Process heating with fuel-fired burners is the most effective and cost-efficient method of heating. Personnel safety, production equipment protection and maintaining production schedules are all best served by understanding safety requirements. NFPA is the consensus safety authority. The 2019 edition of NFPA 86, Standard for Ovens and Furnaces, applies to any heated enclosures used for processing of materials except solid fuel-firing systems and systems supplying less than 150,000 BTU/hr heat.
All equipment in a fuel-fired safety system should be of the listed or approved type and designed for use as a safety device. Listed or approved equipment has undergone testing and classification by Underwriters Laboratories, Factory Mutual, Canadian Standards Association or similar nationally recognized laboratories, with quarterly followup service.
Figure 1 shows a typical combustion safety equipment list. Equipment includes:
- A manual emergency switch, which is required to start a safety shutdown.
- Combustion starters (and recirculation and exhaust starters, if required), which must be interlocked into the flame-safety circuit.
- A low gas-pressure switch, which is required to ensure that the burner is supplied with sufficient gas to safely ignite and maintain the flame.
- A high gas-pressure switch, which is required to ensure that the flame will not blow out due to excessive gas pressure.
- A pressure switch, which is required to ensure that the burner has sufficient air pressure to safely maintain the flame.
- A high temperature limit. It is required to ensure that the maximum temperature specified by the oven or furnace manufacturer cannot be exceeded. It must require manual reset, must be of the indicating type and must be used solely for high temperature limit. (It cannot be an auxiliary contact of the temperature controller.) It must drive upscale upon thermocouple break.
It should be noted that the setpoint of all safety limits must be verified and documented, at least annually. They must be maintained per the manufacturer’s instructions. And, if replaced, they must be tested for function, and the setpoint must be verified. Safety interlocks must not be bypassed.
Why Purge Timing Matters
The purge timer may be one of the most important, yet unappreciated, pieces of safety equipment.
A purge timer can be integral to the combustion safeguard, a stand-alone component or a function of a PLC. PLC use in combustion safety is strictly and specifically restricted in NFPA 86. Because gas valves can and do leak, the combustion chamber should have four system-volumes of air introduced during the purge timing. This timing is calculated on the volume of the combustion chamber and the airflow produced by the blower. The purge time begins when the proof-of-closure (POC) switch on the shutoff valves are proven, and the purge airflow must be maintained during the entire purge time.
Particular attention should be paid for fuel gas that is heavier than air and so could be at the bottom of the furnace. If the purge flow rate is not known, measurement may be required using gas and oxygen analyzers. In no case should the purge time be less than the time necessary to bring combustibles lower than 25 percent LFL (lower flammable limit). Flue-gas recirculation, thermal oxidizers, incinerators and similar heaters must allow for purge with fresh air or inert gas. No valves should be installed downstream of the pressure switch that would restrict necessary purge airflow.
At the end of purge timing, ignition may be initiated. The maximum ignition trial time is 15 sec. If a burner shuts down, or if there is flame failure, purge timing must be repeated before ignition is initiated. (Purge timing need not be repeated if temperature is above self-ignition temperature, which is 1400°F [760°C]. Purge timing need not be repeated if combustible concentration is 25 percent LFL or less.)
FIGURE 1. Basic combustion safety equipment includes a manual emergency stop, combustion starter, a low gas-pressure switch, a high gas-pressure switch and a pressure switch.
The Combustion Safeguard
The ignition-trial timer may be integral to the combustion safeguard, a stand-alone component, or a function of a PLC.
The combustion safeguard — with its flame sensor as input — monitors or supervises the flame. When spark from the ignition transformer and gas from the pilot burner (or main burner, if direct ignition) produce flame, the response is immediate. Before ignition may take place, the combustion safeguard performs a safe-start check. The safe-start circuit prevents ignition trial if the combustion safeguard indicates flame is present due to component failure within the combustion safeguard, the sensor or due to the presence of actual or simulated flame.
If a burner has both main and pilot flames, each should be supervised with independent flame sensors unless the pilot flame is interrupted, or the burner is a self-piloted burner. If the flame goes out, the safeguard must remove power to the gas valves in 4 sec or less.
If a furnace is running at 1400°F (760°C) and above, flame supervision is not required. However, flame supervision is required while temperature is ramping up to 1400°F (760°C) and if the temperature ever drops below 1400°F (760°C). The 1400°F (760°C) instrument should be approved for that purpose, be dedicated only for that purpose, be locked so that the temperature setting cannot be altered, and drive downscale if the thermocouple breaks. A radiant-tube burner that is open at one or both ends or is validated explosion-resistant does not require flame supervision.
Please note that 2019 NFPA 86 added requirements for line burners, pipe burners and radiant burners in Paragraph 184.108.40.206. Line burners, pipe burners and radiant burners longer than 3’ with a pilot must have a flame sensor for the pilot burner at the source of ignition in addition to a main burner flame sensor at the far end of the burner.
A two-burner combustion safeguard can accomplish this requirement. There should be two pilot valves (redundant) piped in series. There also should be two main valves (redundant) piped in series. If the burner system (pilot or main) exceeds 400,000 BTU/hr, at least one valve in that system should have a proof-of-closure (POC) switch that is proven closed before purge can start.
The flame sensor usually is a flame rod or a UV scanner. Neither depends on sensing heat. The flame rod is a stainless steel wire that intersects the flame. It is not required to be listed. The flame acts as high resistance rectifying path to ground. The combustion safeguard amplifies this signal to provide a relay contact output. The UV scanner provides input to the combustion safeguard amplifier when it sees sufficient UV from the flame.
The 2019 edition of the NFPA 86 standard added requirements for line burners, pipe burners and radiant burners. Line burners, pipe burners and radiant burners longer than 3’ with a pilot must have a flame sensor for the pilot burner at the source of ignition in addition to a main burner flame sensor at the far end of the burner.
Required Combustion System Maintenance
As part of the maintenance plan, all safety interlocks should be tested at least annually. All gas valves should be leak tested at least annually. It is recommended that combustion safeguards be tested monthly. Shut off the manual gas cock closest to the burner, and the gas valve should close in a maximum of 4 sec.
If a system is using UV scanners and runs continuously, the above sequence should be performed on a weekly schedule. An alternative is to use a shutter-type, self-check system or a dual/redundant self-check combustion system.
Any shutdown due to a safety function requires manual intervention. For example, if the high temperature limit is exceeded, the operator must manually reset it. In doing so, he should investigate why the temperature was too high. Was the setting wrong? Did the thermocouple break? (A high temperature limit should drive upscale upon thermocouple break.) Did the temperature controller malfunction? Did the product being heated catch fire?
If the flame safeguard was the source of the shutdown, the operator should check the gas/air ratio, flame color, flame signal strength (usually with a micro amp test meter), cleanliness of the UV sensor and cleanliness and positioning of the flame rod sensor.
We have covered only the essentials in this article. For more information about atmospheres, incineration, PLC requirements and the complete safety requirements, it is highly recommended that you purchase and read NFPA 86. The Annex A (explanatory material) is not a part of the NFPA requirements, but it is included for informational purposes.