FMDS0604

May 2006 of 17

OIL- AND GAS-FIRED SINGLE-BURNER BOILERS

Table of ContentsPage

1.0 SCOPE ................................................................................................................................................... 21.1 Changes ............................................................................................................................................ 2

2.0 LOSS PREVENTION RECOMMENDATIONS ....................................................................................... 22.1 Introduction ...................................................................................................................................... 22.2 Equipment and Processes .............................................................................................................. 3

2.2.1 Combustion Air and Preignition Purge .................................................................................. 32.2.2 Igniters, Safety Shutoff Valves, and Combustion Safeguards .............................................. 32.2.3 Additional Safety Controls ..................................................................................................... 42.2.4 Additional Recommendations for Gas Firing ........................................................................ 52.2.5 Additional Recommendations for Oil Firing ........................................................................... 52.2.6 Gas-Fired Multiport Burner Grid ............................................................................................ 72.2.7 Flue Gas Recirculation Interlocks ......................................................................................... 7

2.3 Protection ......................................................................................................................................... 82.4 Operation and Maintenance .......................................................................................................... 10

2.4.1 General ................................................................................................................................ 102.4.2 Alternate and Simultaneous Fuel Systems ......................................................................... 112.4.3 Flue Gas Recirculation ........................................................................................................ 13

2.5 Electrical ........................................................................................................................................ 143.0 SUPPORT FOR RECOMMENDATIONS ............................................................................................. 144.0 REFERENCES ..................................................................................................................................... 15

4.1 FM Global ...................................................................................................................................... 154.2 NFPA Standards ............................................................................................................................ 154.2 Other Standards ............................................................................................................................ 15

APPENDIX A GLOSSARY OF TERMS ..................................................................................................... 15APPENDIX B DOCUMENT REVISION HISTORY ..................................................................................... 15APPENDIX C SUPPLEMENTAL INFORMATION ...................................................................................... 16

C.1 Igniters .......................................................................................................................................... 16C.2 Safety Shutoff Valves .................................................................................................................... 16C.3 Other Standards ............................................................................................................................ 17

List of FiguresFig. 1. Single-burner automatic-lighted gas-fired boiler. Example of a permissive start and firing sequence. . 6Fig. 2. Schematic. Typical gas-fired piping arrangement over 2.5×106 Btuh (732 kW). ............................... 7Fig. 3. Single-burner automatic-lighted oil-fired boiler. Example of a permissive start and firing sequence. . 8Fig. 4. Schematic. Typical oil-fired piping arrangement over 2.5×106 Btuh (732 kW). ................................. 9Fig. 5. Inspection tank for SSOV and automatic vent valve leak checking. ............................................... 17

List of TablesTable 1. Summary of Safety Control Recommendations. .............................................................................. 2Table 2. Safety Shutoff Valve Recommendations for Gas-fired Boilers. See Section 2.2.2.3. .................... 4Table 3. Safety Shutoff Valve Recommendations for Oil-fired Boilers. ........................................................ 4

FM GlobalProperty Loss Prevention Data Sheets 6-4

©2006 Factory Mutual Insurance Company. All rights reserved. No part of this document may be reproduced,stored in a retrieval system, or transmitted, in whole or in part, in any form or by any means, electronic, mechanical,photocopying, recording, or otherwise, without written permission of Factory Mutual Insurance Company.

1.0 SCOPE

This FM Global loss prevention data sheet discusses the fuel hazards inherent in gas-fired or oil-firedsingle-burner boilers. Recommendations are made for the installation, maintenance, and operation ofcombustion safeguards and boilers as related to the fuel explosion and fire hazards. Automatic and supervisedmanual single-burner boilers rated greater than 400,000 Btuh (117 kW) input are covered. This data sheetdoes not apply to natural draft boilers; refer to ASME CSD-1, Controls and Safety Devices for AutomaticallyFired Boilers. This data sheet also does not apply to boilers 2,500,000 Btuh (732 kW) or less that are listedby a nationally recognized testing laboratory agency. Multiple-burner boilers are covered in Data Sheet 6-5,Oil- or Gas-Fired Multiple Burner Boilers.

Related data sheets are 7-32, Flammable Liquid Operations, and 7-88, Storage Tanks for Flammable Liquids.

1.1 Changes

May 2006. Editorial change was made to the recommendation 2.2.4.1.

2.0 LOSS PREVENTION RECOMMENDATIONS

2.1 Introduction

2.1.1 FM Approved equipment and devices such as fuel safety shutoff valves (SSOVs), fuel and airsupervisory switches, timers, and flame failure supervisory combustion safeguards, etc. should be used.Approved means equipment tested by FM Approvals and listed in the Approval Guide, a publication of FMApprovals.

In a few cases, FM Approved equipment may not be available to meet a specific arrangement or particularfield operating condition. In these cases, the equipment should be from a reliable manufacturer and haveproven satisfactory field experience. See Table 1 for a summary of safety control recommendations.

2.1.2 Equipment and devices should be suitable for the environment in which they are installed. Equipmentused in dusty or moist areas requires careful selection.

Table 1. Summary of Safety Control Recommendations.

Safety Device/Control Gas-Fired Boilers Oil-Fired BoilersFan motor interlocks and air flow/pressure switches—forced draft, induceddraft, and/or recirculation fans

Yes Yes

Pre-purge at 50 maximum airflow,minimum 4-volume changes

Yes Yes

Post-purge, 15 seconds minimum Yes, if >12.5 MMBtuh (3662 kW) Yes, if >2.8 MMBtuh (820 kW)Igniter/pilot See Section 2.2.2 See Section 2.2.2Pilot SSOV Yes YesMain burner SSOV Yes. See Table 2. Yes. See Table 3Tightness test connections for mainSSOVs

Yes. See Data Sheet 6-0.

Low gas pressure switch Yes, if > 2.5 MMBtuh (732 kW)High gas pressure switch Yes, if > 2.5 MMBtuh (732 kW)Interlock for low fire light-off Yes, if >2.5 MMBtuh (732 kW) Yes, if >2.8 MMBtuh (820 kW)Nonrecycling combustion safeguard Yes, if >2.5 MMBtuh (732 kW) Yes, if >2.8 MMBtuh (820 kW)10-second pilot flame establishing period Yes YesLimited main flame trial-for-ignition 10 seconds 10 seconds for No.2 & No.4 Oil;

15 seconds for No. 5 & No.6 OilSSOV proved closed prior to and duringpurge

See Table 2 See Table 3

Low oil pressure switch Yes, if >2.8 MMBtuh (820 kW) unless oilpump is integral with burner motor shaft

High/low oil temperature switch Yes (for heated oils)

6-4 Oil- and Gas-Fired Single-Burner Boilers FM Global Property Loss Prevention Data Sheets

©2006 Factory Mutual Insurance Company. All rights reserved.

Safety Device/Control Gas-Fired Boilers Oil-Fired BoilersAtomizing steam/air switch Yes (if applicable)Electric igniter proved fully inserted Yes (if applicable) Yes (if applicable)High steam pressure/temp. Yes (on unattended boilers) Yes (on unattended boilers)Low water cutout* Yes Yes

* Refer to Data Sheet 6-12, Low Water Protection.

2.2 Equipment and Processes

2.2.1 Combustion Air and Preignition Purge

2.2.1.1 Interlock any fans for combustion air, for removal of combustion products, or for recirculation ofcombustion products so that fuel SSOVs are closed and electric ignition deactivated unless fans are inoperation and proper flow is provided. Air flow or pressure switches should be provided. For three-phaseblower motors, the airflow switch should be interlocked with the motor starter. Loss of airflow or fan motor notrunning should cause a complete shutdown of the boiler. Air flow supervision is not needed with oil-firing ifthe fan is integral with the burner motor shaft.

2.2.1.2 Design for a supervised preignition purge of the boiler furnace, passes, and horizontal breechingwith air flow not less than 50% of the air flow at the maximum firing rate for a duration sufficient to achievenot less than four volume changes of the furnace/fire box and boiler passes. Flame failure or failure to igniteon start-up requires a full repurge before attempting a relight.

2.2.1.3 Interlock induced- or forced-draft dampers that are automatically positioned by the combustion controlsystem so that on lighting off, the induced-draft dampers will be open and the forced-draft dampers will bein position for smooth light-off. Adjust the control for these dampers so that proper amounts of air for completecombustion at all firing rates will flow through the boiler furnace. Smooth light-off depends on proper draftsetting and on proper throttling of the fuel input. The burner fuel and air supplies should be suitably controlledto give smooth light-off at low-fire conditions. It is preferred that low fire position switches, when they areused, be located on the controlled device in case of linkage separation. This may not be practical on somesmall package boilers.

2.2.1.4 If there is no ducted air supply to the boiler, provide openings or the equivalent in the boiler roomto permit entrance of sufficient fresh air for complete combustion of the fuel at all firing rates. At least 1 ft2

(0.09 m2) of free opening should be provided for every 2 MMBtuh (586 kW) of fuel burned. For boilersoperating with natural draft, allow 1 in2 (650 mm2) of free opening for every 2,000 Btuh (586 W) of fuel burnedfor boilers with horizontal stacks and 4,000 Btuh (1172 W) for boilers with vertical stacks. Some plants utilizeexhaust fans to rid the area of dust or vapors. If the boiler room is not separate from these areas, thecombustion air fan may not be able to overcome the negative pressures created. In such an instance, thecombustion air supply should be ducted directly from out-of-doors.

Air preheating may be necessary in colder climates as cold air can interfere with proper operation of a boiler,and can cause a freeze-up of pneumatic control lines. Control air should be kept clean and dry. Refer toData Sheet 9-18/17-18, Prevention of Freeze-ups.

2.2.2 Igniters, Safety Shutoff Valves, and Combustion Safeguards

2.2.2.1 Provide igniters that have sufficiently large capacities to ensure prompt ignition of the main burners.It is preferred that igniters be used that are stable with the main burner extinguished and with maximumboiler draft. Do not use direct spark igniters on light oil burners greater than 2.8 MMBtuh (820 kW) input orgas-fired burners greater than 400,000 Btuh input unless the burner lights off at a fuel input less than theseratings. There are four classes of igniters: Class 1, 2, 3, and 3 Special. Igniters are described in Appendix C.

2.2.2.2 Install the pilot, electric-spark igniter, and flame-sensing element of the combustion safeguardsecurely so that the position of each in respect to the others and the main flame will not change. Provideobservation ports so that the positions can easily be observed while the pilot and/or main burner are firing.These units should be readily accessible for inspection and cleaning.

2.2.2.3 Install safety shutoff valves (SSOVs) in the fuel supply lines to both the pilots and main burners.Refer to Tables 2 and 3 for SSOV recommendations. For small pilots or fuel burners having less than 400,000

Oil- and Gas-Fired Single-Burner Boilers 6-4FM Global Property Loss Prevention Data Sheets


Btuh (117 kW) input, one SSOV is adequate. Using an FM Approved automatic leak detection device fulfillsthe requirements for proof-of-closure (as required in Table 2). This device pumps gas above line pressurebetween the two safety shutoff valves and monitors for pressure drop for a predetermined period of time beforepurge can begin. Once the test has been completed, the pressurized gas is bled back into the supply line.See Appendix C for more information on SSOVs.

2.2.2.4 Provide an FM Approved nonrecycling flame-supervisory combustion safeguard arranged to firstprove the existence of a reliable pilot flame before permitting the main burner SSOV to open. Limit thepilot-trial-for-ignition period to ten seconds. When an Approved high-voltage spark igniter is provided, the fuelburning pilot and pilot trial-for-ignition may be omitted. If a retractable type of igniter is used, it should beproved to be fully inserted. After a shutdown occurs due to a flameout, the combustion safeguard is allowedto recycle once on a gas-fired boiler rated at less than 2.5 MMBtuh (732 kW) or on an oil-fired boiler ratedat less than 2.8 MMBtuh (820 kW).

Table 2. Safety Shutoff Valve Recommendations for Gas-fired Boilers. See Section 2.2.2.3.

400,000 Btuh– 2.5 MMBtuh(117–732 kW)

>2.5–5 MMBtuh(732–1465 kW)

>5–12.5 MMBtuh(1465–3662 kW) >12.5 MMBtuh (>3662 kW)

Two SSOVs, or one SSOVwith proof-of-closure

Two SSOVs or one SSOVwith proof-of-closure

Two SSOVs, one withproof-of-closure

Two SSOVs withproof-of-closure

Table 3. Safety Shutoff Valve Recommendations for Oil-fired Boilers.

400,000 Btuh–12.5 MMBtuh (117–3662 kW) >12.5 MMBtuh (>3662 kW)Two SSOVs or one SSOV with proof-of-closure * Two SSOVs with proof-of-closure *

* A relief device should be installed between valves if there is a possibility of pressure build-up due to location in a hot area.

2.2.2.5 Arrange a nonrecycling flame-supervisory combustion safeguard for direct supervision of the mainburner flame only, following proof of pilot flame and opening of the main burner SSOV. Limit the main burnertrial-for-ignition to the shortest practical time, but not more than ten seconds (15 seconds for No. 5 and No. 6oil).

2.2.2.6 Install the combustion safeguard flame-sensing element(s) in accordance with the manufacturer’sinstructions. Locate the main-burner flame-sensing element so that it reliably senses the main flame at all firingrates. The fuel SSOVs must be deenergized within four seconds of a flameout. When a pilot is proved bya combustion safeguard, supervision should be at a location where it will reliably sense the presence of aflame. A main burner should be ignited immediately by its pilot even when the pilot is reduced to the minimumflame capable of holding the flame-sensing relay of the combustion safeguard in the energized (flame-present) position.

2.2.2.7 The safe-start check of the combustion safeguard should not be nullified by the action of operatingor limit controls. A safe-start check, which takes place each time a boiler is started, determines if the flamescanner will detect a flameout. For boilers that are in continuous operation or where an operating cycleexceeds 24 hours, provide an automatic self-checking UV flame scanner (when a UV flame scanner is used).Self-checking is not needed on electronic UV scanners that do not use UV cells.

2.2.2.8 A combustion safety control system may be linked to a programmable logic controller (PLC) approvedfor burner management or combustion safety. The PLC may be used for boiler control such as combustioncontrol and feedwater control, but not for any other process control. Refer to Data Sheet 7-45, Instrumentationand Control in Safety Applications.

2.2.3 Additional Safety Controls

2.2.3.1 High water temperature or steam pressure limit controls should be provided on all boilers, particularlyunattended boilers. Operation of these devices should cause a safety shutdown, requiring a manual reset.

2.2.3.2 Provide interlocked low-fire lighting off on automatic boilers so that on a call for heat, and after thepreignition purge has been satisfied, fuel and air controls are in the correct minimal low firing positions beforethe ignition cycle can proceed for gas burners greater than 2.5 MMBtuh (732 kW) and for oil burners greaterthan 2.8 MMBtuh (820 kW).

2.2.3.3 Provide low water protection interlocks. See Data Sheet 6-12, Low Water Protection.



2.2.3.4 Consideration should be given to provide combustibles/oxygen analyzers, as an aid to safe andefficient operation, on supervised manually operated boilers of 100,000,000 Btuh (29,300 kW) capacity andover, and preferably on smaller units as well.

2.2.4 Additional Recommendations for Gas Firing

The following recommendations apply to new boilers and new conversions to gas firing, and burnerinstallations on any size boiler, new or in service, where a serious production interruption is possible.

2.2.4.1 Provide permanent and ready means for making periodic tightness checks of the main burner safetyshutoff valve. See Data Sheet 6-0, Elements of Combustion Controls, and Safeguards in Industrial HeatingEquipment. The need for tightness checks is lessened if an FM Approved automatic leak testing systemis installed. This type of system pressurizes the space in between the SSOVs, and checks for pressure dropeach time a boiler is started. Perform a manual leak test annually if equipped with the automatic systemto determine that leakage does not exceed the requirements of Data Sheet 6-0, Elements of Combustion,Controls, and Safeguards in Industrial Heating Equipment.

2.2.4.2 Provide high and low gas pressure interlocks for boilers greater than 2.5 MMBtuh (732 kW) by meansof FM Approved pressure switches arranged to shut off and lock out all fuel and electric ignition in the eventof dangerously low or high pressures. Gas pressure switches are optional for boilers less than 2.5 MMBtuh(732 kW) if provided with a pilot. Setting of switches should fall within the turndown or safe operating rangeof the burner. Locate the switches between the pressure regulator and safety shutoff valves. Alternatively,the high pressure switch can be located downstream of the safety shutoff valves, however, account forpressure drops in the fuel train when setting the high pressure switch.

The fuel train should be designed to withstand the maximum pressure that can develop because of a pressureregulator failure. The high pressure switch should be set lower than this pressure. If the fuel train cannotwithstand this pressure or this pressure can exceed 60 psig, then install a relief valve just downstream of theregulator and vent to a safe location. Set the high pressure switch lower than the relief valve setting andno higher than 150% of normal pressure. Preferably, set the switch at about 125% of normal pressure as longas nuisance tripping does not result.

2.2.4.3 Provide manual gas cocks which will only allow handles to be installed such that the handle mustbe perpendicular to the gas pipe when closed.

An example of a permissive start-up and firing sequence for a gas-fired automatically lighted boiler burneris shown in Figure 1. (See also Fig. 2.)

For a supervised manual system, the permissive start and firing sequence is essentially as shown in Figure1. The operator initiates the start of the sequence. Then, as the sequence steps are completed and theinterlocks are satisfied the subsequent step may be initiated. When changes in firing rate are made manually,the fuel and air supplies should be adjusted simultaneously at a pre-established optimum air-fuel ratio. Onlyone control device should be manipulated to achieve the change in firing rate.

2.2.5 Additional Recommendations for Oil Firing

The following recommendations apply to new boilers and new conversions to oil firing, and burner installationson any size boiler, new or in service, where a serious production interruption is possible.

2.2.5.1 Provide a low-fuel-oil supply pressure interlock by means of an Approved pressure switch arrangedto shut off and lock out all fuel and electric ignition in the event of inadequate supply pressure. A low oilcondition should require that an operator correct the problem and then manually reset the switch orcombustion safeguard.

2.2.5.2 Provide a low-temperature interlock for oil that requires preheating to ensure that the oil is at theviscosity recommended by the burner manufacturer before the burner can be fired. A high temperature cutoutswitch should also be provided if the oil can be heated enough to cause vapor lock at the pump. To ensureprompt ignition during trial-for-ignition, arrange the piping and preheating system so that the volume ofunheated oil immediately upstream of the burner tip is at a minimum. Specifications for oils should beunderstood by operators.

2.2.5.3 Interlock the atomizing air or steam supply to the burner (generally used for Nos. 5 and 6 oil) so thatall fuel and electric ignition will be shut off and locked out in the event of inadequate atomizing pressureor lack of proper differential between oil pressure and atomizing medium pressure.



Fig. 1. Single-burner automatic-lighted gas-fired boiler. Example of a permissive start and firing sequence.



An example of a permissive start-up and firing sequence for an oil-fired automatic-lighted boiler is shownin Figure 3. (See also Fig. 4.)

For a supervised manual system, the permissive start and firing sequence is essentially as shown in Figure3. The operator initiates the start of the sequence. Then, as the sequence steps are completed and theinterlocks are satisfied, the subsequent steps may be initiated. When changes in firing rate are made manually,the fuel and air supplies should be adjusted simultaneously at a pre-established optimum air-fuel ratio. Onlyone control device should be manipulated to achieve the change in firing rate.

2.2.6 Gas-Fired Multiport Burner Grid

2.2.6.1 The recommendations previously presented in this data sheet should be applied. However, mainflame supervision (with interrupted pilot) is generally impractical on gas- fired multiport burner grids.

2.2.6.2 The pilots may be either spark-ignited or manually ignited.

2.2.6.3 The light-off pilots should have FM Approved nonrecycling flame supervision combustion safeguardsarranged to prove the existence of reliable pilot flame before permitting the main burner safety shutoff valveto open and during main burner operation.

2.2.6.4 Stabilization pilots should be supervised. The failure of the pilot flame should cause shutoff of thefuel to the pilots and main burners.

2.2.7 Flue Gas Recirculation Interlocks

The following interlocks should be provided to prevent unstable combustion due to an FGR malfunction,and should cause a boiler shutdown and closure of the FGR damper. Otherwise, damage could result to theFGR system or conditions could result that could lead to a boiler explosion due to off-ratio firing or a flameout.

2.2.7.1 An FGR fan motor electrical interlock should be provided, using motor starter auxiliary contacts(three-phase motors only).

2.2.7.2 An FGR fan flow switch should be provided to verify proper gas flow through the system.

2.2.7.3 An FGR control damper position interlock should be provided, if practical. This compares actualdamper position with control position and should not allow the boiler to operate if the damper is not withinthe limits.

2.2.7.4 An FGR circuit trouble interlock should be provided. The boiler should not operate if there is circuittrouble or loss of power.

2.2.7.5 A stack low temperature interlock should be provided.

1. Manual Cock2. Pressure Regulator3. Low Gas Pressure Switch4. Safety Shutoff Valve

5. Safety Shutoff Valve w/Proof of Closure6. Leak Test Connection7. High Gas Pressure Switch8. Combustion Air Pressure Switch

Fig. 2. Schematic. Typical gas-fired piping arrangement over 2.5×106 Btuh (732 kW).



2.3 Protection

2.3.1 Provide automatic sprinklers of proper temperature rating in boiler rooms of combustible constructionand in noncombustible boiler rooms that contain sufficient combustibles, other than fuel oil, to sustain a fire.Refer to Data Sheet 7-88, Storage Tanks for Flammable Liquids.

2.3.2 Locate boilers preferably in a separate noncombustible room or basement. A manufacturing room isnot objectionable for small boilers if the floor area within 15 ft (4.5 m) of the burner and 10 ft (3 m) of the boileris kept free of combustibles and the room is free from readily ignitable fibers, exposed flammable liquids,combustible dusts, or contents susceptible to smoke or fume contamination. See Section 2.2.1.4 oncombustion air.

Fig. 3. Single-burner automatic-lighted oil-fired boiler. Example of a permissive start and firing sequence.



2.3.3 Replace wooden floors with concrete extending 4 ft (1.2 m) beyond the boiler, unless there is agenerous clearance for free-air circulation beneath the boiler sufficient to keep floor temperature below 160°F(71°C). Capping a wooden floor with concrete is not acceptable.

2.3.4 Avoid passing metal stacks through combustible ceilings, walls, and floors. Where this is unavoidable,provide metal collars, rain shields, and clearances or suitable insulation and/or clearances to keep the surfacetemperature of the combustible material below 160°F (71°C).

2.3.5 Clearance between metal stacks and stored combustible materials should be at least 30 in. (0.76 m).Guards that will assure this clearance should be installed.

2.3.6 Provide each oil- or gas-fired boiler with a manually operated fuel shutoff valve for emergency closingin case of fire. The valve should be prominently marked, easily accessible, and preferably located outsidethe boiler room.

2.3.7 Provide fusible-link-actuated shutoff valves in the oil supply line to oil-fired boilers. These valves shouldbe located in the boiler room on the suction side of the pump. For multi-boiler locations with a common oilsupply pump, each boiler should have its cable linked to a separate valve. The fusible link for the valves shouldbe located at the boiler burner front. The valves should be tested periodically to be sure that the weightswill close the valves. An alternative method would be to have a heat detector mounted above the burner andwired into the safety control circuit. Actuation of this device would cause the SSOV to become deenergized.

2.3.8 Recognition should be given to the fire hazards imposed by leakage or rupture of oil piping near theburner. Particular attention should be given to flexible connections, hoses, swivel joints, etc. Goodhousekeeping is important.

Braided oil hoses should be replaced periodically according to manufacturers’ recommendations, and acommitment to a replacement frequency should be made based on usage and consultation with the hoseor boiler manufacturer. Older hoses may not flex properly and may leak at threaded connections in the fuelpiping or result in hose breakage, especially if there are 90 degree bends; hoses that are bulged, stiff, orcorroded need immediate replacement. Hoses are subject to both tensile and compressive stresses, tointernal pressure, and to the extremes of temperature, vibration, corrosive atmospheres, and physical impactand reactive forces. Double braided, noncombustible hoses should be used. Hoses should be designed forthe oil being fired and should be capable of withstanding four times the normal maximum operating pressure.Hose couplings and fittings and minimum bending radius should be in accordance with manufacturers’

Fig. 4. Schematic. Typical oil-fired piping arrangement over 2.5×106 Btuh (732 kW).

1. Manual Cock2. Preheater3. Low Oil Temperature Switch4. Low Oil Pressure Switch5. Oil Circulating Valve

6. Safety Shutoff Valve w/Proof ofClosure

7. Safety Valve8. Atomizing Media Pressure Switch9. Combustion Air Pressure Switch10. Pressure Regulator



instructions. If 90 degree bends are necessary, use 90 degree elbows. Valves should be installed upstreamof hoses. Flexible hoses are a likely place for a fuel leak to develop, and should be examined carefully.

2.4 Operation and Maintenance

2.4.1 General

2.4.1.1 Operators should be trained in the proper operation of the boiler and in the specific functions of thevarious safety controls. Operating instructions should be posted or kept in the boiler room for ready reference.

Posted operating procedures that are faithfully adhered to, combined with a solid operator training programthat prepares operators to handle various emergency scenarios before they happen, will reduce losses.

Operator awareness of critical valve positions and other conditions before taking any actions will greatlycontribute to safe operation of boilers. For example, if an operator checks to see if any valves, manual andautomatic, have been open before lighting off a gas burner, then the operator can determine if there is apossibility of a vapor accumulation in the combustion chamber.

2.4.1.2 Maintain all equipment in proper operating condition. Maintenance details and schedules dependon the equipment and operating conditions. The program should follow a specific routine recommended bythe manufacturer and should include, at scheduled intervals, maintenance of burner equipment, gas SSOVleakage testing, inspection and cleaning of the electric-igniter and/or pilot-burner assembly, adjustment oflinkages and controllers for fuel-air ratio control dampers and valves, and maintenance and testing ofcombustion safeguards and safety controls as recommended by the manufacturer.

SSOVs should be inspected and maintained according to manufacturers’ instructions. Valve stems and guidesshould be kept clean to prevent sticking. Certain soft parts such as packing, seals, and O-rings may hardenwith time, and should be replaced as necessary. Aging valves that have not been maintained may need tobe replaced.

2.4.1.3 Inspect and test safety controls periodically according to manufacturers’ recommendations to ensureproper functioning when emergencies arise. Failure to make periodic checks may result in fire or explosiondamage and may also contribute to accidental shutdown and loss of production.

A carefully planned inspection/test procedure should be drawn up for each boiler. This procedure shouldinclude tests, performed at prescribed intervals, for tightness of gas safety shutoff valves (SSOVs), forresponse to flame failure, and for proper action of fuel-air interlocks. Tests should be recorded on asafety-control inspection and testing report, listing the date, controls tested, their condition, and the propersequence and methods of performing the tests. Refer to Data Sheet 9-0, Preventive Maintenance.

The tests should be conducted by well-trained personnel who are familiar with the equipment and the specificfunctions of the various safety controls. Any defects found should be corrected immediately. Upon completion,covers of all safety controls should be secured in place to minimize tampering and introduction of dust anddirt.

The following is a testing frequency schedule which should be followed whenever practical. Details of thetesting procedures should be developed in accordance with manufacturers’ recommendations if available.

1. Monthly.

a. Flame failure detection system.

b. Fan and airflow interlocks.

c. Fuel safety shutoff valves for leakage. Refer to Data Sheet 6-0, Elements of Industrial HeatingEquipment.

d. Low fire start interlock.

e. High steam pressure or temperature interlock.

f. For Oil: Fuel pressure and temperature interlocks.



g. For Gas: (a) Gas cleaner and drip leg.

(b) High and low fuel pressure interlocks.

Note: Monthly and semiannual tests should be performed if qualified personnel who regularly maintain burnerequipment are available. Otherwise, these tests may be performed annually by a qualified contractor.

2. Semiannually or Annually (if conditions permit).

a. Igniter and burner components. Smooth, reliable light-off in the required period of time should beachieved. Proper flame color and shape and exhaust color are indicative of good combustion.

b. Combustion air supply system. Air flow or pressure switches, and damper high/low fire interlocksshould be checked.

c. Flame failure system components. Flame scanners and all safety controls that interlock with the SSOVthrough the combustion safeguard controller should be checked.

d. Piping, hoses, wiring, and electrical connections of all interlocks and shutoff valves. Check for leaks,corrosion, and loose connections.

e. Combustion control system. Check the fuel-air control system at various loads to see that proper air-fuelstoichiometry is maintained during turndown.

f. Calibration of indication and recording instruments.

3. As Required for Oil-Firing.

a. Disassemble and clean atomizers.

b. Clean strainers.

Cleaning of fuel piping and provision of strainers or cleaners in the fuel supply will prevent foreign matterfrom interfering with the operation of valves.

2.4.2 Alternate and Simultaneous Fuel Systems

2.4.2.1 Alternate Fuel Firing and Transfer System

Some single-burner boilers may fire either gas or oil. The combustion control system is arranged to providea proper fuel-air ratio for individual firing of either fuel, but only one fuel at a time. The transfer to the alternatefuel may be accomplished either manually or automatically through a suitable fuel transfer system. Thevarious interlocks, flame supervision, and safety shutoff valves should be in service for the fuel being fired.The transfer procedure should be compatible with the specific combustion control system and should includethe following sequence: (1) the burner should be shut down; (2) the alternate fuel system with its respectiveinterlocks and the combustion control system adjusted for the alternate fuel should be placed in service;and (3) a light-off procedure compatible for the fuel should be followed. These procedures should be followedwhen switching to other fuels such as propane. Never change over in a high firing position. If simultaneoustransfer is necessary, the boiler must be reduced to low firing.

2.4.2.2 Alternate Fuel System—Simultaneous Fuel Firing Transfer

Some steam demands may require transfer from firing one fuel to another without stopping the fuel inputto the furnace even though the combustion control system is suitable for firing only one fuel at a time. Thetransfer procedure should be accomplished manually by a qualified operator following a procedure that willprevent a fuel-rich condition.

The burner should be capable of burning two fuels simultaneously and the system should be equipped withthe various interlocks, flame supervision, and SSOVs necessary for the combustion safe guard system foreach fuel. The fuel selector switch should be provided with a gas position in which oil cannot be fired, an oilposition on which gas cannot be fired, and a gas-oil position that permits simultaneous firing of both fuelsprovided all interlocks for both fuels are satisfied, including light-off position for both fuels. Manual valvesshould be provided at the burner, downstream of the SSOVs in each fuel line. Pressure gauges should beinstalled downstream of the manual valves.



2.4.2.2.1 Procedure for changing from gas to oil

1. If an intermittent pilot is available, place it in service.

2. Check that the manual oil valve at the burner is closed.

3. Establish oil fuel system to satisfy interlocks.

4. Install oil atomizer.

5. Open atomizing medium shutoff valve.

6. Place combustion control system in manual position.

7. Reduce gas flow to the low firing rate.

8. Place oil control valve in the normal light-off position.

9. Place fuel transfer switching system into oil-gas position.

If the oil safety interlocks are satisfied, the oil safety shutoff valve will open. Fuel oil pressure now will beupstream of manual oil valve at the burner.

10. Observe the gas pressure downstream from the manual gas shutoff valve and slowly close valve untilthe gas pressure starts to drop. At this point, the gas flow rate is controlled by the manual valve instead of bythe normal control valve.

11. Simultaneously and slowly close the manual gas valve while operating the manual oil valve to light theoil flame from the gas flame. Continue to increase the oil firing rate while cutting back on gas firing rate tokeep a constant heat input of the combined fuels to the burner until the manual gas valve is closed and manualoil valve is fully open. Care must be taken to maintain an adequate amount of excess air at all times bycontinuously observing the burner flame, or by observing the fuel-air ratio or oxygen indicator, if provided.During this period, air flow is maintained at a constant rate, with only the manual fuel valves in operation.

12. Place the fuel transfer system in the oil position. The gas safety shutoff valves will now close.

13. Return the combustion control system and burner firing rate to normal operation.

2.4.2.2.2 Procedure for changing from oil to gas

1. If an intermittent igniter is available, place it in service.

2. Check that the manual gas valve at the burner is closed.

3. Establish gas fuel system to satisfy interlocks.

4. Place combustion control system in manual position.

5. Reduce oil flow to the low firing rate.

6. Place gas control valve in the normal light-off position.

7. Place fuel transfer switching system in the gas-oil position.

If the gas safety interlocks are satisfied, the gas safety shutoff valves will open. Gas pressure now will beupstream of gas manual valve at the burner.

8. Observe the oil pressure downstream from the manual oil shutoff valve and slowly close valve until theoil pressure starts to drop. At this point, the oil flow is controlled by the manual valve instead of by the normalcontrol valve.

9. Simultaneously and slowly close the manual oil valve while operating the manual gas valve to light thegas flame from the oil flame. Continue to increase the gas firing rate while cutting back on oil firing rate to keepa constant heat input of the combined fuel to the burner until the oil valve is closed and manual gas valveis fully open. Care must be taken to maintain an adequate amount of excess air at all times by continuouslyobserving the burner flame, or by observing the fuel air ratio or oxygen indicator, if provided. During thisperiod, air flow is being maintained at a constant rate, with only the manual fuel valves in operation.

10. Place the fuel transfer system in the gas position. The oil safety shutoff valve will now close.

11. Return the combustion control system and burner firing rate to normal operation.



2.4.2.3 Simultaneous Firing of Oil and Gas

Some single-burner assemblies and combustion control systems that maintain a proper fuel-air ratio for eachfuel and the combined fuel input are suitable for simultaneous firing of oil and gas. The combustion controlsystem should be of the type that meters and totals to inputs from both gas and oil fuels, alone or in anycombination, and controls air flow proportionally. In most instances, a fuel selector switch is provided to permitfiring of either fuel or both fuels simultaneously. The various interlocks, flame supervision and SSOVs shouldremain in service for the fuel or fuels being fired. During simultaneous firing of two fuels, the interlocks forone fuel should not bypass the interlocks for the second fuel. Such a bypass could permit the introduction ofthe second fuel in a condition unsuitable for proper combustion. The operating procedures should becompatible with the specific combustion control system and should follow the manufacturers’ instructions.

2.4.3 Flue Gas Recirculation

2.4.3.1 Design Considerations

2.4.3.1.1 The flue gas recirculation (FGR) fan, which delivers flue gas back to the burner for the purposeof NOx reduction, should be sized to deliver the correct volume of gas at the required pressure.

2.4.3.1.2 A gas/oil selector switch should be provided for combination-fired boilers to change recirculationrates. The rates are lower for oil than for gas.

2.4.3.1.3 Allow a cool-down period (allowing the fan to run) for the FGR fan after boiler or FGR shutdown,if practical.

2.4.3.2 Fan-Assisted FGR System

2.4.3.2.1 The FGR control damper should be proven fully open before purge can begin. This permits airflow from the forced draft fan through the FGR system and out the stack.

2.4.3.2.2 The FGR control damper should close after purge and be proven fully closed before light-off.

2.4.3.2.3 The FGR fan should start after burner light-off.

2.4.3.2.4 The FGR damper should not be allowed to open until the stack temperature reaches apredetermined value established by the manufacturer. If the gas temperature is too low, the percentage ofgas recirculated will be too high since the system controls volumetric flow rather than mass flow.

2.4.3.2.5 A minimum time delay established by the manufacturer after proper stack gas temperature isestablished should be allowed before the FGR damper is permitted to open to the low fire position.

2.4.3.2.6 A minimum time delay established by the manufacturer after initial FGR damper opening shouldbe allowed before the boiler is permitted to modulate in order to achieve steady state operation.

Note: It is preferable to purge the FGR duct and start up the system as recommended in this section; however,this may not be possible with some systems. Some systems have all of the fans starting at the same time.The FGR damper may also be mechanically connected to the fuel and air controls, making it impossibleto implement some of these recommendations.

When the above procedure cannot be followed, purge time should be of sufficient duration consistent withthe above procedure to ensure that only an inconsequential amount of combustibles are present at the endof the purge taking into account the recirculation through the FGR duct.

2.4.3.3 Induced Flue Gas Recirculation

2.4.3.3.1 The flue gas recirculation damper should modulate according to the firing rate. The combustioncontrols should be designed and tested to ensure that the rate of change of FGR flow matches the fuel andair flows, with the flue gas recirculation matching the fuel in the lead-lag control arrangement if possible.This can be accomplished by means of a direct mechanical linkage to the fuel-air controller or by other controlmeans such as electric or pneumatic. Controls should be evaluated for reliability. The damper should bedesigned to fail in the closed position if there is an actuator failure, linkage failure or loss of actuating medium.A fixed damper (positioned at initial startup) could allow combustion upset to occur during transient conditionsor load changes due to a lag in pressure changes throughout the system, causing a temporary unacceptableincrease in the rate of recirculation. It is preferable that fixed dampers not be used in new installations. If afixed damper is used, operators should understand that rapid load changes could cause flame instability.



2.4.3.3.2 Purge time should be re-evaluated to account for purging the flue gas recirculation duct. Since apercentage of purge air will be recirculated, the purge period will have to be increased in order to purge to fourvolume changes.

2.4.3.3.3 Methods to reduce NOx can cause an increase in other emissions such as carbon monoxide (CO)and unburned hydrocarbons. It should be verified that CO emissions remain well below the lower explosivelimit at all steady state loads and during load changes. With a properly designed and maintained system,CO emissions may actually decrease.

2.4.3.3.4 Operating at very low or high loads may cause combustion instability when using some NOx

reduction methods, especially when using secondary combustion or low NOx burners. Turndown ratio mayneed to be reduced slightly when using these methods. At low loads, although mixing and turbulence may beassisted by FGR, too much flame quenching may cause an increase in unburned combustibles. Systemswith fixed damper systems may be more susceptible to this. At high loads, the fuel-air velocity may overcomethe flame velocity and cause it to move away from the burner and become unstable. Flame scanners mayneed to be relocated if flame characteristics change.

2.4.3.3.5 Maximum recirculation rates should be approximately 30% for natural gas and 15% for oil. Fluegas recirculation is primarily used to reduce thermally generated NOx and is not always as effective for oilbecause oils generally contain more fuel-bound nitrogen than natural gas.

2.4.3.3.6 When installation of flue gas recirculation is planned, calculations should be provided that explainthe percentage of recirculated flue gas in terms of mass flow rates. The rate of recirculation is equal to themass flow of recirculated gas divided by the total of the air, fuel and recirculated gas mass flow rates. Amechanical and thermal energy balance should be calculated to verify the percentage flue gas mass flow ifit is impractical to measure it.

2.4.3.3.7 If flue gas is allowed to recirculate at start-up, it will be at a lower temperature than the normaloperating temperature. Since the system is controlled volumetrically, there will be a slightly higher mass flowrate when the flue gas is cold. Too much FGR at start-up when the furnace is cold and before a stable flameis established could cause combustion difficulties. Therefore, it should be verified by calculation that theincreased flue gas density of cold flue gas does not adversely impact the combustion fuel-air ratio duringstart-up. The percentage of recirculation at start-up may need to be reduced.

2.4.3.3.8 During installation, setup of the system (such as damper linkage adjustments and cam settings)should be verified to be in agreement with calculations by means of on-site testing performed under thedirection of the manufacturer.

2.5 Electrical

2.5.1 Electrical installations should conform to the National Electrical Code® or other statutory local codes.The burner front is classified as non-hazardous. Although there is a possibility of combustible vapors beingpresent, the close proximity of the igniter, pilot, and burner make the additional risk of an explosion froman electrical spark negligible.

2.5.2 AC and dc safety-control circuits should be two-wire, one side grounded, and not over nominal 120volts. All safety-control switching should be in the ungrounded conductor. If a boiler has a 240 volt safetycontrol system, it should be determined if the SSOV coil drops out at about 175 volts. Failure of the coil to dropout at 175 volts could cause the valve to remain open all the time under fault conditions. This recommendationonly applies to countries where 120 volt circuits are used and is mandatory in the U.S. Overcurrent protectionshould be provided. Ground fault protection is also desirable. In addition to circuit grounds, non-current-carrying metal parts, such as equipment enclosures and conduit, should also be grounded.

2.5.3 Where an ungrounded dc power supply cannot be avoided, all switching should be located in oneconductor, and ground fault detection should be provided.

3.0 SUPPORT FOR RECOMMENDATIONS

Although losses during normal boiler operation due to malfunction of equipment are common, properlyinstalled and maintained safety controls will help to prevent or mitigate many of these losses. Safety controlswill also help to ensure a safe lightoff and shutdown. Note that some of the worst losses have occurred aftershutdown because of gas leaking in. This shows the importance of testing safety shutoff valves. Operator



training and following documented, safe operating procedures have also proved to be critical. Therecommendations in this data sheet support the need for safety devices, operator training, proper operatingprocedures, regular testing and maintenance.

4.0 REFERENCES

For more information, please refer to the following data sheets as cited in the text.

4.1 FM Global

Data Sheet 6-0, Elements of Industrial Heating EquipmentData Sheet 6-5, Oil- or Gas-Fired Multiple Burner BoilersData Sheet 6-12, Low Water ProtectionData Sheet 7-32, Flammable Liquid OperationsData Sheet 7-45, Instrumentation and Control in Safety ApplicationsData Sheet 7-88, Storage Tanks For Flammable LiquidsData Sheet 9-0/17-0, Maintenance and InspectionData Sheet 9-18/17-18, Prevention of Freeze-ups

4.2 NFPA Standards

NFPA 54, Installation of Gas Appliances—Gas PipingNFPA 8501, Prevention of Furnace Explosions in Fuel Oil and Natural Gas-Fired Single BurnerBoiler-Furnaces

4.2 Other Standards

ASME CSD-1, Controls and Safety Devices for Automatically Fired Boilers

APPENDIX A GLOSSARY OF TERMS

continuous igniter: An igniter that is in operation at the completion of the igniter trial-for-ignition and remainsin operation through the main burner trial-for-ignition and during the entire operating cycle of the main burneruntil it is shut down. (Defined as ‘‘intermittent’’ in ASME CSD-1.)

intermittent igniter: An igniter that is in operation at the completion of the igniter trial-for-ignition and remainsin operation through the main burner trial-for-ignition. It may, but does not have to, remain in operation forall or part of the normal operating cycle of the main burner. (No equivalent definition exists in ASME CSD-1.)

interrupted igniter: An igniter that is in operation at the completion of the igniter trial-for-ignition and remainsin operation through the main burner trial-for-ignition. It may not be placed in service during the normaloperating cycle of the main burner.

APPENDIX B DOCUMENT REVISION HISTORY

May 2005. Recommendation 2.2.1.2 uses revised editorial changes to section C.3, other standards weremade.

January 2003. Loss information in section 3.0 was revised.

September 2002. Clarification was made to Recommendation 2.2.2.1.

September 2001. A change was made to Recommendation 2.2.1.1.

January 2001. The following changes were made:

1. Recommendation on igniters changed to limit size of gas and light oil burners using spark igniters (2.2.2.1).

2. Recommendations on safety shutoff valves changed to state that using an Approved automatic leakdetection device fulfills requirements for proof-of-closure, but does not eliminate the need for manual leaktesting (2.2.2.3, 2.2.4.1).

3. Recommendation for self-checking UV flame scanners changed to state electronic UV scanners do notneed self-checking (2.2.2.7).



4. Recommendation on PLCs used for safety control changed to state that a combustion safety controlsystem can be linked to a PLC approved for burner management or combustion safety (2.2.2.8).

5. Recommendations on gas pressure switch settings revised, and recommendation on the need for a gaspressure relief valve added (2.2.4.2).

6. A recommendation on induced flue gas recirculation was revised to add a warning on the use of fixeddampers (2.4.3.3.1).

January 2000. This document was reorganized to provide a consistent format.

APPENDIX C SUPPLEMENTAL INFORMATION

C.1 Igniters

An igniter is a fixed device which provides the energy required to ensure prompt ignition of the main burner.A pilot is a fuel-fired type of igniter. NFPA 8501, Prevention of Furnace Explosions in Fuel Oil and NaturalGas-Fired Single Burner Boiler-Furnaces, and ASME CSD-1, Controls and Safety Devices for AutomaticallyFired Boilers, define continuous and intermittent igniters/pilots differently. ASME CSD-1 considers acontinuous pilot to be a constant burning pilot which remains lit all of the time and an intermittent pilot tobe one which remains lit only while the main burner is in operation. NFPA 8501 and this Data Sheet use thedefinitions for igniters found in Appendix A.

There are four classes of igniters; Class 1, 2, 3, and 3 Special, defined as follows:

• Class 1 provides energy generally in excess of 10% of burner fuel input, and should reliably ignite anycombination of fuel and air under any light-off condition. Flame supervision of the igniter should be donein a manner that ensures that when flame is detected, it is of sufficient size and location to reliably ignite themain burner during all operating and transient conditions. Class 1 igniters can be operated as continuous,intermittent or interrupted igniters. Flame supervision of the main burner is not required when a Class 1continuous igniter is used.

• Class 2 igniters provide energy generally between four and ten percent of burner fuel input, and shouldreliably ignite fuel under normal light-off conditions. A Class 2 igniter can be operated as an intermittentor an interrupted igniter. When used as an intermittent igniter to support ignition under low load and tosupport ignition under low load and other normal conditions, independent supervision of the main burnerflame is required (at least two flame scanners).

• Class 3 igniters generally provide energy less than four percent of burner fuel input, and should reliablyignite fuel under normal light-off conditions. This type of igniter can only be operated as an interruptedpilot and must shut off when the main burner trial-for-ignition is ended.

• Class 3 Special (electric igniter) is a high-voltage igniter which will directly light-off the main burner fuel.Power should be interrupted when the main burner trial-for-ignition is ended. If the igniter is a retractabletype, it should be interlocked to be proven fully inserted prior to energization.

C.2 Safety Shutoff Valves

Various types of SSOVs are FM Approved. Pneumatically operated valves can be used, although these areslower closing valves and must have vents sized properly in order to close as quickly as possible withoutdamaging valve seats. There are quick-acting solenoid-operated valves that are normally used as pilot SSOVsor as second main SSOVs. Hydraulically-operated SSOVs are quite commonly used as main SSOVs: thesevalves employ a solenoid and a hydraulic actuator and are slow opening and quick closing. Some SSOVsare electrically operated, but also must be latched in manually to open the valve. There are also valvesavailable with a proof-of-closure interlock using valve seal overtravel: the valve must be proven closed beforethe purge and ignition cycles can begin. This type of SSOV is required on boilers above a certain size (seeTables 2 and 3). SSOVs should be fully closed within one second after deenergization. A five-second closingtime is allowed if the boiler is rated at less than 2.5 MMBtuh (732 kW); however, one second is preferred.

A method for checking gas leaks with this type of arrangement would be to install the vent line to a tankpartially filled with glycerine (see Fig. 5). The vent line should enter the top of the tank and terminate insidebelow the fluid surface. The area above the surface should be vented to a safe location out-of-doors.Operators or maintenance personnel should inspect the tank during their rounds to determine if there is agas leak by checking for bubbles in the tank. This method would provide a continuous leak test of the upstream



SSOV during boiler shutdown. This would also provide a means to check for a vent valve leak during firing.Checking this inspection tank should be a part of the posted start-up procedures for the boiler.

C.3 Other Standards

Small oil- or gas-fired boilers are covered by NFPA 54, Installation of Gas Appliances—Gas Piping. Thereare no known conflicts with FM Global standards. Medium (>400,000 Btuh [117 kW]) oil- or gas-fired boilersare covered by ASME CSD-1, Controls and Safety Devices For Automatically Fired Boilers. There are noknown conflicts with FM Global standards.

Large (>12,500,000 Btuh [3662 kW]) oil- or gas-fired single- burner boilers are covered by Chapters 1-5 ofNFPA 85, Boiler and Combustion System Hazards Code (2004 Edition). NFPA 85 applies to firetube andwatertube boilers having a capacity equal to or greater than 12,500,000 Btuh (3662 kW), 10,000 lb/hr (4,500kg/hr). FM Global exceptions and comments are as follows:

a. Article 5.5.2.6.1 and 5.5.4.2.1

The purge air flow rate and air change quantity should be sufficient to scavenge all gas vaporaccumulations. Flow ratios and quantities which are greater than the FM Global’s recommended minimummay be used. NFPA recommends eight volume air changes at 70% air flow for watertube boilers andfour volume air changes at 70% air flow for firetube boilers.

b. Article 5.5.2.6.8, 5.5.4.2.6, 5.6.2 and 5.7.2

Safety shutdown and nonrecycle lockout on power failure are not considered necessary when theprogramming control automatically recycles to the prepurge sequence after power is restored. NFPA callsfor a manual reset on any interlock shutdown.

c. Appendix

‘‘Typical Schematic Arrangement of Safety Equipment’’ shows the gas pilot piping with two safety shutoffvalves (A), with a vent valve (C) between. Some users, manufacturers, and authorities having jurisdictionprefer this arrangement for the gas pilot piping.

Fig. 5. Inspection tank for SSOV and automatic vent valve leak checking.



FMDS0604

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