Contents
BOILER INSPECTION
Boiler surveys or inspections are carried out at regular
intervals due to following reasons:
Boilers are inspected to maintain the class requirement.
Regular internal and external exarninations during such
survey constitute the preventive maintenance schedule .the boiler goes through
to have a safe working condition.
In the event of a leakage or a repair
Contamination of boiler water with salt or oil
Frequeuey of Surveys:
• Water tube high-pressure boilers are surveyed at 2 ½ year
intervals.
Scope of Surveys:
A complete boiler
survey allows us to check out if any
- build-up deposits has
taken place
- deformations
- wastage of platework,
piping
- any of the various parts
which may compromise the safe working order of the boiler
The survey should include Ending reasons for any abnormality
found and should also ensure that any repair carried out does not affect the
safe working order of the boiler. _
A complete survey means full internal and external
examination of all parts of the boiler and accessories such as super-heater, air-heater and all
mountings.
The examination may lead the inspector to require hydraulic
testing of pressure parts or thickness gauging of tube plate or tubes that appear to be wasted and
eventually assign a lower working pressure.
The seating tools,hangers and rolling stays are also to be
checked for good working condition,
The survey is not
complete until the boiler has been examined under steam and the following items
dealt with:
(a) Pressure gauge cheeked against a test gauge.
(b) Testing of water level indicators and protective
devices.
(c) Safety valves adjusted under steam to blow off at the
required pressures.
(d) The oil burning
system examined
(e) Testing of remote control gear for fuel shut off valves.
_
(g) For gas fired boilers, the Chief Engineer floats the
safety valves at sea at the first opportunity. Survey record is not assigned until a statement is received from
the GJE conforming the pressure at which the safety valve were set.
Survey Consists of:
a)
Examination of the item.
b)
Statement whether a problem or defect exists.
c)
Determining the cause of problem.
d)
Define the repair and whether temporary permanent.
Arrangement before Survey:
Boiler isolated from other boilers and steam systems
Boiler shut down ,drained and opened
Boiler must bcsufficiently
cleaned and dried to make a thorough examination possible.
Boilers should be
manually wire-brushed to clean the internal surfaces.
In case of difficulty in manual cleaning chemical cleaning
with hydrochloric acid plus an inhibitor to preventacid attacking the metal
without affecting removal of deposits is the best procedure
For oil contamination, alkali boil-out using tri-sodium
phosphate solution (which produces a detergent action) is essential prior to
acid cleaning. Thorough water washing must be carried out alia: acid cleaning
to avoid acid concentrating in crevices and captive spaces.
• All internals,
which may interfere with the inspection, has to be removed.
• Wherever adequate visual examination is not possible,
surveyor may have to resort to drilling, ultrasonic orhydraulic testing, _
• All manhole doors and other doors must be opened for a reasonable time previous
to survey for ventilation.
If another boiler is
under steam arrangement oflocking bar and other security devices must be in
position
preventing the admission of steam or hot water to the boiler under survey. The smoke trunking
(separatingdevice), exhaust gas shut-off etc. must be in position and in proper
working condition.
• repairers staff should stand by the manhole in case of
emergency and to take note for
Defects ofrepairs required.
Before survey, the
surveyor should acquaint himselfwith the boiler type in question (drawings carried
on board) and during the survey it is advisable to follow a planed routine in
order not to miss parts ofthe boiler or importantitems.
LOW
PRESSURE TANK TYPE BOILER INSPECTION:
SURVEY OFA
SUNROD CP BOILER
Poor condition in this type of boiler may stem from
• Poor workmanship during construction or repair.
• Deterioration due to leaks or deposits.
• Local overheating,
• Combined effects of mechanical stress and corrosion.
Damage condition would show up in the form of (a) wastage,
grooving or pitting, corrosion or, (b) distortion or, (c) crack.
Survey Routes: -
Any boiler to survey, the inspector must plan out a route
for his movement.
• Almost always, initial inspection must start at the
furnace. The reason being, the furnace reveals the quality of combustion and
its effects and any distortion at the crown or the tube walls signifying the
problems originating at the waterside. In fact, a good clean furnace with no
signs of distortion could assure the inspector that the boiler is in good
running condition.
• The next in succession should be the burner unit, the
bottom header and the boiler bottom.
• Mounting attachments on the upper shell will follow before
entry is made through the manhole to inspect the waterside of the steam space.
• Top mountings will be checked before entering the gas
space through the inspection door for a look at the Sunrod tubes.
• The inspector will end with a check on the dismantled mounting
pans arranged in order for the purpose.
• If in doubt of gas side corrosion, checks on the gas
uptake could be done as a final step in the survey route.
Furnace: Sludge deposits continue to be the prime cause of non-operation
of internal controls and overheating of furnace in vertical boilers.
• Overheating distortion on the furnace crown is due mainly
to a deposit of oil, scale or sludge deposits on the heating surface or due to
water shortage. The lower section of the crown causes overheating distortion
due to the sludge deposits.
• The furnace connecting to the shell may be welding cracks
due to rapid fluctuation of thermal and mechanical stresses, results of
improper starting up} shutting down procedure
Direct flame impingement resulting in deformation of the
crown or a local bulge in wall tubes opposite the burner opening is also
possible.
• Dry cracks in furnace mouth, crown or the furnace tubes
caused by flame impingement is possible due to scale encrustation at the water
side and forcing of the boiler. Areas suffering from poor circulation or
relatively uncooled areas are also susceptible to the above failure.
• Deep cracks in furnace mouth, crown or the furnace tubes
caused by flame impingement is possible due to scale encrustation at the water
side and forcing of the boiler. Areas suffering from poor circulation or
relatively uncooled areas are also susceptible to the above failure.
• Dry cracks on plating should be stopped by drilling a hole
at each end, opening up and then welding,
• indifferent feed
water may cause pitting of the furnace crown. A careful examination through the
bottom manhole door would require to detect- the above grooving.
• Brickwork protecting the foundation, if damaged, may cause
distortion of the bottom plating underneath the furnace. Damaged brickwork need
bc removed to inspect the bottom plating for distortion before repairs to the brick
work is carried out.
• Any sign of corrosion on plating should be chipped clean
and brushed them. It is also possible to build up the weakened areas by means
of electric welding. Pitted areas are difficult to protect from further
corrosion due to the difficulty in maintaining the protective magnetic oxide
layer.
Furnace crowns, which have subjected to a gradual deformation,
can be jacked back or pressing back to their original shape with or without heat.
A sharper deformation may require the plating to be slotted so that the metal
can extrude into tlxegqo during heating and joining. The slot tls brat welded
on completion. A much better repair is to burn out the rejected area and
replace with butt-welded insert section by introducing a new section from the
approve material of for severely damaged furnace with a pronounced large belly
replacement could be the only answer.
Furnace tubes must be inspected for correct alignment and the
tubes together must form a circular tube wall, anywhere the tubes are deformed
furnace shape will show up as misaligned Distortion to a very small extent could be
accepted but beyond that renewal of tubes will be mandatory.
Furnace tubes, J damaged (cracked holed or deformed) need be
renewed with new tubes, only under emergen cyconditions, one could be allowed
to operate boiler at low load with plugged jhrnatse tubes, plugging could be carried
our with tapered steel plugs on each tube ends. The bottom plug will have to be
inserted through the bottom header, difficulty in doing that may also compel
cutting windows on tubes front the furanace end and manipulating the tapered plug
in position (similar to that done in membrane main boiler panels )
Bottom Header: _
This contains the furnace tubes and the down comer tubes. No
hand hole doors are provided for internal inspection and repairs to the tubes.
Inspection for deposits of sludge must be carried out during
the survey.
Regular blowing down from this header will be necessary to
keep it clear of sludge deposits. Sun
rod Tubes:
• Internal wastage due to waterside corrosion and pitting is
the main reason for renewal.
• The reason for corroded tubes is almost always the bed
quality feed used giving rise to heavy pitting and corrosion.
• it is difficult to
determine the condition of tube by visual examination. A metal rod inserted at
the tube ends and worked up or down may reveal a weak tube
• Thermal cracks may develop at the tube ends at the hot gas
entry zone.
• The elements could
be corroded on the outside due to the hot gas containing sodium (Na) and
vanadium (V), referred to as high temperature corrosion. Ash containing Na and
V may promote melting of oxide deposits across the tubes and cause scoring of metal
at the tube externals.
Shell :
• Internal examination is made for cracks, corrosion wastage
or deformation of shell plating.
• Any oil trace must be removed by alkali-boil out corrosion
may be expected at positions with poor circulation of gas and places which can
harbor deposits. high Pitting corrosion
in way of water level to be checked for, especially on idle boilers where
liberated dissolved gas was not removed from the boiler with the steam. Boilers
idle with undisturbed water level for SOIDO length, of time cnn develop serious
pitting.
• External corrosion can be caused by persistent leakage at
mounting flanges and manhole or handhole doors.
Engine room floor underneath the boiler may have occasional
bilge water and possess a damp atmosphere,there may also be oil deposit and
stored rags or paint drums; these are all potential hazards,
• Welding may
reinforce the wasted shell plating but in case of extension wastage, renewal of
plating is the only remedy.
Support and Securing Arrangements:
• Attachment between
boiler and foundation structure should have adequate provision for expansion.
Restriction ofmovement imposes loads on the connections and ifthe part is
unable to yield or bend, cracking will occur.
• Welded attachments such as cradles, feet and rolling stay
lugs should always be inspected periodically. Cracks due to stress concentration at the welded
connections may propagate into the shell plating
Mountings and Fittings:
• Major mounting are removed, dismantled and inspected.
• Gauge glasses safety valves, feed check valves, steams
stop valves, are all checked for corrosion, erosion,strength end correct
operations.
• internal feed and chemical injection pipes are inspected
for oxygen pitting and corrosion.
• Waste steam pipes
are hammer tested and all drains in the exhaust system checked.
• Soot blower nozzles
are vulnerable to burning and to be checked for correct sweep pattern.
• The air registers are to be checked and cleaned.
• Clearance at the manhole and mud hole doors to be checked
and should have a spigot clearance not exceeding 1.5mm all around. Leakage from
manhole doors has been cause ofserious shell wastage. Where this is exceeded,
the clearance can be restored by building up the door spigot with welding and
hand dressing to suit.
• Wastage of manhole landing faces is difficult to rectify
by welding - fitting a false scaling ring could be the recommended repair.
• A careful check is made for strained door studs stripped
and slack nuts and distorted dogs. A badly fitted doorcan cause ajoint to blow
out under pressure. · _
• When under steam, the inspector always checks if manhole
doors have been pulled up when hot and the dogsare correctly positioned.
Crack: can occur in valve body: due to water carryover and
quenching or may originate from shrinkagedefects in the casting propagation in service.
The only positive solution is to replace the manifold entirely
with a similar fabricated construction
Repair by welding the
defects in the steel castings is also possible but this presents the problem of
distortion
MEDIUM
PRESSURE DRUM TYPE BOILER INSPECTION
SURVEY OF A D-TYPE BOILER.
Route for inspection
0l. Inside steam drum..
02. Outside steam drum
03. Super heater.
04. Rear water wall headers and casing.
5. Side water wall headers
6. Furnace a) Roof tubes (b) Screen tubes
c) Water wall tubes
(d) Refectories
(e) Burner front.
07. inside water drum
08. Super heater space.
09. Economizer.
10- Uptake.
Detail ofinspection and Defects:
I. Inside Steam Drum
(a) Certain internal fittings to remove for access
b) Boiler internal to check for erosion and corrosion.
(c) Drums and tubes checked for active corrosion. Active of dissolved gases like O2 and CO2
are mainly responsible for "pitting” of the steam space.
Drum openings and doors.
- Spigot clearance not
exceeding 1.5 mm all around.
- Door studs and nuts to be
free of slackness and stretched threads.
- Door dogs to check for
ovality and distortion
- Drum openings of mountings
to be sighted to check for any “planted” objects in the passage.
2. Out Side Steam Drum
(a) All internal (removed from diurn) diecked and tested.
(b) Feed regulator, feed check valve, water gauge fittings,
and drums safety valves examined. Attention to securing arrangements of seats
in valves covers to v/v chests, and v/v chest to drum nozzles.
(c) Welded connection of drum to casing to check for any
possible damage creating gas leakage.
(d) Areas of drum not protected by tubes lion: heat
radiation are shielded refractory. Thermal tracking of the refractory material
to be cheeked
3. Super heater
(a) internal and
external examination of heaters.
(b) 'I'hermal crack at the headers due to high stressess up
across the thick welded section is possible. _
(c) Super heater safety valve and stop valve. _
d) Super heater drains and vents valves and manhole openings
to check.
(e) Efficiency of the ‘screen’ plates to ascertain these
protect headers from direct heat of furnace.
(t) Super heater tubes are also prone to high temperature
creep failures and thermal fatigue cracking. Sudden quenching can cause fatigue
failure.
4. Rear and Side Wall Headers
(a) Sufficient doors or hand hole plugs to remove for assessment
of internal condition of headers and tubes.
(b) Check for pitting and corrosion of headers, rear wall,
floor, roof and side wall tubes.
(c) Check for casing defect for possible gas or air leakage.
For single casing boiler a complete shutdown is possible due to damaged refractory
and overheated casing.
(d) Check for deposit accumulation in header.
(e) Drain valves from headers to examine.
5. Furnace
a. Screen tubes:
Check for direction due to overheating maximum permissible
deviation or sag may be 2 in in 12 feet before renewal.
Leakage at top expansion usually shows by white stains down
the outside of tubes -these may be re-expanded.
Check for passage through super heater banks for
cleanliness.
Examine furnace end super
heater supports visible through screen tubes. .
Gas baffle conditions above and below the super heater attached
to the rear screen tubes to verify which may cause gas short circuit giving
local over heat in super heater and loss of superheat
Screen tubes exposed to excessive heat due to flame impingement,
forcing etc. Possibility of high temperature
creep cracks or circumferential fatigue fracture due to
rapid thermal cycling.
b. Water Wall Tube:
More possibility of overheating due to restriction in
circulation.
Rooftubes having
horizontal portion show effect ofoverheating due to water shortage very
quickly. Rooftubes with roof tired boilers are also prone to distortion due to
overheat.
around soot blower openings have more chances ofoverheating
due to turbulence at the bents restricting circulation.
Refractory:
- Front wall with its quarls receives full radiant heat and
so deteriorates more rapids and may cause, spalling ofbrickwork, misshapenquarls,
badly burnt registers
- Refractory to protect front part of water drum,
particularly below screen tubes may fail and expose drum todirect heat of
furnace resulting in circumferntial thermal cracking.
(6) inside Water Drum: - Normally no defects are
found
- Ifpitting, corrosion or deposits are earlier discovered in
steam drum or headers then lower part oftubc bores tobe checked from inside the
drum.
- Inspections for manhole covers are as usual.
(7) Superheater Walk-in Space
Supports of
horizontal superheatcr tubes to check for burning away and leave the unit
unsupported and causedrainage problem.
- Superheater support tube may also crack due to effect of
bending fatigue stresses due to misalignment of tubesin the tube holes.
- Build-up of deposit is most troublesome defect in
superheater. These may result in high furnace pressure, lossof superheater, and
poor combustion.
- Special attention and suspicion to be reserved for tubes
through which there still exist a gas path as they operateunder excessive metal
temperature.
- Oxide scaling inside or outside may cause tube failure and
worst case hydrogen-fire when iron burns in stem atabove 700 ° C in an exothermic
reaction, and destroys all the boiler, economizer and air heater.
8 & 9.Ecouomizer& uptake
· 'The major problem at the eoconomiser section is the low
temp corrosion and the problem from the gas sidedeposits.
- Sliding and leaky expansion joints at the casing may allow
accumulation of soot with severe acid attack.
- Inspection of tube bends by opening the inspection covers
needs to be carried out to check these.
- Uptake areas may show cracked expansion bellows or sign of
acid corrosion.
— General cleanliness ofthe areas indicates the combustion
performance in boiler.
Hydraulic
Testing of a Boiler . During Survey:
General Precautions:
• Warm water should preferably be used when a boiler is
subjected to a hydraulic test. If cold water used rolled and riveted connections may leak
because of stresses, which are non-representative ofthe servicecondition. .
• The water temperature should be about 50° C. This
temperature may be obtained by careful heating of water inthe boiler.
• Care should be taken to prevent rapid increase ofthe test
pressure. Damage may occur due to sudden pressureincrease ifa powerful pump is
being used. -
Survey Procedure:
• Hydraulic test should be carried out with a test pressure
not less than the working pressure and notexceeding. 1.25 X the working pressure.
Leakage Test:
• The higher of the value 0. 25 X the working pressure and 7
kp/cm.2 normally be sufficient
• The pressure is not to exceed the design pressure.
ECONOMISER:
The eoconomiser is called because transferring heat from hot
waste gasses to the boiler feed water
effects economy. This heat which would otherwise be lost in the gases escaping
up the funnel increases the temperature of the
feed water, hence less heat is required from the fuel per given mass of
heating of steam generated in the boiler; resulting inincreased efficiency. theeoconomiser
consists of a bank of horizontal tubes in staggered formation sometimes
withexternal gills shrunk on to increase the surface area and assist
transference of heat.
Boiler Repairs:
Replacement of leaky manhole joint:
• Maintain proper spigot clearance -— L 5 mm to position the
door centrally for evenly loading the gasket.
• Never use an old gasket.
• Do not overstrain door studs, which may stretch.
• Pull-up studs by re-tightening the nuts after steam rising
or warm up.
• Avoid causing damage to door by holding it by rope and
gently lowering it inside or taking it out.
• Mark the dogs and nuts to lit back in the same order.
• Checkforanywearandtearonthestudsandnuts.
. Check the matting landing surfaces for corrosion &
erosion on the door and boiler end carefully beforereassembling.
Sketch man hole cover
Procedure of plugging of a Damaged Fire Tube Boiler;
• Hydrostatic testing to mark the leaky tubes.
• Cut the tube on one end and clear of the tube plate. At
the other end the tube is collapsed inside the tube plate
• Pull out the tube from the collapsed end.
• Insert a short tube into the tube plate and weld it in place.
w Lap the spare tapered plugs on both stub ends in the tube
plates.
• Insert the tube plugs and tack-weld.
• Alternatively, the plugscan behold in place bya long steel bar threaded
and bolted atboth the ends.
• Hydrostatic pressure test to conform no leaks.
• Flush up the boiler and re-inspect the plugss for leaks
underfull steam pressure
Sketch boiler tube expanding
Instructicnfor Plugging / Repair ofBoiler/ Economizer.
( Aalborg Marine Boilers & Engineering A/S)
• In ease of tube failure, thesteam pressurehas to be removed from the oil burner
dismantled.
• lf the leakage isreadily visible from the fire hole, the
boiler can be emptied and repairs commenced.
• Otherwise the boiler is put on pressure by means of the
feed pump. The position of the leakage will then beindicated by its water flow.
• This flow may not be visible from the fire hole. lf it is
not visible, remove the inspection door and enter theFurnace. Ifthe tube failure
is still not found, then enter the generating tube section. From here the
bottom of themembrane walls and generating tubes can be inspected for leakage.
•ifthe leakage has resulted from the membrane walls or
generating tube, the inspection doors at the smokeconnection pipe must be
removed, and the generating tube /membrane tube in which the failure has
occurred,is pointed out.
• The leakage may also result from the economizer.
• By removing the inspection door at the bottom of the economiser,
it can be determined which uptake hascaused the leakage.
•if necessary other inspection doors should be removed to
point out the damaged register.
• when a damaged tube or convection register has been
removed, and the remaining tube stubs have beenrepaired plugged, a new tube or
register should be mounted as soon as possible
• Operation for a longer period with one or more registers
missing involves the risk of further damage to theboiler due to increased heat
leads on the parts next to the ones removed.
•The generating tube at the e boiler can be plugged as shown
below:
TemporarySunrod Boiler Tube Repairs: -
If the sunrod tube is leaking, proceed as follows to effect
a temporary repair-
1. Stop the burner, allow the boiler to cool and remove thee
soot.
2. Allow boiler to depressurize, and open the blow-down
valves to drain the boiler.
3. Enter the boiler
flue box and out it hole in the side of the relevant smoke tube.
4. Clean the rim of the smoke tube with a wire brush.
5. Cut a circular plate (15mm thick) of the same diameter as
the smoke tube and chamfer the top edge 30 ° bygrinding. .
6. Fit the plate into the top of the smoke tube and weld it
in position as shown
7. Enter the boiler furnace and cut a similar hole in this
end of the relevant smoke tube as shown
8. Repeat operations 4 to 6 for lower plate.
9. Refill boiler and check for leaks before start-up.
10. Start-up boiler and check for leaks when pressurized.
Note: Temporary repairs to sun rod tubes should receive more
permerentattention as soon as convenientlypossible.
Sunrod Furnace Wall (Risers) Tube Repair: - WINDOW
REPAIR
I . Stop the burner, allow the boiler to cool and drain the
boiler.
2. Swing out the burner and enter the furnace through the burner
aperture.
3. Cut out a window in the relevant section of pipe as
shown. Note that 1200 window
is themaximum allowed.
4. Grind a 30° chamfer on the edges of the window.
5. Using a section of tube of the same diameter and thicknesscut
out a window to match the window cut in thefurnace wall.
6- Grinda30° chamfer on the edges of thenew section.
7. Spot-weld a piece of steel strip to the top of the window
as shown (position A).
8. Fit the new section over the window in the tube wall as
shown and spot-weld the steel strip to the riser inposition B. Note: the piece
of steel strip holds the window in place and allows the new section to fit into
thewindow in exactly the correct position and at the correct depth.
9. Weld around the edges of the window (Note: 60 ° weld
angle).
10. When cool, knock offthe steel strip and clean offthe
spot weld.
ll. Refill the boiler and test for leaks. .
l2. Ifno leaks are found, start the boiler and check that
the weld is not leaking when the boiler is pressurized.
BOILER STRESS:
Stress = ( Pressure x Diameter ) / (2 x Plate thickness ).
Boiler shell is subjected to internal pressure which sets up
the stresses in the circumferential and
longitudinal axes.
• Longitudinalstress = (Pressure X diameter)/(2 x thickness
).
• Circumferential stress
= (Pressure x diameter)/( 4 x
thickness ).
• Longitudinal stress= 2 x Circumferential stress.
Compensation for holes cut in boiler
shell:
• Any material cut from the shell will weaken it by an
external related to the amotmtcut out.
• Any holes cut in the shell, with a diameter greater than
2. 5 x plate thickness + 70 mm must be provided withcompensation for the loss
of strength.
• The largest holes cut in the shell include the manholes,
and where these are cut in the cylindrical portion of theshell they must be
arranged with their ntirtor axis parallel to the longitudinal axis of the
boiler. This is due to thestress acting upon the longitudinal seam being twice
that acting upon the circumferential seam. Thus the shell mustnot be weakettexl
more than necessary along its longitudinal axis.
How the Smoke Tubes are fitted in the Boiler?
The hotgases leaving the combustion chamber pass through
smoke tubes fitted between the tube plate and the fronttube plate, on the to the uptakes. Two types oftubes are used:
Plain and Stay tubes as 3:l
2. Plain Tubes:
- The plain or common tubes are expanded into the tube
plates at both ends.
- The tubes have a diameter of about 65mm with a thickness
of 5mm.
- The front end of the tube is often swelled out to allow
for easier tube removal.
- The back end of the tube is bell-mouthed after expansion,
or may be spot-welded.
b. Stay Tubes:
- The flat tube plates must be supported, so that stay tubes
are fitted by screwed and then expanded into bothtube plates.
- The thickness of the stay tubes must not be less than 5 mm
at the base of the thread.
- After the tube has been screwed and then expanded into the
tube plates, nuts are usually fitted at the frontendbut not in the combustion
chamber to avoid overheating.
- Welding can be used after screwing the stay tubes into the
tube plates but the tubes must be expanded beforeand after welding.
Arrangement of smoke tubes in a boiler
How the Water Tubes are fitted in the Boiler?
· Tubes can be
attached to drums and headers by expanding or by welding.
- In most cases the
generating, screen and water wall tubes are expanded into plain seats, and the
bell-mouthed.
The tube ends must be cleaned and then carefully rolled
expanded into the holes in the tube plate.
- To prevent tubes pulling out of the tube plate, they must
be bell-mouthed.
- This bell-mouthed is to be 1 mm for every 25 mm of outside
diameter + 1. 5 mm. _
— Superheatertubes are also expanded and bell-mouthed up to
steam temperatures of 4500 C; above this valuethe tubes are attached
by welding.
Attachment of expanded tube and welded tube
Expanded 8: Bell-Mouthed Grooved Seat in Welded tube
Expanded tube attachment Welded tube attachment
Boiler mountings
l Safety valves
l Water level indicators
l Water level controller
l Water level alarms & cut-out
assembly
l Remote water level transmitter
l Main steam outlet valve
l Pressure gauge
l Pressure switches
l Feed water valves
l Burner assembly
l Air vent
l Water sampling valve
l Manholes, mudholes& peepholes
l Bottom blowdown valve
l De-foaming (scum) valve
l Furnace drain valve
l Soot blowers
BOILER SAFETY VALVE:
• Safety valves are fitted to protect the boiler from the
effect of over pressure.
• At least two safety valves are fitted to each boiler steam
drum, but ifthere is any superheater another safetyvalve should be fitted on
it. .
• Pressure setting ofsuperheater safety valve should be less
than the design pressure of the boiler i.e. less thanthe steam drum safety
valve to ensure flow of steam through the superheater under blow off
conditions.
• Pressure setting ofone steam drum safety valve should be
same as the design pressureof the boiler.
• Pressure setting of another safety valve should be_2%to 3%
more than the design pressure of the boiler.
• Classification: There are 3 types ofsafcty valves are
use-d-in boiler.
- Improved high lift safety
valve.
- Full lift safety valve,
- Full bore safety valve.
Improved High Lift Boiler Safety Valve:
• Wingless valveimproves steam flow and reduces risk of
seizure.
• Waste steam pressure acting on piston gives increased
valve lift
• Special shaped seat deflects steam towards lip on valve
and increases valve lift.
• The valve lid is able to lift at least l/4 of the valve
bore in order to provide full steam {low.
• As the valve lifts, the force to compress the spring
increases, so the higher the valve lifts the greater theincrease in boiler
pressure. The DOT limits this accumulation of pressure to 10 % of the max
allowableworking pressure
• Waste steam pressure keeps cylinder in place while piston
moves, also by having at floating cylinder seizurerisk is reduced.
• A lip is placed around the valve seat so that, when the
valve lid lifts, escaping steam is trapped in the annularspace around the valve
face; the resultant build—up of pressure acting upon the greater valve lid area
causes the
valve to lift sharply. This arrangement gives another
advantage to close the valve cleanly and sharply with verylittle blow down
effect.
• The improved high lift safety valve makes use of waste
pressure to increase the valve lift this is done byallowing the pressure to act
upon the lower spring carrier, which fits within a floating ring, so forming in
effect a piston. The pressure acts upon this piston causing it to move up, helping to compress the spring,
and soincrease the valve lift.
• Loose fitting key or pad lock is provided to ensure proper
closing of valve.
• Loose pin is provided to secure valve lid and allow
thermal expansion.
• Adjustment of the valve is carried out by means of a compression
nut screwing down on to the top spring plate.
• A compression ring is fitted alter final adjustment to
ensure no further movement takes place.
• A cap is then placed over this compression nut and the top
of the valve spindle, and a cotter is passed throughand padlocked to prevent
tampering by unauthorized persons.
• Clearances between this cap, the valve spindle and the cotter
are such as to prevent the valve being held down -externally
• Easing gear is fitted so that in the event ofan emergency
the valve can be opened by hand to a full lift of 1/4 Dto release the boiler
pressure.
• Valvearea: As = A(l +Ts/555)
As = Aggregate area through the seating of valve( mm2) for
superheated steam.
A= Aggregated area through the seating of valve (mm 2)for
saturated Steam.
Ts= degree of super heat in °C
As is greater than A due to sp. volume ofsteam increases
with increase oftemperature at constant
pressure and more escape area is required to avoid accumulation o f pressure. ·
• the area of the valve chest must be at least ½ A
• Thewastesteampipeandsteam passagemustbeatleastl.l:xA.
Boiler Safety Valve Drain
• Drainpipe must be fitted to the lowest part of the valve chest on the discharge side of the
valve.
• The pipe should be led clear of the boiler.
• The pipe must have
no valve or cock fitted through its length. ·
• The open drain of the pipe should be regularly checked.
• Ifthepipe become choked, there is possibility of
overloading the valve due to hydraulic head, or damage duetowater hammer.
• The waste steam pipe of the boiler safety valve should be
well secured so that no load of the pipe is on thesafety valve, which can be a
cause of additional stress on the valve.
Pressure Setting of Safety Valves:
• Safety valves pressure setting can be done from high to
low pressure or vice versa.
• Take necessary personal safety precautions and arrange
tools i.e. gagging tool and 2 master gauges.
• Slowly raise the boiler pressure and blow off the safety
valves manually few times for thermal expansion and toreduce thermal stress on
the valves.'
• Then screw down all the safety valves higher than the
setting pressure at which you are going to set.
• Raise the boiler steam pressure 2 - 3 % more than the anticipated
set pressure of the boiler, then stop firing andunserew the first valve slowly,
when it blows off at 2--3% more than the design pressure then note this openingand
closing pressure. Keep this setting of the first valve, raise the boiler
pressure and recheck the opening &
closing pressure of the valve and finally gag it.
• Raise the boiler pressure at the design pressure of the
boiler and unsercw the 2nd valve, when it blows off atdesign pressure then note
this opening pressure and check the closing pressure also. Recheck the setting
pressure and gag the valve.
• 'Ihen set the supcrheater safety valve lower than the
design pressure of the boiler in same procedure.
•finally take out the
gagging tools. Pressure setting should be done in presence of the surveyor.
When testing boilers and pressure vessels at pressures
exceeding the set pressure of the safety valve it is necessary to mount a gag
to prevent a valve from opening Gags are usually clamp type as indicated in the sketch they are to be fitted to the valve after
removal of the easing gear and the cap and lifting lever. Gags must only be
fitted hand tight to prevent damage to
spindle ans seat surface.
Sketch improved high lift safety
valve
Safety Valve Llp Clearance: E
·
Controls the huddling chamber (below the valve
disc) to improve the performance.
·
Controls the pop action of the valve.
·
Lower clearance makes the valve simmer before closing, which
increases the valve blow down closingpressure will be lower.
Safety Valve Blow Down Ring:
Controls the huddling chamber pressure of the valve.
When lowered, valve
tends to simmer before opening andredueea valve blow down.
When raised,
increases blow down, valve might simmer before closing, and gives the valve
good pop action.
Sketch Blow down ring
Fig Boiler gauge
glass water level
Uptake FIRES:
An analysis ofsoot fire indicates that they are more likely
to occur during manoeuvring, following staying in port.
There are 3 dominant parameters that will influence the
tendency to go on fire. They are:
1. Gas velocity:
velocities dropping below 10m/ s,
due to poor design, fouling, reduced engine load etc. will increase the
tendency to deposit soot.in a ship designed to operate with high gas velocities
may encount soot fires when they are operating at low loads or when maneuvering
for prolonged period
2. Soot stickiness:
Sootstickiness is insufficient by the unburned residues of fuel and lub oils.
More prominent isduring manoeuvring. These unburned liquids can lower the
ignition temperature from 300 - l50° C. I case of a economizer this can be due
to the engine operating at low load
3. Low circulation rate in boiler: This reduces heart
absorption from the gas, hence gas temperature remains highwith increased risk of
soot ignition.
4. poor soot blowing arrangement
Early Indication of Uptake fire:
• Sparks emission from the funnel.
• Increase in the flue gas temperature or fire alarm.
• High steam generating rate.
Action in case of Uptake fire:
• Slow down or stop main engine.
• Stop the auxiliary blower, blank off T/C inlet ducting.
This reduces O2 supply.
• Restore or maintain forced circulation of water. This
cools inside of the tubes. ·
• Rig fire hoses."Do not use soot blowers”.this cools
out sideof the tubes.
• Remove casing, tackle fire withjet cooling not spray. Check
water drainage point for ovcrflow.
• if not possible to restore forced circulation, drain and
vent the boiler
Uptake fires are avoided by:
• Maintain the correct exhaust pressure difference across the
boiler to maintain gas velocity.
• Soot blow before or after manoeuvring to keep clear tube
bank.
• By-pass exhaust gas
during manoeuvring.
• Clean tube bank, after water washing the T/C.
• Maintain water circulation after shutting down for 12 --24
hours.
• Ensure proper engine operating condition.
Furnace Blow Back (back Fire)
Furnace blowback is prevented by going through a safe start
up procedure, which involves:
• A furnace pre-purge before attempting ignition. This is
usually 25% (100:%) max. air flow through the furnacefor 3 minutes.
• Allowing an oil spray at the correct pressure /temperature
from the atomizer to pass across the ignition forsufficient time to achieve ignition. this time is less than the time
required to cause an explosive air /fuel mixture to form in the furnace, ·
CAUSE OF FLAME FAILURE:
• Fuel oil filter choked or FO pumpfailure.
• Empty of FO tank or low temperature of fuel.
• Water contamination in fuel.
• Defective burner.
• Nozzle choked.
• Air failure.
• Photo cells failure or dirty.
• Electrode clearance incorrect.
• Hair crack in the electrode.
• Electrodes dirty.
Why Steam Pipe is insulated
To protect heat radiation.
Safety of personnel.
To keep cool engine room environment.
To protect weak point of pipe lines from damage.
To protect from fire in case of FO spray due to any leakage
Effect of Air Inside Boiler:
Oxygen ofair or gas at 550° C will react with the tube
material and cause oxidation, which is
the cause of crack and tube failure.
Action in ease of Boiler Low Low Water Level and Tube Red Hot:
• Stop firing the boiler.
• Close the main steam stop valve and take necessary action
for purifier and main engine fuel oil heating.
• Stop feed water pump and do not fill the boiler with
water.
• Allow boiler to cool down to normal.
· Start the boiler feed water pump and fill up the boiler at
low level.
‘ • Star firing the boiler at slow rate.
Electrically Operated Water Level Alarm & Cut out:
• A float within the chamber relates to water level and
moves an attached magnet up and down in response to anychanges of water level
within the steam drum.
• Three micro-switches with magnetically operated a.rms are
fitted on the outside of the balance chamber. Theyare arranged so that, when
the internal magnet comes level with one of these external magnets, like poles
arepresented, and so, repelling each other, operate the micro-switch.
• The positions of the switches are set to correspond to
predetermined water levels within the steam drum.
• One switch will correspond to a low water level condition,
and when this is reached will operate an alarm.
• If then no corrective action is taken, or it prove
ineffective and water level continues to fall, the next switchpositioned to a
predetermined low low water level will trip out the fuel oil shut-offvalve, so
shutting the boilerbefore damage can occur,
• The other switch is positioned in a similar manner for
high water levels, operating an alarm for a predeterminedhigh water level in
the drum.
Water level alarm and cutout
—> Water level in the boiler is critical. If it is too
low, damage may result from overheating; too high and primingcan occur with
resultant carryover of water and dissolved solids into superheater, steam lines
etc.
Spalling -} Due
to flame impingement, the firebrick layer tends to break. _
Slagging} Due to
"Na“ deposits which will reduce the melting point of refractory material.
Bulging -> Due to overheating, deformation oftube banks.
TYPES OF
TUBES IN WATER TUBE BOILER:
Generating Tubes:
• These consists of number of small diameter tubes placed in
the main flow ofhot gases, so forming a large heatexchange surface; the gas
generation of steam takes place mainly by convention.
Screen Tubes:
• These are placed adjacent to the furnace, so receiving
heat from the flame together with the heat from the hotgases leaving the furnace;
therefore they need a relatively large diameter to keep the ratio of steam to
water lowenough to prevent overheating.
• The duty of the screen tubes is to protect the superheater
tubes from the direct radiant heat of the furnace flame.
Water Wall Tubes:
• These are used basically to contain the heat of furnace,
thus reducing the amount of refractory materialrequired.
• in some types of boilers, water-cooled refractory walls
are used. These consist of tubes with studs welded intothem, covered with refractory
material, which can withstand the high temperatures without damage.
Down Comer Tubes:
• These consist of large diameter, unheated tubes placed
outside the gas stream which act as feeders to the waterdrum and headers.
Riser/ Return tubes:
• These are used toreturn steam and water from the top water
wallheaderstothe steam drum.
Superheater Tubes:
These consist of
small diameter tubes placed in the main gas stream, after the screen tubes.
• Their duty is to superheat the saturated steam leaving the
drum to a temperature suitable for use in the mainturbines.
• They must be protected from direct radiant heat, as they
are liable to overheating due to the much smallerspecific heat of steam
compared to that of water.
Superheater Support tubes: .
• These relatively large diameter tubes act basically as
water-cooled supports for the superheater tubes.
VERTICAL SMOKE TUBE BOILER: .
The Cochran is a typical tank boiler of vertical type
suitable for producing relatively small amounts of low-pressuresteam for
auxiliary purposes.
• The cylindrical boiler shell with its hemispherical crown,
together with the hemispherical furnace forming
the bottom of the pressure space, requires no stays.
• Gusset Stays:
The top of combustion {heating chamber requires support, and this is provided
by means of gusset stays which transfers the stresses from the flat top of the
chamber on to the boiler shell.The tube plates are also supported by means of
gusset stays.
• Refractory material is fitted in the combustion chamber,
on the floor and sides of the furnace. The refractory should be high enough to
prevent direct radiant heat coming onto the ogee ring and lower parts of the
hemispherical furnace crown. .
• Internal access to the boiler is provided by a manhole in
the top of the shell, while hand holes in the lower section of the shell
provide access to the lower parts of the water space for cleaning and
inspection.
• Hinged smoke box doors give access to the tubes and tube
plate at the front, while a removable rear panel fitted to the combustion
chamber gives access to the back tube plate
Sketch water tube boiler
Sketch Cochran boiler - vertical
steam composite boiler
Ogee Ring:—-
• Connecting to the bottom of the furnace to the boiler
shell plating is a seamless ogee ring.
• This is pressed out of thicker plating than the furnace,
• Greater thickness is necessary due to poor circulation of
water in this area and deposits can accumulate between it and boiler shell
plating.
• Tell tale holes drilled at equal circumferential intervals
in the boiler shell enable leakage between the ogee ring and boiler shell to be
detected.
Sketch Og ring
Boiler Operation:
Starting up a boiler should not be speeded up and it
normally takes about 4 hours to complete. Strict adherence tothe manufacturers
recommended procedures will make the boiler operation more efficient and safer.
· Check that the boiler is properly closed up ( especially
all the repairs).
* Check (physically) that all appropriate valves are shut
oropen for safe starting of the boiler.
• Boiler filled to slightly below normal level.
• Boiler treatment chemicals may now be added to the boiler
water.
· Check and clear the furnace of any flammable materials.
· Ensure that the boiler uptake passage is clear.
· Pre-purging of furnace for a specified amount of time is
necessary to clear the gas-side of flammable gases, toavoid a starting
explosion.
• On a cold boiler, the firingup must not be speeded up too
much in order not to overstrain the boiler materialunnecessarily by quick,
uneven temperature raises.
¤ Keep the boiler vents open until a heavy steam jet is
flowing out (until e boiler pressure of about l bar isreached).
· Before the boiler is put on load, blow-through the gauge
glasses, test the safety valves using easing gear and tryout the safety cutouts.
Starting ofa Water Tube Boiler fitted withan integral Superheater ~
Integral superheaters fitted to boilers are prone to
overheating as they are normally situated in a high temperaturepath
Superheaters depend on adequate steam-fIow through them to keep their tube
metal temperatures withinlimits. Overheating of superheater tube element
results in sagging and failure of tubes and may even trigger ahydrogen fire in
the boiler. To protect the superheater from overheating during boiler start -ups,
the superheater isdrained of any accumulated condensate and a vent provided on
its outlet header is left partially open until the boileris put on load.
Points to note while the boiler is on load:
· Operate the boiler at a load where its efficiency is the
highest.
·· Maintain correct air fuel ratio; under perfect conditions
a brownish hazy colour of exhaust smoke is noticedfrom uptake.
· CO, CO2; O2, contents as monitored
in the exhaust gas will indicate the combustion condition inside the boiler.
·· Every morning mud is blown from the boiler through the
bottom bl0w—ofi valves and float chambers. (Bottomblow is not recommended for
drum type of water tube boilers while the boiler is onload).‘
• Ensure that all safety cut-outs are operational. ·
· Maintain the feed water quality as observed from the
funnel should be brownish hazy.
Boiler care while running onload Monitoring of perfect combustion ,
· Flame should fill thefurnace without any impingement.
· Colour of flame should be moderately bright orange. -
· Colour of smoke as observed from the funnel should be
brownish hazy.
- Flue gas analysis should give the following readings: .
l. Oxygen:
·
3 ~ 4% by volume (for tire tube boilers)
·
0.5 -1.5% by volume (for water tube boilers with high
efficiency burners)
2. Carbon Dioxide:
·
12 to 14% by volume. Higher the reading, the better is
the combustion efficiency. lt must beremembered that the C02 reading changes
with the fuel type/quality and the level of excess air supplied to the burners.
3. Carbon Monoxide:
• A reading range of 100 - 200 ppm indicates good combustion
efficiency. This reading is very reliableunlike in the case of CO2;
reading of flue gases.
• Black colour indicates improper combustion (due to
insufficient air, poor atomisationetc). Whereas whitecolour of smoke indicates
too much air (or profuse leakages of
steam or water from thedamaged boilerparts).
BOILER LAY UP PROCEDURE
Boilers are more susceptible to corrosion during lay up or
idle periods than during operating conditions, therefore,adequate precautions
must be undertaken during lay up. Several methods may be adopted to combat the
harmful effects of low pH and oxygen attack on metal surfaces and to protect
the fireside against acidic attack from deposits containing moisture,
particularly when sulfur type compounds are present. At least one day before a
boiler is removed from service prior to lay up, an organic sludge conditioner
(COAGULANT) should be dosed to the boiler. The rate of blowdown should be
increased in order to keep the amount of sludge to a minimum and as
non-adherent as possible.
Never drain a boiler while under pressure. Heat from the
refractory etc., will cause remaining sludge to bake on internal surfaces.
Prior adopting one of the following methods, the boiler should be drained and
thoroughly washed down internally. Use a high-pressure hose and preferably hot
water.
a) Wet storage: Used for short layup of less than a month
and the boiler is maintained in a stand —by condition.Not suitable for boilers
exposed to freezing conditions. The boiler is completely filled with hot
distilled de-aerated alkaline water. The water should overflow through the vent
during filling-up. Daily checks are
necessary to ensure fullness and alkalinity are maintained.
b) Dry storage: Used for longer lay-up of more than a month,
The boiler is completely dried out using heaters oron light fire or passing hot
air through the boiler parts. When dry completely all the boiler outlets are
sealed tight after Placingdehydrant(such
as silica gel at the rate of 2.7 kg /Cu meter) inside the boiler blank the blow down line. If boiler connected
to other boiler blank all he connections if the
PROTECTING THE WATERSIDES USING ONE OF THE FOLLOWING WET
METHODS (more detailed method)
It is important to maintain high concentrations of corrosion
inhibitors in the boiler water; the boiler should be kept completely filled to
exclude any air.
Wet Method # 1: concentrations between 2300-3500 PPM of
sodium nitrite (corresponding to 1500-2300 PPM nitrite), plus ALKALINITY
CONTROL if necessary to obtain a pH level above 9,5.
Wet Method # 2: Use HYDRAZINE maintaining concentrations of 250 PPM. This concentration is sufficient to raise the
boiler water pH to a safe and satisfactory level of 9.5, without further
additional chemicals. (It is also advisable to connect an inert gas cylinder on
the upper part of the economizer and maintain a 2 Kg/cm2 pressure to avoid
possible HYDRAZINE leaking. Drain boiler of excess HYDRAZINE and refill before
returning to service. Test concentration of various chemicals (nitrite, pH
etc.) at different water levels (bottom-half, surface etc.) if necessary
circulate boiler water with ships pump or with an additional external pump. In
each case after chemicals have been added and the boiler filled to normal
working level (preferably with hot, deaerated, feed water) the boiler should be
fired and run only long enough to provide sufficient circulation to obtain
uniform concentrations of chemical treatment throughout the boiler, and to
eliminate its oxygen. Steam should be vented during this operation.
Where boilers are equipped with superheaters, the
instructions supplied by the boiler manufacturer for laying up superheaters
should be followed.
HANDLING OF FIRESIDE:
The Fireside of all laid-up boilers should be thoroughly
cleaned to remove all soot and carbon deposits. It is good practice to keep a
small heater in the fireside during the lay up period to prevent corrrosion due
to moisture or humidity
which joins with sulfur laden soot and starts a sulfuric
acid attack on boiler metal.
Treatment for Gross Oil Contamination of Boiler
Feed Water
• Boiler taken off load Shutdown.
• Subsequently to be overflown through top manhole opening,
· Boiling out or chemical cleaning.
• Alkaline soak /degreasing using Sodium hydroxide or
phosphate, NaOH, Na3 PO4.,
• Wash out
• Acid clean to remove mill scale (F e3O4
:Magnetite layer} 30% Concentrations of HCl with inhibitor toneuualise Acid.
· Wash out and neutralise,
- Passivate with Hydrazine or Alkaline chemicals.
• Don‘t drain boiler through bottom blow down valve (to
ovoid contamination).
Treatment for Small Oil Contamination .
l. (non-availability ofspecial chemical onboard):
•
Reduce the boiler load
•
Surface blow .
•
Maintain Alkalinity by adding Alkaline chemicals
•
Add Anti—foam compound (if available) _
2.If oil combating chemicals like "Epsom Salt" is
available onboard:
•
Add 350 gms of Epsom salt with the boiler offload and
100 gms ofEpsom salt if the boiler is on reduced load.
•
Maintain Alkalinity by dosing chemicals an hour after
the dosing ofepsom salt.
•
Add Anti-foam.
•
Give flash blow 5 to 20 sec; periodically, to remove
the precipitated sludge
Prevention of Boiler Accidents
• Never by-pass safety alarms and cut-outs.
* A weekly routine testing ofalarms and cutouts is
essential.
· Strict quality control of feed wares quality and chemical
treatment
• Regular blowdown of a boiler is must irrespective of
boiler water test results.
· A safety check list should be maintained for all normal
and emergency operational procedures for the boilerplant.
· When the boiler is operated with any faulty safety cut-outs
by-passed, manual watch should be kept on theboiler plant..
* Safety valves should checked at least once every three
mouths to ensure their; operational readiness in the event ofover-pressure in
the boiler.
Safety Valve Gagging:
· When testing boilers and pressure vessels, at pressure
exceeding the set pressure of the safety valve, it isnecessary to mount a gag
to prevent the valve from opening.
• Gag: are usually of the clamp type and are fitted to the
valve after removal of the cap and lifting lever.
• Gags must only be fitted hand right to prevent damage to
the spindle and seat surface.
Safety Valve Exhaust Pipe:
• It is recommended to provide the individual safely valve
with a separate exhaust pipe.
• This must have n sufficient inside diameter determined by
the full exhaust quantity of the valve and must be atleast one number larger
than the outlet flange.
The pressure loss in the exhaust pipe of the safety valve
including of the exhaust loss most amount to 10% of the valve set pressure at the most.
· The vertical pipe must run as straight as possible and be thoroughly
anchored to and supported by the structureof the vessel to be able to withstand
the reaction during blowing out.
• It is necessary to mount an expansion between the outlet
bending and the exhaust pipe or in other ways to source that forces from the
latter are not transferred to the safety valve house with subsequent tensions
anddislocation of the house, which may result in leakages or, at the worst,
destruction of the valve.
' The exhaust pipe is normally expanded about 2. 5mm per
meter pipe from the valve to the fixing point The long exhaust pipe or many bending may
necessitate that a pipe with a bigger inside diameter is chosen inorder to
secure that the backpressure will not be too high
The dimension "A" in figure must be as short as
possible. A greater horizontal length increases the riskoftensions in the valve
house during blowing.
Deposits and scales found in boiler water
Definition: material
originating elsewhere and conveyed to deposition site; Oxides formed at the
site are not deposits.
Water formed and steam formed deposits
May occur
anywhere
• Wall and
screen tubes most heavily fouled, superheater has deposits formed elsewhere and
carried with the steam or carryover. Economisers( non-steaming) contain
deposits moved from there original site.
• Tube
orientation can influence location and amount of deposition.
• Deposits
usually heaviest on the hot side of the steam generating tubes. Because of
steam channeling, deposition is often heavier on the top portion of horizontal
or slanting tubes
• Deposition
occurs immediately downstream of horizontal backing rings.
• Water and
steam drums can contain deposits, as these are readily accessed then inspection
of the deposition can indicate types of corrosion. e.g. Sparkling black
magnetite can precipitate in steam drums when iron is released by decomposition
of organic complex agents.
• Superheater deposits ( normally associated with high
water levels and foaming ) tend to concentrate near the inlet header or in
nearby pendant U-tubes
• Contaminated
attemperating spray water leads to deposits immediately down stream with the
possibility of chip scale carried to the turbines.
• At high
heat transfer rates a stable thin film boiling can occur, the surface is not
washed ( as it is during bubble formation ) and deposits may form
• Thermal
stressing can lead to oxide spalling( the exfoliation of oxide layers in areas
such as the suphtr). These chips can pass on to the turbine with severe
results. Steam soluble forms can be deposited on the turbine blades , If
chlorides and sulphates are present , Hydration can cause severe corrosion due
to hydrolysis.
• As deposits
form on the inside of waterwall the temperature increases. This leads to steam
blanketing which in turn leads to reduced heat transfer rate , long term
overheating and tube failure.
Effects on tube temperature of scale deposit
DEPOSITS
Iron oxides
Magnetite (Fe3O4)
A smooth black tenacious , dense magnetite layer normally grows on boiler water side surfaces. taken to indicate good corrosion protection as it forms in low oxygen levels and is susceptible to acidic attack
A smooth black tenacious , dense magnetite layer normally grows on boiler water side surfaces. taken to indicate good corrosion protection as it forms in low oxygen levels and is susceptible to acidic attack
Heamatite (Fe2O3)
is favoured at low temperatures and high oxygen levels can be red and is a binding agent and tends to hold over materials in deposition. This is an indication of active corrosion occuring within the boiler/feed system
is favoured at low temperatures and high oxygen levels can be red and is a binding agent and tends to hold over materials in deposition. This is an indication of active corrosion occuring within the boiler/feed system
Other metals
Copper and Copper oxide
is deposited by direct exchange with iron or by reduction of copper oxide by
hydrogen evolved during corrosion . Reddish stains of copper are common at or
near areas of caustic corrosion. Copper Oxide appears as a black depositi. It
is considered very serious corrosion risk because of the initiation of galvanic
corrosion mechanisms.
Galvanic
corrosion associated with copper deposition is very rare in a well passivated
boiler. Zinc and nickel are very often found near copper
deposition , nickel being a particularly tenacious binder
Rapid
loss of boiler metals can occur. Copper can appear in various forms as a
deposit in the boiler. As a copper coloured metallic deposit, usually in a
corrosion pit, as a bright red/orange tubercules on the boiler metal surface or
as a brown tear drop shaped formation.
Copper
is generally an indicator of corrosion (or possible wear) occuring in the feed
pump whether in the condensate lines or in the parts of a feed pump. A possoble
cause of this is the excessive treatement of hydrazine which decompose to
ammonia carrying over with the steam to attack suc areas as the air ejectors on
condensers.
Copper oxide formed
in boiler conditions is black and non- metallic.
SALTS - The least
soluble salts deposit first
Calcium carbonate-effervesces (Effervescence is the escape of gas from
an aqueous solution and the foaming or fizzing that results
from a release of the gas.)when exposed to HCl acid
Calcium sulphate-Slightly less friable (easily
broken up) then CaCO3
Magnesium Phosphate-Tenacious binder, discoloured by
contaminants
Silicates-Insoluble except in hydrofluoric acid E.G.
Analcite (a white or slightly colored zeolite mineral, Na(AlSi2O6)⋅H2O,
generally found in crystalline form)
Water soluble deposits can only be retained if local concentration
mechanism is severe. Prescence of NaOH , NaPO3 Na2SO3
should be considered proof of vapouration to dryness.
Calcium and magnessium salts exhibit inverse solubility. As the
water temperature rises their solubility reduces, at a temperature of 70'C and
above they come out of solution and begin to deposit. Feed water must be
condition to remove the hardness salts before the water enters the boiler. The
purity of the water is related to the steam conditions required of the boiler.
Hydrolyzable
salts such as MgCl can concentrate in porous deposits and hydrolyze to
hydrochloric acid
Scaling mechanism examples
Calcium
Carbonate
Cacium Carbonate is formed by the thermal decomposition of Calcium BiCarbonate and apperas as a pale cream to yellow scale
Cacium Carbonate is formed by the thermal decomposition of Calcium BiCarbonate and apperas as a pale cream to yellow scale
Ca(HCO3)2 + Heat = CaCO3 +
H2O + CO2
Magnesium
Silicate
Tor form requires sufficient amounts of magnesium and silicate ions coupled with a deficiency in OH- alkalinity
Tor form requires sufficient amounts of magnesium and silicate ions coupled with a deficiency in OH- alkalinity
Mg2+ + OH- = MgOH+
H2SiO3 = H+ + HSiO3-
MgOH- + HSiO3- = MgSiO3
+ H2SO4
Thus this rough tan scale can be prevented by the
maintenace of alkalinity levels
Calcium
Phosphate (hydroxyapatite)
Ca10(PO4)6(OH)2
Ca10(PO4)6(OH)2
Found in boilers using the phosphate cycle treatment method
this is a tan/cream deposit. This is generally associated with overdosing a
boiler but can occur where insufficient dispersing agent reduces the effects of
blow down.
Scales forming salts found in the boiler
Calcium
Bi-Carbonate 180ppm
• Slightly
soluble
• >65oC
breaks down to form CaCO3 +CO2, remaining Calcium carbonate insoluble in water
• Forms a
soft white scale
Magnesium BiCarbonate 150 ppm
• Soluble
in water
• at more
than 90oC breaks down to form MgCO3 and CO2 and then Mg(OH)2 and CO2
• Forms a
soft scale
• Worst
scale forming salt
• >
140oC (sat. press 2.5bar) or >96000ppm will precipitate out
• Forms a
thin hard grey scale
• Precipitates
at high temperatures and about 8 bar
• Forms
sludge
• Breaks
down in boiler conditions to form MgOH and HCl
• forms a
soft white scale Rapidly lowers pH in the event of sea water contamination of
the boiler initiating rapid corrosion MgCl2 + 2H2O---> Mg(OH)2 + 2HCl HCl +
Fe --->FeCl + H 2FeCl + Mg(OH)2 ---> MgCl2 + 2FeOH This series is then
repeated. Effective feed treatment ensuring alkaline conditions controls this
problem
• Soluble <225000ppm o:p="">
• forms a
soft encrustation
• Free
irons promote galvanic action
Amorphous
Silicon dioxide (SiO2) - trace
• at high tempos and pressures (>40bar)
silica can distill from the bioler as Silicic acid and can sublime and pass
over into the steam system as a gas. Here it glazes surfaces with a smooth
layer, which due to thermal expansion crack and roughen the surface.
Troublesome on HP blading.Can be removed only by washing with Hydroflouric
acid.
SCALE FORMATION
The roughness of the heated surface has a direct
relationship to the deposit of scale. Each peak acts as a 'seed' for the scale
to bind to.
Nucleate Boiling
Scale
built up as a series of rings forming multi layers of different combinations. Much
increased by corrosion products or prescience of oil, even in very small
quantities.
Oil also increases scale insulatory properties.
Oil also increases scale insulatory properties.
Departure
form nucleate boiling (DNB)Under normal
conditions steam bubbles are formed in discrete parts. Boiler water solids
develop near the surface . However on departure of the bubble rinsing water
flows in and redissolves the soluble solids
However
at increased rates the rate of bubble formation may exceed the flow of rinsing
water , and at higher still rate, a stable film may occur with corrosion
concentrations at the edge of this blanket.
Dissolved solids in fresh water
|
Hard water
-Calcium and magnesium salts
- Alkaline
-Scale forming
|
soft water
-Mainly sodium salts
- Acidic
- Causes corrosion rather than scale
Boiler water testing
Recommended ranges
Chlorides
— Measure
100ml of sample water into a casserole
— Add
10 drops phenolpthalein Neutralize with sulphuric acid
— Add
10 drops of Potassium Chromate
— Titrate
Silver Nitrate until sample just turns brick red
— ppm
as CaCO3= (ml x 10) ppm
pH
— 100
ml unfiltered sealed water poured into two 50 ml glass stoppered test tubes
— Add
0.2 ml pH indicator to one ( pH indicator vary's according to required
measuring range)
— Use
colour comparator
— Due
to difficulty of excluding air, electronic pH meter preferred
Phosphates
— Fill
one 10 ml tube with distilled water
— Fill
one 10 ml tube with boiler water tests.
— Add
0.5 ml sulphuric acid to each
Add 0.5 ml Ammonium Molybdnate to each
Add 0.5 ml Aminonapthol Sulfonic acid (RE 132) to each
Stir well between each addition
Add 0.5 ml Ammonium Molybdnate to each
Add 0.5 ml Aminonapthol Sulfonic acid (RE 132) to each
Stir well between each addition
— Wait
3 minutes for calorimetric compaison
Alkalinity Phenolpthalein
— 100
ml filtered water
— Add
1 ml phenolpthalein
— If
pH >8.4 Solution turns pink
— Add
H2SO4untill pink disapears
— Ml
0.02N H2SO4 x 10 = ALk in CaCO3 ppm
— Measures
hydroxides and carbonates in sample, bi-carbonates do not show up so sample
should not be allowed to be exposed to the air for too long
Alkalinity Methyl orange
— Alkalinity
Methyl orange
— Bi
carbonates do not show up in the phenolpthalein sample as they have a pH <
8.4. Bi carbonates can not occur in boiler but if suspected in raw feed then
the following test.
— Take
phenolpthalein sample, add 1 ml methyl orange
— If
yellow, bi carbonates are present
— Add
H2SO4 untill red
— Total
0.02N H2SO4 x 10 = Total Alk in CaCO3
TDS
— Measure
100ml of sample water into a casserole
— Add
10 drops of phenolpthalein
— Neutralise
with TDS reagent (acetic acid)
— Temperature
compensate then read off scale in ppm.
Hydrazine
— Add
9ml distilled water to one tube
— Add
9 ml boiler test water to anouther
— Add
1 ml hydrazine reagent to each
— Use
colour comparator
Ammonia in feed
Only necessary where N2H4 used in blr
— Pour
condensate sample into two 50 ml colour comparator tubes
— Add
2 ml Nessler reagent to one
— Wait
10 mins
— Use
colour comparator
Only necessary where N2H4 used in
blr
— Pour
condensate sample into two 50 ml colour comparator tubes
— Add
2 ml Nessler reagent to one
— Wait
10 mins
— Use
colour comparator
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