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APPLICATION NOTE 

1

YS170 Boiler Combustion Control

OVERVIEW

The boiler combustion controlsystem is designed to maintain aproper air/fuel mixture under varyingload conditions and within safelimits. The system should providenearly complete fuel combustion asefficiently as possible. Discussionshere are limited to oil and/or naturalgas fired industrial boilers. Controlsystems that perform these functionscan be classified as positioning or

metering systems. Positioningcombustion systems use mechanicallinkages between the fuel controlvalve and air damper to adjust forthe proper ratio. Both single pointpositioning and parallel positionsystems are common.

ID Numbe07-07

SINGLE POINTPOSITIONING 

One of the most commoncombustion control system on oil

and/or gas fired boilers is the singlepoint positioning system, commonlyreferred to as a jackshaft. Refer tothe drawing below. The fuel valve(s)and air damper are mechanicallylinked to a common rotating drivemechanism controlled by a masterdrive unit. Simple and safe, butrequires a constant fuel pressureand BTU content. Maintaining theoptimum air/fuel ratio throughout the

load range is difficult.

PARALLEL POSITIONING 

This system uses a master pressurecontroller to position the fuel valve(s)and air damper. An air/fuel ratiostation is placed in the air dampercircuit. Advantages are low installedcost and simplicity with only onecontroller to manipulate. Thedisadvantage is that it is not fail-safe.There is no inherent fuel cut back or

air tracking the fuel if either drivewere to fail.

PARALLELMETEREDCROSSLIMITEDSYSTEM

In this combustionsystem, the air andfuel flows aremeasured and arethe processvariables of twocontrollers. The airand fuel controllersreceive a setpointvalue from a mastersteam pressurecontroller. In thefuel controller, thisvalue is compared to a modified air

flow signal in a low signalselector. The lower valueis selected as the remote

set point of the fuel flowcontroller.

Conversely, the pressurecontroller output iscompared to the fuel flowusing a high signalselector in the air flowcontroller. The highersignal is selected.

Refer to the SAMA diagram below.This configuration is called a crosslimited or lead-lag system. If anincrease in firing rate is needed dueto an increasing steam demand, theair flow leads the fuel. As the loadcurtails, the fuel decreases first,followed by a decrease incombustion air flow. A low limiter isused in the air controller to preventthe air flow set point to be reducedless than 25% of full span. Thisminimum setting is required byNFPA code and ensures safeoperation in the event of loss ofsignals.

FT

√

Fuel Flow

Low

Select

K

T A

 ∫

FuelValve

f(x)

>

FT

√

f(x)

Air F

K

T A

 ∫

f(x)

<

Firing RateDemand

HighSelect

A

25%

AirDamper

From Pressure

Controller

The YS170 Dual LoopProgrammable Controller is thelogical choice for boiler combustioncontrol. The controllers can be

easily implemented to perform thefull metering cross limited systemconfiguration. The YS170 fuelcontroller can transmit the fuel signalas an analog output to the aircontroller for the cross limitingaction. Conversely, the YS170 aircontroller can transmit the air flow tothe fuel controller. The limiting

Combustion Air Fuel Valve

Jackshaft

Automatic Adjustment ofBlower Side of Shaft

Single Point Positioning

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APPLICATION NOTE 

2

SUMMARY A load index is created from thefiring rate demand signal. Theamount of excess oxygen varies

inversely with steam demand, i.e.,low loads have a higher excessoxygen. The operator can apply amanual bias to the index set pointwithout changing the index slope orshape. Refer to the SAMA diagrambelow.

signal selectors reside in eachcontroller.

YS170 Programmable Loop

Controllers can be installed in avariety of applications. As discussedhere, boiler combustion control canbe implemented easily and costeffectively. Multiple analog anddiscrete inputs/outputs permitinteraction with burner managementequipment. YS170 controllers havebeen installed in dozens of boilerinstallations. Users are glad theyselected this versatile easy-to-usecontroller.

Using discrete inputs and outputs,the fuel controller can track the aircontrol MANUAL status and logiccan be incorporated to not permitautomatic fuel control until the aircontroller is in AUTO. Thesesignals can be hard-wired betweencontrollers or an optional peer-to-peer digital communicationsnetwork can transfer analog andstatus information over a singlepair of twisted wires.

Diagnostics of flow inputs allowsthe controllers to transfer the fueland air outputs to safe conditionsin the event of transmitter failure.The controllers can be forced intoMANUAL mode (holding the lastcalculated output) and discreteoutputs are available to activatean annunciator or other alarmdevice.

YS170 - The Logical Choice in Combustion Control!  

The output of the percent oxygencontroller is adjusted for gain andbias and applied to the combustionair flow signal. Additionally, low andhigh limits are used to ensurecontinued operation during analyzeroutage.

AT

K

T A

 ∫

A

K +

Percent

Oxygen

FT

√

f(x)

Air Flow

X

Firing RateDemand

f(x)

Operator

Bias

To Air

Controller

If the YS170 controller were to fail,an integral hard manual stationallows immediate backup. By raisingthe front panel assembly of theinstrument, the manual station canbe activated by a slide switch. Aknurled thumb wheel can beadjusted to provide the appropriateoutput to the control element. AFAIL discrete output can be wired toan alarm device.

The YS170 can be programmed toperform two independent P+I+Dcontrol algorithms. This is ideal toincorporate the combustion air andoxygen trim controls into oneinstrument. The multipleinput/output capabilities permit airflow, percent oxygen, and firing ratedemand inputs. Function generatorsresiding in the microprocessor-basedcontroller allow calculation of the

load index described above andcharacterization of the air flowmeasurement. By implementingthese two control strategies in onedevice, the purchase cost is loweredand installation costs are reduced.

FLUE GAS ANALYSIS 

Flue gas analysis is a method ofproviding a more precise control ofthe air/fuel ratio. The intent is tominimize the amount of excess airrising through the stack which resultsin lower operating costs by usingless fuel. An oxygen trim controllercan be used to bias or trim thecombustion air controller.

ID Number07-07