Combustion Safety for Furnace Operation
A furnace can be defined in a broad sense as an enclosure for the combustion of fuel, or more specifically, an enclosure where the heat of combustion is transferred by radiation or by convection to a product within the enclosure. The importance of combustion safety in industrial ovens and furnaces cannot be overstated. Therefore, combustion safeguards are required in most national combustion safety standards, such as the National Fire Protection Association (NFPA) Standard 86: Standard for Ovens and Furnaces. Standard 86 is a comprehensive document covering the safe operation of ovens and furnaces, with sections on explosion relief, ventilation, equipment locations and many other helpful sections.
Standard 86 applies to a wide range of furnaces-from electric-arc melting furnaces to low-oxygen types. However, this article concentrates on combustion safety, therefore, a direct natural gas-fired furnace will be considered to simplify the discussion, which also applies to oil- and propane- fired furnaces. Standard 86 also applies to new installations and alterations or extensions to existing equipment.
Safe practices can prevent problems
The basic cause of furnace explosions is the ignition of an accumulated combustible mixture within the confined space of the furnace chamber and/or the associated ducts that exhaust combustion products to the stack. This entire volume can conveniently be referred to as the furnace enclosure or setting. A dangerous combustible mixture within the furnace enclosure consists of the accumulation of a quantity of combustibles mixed with air in proportions that will result in rapid or uncontrolled combustion when an ignition source is supplied. A furnace explosion can result from ignition of this accumulation. The magnitude and intensity of the explosion depends on both the relative quantity of combustibles and the air-to-fuel ratio at the moment of ignition.
The basic requirements established in Standard 86 for a gas-fired furnace and an explanation of the major components of the furnace's combustion safety system are discussed. Major items include the burner management system controller, the purge cycle, flame monitoring, positive fuel shut-off, training of operating and maintenance personnel and the periodic testing of safeguard devices.
NFPA Standard 86 requirements
Table 1 summarizes NFPA Standard 86 requirements for furnaces. This table provides only an overview of the information contained in the standard, and the standard should be referred to directly for greater detail in specific areas. Alarms are not included in the table because they are not combustion safeguards. However, many devices listed are alarmed or prealarmed on many systems.
Burner management system controller
Modern burner management system (BMS) controllers typically are digital electronic control devices that use preprogrammed logic to monitor and control the safety functions of a burner. In simple terms, a BMS controller operates as an automated brain to monitor and control the safety functions of a burner. Such a controller proves airflow and purge time prior to trial for ignition, enforces minimum firing valve setting at light-off and monitors the presence of a flame and fuel conditions during operation. In the event that the BMS controller determines a condition is out of a preset limit, a master fuel trip is enforced, which closes the main fuel safety shut-off valves, thereby stopping burner operation.
Standard 86 requires that a BMS controller "?hall be listed for use in the service intended," the status of which can be checked by an insurance underwriter for the equipment covered. It further includes the minimum requirements a BMS controller must meet, such as limited access to unauthorized changes in its control logic, safe shut down of the controller under seven different failure modes, safety logic stored in nonvolatile memory and many others. Also, Standard 86 specifies that a trip of the burner system by any safety device requires manual intervention of a trained operator to correct a fault situation prior to restarting the burner for normal operation.
Purge is the flow of clean air through the furnace, which effectively removes gaseous combustibles and replaces them with air. The purge airflow and time must be sufficient enough to allow "?t least four standard cubic feet of fresh air or inert gas per cubic foot of heating chamber" (paragraph 5-4.1.2). The furnace volume to be purged should include all equipment from the burner up to, but excluding, the stack. Standard 86 requires that, "?rior to each furnace heating system start-up, provision shall be made for the removal of all flammable vapors and gases that might have entered the chambers during the shutdown period" (paragraph 5-4.1.1).
Two basic purge requirements are purge airflow adequate and purge time satisfied. Directly measured and indirectly inferred are two different methods used to determine airflow adequate. Direct airflow measurement can be accomplished using either metering devices or by measuring the pressure drop across a fixed portion of the purge airflow path. Metering devices include venturis, annubars, air foils and mass-airflow meters. Indirect infer-red airflow is accomplished by proving the appropriate fan or fans are running (by pressure or speed) and the flow path is adequately open (by damper position switches). Direct measured airflow offers better protection, but may be more expensive.
The purge timer determines the period of time for which the purge requisites must be maintained. Any interruption of the purge airflow or timing cycle should cancel purge and require a new, full purge.
Flame monitoring is performed to ensure there is a flame in the burner at all times when the fuel safety shut-off valves are open, with the exception of the timed trial for ignition. Common flame monitors are thermocouples, flame rods, ultraviolet (UV) scanners and infrared (IR) scanners (Table 2). Each type of monitor is used in a manner where it can sense the measured variable it is designed to detect and convert the measurement into an electrical output signal, which is input into the BMS controller.
Different type scanners are used in different applications because combustion characteristics vary with different types of fuels. A thermocouple is a heat-activated flame monitor, and, as such, has a relatively long response time to flame loss. Therefore, thermocouples are seldom used in this industrial application. A flame rod is a monitoring device used to sense flame conductivity, and is faster than a thermocouple. Flame rods require more maintenance than UV and IR monitors, but are less expensive. Ultraviolet monitors are used to scan clean flames, such as gas- and light oil-fired burners. These fuels have ultraviolet properties and are easy to detect. Infrared scanners often are used to scan dirty flames, such as coal and heavy-oil firing, which can block the ultraviolet properties of the flame. Both UV and IR monitors are optical scanners that look for flame radiation, and, as such, have very fast response times.
Standard 86 requires that each pilot and main flame have independent flame monitoring, but does allow for one flame sensor for both if the pilot is an interrupted pilot or if it is a self-piloted burner. The flame monitoring device should have a maximum flame failure response time of 4 seconds or less and should be interlocked into the burner's safeguard control logic. The loss of a flame signal should cause a master fuel trip, which closes the main fuel safety shut-off valves, thereby stopping burner operation.
It is possible for gas to leak by a closed valve, which, in the case of a furnace, could allow a significant amount of gas to accumulate in the furnace. Such an accumulation of gas prior to start-up could result in an explosive mixture, only needing an ignition source to result in an explosion. Causes of leaking shut-off valves range from a worn valve seat to dirt or scale in the valve.
To prevent gas leaking into a furnace when the furnace is not operating, two automatic shut-off valves are piped in series in the main gas pipe. The main shut-off valves are energized to open and fail closed on loss of a control signal. Typically, the BMS sends the same signal to both valves, which results in these main valves opening together. These valves are powered by motorized and solenoid operators, which typically fully open in 1 to 7 seconds, but close on loss of a control signal in 1 second or less. (Note: some insurance underwriters require an additional feature, that a vent valve is piped between these main block valves and operate in reverse action to the main valves-refer to insurance requirements.)
Standard 86 includes requirements for these safety shut-off valves, such as:
- The valves cannot be used as modulating fuel control valves without special permission to do so
- Position indication is required for shut-off valves on burners (or pilots) having greater than 150,000 Btu/h of heat input
- Two shut-off valves are required for burners (or pilots) having greater than 400,000 Btu/h of heat input, and the valves should not be subjected to pressures greater than the manufacturer's rating. The valves also must be leak tested regularly (at least annually)
Standard 86 requires that all operating, maintenance and appropriate supervisory personnel be thoroughly instructed and trained to ensure knowledge of and practice of safe operating procedures. The standard also requires regular retraining along with recommended initial training to maintain the thorough understanding. Training for gas-fired furnaces should include combustion principles, explosion hazards, ignition sources, confined space entry and functioning of safety and control devices. Operating instructions including piping and wiring diagrams, start-up and shut-down procedures and maintenance instructions also should be included for the furnace.
The author has been involved in the testing of combustion safety interlocks on both boilers and furnaces of many types and can attest that safety devices that go "unexercised" for long periods of time might not work when needed. In a recent survey of the combustion safeguard systems on 21 burners (all on boilers), 15 safety devices were found inoperative. These devices ranged from a flame scanner to high and low gas-pressure switches, which are the basic components of any gas-fired burner management system.
NFPA Standard 86, Appendix B provides a list of operation checks of such items as ignition sparks (ignitor gas pressure also can be added), operating temperatures, and proper ventilation. Appendix B further suggests checks that should be conducted each operating shift, each week, each month and "periodically." These lists of operating and maintenance checks are not Standard 86 requirements, but are provided only for reference and informational purposes. For example, a monthly checklist includes:
- Test the interlock sequence of all combustion safety equipment.
- Verify that each interlock works by manually failing the device.
- Test shut-off valves for tightness.
- Check the pressure setting of all pressure switches against set points.
- Inspect and clean if necessary all electrical switches.
- Test all temperature devices.
- Inspect and clean if necessary all fan filters.
- Inspect and clean if necessary burners and pilots.
The reader is encouraged to review the entire Standard 86 for in-depth, detailed information regarding all oven and furnace questions. Standard 86: Standard for Ovens and Furnaces, 1999 Ed. is published by NFPA, 1 Batterymarch Park, PO Box 9101, Quincy, MA 02269-9101; tel: 617-770-3000; Internet: www.nfpa.org