Tuesday, March 6, 2018

boiler inspection


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.

Gagging
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.
Larger clearance makm the valve simmer before opening and reduces valve blow down —> closing pressurewill be higher.
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.
 Automatic   feed regulatorsare therefore fitted to control the flow of water into the boiler and maintain the waterlevel at its desired value.They are fitted in the feed line, before the main feed check.
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
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

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
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
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
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.
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
  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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