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Combustion & Clean Air Breakthroughs.

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Pilot Flame Rods

Flame rods are used on process burner pilots to confirm that a flame is present. Flame rods work due to the ionization / rectification process to complete a circuit. When the flame rod is energized the current produces a positive charge that attracts the negative ions in the flame. The positive ions normally will be attracted to the grounding area of the pilot tip. The theory is to collect more positive ions, through the ground, than negative ions, from the flame rod, so the flow of electrons is “rectified” or flowing in one direction. This produces the direct current signal that is used to indicate the presence of flame.

Flame Rod Testing


High voltages capable of causing death may be used with this equipment.  Use extreme caution when servicing control cabinets and electrically actuated components. Testing should be performed by qualified personnel only. Use the required PPE, including safety glasses and leather gloves.

Testing was performed to establish a procedure to troubleshoot pilots with flame rods. The test pilot was a ST-1SE-FR which is one of our standard pilots for process burners. The pilot was tested on natural gas at various fuel pressures and air door settings.

ST-1SE-FR Pilot

We used a “Test Circuit” assembly that consists of an 820,000-ohm resistor wired in series with a 1N4004 diode. The red, or positive clip, is the diode end and the black, or negative clip, is the resistor end of the test circuit. The test meter was a “FLUKE 87 V True RMS MULTIMETER”.

Test Circuit

Pilot flame rods work on the ionization / rectification process to complete a circuit. There is always voltage to the flame rod when the power is on. Our relay panel used a small transformer to increase the voltage from 120 VAC to approximately 300 VAC at the flame rod. This voltage is with the pilot off and no presence of flame. This voltage was measured by touching the red probe (+), on the FLUKE meter, to the flame rod extension rod and the black probe (-) to the pilot stand for grounding. It is important to note that this voltage is the result of alternating current (AC).

Relay Panel

Flame Rod Voltage (VAC) / No Flame / Power On


Our system used a FIREYE “Rectification Amplifier – Type 72DRT1”. Different types of amplifiers may result in voltages and currents that are higher or lower from those noted in this procedure.

The Test Circuit was then connected with the red clip on the high-tension wire to the flame rod and the black clip to the pilot mounting plate. The Test Circuit simulates the flame rectification circuit to energize the “Flame On” light on the relay panel.

Positive Lead / Test Circuit

Negative Lead / Test Circuit

Relay Panel / “Flame ON” Light Energized

We also connected the red clip to the flame rod, near the pilot tip and the black clip to the pilot mounting plate and the light came on.

The next step was to light the pilot at 5 psig with the air door 50% open. The light came on and the measured voltage was 193 volts direct current (VDC).

Connecting Test Probes for Flame Voltage

Flame Rod Voltage (VDC)

The current at this point was 44 micro-amps (µA). To measure the current the FLUKE meter must be connected in series with the red lead to the flame rod wire and the black lead to the extension rod.

Flame Rod Current (Micro-Amps µA)

The pilot pressure was increased to 10 psig and the voltage increased from 170 VDC to 240 VDC. Increasing the pressure further to 14.5 psig increased the voltage to 250 VDC.

At this point we adjusted the fuel pressure down to 5 psig and then closed the mixer air door to approximately 25% (0.07”-0.09”). The voltage dropped from a constant 200 VDC to fluctuating between 90 and 150 VDC.

Opening the mixer air door to 100% increased the voltage to 240 VDC.

Recommendations and Conclusions

The pilot fuel pressure did not have a significant impact on the voltage generated in the circuit. Running on natural gas the recommended pressure would be 7 – 15 psig. The light on the relay panel stayed on down to 1.5 psig.

The position of the mixer air door does reduce the flame voltage significantly. The air door should be set between 50% and 100% open for the best signal.

During normal operation the flame rod should generate a voltage of 220 to 250 VDC. The minimum voltage, for a reliable signal, should be 90 VDC. The current should be 20 to 45 micro-amps (µA). Current levels less than 5 micro-amps (µA) may cause “loss of flame” trips.

Field Test Procedure

To prove the system wiring is functional close the pilot fuel valve. Then connect the red clip, on the Test Circuit, to the high-tension wire at the pilot and black clip to the mounting plate. Use extreme caution since the system will have power on. The “Flame On” light on the relay or the panel should come on.

To prove the flame rod is working will require that the pilot be removed from the burner. Close the pilot fuel valve and remove the pilot. Be sure the pilot is grounded. Connect the red clip, on the Test Circuit, to the flame rod and the black clip to the mounting plate. The “Flame On” light should come on. If the light does not come on then the problem is with the flame rod, the insulators or the extension rods. Use extreme caution do not touch the pilot or the flame rod when the power is on.

Inspect the flame rod for any type of corrosion or build-up that could interfere with the ionization process. Moisture build-up on the insulators and connecting rods is a frequent problem. Turn the power off before cleaning or replacing the pilot or pilot parts.

Diagnosing And Solving Problems With Burners On Heaters

Frequently, the operator of the heater must be trained to use knowledge of the equipment and process unit to make adjustments that bring operations back to the required capacity desired by plant management.

It is essential that troubleshooting be done in a systematic, well-organized fashion. Effective and safe troubleshooting involves four basic steps:

  1. Recognizing the problem
  2. Observing indications of the problem
  3. Identifying solutions for the problem
  4. Taking corrective action

When a problem is noted, it is necessary to evaluate its likely effect on the process or product being produced.  Some solutions may require the heater to be shut down for the problem to be resolved.

Once a cause has been determined, standard procedures should be followed to solve the problem.  All personnel involved should be aware of the problem, the planned corrective actions, the ways that safety is addressed, the expected results, and the proper action to take should the problem worsen or not be solved.