WATER MIST IN LARGE MACHINERY SPACES

Water mist systems are extensively used as fire protection of machinery spaces onboard passenger ships, and have almost taken over as fire extinguishing systems after IMO standardized such systems. Water mist systems are documented through full scale fire testing of the manufacturers' solutions. In practice it has been shown that in spaces with a ceiling height above approximately 10m it is necessary with more water mist nozzles to extinguish low seated fires. The IMO-method that describes the test procedures requires that all nozzles are mounted at ceiling level, and gives no opportunity to extrapolate results from fire tests to spaces with higher ceiling heights.

Conventional merchant vessels have large machinery spaces with ceiling heights above 10m. Therefore it is still common to install CO2-systems in these ships, since no alternative extinguishing systems are approved for this application area. Until now several manufacturers of water mist systems have documented that it is possible to extinguish fires in machinery spaces with height up to 10m and a volume exceeding 3,000m3. A research project initiated by IWMA (International Water Mist Association) resulted in a change of IMO's regulations. That means that the test results will be valid also for spaces up to twice the tested volume, but there was not given any possibility to increase the ceiling height.

If it can be shown through testing that water mist systems have sufficient ability to fight fires in larger machinery spaces, such systems will challenge and replace CO2-systems. In practice this means that fire extinguishing can start immediately after a fire has been detected. Today the start of extinguishing may be delayed with 10-15 minutes because CO2 represents a toxic hazard to persons in the machinery space. The CO2 cannot be released before people are evacuated from the engine room and all air supply is stopped and all openings are closed. Every minute with a fire in the machinery space increases the damage to equipment and cables; it has been indicated that the damage increases with €12,000 per minute.

Water mist systems can be released immediately after fire detection, and will quickly cool down the machinery space. Most fires will be extinguished after 10-15 minutes. In addition water mist systems have become popular among engineers and crew because they, besides being possible to use without hazard to people, are easy to test regularly. Water mist systems on passenger ships are often tested every week, and are simultaneously used for cleaning the machinery space. The costs connected to refilling the water reservoir are negligible in contrast to replacement of CO2-bottles after release of a CO2-system.

SINTEF NBL is now working to start projects that can open up for use of water mist system in large machinery spaces, and for this purpose we have the possibility to utilize our large test hall. The test hall is over 20m high and has a total volume of approximately 14,000m3.

Thursday, December 19, 2013
Posted by sanjay swain

DRY DOCKING



DRY DOCKING
·        the major event
·        preparing the vessel for the next 30 months
·        cost

DRYDOCKING MAINTENANCE ALTERNATIVES
·        crew
·        traveling squads
·        shore assistance
·        compare with shipyard
·        heavy work involving high lifts  transportation
·        lowest cost

DRYDOCKING SPECIFICATION (most important document)
·        terms/conditions
·        description of all jobs
·        compiling the specifications as  early as possible

DRYDOCKING SUCCESS FACTORS   (knowing the vessel)
·        good and complete specification
·        planning
·        controlling the yard
·        adequate resources

DRYDOCKING THE SUCCESSFULL DD
·        the specification to cover between 80-90%  of all work
·        no major extra cost
·        within the stipulated time and estimated  cost

DRYDOCKING PREPARATION FOR ENTRY
·        vessel to be free for hot work and   enclosed places to be ventilated as required
·        bunkers / f.water / ballast etc. be suitably distributed to achieve   suggested trim.
·        works in specification identified / marked.
·        supervision duties decided and agreed to.

DRYDOCKING SURVEYS
·         a list of all items to be surveyed is kept ready
·         list of all certificates expiring is ready
·         list of conditions of class to be dealt with is clear
·         list of new applicable regulations to be attended to is available.
·         modifications /fabrications if any as per new regulations should already
·        be in the scope of repairs
·         liase with the surveyor and agree on the scope of inspections / duration of inspections.
·         to keep surveyor informed about the docking surveys and get a list of recommendations at the earliest for completion of statutory  certificates.
·         get surveyor’s approvals for the scope of repairs involving class.


PRE DOCKING SURVEYS
• get the gauging reports verified and establish the scope of steel repairs.
• agree with surveyors’ action plan for crediting of cargo /ballast tanks   for (intermediate/special) surveys
• agree on testing procedures / repair procedures if any and press test of tanks as per    requirements.
• it is advantageous to plan to credit all tanks prior to docking, the trading pattern permitting.
• to credit max possible CSM items prior to docking.

DRYDOCK WORK SCHEDULE
ship staff and yard jobs are clearly understood / planned and carried out without interference
• yard jobs (in process and completion and testing) are properly supervised
• spares supplied to yard by vessel / arrival of new spares etc are properly  recorded and monitored
• decision makings are properly delegated
• onboard timings are suitably altered to get max   productivity.
• shipboard meetings are properly timed to continously   monitor the situation.
• repair teams if any are to be properly utilised and   effectively monitored

dry docking safety
• safety meetings involving yard and ship staff to be properly timed and well attended
• violations of yard guidelines are to be strictly discouraged (hotwork in engine room etc.)
• tank entries etc. are to be done strictly according to procedures and  personnel to be doubly careful while closing openings
• importance to be given to attire.
• extreme care to be exercised while turning engines/operating steering   gear/starting blowers/switching on electrical breakers/prior trials etc.
• systems are to be properly deactivated and rendered safe (depressurise  hydraulic lines/empty oil lines/drain  sea water lines etc)
• boundaries of fuel oil tanks are properly marked
• the tank drain plugs are properly marked / identified and protected from inadvertant opening.
• frame nos are suitably marked on deck/sides and bottom.
• vent pipes are suitably marked/identified.

drydocking economy    
• repairs done through afloat workshops are always cheaper
• most of the specialized jobs are   done thru subcontractors or representatives of OEMs with a surcharge.
• a repair team will always be cheaper if the materials can   be organized cheaply.
• you will loose a fortune if you need to do tank cleaning inside a yard.
• all additional work will be charged at a very high rate.
• the smaller the fabrication jobs/steel jobs on deck--the more   uneconomical it becomes.
• quantities of steel work/pipe work/hull painting etc are to be determined with respect to conditions in tariff like
           • minimum kgs per location
           • minimum meters per location.
           • charges for inway and access work
           • minimum surface (blasting)
           • minimum no of points per location (gauging)

• WATCH OUT FOR THESE DANGER AREAS
          pipe clamps
          transport to workshop
          machining/fabrication items
          cleaning
          ventilations / bilges / services.

 it would be cost effective to take a quote for the following
     - unshipment of rudder
     - renewal of carrier bearing with yard material
     - renewal of pintle bearing with yard material
     - standard tariff for o’haul of motors (kw basis) /pumps (kw basis)/pipes (meter basis)/ testing of pipes (meter basis) /testing of welds (meter basis) / supply of skilled and unskilled manpower (hourly basis) magnaflux / radiography and other tests / staging (tower and block/buildup of pits/reweld bottom plates

• best economy is an outcome of a tight specification

drydocking   commercial

the following important aspects are covered by   the contract document
•       time of completion
•       discount offerred
•       penalty clauses
•       repair specifications
•       repair specifications
•       payment schedule & redelivery of vessel
•       owners rights
•       yards responsibilty and limitations
•       guarantee
•       engagement of subcontractors and surcharge.
•       work done certificates
•       authority and owners representative
•       cancellation clause / terms
•       facilities to be provided to owners representatives
•       change of scope of repairs
•       spares and stowage
•       pilotage tugs etc for shifting for yards convienience
    •       the owners enquiry contains the conditions   of the owners
    •       the yards offer contains the yards rules and conditions
    •       the conditions in the yards offer stand valid even if they are
contradictory to owners   request once the vessel has been stemmed
    •       the areas of contradictions are to be agreed upon / thrashed out prior stemming the vessel.

drydocking technical -  corrosion protection
anodes
-       choice of the correct type and quantity of anodes.
-       replenishing of electrodes for impressed current   systems if fitted.
    -       protection of anodes while painting
    -       correct placing of anodes under guidance from supplier.
-       removal of old anodes

drydocking technical
• steel work
           - inspection of weld quality / fitups
         - keep track of the dimensions and locations (for commercial reasons)


• rudder
            - bush clearance
          - drop
          - sealing 0 rings for the taper surface of the pintles
          - proper cementing of palm bolts
          - water box closing plate water tightness
          - jumping bar /clearance
          - key ways
• propeller
       -  push up and pull up hydraulic pressures (push up curve)
      -  key ways
      - polishing

• anchors and chain-
      - gauging and reversal if reqd
     - cleaning of chain locker and insp


• HULL
      - gauging of stiker plates
      - build up of pits
      - wee out/gouge and build up weld seams of bottom   plates
      - inspection of drain plugs

drydocking technical (machinery)
• cleaning of coolers and heaters
• o’haul of large electric motors
• balancing of rotating equipments as reqd.
• o’haul of machinery as reqd
• recheck alignments of machineries if disturbed.
• remove deficiencies in automation and control equipmen as reqd.

drydocking instrumentation and navigating equipments
• callibration of electrical switchboard meters/other electrical and mechanical measuring devices
• o’haul of main breakers and callibrating safety trips.
• rectify deficiencies in navigating instruments
• rectify deficiencies in safety systems if any.

drydocking prior flooding(important items prior flooding)
-       all tank drain plugs are inspected and vacuum   tested after fitment
-       all anodes are functional
-       sea chest gratings etc are secured
-       correct ballast is taken.
-       inspect rudder (water box) / propeller /rope/ guard
-       all cargo/ballast tanks are clean from remnants of    steel work
-       cargo lines etc pressure tested / pumps are tried out
-       all tank valves (if remote operated) are tried out
-       hatch covers etc and other type of cargo gear are tried out and properly secured
-       all sea chests are properly purged of air
-       all equipment in engine room are tried out
-       main engines/boilers are preheated and ready for trials


drydocking sea trials
• sea trials
     - engines are run to required speed, movements are checked.
     - steering is confirmed to be performing satisfactory
     - stern tube oil is checked for maintaining level.

• the vessel may proceed to sea on   satisfactory completion in some cases


Saturday, October 26, 2013
Posted by sanjay swain

SAFETY EQUIPMENT'S ONBOARD



HYPERMIST SYSTEM

1) CHECK THAT THE SYSTEM IS LINED UP CORRECTLY.
2) CHECK PUMP IN AUTO MODE AND NO ALARMS ON THE FIRE CONTROL PANEL IN THE MSB ROOM.
3) TRYOUT AT LEAST ONE ZONE SPRINKLER RELEASE IN CONTROLLED MODE.
4) CHECK PUMP PRESSURE WHEN SYSTEM IS TESTED.
5) CHECK SPRINKLER HEADS FOR CLARITY (NO PAINT DEPOSITS ETC..)

CO2 SYSTEM

1) CHECK KEY IS IN PLACE.
2) CHECK INTEGRITY OF ALL THE CONNECTIONS
3) CHECK ALL CO2 HEADS FOR CLARITY
4) CHECK ROOM DOORS AND CABINET DOOR.
5) BLOW THROUGH WITH AIR.

QUICK CLOSING VALVE

1) VISUALLY INSPECT THE SYSTEM.
2) CHECK THE AIR PRESSURE IN THE BOTTLE
3) TRY OUT AT LEAST ONE SECTION OF QCVS. OR INDIVIDUAL VALVE FOR PROPER OPERATION

REMOTE TRIPS

1) FUNCTION TEST ONE SECTION AT A TIME..

EM'CY FIRE PUMP

1) ENSURE PUMP IS LINED UP AND READY FOR IMMEDIATE USE
2) TRIAL RUN FOR 10MINS AND RECORD THE PRESSURE GENERATED WITH TWO FIRE HOSES RIGGED.
3) CHECK FOR LEAKAGES

EM'CY GENERATOR

1) CHECK  LO, FO, COOLING WATER LEVELS
2) CHECK E/GEN ON AUTO MODE.
3) TEST RUN THE ENGINE ON BATTERY MODE AND HYDRAULLIC STARTING MODE
4) CHECK OIL LEVEL IN HYD OIL TANK
5) TEST RUN ON LOAD FOR AT LEAST 30 MIN AND CHECK AVAILABILITY OF POWER AT SERVICES PROVIDED BY E/GEN.

SCBA COMPRESSOR

1) CHECK THE CONDITION OF CHARGING HOSES AND THE CONNECTIONS.
2) ENSURE ALL SCBA BOTTLES ARE FULLY CHARGED
3) CHECK OIL LEVEL IN THE SUMP
4) CHECK THE COMPRESSOR CUT OFF FUCNTION AT 300 BAR.

LIFE BOAT ENGINE

1) CHECK LO, FO AND COOLING WATER LEVEL.
2) TEST RUN THE ENGINE IN ALL RUNNING DIRECTIONS
3) CHECK SPRINKLER PUMP DRIVING MECHANISM

RESCUE BOAT ENGINE

1) CHECK LO, FO AND COOLING WATER LEVEL
2) TEST RUN THE ENGINE IN ALL RUNNING DIRECTIONS.

VENTILATION FLAPS

1) FUCNTION CHECK.
2) CHECK FOR ANY AIR LEAKS.

FIRE HYDRANTS AND HOSES

1) CHECK THAT ALL HOSES ARE IN PLACE AND GENERAL CONDITIONS ARE SATISFACTORY
2) CHECK FOR FREENESS OF NOZZLES, AND GREASE ACCORDINGLY.
3) PRESSURE TEST ALL HOSES ONCE IN THREE MONTHS

EM’CY BILGE SUCTION

1) OPERATE AND GREASE.

S.W. RECIRC. V/V

1)  OPERATE FROM REMOTE AND LOCAL STATIONS AND CONFIRM THE OPERATION.

STEAM SMOTHERING SYSTEM

1) CARRY OUT VISUAL INSPECTION OF THE SYSTEM AND CHECK INDIVIDUAL
UNIT V/V FOR FREENESS

INCINERATOR

1) TRY OUT TRIPS AND ALARMS.

SHIP SIDE V/V

1) OPERATE AND GREASE

FIRE AND GAS DETECTION EQUIPMENTS

1) TEST ALL THE SENSORS ONCE IN THREE MONTHS.

BILGE ALARMS

1) FUCNTION CHECK

BATTERIES AND CHARGERS


1)CHECK THE BATTERY TERMINALS,APPLY PETROLEUM JELLY.

2) CHECK THAT THE BATTERY IS FULLY CHARGED.
3) EVERY QUARTER DISCHARGE ROUTINE TO BE CARRIED OUT.
4) AFTER STARTING EM’CY GEN KEEP THE BATTERY IN EQUALISING  CHARGE
TILL BATTERY VOLTAGE REACHED TO 27 V, THEN CHANGE OVER SWITCH TO
FLOATING CHARGE POSITION.

CRANES

1) FUCNTION CHECK THE LIMIT SWITCHES

REF. CHAMBER ALARM

1) FUCNTION CHECK

O.W.S. 15 PPM

1) FUCNTION TEST OF 15-PPM ALARM AND CHANGING OVER OF O/B V/V

ELE. EM’CY TRIPS

1) CARRY OUT FUNCTION TEST
2) CONFIRM  ALL BREAKERS ASSOCIATED WITH  THE GROUP HAVE TRIPPED.

EM’CY LIGHTING

1) FUCNTION CHECK

COMMUNICATION EQUIPMENTS

1) FUCNTION CHECK

HAZ. AREA EQUIPMENTS

1) CHECK PHYSICAL CONDITION OF THE EQUIPMENT
2) CHECK THE BONDING

M/E EM’CY MANOEUVRING

1) TRY OUT M/E FROM LOCAL MANOEUVRING STATION.

EM’CY STEERING

1) TRY OUT STEERING FROM LOCAL STATION.

MACHINERY TRIPS

1) TRY OUT TRIPS AND ALARMS FOR MACHINERIES AS PER INDIVIDUAL SCHEDULES

BLACKOUT TEST

1) CARRY OUT BLACK OUT TEST AND CHECK SEQUENTIAL START

EMERGENCY SHOWER

1) OPERATIONAL CHECK TO BE CARRIED OUT.
2) OBSERVE THE COLOUR OF WATER.
Posted by sanjay swain

BOILER FEED WATER MANAGEMENT / CORROSION FIGHTING

CORROSION FOUND IN BOILER AND FEED WATER SYSTEM


CORROSION AND TUBE FAILURE CAUSED BY WATER CHEMISTRY
Metals obtained from their oxide ores will tend to revert to that state. However , if on exposure to oxygen the oxide layer is stable , no further oxidation will occur. If it is porous or unstable then no protection is afforded.

Iron+O2 --- magnetite(stable and protective) + O2----ferrous oxide (porous)

TWO PRINCIPLE TYPES OF CORROSION

Direct chemical
                      Higher temperature metal comes into contact with air or other gasses (oxidation, Sulphurisation)

Electrochemical
                      -e.g. Galvanic action , hydrogen evolution , oxygen absorption

Hydrogen Evolution (low pH attack)




Valency = No of electrons required to fill outer shell










Pure water contains equal amounts of hydrogen and hydroxyl ions . Impurities change the balance. Acidic water has an excess of hydrogen ions which leads to hydrogen evolution





For hydrogen absorption to occur no oxygen needs to be present, a pH of less than 6.5 and so an excess of free hydrogen ions is required.

The Protective film of hydrogen gas on the cathodic surface breaks down as the hydrogen combines and bubbles off as diatomic hydrogen gas.


Oxygen Absorption(high O2 corrosion)



pH between 6- 10, Oxygen present. Leads to pitting. Very troublesome and can be due to ineffective feed treatment prevalent in idle boilers. Once started this type of corrosion cannot be stopped until the rust scab is removed , either by mechanical means or by acid cleaning. One special type is called deposit attack, the area under a deposit being deprived of oxygen become anodic. More common in horizontal than vertical tubing and often associated with condensers.

BOILER CORROSION

General Wastage

Common in boilers having an open feed system.

.

.
Pitting
-Most serious form of corrosion on the waterside

-Often found in boiler shell at w.l.

-Usually due to poor shape

-In HP blrs found also in screen and generating tubes and in suphtr tubes after priming.

CORROSION FATIGUE CRACKING


Cases found in water tube blrs where due to alternating cyclic stresses set up in tube material leading to a series of fine cracks in wall. Corrosive environment aggravates. Trans crystalline

more in depth: Occurs in any location where cyclic stressing of sufficient magnitude are present

Rapid start up and shut down can greatly increase susceptibility.

Common in wall and supht tubes, end of the membrane on waterwall tubes, economisers, deaerators . Also common on areas of rigid constraint such as connections to inlet and outlet headers

Other possible locations and causes are in grooves along partially full boiler tubes (cracks normally lie at right angle to groove ), at points of intermittent stm blanketing within generating tubes, at oxygen pits in waterline or feed water lines, in welds at slag pockets or points of incomplete fusion , in sootblower lines where vibration stresses are developed , and in blowdown lines.

CAUSTIC CRACKING (EMBRITTLEMENT) or STRESS CORROSION CRACKING



Pure iron grains bound by cementite ( iron carbide).

Occurs when a specific corrodent and sufficient tensile stress exists

Due to improved water treatment caustic stress- Corrosion cracking ( or caustic embrittlement ) has all but been eliminated.

It can however be found in water tubes , suphtr and reheat tubes and in stressed components of the water drum.
The required stress may be applied ( e.g. thermal, bending etc. ) or residual ( e.g. welding)

Boiler steel is sensitive to Na OH , stainless steel is sensitive to NaOH and chlorides.

A large scale attack on the material is not normal and indeed uncommon. The combination of NaOH , some soluble silica and a tensile stress is all that is required to form the characteristic intergranular cracks in carbon steel.

Concentrations of the corrodent may build up in a similar way to those caustic corrosion i.e.
·         DNB
·         Deposition
·         Evaporation at water line
·         And also by small leakage

Caustic corrosion at temperatures less than 149oC are rare

NaOH concentration may be as low as 5% but increased susceptibility occurs in the range 20- 40 %

Failure is of the thick walled type regardless of ductility.

Whitish highly alkaline deposits or sparkling magnetite may indicate a corrosion sight.

To eliminate this problem either the stresses can be removed or the corrodent. The stresses may be hoop stress( temp', pressure) which cannot be avoided bending or residual weld stresses which must be removed in the design/ manufacturing stage.

Avoidance of the concentrations of the corrodents is generally the most successful. Avoid DNB , avoid undue deposits prevent leakage of corrodents, prevent carryover.
Proper water treatment is essential.

CAUSTIC CORROSION
·         Takes place at high pressure due to excessive NaOH
·         In high temperature, high evaporation rates leading to local concentrations nearly coming out of solution and form a thin film near heating surface.
·         Magnetite layer broken down
·         Soluble compound formed which deposits on metal as a porous oxide
·         Local concentrations may cause a significant overall reduction in alkalinity.
·         If evaporation rate reduced alkalinity restored.
More in depth:
Generally confined to
1.   Water cooled in regions of high heat flux
2.   Slanted or horizontal tubes
3.   Beneath heavy deposits
4.   Adjacent to devices that disrupt flow ( e.g. backing rings)

Caustic ( or ductile ) gouging refers to the corrosive interaction of concentrated NaOH with a metal to produce distinct hemispherical or elliptical depressions.

Depression are often filled with corrosion products that sometimes contain sparkling crystals of magnetite.
Iron oxides being amphoteric are susceptible to corrosion by both high and low pH enviroments.


High pH substances such as NaOH dissolve the magnetite then attack the iron.

The two factors required to cause caustic corrosion are;
·         the availability of NaOH or of alkaline producing salts. ( e.g. intentional by water treatment or unintentional by ion exchange resin regeneration.)
·         Method of concentration, i.e. one of the following;
                                           i.  Departure form nucleate boiling (DNB)
                                         ii.  Deposition
                                       iii.  Evapouration

i)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.
The magnetite layer is then attacked leading to metal loss.
The area under the film may be relatively intact.

ii) Deposition
A similar situation can occur beneath layers of heavy deposition where bubbles formation occur but the corrosive residue is protected from the bulk water

iii), Evaporation at waterline

Where a waterline exists corrosives may concentrate at this point by evaporation and corrosion occurs.



PREVENTIONS
·         Rifling is sometimes fitted to prevent DNB by inducing water swirl.
·         Reduce free NaOH by correct water treatment
·         Prevent inadvertent release of NaOH into system (say from an ion exchange column regenerator )
·         Prevent leakage of alkaline salts via condenser
·         Prevent DNB
·         Prevent excessive waterside deposits
·         Prevent creation of waterlines in tubes- slanted or horizontal tubes are particularly susceptible to this at light loads were low water flows allow stm water stratification.

If the magnetite layer is broken down by corrosive action, high temperature hydrogen atoms diffuse into the metal, combine with the carbon and form methane. Large CH-3 molecules causes internal stress and cracking along crystal boundaries and sharp sided pits or cracks in tubes appear.

more in depth: Generally confined to internal surfaces of water carrying tubes that are actively corroding. Usually occurs in regions of high heat flux, beneath heavy deposits, in slanted and horizontal tubes and in heat regions at or adjacent to backing rings at welds or near devices that disrupt flow .

Uncommon in boilers with a W.P.of less than 70 bar

A typical sequence would be ;
·         NaOH removes the magnetite
·         free hydrogen is formed ( hydrogen in its atomic rather than diatomic state) by either the reaction of water with the iron reforming the magnetite or by NaOH reacting with the iron
·         This free hydrogen can diffuse into the steel where it combines at the grain boundaries to form molecular hydrogen or reacts with the iron carbide to form methane
·         As neither molecular hydrogen or methane can diffuse through the steel the gasses build up , increasing pressure and leading to failure at the grain boundaries
·         These micro cracks accumulate reducing tensile stress and leading to a thick walled failure. Sections may be blown out.
·         This form of damage may also occur in regions of low pH
·         For boilers operating above 70 bar , where high pH corrosion has occurred the possibility of hydrogen damage should be considered

Loss of circulation , high temperature in steam atmosphere, or externally on suphtr tubes

Concentrated chelants ( i,e. amines and other protecting chemicals) can attack magnetite , stm drum internals most susceptible.
A surface under attack is free of deposits and corrosion products , it may be very smooth and coated with a glassy black like substance
Horse shoe shaped contours with comet tails in the direction of the flow may be present.

Alternately deep discrete isolated pits may occur depending on the flow and turbulence

The main concentrating mechanism is evaporation and hence DNB should be avoided

Careful watch on reserves and O2 prescience should be maintained

Low pH attack
Pure water contains equal amounts of hydrogen and hydroxyl ions . Impurities change the balance . Acidic water has an excess of hydrogen ions which leads to hydrogen evolution.See previous notes on Hydrogen Evolution

For hydrogen absorption to occur no oxygen needs to be present, a pH of less than 6.5 and so an excess of free hydrogen ions is required.
The Protective film of hydrogen gas on the cathodic surface breaks down as the hydrogen combines and bubbles off as diatomic hydrogen gas.
May occur due to heavy salt water contamination or by acids leaching into the system from a demineralisation regeneration.

Localised attack may occur however where evaporation causes the concentration of acid forming salts . The mechanism are the same as for caustic attack. The corrosion is of a similar appearance to caustic gouging

Prevention is the same as for caustic attack . Proper maintenance of boiler water chemicals is essential

Vigorous acid attack may occur following chemical cleaning . Distinguished from other forms of pitting by its being found on all exposed areas.

Very careful monitoring whilst chemical cleaning with the temperature being maintained below the inhibitor breakdown point. Constant testing of dissolved iron and non ferrous content in the cleaning solution should be carried out.

After acid cleaning a chelating agent such as phosphoric acid as sometimes used . This helps to prevent surface rusting , The boiler is then flushed with warm water until a neutral solution is obtained.

OXYGEN CORROSION
Uncommon in operating boilers but may be found in idle boilers.
Entire boiler susceptible , but most common in the superheater tubes (reheater tubes especially where water accumulates in bends and sags )

In an operating boiler firstly the economiser and feed heater are effected.

In the event of severe contamination of oxygen areas such as the stm drum water line and the stm separation equipment

In all cases considerable damage can occur even if the period of oxygen contamination is short

Bare steel coming into contact with oxygenated water will tend to form magnetite with a sound chemical water treatment program.
However , in areas where water may accumulate then any trace oxygen is dissolved into the water and corrosion by oxygen absorption occurs( see previous explanation )

OXYGEN ABSORPTION
in addition to notes above pH between 6- 10, Oxygen present.
Leads to pitting. Very troublesome and can be due to ineffective feed treatment prevalent in idle boilers. Once started this type of corrosion cannot be stopped until the rust scab is removed , either by mechanical means or by acid cleaning.

One special type is called pitting were metal below deposits being deprived of oxygen become anodic . More common in horizontal than vertical tubing and often associated with condensers.

The ensuing pitting not only causes trouble due to the material loss but also acts as a stress raiser
The three critical factors are
                           i.  the prescience of water or moisture
                         ii.  prescience of dissolved oxygen
                       iii.  unprotected metal surface

The corrosiveness of the water increases with temperature and dissolved solids and decreases with increased pH

Aggressiveness generally increases with increased O2

The three causes of unprotected metal surfaces are
                           i.  following acid cleaning
                         ii.  surface covered by a marginally or non protective  iron oxide such
as Hematite (Fe2O3)
                       iii.  The metal surface is covered with a protective iron oxide such as
magnetite (Fe3O4 , black) But holidays or cracks exist in the coating, this
may be due to mechanical or thermal stressing.

During normal operation the environment favours rapid repair of these cracks. However, with high O2 prescience then corrosion may commence before the crack is adequately repaired.

FEED SYSTEM CORROSION

Graphitization
Cast iron , ferrous materials corrode leaving a soft matrix structur of carbon flakes

Dezincification
Brass with a high zinc content in contact with sea water , corrodes and the copper is redeposited. Inhibitors such as arsenic , antimony or phosphorus can be used , but are ineffective at higher temperatures.
Tin has some improving effects

Exfoliation (denickelfication)
Normally occurs in feed heaters with a cupro-nickel tubing ( temp 205oC or higher)
Very low sea water flow condensers also susceptible.
Nickel oxidised forming layers of copper and nickel oxide

Ammonium corrosion
Ammonium formed by the decompositin of hydrazine
Dissolve cupric oxide formed on copper or copper alloy tubes
Does not attack copper, hence oxygen required to provide corrosion,Hence only possibel at the lower temperature regions where the hydrazine is less effective or inactive,

The copper travels to the boiler and leads to piting.
Posted by sanjay swain

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