Fired Heaters

CONTROLLING FIRED HEATERS © Walter Driedger, P. Eng., 2000 May 20. [email protected] First published in Hydrocarbon Processing, April 1997. This Office 97 file is available for download at http://www.cadvision.com/driedgew/. INTRODUCTION. The purpose of a fired heater is very simple: To add heat to a proces process s fluid. fluid. Its repre represen sentat tation ion on a process flow diagram is also very simple. But, of course, fired heaters are among the the most most comp comple lex x piec pieces es of proc proces ess s control equipment. equipment. Each furnace furnace is, after after all, at least two pieces of equipment in one. one. Firstly, Firstly, it is a specia speciall variant variant of the shell and tube heat exchanger since its purpose is to exchange heat. Secondly, it is a chemical reactor in which fuel and air und undergo rgo extre xtrem mely exoth othermic rmic reactions to produce the required heat.

PROCESSF SFLUID

In previous articles of this series 1, 2, 3, 4, the process aspects of controlling a piece of equipm ipment were prese resent nted ed befo before re dealing dealing with protec protection tion and safety. safety. This time the topics will be reversed: In the case of fired heaters, it must be safety first!

TIC

TV

SAFETY. If fired heaters had not been inven invented ted and were were being being propo proposed sed for the the firs firstt time time,, I woul would d prob probab ably ly say, say, Fig. 5-1. A Fired Heater as Seen by a Process Engineer "You've got to be kidding. That thing thing will will blow up in your face the first time you throw a match match in it." However, at least a half a billion gas fired heaters are in service service around the world (according (according to the American American Gas Association). Association). Most of them are operated by people with no technical experience whatsoever; few heaters blow up. Still Still,, the the averag average e domest domestic ic water water heater heater is not in the same same league league as a hydro hydrogen gen reform reformer er furna furnace. ce. The The fact fact that acciden accidents ts and disas disaster ters s are as few as they they are, are, is due to the the long long experience the human human race has in dealing dealing with fire. A million years, years, I'm told. told. For the last century, century, this experience has been embodied in various codes and standards that have been written into law and are en-forced by inspectors around the world. THE CODE. The most popular, or notorious, of these codes in North America is the NFPA 8500 5, 6, 7 series series issued by the National National Fire Protection Protection Associati Association. on. These These have been considera considerably bly updated in recent years, years, especially in terms of of clarity. Nevertheless, Nevertheless, there is still the problem of interpre interpretation tation.. The code is not at all easy to read as it combines combines many facets facets of construc construction tion,, Controlling Fired Heaters

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Page 5-1

instrume instrumentati ntation on and operation operation in a single document. document. Not only that, that, but the code 5 contains the following disclaimers: disclaimers: It is not possib possible le for these these standa standards rds to encom encompas pass s specif specific ic hardw hardware are applications, nor should these be considered a "cookbook" for the design of safety systems. and: This standard applies to boilers with a fuel input of 12,500,000 Btu/hr (3663 kW) or greater. greater. This standard standard applies applies only to boiler-fu boiler-furnac rnaces es using single burners firing: a) Natura turall ga gas only only as defi define ned d in Chapt hapte er 3. 3. b) Othe Otherr gas gas with with a BTU BTU val value ue and and cha chara ract cter eris isti tics cs sim simil ilar ar to to nat natur ural al gas gas.. c) Fuel oil of No. 2.... and: Furnaces such as those of process heaters used in chemical and petroleum manufacture, wherein steam generation is incidental to the operation of a processing system, are not covered in this standard. What is an engineer to use for a guide when the furnace is not a boiler, but a feed heater; does not exceed 12½ million Btu/hr, but is only four million; does not burn natural gas as defined in Chapter 3; but refinery off-gas with a high hydrogen hydrogen content? Despite the disclaimers, disclaimers, the NFPA series is still an excellent guide to the instrumentation instrumentation and control of any furnace. FUEL GAS FIRED, SINGLE BURNER FURNACES. The NFPA standards deal with a variety of fuels, fuels, both oil and and gas. There There is one standard, standard, NFPA NFPA 8501 5, for single burner furnaces; and another, NFPA 8502 6, for multiple, multiple, independent burners burners in a single furnace. furnace. The discussion that that follows follows restricts restricts itself itself to fuel gas fired, single burners. burners. NFPA NFPA standards standards have been followed followed as much much as possi possible ble and have have some sometim times es been been exceed exceeded ed by addin adding g compo componen nents ts and and contro controll functions where the special requirements of process control make it advisable. The diagram on the following page, Figure 5-2, shows the in-line instruments typically installed on a burn burner er fuel fuel gas gas trai train. n. Diam Diamon ond d symb symbol ols s with with an "I" in them them refe referr to I/O I/O of the Burn Burner er Management Management System (BMS). FUEL GAS SUPPLY INSTRUMENTS PCV-1

The fu fuel ga gas su supply re regulator is is on only re required wh when th the fu fuel ga gas pr pressure must be be reduced reduced in two stages. stages. This is often often the case case in refinery refinery service service.. See Figure 5-3 for typical regulator settings.

PI-1

Every regulator should have a gauge so that the operator can set the regulator properly and so that he can know that it is doing its job.

PSV-1

Many standard fuel gas train components have an upper pressure limit of approxim approximately ately 100 100 psig (700 (700 kPag). kPag). Failure Failure of both both PCV-1 PCV-1 and PCV-21 PCV-21 would would overpressure the fuel gas train if the supply pressure exceeds the rating of any downstream component. component. In such cases cases provision for pressure pressure relief is required. required.

Controlling Fired Heaters


Page 5-2

Note that it is not unusual to consider double jeopardy in burner safety analysis.



Page 5-3

I 4 P 4

E 3 A 4

R E D T E A R I E F H

D I U L F S S E C O R P

H H 2 S 4 T

L L 9 S 2 B

V

V I

V

L L 9 S 1 B

E 1 T 4

A

V A G 5 B 4

Y 8 1 B

I 8 P 2

H 5 S F 3

R O T I N G I

C S 3 Z

O F

V

I

O 6 S 1 Z I

V M

T N E V

M

O C 5 5 S S 2 Z 2 Z

V M V I

O F

M

O 5 C 5 S S 1 Z 1 Z

C F

E 3 F 3

6 V 1 B

V

T N E V

O F

V 4 B 2

V 5 B 2

C 6 S 1 Z

V

C F

V 6 B 2

V

O S 3 3 Z

I 8 P 1

C 6 S 2 Z

V I

V

V I

H 7 H 2 S P

O S 6 2 Z

3 V F 3

V

V F 3 V

L 6 L 3 S F

V I

C F

I

S A

5 G 4 F

4 V 1 B

5 V 1 B R

V V

I

V

V M

C F I

C 4 S 2 Z

I

S

O 4 S 2 Z

L L 2 S 2 P

V

N I A R T S A G N I A M

C F

C 4 S 1 Z

I 1 1 P

1 V 1 C P

N I A R T S A G T O L I P

S 4 Y 3

M

Y 4 V H 3

N I A R T S T N A E G V N I A M

I 1 P 2

V 1 C 2 P

V

I

O 4 S 1 Z

O F

2 V F 3

H H 4 S 3 V

T 3 P

E 3 F E 2 F 3 V 1 S P

/ // I 1 P

T N E V

V 1 C P

S A G L E U F

R I A N O I T S U B M O C

V I

H S 1 3 D P

Fig. 5-2. Burner Fuel Gas Train In-Line Instruments



Page 5-4

FE-3

This is is th the pl place to to pu put fu fuel ga gas me metering, ifif re required red. NFPA pu puts the fu fuel ga gas meter meter on the main gas line down-stre down-stream am of the main gas regulat regulator. or. But when using accurate fuel/air ratio control it makes sense to include the pilot as part of the total fuel supply as it may supply as much as 10% of the total fuel.

PT-3

Larg Large e bu burne rners with a va varia riable fuel uel ga gas su supply pply pres ressure sure may re require uire press ressur ure e, an and possibly even temperature, compensation for the fuel gas measurement. measurement. PSV 1

SET @ 75 PSIG

/ / / TIC 3

VENT

PCV 1

Supply 105 PSIG

SET @ 45 PSIG

PI 1

SET @ 15 PSIG

PCV 21

PI 21

/ /

PI 28

TV 3

FUEL GAS

FC

M A IN G A S T R A I N

PCV 11

SET @ 2" WC

PI 11

PILOT GAS TRAIN

Fig. 5-3. Fuel Gas Regulation

PILOT GAS LINE INSTRUMENTS PCVPCV-11 11

The The pil pilot ot gas gas reg regul ulat ator or is set set for for the the act actua uall req requi uire red d pil pilot ot gas gas pre press ssur ure. e.

PI-11

Of course the regulator has a gauge!

BV-1 BV-14 4

On smal smalll bu burner rners, s, the the fir first st pilo pilott shut shutof offf valv valve e ma may be be a sta stand ndar ard d ind indus ustr tria ial, l, two two port port,, solenoid valve. Limit switches switches can be considered considered optional on solenoid valves. On large burners, a burner safety shutoff valve is generally the preferred choice. These valves have an internal spring to force the valve shut, a solenoid to hold them them open, open, and a smal smalll motor motor to re-op re-open en the valve valve.. Item Item BV-24 BV-24,, follow following ing,, provides more detail concerning this type of valve. Some installations installations use a standard, industrial, industrial, diaphragm operated operated control valve. In either case, the valve must be fail-closed.

ZSC/ ZSC/O-1 O-14 4

At leas leastt one limi limitt switc switch h is requi require red d on BV-1 BV-14 4 if it is anyt anythi hing ng more more than than a simpl simple e solenoid solenoid valve. valve. This is needed needed to prove that the valve is shut shut during the purge



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phase. For failsafe operation operation it is best to have a limit limit switch at each end of of travel. The upper limit switch proves that the valve is fully open at all times when the pilot flame is supposed supposed to be on. The article, “Limit “Limit Switches Key to Valve Valve Reliability” 8, explains exactly how to connect double, failsafe, limit switches. BV-15 -15

The pil pilot vent val valve mak makes cer certai tain tha that the there is nev never any any gas gas pre pressure on the the second second shutoff shutoff valve despite despite any leakage leakage through the first one. one. Since Since its only purpose is to vent leakage, the vent line has a smaller bore than the supply line. It must be fail-ope fail-open. n. The Canadian Canadian Gas Associati Association on code CAN/CGA-3.9-M CAN/CGA-3.9-M87 87 9 repeats the following table from NFPA 86A 7 for determining vent line sizes: Gas Supply Line Size

Gas Vent Line Size

NPS

mm

NPS

mm

1½

( 40 )

¾

( 20 )

2

( 50 )

1

( 25 )

2½

( 65 )

1¼

( 32 )

3

( 80 )

1¼

( 32 )

3½

( 90 )

1½

( 40 )

4

(100)

2

( 50 )

5

(120)

2

( 50 )

6

(150)

2½

( 40 )

8

(200)

3½

( 90 )

>8

(>200)

>15 % line crosssectional area

For low molecular weight fuels such as hydrogen (Mol. Wt. = 2) or methane (Mol. Wt. = 16) it is sufficient sufficient to vent the valve to atmosphere outside the building. building. For fuels heavier than air, such as ethane, propane and butane (Mol. Wt. = 30, 44, 58 respectively), the vent should be piped to a flare header. Using the table above, it may be determined that a ¾" or 1", simple solenoid valve is sufficient. sufficient. If not, a fail-open fail-open burner safety safety valve should should be used. used. It operates in a similar, but opposite, manner to BV-14. ZSC/O-1 ZSC/O-15 5

Limit Limit swit switche ches s should should be be includ included ed and and incorp incorpora orated ted in in the the BMS logic, logic, ifif a fullfull-siz sized ed vent valve is required.

BV-16 -16

The The se second cond pilot ilot shuto hutoff ff valve is ide iden ntica ticall to to the the fifirst rst, BV BV-14 -14.

ZSC/O-1 ZSC/O-16 6

Limit Limit swit switche ches s shoul should d be inclu included ded and and incor incorpo porat rated ed in the BMS BMS logic logic,, if a ful full-s l-size ized, d, second pilot valve is required.

PI-18 -18

The The fin final pre pres ssure sure gau gauge ge co confirm firms s th that all all va valve lves are are in in the their ir co correc rrectt po positi sitio on and and



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that the appropriate pressure is available for the pilot flame. BY-1 BY-18 8

The The ign ignit ite er itse itself lf is is ess essen enti tial ally ly a spa spark rk plu plug g pow power ered ed by by a hig high h volt voltag age e tran transf sfo orme rmer. It is capable of sparking continuously as required by the BMS.

BSLL BSLL-1 -19 9

The The pilo pilott flame flame det detec ecto torr is used used to con confi firm rm tha thatt the pil pilot ot ligh lightt has has igni ignite ted d and and is burning in a stable manner. manner. A variety of types exist exist depending on the size of the burner and the type of fuel. A flame rod is a simple simple electrode electrode that projects into the flame. flame. An electric current current passes through the flame to the pilot gas nozzle and energizes a sensitive relay or electronic circuit. circuit. Since it only senses flame flame at a point, it will not not detect the main flame flame.. A disadvan disadvantag tage e is that that the tips tips burn burn off after after a perio period d of time and and a nuisance trip of the furnace will result. An ultra ultra-vi -viole olett (UV) (UV) flame flame detect detector or is probab probably ly the most most popula popularr on proces process s heaters. It is, however, a rather rather complex device device that requires certain certain precautions for reliable reliable operation. operation. UV detectors detectors are optical devices. devices. A lens lens in front front must be aimed directly at the flame. flame. Depending on the particular particular arrangement used, it may may or may not be desirable for the detector to "see" only the pilot flame or the main flame as well. well. One or more viewing viewing windows, BG-45A BG-45A to X, must be provided provided by the furnace fabricator to assist assist in aligning the detector. UV detectors are mounted mounted on ball swivels to permit accurate accurate alignment alignment.. An instrument instrument air purge complete complete with a rotameter and a needle valve should be connected to the tube between the lens and the flame in order to prevent dust from accumulating on the lens and to cool it. Some units also require require a supply of cooling cooling water.

FUEL GAS LINE INSTRUMENTS PCVPCV-21 21

The The main main gas gas reg regul ulat ator or is is set set to the the max maxim imu um allo allowa wabl ble e fuel fuel gas gas pre press ssur ure e for for the the main burner.

PI-21 -21

Eve Every reg regula ulator tor re require ires a press ressu ure gauge. uge. Thi This ga gauge uge is is al also used sed to to ad adjust just the the setpoints of PSLL-22 and PSHH-27.

PSLL PSLL-22 -22

The The fuel fuel gas gas low low pres pressu sure re swi switc tch h is is use used d to shut shut dow down n the the bur burne nerr in cas case e ther there e is insuffici insufficient ent pressure pressure to maintain maintain a steady steady flame. Once a trip has occurred, occurred, the manual reset on the first safety shutoff valve will prevent the burner from reigniting. This is an extremely extremely important safety feature. Even if there is insufficient insufficient pressure to maintain a flame, unburned gas may still collect in the firebox and ignite with explosive force once a large quantity has accumulated.

BV-2 BV-24 4

The The fir first st fuel fuel gas gas sa safety fety shut shutof offf val valve ve is one one of of the the "saf "safet ety y shu shuto toff ff valv valves es". ". It must must be especially certified certified for fired heater use. use. The first of the three three valves in the main main gas train has a manual reset and a solenoid that function as follows: -


When When the the sol solen enoi oid d is is dede-en ener ergi gize zed d the the val valve ve is is shu shutt and and the the man manua uall rese resett http://www.cadvision.com/driedgew/CE5_FH.html

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-

has no function whatsoever. When the the sole solen noid is ene energ rgiz ized ed,, the the valv valve e rem remains ins shu shut unti untill the the man manual ual reset is lifted. Lift Liftin ing g the the manu manual al rese resett ope opens ns the the va valve. lve. The The va valve lve the then n re remain mains s ope open n as as long as the solenoid is energized.

A valve with this function is sometimes termed a "free handle" valve. ZSC/ ZSC/O-2 O-24 4

In addi additi tion on,, BV-24 BV-24 requ require ires s a limi limitt switc switch h to prove prove that that it is shut shut duri during ng the the purge purge and ignition phases. phases. Item ZSC/O-14, ZSC/O-14, above, details the requirements requirements of fail-safe fail-safe limit switch arrangements. arrangements.

BV-25 -25

The The fu fuel gas gas vent vent valv valve e ha has sim simiilar lar cri critteria eria to tho those se of BV BV-15 -15. The The sa same si sizin zing table applies.

ZSC/O-2 ZSC/O-25 5

These These limi limitt switc switche hes s are are optio optional nal,, depend depending ing on on the the size size of of the the vent vent valv valve. e.

BV-2 BV-26 6

The The sec secon ond d ma main gas gas sa safety fety shut shutof offf val valve ve is a lit littl tle e dif diffe fere rent nt fro from m the the firs firstt in in tha thatt itit does not require a manual manual reset. The automatic reset reset feature is accomplished accomplished by a small electric motor motor that opens the valve as as soon as it is energized. energized. The valve fails closed upon de-energization. de-energization. Automatic reset valves valves are identified by the little "M" instead of the "S" "S" in the symbols symbols on Figure 5-2. The valve may incorporate incorporate a slow opening feature. feature. Power to the reset motor motor should be disconnected disconnected as soon as the valve is open so that a momentary power loss to the solenoid does not cause the valve to cycle.

ZSC/O-2 ZSC/O-26 6

BV-26 BV-26 sho should uld also also have have limit limit switc switche hes s whose whose status status is inco incorpo rporat rated ed into into the the BMS BMS logic.

PSHH PSHH-2 -27 7

The The fuel fuel gas gas high high pre press ssur ure e swit switch ch shu shuts ts down down the the main main furn furnac ace e when when the the fuel fuel gas gas pressure pressure exceeds exceeds the maximum maximum allowab allowable le for a stable flame. flame. Excessive Excessively ly high pressure pressure can blow the flame flame out only to have it reignite, reignite, perhaps perhaps explosiv explosively, ely, higher up in the furnace. There is a trend toward the use of analog transmitters instead of switches for sensing sensing process process values, values, even for shutdown shutdown purposes. purposes. The trip value value is then progra programm mmed ed into into the contr control ol syste system m instea instead d of being being adjust adjusted ed at the switc switch. h. Transmitters have become more reliable than switches and also provide much more information 10. If this is the policy policy at your installation, installation, the functions of PSLL-22 and PSHH-27 PSHH-27 can be combined combined in a single transmi transmitter tter located located at PSLL-22. PSLL-22. A second second trans transmit mitte terr can then then be used used to provid provide e pre-a pre-ala larms rms or an autom automati atic c redundancy scheme.

FV-3

Most Most larg large e in industri stria al fu furna rnaces ces ha have a ga gas flo flow w co contro trol va valve to mo modulate late the the he heat input. Details of the control control system are discussed in the sections sections on fuel control. The gas flow control valve must be fail-closed but must also have some means of ensuring that it does not shut too closely to the seat or an unstable flame will



Page 5-8

result. result. Approxim Approximately ately 35% 35% of full full flow is a typica typicall minimum minimum.. Some Some engineers engineers program a minimum minimum output into the control system. Others weld a small stop onto the shaft. Software blocks can be easily easily altered and welded stops stops may not be reinstal installed led when when the valve valve is replac replaced ed.. I use both. both. The softw software are limit limit has has the advantage advantage that it can be raised if the original original setting setting is too low. It should should be set slightly higher than the welded stop in order to inhibit reset windup before the 0% controller output is reached. Furnaces with cyclical service, such as those used in dryer regeneration service, may have have the main flame flame turned turned on and off on a regular regular basis. basis. In such cases, cases, there cannot cannot be a minimu minimum m stop on the control valve. valve. Instead, Instead, logic should should be provided that shuts the valve fully whenever the controller tries to position it below the minimum minimum flame flame setting. setting. The controlle controllerr must also be switched switched to manual manual to prevent reset windup whenever this is done. ZSC-3

The gas gas flo flow con control rol val valve req requires res a limit swi switch to co confir firm tha that it is is at th the minimum flow position before before lightoff. The switch should should be set slightly above the minimum stop.

PI-28 -28

A pre pressure gau gauge sho should be pl placed las last bef before the the fue fuel noz nozzle its itself to pr provide assistanc assistance e in diagnosi diagnosing ng mysterious mysterious problems problems.. Dirt does collect collect in valves and other places!

BSLL BSLL-29 -29

The The main main flam flame e dete detect ctor or is gen gener eral ally ly of of the com combi bina nati tion on ultr ultraa-vi viol olet et and and infra infra-re -red d (UV/IR) type. The same requirements requirements apply as to the UV UV detector for the pilot. pilot. Burners in cyclical se service rvice do not require a main flame flame detector. They rely entirely on the pilot flame detector, which may be adjusted so that it can see both flames.

AIR SUPPLY INSTRUMENTS. There is considerable variation in the air supply system among furnaces of different different sizes. Small heaters, heaters, and some not so small, small, may be entirely natural natural draft and are controlled only with with a manually set air air damper. For furnaces with forced forced draft (FD) fans fans some, or all, of the following instruments are required. PDSH PDSH-31 -31

If the the air int intak ake e has a filt filter er,, it must must be fit fitte ted d with with a diffe differe rent ntia iall press pressur ure e switc switch h or transmitter connected connected to an alarm to warn of plugging.

FE-32

If the the FD fa fan is la large, a flo flow mea measurement device is ne needed at the the int intake to measure flow through through the fan so that surge can be prevented. prevented. A simple averaging averaging pitot pitot tube tube (Annub (Annubar) ar) or a therma thermall device device provid provides es suffi sufficie cient nt accura accuracy cy withou withoutt significant loss of head.

FV-32 -32

Larg Large e fan fans s re requir quire e a minim inimu um flo flow w blo blow w-of -off va valve lve to to pre preve vent nt surg urge.

FE-3 FE-33 3

The The act actua uall air air flow flow to the the bur burne nerr mus mustt be be mea measu sure red d ifif an an acc accur urat ate e fue fuel/ l/ai airr rat ratio io is to be maintained. maintained. This is generally done done for large large furnaces. Smaller packaged packaged units have the fuel fuel and the air linked with cams cams and no air measurem measurement ent is done. A



Page 5-9

previous article 1, explains why it is not possible to use same flow element for both minimum flow and throughput flow control. control. It may be required to use a venturi venturi to achieve both high accuracy accuracy and low head loss. On very large burners, burners, arrays of pitot tubes or thermal flow meters distributed across the duct, are the preferred metho method. d. Howeve However, r, these these can can be some somewha whatt time-c time-cons onsum uming ing to insta installll and maintain. The associated transmitter is also used to provide the pre-alarm for FSLL-36. FV-33 -33

A fai fail-op l-open en co combustio stion n air air con conttrol rol valv valve e is plac laced at th the inle inlett to th the wi wind bo box for for those burners requiring requiring external air control. control. A butterfly valve valve is a popular choice. choice. Very large fans may have provision for inlet guide vanes which provide a more efficient efficient means means of doing doing the same same thing. The valve valve requires requires a minimum minimum stop stop to prevent complete closure.

ZSOZSO-33 33

FV-3 FV-33 3 has has a lim limit it swit switch ch to conf confir irm m that that it is full fully y ope open n dur durin ing g pur purgi ging ng..

VSHH VSHH-3 -34 4

Larg Large e fans fans sho shoul uld d be pro provi vide ded d with with a seis seismi mic c type type vib vibra rati tion on swi switc tch. h. A pre previ viou ous s 3 article discusse discusses s these devices. devices. The switch switch should be connected connected to shut down the entire entire system as the heater heater cannot cannot be operated operated without without the fan. Very large large fans may even include an entire bearing vibration monitoring package.

YS-3 YS-34 4

The The FD fan fan mot moto or sta statu tus s con conta tact ct is used used to provi rovide de info inform rmat atio ion n to to the the BMS BMS..

FSH-3 SH-35 5

The The purg purge e air air swit switch ch is is use used d to in indica icate ad adequat quate e purg purge e flo flow. It is is usu usua ally lly a differential pressure switch across the wind box, if any, of the furnace or it may be a signal from the flow flow transmitter, transmitter, FT-33. As can be seen in Table Table 5-1, there is a variety of opinions concerning minimum purge requirements. Specification

Time

Volume

Flow

ASME Section VII 11

> 5 minutes

> 5 volumes

> 25% full flow

API(RP 550 – Part 4) 12

> 5 minutes

> 5 volumes

> 25% full flow

NFPA 8501 5

not sp specified

> 8 volumes

> 70% fu full flflow

CGA B149.3 9

not sp specified

> 4 volumes

> 60% fu full flflow

Table 5-1 Minimum Purge Requirements FSLL FSLL-3 -36 6

The The comb combus usti tion on air air low low flo flow w shut shutdo down wn swit switch ch shu shuts ts dow down n the the burn burner er ifif the there is insufficient flow for safe combustion. combustion.

FURNACE INSTRUMENTS TE-41 -41

One One or mo more sk skin the therm rmo ocou couples les shou should ld be be pro prov vide ided on th the tub tubes of of the the furn furna ace. ce. The thermocou thermocouple ple element element is welded directly directly to the tubes. Note that that the device device that receives the T/C signal must be capable of accepting grounded signals.



Page 5-10

TSHH TSHH-4 -42 2

High High sta stack ck gas gas tem tempe pera ratu ture re is is caus cause e for for shut shutdo down wn.. The The furn furnac ace e shou should ld als also o be isolated from the process process feed if the feed is flammable. flammable. Tube rupture may be be the cause for the high stack temperature.

AE-4 AE-43 3

For For a bur burne nerr to to ope opera rate te at at pea peak k eff effic icie ienc ncy, y, stac stack k oxy oxyge gen n and and comb combus usti tibl bles es must ust be be measured and controlled. controlled. However, the additional additional cost and maintenance maintenance of stack analyz analyzers ers limit limit their their use to large large burners burners.. High High and low alarms alarms should should be programmed.

PI-44 -44

A dra draft ft gaug gauge e mu must be be pro provi vid ded to to gi give a gra grad de le level vel in indica dicati tio on of of the the pres ressure sure in the furnace. This is a special special type of pressure pressure gauge sensitive to inc inches hes of water pressure.

BG-45 BG-45 A to X Viewing Viewing windows windows were mentione mentioned d previously previously with respect respect to aligning aligning the flame FG-45 FG-45 A to X detect detectors ors.. Other Other window windows s may be needed needed as well. well. These These are usua usually lly made made of glass and require a small small air purge purge to keep them cool and and clean. Regulators are often used control the air flow. This is rather pointless pointless since it is flow control that is required. The presence presence of pressure pressure may only mean mean that the line is plugged. The absence of pressure may mean that the purge is flowing freely at a rate beyond the capacity capacity of the the regulator regulator.. Pressure Pressure alone alone proves proves nothing. nothing. The appropr appropriate iate instrument is a rotameter rotameter with a needle valve. 15 SCFH is a common common flow setting. The rotameter provides proof that purge is actually occurring. A number of purge rotameters can be mounted on a plate and pre-tubed to a small instrument air header. header. This greatly reduces reduces field labour and provides a convenient convenient way of checking all purge rates from a single location. PROCESS PROCESS RELATED RELATED SAFETY SAFETY INSTRUME INSTRUMENTS. NTS. The The deta detailils s of proc proces ess s rela relate ted d safe safety ty instrume instrumentati ntation on depend depend very much much on the individu individual al process. process. It is hard to genera generalize. lize. The majority of of furnace incidents occur occur during light-off. Therefore, it is not not necessarily a good idea to add every automati automatic c shutdown shutdown function function that might might suggest itself. itself. The safety safety value of each interlock must be weighed weighed against the risk of unnecessary shutdown shutdown and relight cycles. At least one nuisance trip is bound to occur during the life of the equipment for every shutdown device installed. Furnace isolation valves valves are especially especially problematical. TO FLARE PSV 54

XYC 52

// /

XV 52

S

AS FL AS

AS

V S

AS

V

I

XYO 51

V

S

XYC 51 XV 51

FL

V

ZSO 51

FIRED HEATER

V

S V

52

minimum 50 feet

PROCESS FLUID

52 V

I ZSC 51

XYO

V

V

ZSO 52 ZSC 52

I

V

TSHH 53

FSLL 53

V

Controlling Fired Heaters http://www.cadvision.com/driedgew/CE5_FH.html Fig. 5-4. Process ESD Valves

Page 5-11

XV-51

It may sound desirable to automatically shut isolation valves if it appears that a tube rupture has occurred. occurred. However, remember remember that any closure closure of an isolation valve during operation results results in loss of flow. Further interlocks interlocks must be provided provided to shut down the burner if this occurs. occurs. Even after the shutdown, shutdown, radiation from the refractory refractory continues continues to heat the trapped trapped fluid. fluid. Boiling Boiling may result in over-heated over-heated tubes. If the fluid is susceptible to coking, coking, tubing damage damage may accumulate accumulate even after the immediate incident is past. If careful analysis shows that an isolation valve would provide a net contribution to safety, the valve must must be installed correctly. correctly. Details of XV-51 are shown shown in Figure 5-4. 5-4. The The valve valve must must be a fire rate rated d valve valve.. That That is, it must must be capa capabl ble e of 13 14 maintaining a seal seal in the presence presence of fire. API SPEC 6FA and 6FC outline the requirements. The wiring for both solenoids solenoids and for the associated limit limit switches should also be fireproof. fireproof. Mineral insulated insulated (MI) cable is an appropriate appropriate choice. XV-51 should be located at least 50 feet (15 meters) meters) from the furnace. furnace. Provision must be made to manually initiate valve closure from a location at least another 50 feet from the the valve itself. itself. The DCS console console in the main control control room is an ideal location.

XYO/ XYO/CC-51 51

If the proce process ss fluid fluid is suffi suffici cien entl tly y haza hazard rdou ous s that that isola isolati tion on valve valves s are are cons consid ider ered ed necessary, a difficult issue issue must be addressed: addressed: What should the failure failure mode of the valves valves be? If the valve fails fails to shut shut durin during g an emerge emergency ncy,, a hazar hazardou dous s situation exists. If the valve fails closed closed due to a power or instrument instrument failure, other hazards hazards occur. One solution solution to this problem problem is to use double-actin double-acting, g, fail-last fail-last (locked) valve actuators. These should be installed with two separate solenoids: solenoids: XYO to open the valve and XYC to shut shut it. If this is done, two failures failures are required to cause an undesired valve valve action. For example, example, if the valve is required required to be in the open state, XYO will will be energized. energized. A signal failure failure results in both both solenoids being de-energized. However, the valve will not move since air pressure on the piston is still balanced. balanced. It requires energization energization of XYC before anything anything happens. A valve that has been shut because of a fire will not re-open due to burned out wiring. Incidentally, double-acting double-acting actuators are considerably considerably less expensive expensive than spring-return actuators.

ZSC/O-51

Limit switches 8 are provided on the valve to ensure that it is fully open before furnace ignition is permitted, and fully shut when necessary.

XV-52 -52

If the the fu furna rnace con conta tain ins s a large rge inve invent ntor ory y of fla flammable flfluid, id, a depress ressu uriz rizatio ation n valve may be required. required. It should be capable of dumping dumping the entire contents of the furnace to the flare. flare. A check valve should be added at the the outlet of the furnace furnace so that the blowdown valve does not attempt to depressure the entire downstream process.

XYO/ XYO/CC-52 52

The The prob proble lems ms asso associ ciat ated ed with with a fail fail-o -ope pen n depr depres essu suri riza zati tion on valve valve make make it very very unlikely that it would would be acceptable to operations. operations. Any power or air air failure would



Page 5-12

result in massive flaring. flaring. Thus a fail-last valve valve actuator arranged in same way way as XV-51 is the most reasonable choice. ZSC/O-5 ZSC/O-52 2

Limit Limit swi switch tches es on on XV-52 XV-52 shou should ld be inter interloc locked ked to to preve prevent nt open opening ing XV-5 XV-51 1 when when the the depressurization valve is open.

FSLL FSLL-5 -53 3

Meas Measur urin ing g the the pro proce cess ss out outle lett flow flow is an exce excellllen entt safe safety ty fea featu ture re.. Reme Rememb mber er that that a process process furnace furnace is essential essentially ly a heat heat exchange exchanger. r. If there is no process process flow, the furnac furnace e tubes tubes will overhe overheat at and and may may coke coke or even even rupture. rupture. The The flue flue gas gas will will overheat upper section section of the furnace and the stack. No furnace can be allowed allowed to operate under these conditions. If the inlet process flow, FE-33, is high and the outlet is low, it can only mean that a tube has ruptured. Immediate Immediate shutdown is imperative. imperative.

TSH TSHH-53 H-53

High High proce process ss outle outlett temp temper erat atur ure e mean means s that that the tempe tempera ratu ture re cont contro rols ls are not not functioning properly: properly: The furnace must must be shut down. down. The note under PSHH-27 concerning the use of transmitters instead of switches also applies here. here. Modern programmable programmable logic controllers controllers (PLCs) (PLCs) and distributed control systems (DCSes) are quite capable of interpreting thermocouple and RTD signals directly. Consideration should should be given to using them instead instead of the less reliable temperature switches or the combination of thermocouple and millivolt trip relay10.

PSVPSV-54 54

In all all pro proba babi bili lity ty the there re wil willl be blo block ck val valve ves s some somewh wher ere e on eit eithe herr side side of of the the heat heater er so that it can be isolated isolated manually. If these exist, a relief relief valve must be provided. provided.

SUPERVISED MANUAL. The NFPA standards 5, 6 specify several types of burner management systems: • • •

Automatic (Recycling) (Recycling) Automatic (Nonrecycling) (Nonrecycling) Supervised Manual.

The type most commonly used in industrial process furnaces is supervised manual, often called semiautomatic. semiautomatic. NFPA describes describes it best: best: Supervised Manual. Manual. A system by which the furnace furnace is purged and a burner burner started, ignited, and stopped manually, with interlocks to ensure that the operation follows proper procedures. What What this this mean means s is that that the operator operator must be presen presentt at the burner burner to contr control ol the the igniti ignition on procedure. At various key points points he pushes a button or takes takes some other action to advance advance to the next step. The BMS supervises supervises the operator and confirms confirms that all conditions conditions are appropriate for that step before allowing allowing it to proceed. If conditions are wrong, wrong, the BMS shuts down the entire sequence sequence and the operator operator must rectify rectify the problem and start start all over again. It's a little like like Controlling Fired Heaters


Page 5-13

playing Snakes and Ladders except except that there is no quick way to the top. A detailed example of the ignition ignition sequence sequence for a furnace furnace with an FD fan is given below. below. The indented indented paragraphs paragraphs marked "•" indicate automatic action. STEP 1: 1:

Close the gas flow control valve, FV-3, to its minimum minimum stop. Open the combustion air valve, FV-33, full. Push the Start Fan button, HS-34. (The fan may be started from from the main control panel or by some other means.) •

The green Fan Running light, YL-34, comes on.

PAUSE

The sequence may wait indefinitely at this point.

STEP 2: 2:

Switch the Enable switch, HS-2, to ON. (This can also be done before the fan is started.) started.) Any other shutdown shutdowns, s, such as from the Main Control Control Panel or a plant plant emergency shutdown (ESD) must be cleared before this has any effect. The green Ready light, XL-2, comes on if there are no active shutdowns. Automatic functions functions are enabled once the switch switch is ON. Until that point, no lights are on except the Fan Running light, and no push buttons function except the Lamp Test. Test. All block and vent vent valves are de-energized. de-energized.

• •

H-101 MAIN PROCESS HEATER XL-2

YL-34

FLH-35

BURNER READY

FAN RUNNING

PURGE COMPLETE

PILOT FLAME ON

MAIN FLAME ON

HS-2 OFF ON

HS-34

HS-35

HS-14

HS-24

HS-9

START FAN

START PURGE

START IGNITION

OPEN MAIN GAS

LAMP TEST

ENABLE

BLH-19

BLH-29

LP-7 Fig. 5-5. The Basic Local Burner Panel

PAUSE

The sequence may wait indefinitely at this point.

STEP 3: 3:

Push the Lamp Test button, HS-9. •


All lights should come on.


Page 5-14

STEP 4: 4:

Push the Start Purge, HS-35, button. •

•

•

The follow following ing condit condition ions s must must be met met befor before e the purge purge sequen sequence ce can begin: Both sa safety sh shutoff va valves are are confirmed sh shut. The The ga gas flo flow w co control rol va valve lve is is co confirm firmed ed on minim inimu um sto stop. p. The The co combusti stion air contr ontrol ol valve lve is is co confirm firme ed wi wide op open. The fan is confirmed running (YS-34). The purge flow signal, FSH-35, is on. Once these condition conditions s are met, a timer, timer, KC-35, is started. started. The timer is set set so that a minimum minimum of eight air changes changes of the furnace is assured. assured. (See Table Table 5-1, Minimu Minimum m Purge Requireme Requirements.) nts.) The green Purge Purge Complet Complete e light, light, FLH-35 FLH-35,, flashe flashes s while while the timer timer is running running.. Any Any time time any any of the conditions are not met, the timer is reset and the light goes off. Once Once the the time timerr has has comp comple lete ted d the the purg purge e inte interva rval, l, the the gree green n Purg Purge e Complete light is on steady.

PAUSE

The sequence may wait indefinitely at this point as long as the fan is running.

STEP 5: 5:

Push th the St Start Ig Ignition bu button, HS HS-14. A twelve second timer, KC-14, is started. The ignition transformer, BY-18, is turned on. • The green Pilot Flame On light, BLH-19, is flashing. • • The pilot vent valve, BV-25 is shut. • The first pilot shutoff valve, BV-24, is opened. • The second pilot shutoff valve, BV-26, is opened. Pilot flame, BSLL-19, must be confirmed for two seconds during the twelve • second interval or the BMS reverts to Step 4. Once the pilot flame, BSLL-19, is confirmed. • The ignition transformer, BY-18, is turned off. • The green Pilot Flame On light, BLH-19, is on steady. •

PAUSE

The sequence may wait indefinitely at this point as long as pilot flame is detected.

STEP 6: 6:

Push th the Open Main Fuel bu button, HS HS-24. •

• •

STEP 7: 7:

A second twelve second timer, KC-24, is started and the green Main Flame On light, BLH-29, is flashing. The fuel gas vent valve, BV-25, is shut. The first fuel gas safety shutoff valve, BV-24, is energized.

Lift Lift the the leve leverr on the the firs firstt fuel fuel gas gas saf safet ety y shut shutof offf valv valve, e, BV BV-24, -24, wit withi hin n ten ten seco second nds. s. The valve valve will then then stay stay latch latched ed in the the open open posit position ion.. It will not not open open at all if conditions are not safe. •


If this is not confirmed by ZSO-24 within the allowed ten seconds, the http://www.cadvision.com/driedgew/CE5_FH.html

Page 5-15

•

•

sequence sequence reverts reverts to Step 4. If it is, the open limit switch switch will trigger trigger the BMS to open the second fuel gas safety shutoff valve, BV-26. Main flame must be confirmed by BSLL-29 for two seconds during the twelve second interval or the BMS reverts to Step 4. Once the Main flame is confirmed, the green Main Flame On light, BLH-29, is on steady.

PAUSE

The furnace furnace is now running running on minimum minimum flame (also (also known as "low fire"). fire"). The sequence may wait indefinitely at this point.

STEP 8: 8:

The The ope opera rato torr may may open open the the gas gas flow flow cont contro roll val valve ve at any any tim time. e.

EMERGENCY EMERGENCY SHUTDOWN. SHUTDOWN. Various trouble indications will trigger a partial or total furnace shutdown. They are tabulated in Table Table 5-2. These shutdowns, shutdowns, except the two flame failures, are are active active at all times, times, including including during the ignition sequence. sequence. The flame detectors detectors are bypassed bypassed during the trial for ignition period. Burner E S D cause

Main Gas BV-24/25/26

Pilot Gas BV-14/15/16

VSHH-34 fan

De-energize

De-energize

HS-2 manual

De-energize

De-energize

FSLL-36 air

De-energize

De-energize

BSLL-19 pi pilot

De-energize

De-energize

BSLL-29 main

De-energize

PSLL-22 fuel

De-energize

PSHH-27 fuel

De-energize

TSHH-41 tube

De-energize

TSHH-43 stack

De-energize

FSLL-53 -53 pro process

De-energize

TSHH TSHH-5 -53 3 proce rocess ss

De-e De-ene nerg rgiz ize e

FD Fan Stop

Manual restart at Step 1 Step 4

Step 6

Table 5-2 Furnace ESD Key Other, process related ESD functions such as isolation and depressurization of the furnace, may also be implement implemented. ed. A thorough thorough analysis of the entire process process is required required to determine determine what else, if anything, may be required. LOCAL CONTROL PANELS. Figure 5-5, The Basic Local Control Panel, shows the minimum status lights, push buttons and switches switches required at the burner. This minimum minimum assumes assumes that all alarms and shutdowns are displayed on a main control console, perhaps a DCS, somewhere else in the plant. The local operator and the console operator operator must be in close communication communication during the light-off sequence. sequence. At other times the bu burner rner is unattended. Many burners are supplied supplied as self-contained self-contained packages. For such installations, installations, a more complete complete Controlling Fired Heaters


Page 5-16

local local panel is required required.. Every Every pre-ala pre-alarm, rm, shutdown shutdown alarm and valve status status is displayed displayed as a separate indicating indicating light on the panel. In general, a good colour colour code is: Pre-Alarm Shutdown Alarm Ready Light Valve Open Valve Shut

Orange Red Green Green Red

Avoid Avoid the use of blue blue indicatin indicating g lights as they are practicall practically y invisible invisible in bright bright daylight. daylight. Alarm Alarm logic must be provided to drive the alarm indicators and a horn according to the standard ISA Automatic Reset Reset sequence A. The First Out and and Lamp Test features (ISA sequence sequence 15 F3A-14) are often requested These can be accomplished accomplished by dedicated alarm annunciator annunciator logic or by a PLC. Logic and control hardware may be housed housed within the local panel or elsewhere in the plant. FEEDBACK FUEL CONTROL. The simplest simplest combust combustion ion control control arrangem arrangement ent is to have a temperature controller controller (TIC) on the process process outlet controlling the fuel gas gas firing rate. This system is essentially like the medium side, inlet throttling, arrangement described “Controlling Shell and Tube Exchangers” 3. The TIC may drive the fuel valve valve directly, directly, in which case case the valve is tagged "TV", or it may cascade to a flow controller (FIC) on the fuel gas. FEEDFORWARD FEEDFORWARD FUEL CONTROL. CONTROL. In theory, feedback fixes everything. In practice, practice, there are problems. Fired heaters have a considerable considerable response lag to changes in either either the firing rate or process process inlet conditions. conditions. There There will be transient transient dips or bumps whenever whenever a significa significant nt change occurs on any process, process, fuel, or air variable. variable. If deviation from setpoint setpoint is the disease, then feedfeedback is the cure. But an ounce (28 grams) grams) of prevention is worth a pound pound (454 grams) of cure. cure. Feedforward is the technique of preventing the problem before it occurs. If there there is signif significa icant nt variat variation ion in feed feed rate, rate, feed feed forwa forward rd can be appli applied ed as is descr describe ibed d in 3 “Controlling Shell and Tube Exchangers” . Similar methods methods can be used used to compensate compensate for rapid variations in feed temperature. Dire Direct ct cont contro roll of the the fuel fuel BTU valve is not enough for any Controller Set Point FIC 3 but but the smal smalles lestt burne burners. rs. Process Variable The fuel must be controlled FY to provide the exact amount 3 of energy that the process requires requires.. This implies implies that FIRED FT PT the heat content of the fuel AT TE HEATER FV 3 3 3 3 3 V as well as its flow rate must be kno known. If the the fue fuel FUEL GAS pressure pressure,, temperatu temperature re and FE composition are constant, a 3 simple flow meter is Fig. 5-6. Fuel Gas Heat Content Measurement suffic sufficien ient. t. If signif significa icant nt vari variat atio ions ns are are expe expect cted ed,, more more complex complex feedforwa feedforward rd

V

V

V

V

V

V V



Page 5-17

techniqu techniques es can be applied. applied. Figure Figure 5-6 shows shows the "full "full meal meal deal". deal". It is seldom seldom necessa necessary ry to include include all this instrum instrumenta entation tion.. A chromatog chromatograph raph measures measures the compo compositio sition. n. From this this the density, density, heating heating value, value, and oxygen oxygen demand can be calculate calculated. d. Combini Combining ng these calculations calculations with the PT, TE and FT readings readings results in an exact figure for heat input per unit time. time. The result of this arrangement is that any changes in fuel gas condition will be corrected before any change in the process temperature temperature occurs. In the past, a calorimeter calorimeter has been used instead of a chromatograph. chromatograph. It was specially designed designed to produce a single figure figure called the "Wobbe Index". Index". (I don't know if this rhymes rhymes with "lobby" "lobby" or 1/2 "Toby".) This index is defined as Btu/scf/(specific Btu/scf/(specific gravity) . The square root in the the denominator denominator autom automati atical cally ly accou accounts nts for the the effec effects ts of molecul molecular ar weigh weightt change changes s on an orific orifice e plate. plate. A calorimeter calorimeter with such an output is frequently called a "Wobbe meter". Wobbe meters function by actually burning a small amount of fuel that has been metered through a fixed restriction orifice. orifice. In this manner manner the effect of changing density on the flow meter meter and the contro controll valve is mimic mimicked ked.. They They serve very very well well with mixed mixed hydroc hydrocarb arbon on fuel gas. gas. The The assumption is that there is a proportional relationship between heat content and oxygen demand. This assumption is reasonably accurate for hydrocarbons but is not reliable if there are large, variable proportions proportions of hydrogen in the fuel. In such cases, more complex complex analyzers analyzers are needed to provide an exact oxygen demand figure which can be used to adjust the ratio of the air controller, FFIC-33. AIR CONTROL. Air must must be supplied to the burner burner in proportio proportion n to the fuel. In small, small, simple units this is accomplished accomplished by having the two valves valves linked by a cam. The ratio between the two flow rates is set by mechanically mechanically adjusting adjusting the cam. Several significant significant assumptions are made made when this arrangement is used: The pressure of both the fuel and the air is constant. • The temperature temperature of both the fuel and and the air is constant. constant. Note that a 90F 90F ° (50C °) swin swing g in temp temper erat atur ure e resu result lts s in an 18% 18% varia variati tion on in dens densit ity. y. Because of the square root relationship between density and the flow rate, this will result in an error of 9% in the measurement and hence a 1.8% change in excess oxygen. The composition of the fuel, and hence its air demand, is constant. • If these conditions conditions are not met, the fuel/air fuel/air ratio will vary. A fuel-rich fuel-rich mixture results results in serious serious waste of fuel and possibly possibly a hazardous condition. condition. An air-rich mixture mixture also results results in inefficiency due the volume of air that must be heated and blown out the stack while reducing heat transfer. •

Natur Natural al draft draft burne burners rs may may have have a set set of actua actuator tor-dr -drive iven n louvre louvres s contro controlli lling ng combu combusti stion on air. air. Accurate Accurate flow measureme measurement nt may not be possible possible for such installati installations. ons. The correct correct louvre position can be controlled by sending it a signal that is proportional to the signal sent to the fuel gas valve. Exact adjustment adjustment of the ratio can only be done on-line on-line with the assistance assistance of the stack O2 analyzer. LEAD/LAG COMBUSTION CONTROL. In order to control the fuel/air ratio on a continuous basis through all load changes, changes, a flow ratio controller controller is used. This involves measuring measuring both the fuel and the air. Figure 5-7 shows the complete complete arrangement; it has a number number of specialized features. Controlling Fired Heaters


Page 5-18

Firstly, the total heat demand is determined by the load controller, XIC-99, responding to the demands of of the process. process. Most probably probably it is a temperature temperature controller controller on the the outlet. The XIC is cascaded to both the fuel and air controllers. Since these two controllers controllers are measuring different



Page 5-19

R E D T E A R I E F H E 3 A 4 2 O

V

T O L I P

N I A M

T I 3 A 4 H L A A AV A I 3 A 4

V 3 F V V 3 V 3 F

V 1 2 C P

T 3 T 3

V 1 1 C P

3 E 3 F

V C I 3V F 3 F

o

A Y 3 V F 3

T 3 F 3

V

V

P S

T 3 P 3

M

E 3 T

V V C I 3 F

o

V Y A F 3

T 3 P

P S

T N E V

E 3 F

V

V

T 3 F

V 2 F 3

K

V

C Y 3 F 3

V Y B F 3

V

V B V Y F 3 3

o

L A D F C I 2 F 3

C I 9 X 9

V

E 2 F 3

V 1 S P

/// T 2 F 3

T N E V

V 1 C P

R I A N O I T S U B M O C

S A G L E U F

Fig. 5-7. Combustion Controls Controlling Fired Heaters


Page 5-20

flows, flows, the signal from the XIC must be in units units that both controll controllers ers can understand. understand. The most convenient set of units is to have the 0 to 100% signal equal to the span of the fuel gas controller, either in SCFM, BTU/min BTU/min or some some other appropriate unit. unit. Some DCSs have have the features to do this automatic automatically ally.. Ignoring, Ignoring, for a moment, moment, the two selectors selectors FY-3B FY-3B and FY-33B, FY-33B, the XIC-99 XIC-99 signal becomes becomes the setpoint of both the fuel controller, FIC-3, and the air controller, controller, FFIC-33. The latter is configured configured as a ratio controller which has has the appropriate air/fuel ratio built built in. Be careful here: Sometimes, for reasons of tradition, fuel is measured in SCFM and air is measured in lb/min. lb/min. Ensure Ensure that the ratio programmed programmed into the control control system takes takes such peculiari peculiarities ties into account. Once the furnace is in operation, the ratio may be adjusted adjusted with the help of the flue gas analyzer. So far, so good. good. But this arrange arrangeme ment nt does does not not allow allow for eithe eitherr rapid rapid transien transients ts in the fuel fuel demand nor the possibility that either the fuel or the air valves may not be responding correctly. This is where the two limiters come come in. They are arranged in what is commonly commonly called a "lead/lag" arrangement. arrangement. (Note that the expression "lead/lag" "lead/lag" has at least three totally unrelated meanings meanings in process control: The first refers to a specific form of dynamic signal conditioner; the second to a two-stage pump or or compressor control; control; the third applies to burners and is explained explained here.) Some call this arrangement a "cross limiter". The purpose of the high selector on the setpoint of the air flow is to pass the greater of the process driven air air demand and the actual fuel rate. rate. On a rising demand demand signal, the air will lead lead the fuel since it responds immediately immediately and does not wait for the fuel to rise. On a falling demand signal, the air setpoint will not fall until the fuel, as measured measured by FT-3, has fallen. Thus the air will lag the fuel on the way down. Both the fuel and the air controllers controllers have their setpoints setpoints based on the same fuel fuel rate. In the event that the fuel valve opens without the setpoint to the air controller rising a corresponding amount, the increased increased fuel rate will pass through through the high selector selector to increase increase the setpoint setpoint to the air controller to agree with the actual fuel injected into the burner. The purpose of the low selector on the setpoint of the fuel controller is to prevent the fuel flow from rising before before the air flow has risen. The fuel lags the air on the way up and leads on the way down. If the air supply should fail, the fuel setpoint will fall with the air, regardless of the demands of XIC-99.

One detail remains: remains: FIC-3 is not a ratio controller. controller. Its setpoint is defined in terms terms of the fuel flow rate. Both inputs inputs to the selector FY-3B must must have identical units and range. range. FY-33C is a simple simple scaling function with the scale scale constant constant "K" exactly exactly equal to the reciprocal reciprocal of the air/fuel ratio of FFIC-33. FFIC-33. In a DCS it is possible to configure this function block to automatically read the value of the ratio from FFIC-33 and apply apply it directly in the formula. formula. This is importa important nt because because the value of the ratio may be changed changed in response to changes changes in the fuel gas compositio composition n or the combustion combustion efficienc efficiency. y. It is easy to overlook the K factor when making this adjustment. FLUE GAS MEASUREMENT. MEASUREMENT. Most modern process furnaces, even relatively small ones, have some form of continuous continuous excess air measurement in the flue. This is important for both efficiency and and safet safety. y. In the past, past, such instru instrume ments nts have had a well well dese deserve rved d reputa reputatio tion n for for high high Controlling Fired Heaters


Page 5-21

maintenance and low reliability. reliability. This is no longer longer the case. Today's instruments instruments are based on the electrochemical response of zirconium oxide (ZrO 2), also also known known as zircon zirconia. ia. A probe probe is inserted directly into the flue and does not require any sampling system. An explanation of the meaning of excess air is in order. order. Excess air is combustion combustion air in excess of the theoretical, or sto stoichiometric, ichiometric, air. air. This is approximately, approximately, but not exactly, exactly, the same as what is measured by the stack O 2 analyzer. The difference is that there will be at least some some oxygen in the stack that could have burned burned but did not because of poor combustion combustion conditions. This implies implies that it is possible to have some O 2 as well as unburned unburned fuel going going up the flue. This This condition condition cannot be cured by instrumentation. Air is about 21 volume volume percent oxygen. oxygen. This means means that a 10% excess excess O 2 reading implies that only half the oxygen is being consumed and that twice as much air is being blown into the furnace, heated up, and sent up the stack, as is necessary necessary for combustion. combustion. That wastes a lot of energy energy and reduces the capacity capacity of the furnace. furnace. Since Since the cost of unburned unburned fuel is higher higher that than of excess air, a typical optimum is approximately 2% excess O 2 representing approximately approximately 10% excess excess air. Natural Natural draft furnaces furnaces without without continuou continuous s air control control should be operated operated at approximately approximately 6% excess O 2. This ensures complete complete combustion combustion under upset conditions. conditions. Large furnaces such as power boilers, where the cost of inefficiency is high, often include a combustibles analyzer in the same unit as the excess O 2 analyzer. These devices devices can also also make in-situ measurements measurements without sampling. sampling. They use an infrared beam beam directed across the stack to meas measure ure the carbo carbon n monox monoxide ide (CO) conten content. t. Exces Excess s air air levels levels below below 1% give no useful useful indication of combustion efficiency as there is no longer any correlation between the amount of residual oxygen oxygen and the amount of unburned fuel going going up the flue. A combustibles combustibles analyzer can provide that information. information. They are expensive, expensive, however, and must must be justified on a co cost/benefit st/benefit basis. Sometimes the flue gas analyzer is used to automatically adjust the air/fuel ratio of FFIC-33. Great Great care should should be exercised exercised in taking taking this obvious-l obvious-looki ooking ng step. step. Despite Despite their considera considerable ble improvement in terms of accuracy and reliability, flue gas analyzers are still complex instruments and do not approach approach the reliability reliability of flow transmitt transmitters. ers. This step step should only be taken if the benefit justifies the necessary necessary commitment commitment to maintenance. Even then, high and low limits must must be placed on the signal going to the ratio controller so that failures or inaccuracies cannot drive the air/fuel ratio to dangerous extremes. BURNER CONTROL and MANAGEMENT SYSTEMS. systems:

NFPA5, 6 defines three distinct control

Boiler Boiler Control System. System. The group group of control control systems systems that regulates regulates the boiler process including the combustion control system but not the burner management system. Combustio Combustion n Control Control System. System. The control control system that regulates regulates the furnace furnace fuel and air inputs to maintain air/fuel ratio within the limits required for continuous combus combustio tion n and and stable stable flame flame throu througho ghout ut the operat operating ing range range of the boiler boiler in accordance with demand. demand. This control system includes the furnace draft draft control Controlling Fired Heaters


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where applicable. Burner Burner Manag Managem ement ent Syste System. m. The control control syste system m dedic dedicate ated d to boile boiler-f r-furn urnac ace e safety, safety, operator operator assistance assistance in the starting starting and stopping stopping of fuel preparation preparation and burning burning equipment, equipment, and prevention prevention of damage damage to fuel preparation preparation and burning burning equipment. For a process process heater, the various various process process related controls controls would would be equivalent equivalent to the boiler boiler control system. Contrary to common belief, neither PLCs nor DCSs are mentioned in any of the NFPA 8500 series. They are not not mentioned in the CGA Code Code either. What NFPA says says is The logic system performing the safety functions for burner management and boiler control systems shall not be combined with any other logic or control system. Unfortunately it is not at all clear what the word "combined" means, nor is there any explanation of the word "system". "system". There seem to be many many interpretations, interpretations, some of of them quite extreme. One I have have heard heard is that that "NFPA "NFPA requir requires es that that the combust combustion ion contr controls ols shall be in the DCS, the sequenced start shall be in a dedicated PLC and there shall be hardwired hardwired shutdown relays." The problem with this this approach is that there are are too many links between between these three systems. systems. Each link adds complexity complexity that reduces the reliability reliability of the whole. whole. For example, example, the sequenced start requires that the flame detectors detectors must be bypassed bypassed during the trial for ignition period. period. Once the relay relays s have have bypass bypasses es from from the PLC, they no longer longer have have any indepen independen dentt reliab reliabili ility. ty. And And 10 besides, who says that relays are safer than PLCs ? There There are also links between between the PLC and the DCS. DCS. The start start sequence sequence turns the fuel gas control control valve down down to its minimum minimum stop. It then manipulate manipulates s the air flow for purging. purging. Whether Whether these links have the form of signals passing between the PLC and the DCS or they take the form of solen solenoid oid valves valves inter interrup ruptin ting g the air signal signals s to the actua actuator tors, s, compl complexi exity ty is added added that that compromises compromises the integrity of both the BMS and the combustion combustion control system. Firstly, the start sequence sequence and the safety interlocks interlocks are one inseparable control control system. Normal operation is simply simply the final stage of the start sequence. The main flame is now on, on, and all safety interlocks are fully operational. Programmable Programmable control systems commonly used in today's process industries are built of large and powerful units that can handle logic and control functions far more complex than control of a single burner. A DCS, in particular, particular, consists of many many independent function blocks blocks that are either linked together or kept separate separate according to the requirements requirements of the control strategy. strategy. There is no reason why all aspects of a burner control cannot be carried out within a single physical controller. Certain precautions precautions must be taken to ensure that the NFPA requirement requirement for separation separation is met: 1.

The BMS BMS must must reside reside in a clea clearly rly identi identifie fied d bloc block k of of log logic ic functi functions ons.. This This block block must must have have all inputs inputs and outputs outputs clearly clearly identif identified ied within within the the block block.. A DCS that require requires s this this identifica identification tion as part of its programm programming ing approach approach is acceptabl acceptable. e. A DCS that requires requires



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"hardware" addressing is not. For example: Assume Assume that Logic Block 13 is the BMS. The statement "Set "Set Register 21 in Logic Block 13 ON" appearing elsewhere in the DCS is an example of risky programming practi practice. ce. An inspe inspecti ction on of Block Block 13 would would not reveal reveal the connec connectio tion. n. This This type type of statement is even more more dangerous if it is made made in error in a totally unrelated unrelated function. A safe method of accomplishing the desired function is to include the statement "If HS-12 is ON, then set BV-24 BV-24 ON" within the body body of Block 13. Anyone Anyone examining examining the BMS can readily see the statement statement and judge its validity. Such statements statements cannot accidentally be included in Block 13 by someone working on a different block. 2.

Outg Outgoi oing ng link links s betwe between en the the BMS and and othe otherr contr control ol func functi tion ons s must must alw alway ays s be "pus "pushe hed" d" by the BMS. For example: example: The BMS forces the fuel gas valve shut shut and then puts it back on automati automatic c at the appropriat appropriate e times. The fuel gas controller controller does not check the BMS to see what it should do.

3.

Inco Incomi ming ng link links s betwe between en the the BMS BMS and and other other cont contro roll funct functio ions ns mus mustt alwa always ys be "pu "pulllled ed"" by the BMS. For example: example: The BMS checks the status of a bypass bypass switch to see if it should ignore a high discharge discharge temperature. The bypass switch does does not force the BMS.

4.

No unr unrel elat ated ed log logic ic fun funct ctio ion n may may resi reside de in in the the same same bloc block k as a BMS. BMS. It mus mustt alw alway ays s be extremely clear to anyone making modifications whether or not they are in a BMS block. Since there may be times when it is necessary to disable some unrelated function, it must always be possible to do this without disabling any part of the BMS.

5.

Input Input and and outpu outputt modul modules es for BMS relate related d signa signals ls should should not not also also handle handle other other sign signals als..

6.

A har hardw dwir ired ed "ki "killll"" swit switch ch sho shoul uld d be inc inclu lude ded d on the the ope opera rato tor's r's cons consol ole. e. This This swi switc tch h dedeenergize energizes s all gas safety shutoff shutoff valves. This This can easily easily be accompli accomplished shed by cutting the power to the output modules. modules. If rule 5 is followed followed this can easily be accomplished accomplished without disturbin disturbing g unrelated unrelated functions functions of the DCS. The kill switch switch should should also have a second second contact wired to provide provide input to the BMS logic in the DCS. The BMS will then then carry out an independent independent shutdown. In this way way full redundancy is accomplished. accomplished. In addition, addition, the DCS logs the use of the kill switch.

7.

A DCS DCS cont contro roll unit unit that that carri carries es out out any any func functi tion ons s beyon beyond d the con contro troll of a sing single le furn furnac ace e must be fully dual redundant with automatic transfer to an identical, on-line, backup unit. All communications between the control unit and the operator's console must be fully dual redundant with auto transfer. transfer. The consoles themselves themselves must be at least dual dual redundant with no shared components.

It is my own opinion that a BMS can be integrated into DCS control unit safely, reliably, and legally if the above rules are followed. But... But... I realize that this a complex subject and that many users have had different experience. experience. I would very much welcome welcome feedback feedback on this controversial subject. Because of the wide range of interpretations of the functional separation rule, it is very wise to Controlling Fired Heaters


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outline outline any proposed proposed control system system in detail detail and have it approved approved by the "authori "authority ty having jurisdiction" before before proceeding with implementation. implementation. I understand it is not very pleasant to have a design refused during commissioning. commissioning. COMMISSIONING. No Burner Management System can be certified safe until it has been field tested. NFPA is quite quite clear on this this point: The safety interlock system and protective devices shall be jointly tested by the organiza organization tion with the system design design respons responsibilit ibility y and those who operate and maintain maintain such systems systems during the normal normal operating operating life of the plant. plant. These These tests shall be accomplished before initial operation. The proof of the pudding pudding is in the eating. eating. No safety system system is safe until until it is demonstrated to be safe. REFERENCES 1.

Drie Driedg dger er,, W. W. C., C., “Con “Contr trol olli ling ng Cent Centri rifu fuga gall Pum Pumps ps”; ”; Hydrocarbon Processing , July 1995. (http://www.cadvision.com/driedgew/CE1_CP.html)

2.

Drie Driedg dger er,, W. C., C., “Con “Contr trol olli ling ng Pos Posit itiv ive e Disp Displace laceme ment nt Pump Pumps” s”;; Hydrocarbon Processing , May 1996. (http://www.cadvision.com/driedgew/CE2_PD.html)

3.

Drie riedger, ger, W. C., C., “Con “Control rollin ling She Shell and and Tub Tube Exch xchange ngers”; rs”; Hydrocarbon Processing , March 1998. (http://www.cadvision.com/driedgew/CE3_STX.html)

4.

Drie Driedg dger er,, W. C., C., “Con “Contr trol olli ling ng Stea Steam m Heat Heater ers” s”;; Hydrocarbon Processing , November 1996. (http://www.cadvision.com/driedgew/CE4_ST.html)

5.

NFPA NFPA 8501 8501,, Stand Standard ard for Single Single Burne Burnerr Boile Boilerr Opera Operatio tion, n, 1997 1997 Editi Edition on.. (For (Forme merly rly 85A) 85A) (http://catalog.nfpa.org/)

6.

NFPA NFPA 8502 8502,, Stand Standard ard for Preven Preventio tion n of Furna Furnace ce Expl Explosi osion ons/Im s/Implo plosio sions ns in in Multi Multipl ple e Burne Burner r Boiler-Furnaces, Boiler-Furnaces, 1999 Edition. (Formerly 85C) (http://catalog.nfpa.org/)

7.

NFPA NFPA 86A, 86A, Stan Standa dard rd for for Ov Oven ens s and and Fur Furna nace ces, s, 199 1999 9 edi editi tion on.. (http://catalog.nfpa.org/)

8.

Drie Driedg dger er,, W., W., Limi Limitt Sw Switch itches es Key Key to Valv Valve e Re Reliab liabil ilit ity; y; Intech, January 1993. (http://www.cadvision.com/driedgew/)

9.

CAN CAN/CGA /CGA-B -B14 149. 9.33-M8 M89, 9, Code Code for for the the Fiel Field d Appr Approv oval al of Fuel Fuel-R -Rel elat ated ed Compo Compone nent nts s on Appliances and Equipment. (http://www.csa-intl.org/onlinestore/welcome.asp)



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10.

Gruh ruhn, P. and and Chedd eddie, ie, H.L. .L., Safety fety Shutd hutdo own Sys System tems: Justification, ISA (http://www.isa.org/reference/index.cfm)

11. 11.

ASME ASME Boil Boiler er and and Pres Pressur sure e Vesse Vessell Code, Code, Sect Section ion VII, VII, Care Care of of Powe Powerr Boile Boilers. rs. (http://www.asme.org/catalog/)

12. 12.

API API RP 550, 550, Manua Manuall on Instal Installat lation ion of of Refine Refinery ry Instru Instrumen ments ts and and Contro Controll System Systems s - Part Part 4: Steam Generators ( API has withdrawn this standard without a replacement. ) (http://www.cssinfo.com/apigate.html)

13. 13.

API API SPEC SPEC 6FA 6FA-8 -85, 5, Spe Speci cifi fica cati tion on of of Fire Fire Tes Testt for for Val Valve ves s (http://www.cssinfo.com/apigate.html)

14. 14.

API API SPEC SPEC 6FC-8 6FC-89, 9, Speci Specific ficati ation on of Fire Fire Test Test for for Valve Valves s with with Selec Selectiv tive e Backs Backseat eats s (http://www.cssinfo.com/apigate.html)

15. 15.

ISAISA-S1 S18. 8.1-1 1-197 979, 9, Annu Annunc ncia iato torr Seque Sequenc nces es and and Speci Specifi fica cati tion ons s (http://www.isa.org/reference/index.cfm)

16. 16.

ISA-S7 ISA-S77.4 7.41, 1, Foss Fossilil Fue Fuell Powe Powerr Plan Plantt Boil Boiler er Combu Combusti stion on Cont Control rols s (http://www.isa.org/reference/index.cfm)

17. 17.

CAN/ CAN/CG CGAA-3. 3.99-M8 M87, 7, Aut Autom omat atic ic Saf Safet ety y Shut Shut-Of -Offf Gas Gas Valv Valves es.. (http://www.csa-intl.org/onlinestore/welcome.asp)



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