Boiler Flame Failure Detectors PDF

Objective 2 Describe the construction and operation of flame detectors. BOILER FLAME FAILURE DETECTORS On boilers, various methods are used to detect flame failure. These include: • Thermocouples • Flame rods • Thermopiles • Photoelectric cells Thermocouples Thermocouples are found on small gas...

Objective 2 Describe the construction and operation of flame detectors. BOILER FLAME FAILURE DETECTORS On boilers, various methods are used to detect flame failure. These include: • Thermocouples • Flame rods • Thermopiles • Photoelectric cells Thermocouples Thermocouples are found on small gas burners that use continuous pilots. Thermocouple flame safeguards are limited in use to pilot burners for small boilers up to 120 kW input (400 000 Btuh or 9.5 BoHP). This is because thermocouples have a slow flame failure response time. It can take up to 90 seconds for a thermocouple to cool off sufficiently to close a safety shut-off valve. In that period of time, significant amounts of unburned fuel can enter a furnace. A thermocouple consists of two dissimilar wires welded together at one end to form a measuring (or “hot”) junction. The opposite ends are connected to an electric circuit, called the reference (or “cold”) junction. A basic thermocouple circuit is shown in Figure 8. Side Track Construction and operation of thermocouples are discussed in Fourth Class Part A Unit 9 Chapter 2 “Introduction to Process Measurement Figure 8 – Basic Thermocouple Circuit Page 15 of 49 A thermocouple designed for flame monitoring is made of a copper outer sheath and an insulated inner wire of a different metal. The outer copper sheath acts as one of the two conductors. The inner wire is insulated so that it does not contact the copper. The two wires are welded together at the tip of the copper sheath, forming the hot junction. When the hot junction is exposed to a flame, its temperature increases above that of the cold junction. This generates a small voltage (about 20 to 30 millivolts in an open circuit), capable of causing a small current to flow. This energizes a sensitive relay, called a pilotstat, which may be built into an SSOV. When the pilotstat is energized by the thermocouple, an external low voltage power supply energizes the main SSOV, causing it to open. The SSOV stays open until the thermocouple voltage drops below about 7 millivolts, which may occur due to pilot flame failure. Figure 9 shows a pilot burner assembly that Figure 9 – Pilot Burner with Thermocouple uses a thermocouple to supervise the pilot. Quite often, pilotstats are combined into a single valve body with the following components: • Gas pressure regulator • Safety shut-off valve • Manual gas valve This type of SSOV is called a combination gas valve. These are common on smaller gas appliances with inputs up to 120 kW. Page 16 of 49 Pilotstats can also be external to the SSOV. These external pilotstats may function to interrupt the main flame only (called 50% shutoff), or both the pilot and main flames (called 100% shutoff). Figure 9 shows an external pilotstat that provides 100% shutoff. It interrupts the gas to the pilot burner, and interrupts the main SSOV circuit, when the pilot flame is extinguished. Combination gas valves, and pilotstats that provide 100% shutoff, must be manually reset in order to light the pilot. Refer to Figure 10. To relight the pilot: 1. Close the main manual shutoff valve (the “A” valve). 2. Open the pilot gas valve (the “B” valve). 3. Provide a source of ignition to the pilot burner. 4. Depress the pilotstat manual reset button, for about 90 seconds, until the pilotstat remains energized. If the pilot flame fails, the thermocouple does not generate electric current. This interrupts the 24 V control circuit at the pilotstat, and prevents the main SSOV from opening. During main burner operation, if the thermocouple does not detect a flame, the pilotstat deenergizes. Then, the pilotstat stops the flow of gas to the pilot burner, and de-energizes the SSOV. This stops the gas flow to the main burner. Figure 10 – Gas Burner Control System Figure 10 shows a limit switch in the SSOV circuit. In a boiler, there can be several limit switches in series. Any one of these switches can prevent the burner from operating. Switches may include low water cut-offs, high temperature cut-offs, high pressure cutoffs, aquastats, and pressuretrols. If any of these switches open, the main SSOV will close. However, the pilot burner remains lit. Page 17 of 49 Thermopiles The voltage supply generated by a thermocouple is usually not more than 30 millivolts, so its electrical power is not sufficient to operate an SSOV directly. For the SSOV to operate, a transformer must supply power from the electrical main. However, multiple thermocouples in series can Figure 11 – Basic Thermopile Construction develop enough voltage to operate and SSOV. Such a flame detector is called a thermopile (see Figure 11). Thermopiles develop sufficient voltage to operate an SSOV independently from any external power source. This system is called a millivolt system. Typical thermopiles generate around 750 millivolts. Special combination gas valves are designed to operate on this small voltage. An advantage of a millivolt system is that the SSOV and burner control system operate without external power. Small residential boilers with millivolt systems can function even during a power failure. Like thermocouples, thermopiles are limited to burners not exceeding 120 kW input. Flame Rod The flame rod is a flame detection device that operates on the principle that flames can conduct electricity. A complete flame circuit consists of three elements: two electrodes and a flame. One of the electrodes is the flame rod. The other electrode is the burner itself. The flame conducts electricity between the flame rod and the burner. A flame rod is a heat-resistant, small diameter metal rod, supported by a ceramic holder that keeps the rod electrically insulated from the burner. For continuous or intermittent pilots, the rod is located in the path of the pilot flame. With interrupted pilots, the rod penetrates the main and pilot flames. Therefore, the flame rod can be used to detect both pilot and main flames. Page 18 of 49 Alternating current (AC) is applied to the circuit. The current that flows through the flame is a pulsating direct current (DC). When AC is converted to DC, the process is called rectification. There are two reasons why the AC gets rectified to pulsating DC: 1. The flame rod and burner have very different surface areas. The burner can emit more electrons than the flame rod. For this reason, electrons flow primarily from the burner to the flame rod, effectively rectifying the applied AC. 2. The flame pulsates at a particular frequency while in operation. The pulsations cause pulsating DC flow from the burner to the flame rod. The burner monitoring circuit is designed to recognize a pulsating DC current. If the flame is extinguished, current stops, and the main and SSOVs close. If the flame rod is accidentally grounded to the burner, AC current flows through the circuit. The burner monitoring circuit does not accept the AC signal, and shuts off the SSOVs. Grounding can occur due to: a) Soot bridging between the flame rod and the burner. b) A cracked flame rod insulator. c) A flame rod that has been warped or knocked out of position, so that it contacts the burner or other grounded metal part. A basic flame rod system is shown in Figure 12. The flame rod extends into the burner flame. When the flame is present, the rod, burner, and flame close the flame relay circuit. This relay amplifies the signal from the flame unit, and provides sufficient power to keep the safety shut-off valve open. If the flame fails, the circuit opens, and the SSOV closes. During boiler startup, the main SSOV cannot be opened unless the rod senses the pilot burner flame. Figure 12 – Basic Burner with Flame Rod Page 19 of 49 Unlike thermocouples and thermopiles, flame rods offer fast flame failure response time. Because of this, they are used in burners over 120 kW input. For automatically fired boilers, flame rod monitoring systems are more complex than that shown in Figure 12. The firing equipment will also include a pilot burner, pilot SSOV, and an ignition system. In such a system, the relay is only a component of the burner management system (BMS). The BMS sequences the startup and the shutdown of the firing equipment, as well. Flame rods are commonly used with gas burners. When used with oil-fired burners, the rod can become coated with carbon deposits, which may cause soot bridging and nuisance outages. Photoelectric Cell A photoelectric cell is a device that reacts to infrared, ultraviolet, or visible light emitted by fire. The cell is mounted on the boiler in such a way that it can observe the flame. Photoelectric cells are commonly called flame scanners. Figure 13 shows a scanner. The lens at the end is inserted into a carefully aimed sight tube, so that the scanner can see the flame. A threaded ring secures the scanner on the sight tube. Figure 14 shows a scanner mounted on a power burner assembly. Figure 13 – Photoelectric Scanner Figure 14 – Scanner Mounting Page 20 of 49 There are various types of photocells in use, including: • Infrared scanner • Ultraviolet scanner • Rectifying photocell Infrared (IR) Scanner The scanner illustrated in Figure 15 uses a lead-sulfide cell that responds to infrared rays. The cell is a semiconductor whose electrical resistance decreases with an increase in the amount of infrared light it receives from the flame. Flame pulsations cause the resistance of the cell to fluctuate. This causes a fluctuating voltage (called the flame signal), which is amplified by an electronic amplifier to hold the flame relay in the closed position. Figure 15 – Sulfide Cell Scanner The flame relay controls the power supply to the main SSOV. When a flame failure occurs, the scanner does not sense a flame. Its resistance increases, and causes the flame relay to open. The power supply to the SSOVs is cut-off, which shuts off the fuel to the burner. During boiler startup, when the scanner senses the pilot flame, the flame relay closes and the main SSOV opens. This permits fuel flow to the main burner. If the main burner does not light, or if the pilot flame goes out, the scanner opens the flame relay circuit, which causes the SSOVs to close. Most boilers equipped with a photoelectric safeguard system use a single scanner to sight both the pilot flame and the main burner flame. A basic diagram of a lead sulfide scanner installation is shown in Figure 16. The scanner is mounted on the end of a sighting tube in Figure 16 – Scanner Installation such a way that its temperature will not exceed 50°C. The tube is aimed so the scanner can observe the pilot and the main burner. Figure 16 also shows the size of pilot flame required to keep the burner circuit energized. Page 21 of 49 Figure 17 shows an end view and a side Figure 17 – Scanner Installation view of a firetube boiler burner assembly. The scanner sight tube is clearly visible at the top of the photo. The spark igniter electrodes are adjacent to the pilot burner. The pilot burner is surrounded by the main burner, which is a donut-shaped metal casting. Note the angle of the sight tube. This permits the scanner to observe the pilot and the main burner. Ultraviolet (UV) Scanner The principle of operation and mounting of the ultraviolet scanner is quite similar to the infrared. However, this scanner responds only to the ultraviolet rays emitted by the flame. UV scanners are mounted as close as possible to the flame, without being subjected to a temperature over 100°C. Igniter sparks give off UV light, and can fool a UV scanner into mistaking the spark for a pilot flame. Newer burner management systems may be able to recognize the frequency of a spark, and can distinguish sparks from flames. However, it is best practice to ensure the scanner cannot see the igniter spark. Page 22 of 49 Case Study the boiler was down for regular maintenance. During shutdown, or replace the furnace sight glass. The old one is dirty and cloudy, so I put in a shiny new one. After the boiler was back together and ready to go, it was time to check the burner safeguard controls. I shut the main and pilot test firing valves and did an ignition spark response test. I could not believe that I got a flame signal of 3.5 volts off of the spark! The programming controller opened up the main safety shut off valves, too! Cortana test firing the valves were closed. I can imagine the explosion if I tried to light off the main burner from that little spark. I tried to test again, but this time I pulled the scanner and looked down the site tube. it turns out that the new sight glass was so shiny that the igniter spark reflected off of it back to the scanner. Sight lines are just right! Rectifying Photocell The rectifying photocell is also known as a cad cell. It consists of a glass vacuum tube containing a curved metal cathode made of cadmium oxide and an anode wire, which form part of an electronic circuit. When the cathode is exposed to visible light from a flame, it emits electrons that are picked up by the anode. This creates a signal that closes the contact points of a flame relay. This relay keeps the main safety shut-off valve energized and open. Failure of the flame causes it to close. The cell is often mounted inside the combustion air supply tube of small packaged oil burners. Here, it is kept relatively cool and clean, while still having an unobstructed view of the flame. Page 23 of 49