Two types of flame sensors generally are used on industrial burners: flame rods and ultraviolet scanners. While each can perform the essential role of safeguarding industrial combustion systems, this article will focus on ultraviolet scanners.
First, a review of flame rods allows comparison of the technologies. Flame rods are simply stainless steel rods that intersect the burner flame. A voltage potential from the combustion safeguard is applied to the flame rod. When a flame is present, an electrical current — measured in millionths of an amp — flows from the flame rod through the ionized gases of the flame to the burner, which is grounded. This current is amplified in the combustion safeguard and energizes a relay output to power the fuel valves (figure 1).
An ultraviolet scanner (figure 2) consists of an ultraviolet tube constructed of a fused silica or ultraviolet glass envelope, two electrodes and a gas contained in this envelope. This is called a cold-cathode gas-discharge tube. Basically, it conducts or ignites when it is irradiated with ultraviolet light and when sufficient voltage potential exists across the two electrodes. The electrodes are made of tungsten, molybdenum or nickel. When a photon of sufficient energy is absorbed into the cathode electrode, electrons are emitted and are drawn to the anode. A larger cathode allows more electrons to avalanche, causing higher current flow and thus higher sensitivity to UV.
The tube of the ultraviolet scanner responds only to radiation in the spectrum of 1,900 to 2,300 angstroms (figure 3). Peak response is at 2,100 angstroms (210 nanometers). Solar ultraviolet starts near 2,800 angstroms; solar radiation, of course, extends into the visible spectrum (4,000 angstroms) and extends into the infrared spectrum.
Some companies make high sensitivity ultraviolet scanners for special burners that produce low ultraviolet radiation. The gas in the tube is usually a helium-hydrogen ionizable mix. Electrons released by the cathode release electrons in the ionized gas, becoming a self-sustaining discharge much greater than that of the originally generated electrons and producing a high current gain or avalanche effect.
The sensitivity of a tube will slowly decrease over a period of time. Replacement commonly is recommended after 8,000 hours of operation. The current produced by the photoelectrons is measured in millionths of an ampere, so this current is amplified in the combustion safeguard in order to energize a relay that can then energize the fuel valves.
While ultraviolet scanners are time proven and reliable, tube gas contamination may occur with large temperature shock or large physical shock. (For instance, a 2” drop may cause 100 G shock.) These shocks can cause the electrode-to-ultraviolet-glass-envelope seal-integrity to be compromised, so it is possible for an ultraviolet tube to conduct current when no ultraviolet is incident upon it. This would normally be detected during the flame safeguard safe-start check. When an indicated flame-on condition exists prior to purge or ignition, the safe-start check relay prevents ignition and gas valve energization.
In addition to safe-start check before every heating cycle, a monthly preventive maintenance schedule should be followed if the burner is used on a daily basis. This consists of closing a manual gas valve. The electrically powered gas valves should close in two to four seconds as the ultraviolet scanner and combustion safeguard respond to loss of flame. If a burner is in continuous service, this maintenance schedule should be performed on a weekly schedule.
An alternative to this is to use a self-checking ultraviolet scanner and control. This type of scanner involves an electrically operated shutter that alternately blocks and allows ultraviolet to the tube. However, having a mechanical device operating close to the burner heat and vibration is a recipe for frequent and premature failures. They typically are rated for only 140 to 175°F 60 to 79.4°C) maximum, and they are quite expensive.
Self-Check UV Flame Sensor
One alternative is a dual/redundant, self-check ultraviolet flame sensor and combustion safeguard control. With one design, there are no moving parts, so reliability and durability provide an advantage over shutter-based self-check ultraviolet controls.
The redundant self-check ultraviolet flame sensor and combustion safeguard control has two ultraviolet tubes in one ultraviolet sensor to monitor one burner flame. Of course, ultraviolet tubes can respond to welding sparks, ignition sparks, lightning, bright incandescent or fluorescent light, solar radiation, gamma rays and X-rays.
Because ultraviolet tubes produce ultraviolet rays when they conduct, two ultraviolet tubes in one sensor would not normally be suitable for sensing a burner flame because one ultraviolet tube could be responding to the other tube and not the flame.
The redundant self-check ultraviolet flame sensor and combustion safeguard control includes two voltage supplies to the ultraviolet tubes that are out of phase with each other. When one ultraviolet tube is powered and may respond to ultraviolet rays, the other ultraviolet tube is off. The two ultraviolet tubes are powered through two rectifier circuits from two transformers that are out of phase with each other. The two ultraviolet tubes are powered and sense ultraviolet from the flame on alternating half cycles (figure 4). Each ultraviolet tube and rectifier circuit provides input to its own amplifier. Each amplifier provides input to its own flame relay (figure 5). Upon burner startup, before burner ignition or if either ultraviolet tube is in conduction, the safe-start check circuit does not permit powering the fuel valve.
During the burner run cycle, if either ultraviolet tube fails in the conduction state, the cycle will safely continue with the other ultraviolet tube sensing the burner flame.