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Show AFRC 2017 Industrial Combustion Symposium Combustion Flame Safeguarding Paper Are your current flame scanners or flame rods tripping your fired process heaters? Do you have to clean your flame rods so frequently to get them to work properly your maintenance costs are going through the roof and your downtime is going through the floor? Do you have to program your flame scanners to see a flame using a keypad "learn" sequence or a laptop? Does low BTU gas, high Hydrogen fuels or steam injection cause your flame scanners to trip the burners? The ZEECO® ProFlame™ integrated flame scanner revolutionizes the fired process heater market by providing reliable flame detection and instant flame status for safer operation. A unique photoelectric sensor in the ProFlame scanner converts the flickering Ultra-Violet (UV) radiation, pulsing from a burner‘s flame, into electrical signals. These electrical signals are used to determine the presence or absence of a target flame. Positive indication of the burner's main flame or the pilot flame allows the fuel valves to remain open so the desired process temperature in the furnace is reached. Steam injection and water vapor as a by-product of combusting high Hydrogen-bound fuels, can "blind" many UV tube style scanners, causing the scanner to trip the burner, leading to temperature swings in the furnace with the loss of the main burner. Flame rods (and their ground surface), can short circuit or oxidize to the point where a flame signal is no longer transmitted, causing the flame rod system to trip the pilot and preventing operations from starting up the burner. The ProFlame scanner's unique sensor picks up on a specific UV wavelength where steam and water vapor will not affect the detection of UV from a gaseous or liquid flame. Detection of this wavelength allows the ProFlame scanner to provide a reliable ‘Flame-On' output, as long as the pilot or main flame is present. ProFlame also eliminates the worry regarding flame rods shorting out on the ground surface or being too far from the ground surface to conduct a signal, since the flame scanner's sensor optically picks up on the UV radiation through a forgiving eight-degree field of view lens. Amplitude and flicker frequency are the two primary components of any flame signal. Amplitude is simply the intensity at which fuel and oxygen combine under heat; and flicker frequency is the rate at which the flame fluctuates. With a flame rod, the flame signal's amplitude is determined by the area of the ground surface, the distance of the rod from the ground surface, and the location of the flame in relation to the flame rod and the ground surface. A flame rod carries a high voltage on the tip of the electrode supplied by the flame rod's remotely located amplifier. If a flame is present, the voltage can ground out through the ionized flame to the ground surface, and create a micro-amp flame signal used by the flame rod's amplifier. If the flame rod or ground surface oxidizes, the current trying to flow through the ionized flame will not be conducted. If the flame is too far from the ground surface or the flame rod, no current can be conducted. If water vapor as a byproduct of combustion is present, the water vapor will decrease the amplitude of the flame signal, and cause the flame rod or ground surface to oxidize prematurely, leading to the flame eventually not being detected. With a flame rod system, the flame rod cannot touch the ground surface, as this short circuit will not be detected as a fluctuating flame signal in the micro-amp level. Flame rods also have the issue of the ceramic insulator cracking and leaking the current to a ground surface rather than through the flame. If the flame rod is not well designed with properly spaced insulators, the flame rod can droop or sag, ultimately shorting out against the ground surface. When the flame rod is transmitting the flame signal through the flame, the wiring between the flame rod and the amplifier are also critical components. High voltage cable trays or improperly grounded cables/ignition transformers can cause the flame signal to pick up electrical noise. Electrical noise can drown out the micro-amp flame signal, causing the flame rod amplifier to not detect the flame properly. The distance between the flame rod and the flame rod amplifier can also decrease the flame signal due to capacitive coupling. Flame rods have a low initial cost, but constitute a variety of maintenance, troubleshooting, and technical challenges that can be operationally costly for a plant. Optical flame scanners offer more reliable detection of a flame than a flame rod. UV tube scanners use technology developed in the 1940s and face many issues with not only steam or water vapor, but also xrays, gamma rays, UV from the sun, and spark pick up. Since the UV tube is only designed to last for 10,000 operating hours, the use of an electromechanical shutter is required for the scanner to detect when the UV tube is no longer responding to the UV from the flame, and is only responding to the UV tube's inherent nature of failing in a "flame-on" condition. The electromechanical shutter can get stuck in the open or closed position, and cause the burner to trip. This technology is also relatively inexpensive, but has many shortcomings. To combat the shortcomings of a UV tube, many of the existing manufacturers use a solid-state sensor and microprocessors. However, the microprocessors are used to calibrate the gain of the sensor or utilize algorithms to mask background signals such as electrical noise, solar radiation, line voltage frequency, and other sources that can be mistaken as a flame signal due to the type of sensor used. Furthermore, to properly utilize these scanners, they must be "taught" what a flame signal looks like, forcing the operator to go through a "learning" sequence of starting and stopping the burners on a heater. Some manufacturers use a keypad on the scanner, and some force you to use a laptop with dedicated scanner software to tune the flame scanners. Although this learning sequence can be beneficial in the boiler market with up to 350 mmbtu/hr burners, the "learning sequence" is burdensome for the process heater market. Zeeco addresses these issues by using a solid-state UV only sensor that allows the scanner to zero in on the natural UV properties of a flame. The majority of UV in all flames, regardless of fuel, is located in the first 1/3rd of the flame because UV is a byproduct that indicates the start of the combustion process. Regardless of fuel, the O2 molecule sweeps over the Hydrocarbon molecule, and tears an electron out of the hydrocarbon's outer valence under the right amount of heat. The energy that held the electron in place is released in the form of UV radiation. As the hydrocarbon molecule is further torn apart and combined with the O2 molecules, more heat is released. The release of this heat energy consists mostly of infra-red (IR) radiation. With a cigarette lighter, as an example, the base of the flame is blue (beginning of combustion), and the rest of the flame is yellow (final stages of combustion). Because IR radiation is a longer wavelength, using an IR sensor or visible light (VL) sensor to detect a flame in a heater will not only cause the sensor to pick up the tips of other flames in the heater, but also the IR from glowing insulation or refractory inside the heater. The use of a UV only sensor not only avoids these issues, but also helps the scanner focus on the flame it should be detecting. However, not all solid-state UV sensors are the same. All other manufacturers use a broad band sensor, meaning the sensor not only picks up UV, but also VL and IR. To focus on the UV, an optical filter is used to block out unwanted VL and IR. Some filters have leaks in the IR spectrum, requiring the use of an additional IR filter. These filters cause significant signal loss by the time the UV radiation reaches the sensor. By using an unfiltered UV only sensor, the ProFlame scanner can be amplified in a way that allows for increased sensitivity to low BTU fuels, high Hydrogen Fuels, liquid or gaseous fuels, and even processes with steam injection. By using an unfiltered solid-state UV only sensor, no moving shutters are needed to perform the scanner's self-check functions. Plus, the IR signal from the heater's other flame tips, nor the hot refractory or glowing insulation are not read as false positives of the burner flame the scanner is targeted to monitor. To help further dial out the background radiation, the ProFlame uses the properties of the flame's flicker frequency to distinguish the target flame from the other flames. The high flicker frequency through the base of the flame is used to determine the presence of the target flame. The filtering out of the low flicker frequencies - occurring at the tips of the adjacent flames - is used to dial out these background flames. The only tuning tool required is a screwdriver, which is used to increase the gain (amplification) via rotary switches until the flame signal reaches the green LEDs in the bar graph display. If needed, the screwdriver can also be used to dial out the background signals using the dipswitches for the flicker frequency. Emergency spare ProFlame scanners can be kept on the shelf with the commissioned settings from the previous ProFlame and will not require the operator to spend time re-learning the different flame parameters in order for another manufacturer's scanner to simply see the flame. Removing flame rods from pilots requires the entire pilot assembly to be removed, costing time and money to troubleshoot, clean, and re-install. The ProFlame scanner's revolutionary technology allows the plant to free up time and money, improve heater reliability and safety, and can be commissioned using a simple screwdriver. |
