flame safeguard control phần 9 ppsx

lOll 1000 10.000 100 000 1 000 000 -'~·11!tl'·'-I'!·I·I'!·!'!·I·I'!II'O~! -!'!r,I·I'I·,'1'!'I'~I'I'~I'I'!II'I'!'I'i'I'III'I'I~I'I"'I'!'I'I' , -' , i I . I 'I -or" u 'r '~ii . r!;li VTlJ VI: • I " U ,mm~~ z la, ., ., -~ ~ -\. tit ~: , + ~ ~ " Q , " 1 ~+-c -t-wu- .; ,i ! I 1/, ' . " : , • , ." •• 1 T -, ~ '"1 r-j-' XiUUM <Ii 'I OI!ENlIto, • , , y If "'7G L l!:i . , 5-67UI ~678~1 ;: J ~S6!a9' 2 l4S67890 JJ 4 , " 1000 I 10.~GO \00,000 , ,ooo,oo~ "" , PI2 L < 4 ' CAPACITY. IN CFH WITH064 5P GR GAS n +, I ~/II; ~/ , FIG. 8- PRESSURE DROP VS. CAPACI'N FOR A 'NPICAl BUlTERFl Y 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 10,500 294 cfh at standard conditions. a. If SSOV estimaled <md actual pressure drop are lha ~. Mark the intersection oflhe estimated Butter- fly 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 " " , 'r , i VAL\I~ sin ,. J , , ,r ., 'Rf~1U1U ORO' "' " ,. we ., LO ,. ; ,. ; . , 1 ,. l4UOO V5055 VALVE 100.000 SIZING CHART J'!C'~'C CIlA"'TY COIlAtel'ON , , • , • , " • lUDD "LlHi:< · , 0 .IlUSUllli <'II G , (I · , HD~ " , , " 0 , • " • · • , • " 1.000 , t ·• • , • , , , , RfOUIRW > fOA",,, TID" T,,,, 01 0 • • ;00 · , " s ",«o ·", _. 0 Bu,"o' G. Cl~ . _ · • , , " rc'"· 8",,0, ~ O'olf,l, \I5"~S Inl.' ' '.1 ,,",, , · , , • V~5 Ou'''' Ro""" P,."y'"O,""'oco I 0",.1' " " '" . • , _ ," , "', __ '" ,-, __ ,n w' 'P ,"" d.". ,",." "~, 111 ,,, •• ~ , ., " '00 , V".,5< "."ho. Yl o.~ ,". So>o."f",.~ No .,,5055 I INITRuCT<CNI 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,"" (!) ."d 101 '0 ' ·· C",·· ,.t , 'h' t" , 0"', 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. EXAMPLE 1 These are the specifications from our original example: Type of Gas Milmd SpeCifiC Gravity 0.72 sp gr Gas Flow Capacity 7.200,000 Btuh Heat Content of Gas 800 Btu per Cu. fl. Estimated Pressure Drcp across V5055 Single Valve 12.2 in. we 2 Valves Piped in Series 5.5 in. we 295 For this chart, we need to know the inlel and outlet pressure for the V5055. 1. Determine inlet pressure for V5055. When 2 valves are used, lind the inlel pressure of the first valY'6. From Fig. 2 we see thai the V5055 inlel pressure equalS the pressure regulator oullet pressure less lhe piping pressure loss from the pressure regulator outlet to the V5Q55 il'l- Ie!. Find the piping lOSS flQJre tram Fig. 3. It we haY'6 5 leet at 2 incll pipe, tM pressure drcp will be 1 inch wc. 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 , • vuv£ SIZE , "' PAE~~lJAE ORO' ", " '" Ito ltuoo 101.090 "' V5055 VALVE SIZING CHART EXAMPLE 1 SPH'FIC G~""'TY COIIIIECTlO • Dc TUT PRESSURE IPS' ~"UGEI " '" " , , r ~ t ~ ~ • 8 f ~ , , ," ,, ! "' • , • , 1 .00i '" ". ". " o " " "". · , Inlel Pressure - Estimated Pressure Drcp '" Oullet Pressure 26.6 - 12.0 = 14.6 in, we 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:lElciflcations: capacity 7,500,000 Bluh Type of Gas Prcpane Specific Gravity 1.53 spIJr Heat Conlent of Gas 2,500 Btu/cu. ft. Inlel Pressure Available at 1,'5055 13 Inches wC Eslimated Pressure Drop across 1,'5055 7 inches wC 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. 6 In. we )( 0.0361 '" .216 psi 4. Draw line 4 from adjustecl gas flow on Bumer CFH Gas Flow scale (hrou~ Ihe inlersection of line 3 and M1 to the Valve Size scale (101/2 Inches). , , "' V5055 VALVE VALVE SOl( P~E~SU~l IOU" SIZING CHART OAOP "' EXAMPLE 2 , SPlt."C " ., eRA V"" CORR!tno ,. , ,. , ~ " ; ,1 , OuTLET PRESSURE 'PS' GAuGll '" • , r ,., ,. ,, , ". ! · ~ , • ~l ~ , 1.000 • , , , 297 71·97558-1 APPENDIX 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 = Bluh Btu/cu. ft. 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 i , i I ::~~ !:_-:-4 Y I / - , c~ i i , I 'D'r '" I , , I I , / I '''C_c ~, __ +-_-'-_, _~ +_-~.L ~ c ,, I I iii I I , I /' I I ~-_-_ i ''',-_c ~-~·~~~i~·-!C-I-+-I +,I_-_-_-+,,'-_-_-+-!C-_ ;;-;'I/i:~~,::=~~~:~_-+-,-_-_- , r: L Yi I I ,c - . I Y i I , L.Ce~I~"'.=i=~ ~~;;-f-/ t- O.7:>f I-t t r t t J t- , ! /./ I I 1 o~o l +-7"l"'~ i-++ + + + + +- I ! , ! i I I , "'f +-!i-!-~ +i -UE:, =:1 =:i =='f l~:~:: ;, I i L l._ "" ~O"' ~'~'-~-'Oc", Cl.o 1.1 u' n I '-:> i c, i c ' 'c, '0 CO""V[ltSIOo,r faCTORS " To find cFh itt .64 op gr, mwtiply cfh at "x" ,p, qt, by conversion factor. iD find "Fh at "J<" sp qt, divide cFh at .64 "p. gr. by conversi\ln faclor. >.11 298 CONVERT CAPACITY TO SEA LEVEL ALTITUOE 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 1 087 ~ cfh, 7000 60QO 2000 1000 , I r., , , ' : : J' . . , , , H , ~ , - ,=' I J_.LLL _c.; : +-i , , ' , - ,~ , - : ,. , ' : +- o 1.000 I.oZO 1.060 1.080 1.100 CONVER910" FACTO" 1.120 1.140 ,",60 Ja 299 71-97558-1 PRESSURE DROP CONVERSION 10,000 cfh 10,000 cfh == 5,000 cfh ; 4 in. wc 2 This conversion IS nol otten needed because rf105t valve sizing charts list a range 01 pressure drops In inches When lhe pr9SSUre drCll is raled in pounds per square wc. I-fOWever. if a chart is not available, divide the cft1 by inch (PSij. kilOQfams per SQJ8re centimetre (kscj, or other the square root 01 the available pressure drop in inches pressure units, mUltlplV by known unit by the conversion wo. factors below. CONVERSION OF PRESSURE UNITS (Convert by multiplying value in known pressure units by laclor lisled under required pressure unit) KH0WN PRESSURE UNIT REOUIRED PRESSURE UNIT POUNDS PER SO. IN. OUNCES PER SO. IN. MILLlMHRES 0' MERCURY KILOGRAMS PER sa. CM. INCHES 0' W,o.TER INCHES 0' MERCURY FEET 0' W,o.TER CENTIMETERS 0' WATER Centirnel8J11 of Water 0.0142 0.227 0,735 0.000999 0.394 0.0289 0.032& - F", r:A Water 0.433 6.94 22.4 0.0305 12.0 0.883 30.5 Inche6 of. Mercury 0.491 7.86 25.4 0.0345 13.6 1.13 34.6 Inches or Water 0.0361 0.578 1.87 0.00254 00735 0.0833 254 Kilograms per sq. em. 14.2 228.0 735.0 394.0 29.0 328 HXXlO Millimetres 01 Mercury 0.0193 0.308 0.00136 0.535 0.0394 0.0446 US Ounces per sq In. 0,0625 3.2:4 0.00439 1.73 0.12:8 0.144 440 Pounds per sq. irI, 16.0 51.7 0.0703 2:7.7 2.04 2.31 704 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. ' 300 PART I-FIRING RATE (COMBUSTION) CONTROL INTRODUCTION 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 firing 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 extinguished. Flashback limils the minimum firing rate. A flame movesbaCk ""rough a burner land possibly back fa theair· fr el mixinQ lXlinl) when the flame propaoation rale is grealer lhan the velocity of the entering alr'fUel mixlure. DRAFT CONSIDERATIONS 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 condi- tioos. 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 chamber. 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 regulation, a fireman allends \0 a ballery of boilers and/or furnaces. 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 system 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 provide firing rate control. Initially, the syStem is Iriggered by a disturbaAce In the'CCintrolled variable, Such as steam pressure 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. For exa~le, in a parallel system, a change in steam 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 calorimeter 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 circuit 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 con- Imls 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 ex- cessively 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. ~ AIRFLOW , , FIG. 1-PARAl.l.El. CONTROl. SYSTEM. 302 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 sig~ that controls the fuel flow. Thus. if the airf/(lW fails, 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 ac- cumulating 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 re- duction 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 OYElr- come 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 ERROR SIGNAL I AIJ'lfLOW ERROR SIGNAL FUt:L FLOW RATIO REGULATOR AIRfLOW I RATIO REGULATOR I "., FIG. 2-SERIES-FUEL CONTROL SYSTEM. 303 I fUEL flOW " FIG. 3-SERIES-AIR CONTROL SYSTEM. 71-97558·1 [...]... M941 However,lhe Super Mod Motors cannot replace Slav" m0 lars in master slave systems usIng motors with mechani· cal balancing relays (such as the old style M941); they can replace the master motor For fUrther infotmation, reter 10 the awrC(lriate instruction Sh9&l: M954A,D - form 60-2413; M955A,C-torm 60-2414 325 71 97 558·' TABLE V-FEATURES OF M954 AND M955 SUPER MOD MOTORS FiElD SPRING MODEl M954A... awJiad The actuator determines the method Of f1ring-a V4055 is used for on-off tiring, a V4062 tor high-low tiring, ard a V9055 for modulated tiring The V9055 Actuator can be controlled directly from a series 90 controller (except Ihe T 991 family), just like the M941 Modutrol Motor The V9055 has a solid state balancing relay, so it is not sensi tive to normal burner vibration Valvalactuator corrtlina... M954 AND M955 SUPER MOD MOTORS FiElD SPRING MODEl M954A M954D M955A M955C - AUXIl.IARY SELECTABl.E RETURN SWITCHES MOTOR STROKE No No None 90 or 160 Yos Yos degrees 2 , None ConlinuOJSfy adjustable from 90 to 160 degrees P RING UTURN - FIG 52-M954 AND M9SS SUPER MOD MOTOFlS FIRING RATE VALVES - - - - - - - - - - - - - - A firing fale vallfe controls the amount 01 ~ entering lhe corrtJuslion chamber,... en or close tile firing rale motor as much as desired, which facilitates set up and linkage adjustments MODULATING LOW LIMIT A modulating low limit is used 10 keep the t9fT'4)9ralure althe heating medium (air hoi water, or steam) aoove a 3 19 71 -97 558·1 '-ow l, ,r... usa:lto prevent exces sive 19l1lJ9rature 01 the heating medium The cperation of a high limit in a series 90 circuit Is very similar (oa low limit, 320 - COHTROUlR LIMIT ACTION CONTROllER I I€) I' CONTROLLER , ", LIMIT Ill'H liMn I (" " MOTOR I "" , ,,'" I FIG 43-MOOULATING HIGH LIMIT FIG 44 - TWO-POSITION LIMITS IN SERIES 90 CIRCUITS limitaclion Simply... FIG 22-USING SERIES 60 CONTROLLERS AND LIMITS IN PLACE OF SERIES 40 314 SERIES 90 (MODULATING) C O N T R O L S - - - - - - - drop in temperature, indicating a need for heat, ltle lollow ing memory device is onen used: series 90 controls are commonly used in 1lame safe guard systems to provide modulating control 01 a burner's firil"lQ rale In 2-lXlsi\ion conlrol systems a final control ele ment, once... resented by a few flame sa1eQ.Jard primary Controls (RA 890 '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 . Water 0.0142 0.227 0,735 0.00 099 9 0. 394 0.02 89 0.032& - F", r:A Water 0.433 6 .94 22.4 0.0305 12.0 0.883 30.5 Inche6 of. Mercury 0. 491 7.86 25.4 0.0345 13.6 1.13 34.6. sq. em. 14.2 228.0 735.0 394 .0 29. 0 328 HXXlO Millimetres 01 Mercury 0.0 193 0.308 0.00136 0.535 0.0 394 0.0446 US Ounces per sq In. 0,0625 3.2:4 0.004 39 1.73 0.12:8 0.144 440 Pounds. ,. , ' : +- o 1.000 I.oZO 1.060 1.080 1.100 CONVER910" FACTO" 1.120 1.140 ,",60 Ja 299 71 -97 558-1 PRESSURE DROP CONVERSION 10,000 cfh 10,000 cfh == 5,000

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Số trang	37
Dung lượng	1,3 MB