National Workshop on

“Efficient Operation and Maintenance of Boilers”

organized by National Productivity Council

at Visakhapatnam(AP)

Presented By:

Vikas shrivastava & Akhilesh Tiwari

under guidance of Mr. R N Yadav and Mr. A K Saxena

From JAYPEE SIDHI CEMENT PLANT

, SIDHI,MP

EFFICIENT AND SAFE OPERATION OF BOILERS AND PREVENTION

OF BOILER EXPLOSION

BY JAYPEE SIDHI CEMENT PLANT, SIDHI (M.P.)

PARAMETERS REQUIRED TO BE MAINTAINED STRICTLY FOR EFFICIENT OPERATION OF AFBC BOILERS

Bed Height/FD Air Pressure

Primary Air Pressure

Bed Temp./Furnace Temperature

Fuel Size

Bed Material size & Specifications

Air & Fuel ratio

BED HEIGHT/FD AIR PRESSURE Static height for AFBC Boiler is to be maintained 300

to 325 mm during initial light up of boiler.

Expand bed height for AFBC boiler is to be maintained 500 mmwc to 600 mmwc.

FD Air discharge Pressure 600 to 650 mmwc.

Wind box or Air Box pressure 500 to 600 mmwc.

NOTE: High W.B. pressure increases the erosion of bed coils & low W.B. stops fluidization.

PRIMARY AIR PRESSURE P.A. fan air header pressure for AFBC boiler is to be

maintained 1000 to 1150 mmwc.

Difference of primary air pressure & F.D. wind box pressure is to be maintained approximately 500 mmwc.

NOTE: High P.A. pressure increases the bed coil erosion and carryover to enhance secondary combustion.

BED TEMPERATURE/FURNACE TEMPERATURE FOR AFBC/CFBC

Depends upon the type of fuel & firing method.

Indian coal 850 to 900 Deg.C.

Imported coal 875 to 925 Deg.C.

Lignite fuel 800 to 850 Deg.C.

Pet coke 875 to 950 Deg.C.

Rice husk and other biomass fuel 850 to 900 Deg.C.

Stoker firing 1100 to 1200 Deg.C.

Pulverized fuel firing 1100 to 1300 Deg.C.

For oil & gas firing 1100 to 1500 Deg.C.

FUEL SIZE When firing with Indian/Imported coal following sizes are

to strictly maintained as it will affect on performance of boiler.

For AFBC/CFBC Size : 0 to 8 mm Less than 1 mm not allowed more than 20%. Distribution: 1 to 5 mm – 70% 5 to 8 mm -20% 0 to 1 mm – 10% NOTE: Lower the size of coal, higher the unburnt in fly

ash, higher the size of coal, higher the erosion of bed coils and blockage of fuel air pipe & clinkerization in boiler.

FUEL SIZE CONT’D For STOKER FIRED BOILERS Size: 5 to 25 mm

Less than 5 mm not allowed more than 5%

Distribution: 5 mm to 15 mm-25%

15 mm to 25 mm-70%

0 mm to 5 mm -5%

NOTE: Lower the size of coal higher unburnt in flyash & carryover through boiler.

FUEL SIZE CONT’D PULVARIZED FIRED BOILERS

Pulverized coal size: mesh powder

(μm) and, for a bituminous coal

Size Distribution: 2% is +300 micro metre (μm)

70-75% is below 75 microns

20% is about 75-100 microns

NOTE: Higher the size of pulverized coal below 100 mesh results unburnt in bottom ash.

BED MATERIAL SIZE & SPECIFICATION

Bed Material Size: 0.85 mm to 2.36 mm

Distribution : 0.85 mm to 1.00 mm – 10%

1.00 mm to 1.50 mm - 50%

1.5 mm to 2.36 mm - 40 %

BED MATERIAL SIZE & SPECIFICATION CONT’D

Bed Material Specifications:

Crushed fire bricks castables IS8 grade bricks or river silica sand

Fusion Temp. 1300 Deg.C Shape: Spherical Angular

Bulk Density 1050 Kg/M3 Silica : 65%

Al2O3 28% Fe2O3: 1.05%

PbO2 1.67 % MnO : Trace

MgO 0.23 % P2O5 : 0.08 %

V2O 0.22 % K2O : 0.45 %

AIR TO FUEL RATIO Theoretical Air for combustion:

Theoretical Air Required:

4.31(8/3C+8(H-O/8)+S) Kg/Kg of fuel burnt.

To Understand the basics of efficient boiler Operation, the combustion process must be understood. Stable combustion condition requires the right amount of fuels and Oxygen, combustion products are heat energy,CO2,water vapour, N2,Sox,Nox and O2. In theory there is a specific amount of O2 needed to completely burn a given amount of fuel. In practice , burning conditions are never ideal, Therefore excess air must be supplied to burn the fuel completely depending upon the type of fuel.

TYPICAL EXCESS AIR TO ACHIEVE HIGHEST EFFICIENCY FOR DIFFERENT FUELS

Captive Power plant(Coal) boilers normally run about 15 to 20 %.

Fuel oil fired boilers may run as low as 5 to 10 %.

Natural gas fired boilers may run as low 5 to 8%.

Pulverized coal fired boilers may run about 10 to 15%.

OXYGEN AT BOILER OUTLET To Ensure complete combustion of the fuel used,

combustion chambers are supplied with excess air.

Excess air increase the amount of oxygen and the probability of combustion of all fuels.

EA= (O2% /21- O2% )x100

Stoichiometric Combustion

fuel + Oxygen in the air are in perfect balance-.

CO2% or O2% in flue gas is an important indication of the combustion efficiency.

BOILER EMERGENCIES Various Emergencies situations during Operation with

a special emphasis on the safety aspect like boiler protection systems controls and interlocks.

Drum level low and low-low.

Drum level high and high-high.

Furnace draught high and high-high.

Bed Temp. high.

Bed Temp low.

Water wall/screen tube/Evaporator tube failure.

BOILER EMERGENCIES CONT’D Super heater tube failure.

High Super heater Temp.

Low Super heater Temp.

Flame failure.

Furnace Explosion.

Boiler pressure high.

Coal feeder failure.

PAH/SAH tube failure.

Boiler feed pump failure.

Fan Failure.

DRUM LEVEL LOW AND LOW-LOW (A) CAUSES: Failure of BFP. Failure of drum level controllers. Excess opening of CBD/IBD. Sudden change in load(sudden red’n in load) Water tube failure (B) EFFECT: Boiler pressure parts may damage badly. (C) ACTIONS: Run the boiler if drum level is within safe limit otherwise

allow boiler to trip when the water level goes low-low limit to protect the boiler pressure parts.

DRUM LEVEL HIGH AND HIGH HIGH (A) CAUSES:

Failure of drum level controller. Sudden increase in load. Sudden increase in firing rate. (B) EFFECT: Water may enter in to the turbine and serious damage of turbine blades and

thrust pads. Carry over in Super heater and sharp fall in S.H. temp. Flange gasket may be failure. (C) ACTIONS: Run the boiler if drum level within safe limit and control the FCV. Open the CBD to maintain drum level in safe limit. Open the TG side main steam & Turbine drains to avoid the water entering into

TG. Trip the TG when steam temp gets below the safe limit.

FURNACE DRAUGHT HIGH AND HIGH HIGH

(A) CAUSE: Due to faulty operation of fan control. Disturbed combustion . Uncontrolled fuel entry. (B) EFFECT: Boiler may damage due to high furnace pressure. Weak part of furnace(ducting & Enclosure)may explode high

furnace pressure. (C) ACTION: If it is due to faulty operation of ID/FD/PA/SA fan control , take

it on manual mode and maintain furnace in suction. If furnace pressure has increased beyond limit allow boiler to

trip on furnace draught high-high.

BED TEMP. HIGH (A) CAUSE: High CV & low ash coal Low PA/FD/SA flow Sudden change in load

Ash recirculation system trouble.

Faulty bed thermocouple (B) EFFECT: Chances of clinker formation Chances of refractory failure. Chances of Screen tube failure. (C) ACTION: Control bed temp. by recirculation of ash. Increased PA & SA flow and reduce the load by cutting coal feeder. Coal feeder should be trip if bed temp increases beyond 975 deg.c If bed temp. exceeds further then allow boiler to trip to avoid clinker formation. Check the bed thermocouple.

BED TEMP. LOW (A) CAUSE: High PA/FD/SA flow w.r.to load. Low CV & high Ash content coal used. Coal feeder trips or overfeeding of coal in to furnace. Faulty bed thermocouples. Water /screen/evaporator tube leakage. (B) EFFECT: Boiler steam flow reduce. Super heater temp. drops. Furnace draught fluctuates. (C)ACTIONS: Boiler PA/FD/SA flow reduced if excessive. Check bed thermocouple. Stop bed material supply, if running. Check any leakage sound from furnace.

WATER WALL/SCREEN TUBE/EVAPORATOR TUBE FAILURE (A) CAUSE: Starved water wall. Block tube ,erode tube, pitted tube, salt deposits. (B) EFFECT: Hissing steam leakage noise from boilers. Unstable flame fluctuating draught. Bed temp. drops sharply. Increase ID fan loading. Flue gas outlet temp. decreased. (ACTIONS): Take shut down the boiler when boiler tube leakage

noticed and maintain the drum level.

SUPER HEATER TUBE FAILURE (A) CAUSE: Inadequate steam flow and high gas temp. during hot start-up. Erosion of tube due to high excess air. Blocked tube. Starvation of tube. Salt deposition due to high water level in drum. (B) EFFECT: Hissing noise noticed. Flue gas temp drops & high FW consumption than steam.. Overloading of ID fan. (C) ACTIONS: As soon as leakage noticed start reducing the load and trip the boiler. Try to locate leakage through manholes before the boiler depressurized. Boiler is to be forced cooled when S.H. leakage noticed.

HIGH SUPER HEATER TEMP. (A) CAUSE: High Excess air. Low feed water temp or HP heater not in service at constant firing

/load. Sudden increase in firing rate to increase steam pressure. Inadequate spray water. (B) EFFECT: +ve turbine expansion. Creep rate increase in tube metal ,turbine parts & steam piping. (C) ACTIONS: Always keep HP heaters in line when optimum loading of TG. Slow down firing rate to limit the S.H. Temp. Reduce excess air if more. Check Spray control.

LOW SUPER HEATER TEMP (A) CAUSE: Soot deposit on super heater tube. Inadequate air flow. High spray. Sudden increase in load and pressure drops. High Drum level. (B) EFFECT: Turbine expansion may be –ve. May induce thermal stresses in S.H. (C) ACTIONS: Check air flow, increase, if necessary. Reduce spray, if more. Avoid sudden rise in load to boiler pressure drop. Check feed water temp.

FLAME FAILURE (A) CAUSE: Dirty Oil/gas burner. Faulty flame sensor. Furnace pressure high. Low combustion Air. (B) EFFECT: Boiler will trip on flame failure. Chances of furnace explosion if unburnt oil/gas/coal moisture entered in furnace. Steam pressure & temp. may fall. Variation in drum level. (ACTIONS): Purge the boiler putting burner back and purge burner as per cycle time( minimum 5

minutes). Check the flame sensor & clean the photocell if found dirty. Check the igniter circuit & H.V. transformer . Clean the burner tip & nozzles regularly. Ensure the healthiness of explosion vent & door.

FURNACE EXPLOSIONS (A) CAUSE: Accumulation of unburnt fuel during lit up /start up of

boiler. Improper burning. Inadequate air. Secondary combustion. (B) EFFECT: Furnace explosion can cause extensive damage. (C) ACTION: Always purge the boiler with min 40% full load air for

about 5 minutes. No cut short in purging allowed. Adjust fuel air ratio.

BOILER PRESSURE HIGH (A) CAUSE: Sudden drop in load/steam flow. Uncontrolled fuel entry. Turbine/prime mover trips. (B) EFFECT: Disturbance in drum water level. Safety valve may disturbed if pressure rise in frequent way. Boiler may trip at high high pressure. (C) ACTIONS: Open start up vent to control the pressure. Control fuel ,air input & drum level. If TG /prime mover has tripped first, allow boiler to trip but safety valve may

lift. TG warm up vent put in auto, if pressure exceeds then it will open accordingly. Use Electromagnetic safety valve to limit the frequent operation of spring

loaded safety valve.

PAH/SAH TUBE FAILURE (A) CAUSE: Erosion of Air heater tubes. Corrosion of Air heater tubes. (B) EFFECT: Flue gas temp. after APH will fall down. Increase in O2% in at Air heater I/L. Air heater completely in line during initial start up. (C) ACTIONS: Control flue gas temp. bypasses PAH. Reduce coal feeding /air to maintain O2%. If leakages of tubes are more then stop the boiler and plug

that tubes.

COAL FEEDER FAILURE (A) CAUSE: Electrical supply failed. VFD faulty. Bed temp.high. Furnace draught low. Drum level low. (B) EFFECT: Boiler pressure may fall down. Steam temp. fall sharply. Bed temp.will decrease. Variation in furnace pressure. Variation in drum level. (C)ACTION: Control boiler pressure by reducing the TG load & control S.H. steam temp. by closing

the spray CV. Reduce PA/SA air to control bed temp. Control furnace draught & drum level. Check the electrical fault or emergency stop button. Check the VFD fault, if any. Restart the coal feeder after detecting the cause of failure.

BOILER FEED PUMP FAILURE (A) CAUSE: Motor protection relay operates. Lube oil temp. high. Discharge flow less. Motor bearing temp. high Deaerator level low. BFP Suction DP high. (B) EFFECT: Stand by pump will start in Auto/manual. (C)ACTIONS: Start the stand by pump ,if it does not start on auto ,adjust the load to

maintain the drum. Analyze and rectify the fault in the main feed pump & put it in auto. Check the BFP suction strainer & clean it ,if found chocked.

LOSS OF FANS (A)CAUSE: Electrical motor protection relay operated. Fan bearing temp. becomes high-high. Motor bearing temp. becomes very high. Drive fault. Boiler trip. ID fan trips. SA trip. (B) EFFECT: Boiler will trip on. Furnace draft either low or high. (C)ACTIONS: Rectify electrical fault, if any. Check cause for boiler trip and normalize it. Check fan/motor bearing RTD. Restart the fan(ID/FD/SA) after checking the cause of tripping and

taking corrective actions.

EXPLOSIVE POWER OF BOILER It will not be false to state that power librated by the

explosion of Lancashire boiler 7.5’ dia x 30’ length, working at 7 Kg/CM^2 is sufficient to project it to a height of 3.29 KMS. Therefore hazards of boiler explosion may well be imagined.

As a thumb rule, it could be stated that destruction hazards of 28.3 liters of water at 4.23 Kg/CM2 and sat.temp in a steam boiler is equivalent to 0.45 Kg of gun powder.

Introduction to furnace explosions In CFBC Boilers

Many CFBC Boilers have suffered/reported furnace explosion in the past, Apart from causing severe losses to the business concerned , the occurrences have shaken the confidence of CPP professionals , however PF boilers are more prone to the such type explosions than CFBC boiler but the Operation philosophy of PF boiler is clearly understood and established due to history of centuries.

As regards of CFBC boilers, these are completely newer generation of technology and explosions avoidance measures are not clearly understood by the operating Engineers.

TYPE OF EXPLOSIONS IN CFBC BOILERS

Most of the explosions faced in CFBC boilers are dust explosions caused by small particles of coal in the bed and in the free board kept under suspension by fluidizing air fans.

Explosions due to FO/HSD/LDO used in duct burners and /or load carrying burners has also been reported.

DUST EXPLOSIONS A dust explosions is the rapid combustion of a dust cloud .

In a confined or nearly confined space, the explosions is characterized by relatively rapid development of pressure with flame propagation and the evolution of large quantities of heat(coal) and reaction products. The required oxygen for this combustion is mostly supplied by the combustion air.

The condition necessary for a dust explosions is a simultaneous presence of a dust cloud of proper concentration in air that will support combustion and suitable ignition source.(Coal/HSD/LDO/FO).

Minor flue gas explosions are called puffs or blow backs.

FIRE TRIANGLE AND EXPLOSION PENTAGON

There are three necessary elements which must occur simultaneously to cause a fire FIRE TRIANGLE EXPLOTION PENTAGON Ignition Source(Coal/LDO/HSD/FO) Coal/LDO/HSD/FO

,Suspension Confinement

Air or Oxygen Heat (Temp.) Air or Oxygen Heat (Temp.)

On the other hand, for an explosions to occur,5 elements Fuel, heat, Oxygen, Suspension and confinement must occur simultaneously :.

These form the five sides of the explosions pentagon like fire triangle, removing any ignition source one of these requirements would prevent an explosions.

Remembering the three sides of the fire triangle (Fuel, heat & Oxygen) and five sides of the explosion pentagon(Fuel, heat, Oxygen, Suspension & Confinement) is important in preventing fires and explosions at any facility.

By eliminating the possibility of either suspension or confinement , an explosion can not occur, but a fire may occur. By eliminating the fuel, the heat ,or the Oxygen requirements , neither a fire nor an explosion can occur.

BASIC PHILOSPHY OF EXPLOSIONS PREVENTION

Basic Principles of avoidance of explosions are: Fuel should never fed in to the furnace continuously for than 12

seconds when there is no fire and coal should be added in a small quantity at ignition temp of coal.

Furnace is completely purged of the explosive mixture and then fired.

Fuel supply should be stopped immediately if fire/flame is not established and repurging is surely done before restart.

Correct air fuel ratio is to be maintained so that dust concentration with explosive limits is never achieved.

Explosion doors/vents/bleed valve (IN AFBC) must be perfectly operational and all protections and interlocks and fan drives sequence to be check in each shutdown as per OEM schedule/recommendations.

BOILER EXPLOSIONS PICTURES

EXPLOSIONS PICTURES CONT’D

BOILER FURNACE EXPLODED

BOILER STRUCTURES,BUILDINGS, & CABLE GALLRIES COLLAPSED

THE END