Date post: | 22-Apr-2015 |
Category: |
Documents |
Upload: | nithin-varghese |
View: | 183 times |
Download: | 4 times |
!. Describe with the aid of a neat sketch, the following:
a. blowing down a boiler gauge glass
b. Boiler safety valve. What is the lifting pressure of the safety valve? What is meant by
accumulation of pressure test?
Gauge glass is a kind of a level indicator which shows the amount of fluid in a boiler or any other storage place on
a ship.
Refer the diagram for better understanding of boiler gauge glass.
� Check the nut (1) and tighten if necessary.
� Bolts on the boiler flanges are to be checked, if loose tighten them.
� Check if union nuts are loose.
� Bolt from screw 7 with cocks in open position.
� If the leak cannot be stopped from water side on the gauge glass by tightening the screw 7 then the sealing
surface of the cock plug 8 may be damaged or corroded.
Gauge glass blow down procedure
Gauge glass should be blown before lighting up of boiler, after stopping the boiler and regularly if the level in gauge
glass is suspected to be wrong.
Cleaning the water side of gauge glass
� Close the valve S and W as shown in the figure.
� Now open the cock W and see if the water is coming out of the drain valve D indicating the drain line is clear.
� Now close the drain valve D and keep the cock W open and see if the water level rises in the gauge glass; this
indicates the line to gauge glass is also clear.
� Repeat the steps two to three times to remove nuts and deposits inside.
Cleaning the steam side of gauge glass
� Close both the cocks S and W.
� Now open the cock S and open the drain valve D and see the steam is coming out. The drain is opened only for
1-2 seconds only as steam may damage the sealing and service life decreases.
Putting the gauge glass in normal operating position
� Close all the valves S, W and the drain valve D.
� Now open the cock W and let the water fill inside the gauge glass.
� Now open the cock S and then the level can be seen as the pressure equalizes.
b. Safety valve:
The lifting pressure of the safety valve is 3% of working pressure.
The auxiliary boiler should be able to withstand 10% over and above the working pressure, without the
shell and the structure getting damaged. The pressure test done to test this feature is called
accumulation of pressure test.
2. What are the steps to be carried out for warming up and firing a boiler from a cold condition? How
long does it take to raise the steam from cold condition?
1. If the boiler has been opened up for cleaning or repairs, check that all work is completed and carried
out in a satisfactory manner. Ensure that all tools etc have been removed. Any blanks fitted are to be
removed and the boiler is in order. Keep the top manhole door opened.
2. Fill boiler with water to about one quarter of gauge glass. If possible fill it with hot water heated by
means of a feed heater. If required any treatment chemicals are to be added, they can be mixed up
with water and fed into the boiler through top manhole door. Fit back the top man hole door.
3. Keep the air vent open.
4. Start the burner and keep the burner burning at the lowest rate. Preferably fire for 1 min. and stop
for 5 minutes. Continue this till the temperature around the boiler shell have become uniform. (if
provision is there, then the water can be circulated to attain uniform temperature)
5. Boiler temperature is raised at a rate of about 6°- 7°C per hour. Once the temperature is uniform
around the boiler, fire the burner for 2’ and rest for 5’.
6. Tighten all the loose flanges (only hand tight).
7. Continue firing at this rate till steam escapes through the air vent.
8. Increase the firing to continuous firing (lowest rate), till the steam comes out clearly through air vent
and the pressure gauge indicates atleast 1.2 bar. Close the air vent.
9. continue firing at the lowest rate, till the boiler builds up the required pressure. Any leaky joints,
should be nipped up.
10. The water would have expanded by now and check the water level by blowing through the gauge
glass. Take water if required. Maintain around 60 -65% water in the gauge glass.
11. If a circulation system is available, then it takes about 12 hours for this procedure. If no circulation
system is provided, then it will take somewhere between 18 – 24 hours. This is due to the fact that
water is a very poor conductor of heat, and heat from the furnace will be carried up by convection
currents leaving the water below the furnace in cold condition. This will lead to severe stresses and may
lead to boiler failure.
3. With respect to steam system on board ship, explain the following:
a. Blowing down the gauge glass of the boiler
b. soot blowing of an exhaust gas boiler
c. damage that can be done on system if water is carried along with steam.
d. use of sight glass and hot well in feed system
e. why regular testing of boiler water is required?
a. Blowing down the gauge glass of the boiler: This is done to ensure that the boiler has safe level of
water and that the level reading is correct.
Procedure: 1`. Close steam and water cocks.
2. Open the drain cock. Water will drain out from the gauge glass.
3. Open the water cock and ensure that the water flows freely from the drain. This
ensures that the water side is clear.
4. close the water cock.
5. Open the steam cock and ensure that steam is flowing clearly through the drain. This
ensures that the steam side is free and clear.
6. Close the steam cock.
7. close the drain
8. Open the water cock. Water will slowly fill up the gauge glass.
9. Open the steam cock. The steam pressure will act on the water and show the correct
level.
b. soot blowing of an exhaust gas boiler:
it is done to effectively reduce combustion deposits and to improve the heat transfer area of the
exhaust gas economizer / exhaust gas boiler. Soot blowing is performed 2-3 times
a day as a minimum, at 75% engine load (high gas velocity will help carry out the soot), if a soot
blower is installed.
Steam soot blowing
A steam soot blower consists of a number of nozzles on a pipe that
rotate and sometimes moves axially between the banks of tubes.
Designed to be used during boiler operation
Water soot washing
A water washing devise is normally a number of water nozzles placed
over the tubes and the water washes the tubs and the sooty water is then
collected in a pit below the exhaust gas boiler.
The water washing system is designed for soot cleaning during non-
operation and for fire fighting. The drain from the water collecting pit has
to be open before the washing start.
A water washing devise is often installed as a complement to other
methods that can be used during boiler operation.
c. damage that can be done on system if water is carried along with steam:
Entrained boiler water is the most common cause of steam contamination. The entrained boiler water
contains dissolved solids and can also contain suspended solids.
Steam Ouality-The amount of moisture in the steam. It is the weight of dry steam in the mixture of
steam and water droplets. It is reported as a percentage.
Some of the effects of carryover are:
1. Deposition in regulators and valving
2. Deposition in superheaters
3. Deposition in control valves and turbines
4. Process contamination
Carryover can contaminate process streams and affect product quality. Deposition in superheaters can
lead to failure due to overheating and corrosion,
Superheated steam turbines are particularly prone to damage by carryover. Sticking of governor and stop valves
due to deposits can cause turbine overspeed and catastrophic damage. Solid particles in steam can erode turbine
parts, while deposition on turbine blades can reduce efficiency and capacity. Losses of 5% in turbine efficiency and
20% in turbine capacity have occurred due to deposition. When large slugs of boiler water carry over with steam, the
resulting thermal and mechanical shock can cause severe damage.
Loss of production may result from reduced capacity or equipment failure caused by carryover. In some instances,
the effect of carryover on production overshadows all other considerations.
d. use of sight glass and hot well in feed system: Hot Well recollects the steam after the work is done and it is condensed. Boiler water tank is known as
the hot well because boiler feed pump takes suction from the hot well and gives it to the boiler through
feed check valve. It can be called by three different names, they are:
Hot well - because the water collected is hot
Cascade tank - because it collects the water from the condenser
Observation tank - because it is used for observe for any oil or dirt entering the system
If any traces of oil are found in the system, it indicates that there is a crack in the steam heating line in
side the fuel oil tanks. A sight glass is placed to observe the traces of oil or dirt present in the system.
If oil is present in the system then it forms a coating in tubes of the boiler, which may lead to lesser heat
transfer to the water in the boiler.
e. why regular testing of boiler water is required?
To determine the levels of impurities and the type and amount of treatment necessary to eliminate
corrosion, scale and loss of heat transfer ability.
Regular Water Analysis should be made as often as possible to assure water treatment is properly
regulated. If there are indications of scale, corrosion, or any other unfavorable condition, water analysis
may need to be performed more often until condition is corrected.
Boiler water testing is an integral part of the routine boiler tests carried out on board a ship. Precise
boiler water test assures higher efficiency and reduced maintenance of boilers. Careful monitoring of
boiler water is of utmost importance in order to maintain correct water conditions. Failure to do so,
leads to scale formation, reduction in efficiency and steam purity, and corrosive water conditions.
For this reason, boiler water tests are carried out with the help of specialized instruments, by checking
the constituents of water before feeding it inside the boiler.
4. a. Why boiler water is tested? Name any 4 boiler water tests carried out on board.
b. Sketch and describe a composite boiler which can be used in port as well as during sailing?
a. To determine the levels of impurities and the type and amount of treatment necessary to eliminate
corrosion, scale and loss of heat transfer ability.
Regular Water Analysis should be made as often as possible to assure water treatment is properly
regulated. If there are indications of scale, corrosion, or any other unfavorable condition, water analysis
may need to be performed more often until condition is corrected.
Boiler water testing is an integral part of the routine boiler tests carried out on board a ship. Precise
boiler water test assures higher efficiency and reduced maintenance of boilers. Careful monitoring of
boiler water is of utmost importance in order to maintain correct water conditions. Failure to do so,
leads to scale formation, reduction in efficiency and steam purity, and corrosive water conditions.
For this reason, boiler water tests are carried out with the help of specialized instruments, by checking
the constituents of water before feeding it inside the boiler.
1. Phenolphthalein alkalinity and total alkalinity
2. chloride test
3. Phosphate reserve
4. Condensate PH - Measuring the acidity or alkalinity of the water.
5. Neutralized Conductivity - For testing the presence of ferrous ions and metal contents.
6. Feed Water Hardness - For testing the presence of carbonate compounds.
7. Dissolved oxygen content
Composite boilers are combination of oil-fired boilers and exhaust gas economizers. When the diesel
engine is at full load the fuel oil burner only starts if the steam demand exceeds the steam production
achieved from the diesel engines exhaust gases. Most composite boilers nowadays have separated
sections for the diesel engines exhaust gases and the flue gases from the fuel oil burners. Composite
boilers also works as a silencer for the diesel engine, but often an additional silencer is required.
Composite boilers that mixes the diesel engines exhaust gases and the flue gases from the fuel oil
burner have existed, and may still exist. Using the diesel engines exhaust gases as combustion air for
the fuel oil burner is quite economically although it makes a rather complicated unit.
Starting, stopping and modulating the burner normally controls the steam pressure in the composite
boiler. In the old days a huge damper was used to by-pass the composite boiler when the steam demand
was less than the quantity produced by the diesel engines exhaust gases. Nowadays a pressure
governed control valve dumps excess steam into a dump condenser.
5. a. Describe the process of preparing a boiler for survey
b. Describe the firing of the boiler to raise steam.
a. Boilers and thermal oil heaters are to be surveyed twice in every five-year period. The
periodicity of the boiler survey is normally 2.5 years. Steam boilers, superheaters and economisers are
examined internally and externally. To this end, boilers are to be drained and suitably prepared for the
examination of the watersteam side and the fire side. Where necessary, the external surfaces are to be
made accessible for inspection by removal of insulation and lining
All mountings are dismantled and examined, before being passed as fit for continued use. Safety valves
are examined and set, to the correct working pressure by the Surveyor. All safety controls and trips are
tested. Auxiliary boilers are surveyed every 30 months. Main boilers are surveyed every 30 months
upto 8 years, and every year thereafter.
Lifting pressure of safety valve is 3% of working pressure.
Procedure:
1. Allow the boiler to cool down. When pressure is around 300-400 kN/m2, blow down to empty the
water.
2. When pressure reaches slightly above atmospheric pressure, open the air vent. This will prevent
vacuum formation in the boiler when steam pressure becomes zero.
3. Keep the drain cock of the boiler water gauge glass open.
4. Remove the top man hole door, depending on the weight of the door, rig necessary lifting block as
necessary.
5. Ensure the water space is empty and open the bottom manhole door.
6. With both doors opened, there will be clear draft which will enable to boiler to be ventilated.
7. A preliminary inspection is carried out internally before cleaning is commenced , note scale deposits
and other observations.
8. plug the orifice to the blow down valve to ensure it does not get choked during cleaning operations.
9. Remove the mountings from the boiler and dismantle all the mountings.
10. Clean all the parts including the internals of the boiler.
11. After cleaning is over, keep the parts of mountings in an neatly arranged wooden board or cloth to
exhibit the condition to the surveyor.
12. Safety valves while dismantling, the marking is noted for tightening position. All other valves are
also marked to enable proper boxing back.
13. Keep all the packing and gaskets (new) ready to enable boxing back after the survey is over.
b. this question is already answered in one of the previous questions.
6. Sketch an auxiliary boiler and label the mountings.
Explain the use of following: a. Economiser b. Super heater.
a. this question can be answered from the answer given in one of the previous questions.
BOILER MOUNTINGS:
1) Main steam Stop Valve: The steam generated in the boiler is supplied to the main system through this v/v. It is
normally non-return type of valve.
2) Auxiliary steam Stop Valve: If there is separate steam line provided for small auxiliary system, it is supplied
through this valve. The valve is smaller in size and usually of a non-return type.
3) Safety valve: In the event of unsafe excessive pressure inside the boiler, the safety valve comes in action to
release the overpressure. The lifting pressure of the valve is set prior to its installation and locked in the presence of a
surveyor so that it not changed later on. Safety valves are generally installed in pairs.
4) Water level gauge glass: Gauge glasses are also fitted in pairs for manually checking the water level inside the
boiler drum. It is on the basis of the boiler pressure that the construction of the gauge glass is decided.
5) Air release valve or boiler vent: This valve is fitted in the headers, boiler drum etc, to avoid imploding of boiler
when it is depressurized or when initially raising the steam pressure.
6) Feed check and control v/v: This valve controls the supply of steam supply as per the demand and is fitted in
both main and aux steam line after the stop valve. They are non-return valves with a visible indication of open and
close position.
7) Pressure gauge connection: This pressure gauge can be fitted in super heater, boiler drum and wherever it is
necessary to read the pressure reading.
8) Blow down valve: It is used to empty the boiler completely for maintenance purpose or for water treatment of
boiler when the chloride level becomes very high.
9) Scum blow down valve: It’s a shallow dish type arrangement fitted at the normal water level which allows the
blow down of floating impurities, oil foaming etc from the water surface.
10) Sampling connection: Generally, a sampling water cock arrangement is also fitted with cooler in series so that
water sample can be collected at any time for feed water analysis.
11) Whistle valve: If there is a provision for steam whistle in the ship, then steam is supplied directly from the boiler
through a small bore type non-return valve known as whistle valve.
12) Low Level alarm: A device used to initiate audible warning at low water level condition.
13) Soot blowers: Required to blow the soot and the combustion products from the tube surfaces. It is operated by
steam or compressed air.
14) Automatic feed water regulator: Device which is essential to ensure appropriate water level in all load
conditions and is fitted in the feed line. Multiple element feed water control system is used in boilers with high
evaporation rate.
7. a. Sketch a simple smoke tube boiler using heat from both main diesel engine exhaust gas and fuel
oil fire
b. For efficient running, what maintenance is required with regard to fire side and water side of a
boiler?
a. Answer is given in question 4 b.. Draw the composite boiler and explain.
b. Maintenance to be carried out:
The fire side and the water side are thoroughly cleaned of scales, deposits and soots. The external
and internal cleaning will improve heat transfer, thereby improving the efficiency of the boiler.
Once the boiler is cooled and all the enclosed space entry precautions observed, personnel can
enter both water side and fire side for cleaning. During sailing, these soots on the fire side are
cleaned by soot blowing, but if it is not efficient, the manual wire brushing and water washing may
be required. Washing will require hot water, preferably fresh water, under pressure and delivered
by suitable lances. The water serves two purposes, dissolving the soluble deposits and then
breaking up and flushing away the loosened insoluble residue. While water washing, the
refractory should be protected with some bituminous coating. Any hard deposits can be
chemically cleaned, but after cleaning, pressure testing may be requirted.
Internal cleaning also may require chemical cleaning, but after cleaning with chemicals, draining
and thorough flushing will be required.
On the fire side, the burner and its attachments should be properly overhauled. The damper, FD
fans are also checked up for proper functioning.
Refractory should not be damaged, it is checked and if found deteriorated in its physical condition,
same has to be renewed. Normally they are made of fire bricks and joined with fire clay cement.
8. a. Sketch an auxiliary boiler and label / mention all its mountings
b. State the various boiler water tests carried out by ship’s staff
Both parts are answered in one of the previous questions. Same can be repeated
9. a. Explain with the help of a simple sketch, the working of a water tube boiler.
b. What is the purpose of each of the following: (i) Air heater (ii) super heater (iii) economizer
STEAM DRUM is the drum where steam separates from water, and the steam outlet is taken. It
is a cylindrical drum with dished ends. Natural circulation of water within the boiler is due to
the differing specific gravities of the water at the differing temperatures, the steam drum
provides a reservoir of water to give the gravitational head necessary for the natural circulation.
The water entering the steam drum via the feed lines provides the motive effect for the
circulation distributing it to the downcomers.
WATER DRUM distributes the feed water from the down comers to the headers and generating
tubes. Provides a space for accumulating precipitates and allows them to be blown down.
HEADERS these have similar purpose to the water drum but are smaller in size.
GENERATING TUBES are getting heated by convection heat and they contribute in generating
hot water, which will collect in the steam drum and then the steam and water will separate.
SCREEN TUBES these are larger bore tubes receiving the radiant heat of the flame and the
convective heat of the hot gasses. The large diameter keeps the steam/water ratio down hence
preventing overheating. Their main duty is to protect the superheater form the direct radian
heat.
WATERWALL TUBES contains the heat of the furnace so reducing the refractory and insulation
requirements.
DOWNCOMERS these are larger diameter unheated, i.e. external to the furnace, their purpose
is to feed water from the steam drum to the water drum and bottom headers.
RISER/RETURN TUBES these return steam from the top water wall headers to the steam drum.
SUPER HEATER TUBES these are smaller diameter tubes in the gas flow after the screen tubes.
Due to the low specific heat capacity of the saturated steam they require protection from
overheating in low steam flow conditions.
Air Heaters are fitted for (i) to increase the efficiency by recovery of heat from the flue gases.
(ii) accelerate rate of combustion (iii) avoid effect of cold air impinging on boiler surface (iv)
as a by product air heaters also form a convenient way to warm up a standby boiler before
initial firing.
Super heaters : Increases the steam temperature and also increases the heat energy stored. It
delays the onset of condensation through the turbine. For turbine operations, it is preferred to
use the superheated steam. Superheated steam, having a lower specific heat capacity than
water, does not conduct heat away as efficiently as in water cooled tubes, and hence the tube
metal surface temperature is higher.
Economiser : The purpose of the economizer is to increase plant efficiency by removing heat
that would otherwise be lost in the flue gas and use it to indirectly feed heat the water. By
heating the feed water it is also helping to prevent thermal shock as the water enters the steam
drum.
Procedure for Scum and Bottom Blow Down
Below is the procedure for boiler blow down using the blow down valve located at the bottom of the boiler. In order to
do scumming, instead of bottom blow down, the scum valve is to be opened.
Steps for blow down procedure are as follows:
Kindly refer the diagram to understand the blow down procedure properly.
1. Open the overboard or ship side valve(1) first.
2. Open the blow down valve (2), this valve is a non-return valve.
3. The blow down valve adjacent to the boiler (2) should be opened fully so as to prevent cutting of the valve seat.
4. The rate of blow down is controlled by the valve (3).
5. After blow down close the valve in reverse order.
6. A hot drain pipe even when all valves are closed indicates a leaking blow down valve.