This document discusses instrumentation and controls used on shell boilers. It describes common instruments like pressure gauges, level indicators, and temperature sensors. It explains how these instruments monitor key parameters like pressure, water level, and flue gas temperature. The document also summarizes common control systems for processes like combustion, steam pressure, and feedwater supply that regulate operation and shut the boiler down if safety limits are exceeded.

1. ID IDC Technologies - Technical Training That Works
BOILER PLANT OPERATION
Presented By Mr. Sivanand Ray
Fleet Management Training
Institute

2. ID IDC Technologies - Technical Training That Works
INSTRUMENTATION USED ON A SHELL
BOILER
 Fire-tube type boiler reviewed for simplicity
 Instrumentation:
» Items that provide operator with indication of
condition within the boiler
» Ensure indications are within SAFETY limits and
design OPERATIONAL PARAMETERS
Operating pressures, temperatures, flows, water
level
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INSTRUMENTATION
 PRESSURE GAUGE
» Normally Bourdon type of gauge
» Large with large numbers
» Graduated approximately double the pressure
of safety valve setting, not < than 1,5
» Located on top of shell/drum on an
independent nozzle
» Supplied with a siphon (“pigtail”) to develop a
water seal
» Must be calibrated at least once a year
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INSTRUMENTATION
 PRESSURE TRANSMITTER
» Where a remote pressure indicator required,
an electrical signal used
» Separate transducer
» Mounted on same branch as pressure
indicator
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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 >28
bar
» Normally simple gauge glass type
 Installed such that with lowest reading on gauge glass, 75
mm water over highest point of tubes
 Visible range of water level approx. 125mm from NWL
» Daily routine blow down to remove debris, scale,
sludge
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Typical Water Level
Gauge for Low Pressure Boiler
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Electronic and digital water level indicators
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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 mandatory
Low level alarm
Low-low-level shut down
» Weekly routine test to check correct
operation of switches
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INSTRUMENTATION
 TEMPERATURE MEASUREMENT
» Flue gas exit temperature
» An increase indicates fouling problems
» Thermocouples, insulated from gas sampling
tube
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TYPICAL PROCESS AND
INSTRUMENTATION 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 SYSTEM
OPERATING PHILOSOPHY
 CONTROLS :
» Regulate the various quantities indicated by
the instruments
» Shut down (with interlocks) the plant if
parameters exceeded
» Manual operation to sophisticated automatic
computer operation (DCS)
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CONTROL AND SYSTEM
OPERATING PHILOSOPHY
 To control a boiler, following quantities
require 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 control
combustion 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 slightly
depending on fuel fired
 No matter what fuel, four aspects that are
regulated:
 Air supply; Fuel supply; Ratio of fuel to air; Flame safety
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COMBUSTION CONTROL
 Two basic types of combustion control
systems:
» 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 feed
control drive for coal
Combustion air fan inlet vanes / outlet damper
operator
» Simplistic
cannot maintain fuel – air ratio at optimum over
full load range
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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 where
fuel pressure may vary
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General Selection Chart for
Combustion Control Systems
Type of Fuel Type of Control Steam Generation
System Capacity Ranges
(kg/hr)
Coal Modulating positioning to 27 500
Full metering (Steam
/air flow) 18 000 – 36 000
Full metering with
Oxygen compensation above 36 000
Fuel Oil & Gas Modulating positioning to 11 500
Full metering (Steam
/ air flow) 11 500 – 45 000
Full metering with
Oxygen compensation above 45 000 M4-17

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STEAM PRESSURE CONTROL
 Main purpose of a steam boiler is to
produce steam within a definite pressure
range
 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 and
lower pressure limits
» On-off pressure switch – low or high fire
burner
» Definite limitation exists on burner minimum
firing 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 flow
to ensure all heat transfer surfaces are cooled
with 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
» Thermostatic
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FEED WATER SUPPLY CONTROL
 Typical arrangements of feed water plant
:
» Two or more pumps (standby) from hot well
tank (10-15 min. storage)
Either on-off operation or a feed valve station
» Feed valve station :
Check valve
Stop valve
Feed control valve c/w isolation and bypass valves
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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 or
low water cut off
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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 water
continuously to the boiler, stabilising the
water level
» THREE types of modulating controllers :
Single Element Level Controller
Two Element Level Controller
Three Element Level Controller
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Schematic of Three
Element Level Controller
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SOME CONSIDERATIONS REGARDING
WATER LEVEL CONTROL
 Rapid changes in steam flow rate can cause
SURGING 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 OPERATING
characteristics of water level controlling system
 Highest water level, NWL, lowest level for safe
operation should be MARKED on boiler outer
shell
M4-25

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FURNACE PRESSURE CONTROL
 Only really required on coal fired boilers
with 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 too
narrow
» typically +250 to –1250 Pa
» ideal control point –100 Pa M4-26

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AUTOMATIC CONTROLS
 Instrumentation employed same as that
for manual operation
 Rapid and continuous development of
electronic control systems means systems
for boilers are now DIVERSE
 Selection shall depend on owner /
purchaser present complexity of control
systems
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AUTOMATIC CONTROLS
 ASPECTS that do not vary whether
auto/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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