boiler plant operation

8/12/2019 Boiler Plant Operation

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IDC Technologies - Technical Training That Works

BOILER PLANT OPERATION

IDC Technologies Tech Briefs

(Mechanical Engineering)


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INSTRUMENTATION USED ON A SHELLBOILER

Fire-tube type boiler reviewed for simplicity

Instrumentation:

Items that provide operator with indication ofcondition within the boiler

Ensure indications are within SAFETY limits and

design OPERATIONAL PARAMETERSOperating pressures, temperatures, flows, water

level

M4-2


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INSTRUMENTATION

PRESSURE GAUGE Normally Bourdon type of gauge

Large with large numbers

Graduated approximately double the pressureof safety valve setting, not < than 1,5

Located on top of shell/drum on anindependent nozzle

Supplied with a siphon (pigtail) to develop awater seal

Must be calibrated at least once a year

M4-3


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INSTRUMENTATION

PRESSURE TRANSMITTER Where a remote pressure indicator required,

an electrical signal used

Separate transducer Mounted on same branch as pressureindicator

M4-4


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INSTRUMENTATION

LEVEL COLUMNS Provides VISIBLE water level indication Minimum 25NB pipe line connecting column to boiler

TWO water gauge glasses for boiler pressure >28bar

Normally simple gauge glass type

Installed such that with lowest reading on gauge glass, 75mm water over highest point of tubes

Visible range of water level approx. 125mm from NWL

Daily routine blow down to remove debris, scale,sludge

M4-5


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Typical Water LevelGauge for Low Pressure Boiler

M4-6


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Electronic and digital water level indicatorsM4-7


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INSTRUMENTATION

LEVEL SWITCHES Float type (Mobrey)

Single-element control (on-off or modulating)

Normally located in an external chamber TWO chambers mandatoryLow level alarm

Low-low-level shut down

Weekly routine test to check correctoperation of switches

M4-8


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INSTRUMENTATION

TEMPERATURE MEASUREMENT Flue gas exit temperature

An increase indicates fouling problems

Thermocouples, insulated from gas samplingtube

M4-9


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TYPICAL PROCESS ANDINSTRUMENTATION DIAGRAM (P&ID)

COAL FIRED LP BOILER

Instrumentation

Control loops

OIL OR GAS FIRED LP BOILER

Instrumentation

Control loops

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CONTROL AND SYSTEMOPERATING PHILOSOPHY

CONTROLS : Regulate the various quantities indicated by

the instruments

Shut down (with interlocks) the plant ifparameters exceeded

Manual operation to sophisticated automaticcomputer operation (DCS)

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CONTROL AND SYSTEMOPERATING PHILOSOPHY

To control a boiler, following quantitiesrequire to be regulated : Fuel/air ratio to maintain optimum

combustion conditions Water flow to match steam flow from boiler

Steam flow and pressure to regulate heat

input Combustion chamber pressure (balanced

draught boilers)

Combustion safety M4-12


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COMBUSTION CONTROL

Manual control cannot continuously controlcombustion in a reliable and efficient manner

Automatic control systems used : maintains fuel-air ratio to furnace at an optimum

maintains a relatively constant steam pressure

Combustion control mechanisms vary slightlydepending on fuel fired

No matter what fuel, four aspects that areregulated:Air supply; Fuel supply; Ratio of fuel to air; Flame safety

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COMBUSTION CONTROL

Two basic types of combustion controlsystems: Positioning

Metering Pneumatic or Electrical control drives

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COMBUSTION CONTROL

Positioning Type Control: Designed to control based on a change in

steam pressure

Components :Master steam pressure controller

Fuel control valve with cam for oil / gas or feedcontrol drive for coal

Combustion air fan inlet vanes / outlet damper

operator Simplisticcannot maintain fuelair ratio at optimum over

full load rangeM4-15


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COMBUSTION CONTROL

Metering Type controller : Components as per Positioning type with :

Fuel flow measuring device

Air flow signal Fuel-air ratio is controlled by a relay

Capable of adjusting fuel-air ratio and wherefuel pressure may vary

M4-16


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General Selection Chart forCombustion Control Systems

Type of Fuel Type of Control Steam GenerationSystem Capacity Ranges

(kg/hr)

Coal Modulating positioning to 27 500

Full metering (Steam/air flow) 18 00036 000

Full metering with

Oxygen compensation above 36 000

Fuel Oil & Gas Modulating positioning to 11 500Full metering (Steam

/ air flow) 11 50045 000

Full metering with

Oxygen compensation above 45 000 M4-17


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STEAM PRESSURE CONTROL

Main purpose of a steam boiler is toproduce steam within a definite pressurerange

Steam pressure dictated by : process plant steam demand heat input to the boiler furnace

Pressure fluctuates between minimum

and maximum settings

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STEAM PRESSURE CONTROL

Modulating control linked to upper andlower pressure limits

On-off pressure switchlow or high fireburner

Definite limitation exists on burner minimumfiring rate

turndown ration varies : 4:1 gas; 2:1 oil

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FEED WATER SUPPLY CONTROL

Purpose is to provide sufficient feed water flowto ensure all heat transfer surfaces are cooledwith water

Visible water level provided by :

Two simple gauge glasses More sophisticated devices (electrodes, capacitance

or conductivity type)

Water sensor controls :

Normally two independent

Float operated

ThermostaticM4-20


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FEED WATER SUPPLY CONTROL

Typical arrangements of feed water plant:

Two or more pumps (standby) from hot welltank (10-15 min. storage)

Either on-off operation or a feed valve station

Feed valve station :

Check valve

Stop valveFeed control valve c/w isolation and bypass

valves

M4-21


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FEED WATER CONTROL SYSTEMS

ON AND OFF FEED WATER PUMP Small boilers with relatively large water

storage, slow load changes

Typical float operated water level controller,mounted at water line

Also serves either as low-water level alarm orlow water cut off

M4-22


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FEED WATER CONTROL SYSTEMS

MODULATING LEVEL CONTROLLERS Boilers with relatively small water volume

relative to steam demand

Thermostatic type regulator feeds watercontinuously to the boiler, stabilising thewater level

THREE types of modulating controllers :

Single Element Level ControllerTwo Element Level Controller

Three Element Level Controller

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SOME CONSIDERATIONS REGARDINGWATER LEVEL CONTROL

Rapid changes in steam flow rate can causeSURGING of the water level and carry-over

VERY IMPORTANT that the :

steam qualities characteristics of boiler (size, firing system)

MUST be properly matched to the OPERATINGcharacteristics of water level controlling system

Highest water level, NWL, lowest level for safeoperation should be MARKED on boiler outershell

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FURNACE PRESSURE CONTROL

Only really required on coal fired boilerswith FD and ID fan

Required to : prevent excessive negative pressure and

uncontrolled air entering furnace products of combustion to percolate back to

coal supply

Controller transmitter range not toonarrow typically +250 to1250 Pa

ideal control point100 PaM4-26


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AUTOMATIC CONTROLS

Instrumentation employed same as thatfor manual operation

Rapid and continuous development of

electronic control systems means systemsfor boilers are now DIVERSE

Selection shall depend on owner /

purchaser present complexity of controlsystems

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AUTOMATIC CONTROLS

ASPECTS that do not vary whetherauto/manual : Principles of operation

Areas of control (control loops) Direct reading of instruments

Safety devices and interlocks

Selector for manual or automatic

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boiler plant operation

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