big hydrogen fire may result.
- The applied steam dissociates into hydrogen and oxygen and accelerating the fire.
- Once such a fire has started, resulting two kind of fires may take place simultaneously, one kind, iron burning in steam, and the other, the hydrogen burning in an air exothermic way.
- This combined fire being self supporting and lasting until the supply of steam is exhausted.
- The primary objective of dealing the fire is to cool the surface and burning material as quickly as possible.
- supporting and lasting until the supply of steam is exhausted.
- The primary objective of dealing the fire is to cool the surface and burning material as quickly as possible.
The boiler maintenance should always be executed with skill and in accordance with valid rules and regulations from the authorities, and below are given some recommendations for periodical inspections and maintenance.
During normal operation of the boiler some work and check procedures have to be considered every day.
- Check the boiler steam pressure and the water level.
- Check that the feed water control system is operational.
- Check the water condition and make necessary countermeasures with regard to the feed and boiler water treatment. If necessary blow-down the boiler.
- Check the function of the oil burner at different capacities through the inspection holes on the boiler.
- Check the flue gas temperature after and/or the draft loss across the boiler. If either the temperature or the draft loss is too high, the pin-tube section must be cleaned.
Weekly Routine Checks
- Drain each water level glass for about 10-15 seconds.
- In case of contaminated boiler water or insufficient water treatment, the draining of the water level glasses must be done more often.
- Check the safety water level device.
- Depending on the boiler water tests blown-down the boiler. Open the blow-down valves quickly for a few seconds, and then close and open again for about 5-10 seconds.
- Repeat this operation when required according to the boiler water tests.
- Perform scum blow out by means of the scum valve when required. The scum blow out must be carried out until the drained water is clean.
Monthly Routine Checks
- Test all stand-by pumps.
- Check all boiler mountings for damage or leaks and repair/replace if necessary.
- Check the function of the high steam pressure switch by lowering the set point or by raising the steam pressure, e.g. by closing the main steam valve slowly. The burner must stop automatically.
- Check the function of the high steam pressure switch by lowering the set point or by raising the steam pressure, e.g. by closing the main steam valve slowly. The burner must stop automatically.
Inspection of Furnace
The furnace should be inspected at least twice a year. During this inspection the following issues should be taken into consideration:
- Check for cracks at the refractory lining and that the furnace walls are free from excessive soot deposits.
- Examine carefully the area opposite the burner. Too much soot deposits indicate that the burner should be adjusted.
- Check that the pin-tube elements are intact and that soot deposits are within normal limits.
Inspection of Boiler Water Side
- The boiler water side (interior) must be carefully inspected at least twice a year. This inspection of great importance and no doubt the most important of all the maintenance measures, since it has a direct influence on the boiler longevity and on the security.
- At these inspections, hard deposits, corrosion and circulation disturbances can be found at an early stage, and preventive measures must be taken to avoid unexpected material damage and boiler breakdown.
- Presence of hard deposits at the furnace wall and the pin-tubes reduces their heat transfer properties and decrease the capacity of the boiler.
- Further, it is possible to make out if the feed water treatment has been satisfactory and if the blow-down has been carried out sufficiently.
- Incorrect feed water treatment is commonly causing hard deposits or corrosion.
- Insufficient blow-down will cause sludge deposits in the tubes and accumulation of sludge in the bottom of the boiler.
- If hard deposits are not removed, it may lead to overheating in the boiler plate material which is exposed to the flame in the furnace wall area. This may cause material damages.
- Incorrect feed water treatment does not always lead to hard deposits. For example, a too low or too high a pH-value may give an electrolytic reaction, causing corrosion in the boiler.
- When the boiler interior is inspected, examine all parts carefully and be attentive to deposits, corrosion and cracks. It is advisable to pay special attention to this inspection.
Procedure and Remarks for Inspection
- Shut off the boiler and allow it to cool (below 100°C). The boiler should NOT be depressurised by lifting the safety valves and then filled with cold feed water since the stress induced by too rapid cooling may cause damage.
- Empty the boiler and close all valves. If the boiler is connected to a second boiler, check that the valves between them are closed.
- Unscrew and remove the manhole hatch(s) on the boiler and enter the boiler when it is sufficiently cold. Check the welding in the boiler. A careful examination should be carried out with respect to any possible corrosion or crack formation.
- Special care should be taken to the water line area in the pressure vessel where oxygen pitting may occur.
- If deposits are found to be forming in the boiler tubes, the tubes should be chemically cleaned.
- It is advisable to consult a company of cleaning specialists who will examine the boiler deposits and treat the boiler accordingly.
- After chemical treatment the boiler should be blown-down at least twice a day for approximately one week. This will ensure that excessive sludge deposits due to chemical treatment do not collect in the bottom of pressure vessel.
If the water is contaminated with foreign substances like oil, chemicals, corrosion products etc., it is very important to act immediately to avoid damages to the boiler.
Layers of thin oil films, mud, etc. exposed to the heating surfaces causes a bad heat transfer in the boiler, leading to overheating followed by burned out pressure parts. In order to remove such contamination, a boiling out or acid cleaning have to be performed immediately.
Corrosion products from the pipe system or insufficient boiler water treatment may result in corrosion in the boiler itself. It is therefore important to observe that such circumstances do not occur in the system.
Corrosion can be broadly classified into two.
Metals have a tendency to return back to their natural state of metal oxides. These metal oxides form a thin layer over them and it prevents further corrosion attack.
Every metals have a specific potential when placed in an electrolyte. When two metals of different potential are placed in an electrolyte, it forms an electro-chemical cell. This results in flow of electrons to and from the metals. Even different parts of a single metal can have different potentials in certain conditions. This also may set up an electro-chemical cell or galvanic cell or corrosion cell.
Whatever be the case a cell means there is an anode and cathode, in which anode loses electrons and deteriorates with time. While at the cathode two conditions are there:
- If electrolyte is acidic, hydrogen gas is released.
- If electrolyte is alkaline, polarizing layer of hydrogen is formed, which prevents further corrosion.
Another thing to be noted is that when there is dissolved oxygen in the electrolyte, the polarizing layer of hydrogen is reacted with oxygen to form water. So no more protection for the metal.
The variation in rate of corrosion with alkalinity at different dissolved oxygen content is shown below.