flame safeguard control phần 9 ppsx

pressure may cause simultaneous adjustment of fan speed and 'damper opening to change the airflow, and pu~ speed and valve opening to control the flow of fu&: all.. a change in steam pre

FIG 8- PRESSURE DROP VS CAPACI'N FOR A 'NPICAl BUlTERFlY VALVE MAXIMUM OPENING

ANGLES SHOWN 'ARE USEO AS TRIAL SETTINGS WHEN AOJUSTING VALVES FOR HtGH FIRE

sure regul!'llor oullel pre,ssure by O~ in \Of<: to allow for the

additional pressure drcp across the 2 inch SSQVs

This requires estimating a new pressure dfcp for the

pressure regulator SSOVs and Firing Rate Control, plus

repealing the sizing procedure

SIZING CHART FOR FIRING RATE

CONTROL VALVE

Butterfly Valves are otlen used for Firing Rale Control

Valves SinCe a Butterfly Valve ctleS no! provide light clo­

sure, a safely shutoff can!rol valve (SSOV) must be used

l4)slream

Inaddilion 101M buUerlly control valve size, we neeO 10

know lhe mallimum QQenino angle (in degrees) U'OOd as a

trial selting when adjusting a BUllerfly Valve to hi~ fire

In our ellample the eslimaleO pressure drq:J across the

liring rate control is 6.5 inches wC wilh a capaCity of 10,500

cfh at standard conditions

a If SSOV estimaled <md actual pressure drop are lha

~ Mark the intersection oflhe estimated Butter­fly Valve pressure drq:J and capacity, X on Fig 8 Use Ihe valve size and opening angle indicaled IYy

lhe nearest slanted line below X In this case, ar;ply· ing the estimated pressure drq:J results 'n a 2 inch valve wHh a mallimum opening angle at 45 degrees

b If SSOV aclual pressure droo is less than lbe esli· malad pressure droo

Add lhe pressure drq:J difference (0.8 inch we) to the pressure Orq:J available for the Bullerlly Valve

0.8 + 6.5 '" 7.3 in we

Mark the intersection 017.3 in wc and lhe capacity

y, Use the valve size and opening angle indicated by lhe nearest slanteO line below In lhis case, we have a 2 inch valve with a 40 degree opening

V5055 VALVE SIZING NOMOGRAPH

Non, II ",,",.1 ' "1'0.,10< " " 0."1 ,,~ o J Dr I G) ''0'" ··C~" ~ '0

,k'Q , , _ on• •nd , ' " "t ,~ .1'r " D'QQ·· c -" _, I,.," ,.<h "" '0

0 ',n 0 ',,,,,,.I,, _ ··S «'« <;",,' II_ )-l"

r:o, ',Qn" '0 ' ""'d "Clh G_ fh :· Dr I;., ~ ''0"' "B.,n", Clh," 'h,,,,,"

D.,.,,,,,, 'J) ,.Q ·" .<,(OO.~.,,'"'··O' '"_ , "oeM 01 101 " d I,,,, @ , 1

So ,'· _ _ "",n' "II, bo,_.n , , .>0 ,i,,,.

, _ '~"""'''Q_ aI t,"" (!)

·· C",·· ,.t ,'h' t" ,

The correct size V505 5 Industrial Gas Valve can be quiCkly selected usinog this nomograph The nomograph is available in pads at 25 under form number 70-86.27

Tha 1oll0winog 8)(amples show how to use the VS055 Valve Sizing Char1

2 Valves Piped in Series

Regulator Outlet Pressure - Piping Pressure Drcp

= V5055 Inlet Pressure 27.6 in we - 1 In we = 26.6 in WC

2 Determine tile oullel pressure for V5055 The outlet pressure equals lhe inlel pressure minus the pressure drcp across the valve

71·97558-1

NOTE: Since we calculated the corrected capacity

under standard conditions, 10,370 cfh we can skip

lines 1 and 2 In lhe 1,'5055 Valve Sizif'l;l Chart

direcllol"6

3 Draw line 3 from 14.6 inches we on the Outlet Pressure

scale 10 12.0 Inches we on the Pressure Prep scale

Since Qutlet Pressure scale is In psi un/Is, multiple 14.6

inches we by (he conversion 1aclor1rom IheAPPENPIX

10 gel the correct psi unl!

14.6 )( 0.0361 = .527 pst Draw line 3 from 527 psi on QVllel Pressure scale \0

12.0 Pressure Droo scale

4 Draw line 4 trom 10,370 cfh on the Burner CFH Gas

Flow caoacity scale through intersection of Ml and line

3, fa the valve site scale 12 inches)

EXAMPLE 2

Determine the 1,'5055 Valve sizQ from lhis job's

5j:lElciflcat ions:

Heat Conlent of Gas 2,500 Btu/cu ft Inlel Pressure Available at 1,'5055 13 Inches wC

Eslimated Pressure Drop

OJtlet Pressure al 1,'5055 6 Inches wc

••

SOLUTION

calculate cfh required

7,500,000 Btuh - 2,500 Btu/cu ft '" 3,000 cfh

1 Draw line 11rom 0.64 on "SpecifiC Gravity Correclioo~

'scale 10 3000 on "CFH Gas Flow· scale

2 Draw line 2 tram 1.53 on -SpecifiC Gravity SCare,~

through interseclion of line 1 and M2, 10 • Bl,rrner CFH

Gas Flow." This is the adjuslecl gas 110w

3 Draw line 31rom I) inches we on Outlet Pressure scale

107 inches we on the Pressure Droo scale Since the Outlet Pressure scale is in psi units, mulliply 6 inches

we by Ihe conversion taclor In (he APPENDIX to get psi units

CONVERSION TO STAHDARD CONDITIONS

STANDARD CONDITIONS

-"'061 valve sizing charts provide coordinates under a

sal of slandard conditions This allows systematic selec­

tion of gas valves for various applications These standard

corditlons are:

1 Capacity - cubiC; feel per hour lcfh) ConverSion 10r­

mula on page 14

2 Specific Gravity - 0.64 sp gr

3 AWtude ~~sea level

CONVERT CAPACITY TO 0.64 SPECIFIC GRAVITY

4 Pressure Drop - , inch walEir column (lin we) across valve

These condilionsare seldom fOUnd on an actual job To use the valve sizing charts, we must convert the jd:l condi· lions in this BKample to the equivalent cfh gas rating under the standard conditions

CONVERT CAPACITY IN BTUH TO CFH

CftJ =

Btu/cu

CONVERT CAPACITY TO 0.64 SPECIFIC GRAVITY

HOW TO USE CHART

Listed valve·capaciTy ratino;:; ~re based on O.6'llp gr 9U W})en the required cfh ""pa";I~ is kncwn (or 911 of other

specific <:IfavitY it can be convened to the 0.64 'p g< tQw~al~nt by use of correcl multiplying faclor Obtained from

this charI Example; A nfve capacity of 2670 cfh blwd on 0.72 lp <:If gax is ~C1ui~d_ Whit valve capacity based on

0.64 sP \IT ","S will be nlCluved? Solution: On wrtSul sc.&Ie of chart find 0.72 sp gr From that point, move horitonlillly

10 ri'ilhl 10 intenecl the cUJVe; lhen move Itn.ighl down 10 bottom seale and !ud lhe conversion factor, 1.06

Mwtiply the 2670 cfh by the conve";"n factor: 2670 dh" 1.06 = 2830 d'h

When the raled capaeity of ill ulve ror 0.64 'P \IT gas is 'b1own ilS equivalent Cap'C)ty for 'l"" of other <peeific

gravity may be determined by dlvidinq the raled capacitY by Ihe conversion faclor ElIa~ The rated capacifY

of II cenain valve is 3500 cFh What is ill; equivalent capacity lor 0.72 lp \IT gas? Soluticnc 3500 cfh

~'" 3301 cFh

I

i ,

To find cFh itt 64 op gr, mwtiply cfh at "x" ,p, qt,

iD find "Fh at "J<" sp qt, divide cFh at 64 "p gr by conversi\ln faclor

CONVERT CAPACITY TO SEA LEVEL AL TITUOE

CONVERT CAPACITY TO SEA LEVEL ALTITUDE

HOW TO USE CHART

When required valve capacity in cfh at sea levrl is known lilt equivaltnl rtquinod capacity at hiqher elevationJ r.1ay

be dele,mined by use of correct multiplyinq factor obt.tintd from litil chari

Example: A valve capaaty or:sooo dh ill required It 5 ltvel Whll would be the required capacity at an elultion

~t above sea lewl?

Solution: On vertical scale of chart, find 4550 ft From lhat point, move horizontally to riqhl to intersect lile curve;

lilen move straiqht down to bottom scale and read lile conV'trslon flctOt, 1.087 Multiply Ihe 3000 cfh by lil factor;

3000 cfh ill: L087 = 3261 cfh

NOTE: To find the capacity at sea level when the capacity at I hiqhet eltvation is known, ~ the known capacity

by the conve,.,.jon factor 3261 cfh = 00

PRESSURE DROP CONVERSION

REOUIRED PRESSURE UNIT POUNDS

Absolute Pressure = Gauge Pressure + 14.74 psi

tf lhe available pressure drop across the valve was 1/4 25 psi x 27.7 (conversion factor for inches wc) =

psi, the equivalent pressure in inches wc would be: apprOXimately 7 in wc

'.­

I N T R O D U C T I O N - - - ­

DEFINITIONS

1119 firing Crate is Jhe combustion rate II is the rate at

which air, fuel, or snair-fuel mixture Is supplied loaburner

or furnace It may be expressed in volume (cubic feel of

gas or gallons of oil), weight (tons of coal), or heal units

(Btu's) sul=9lied par unit lime (usually per h::lur) FirinrJ rate

control (or combustion control) is simply a means of regu­

launQfUel supply, air supply, and the ratioofairto1uelsup­

ply according to load dernard

PURPOSES

Automatic Mng rate conlrols are offen used 10 $illl'Jify

operatioo and to relieve operators from tedious monitoring

dUlies However, their primary purpose is lor economy

To proouca Ihe mosl economical ep8ralion, lh&cOIltrol

system must mainlaln the air-fuel ratio at an optimum

value over the entire load range Usually, The system must

also control other imp::msnl facfOfs such as steam pras­

sure, furnace draft, waler level and steam lemperature It

is virtually impoSSible for an operafor to maintain the pre­

cise control necessary to achieve the gteatest econany

FACTORS AFFECTING THE FIRING RATE

Although the actual neaullQ load pfaceQ on the plant is

the primary consideration other faclors also affect the fir­

ing ral& Since ducts, pipes, and flue passages absorb

much of the lotaf heal produced, opening and closing

lhem in ditf~rent palterns varies the heat loss immensely

The pickup demand or the furnace or boiler also con­

sumas part of the heat output When a bJrr.er syslem is

started, the mass of metal that comprises me furnace or

bOiler absorbs a great amount 01 heat before all)' enters

the system, and it also radiales a portion of the heal 10 lhe

surrounding surfaces Finally, lhe efficiency of the fUrnace

or boiler ilself has a bearing on the amOl.n1 at fuel that is

burned The efficiency depends to a great exlent on the

he-at transfer qualities of the boiler or furnace

LIMITATIONS ON THE FIRING RATE

Turndown is lhe ralio or tna maximum firing rate (high

fire) 10 Ihe minimum /iring rate (Jaw fire) al which a burner

will operate satisfactorily It is also expressed as the range

of firing rales over which satisfactory combuslion can be

obtained For ax~le, lhe firillQ rale 01 a burner with a 4

to 1 turndown range can be varied from ils maximum (100

pl3rcent) clown to 1{4 at ils maximum (25 percenl), A high turndown ralio Is particularly desirable for batCh-type fur· naces or others lhat are shut down periodically A high fir· ing rale can be usecs to Mal lhe furnace rapidly after il is started up again After the flIrnal:e is heated up, the firing rale can be turned down 10 normal

Flame t::Jow-off limilS the maximum firing rate A Ilame moves away from a burner when the velocity at lheair-tuel mixture is greater than the velOCity Of the Ilame front (flame propagation rate) Blow-cff often resuHs in the flame being

Drat!: is lhe movement of air into and through a CON'Ous­

{ion cflamber, brEl8ctling stack, and chimney

Natural draft results from the difference in density of the heated air rising through the slack or chimney and the

COOler displacinQ air Mechanical draft is crealad try mao chinery, such as a fan or blower Types of mechanical draft are forced and indUCed draft

Forced draft is produced by a fan or blower located at lhe inlel air passage 10 the combustion charrt:lel' Induced

draft is produced by a partial vacuum within the corrous­

tion chaiTtler, created by a fan al the outlet at the

cMlTtler

Natural draft depoflds on ma", variables $Uch as 1hE! temperature of the atmosphere, height Of the stack, direc· tion and force Oflhe wind, and other environmenlal condi­tioos Blowers or fans supply a conslanl draft lhat is

independent 01 these conditions Therefor'll, mechanical

draft is used as the main source of air

Every tiring rale and type 01 fuaJ fequirss lhe prcper

amount 01 draft for best results 1he draft that is needed

also depends a lot on Ihevolume ofth'll cO'T'Ouslion cham­

ber Da"l)f¥s in tl"l& air passages are use::llo control the

draft Darrpar p::lSilions are varied as tne firing rale is varied

FIRING RATE CONTROL METHODS - - - ­

In large plants, mothodl; of regulating the firing rate are

(1) fland, (2) base load and (3) automatic In hand regula­

tion, a fireman allends \0 a ballery of boilers and/or fur­

naces He adjusts lhe valves and da,rrp.:lrS manually to

301

keep the pressure ancVor temperature constant In base­

load regulation, most Of a gr~ of boilers and/or furnaces are cperated at a ste&'t, hil1l firing rare but one or fTIOfe are operated at a variable rate \0 handle peak loads In

71-97558-1

automatic regulation, an automatic firing rale control sys­

tem ts used to provide smoother reQ.Jlatlon and beUet

cOl'l'b.Jsl:lon Some of the automatic systems used will be

discussecUn this section

AUTOMATIC FIRING RATE CONTROL

SYSTEMS

Different arranoemeF'lts of elements are used 10 pro­

vide firing rate control Initially, the syStem is Iriggered by a

disturbaAce In the'CCintrolled variable, Such as steam pres­

sure in,aooiler or wall temperature In a furnace The initial

disturbanca then causes a sequence of adjl lStJTlentS, In

parallel or in cascade (series), to control the various parts

of the syslem

pressure may cause simultaneous adjustment of fan

speed and 'damper opening to change the airflow, and

pu~ speed and valve opening to control the flow of fu&:

(all) In a cascade system a change in steam pressure

might lnlllate a change In cCllTbuSlion airflow, and the

dlange in airflow might lhen adjust the rate of fuel flow

Funher cascading might occur it the airflow is changed by

an adjus1manl of Ihe induced draft, and lhe resulting

ct\aIlgEl in furnace pressure is then corrected by a change

in the forced draft

A firing rate control system usually falls inlo 1 of 4 gen­

eral classes-paranat, serieas·fuel, sories-air, or calorime­

ter syStem Each System may not fall exactly Into 1 of the

classes, but l1'I8y be modified to be a combination of 2

classes Usually, one particular syslem (or combination)

will be the best lor a panicular plant based on economy,

type 01 fuel safety required, or lhe type of cperallon

In each system, a Change in the controlled variable

(such as sleam pressure in a boiier plant or temperature in

a healing furnace) generates an error signal The error Sig­

nails then senT 10 adjust the fuel flow and air flow To c0m­

pensate for varial.ions in fuel condlion or lor tube foUling in

boilers a ralio regulator (see belOW) is included lO-make

adjustmenls in the air-fuel ratio The ralio regulator maybe

as Sirl1Jte as the linkage between 2 valves, it may be a

valve-type device, or it may be a complex eleclrooic

device

PARAl.l.El CONTROl SYSTEM (FIG 1)

In the parallel control system, the error signal is sent si·

multaneously to adjust both fuetflow and airllow The ratio

regulator may ~ in either circuil It is generally in the cir­

cuit having the greater capacity, so the capaciTy of theboil·

er or furnace will not be reduced if the ratio of fuel 10 air is

decreased Thus, ilthe blower or fans have greater capac­

ily than tha fuel feeding equipment, the ratio regulator is

pul in the airflow circuit

The parallel syStem is the simplest and requires the

least hardWare II is nol dependent on signals indicative of

the actual flOW of fuel or air Therefore, it is most used in

plants where the fuel buming rate would be difficult to

measure directry, such as a plant using grate firing of coal

SEAlES-FUEl CONTROL SYSTEM (FIG 2)

In the series-fuel control system, lhe error signal con­Imls only the fuel flOW Measurement of the fuel flow then prcdJcBS a Signal that controls the airflow The fuel flow must be readily measurable, which Is true for most gases

liq,Jid fuels, and even pulverized coal A series sysIem is generally considered to be very effective in controlling the air-fuel ratio because the flow of one is used 10 determine the flow requirement of the other

Because airflow Is the dependenl variable, air will not Increase if fuel is not available, wen t~ the SyStem may be calling for more heal This Is advantageous 00

equlJ:YT19nt fired wllh a by-prcdJct fuel that may be in short supply at limes When lhere is a fuel shortage, the system prevenls the InlrcducUoo of large quantities of air, which would deCrease the effectlve heat El\l'9n further

However, if the air SlWly fails or airflOW' Is reduced ex­cessively by a fan failure, there is a danger of filling the fur­nace with unburned fuel sinCe the error signal can still cause the system to continue fuel flow, Airflow failure will decrease the heat release in the furnace, further aggravat­

.I,!lg the situation by causing a demand for even mora heat and fuel Electronic circuits may be used to overcome this problem by prOViding fuel cutback If eirflow ~g; fuel flow

by a predelermined amounl Also, a flame safeguard con­trol can be used 10 Shut clown the burner; the flame will go

OU! when the air supply drops too low to su~n

combus-RATIO REGULATOR

FUEL FLOW

ERROR SIGNAL

_.J_-,

I ALTERNATE I

'LOCATION •

OF RATIO • : REGULATOR

lion Many burners also have airflow interlocks 10 shut

them down if airllow decreases to a predelermined value

SERIES-AIR CONTROL SYSTEM (FIG 3)

"the Series·alr control system overcomes thQ disadvan­

taQ3 of the series-fuel system; cutback of fuel is lDb8rEl!J(

upon a decrease In airflow The error signal controls only

IhQ airflow Measuremenl of thQ ail1low then produces a

the fuel flow will be decreased accordingly preventing the

furnace 1rom bEIIng filled with unburned fuel This system

is vel)' :satisfactory and IXlPUlar when fhe ail1low can be

readlly measured

CALORIMETER CONTROL SYSTEM (FIG 4)

In lhe calorimeter control system, the error signal CCll'l­

trolsoOnly lhe fuel flow, and measurement of the steam now

produces a signal thai is used 10 control ail1low, The

amount 01 steam produced is proportional to the firing rale

(or fuel flow) so fuel flow is measured indirectly by meas­

uring steam flow Because stearn flow is substituting for

fuel flow, the ratio regulator is in the steam flow clrcuil

It lakes a definite amount ot air to burn a quantity of a

given fuel that will release a certain amount 01 heal If the

rate of heat release is known, the amount of air required

can be determined The amount of steam prodJced is pro­

p::>rtionalto the rate of heal release, so a measurement of

steam flow can be used to control the airflow Since a

measur<lment at steam flow is indirectly a measurement at

heat, the system derives its name from lhe calorimeter, an

apparatus for mensurfng amounts of heat

Steam leaVing the SuPerheater is usually held wilhin

close limits of pressure and temperature so its enerlJll

content per p::>und will not vary appreCiably Therefore,

each p::>und of steam carries the same amount of heat

energy, so steam flow is proportional to the firing rate It steam pressure or temperature vary el'lOUl;1l 10 causa appreciable error, compensating devices can be added to the system to correct the steam flow for standard conditions

As with the series-fuel system there is the danger of ac­cumulating unburned fuel In the furnace if the air sowly fails A decrease in airllow will decrease the heat release, which will deCrease lhe steam !low, causing a further re­duction in airflow Meantime, the system is calUng for more heat and more and more fuel is being &4JPlled As in thQ series-fuel system eleclronic circuits may be used to OYElr­come this problem by prOViding fuel cutback; but in this case, steam flow instead of fuel flow is CClI'1l)3.red with airflow

AIR-FUEL RATIO REGULATORS

Besides complex electronic systems (whiCh are be­

yond the scC9'3 of this reference), there are 3 basic types

at air-fuet ratio regulators - Pl area control (2) pressure control, and (3) flow control All 3 actually keep the flow rates at the air and fuel proponlonal They differ In the ba­sic prq:'l8rty that is controlled direCtly to achieve a cO"lStanl air-fuel ratio

AREA CONTROL (FIG 5)

A simple mechanism is used to cause the opening area

of 2 valves, one controlling airflow anclone controlling ruel flow, to vary in proponion fa each other For 2 varves with identical characteristics, a mechanical connection be­tween them will produce directly proponional rT'lO\IeITl9Ot

If one valve rolates through a 45 degree angle, the other will also rolate through a45 degree angle: and If this move­ment causes a 25 percent chaf1Q8 in the flow rate of ona fluid it will cause a 25 percent change in the flow rate of

AIJ'lfLOW

ERROR SIGNAL

FUt:L FLOW

RATIO REGULATOR

AIRfLOW

I

RATIO REGULATOR

"

FIG 3-SERIES-AIR CONTROL SYSTEM

71-97558·1

ERROR STEAM

I

RATIO REGULATOR

MfQ Co., Qeveland, Ohio.)

1M other If the vatve characleristics are not the same, the

now rales of the air and fuel will match only aI one lXlinl,

resulling In a lean mixture aI some firing rates and a rich

mixture at others

Two·rotary valves on a common shaft may be used, but

a ~de arrangement with a parallel ann or Hype

lInkage Is preferred because the linkage can sometimes

be adjl.lSted to correct tor different characleristics In lhe 2

valves one or, preferably, both valves should have man­

ual means of adji"lSling their cpenings for setting the air·

tueI ratio Inilially

The L.9Cilream pressurfil!S 01 both air and fuel must be

constant, reqJlring an air blower with a constant pressure

characteristic and a fuel pressure regulalQ{ ahead of Ihe

fuel control valve For all, ils temperalure must be con­

stanl fa avoid changaS in Its flow rate 0Je 10 viscosity

variali~

PRESSURE CONTROL (FIGS 6 AND 7)

The resistance to flow downsTream trom the clYltrol

valves Is a constant in boIh the fuel and air lines of any

bum&l' system The flow through a constant resistance is

prc:portlonaJ (0 the &q.Jare root 01 fhe pressure ditterentlat

across lhal resistance By keeping the air and fuel pres­

sures equal (or proportional), their flow rates are kept pro­

IXlrtional thrOU\1lOUI lhe entire turndown range of the

burner This type of ratio regulator works with constant ar·

eas and variable pr8SSUres, which is lTlOre accurate and

usually less expensive lhan the area control msthod

Wl1EIn this type of ratio regulator is in the fuel line, il is

cross-conneclad 10 the air Ii.ne by a small impul6B line, and

vice versa It c.onsi&ts 01 a globe-type valve in which Ihe

plug is-aitached to and movecl by a diaphragm 1'he pres­

sure on one side of the diaphragn is proportional to that 01

the air line, and the prassure on the other side is prC4'O'­

tional to lhat Of the fuel line If the pressures are not the

same, the urCialance causes the diaphragm to move The

diaphragn mc:wes the valve plug, adjusllng the flow 01 fuel

(01' air if the regulalor Is In Ihe air nne) until the pressures are the same

The air gas rallo regulator shown In Fig, 6 is connecled

In the gas line An air I~lse line (usually atxlul '/4 InCh pipe) Is conneclecl 10 the CClfTPartment below ttlE! dia­phragm The air pressure In the i~16B line Is adjl.lSteclto achieve the desired alr gas ratio If the maximum air pres­

$Ute Is more than the maximum available gas pressure, a

tieed"f is used to permit a cenain amount of leakage1rom

the 1l"rpU16B Une 10 ensure tM correci ratio at hiQh firing rates If the i~lse line were left open to the almosphere,

111El reguJalor would prOduce zero gas, used In ~ng

(zero governor) types 01 premixing burners The spring is used for counlerba.lancing the weights of lhe shaft, plug and diaphragn assElITbly so that the diaphragm 1Joals freely The balancIng diaphragn prevents t.pStream gas

pressure acling on the underside 01 the valve plug from lift­ing the plug

When b.Jrning all In low-pressure air atomizing burners,

it is desirable 10 maintain an oil pressure several times

greater than lhe COrTtx.lslion air pressure The air-oil ralio

regulator shown in Fig 7 prOduces a oownslream oil pres­sure proportional to Ihe pressure elrertecl on the tc:p side of 1M air diaphragm The j~lseair pressure p lShes down

on the air diaphragn, lending 10 move the valvs shaft as­serTt:lly downward The downstream oil prassure pushes

up on the oil diapnragn, tending 10 raise the valve shaft assembly The space between the diaphraQ'Tls is ventoo

to the atmosphere If the area of 111EI air diaphragm is 12 limes [he area oflhe oil diaphragm, the oil pressure has to'

be 12 limes the air pressure in order to balanc9 it The oil valve opens wider untillhe 12:1 pressure ralio is attained

An adjustable lension spring balanCes the weight of the diap,ragms, shaft and valve plug

FLOW CONTROL (FIG 8)

A llow control system actually measures Iheair1/aw and

fuei flaw and conlrols the flow 01 one of them accordingly

A conslriction, such as an orifice, is plaCed in both the air

and fuel lines The pressure differential across the con,

striction measures the flow thtough that line Both pres'

~re differentials are Iransmitled to some controliing

d9vJce that adjusts Ihe 'low 0' either the air or fuel to main­

tain the desired air-fuel ratio

The alr-gas rallo re(JJtalor shown in Fig 8 conltols the

flow of air An Input coolrol valve controls the flow at gas

upstream and downstream Il'Jl=Iulses from an orifice in the

gas line act on qJpOSite sides oflhe gas diaphragm Simi­

larly', impulseS from the air line act on the air diaphragm so

as'to oppose the action on lhe gas diaphragm lhe result­

ing movement of the shaft between the 2 diaphragms is

amplified and transmitted hyc!raulically (by the hydraulic

,"

L'''(

+OAS a"L"""""'"

O, 9>tR "

'"'".:O"'-~:=:?:::§~SPR, ",

Q.R''''''

FIG 6-iYPICAL AIR-GAS RATIO REGULATOR

USING PRESSURE CONTROL, (From

Combustion Handbook try North American

Mfg Co., Cleveland, Ohio.)

relay and crank type cylinder} to the butterfly valve in the air line The butterfly valve moves untillhe now (pressure dif1erenlLal) across the air orifice balances out the flow across the gas orifice The orifices are usually sized so that equal pressure drops correspond to the correct ratio

01 flow rates The ratio regulator also has a manual adjust·

ment tor selecting the desired air-fuel ratio

,

FIG 7-iYPICAL AIR-OIL RATIO REGULATOR

USING PRESSURE CONTROL (From

Combustion Handbook by North American

Co., GJevs(and, Ohio.)

"R ORIFICE BUTTERFLY VALVE

FIG a-TYPICAL AIR·GAS RATIO REGULATOR USING FLOW CONTROL (From Combustion

Handbook try North Amen'can Mfg Co., Oeveland, Ohio.)

FIRING RATE

TIle size and aw1icalion 01 a burner determines how II

iG fired Ditlerent rnel!nds 01 flrino result In various

prepurge configurations An a,wrovall:lOC¥ may require a

specifiC prepurge COnfiq.Jtalion for a given awlica1ion

The swilching required fa achieve a certain prepurge corr flq.Jralion is providedby the proper model 01 a flame sale­

guard pr~ammlng conlrol programmer)

METHOD OF FIRING

CONnNUOUS FIRING sinal1~t burners, such as dtyQrs are usuarty let! on continuously There afe also continuous InciJslrial fur­

naces Ih~ which the work loads are conveyed al a constanl speed

ON-OFF FIRING TIle most economical method of firing is sirrq::lly to turn the burner on when heal is required and oft when there is

er"lCJl 91 heat This is the most efficient method because

the air-fuel ratio can be sel exactly at its optimum value, an::! it remains constant while the burner is on Fewer con­

trols ars requlrec:l, so It is also less expansive initially Orr 01'1' filing is used mostly on warm air turnaces in residential and corrmerclal buildings, and also for residential awn­

aness such as hot waler healers

HIGH-LOW FIRING

Hig-.-Iow firing provid9s 2 firing rates, high fire and low fite, according to "eat demand The high fire rale is 00­

tained by simultaneously PJs!tioning lhe dampers 10 aO'nil more combustion air and the fuel valve to admit more fuel

(In a 2·slage bur~r, a secord fuell/alva is cpened 10 aO'nit more fuel.l High·low firing is used on large, commerCial and industrial burners to provide a safe Ii~t-off A higher firing rare is r8QUired than Is safe for on-off firing, so the burner must light off at low fire to al/Oid a possible eKp1o­

sion High temperalure blast turnaces, such as brick kilns

use this melhod of firing They are f~ed as high as possible for a short time, and then turned down to low fire for a wnile This melhQj has been found 10 save 10 to 15 per·

Cent of the fuel required 101 modulalac:l firing

MODULATED FIRING Modulaled firing provides a gradually varying firing rate belween high fire and low fire, according to heal de­

marlCt The burner lights off at low tire 1l1en a controller varies ll\EI firing rate as necessary to k~ the conI rolled variable (usually prBSSUre or temperalurel at the controllEi"

sel PJlnt Modulaled firing provides precise coolrol, but it

is the least efficienl because it is ditficull 10 keep the air­

fuel ratio conSlanl over the entire mlX1Jlating range It is used on industrial furnaces or bOilers 'or applicatlOflS re­

quiring close pressure or temperature tolerances, such as chamical process heaters used tor tleal-trQ8ling metals

PREPURGE CONFIGURATIONS

AJ:proval bodies require a pr~rge period, prior 10 oorner Iighl-otl, to remove any fuel or fuel vapors that may have accumulated In th$ butner or stacks By incrQ8sing lhe amounl of air lTlOYed through the combustion chafT', ber, the possitillly of a rough IiQtll-cN or explosion clJa 10 accumulated fuel Is decreased This is why a ~hlgh fire" prepurge Is usually required tor hi~-Iow or mcx)Jlalec! methOOS at tiring The ~high fire" Is really a l!!i~().I!I.!§Ir

since lhere is ro fire during prElPJrge What it really means

Is that the darrper controUing corrbustion air is c:penac:lto

lis maximum ~ition to move> as much air as possIble

same as the high lire position of the damper during the run period when the burner Is firing

It would seem Ihal the longer lhe prepurge period the beller However a burnar system starts ~ when there is a call for heat Be10re the burner can be igniled to provide heat, the prepurge period aclually COOls the system OOwn ev81'l more Therefore, [he beSt pr~rQ8 coofiguratioo for

a gi"'en system is one that provides sufficierll airflow in the shortesllime, taking into consideration ~roval 00Cl'f re­quirements, I~ 01 equipmenl, and cost 01 conlrOls Prepurge types and firing rate sWi~Chjng necessary 10 meet awroval 00Cl'f req Jlrements for vaiiOU'S methods of flrinQ are summarized in Table I Insurance comp9.r'1ies in­sure bolh prcperty loss due to explosions and dowr'1time causing loss of productior'1 It they requIre safely controls thal causa urnecessary down lime they pay, Since they reqUire cerlain firing rate contrOls, you can see the imper·

tance of IheS4 controls to the safety of the Ourner system

STANDARD ON-OFF PREPURGE

No firing rate SwilCtling is rElQJired The amount 01 air admiUed during the prepurge pariod is Itle same as the amount of combuslion air provided during the run period witr the burner firing

OPEN DAMPER PREPURGE Firing rale swilChlne in fhe programmer drives a 2-posi­lion damper molar open 10 its hi~ fire p.:lsiliOn near the beginning of the prepurge periOd, aUOW's it 10 close to lis

low fire p.:lSihon before ignition trials, and drives il back

open 10 high fire for the run period

LOW FIRE PREPURGE This Is !in outdated type 01 prepurge that is seldom used anymore in flame safeguard awlicalions Firing rate switching in lhe programmer keEpS a proportioning damp­

er motor at ils low fire position until after i'7lillOn trials, when switching releases the motor 10 modJlate UI"lCeT the control of a series 90 pressure or temperature conlroller

TABLE I-PAEPURGE CONFIGURAnONS

1-wireb

UL Modulating

(2-stage firing)

or 4-wire Fire Fire

R4181A1042,-A1059

& UL - Underwriters l at:x:lratories Inc requirements

<&"-Factory Mutual requirements

IAI-Industrial Risk Insurers (formerly F.I.A.) requirements

b Firing rate motor mUSI close by itselt (spring-return) when power is removed

C Outdated type; seldom used anymore

d Low fire switch stops sequence before ignition trials until low fire position is proven

ill Same as LQw·High-Low except additional high fire switch stops sequence near begiMing of prepurge until hi1tl tire p:lSil'ion is proven '

I Switching returns firing rale motor to its low tire position when the run period is OVer

LOW-HIGH-LOW PREPURGE

Firing rate swllchlng in the programmer drives a pro­

p:lrtioning damper molar o~n to its high fire position near

the beginning of the prepurge perioo, closed to its low fire

position before Ignilion trials, ard releases it to modulate

under the control at a series 90 pressure or t9ITperature

controller lor the run period The programmer has provi­

sions for a proved 10w-firl;l-5tart interlock Oow fire switch),

which stops the sequence before ignition trials until the

damper motor has closed to its low fire position

LOW-HIGH-LOW PROVEN PREPURGE

This is the same as lOw-high-low prepurge, excepl that

Ihe programmer also has provisions for a proved high fire

interlock (high fIre switch), which slops the sequence near

the beginning of the prepurge perioo until the clarrper m0­

tor has opened to Its hit;jl fire posillon

PROVED LOW-FIRE-START INTERLOCK

A proved low-fire-start interlock Is r9QJired for hl~-klw

Of modulated burners (except those with low fire prepurge, which is outdated and seldom used anymore) This inl9f­tock must ensure that allliring rale controls are In the low fire position (a firing rate no! eXCeeding 33 percenl of the

maximum) betote the burner can be iglited This is usually accompllshed by means at an auxiliary switch {low fire SWitCh) mounted on the drive Shaft or the firing rale motor The auxiliary switCh is wired inlo the safety conlrol (pro­grammer) circuit

PROVED HIGH FIRE INTERLOCK

In addition 10 a proved low-flrl;l-5tart interlock, Factory Mutual and In:iJstrlal Risk Insurers (formerly F.l.A.) also reQUire a hit;jl fire swilch 10 prove thai the ~r is tully cpen wring prepurge fo ensure that 9I"lCll.Q1 air Is moved

!hrOU<tl the combustion charrber The hit;1'l tire swllch Is usually also an auxll1ary switch on the firing ratelOOlOf'

FIRING RATE SWITCHING IN PROGRAMMERS -­

By means of simpllfiod schematic dlagrams and timer

seq Jente charts, we wlU show how fhe PfOlJfammers ae­

CCllT'9liSh firing rate switching For simpliCity, only the cir·

cuits and conlaCts nec~ry 10 de6Cribe the basic

operation are shown For a complete description ollhe 0p­

eration cif ~ actual programmer, Jefer 10 the Detailed Orr STaling ~nce 01 an A4140G1007 (1orm 6D-0443) or an

R4140L"'-4"7 (form 60-{)4.44)

TImer contacts are oesicnaled M1B, M3A, etc Termi·

nals L1 and l2 are the -hoi- and ~common· terminals 01 ttle power SLWly External oevices are shown in 001(95

Operation 01 the firing rale motor Is deScribed in detail In the seedndpar1 of this section covering Honeywell Control

A 2-posJl!on, spting·t~um llcluator is used to control the dar'rper When p:rNer is removed, a spring en the ac­

hator Shaft m~chanicaJJy returns lhe aclualor to its closed

position

Prepurge starts on a cal! for heal As saonas proper air­

"flow is e!tab\ished, Ihe airtlow switch close's, er'oergili~

',' the damper actuator on terminal 10 (through the airflow

switch, jurrper, and M108) TIle actuator drives the darrq>­

er open ~o high lire ~ition)

At 30 seconds, M10B opens The darrper actualor is d8-energiz.ed an::! the sprinQ-relurn drives Ihe darrper clQSEld Al ~ ~, lnlern.al programmer switching s:ops the timer until the low fire swilCh closes, proving thai the daf1lJ9r is in ils low fire position before ignition trials

Ignition lrlals begin at 42 seconds Tl1e darrper stays closed during most of this period Near lhe end of ignition trials, al ti9.5 seconds, M108 closes The damp€lf actuator

Is -.ergi.!:ed anddrives Ihe damper open (10 high fire PJSi

& ~T""To AM , ,~ 5"' v ""''' lOON IUAC S.CC"""

& , -A , T ,.UA"lICI.""ffN U 'NAU" .O

A IN CA""~ ACTUATM ,S"IlC,

11A'·I'O:I'''''''._'''~·.I'''''N AUU.HOII " D rocoorr"OL tNE

FIG '0- TYPICAL TIMER SEQUENCE CHART

FOR '-WIRE (OPEN DAMPER) FIRING RATE SWITCHING

tion) for the run period The timer steps for !he run periOd, and I'" damper stays open during the run period When the tun period ands, the limer starts The caU for

heat ends and power is remoVEldfrom terminal 10, deener·

gillng the acll.la~or The spring-return drives tM damper

closed

3-WIRE FIRING RATE SWITCHING

Figs '1 and 12 show the firing rate sWitchi~ at old

fire and lOW fire proving circuits is nOI shown because it is similar to the opera.tion for 4-Wire switching, whiCh follows Notice lhat the prQgramrner SwiTching (Fig 111 doeS not

awly power to lhe motor, but cnly shorts between molar and c()l"(roller terminals, or opens them

A proportioning, sptfng-return or spring·biased motor

must be used to control the damper When the red ·R~

(comrrn:n) lag of the circuli fa a spring·rerum motor is

FIG 9- TYPICAL 1-WIRE (OPEN DAMPER)

SWITCHING

FIRING RATE SWITCHING,

FIG 12-TYPICAl TIMER SEQUENCE CHART

FOR 3-WIRE FIRING RATE SWITCHING

opened, a spring on the molor shaff mechanically returns

the molor 10 its closed position The spring·tiased molDl'

has a spring allached\o ilsbalancing relay When the "R"

leg is coened, the spring pulls the balancing rela.y to its

closed position, and the molor is than electrically returned

to ils closed posilion

As usual, prepurge starts on a call for heat At 2 sec­

Or'lOs, M98 closes, shorting between terminals 10 and' 1

on lheprogrammer and between terminals Rand B on the

firing rate molor The motor dlives the damper cpen 10 its

high fire posi\ion At 7 seconds internal prCJgrammer

switching stops lhe timer unlillhe high fire switch closes,

prOVing that the damper is open 10 provide maximum air·

flow during prepurge

At 42 secondS, M9B opens, opening Ihe • R· leg of the

motor circuit The spring (or spring-bias) drives the motor

and darnpel' closed At 60 seconds, internal prCJgrammer

switching stops the timer until the lOW fire switch closes,

proving that the damper is in its low fire position before ig­

nition trials

Ignition trials begin at 72 seConds, and the damper

stays closed during mas! of this period Near Ihe end 01 ig­

nition trials, at 101 seconds, M9A closes, shorting be-­

t een terminals 11 ard 12 on Ihe programmer, and

connecting the A terminals of the molar ar.d controller

The firing rate motor is released to mcx:::Iulate under conlrol

of the series 90 coni roller The limer slops for Ihe run

period

The firing rale motor modulates according 10 heat de­

mand during lhe run period When the run period endS,

the timer slarts Al 118 seconds, M9A opens, cpening the

• A· leg 01 the malar circuit The spring (or spring-bias)

drives the molar and damper closed

4-WIRE FIRING RATE SWITCHING

The 3-wire firing rale switching Circuit restricts users ~o

a firing rate molar .ith a spring-return or spring-biasecl

balanCing relay Therefore newer A4150G ard Lmcdels,

and all A4140G and L models, have 4-wire firing rate

switching circuits These models also allow Ihe use of m0­tors thai are electrica.11y driven in both directions

FiQS 13, 14, and 15 show the firing rale switching 01 a typical R4140L Programmer Operation 01 both the higl fire and low fire proving circuits Is shown (Fig 14) R4140G I'T'ICX1els have provisions only for the low 11re switch Prepurge starts normally on a call 10r heal lhe timer motDl" (Fig 14) Is energiZed IhroU(1l MJA Al 3 seconds, M10B opens ard Ml0A closes (Fig 13), shorting between terminals 10 and 11 on the programmer, and between ler·

minals Rand B on lhe 1iring rate motor The motor drives lhe oamper open to its high tire position At 10 secondS,

M3A opens (Fig 14), stc:«>ing the timer molar When the high tire switch closes, prOVing thallh8 damper Is cpen 10

provide maximum airtlow during prepurge, the timer motor

is energiZed thrOUQh tha high fire $Wilch ard M5B; prspurga continues

At 20 secondS, M3B closes (Fig 14), bypassing the high fire switch At 26 seconds, M10A opens (Fig 13) cpening the A leg 01 Ihe motor circuit If a spring-return or

spring-biased molar is used, it starts to drive the damper

FIG 14- T'(PICAL OPERATION OF HIGH FIRE

AND LOW FIRE PROVING CiRCUITS

FIG 15- TYPICAL TIMER SEQUENCE CHART FOR

4.WIRE FIRING RATE SWITCHING

closed At 30 seconds, M10S closes, shorting between

(ermlnals l ' and 14 on the programmer ~hrough M10S

and MBS) and between terminals Rand W on lhe firing

rate motor If an electrically driven motor is used il drives

the damper closed Al 5' seCOl'lds M5B opens (Fig 14),

st,,",lng the timer motor MSA closes When the low fire

swllch closes proving (hat the damper Is in its low fire IX>"

slim before Ignition Irials, lhe limer molor is energized

Of the low fire switch is connecled in the alternate Iocallon the limer molar 16 energizecJ through M3B, the low tire switch and M5A.)

Ignilion trials begin al 60 seCO'ldS and the damper slays closed during most of this period Al 66 seconds M3B opens and M3A closes (Fig 14), bypassing oolh Ihe high fire and low fire swilCheS The timer motor keeps run­nil1Q and Ignilion trials continue, Near the and of ignition trials, al95 seconds, M8Bopens and M8A closes (Fig 13),

shortinQ belween lerminall!l" and 120n the programmer (through M1 OB and MBA), and conneCllng the R terminals

of the motor and controller The firing rale molar Is re­

leased to l1"lCllislale under control at the sefles 90 control­ler The limer st~ for Ihe run period

The firing rale molar modulates according 10 heal de­

mand during lhe run period, When Ihe run period ends,

the limer statts AI '12 seconds, MaA opens (Fig 13), openll1Q lhe R leg o1lhe malar circuil 11 a sprinQ'relurn or sprinQ"biased molar is used, Il starts 10 drive the damper closed At '16 seconds, MBS closes, shorting be/ween terminals l ' and 14 on Ihe programmer (through "-1108 and MBSl and between lerminals Rand W 01 lhe firing rale motor If an electrically driven molar is usecI il drives the damper closed

HONEYWELL CONTROL S E R I E S - - - ­

The electrical series of a control-20, 40, SO, 60, 70, 80,

SO-identifies the vqllage leve! on which a conlrol ~r­

ales and Ihe switching or modQ of operation it provideS

Cqnlrollers provide spst or SIXft swilching, and switch

eilher line or low v01laga (For further information on con­

trolla'rs, refer 10 Ihe Flame safeguard Reference on Auxil·

lary Equipment, form 70-8120.) We can see that by

combining Ihese 2 factors, we can identify 4 dil1erent S&

rias -spsf line, spst low, SIX1I fine, stXfI [ow These 4 series

and their characteristic controller switching actions Bfa

Series 20 (low voltage, spjI) is not USEId in flame safe­guard applications series 50 (mechanical) is represented

by V50S5 and V51 E gas valves, which have separa:e ac­tuators, and by the VS060A Supervisory Fuel Cock Series

70 (electronic) Is represented by name detectors and

flame signal amplifiers Series 80 ~ow voltage SPSI) is rep­resented by a few flame sa1eQ.Jard primary Controls

(RA890'S) indicaling simply thai they have a low voltage contro circuit and by a tew SQlenoid-opllrated gas valves Series 40 and 60 (line voltage), used in on-otf or hiQ1·low Systems, will be discussed briefly Series 90 (mociJ~ting\

is the hearl at mosl firing rale control systems, so it will be

described in detail Table II summarizes Honeywell control series

SERIES 40 (LINE VOLTAGE, SPST) CONTROLS ­

Series 40 controllers ard limits used in flame saf9QJS.rd

awlications are commonly temperature or pressure sens­

Ing devices They provide ::.pst (singla-pole, single-throw)

swaching In olher words, they make a set at contacts to

start SyStem operption, and break the sel of contacts 10

stop' SyStem operation

Se(ies 40 controll(Jrs and limits are rated for direct

switching of line voltage loads They commonly use either

mercury switch or open contact switChing mechanisms

Open contact mechanisms must be snap-acling 10 pre­

vent arcing across the contacts

FINAL CONTROL ELEMENTS

series 40 final conlrol elements - solenoid valves, re­

lays, contactors, and motors- have only 2 positionS They

remain In the normally de-energized position until pow·

ered; at which time, they g) 10 Ihe energized position,

When pOwer is Interrupted, they return autanallcally 10

the de-energized position A normally closed series 4()

solenOid valve, ror e)llample, opens when powered, re­

mains open as long as power Is aw1ied ard closes as

soon as power is cut off

311

Fig, 17 shows a series 4() relay CC{11roJ!ed by a series

40 cootroller with a series 40 limit TIle relay shown pro­

vides spst switching; it could provide more COf1'l>lex switching control, The S91les desiglalioo, series 40 re1ets

10 the control circuit voltage ard control Circuit switching

action If the sWitching action 01 the relay shown were

spdt, the relay would no! be desiQr1aled serles &l eYen

tl10uQh it provides line VOltage spdt: swilching, It wwld re­main series 40 after the Coil circuit TIlere are, in fact, no series 60 relays and no series 60 soJ9I"lOld valves since

lhese devices are, by nature, on-off (spst) controls

de-energized Fig 18 shOws a series 4() motor under cen­

Irol 01 a series 40 controller

The limit switch cuts off power to the motOl' windirv;>

When the motor reaches ils full open po:sillon The braJlfI

'rVinding holds the armature agaInst a brake shoe 10 hold

71·97558-1