Furnace 3i2y1s

FIRED HEATER 

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Plays and important role in Petroleum and Petrochemical Industry. Provide heat to hydrocarbon of all types to increase temp : To it into vapour state so as to separate into narrower boiling range cuts by fractionation To the point where thermal reaction will occur To the point required for catalytic reaction to occur.

FIRED HEATER GENERAL OVERVIEW  DESIGN  INSPECTION & FAILURE ANALYSIS 

FIRED HEATER MODE OF HEAT TRANSFER  FURNACE AND COIL  BURNER  AIR PREHEATER  DAMPER 

FIRED HEATERS TYPES OF FIRED HEATER  INDIRECT FIRED HEATER  DIRECT FIRED HEATER  INDIRECT :  BOX TYPE  CYLINDRICAL TYPE  CABIN TYPE 

FIRED HEATER BOX TYPE  HORIZONTAL TUBE  VERTICAL TUBE  CYLINDRICAL TYPE  HELICAL COIL  VERTICAL TUBE 

FIRED HEATER TUBES RADIANT - Radiant heat from heating flame & incandescent refractory.  SHIELD - Radiant & Convection heat (Receives highest heat flux)  CONVECTION TUBES - Convection heat from combustion gases. 

CONVECTION SECTION After oil convection tubes are installed , usually tubes are often installed to preheat air, superheat steam for process requirement.  Corbelling is provided to minimise flue gas by. 

DESIGN CONSIDERATION PROCESS  COMBUSTION  MECHANICAL 

DESIGN - PROCESS For uniform heat distribution  No. of es to be minimised  Each shall have single circuit from Inlet to Outlet  Maximum allowable inside film temp. for process shall not be exceeded 

DESIGN - COMBUSTION Draft :  Natural : Efficiency shall be based on 20% excess air when fuel gas is primary fuel & 25% when fuel oil is primary fuel.  Forced : Efficiency shall be based on 15% excess air when fuel gas is primary fuel & 20% when fuel oil is primary fuel. 

DESIGN - COMBUSTION VOLUME OF HEAT RELEASED:  For oil fired heater : max. 124 KW / Cu.mt.  For Gasfired heater : max. 165 KW / Cu.mt. 

DESIGN - COMBUSTION 

Designed for a negative pressure of at least 0.10 inch of water ( ) is maintained in radiant and convection sections at maximum heat release with design excess air.

DESIGN Size and arrangement of tube are determined by  type of operation  Amount of heating surface required  Flow 

DESIGN MECHANICAL Thermal expansion considerations including steam air decoking.  Convection section tube layout : for future addition of two row of tubes.  For fuel oil firing soot blower shall be provided for convection section.  Vertical cylindrical heater max.H/D - 2.75 

DESIGN - MECHANICAL Shield tube shall have at least three row of bare tubes.  Convection - Corbels or baffles are employed to minimise flue gas bying.  For Vertical heater : Tube L - 60 ft. max.  For Horizontal heater : Tube L- 40 ft. max.  Heater shall allow replacement of tube without damage to adjacent tubes. 

DESIGN- TUBES Tube wall thickness - API RP 530.  Calculation shall include erosion and corrosion allowance.  Maximum metal temp. as per API 530.  All tubes shall be seamless .  Shield tube material and thickness shall be as connecting radiant tube.  Design life : 100,000 hours. 

TUBE MATERIAL - TEMP. (OISD)       

Metallurgy Carbon steel 5 Cr 1/2 Mo 9 Cr 1 Mo 18 Cr - 8 Ni 18 Cr 8 Ni 2 Mo 25 Cr 20 Ni

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Normal 565 Deg C 650 685 870 870 1150

Decoking 620 Deg C 730 750 925 925 1185

TUBE MATERIAL Standardisation of material: To keep low stock.  Generally used material :  CS upto 300 Deg C  Low alloy steel upto 1200 Deg F  Austenitic SS higher than 1200 Deg F. 

TUBE MATERIAL 4-6 % Cr are superior to plain CS in resistance to sulphur corrosion .  Oxidation resistance of CS decreases rapidly above 650 Deg C.  Cr addition increases corrosion resistance at high temp. 

TUBE MATERIAL 

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For Heaters in Hydrodesulphurisation and hydrocracker : Austenitic stainless steel is used due to their higher temp.strength and high corrosion resistance.

DESIGN - CONVECTION Convection - Stud or finned type tubes  Stud to be arc or resistance welded.  Fins to be helically wound and continous welded to tube.  Stud material : CS - 510 Deg C ( Tip temp) 

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Stud size : 1/2” dia X 1” height

DESIGN - PIPING All flanges to be weld neck type.  Threaded connection are not acceptable.  Low drain and high vent point shall be accessible from outside heater casing.  Crossover piping shall be of same material as per process heater tube. 

DESIGN - TUBE Uned length of tube : 35 OD or 20 ft. whichever is less.  Min.thickness of end tube sheet shall be 1/2”.  Material : CS upto 427 Deg C  25Cr- 20 Ni-1093 Deg C  50 Cr 50 Ni - Nb above 649 Deg C if fuel has Sodium + Vanadium more than 100 ppm 

REFRACTORIES AND INSULATION Design temp. outside casing : Max. 82 Deg C with still air at 27 deg C.  Wall arches and floor : Proper expansion .  Min. Service temp of refractories in radiant section : 982 deg C.  Min service temp for burner block : 1482 Deg C. 

REFRACTORIES AND INSULATION BRICK AND TILE TYPE.  CASTABLE CONSTRUCTION.  CERAMIC FIBRE CONSTRUCTION. 

BURNER A device for introduction of fuel and air into a heater at desired velocities , turbulence and concentration to establish and maintain proper ignition and combustion.  Types :  Oil  Gas  Combination of oil and Gas. 

DESIGN - BURNERS Shall ensure against flame impingement.  Gas pilots shall be provided for all burners firing liquid fuels.  Burner block shall be able to expand and contract as a unit.  Burner , gas tip and oil guns shall be removable in operation. 

DESIGN - BURNER Time  Temperature  Turbulence  Mix the fuel and air  Maintain ignition  Mould the flame  Minimise emmisions. 

BURNER SELECTION Furnace configuration and tube location  Fuel(s) available.  Heat release and burner spacing  Process stream consideration  Draft condition  Altitude  Air temperature 

BURNER Fuel gas pressure : 15 Psig ( Premix burner)  Fuel oil Pressure : 150 Psig.  Steam / oil : 0.15 lb/ lb of oil for natural draft furnace 

DRAFT Draft is the negative air pressure generated by buoyancy of hot gases inside the furnace.  Hot gases are less dense than outside air.  This causes hot gases to move upward out of stack creating a slight vacuum inside the furnace.  Smallest negative pressure is at conection section 

DRAFT Too little draft can damage metal structure and snuff out the burners due to heat build up just under furnace arch and roof.  Too much draft can pull excessive air into the furnace wastes fuel. 

FURNACE TYPE - DRAFT Natural Draft Heater : Stack effect.  Induced Draft Heater : Fan to remove flue gases.  Forced Draft Heater : Fan to supply combustion Air  Balanced Draft Heater : Combination of ID & FD . 

COMBUSTION Combustion is the reaction which takes place when fuel ( C , Hydrogen or Sulphur) unites with oxygen ( from air) to liberate heat.  Well maintained burner and properly designed can reduce heater fuel cost by as much as 10%. 

GOOD COMBUSTION • Uniform flame pattern • No haze

POOR COMBUSTION Non uniform flame pattern  Haze - Unburnt fuel  Bottom burner needs air  Potential impinging on tubes 

Some Indication of Faulty Operation 

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Positive press. At top of firebox . Excessive temp. in fire box High flue gas pressure

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ID fan not running Excessive firing rates Overfiring Heat release too high Incorrect combustion air flow Fouled convection section

Some Indication of Faulty Operation 

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Variation in outlet temp. of multi High Press. Drop in coils

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Low press. Drop in coil High flue gas press. Drop across convection

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Unequal flow Flame impingement Coke build up/ fouling High flow rate High vapourisation Low flow rate. Excessive fouling

DETERIORATION OF COIL MAJOR FACTORS :  Type of Process  Characteristics of charge feed.  Velocity of flow thru coils  Pressure & Temperature  Combustion product  Mechanical damage. 

FORM OF DETERIORATION OF HEATER COIL Deterioration due to corrosion.  Metallurgical deterioration.  Mechanical Deterioration. 

Deterioration due to corrosion High Temp. Oxidation :  Reason : High tube metal temp.due to high process temp., flame impingement, high heat flux, internal coking / fouling , flow starvation.  Location : Fire side of radiant / shield tubes  Results in hard brittle oxide scale on tube surface resulting in tube wall thinning / bulging and cracking. 

Deterioration due to corrosion SULPHIDATION :  Conversion of metal to sulphide scale due to sulphur reaction at high temp. ( 316 to 427 Deg C)  Rate of corrosion of sulphur increases with temperature.  Results in internal pitting. 

Deterioration due to corrosion Low Temp. Acidic Corrosion ;  Flue gases contain SO2 . At low this may combine with moisture and condense as acids which is corrosive.  Convection section is prone to acidic corrosion. 

Deterioration due to corrosion Fuel Ash Corrosion:  If fuel contains more than 50 ppm Vanadium and sodium salts , the deposits formed are corrosive above 650 Deg C. 

Deterioration due to corrosion Polythionic acid Corrosion :  Generation of Ploythionic acid from wet sulphide scaled during downtime may pose similar corrosion problem to SS. 

Metallurgical Deterioration Rare cause of thickness reduction however reduces mechanical strength / ductility which may cause tube failure.  Grain Growth : Smaller Grain size has high strength and low creep strength. Grain growth above 600 Deg C for Steel.  Graphitisation : Carbide breaks into iron (Soft) and graphite ( Brittle) when operated for long period in range of 440 Deg C. 

Metallurgical Deterioration Carburisation : Diffusion of elemental carbon into solid steel. Operating at high temp. / presence of coke . Forms hard and brittle structure resulting in cracking.  High Temp. Hydrogen attack : Above 230 Deg C hydrogen dissociate into atomic hydrogen and diffuse thru metal . Carbon migrates to grain boundary and form methane. Causes internal pressure rise at grain boundary. 

MECHANICAL DETERIORATION 

Due to Overstressing , weakening , poor workmanship , vibration ,thermal shocks.

MECHANICAL DETERIORATION Bulging : Due to localised hot spots.  Sagging : Due to reduction in structural strength of tube on overheating , unequal metal temp.  Bowing : Unequal metal temp. and restriction in thermal expansion.  Vibration : Due to flow fluctuation , rapid evaporation . Wear of tube at tube . 

OTHER DETERIORATION Tube : Overloading , Vibration.  Refractory Lining : Spalling , thermal shock, vanadium .  Casing : Spalling of refractory.  Air preheater elements : Acid dew point corrosion. 

ON STREAM INSPECTION Frequency( OISD) :  Daily : Visual inspection of firing condition, flame pattern , tube condition,TST , Temp., Pressure drop, flue gas temp.  Monthly : Thermography for hot spot on stack , casing. 

ON STREAM INSPECTION Structurals.  Ducting.  Insulation / painting.  Stacks  Heater tube - Condition / Hot spot/ TST.  Tube .  Internal refractory lining. 

ON STREAM INSPECTION Main burner .  Vibration of tube.  Flue gas leakage from ts / expansion bellow. 

PLANNED S/D INSPECTION Visual Inspection : Sulphur deposits.  Sagging /Bowing : > 1.5 OD - replaced.  Bulging/ scaling / cracking / splitting.  Bulging upto 5% is allowed.  Mechanical Damage - At tube .  Weld t .  Welding of TST points : DP  SS tubes - ivation shall be done before opening. 

PLANNED S/D INSPECTION TUBE THICKNESS :  UTM at four or more locations.  Check UT on locations close to flame, return bend, Excessive oxidised area,Bulged location, hot spot . 

PLANNED S/D INSPECTION Other Type of Examination :  Hardness testing : Spot check (Carburisation , Hardening).  OD Measurement.  Metallurgical Examination. 

PLANNED S/D INSPECTION Additional Inspection of Convection tubes :  External corrosion due to lower flue gas temp/ downtime sulphurous scales/bowing / sagging.  Inspection of tube s : Crack / Metallurgical degradation/ oxidation.  Refractory Damage : Spalling / cracks. 

PLANNED S/D INSPECTION Foundation.  Structural.  Casing.  Flue gas ducts : Internal corrosion.  Soot blowers.  Air preheaters : Corrosion/ fouling.  Dampers : Operability.  Stacks : Internal lining/ UTM/ Foundation. 

AIR PREHEATER Air preheater is used to heat the air required for combustion recovering the heat of flue gases which otherwise would be lost with stack gases.  Increases heater efficiency. 

AIR PREHEATER Advantages :  Improved control of combustion air flow.  Reduced oil burner fouling.  Better flame pattern control.  More complete combustion of difficult fuel. 

AIR PREHEATER Disadvantages :  Increased corrosion.  Formation of acid mist , if fuel sulphur content is high.  Increased maintenance.  Reduced stack effluent velocity. 

AIR PREHEATER- TYPE Direct : Direct heat transfer  Indirect : Intermediate fluid is used. 

AIR PREHEATER Direct : Regenerative and Recuperative.  Regenerative :  Matrix of metal or refractory  May be stationary or rotary.  Recuperative :  Separate age for flue gas and air  Tubular 

AIR PREHEATER Temp at which corrosion and fouling becomes excessive are affected by:  Fuel sulphur and other contaminant.  Fuel or flue gas additives  Flue gas oxygen and moisture  Combustion temp.  APH / Burner Design  Furnace cleanliness  Ash content in heavy fuel. 

AIR PREHEATER COLD END TEMP. :  Corrosion due to condensation of sulphuric acid vapours fromed from sulphur of combustion product.  Desirable to operate APH at temp. above acid dew point level. 

AIR PREHEATER COLD END TEMP. CONTROL  Cold air by  External preheat of cold air  Recirculation of hot air 

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STACK TEMP. CONTROL : Curve

HEATERS IN DESULPHURISATION UNIT Uses Austenitic stainless steel tubes.  The sulphide scales formed inside may react with water and oxygen to form weak sulphurous acid called polythionic acid.  This acid attack steel and cause Intergranular Corrosion at location of high residual stresses such as heat affected zone of Weld, resulting in cracking of ASS. 

HEATERS IN DESULPHURISATION UNIT Protection against Polythionic acid attack :  PREVENTION :  Preventing formation by maintaining temp. of ASS equipment above dew point of water.  Nitrogen purging in the system to prevent any air from entering the system. 

HEATERS IN DESULPHURISATION UNIT NEUTRALISATION :  Use of soda ash solution ( Na2CO3) in range of 2 to 5% by weight.  Fill the system with the soln. And soak for minimum 2 hours before draining soda ash soln. And exposing equipment to air.  Wash the system with soda ash soln.  Protective film of soda ash should not be rinsed with steam / water. 

REFORMER Operating temp. 815 to 980 Deg C.  Tube metallurgy : Incoloy 800 / HK 40/Para alloy.  Down fired from roof / side fired for even heat distribution.  Tubes are centrifugally cast.  Pigtails - Connecting pipe between inlet / outlet header and tube. 

REFORMER FAILURE : Due to stress rupture at hottest and highly stressed portion.  Thermal stresses highest at midwall.  Fissuring starts from midwall and progresses inside, final stage of rupture occurs when fissure reaches outside wall.  Pigtails fails from bends. 

DAMPERS Provided in duct system design for control and isolation of various elements of the system.  Consideration must be given to operating differential pressure and temp. across. 

DAMPERS Tight shut off : Low leakage ( max. 1/2%)  Isolation or guillotine ( slide gate) : No leakage ( 0%)  Flow control or distribution : medium to high leakage.  Natural draft air inlet doors : Low leakage to full open. 

DAMPERS 

Multiple Louvers : Preferred for control application.