FLAME SAFEGUARn CONTRULS phần 3 pdf

REFERENCES

•All Aboul Boilers,· Marine Engineering/Log, February

1974, pp. ~53.

Bender, Rene J. (ed), ·Steam Generation,· Power,

Special Report, June 1964, 48 pages.

Burkhardt, Charles H., Domestic and CommerciaJ Oil

Burners, 'Third Edition, New York et ai, McGraw-Hili Book

CompanV. 1969,

•Electrode Boilers Make sense.· Enert:t Marketing

cover slory, Electrical World, 8ep1801ber " 1971. pp.

70-72.

Faust, Frank H. and Kaufman, G. lheOOore, (ed.),

Handbook of Oil Burning, Oil-Heat lnslilule of America,

Inc., 1951.

Griswold, John, Fuels. Combustion and FurnactJ$. Arst

EdItion, ThIrd Impression, New York and l.onQ::ln,

McGraw-HIli Book· Company, Inc., 1946.

Guide for the Selection, Installation and Operation 01 Oil

Burning Units, Newark, New Jersey, American Boiler

Manufacturers Association, 1971.

LeJdcon, Steam Generating Equipment, Third Edition,

Artingta'l. Virginia, Arrllilrlcan Boiler Manufacturers Ass0­

cIatiOn. 1974.

North American Combustion Handbook, Firsl Edilion,

Third Prlnling, Cleveland, Ohio, The North American

Manufacturing Co., 1965.

• Power From Coal" - Parts I, II, and Ill, Power, Special

Repor1 t;7; the edllorsi, February, March, and April, 1974,

64 pages.

Schaphorst, W., "ThouQhts About Fir&-Tube vs.

Water-TUbe BoIiEll'S," Power, september, 1972, p. 167.

SImpson., James H., -Conversion of Boilers to Dual·

Fuel Systems." ASHRAE Journal. May 1973, pp. 46-54.

Steiner, Kalman, Oil Burners, Third Edition, New York,

Fueloil & 011 Heat, 1960.

Trinks, W., and Mawhinney, M.H., Industrial Furnactls,

Volume II, Fourth Edition, New York, London, Sydney,

John Wiley & Sons, Inc., 1967.

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.~•

PART I-TYPES OF FLAME DETECTION SYSTEMS

The purpose of a flame detection system Is 10 delect

the presence or absence 01 a safe flame so lhal bJrner 0p￾eration may be conllnuedU condil1ons are safe. and Inler￾fl.4'led if/hey are ~t

FLAME CHARACTERISTICS USED BY FLAME

DETECTION SYSTEMS

All flames have certain characteristics In CCM1VT1OI'l in￾ch.Jding Ihe following:

ProducUO'1 of heal.

Expansion 01 gases.

Prod.!cllon' of by-products

Emission of Ughl Onfrared 10 ullraviolet).

Ionization of the atmosphere in and around the flame.

Flame detection systems have been developed using

several of these characteristics with Ihe flame detect￾ing ponlon 01 Ihe system emitting a signal or originaJing

soma physical action In Iha presence 01 Ihe--detected

characteristic.

Many flame detection systems designed for use on cb￾mastic healing systems use the thermal effect of/he flame

as the method of detection. The detecting eiement must

be healed by Ihe flame for operation 01 the system to con￾tinue. This ,Is true whelher Ihe heat is converted 10 a physi￾cal force, as in a bimetal or hydraulic pilot SElnsor, or to an

electrical signal as in a Ihermocouple. Considerable lime

)s required for the SElnsor to heal, and a similar time pericx:1

Is required for It to cool on toss or flame.

Larger systems (commerCial and industriaQ require

faster flame proving leChnlQJ8S. Fast responding systems

have been devel~ that use Ih!I Ught emitted by the

flame On1rared, Visible, or unraviolet) and the ionization

characteristics of the flame.

FLAME ROD VS THERMAL SENSORS

Flame rod systems dppend on the abllily of the flame to

conduct a CUrretlt when a potenltal Is awlied across il

(flame lonizalion).

The flame rod must be used wllh a suitable electronic

flame safeo;uard control to al11=l1l1'1 the sig'\3l frcm lhe

flame rod. The flame rod usually is used to detect a gas

flame. 011 flames are not generally suitable for the awli￾calion of a flame rcd because of their higher cperating

temperatures.

Flame rod defection systems have 4 ifTlXlrtant advan￾tages over thermal type pilot sensors:

, . .o.u..!.CKAESPONSE TO FLAME FAILURE-The bi￾metal pilOI and the thermocQt.4lJe pilot have a response

lime of up to 3 minutes. Rarely t:bes this type or pilot r&­

spend In less than 1 minute. On domesllc Installations,

wtlere these flame detection devices are normally em￾ploved, low fuel consufT1)tion makes response time less

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critical. Hence the maxil'l"lUTl r8SjX)1"1S8 timings for (his

type at equipment have been eslabllshed at 3 miAJtes.

Thermocouple systems are often used on larger installa·

lions for gas pilot supervislO1 only, as an auxiliary 10 hl\1l

speed rectifying flame rod systems.

On larger installations, this slow response to flame fall·

ure would be dangerous. For exarrple, on a typical larger

Installation burning 600 cLblc feet at gas an hour, wring

the minute or so It takes the flame sensing device to reccg￾nlze (hal the flame has been /ost, at least 10 cLblc teet 01

gas can be Introduced Info the conbJslion ct\atrtle(. kr

suming that natural gas is used. it will take an ad::filional

100 cubic feel of air for Pfoper corrbustlon. ThiS amounts

10 a tolal ot , 10 cLblc feet at cornbuslj~ mixture intra￾wced into the combustion Chamber, 10000Ing for a means

10 be ignited. If delayed ignition takes place, lhal voll.lTle of

fuel·air mixture COUld caUSEI a SElrious elCplosion. For thIS

reason, larger jobs need electronic flame safE9J3rd sys￾tems that have a response timing of 2 to 4 SElcords.

2. PRQVES FLAME AT IGNmON POINT With the bi￾melal pilot and Ihe thermocouPle pilaf, the pilot is essen￾tially proved at the source.

BecauSEI of lhe flexibility of poslllonlng a flame rod, a

pilot flame can be proved aJ the point of interseclion with

the main flame.

3. PROTECTS ITSELF AGAINST FAILuRE OF ITS

CQM.!'ONENT PARTS A bimetal pilot or lherrTlOCOl.ple

pilot Installed on a large burner Is 5/.bjec1 to the intense

heat of the corTt:Iustion c/1aIrt:ler and reflected h&aI frcm

radiant brickwork:. This heal can cause melal f~, lead￾ing 10 Sluggish operation, nuisance shuldJwns, or eYElrt

failure or the sansing element. In some cases, the bimetal

pilot has actually failed In the -on- posllion. This, Of

courSEl, causes a hazardouS condition by allowing lhe

main gas valve to remain open or be opened with no real

proof that a pilot flame is present

Wflh eleclronic flame safeg.Jllfd systems, a checking

circuit can be built into the syslem. If abnOrmal cordlJons

occur In the flame delee/or circuit-SUCh as c:pen circuits,

shOrt circuits. or leakage resistance to grOU'"d-they

simulate absence. not presence, at flame and cause the

system to fail safe.

4. LONG WFE Of CQNSISTE!'lT OPERATION-As

pointed out abOve, the intense heat encot.rIlered on larger

installations causes metal fatig.Je in thermocouple pilots

and bimetal pilots.

With electronic flame safE9J3rd syslen'lS, hoW8\'8r, a

flame rod or photocell is the flame sensing device.. TIle

flame rod normally has a temperature raUng In excess

or 2,000 degrees, so it can withsland the hi\1l 1Iame

lempel'alures.

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FLAME CONDUCTIVITY VS FLAME

RECTIFICATION SYSTEMS

There are 2 basic principles In flame rod delecllon sys￾tems-flame conductivity and flame rectification. Conduc￾tivity systm are, for the mOSI part, no longer used.

Eilher lype of system depends on the ab.lllty at the

flame 10 conduct current when a voltage Is applied across

The ac VOltage ~Iied 10 the electrodes looks like this:

In a 60 Hzsyslem, II changes its direction /polarity} 120

times a second. At one instant, one of the electrodes is

positive, and 1/120 01 a second later it is negaliva. /J.s the

voltage Changes polarity, the flame current (ion flow) will

Change direction.

For a CondJClivity system, the areas 01 the 2 electrodes

(called flame and ground electrooes) are equal and the

flame current between them is the same In both directions.

This is the principle 01 a conduclivity system, When an ac

voltage is applied across the flame electrode and the

ground electrode, alternating currenl proportional to the

awtled voltage flows through the flame.

Because the flame current in a conductivity system Is

ac, this system cannot differentiate between a leakage

current and an actual flame current. It i!$ possible for the.

system to falsely indicate the presence 01 a flame (with

possibly dangerous results) if the flame electrode is

shQrted to grcx.nd through a leakage circuit with about the

same resistance as the impedance of a flame. A carton

depOSit on the base of the flame electrode could form a

very effective leakage path and cause a false flame indica￾lion. (A direct short of low impedance would, 01 course,

make the system Inoperative.)

lhe flame rectification system also uses 2 electrooes,

bul with 1 irfllortant difference-lhe grOUnd eleclrode is

+

y >

6 - ,

L.c . ~UU£NT Feow _£M

,....

2 eleclrlXies in the flame. Heat frem the flame causes

!!lQIawJ~ be~weE!ln the electrOCies 10 colli.de wllh each

other so forcibly as to knock; some electrons out of the ai￾oms, proC:liClng ions. ThiSts called flame Ionization. Posi￾tively Charged·,1ons flow to the negatively charged

electrocJe; negatively charged electrons flow to the posi￾tively charged eleclrode.

always designed to be much larger than the flame elec·

trode (flame r~. FOI" effective operation, the area of the

ground electrode must be atleasl4 limes thai 01 the flame

rod. Usually, the ground electrode will be the burner head.

Because of the difference In electrode size, more cur￾rent flows In one direction than In the other. When the

flame rocl is positive, more current flows.

:----f------'------'~

When the flame rod is negative. less current flows.

6

•

~I ...•-. ...~_"~O· ..,,

'pO ".

With the current In one direction so much larger than

the current In the other direction, the resultant currenl is,

effectively, a pulsating dIrect current which operates the

electronic netWork. The flame relay pulls in, indicating the

presence of a flame and allowing the bUrner sequence to

continue. The larger the ratio of ground area to flame elec·

trode area, the greater the flow of current in the proper di￾rection-in other words, a reclified current.

Only the ionized path through a flame and the different

sized electrodes can provide the rectified current required

for the operation of the electronic network in a rectification

system. Should a high resislance leakage to ground occur

in the flame circuit, it senc:ls an ac signal Into the network,

and lhe system shuts down safely. The rectification sys￾tem does re'Cognize the difference between a high resis·

tance leakage to ground and the presence of a flame.

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