The document discusses the design and configuration of a combustion control loop using cross limited control for a boiler. It explains that cross limited control works by first adjusting air combustion when increasing load and first adjusting natural gas when decreasing load. It provides details on the hardware including the boiler specifications, burner specifications, and flow transmitter ranges. It also includes diagrams of the combustion control loop, combustion curves showing operating areas, and limits for the control loops.

1. COMBUSTION CONTROL LOOP - CROSS LIMITED
DESIGN & CONFIGURATION
Carlos Alberto Bertoni
Automation Engineer - Control & Safety
FSEng #8669/14 by TÜV Rheinland

2. INTRODUCTION
We have often seen many literatures that show methods for optimization and control for combustion applied in boilers
e Industrial Ovens where Single Point Positioning Control, Parallel Positioning Control and Cross Limited Control are the
typical methods used.
The focus this document is to explain how The Cross Limited Control works and what are necessary know to implement
and configure correctly.
Increase Load => First adjust Air Combustion after that
adjust Natural Gas.
Decrease Load => First adjust Natural Gas after that
adjust Air Combustion.

3. The Combustion Control Loop.
INTRODUCTION

4. The Architecture proposed for this solution.
IHM => Operation and Supervising.
Safety PLC => Interlock and Safety Shutdown.
Burner Control Relay => Safe-start Sequence and Monitoring of Flame.
Combustion Control => Control Loops - Water and Fire Side
HARDWARE

5. BOILER Water Tube Boiler
Operating Pressure: 135psig = 10,5 kgf/cm2
Steam Production (Water 20°): 25000 kg/h
BURNER Fuel: Natural Gas
Capacity: 3525000 - 17625000 kcal/h ( 20 - 100 % )
Air Excess: 10 - 30% see table below
Turndown Ratio: 5 : 1
Stoichiometric Air for Natural Gas: 10,055 “ in m3/m3 “
HHV: 9400 kcal/m3
MAIN FEATURES

6. FLOW TRANSMITER for Natura Gas Range of work: 375,00 - 1875,00 Nm3/h ( 18,75 - 93,75 % )
Range of Calibration: 0,00 - 2000,00 Nm3/h ( 0 - 100 % )
FLOW TRANSMITER for Air Combustion Range of work: 4901,81 - 24509,10 Nm3/h ( 18,15 - 90,77 % )
Range of Calibration: 0,00 - 27000,00 Nm3/h ( 0 - 100 % )
SAFETY CONDITIONS INCREASE LOAD: FIRST ADJUST AIR COMBUSTION AFTER THAN ADJUST NG
DECREASE LOAD: FIRST ADJUST NG AFTER THAN ADJUST AIR COMBUSTION
COMMAND SIS => BPCS MODULATION ENABLE
POSITIONING DAMPER AIR COMBUSTION IN 100 % “ PURGE “
POSITIONING BURNER IN 20 % “ LOW FIRE “
AIR COMBUSTION CONTROL DAMPER NATURAL GAS CONTROL VALVE
Range: 0 - 27000 m3/h Range: 0 - 2000 m3/h
MAIN FEATURES

7. PIC-001 STEAM PRESSURE PV = 0 - 15 kgf/cm2
MV = 18,75 - 93,75 %
SP = 0 - 15 kgf/cm2
FIC-002 COMBUSTION AIR FLOW PV = 0 - 100 %
MV = 0 - 100 %
SP = 18,75 - 93,75 %
FIC-003 NATURAL GAS PV = 0 - 100 %
MV = 0 - 100 %
SP = 18,75 - 93,75 %
FLOW AIR COMBUSTION =>
AIR EXCESS =>
Therefore
RATIO =>
FLOWAIR COMBUSTION = ( FLOWNATURAL GAS * AIRSTOICHIOMETRIC ) + AIREXCESS
RATIO = FLOWAIR COMBUSTION / FLOWNATURAL GAS
AIREXCESS = ( FLOWGAS NATURAL * AIRSTOICHIOMETRIC ) * ( %AIR EXCESS / 100 )
COMBUSTION - DEFINITIONS
FLOWAIR COMBUSTION = ( FLOWGAS NATURAL * AIRSTOICHIOMETRIC ) * ( 1 + %AIR EXCESS / 100 )

8. LOW FIRE – 20 %
HIGH FIRE – 100 %
COMBUSTION - DATA CALCULATED
Column “ %exc = 0 “ the Air Combustion Flow in m3/h and Percent
Column “ %exc ≠ 0 “ the Air Combustion Flow in m3/h and Percent
Ratio is calculated in m3 / m3, therefore it is 13,0715 when Air Excess = 30%
FLOWAIR COMBUSTION = 10,055 * FLOWNATURAL GAS
FLOWAIR COMBUSTION = 13,0715 * FLOWGAS NATURAL

9. RICH MIXTURE
“ EXCESS NG “
AREA
POOR MIXTURE
“ EXCESS AIR COMBUSTION “
AREA
REGULATION
AREA
0 - 30 %
EXCESS AIR
Combustion Curve plotted bellow shown the caractersitics of the burner apllied where we see the combustion curves
and safe limits of operation.
COMBUSTION - GRAPHICS
HIGH FIRE
LOW FIRE

10. #1 #2
#3 #4
NATURAL GAS
AIR COMBUSTION
HIGH FIRE
LOW FIRE
EXCESS AIR = 0 %
CURVE
EXCESS AIR = 30 %
CURVE
There are two Combustion Curves plotted on the Combustion Graphic below where one with 0% excess air and other
with 30% excess air. The área between this two curves is the regulation área or operation rate. Note that are two
limits, Low fire and High Fire too.
COMBUSTION - OPERATION AREA

11. Control Loop with Ranges and Limits of Set-points “ eng. unit “ and “ % “.
COMBUSTION - LIMITS OF CONTROL
18,15 - 90,77 %
4901,81 - 24509,06 m3/h
18,75 - 93,75%
375 - 1875 m3/h
18,75 - 93,75 %
375 - 1875 m3/h
18,75 - 93,75%
18,75 - 93,75%
RATIO = 13,0715
18,75 - 93,75%
375 - 1875 m3/h18,75 - 93,75%
375 - 1875 m3/h
18,75 - 93,75%
375 - 1875 m3/h
18,75 - 93,75%
375 - 1875 m3/h
18,75 - 93,75%
375 - 1875 m3/h
0 - 100 %0 - 100 %

12. COMBUSTION - LOOP CONTROL

13. INTERFACE - SAFE PLC => COMBUSTION CONTROL SYSTEM
The Safe Plc is responsable for Safe-start Sequence, Interlocks and safe shutdown and there are 3 commands between
Safe PLC and Combustion Control Loop, as described below.
Low Fire Command => Command to position FIC-002 Air Combustion Flow Controller and FIC-003 Natural Gas Flow
Controller in Manual Mode with MV in 20 % “ Locked “.
Purge Command => Command to position FIC-002 Air Combustion Flow Controller in Manual Mode with MV in 100 % “
Locked “.
Note that FIC-003 Natural Gas Flow Controller remains in Manual Mode with MV in 20 % “ Locked “.
Modulation => It is responsable to Enable the Modulation of Burner, therefore this Command is responsible for
releasing the Burner operation, that is, FIC-002 Air Combustion Flow Controller and FIC-001 Natural Gas Flow
Controller in Automatic Mode “ Locked “and Set-point Remote “ Locked “ but PIC-003 Steam Pressure Controller is
indiferente.

14. REFERENCES
- NFPA 85 Boiler and Combustion Systems Hazards Code 2019 Edition
National Fire Protection Association
- NFPA 86 Standard for Ovens and Furnaces 2019 Edition
National Fire Protection Association
- IEC 61511:2016 2nd Edition Functional safety - Safety instrumented systems for the process industry sector
International Electro Technical Commission
- ANSI/ISA 5.1 - 2009 Instrumentation Symbols and Identification.
International Society of Automation
- Controles Tipicos de Equipamentos e Processos Industriais Editora Edgard Blucher
Mario Cesar M. Massa de Campos e Herbert C. G. Teixeira
- Combustivel e Combustão Industrial Editora Interciencias
Roberto Garcia
- Instrumentação Aplicada ao Controle de Caldeiras Editora Interciencias
Egídio Alberto Berga