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| Composite Cochran Boiler |
What is the meaning of boiler ?
It is a pressure vessel 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.
1. What is the boiler mounting ?
It is directly fitted to the boiler shell. They are 1. Safety valve 2. Main steam stop valve 3.Air vent cock 4. Two gauge glass level indicators 5. Feed check valve 6. Scum blow down valve 7. Bottom blow down valve 8. Salinity cock or test cock 9. Man hole doors.
2. 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-
1. To release air from the boiler either filling the boiler water or raising the steam.
2. To allow air to enter in the event of boiler cooling down or blowing down .( to prevent
vacuum effect)
3. Boiler safety devices ?
1) Safety valve
2) Low / high water level alarm
3) Too low water level alarm and shut down
4) Water level indicators
5) Pressure gauge
6) Low fuel oil pressure alarm
7) Low / high fuel oil temperature alarm
8) Flame failure alarm
9) Smoke density alarm
10) Easy gear arrangement
11) Air vent
12) Force draught fan stop alarm
13) Low / high steam pressure alarm
4. 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.
5. Boiler gauge glass blow down procedure ?
1) Shut steam and water cocks than open the drain cock.
2) After draining water from the sight glass, there is nothing come out, both steam and water
cocks are good in order.
3) Open the steam cock side, blow out the steam.
4) Close the steam cock side, the steam connection is clear.
5) Open the water cock side, blow out the water.
6) Close the water cock side, the water connection is clear.
7) Close the drain cock.
8) Open the water cock, water should then gradually come rise up to the top of the gauge glass.
9) Open the steam cock, the water in the glass should fall to the level of the water in the boiler. # Working position – all cocks handle are in downward position.
6. 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.
7. Why do scum blow down and bottom blow down ?
Scum blow down ( surface blow down )
To remove accumulated, suspended and floating solids, impurities and also remove dissolved solid concentration, they hinder the formation of steam.
Bottom blow down
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.
8. Purpose of boiler water test ?
1.To ensure that proper chemical treatment are maintained at all time.
2.To detect the present of contaminants in the water that may be injurious to boiler and system.
9. Enumerate the boiler water test
1) Alkalinity test
a. Phenolphthalein ( p) alkalinity test
b. Total ( T ) alkalinity test (2 x P)
2) Chloride test
3) Condensate PH test
4) Amerzine test ( Hydrazine test )
5) Excess phosphate test
6) Conductivity test (Total dissolve solids test)
7) Hardness test
10. Purpose of Boiler water test ?
1) Alkalinity test
This is to ensure that the boiler water prevent corrosion by neutralisation of acidic gases
(a) Phenolphthalein ( 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.
2) Chloride test
>To know the amount of salt in boiler water. ***
>To minimize chloride level and to adjust the blow down.
3) Condensate PH test
>To control condensate PH value within a limit. ****
>To minimize corrosion in steam and condensate system.
4) 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.
5) Phosphate test
>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.
6) Conductivity test
>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.
7) Hardness test
Ö 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.
11. Purpose of boiler water treatment
1) To prevent the scale formation in the boiler and feed system
2) To prevent corrosion in boiler and feed system.
3) To control the sludge formation and prevention of carry over with the steam.
4) To maintain the boiler water in alkaline condition and free from dissolved gases.
5) To prevent of entry into the boiler of foreign matter such as oil, waste, mill scale, FeO, Cu , Sand etc.
12. What is foaming in boiler 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:
1.High concentration of impurities.
2.By the animal or vegetable fats in feed water carry over from the oil heaters.
3.Increase in level of dissolved & suspended solid TDS level.
4.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.
13. 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 1. Excessive foaming 2. Improper amount of steam space 3. 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 ?
1. Scum blow down
2. Reduce boiler fire rate
3. Check whether boiler chemical added are in excess
4. Detect the source of contamination
5. In case of bad priming the boiler may have to be taken out of service, shut down.
For contamination due to oil have to be chemically cleaned.
14. Boiler blow down procedure ( cool down )
1) Change the fuel oil burning system from HO to DO and then shut down the burning system.
2) Stop feed p/p close feed check valve.
3) Drain down the boiler after allowing it to cool down.
4) If no sufficient time to do this, lower the boiler pressure to 3 to 4 bar.
5) Shut the main steam stop valve.
6) Open the ship side valve then open the blow down valve.
7) Banging noise will appear when boiler is empty.
8) Close the boiler blow down valve and ship side valve.
9) Then release the steam pressure through safety v/v by means of easing gear.
10) When pressure is off, open the air vent and the boiler to cool down.
Opening up and Inspection Procedure
Follow as procedure above, After making sure no vacuum in it, first---
1. To remove top man hole door, slacken the dog holding nuts but do not remove them until first broken the joint
2. Remove nuts and dogs and take out the door.
3. The bottom door can be removed after warning personnel to keep clear of the top door.
4. Make ventilation before entering. Do not allow naked light near the boiler.
5. Preliminary internal inspection carried out before cleaning, to check scale deposits and any special
points.
6. Plug orifice to blow down valve to prevent choke, place guards over the manholes landings to prevent damage.
7. 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.
15. Describe the boiler fresh up procedure from cool condition ?
The boiler is carried out firing from cold condition to normal working pressure condition to avoid thermal stress.
1) Check the boiler blow down valve in close position.
2) Shut the main steam stop v/v.
3) Open the air vent cocks.
4) Open the feed check valve and pumping up to ¼ of gauge glass level.
5) Start the force draft fan with dampers, open correctly to purge the furnace and combustion space of any foul gas.
6) Light the burner after closing the recirculating valve.
7) Normally allow the fire to burn for 5-minutes and stop for 30 mins. This step continues until steam come out.
8) When steam coming out from the air vent close the air vent.
9) Rise up the working pressure step by step slowly. (Allow the fire to burn for 30 mins and stop the fire for 10 min.)
10) When the steam pressure reached Is working pressure drain the steam line. (to avoid water hammer)
11) Main steam stop valve open slowly (crack opening)
16. How do 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.
1. The fire must be immediately stopped.
2. The main steam stop valve must be closed.
3. Blow down and cool down the boiler.
4. Check leakage, drum outside, Located the cause of trouble and make necessary repair.
5. Enter the boiler after it has cooled and examined any possible damage.
6. After repair, water fill up slowly and restore to normal operation.
7. If no damage occurred, inject the water slowly into the boiler and restore it to operation.
Possible causes:
- One gauge glass defective
- Boiler tube crack and leaking
- Feed regulator jammed
- Fail feed pump (Air in feed water line or pump)
- Level controller malfunction
- Steam consumption is too much
16. 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.
Erosion
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.
Corrosion
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.
Electro-chemical action.
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.
Caustic embrittlement.
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 intention (]yif:]yif: xefxef oufa7mufonf), 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)
pH value
Logarithm of reciprocal of hydrogen iron concentration.
17. What do you do for preventing of oxygen ?
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 pr boiler, by using the sodium sulphite, the sulphite can break down to give hydrogen sulphide which can attack steel brass and copper.
18. 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.
19. What do you understand composite, alternative, and economizer boiler ?
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 nest of tube for exhaust gas is provided, situated above the return tubes from the fire furnace. Uptake from the tube nest are separated.
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.
Economizer
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.
20. What is meant of accumulation pressure ?
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.
21. 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.
22. Procedure of accumulation pressure test ?
This test is carried out a new boiler or new safety valve.
1) Shut off feed water
2) Closed main steam stop valve.
3) Arrange the boiler fire rate to a maximum.
4) Safety valve will be lift during the test.
5) The test is carried out as long as the water permits in the boiler. During of the test is 15 mins for a tank boiler and 7 mins for water tube boiler
6) Accumulation pressure should not exceed 10% of working pressure.
23. Safety valve setting under steam pressure ?
Setting of safety valves:
1) Take standard pressure gauge (approved by surveyor) for accuracy.
2) Fill up water up to ¼ of gauge glass level, and shut main steam stop valve, feed check valve.
3) Without compression rings, hoods and easing gears, reassembled the safety valves with spring compression less than previous setting.
4) Raise the boiler pressure to desired blow off pressure.
5) Screw-down spring compression nuts of any lifting valves, until all are quite.
6) Arrange to have the desired steam pressure
7) Adjust each valve in turn:
a. Slacken compression nut until the valve lifts.
b. Screw-down compression nut sufficiently enough, so that when the valve spindle is
lightly tapped, valve return to its seat and remain seated.
c. Measure gap between compression nut and spring casing.
d. Make a compression ring equal to this gap, and insert under compression nut.
e. Gag the spindle of this safety valve, to prevent opening, while remaining valve is being set.
8) Remaining valve is again set and insert compression ring.
9) Remove gag and retest both valve to lift and close together.
10) Cap, cotter and easing gear to be refitted
11) Caps and cotter pins padlocked to prevent accidentally altering the setting.
12) When the surveyor satisfied the setting pressure, easing gear should be tested.
13) All safety valves set to lift at not greater than 3% above approved working pressure (design pressure).
24. 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 stack.
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.
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.
25. Causes of furnace blow back ?
>Insufficient purging time,
>Accumulation of oil in furnace from the leaky burners,
>Boiler tubes and uptake have full of soots.
>Air registers control not operating for the high flame mode.
>Too little air
>Insufficient oil temperature
26. 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 and phenolphthalein in the range of recommended value.
- If the boiler is to be taken out of service for long periods.
- It should be drained completely and open up.
- Dried out by means of heater units.
- Then the trays of quick line 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 line should be renewed at least once every two months.
27. 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 or sodium carbonate is used )
- To reduce risk of caustic cracking ( sodium sulphate or sodium nitrate is used.
28. Difference between safety valve and relief valve ?
Safety valve
1) Directly open the design lift at set pressure.
2) Can open manually by easing gear.
3) Setting pressure is just above the working and not more than 3% above the approved working pressure.
Relief valve
1) Setting pressure is 10% above working pressure .
2) Opening is proportion to the increase in pressure.
3) Can not be open manually.
29. Marking on safety valve ?
- Manufacturer’s name
- Serial number
- Inlet diameter
- Operating pressure
- Discharge capacity
- Safe working pressure
- Blow off pressure
30. 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
31. What is water hummer / how will you prevent it ?
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 –
a) Install steam trap in the line
b) Open the drain first before allowing the steam into the line.
c) Crack open of steam valve at first.
32. 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.
33. 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.
34. How to know scum 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.
35. Where is manhole doors fitted ?
They are fitted at one at steam side and other one for steam side.
36. Effect of foaming and priming ?
a) can occur water hummer
b) can cause contamination and scaling.
c) Can cause fluctuation of working water level.
37. What is blowback ?
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.
38. Procedure for hydraulic pressure test ?
Hydraulic pressure test is 1.25 times working pressure, ( 10 minutes maintain ) Close all opening.
1) Open air vent cock.
2) Fill up boiler water fully
3) Close the air vent cock
4) Place hydraulic jack to feed water line.
5) Fit standard pressure gauge.
6) Applied hydraulic pressure 1.25 times of working pressure and maintain 10minutes.
39. What is liquid coagulant ?
It is more molecular wt colorless solution, such 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.
40. 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.
41. Boiler maintenance:
- 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.
- Adjust the fuel air ratio.
- Clean fuel oil fitter.
- Check the fuel oil pressure.
42. When boiler safety valve setting made ?
- Every boiler survey.
- After safety valve overhaul.
43. 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 injurious to watch keeper.
44. If no test and treatments is done to boiler, the following may rise.
(01) Reduction in boiler efficiency due to poor heat transfer
(02) Reduction in tensile strength
(03) Reduction in factor of safety
(04) Overheating of metal resulting distortion and eventual failure
(05) Increase in fuel consumption
(06) Excess concentration of NaOH (caustic soda) may cause caustic embrittlement on boiler metal and tend to failure of boiler metal
(07) Corrosion
(08) Scale formation
(09) Foaming, Priming, carried over (due to increase T.D.S level, foaming is present and tend to carried over and priming)
45. Control of Evaporation (How to control / reduce excess EGE pressure)
01) By regulating the amount of gas flowing over the extended surface, i.e. by damper regulation.
02) By dividing the unit into sections so that each section is controlled by an inlet valve.
03) By passing steam through an automatic pressure controlled surplus valve to a dump condenser.
04) By a system designed for a higher pressure than that it is to operate.
Soot Blow
Purpose :
- To remove soot, to prevent EGE fire
- To get steam generating efficiency good
- To increase heat transfer efficiency
Time:
- Day time at sea 2 -3 times/day
Procedure:
- Inform to bridge
- Check wind direction, good it transverse direction
- Raise boiler pressure
- Open drain cock until drain clear
- Open soot blower valve
- Open steam valve & carried out by turning wheel 20 -30 sec
- Then close steam valve, soot blower valve
- Open drain valve
Boiler safety valve closed 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 (tzHk:) 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.