Boiler Operations and Control

8/2/2019 Boiler Operations and Control

1/41

Prof. P. K. Das; Dept. of Mechanical Engineering; IIT KharagpurProf. P. K. Das; Dept. of Mechanical Engineering; IIT Kharagpur

A Course on Thermal Power PlantA Course on Thermal Power Plant

Prof. P. K. Das

Professor

Mechanical EngineeringDepartment

IIT Kharagpur


2/41



2

Boiler Operations

Objectives of boiler operations:

Satisfy the current power demand according tothe load curve Ensure the highest service life Ensure highest efficiency of the boiler plant


3/41



3

Operation procedures depend on

Type of system Circulating, Once through Coal fired, Gas fired, Oil fired Stokers System, Pulverized Fuel System

Applications

Power, Process Utility Size

Todays operation principle depends on; sensor technology,

control software, improved design of generator orhardware

A balance is sought between best equipmentperformance with safety requirements and


4/41



4

Fundamental Principles:In spite of massive developments in boilerdesign and power production basic operatingprinciples till applyCombustion safety and proper steam/water

cooling of boiler parts are essential

History of accidents:

Accident due to high pressure, accident infurnace


5/41




6/41



ASME CODES

1884 Code for the Conduct of Trials of Steam Boilers first Performance

Test Code

1911 Establishment of a committee to propose a Boiler code

1956 Committee established for ASME Pressure Vessel Code for Nuclear

Age

1963 Proposed Section III (Nuclear Power) of ASME Boiler and Pressure

Vessel Committee

1983 ASME Boiler and Pressure Vessel Code published in bothconventional and metric units

1989 Boiler and Pressure Vessel Code published on CD-ROM


7/41Prof. P. K. Das; Dept. of Mechanical Engineering; IIT KharagpurProf. P. K. Das; Dept. of Mechanical Engineering; IIT Kharagpur


Combustion Safety and Steam/water cooling Requirements

Purging Before firing a furnace there must be no lingering

combustible materials inside the unit

Purging ensures furnace is ready for firing

Standard Operating Rule for Multi-burners - No less than 25%

of maximum continuous rating (MCR) mass air flow for at

least 5 Minutes




8

Once combustion is established, Correct air/fuel ratio must be maintained Insufficient air flow forms pockets ofcombustible gas- Potential for explosion Sufficient air flow with a small amount ofexcess air- covers imperfect mixing, promotes

good air and fuel distribution Over fire air (OFA) ports installed in the uppercombustion zone of furnaces- emission reduction Controlled introduction of combustion air does

not eliminate the need of excess air for safetyIt is important to verify the boiler water levels Combustion should never be established untiladequate cooling water is in the tubes and thesteam drum, particularly in NCB

Or minimum flow rate is there in once




9

General Safety Consideration: Pressure for failure- A major concern and commonfeature in the boiler industry all over the world Codes by ASME, National Board of Boiler andPressure Vessel Inspectors, Other Licensing andInspecting Authorities have issued extensive codes

to minimize such failures Codes are continuously updated

Safety Attentions to be paid to combustion products-CO2, CO, SO2 Chlorine compounds

CO2 reduces O2 availability in the flue gas SO2 forms acids when breadth into the lungs

Chlorine compounds can form HCl acids orCarcinogens such as de-oxine




10

Enough safe guards to be taken for maintenanceand work on out-of-service boilers Confined space- Insufficient Oxygen Insufficient Lighting Ash accumulation high temperature andother health hazards

Safety of fuel ash should be considered

Ashes contain:Hazardous materials- Heavy metals, ArsenicResidual combustion may continue in ashcollected in hoppers

Carbon- In -ash characteristics may be alteredunder low Nox firing configurations




11

Operator Requirements:Good operation begin before equipmentinstallation is complete

Training of operators Preparation of equipments

Every operator should have a knowledge of all

components Functions Design Limitations and relationship to other

componentsthrough inspection of the equipments, studying,drawing and instructionsDistributed control system (DCS) integrates theindividual process controllers




12

Simulator Training Offer real time environment

Working understanding of the various steam generationsystem Reaction time and the rate of the equipment Can be combined with simulated equipment failures andwhat-if-events simulators

Preparation for Start Up:Systematic approach is required for the installation of thenew boiler or when any boiler has undergone majorrepairs or alterationsThe steps are

Inspection Cleaning Hydrostatic tests Pre calibration of instruments and controls Auxiliary equipment preparations Chemical cleaning

Steam line cleaning (Blowing)




13

Inspection: Familiarizes the operator with the equipments

Verifies condition of the equipments Provision or lack of provision for expansion (Thermaland pressure) should be carefully checked Temporary braces, hangers, ties needed forconstructions should be removed

Cleaning:Collection of debris, foreign materials duringtransport, storage, erection and repair

Accumulated debris chokes the water line Alters gas or air flows May ignite at elevated temperature

Special attention should be given for the cleaning of

oil and gas lines, burnersSteam cleaning is recommended for all oil and gas

A C Th l P Pl




14

Hydrostatic test is conducted first time in new boilers fillingit with waterWater with high level of purity to prevent internal foulingand corrosionsDemineralized water with 10 ppm of ammonia for pH control

and 500ppm of hydrazine for the control of oxygen shouldbe used for all non drainable reheaters and superheatersClean filtered water is suitable for components that will bedrained immediately after the test ASME code suggests water temperature of 210C but itshould never be higher than 560C No air should be trapped during the test

Hydrostatic Test: To be conducted after the pressure parts are assembled

but before the refractory and casing are installed 1.5 times the boiler design pressure is applied to allpressure parts Pressure is maintained for sufficient time to detect leaks Testing is equally important following pressure part

replacement

A C Th l P PlA C Th l P Pl t




15

Instrumentations and Controls:Natural Circulation Boilers-

water/steam pressure gauge Water level gauge glass Temperature gauge if superheated are used

Once through type boilers- steam pressure gauge flow meter temperature indicator

operation should not been attended without calibrationcalibration should include corrections in the actualworking range

Most modern boilers are controlled automatically oncethey are fully commissionedBefore start up controls must be operated manually untilautomatic controls have been adjusted for site specific

conditions

A C Th l P Pl tA C Th l P Pl t




16

Auxiliary Equipments:

Fan feed water pumps fuel equipments air heaters

economizer ash removal equipment drain system to drain the boiler shoot blowing system for clearing heattransfer surfaces

post combustion environmental controlequipments

Auxiliary equipments must also be prepared

for operations

A C Th l P Pl tA C Th l P Pl t




17

Chemical Cleaning:Lack of water side cleanliness can lead to

Boiler tube failure Carry over of solids in the steam- failure ofsuperheaters and turbine blade

damage

Flashing of all lose debris from feed water system andboiler and use of high quality water for hydrostatic testmust be supplemented by proper water side cleaning beforestart up. It removes grease paints oilNatural circulation boiler is given caustic and phosphateboil out after the feed water system is given a phosphateflush

After flashing out products of corrosion is still remain inthe form of iron oxide and mill scaleAcid cleaning is recommended for scale removal

A C Th l P Pl tA Course on Thermal Power Plant




18

Steam line blowing:Cleaning steam lines of loose solid particle and mill scale is

essential for protecting the turbine blades

Fine mesh strainers in turbine inlet prevents turbineblades and valves from scale and the other solid materials High velocity steam cleaning before coupling the steam

line to the turbine Temporary piping for flow bypass to the atmosphere

Several Methods for blowing steam linesHigh velocity air and steam mixture removes loose scale

due to temperature shock and velocity effectSufficient shock is obtained with a series of blows byvarying steam temperature

Source of steam for blowing: High pressure steam from boiler Steam from flush tank for forced circulation unit A Course on Thermal Power PlantA Course on Thermal Power Plant




19

Safety Valves: The set point of each safety valve should be

checked and adjusted after reaching the fulloperating pressure first time Safety valves seats are susceptible to damagefrom wet steam and grit. Cleaning of boiler and steam blowing should bedone before testing safety valves

Testing of safety valves always requires cautionValve exhaust piping and vent piping should not

exert any excessive forces on the safety valve. Gagsshould always be used while making adjustment tothe valves

Alternately, they can be tested with boiler pressure

below the design pressure with a hydraulic liftattached to the valve steam. This will reduce the





20

Start up:

Objectives to be included in all operating procedureof every boiler,1. Protection of pressure parts against corrosion,over healing and thermal stress.

2. Prevention of furnace explosions.

3. Production of stream at desired temperature,pressure and purity.

4. Compliance with environmental regulations.

Filling

High quality water to be used for fillingTemperature of the water is controlled to reducetemperature differential (< 56C)

Air venting assure complete filling, reduceoxygen corrosion

Correct water level in drum type boilers. At cold



21/41



21

CirculationCertain amount of circulation is needed to avoidoverheating of the pressure parts. Gradualincrease in firing rate can achieve this in NCBIn case of once through boiler the turbines should

be by passed still the steaming rate is reached.

PurgingCaution against furnace explosion.Burning fuel in suspension is more vulnerable.

Most explosions during start up and low load.Accumulation of combustible gases or dirt is to be

avoidedBurners to be lighted only after a thorough

purging.Purging rate should be at least 25% of the full air



22/41



22

Protection of economizerDuring pressure rising period amount of water is

very littleNo feed water flow through the economizer steam

formed in them can remain trapped. Makes drumlevel control difficult, produces water hammer.To overcome this, difficulty feed water should be

supplied continuously by venting the economizer orby re-circulating through the economizer.Protection of primary, secondary and reheat

super-heaters

During start up before stream flow through eachtube combustion gas temperature is to becontrolled.Maximum metal temperature: Carbon steel-

482C

Alloy steel- 510C-538C



23/41



23

It should be ensured that water flow througheach section and total stream flow > 10% of the

rated flow.Water should be drained from drainable parts or

should be boiled off.No steam flow through partially filled tubes.Super-heaters adjacent to side walls and division

wall are subjected to late boil-of. Theirtemperature should be monitored.



24/41



24

Protection of drums and headers:In most cases start up time time to raise pressure

and to protect super headers from thermal stress.In some case start up and shut down time time

required to limit stress in drum headers

Drum & headers made of rolled tube joints thin

section quick contraction expansion tube seat leakTemperature rise should be controlled. 56C/hr. Largetemperature differential in the tube headers to beavoided.Tubes connected to drum have water below and steam

at the top. Allowable temperature difference should beselected based on drum material.Additional inside and outside temperature of the

drums should be monitored.There are two periods when steam at the top of drum

is hotter than water in the lower half

Soon after firing begins



25/41



25

Steam drum temperature can be equalized by flooding thedrum with water

Operating Techniques for maximum Efficiency Fuel is the major cost Minimize fuel consumption and maximize steamgeneration Boiler efficiency depends on design It can be maintained by reducing loss in stackand ash pit

Stack Loss: Heat loss depends on mass flow rate and temperature of

flue gas Mass flow rate depends on mass of fuel+ excess air Excess air should be controlled efficiency Temperature of the flue gas affected by cleanliness ofboiler and heat transfer

surfaces Pro er o eration of soot blower and air heater



26/41



26

Pressure and Temperatures: Pressure and temperatures are unique parameters for the

turbine output or process requirement Deviation from set point indicate loss of efficiency

For Power generation, modern controllers adjust the supplytemperature to turbine within 60C

Each 280

C reduction in temperature reduces plant efficiencyby 1% in supercritical units

Variable pressure operation is another option forincreasing efficiency Can improve cycle efficiency at low loads Boiler pressure can be varied to meet the turbinerequirements- slow dynamics

For fast response throttle valve control is used. Boilerpressure is increased in the next instant.

Emission Requirements:



27/41



27

To ensure minimum ammonia usage, thegeneration of NO

xby combustion should be

controlled.Trade off between NO

xgeneration and

increase in CO and un-burnt carbon in ash to bemadeCombustion operation

Modern distributed control systemrequires more information and analysis.Closed loop neural network and other intelligentcontrol techniques are used.

Special attention to be paid to the followingpoints



28/41



28

Burner adjustmentsBurner adjustments not only affect the

performance of the said unit but also theperformance of the other burners.

All the burners should be identical. All the burners should be equallyadjusted.

Burners should be adjusted toincrease turbulence Increased air-fuel mixing Increased combustion intensity

Faster heat release Lesser demand for excess air Lesser un-burned carbon But it also amounts to

Increased NOx formation

Higher fan power requirement



29/41


A Course on Thermal Power Plant

29

Overfire air (OFA) port adjustment

For NOx reduction burners are usuallyoperated with less than theoretical air forcomplete combustion.The remaining air needed for completingcombustion is introduced separately abovehighest burner zone. The OFA is typicallyvaried as a function of load based on testsconducted during commissioning.Adjustment is needed if type of fuel is

altered.

Excess Air Total combustion air by adjusting FD and ID fan dampers

Excess air gives combustion intensity, reduced carbon loss and COformation

Reduced slag formation Increased fan power Increased stack loss Increased tube erosion and possibly increased NOx formation



30/41

Prof. P. K. Das; Dept. of Mechanical Engineering; IIT KharagpurProf. P. K. Das; Dept. of Mechanical Engineering; IIT Kharagpur 30

Abnormal Operations: Low Water

In case water level falls below the prescribed minimum,fuel firing should be stopped Caution should be taken while adding water to avoidtemperature shock Water should be added periodically with soak timesprovided to allow drum temperature to equalize Thermocouple readings from the top and bottom of thedrum should be taken time to time to control the process ofcleaningTube Failure: Operating a boiler with a known tube leak is not

recommended Steam or water escaping from a small leak can cut othertubes by impingement and set up a chain reaction of tubefailures Loss of water and steam alters boiler circulations- over

heating For this furnace risers of once throu h t e of boilers



31/41


Detection of Tube failures:

Unusual rise in temperature of furnacerisers Abnormal loss of water from the system Loss of chemical from a drum type boiler Sound made by the leak Boilers should be shut down as soon asnormal operating procedures permit after aleak A leak is then detect by hydrostatic

testing

A thorough inspection is needed after a tubefailure is located. It includes

a visual inspection of the failed tube laboratory analysis Background information like locations offailure, length of time the unit is in operation,

load conditions start up and shut down



32/41


Shut Down Operations:Boiler shut down is less complicated than start upEmphasis is on safety and protection of equipment

Two shut down situations: Control shut down shut down in an emergency

Control shut down: Firing rate is gradually reduced. Fuel is shut off when combustion equipments at itsminimum capacity

Boiler is purged with afresh air If some pressure is to be maintained, the fans areshut down and the dampers are closed Drum pressures are gradually lower as heat is loss

from the boiler setting Minimal amount of air drifts out of the stack



33/41


Emergency Shut Down:

Fuel is immediately shut offBoiler is purged of combustible gases

Additional procedure may apply to the fuel feed

equipment

Boiler may be held at a reduced pressure or may becompletely cooled as described above



34/41


Different Control Strategies for Steam Generation

Boiler following control system

Boiler follows the turbine response

MW load control is the responsibility

of the turbine-generator

Boiler is assigned the secondary

responsibility of throttle control

Load change repositions turbine

control valve

Boiler control modifies firing rate and

restores throttle pressure

It has a rapid load response storedenergy in the boiler provides the initial

change in load

Less stable throttle control



35/41


Turbine follows the boiler response

MW load control is the responsibility of

the boiler

T-G is assigned the secondary

responsibility of throttle control

With load change boiler control

increases the firing rate, this increases

the throttle pressure.

To maintain the throttle pressure,turbine control valve open increasing

Megawatt output.It provides minimal steam pressure and

temperature fluctuations during a load

change

Turbine following control system



36/41


Coordinated-boiler turbine generator control system



37/41


Coordinated boiler turbine control

Both of the above systems can provide satisfactorily control the boiler

operations but have inherent disadvantages

None of them can fully exploit the capabilities of both the boiler and

turbine generators

The two control options can be combined to do so

boiler is not capable of producing rapid changes in steam generationat constant pressure

Turbine is used to provide the initial load response

For a change in load throttle pressure set point is modified using MW

error (demand)

Turbine control valve responds to this change in set point and give the

new load quickly

Boiler modifies the firing rate to reach new load and restore throttle

pressure



38/41


The throttle pressure set point returns to its normal value

The result is fast and efficient production of electric power by a proper

coordination between the boiler and turbine

The response is much faster than turbine following system

But not as fast as boiler following system because the effect of MW error is

limited to maintain a balance between boiler response and stability

Wider limits would provide a faster response but a narrower limit will give

more stability



39/41



Turbine3000 rpm


40/41


Pl

S

H

R

H

R

H

F

S

H

Eco

PSH

147Kg/cm25400C

Generator

TransformerGenerator

Boiler

Coal Bunkers

Primary Air Fans

Coal

Mills

Raw

Coal Fdr

Chimne

y

Flue Gas Duct

Induced Draft Fan

Coal

Conveyer

SECL

Coal

Bunker

CSEB Crusher

House

400/220 KV

15.75KV

Condenser

Coal Handling Plant

Boiler

Drum

3000 rpm

210MW

ESPESP

AH



41/41

Documents

Boiler Operations and Control