Boiler Low Water Level Fuel Cut-Off Equipment PDF

Objective 1 Describe the construction and operation of boiler low water level fuel cut-off equipment. PURPOSE OF THE LOW WATER FUEL CUT-OFF The low water fuel cut-off (LWCO) is a float or probe-operated switch that directly detects the level of water in a boiler. Its function is to shut off the bo...

Objective 1 Describe the construction and operation of boiler low water level fuel cut-off equipment. PURPOSE OF THE LOW WATER FUEL CUT-OFF The low water fuel cut-off (LWCO) is a float or probe-operated switch that directly detects the level of water in a boiler. Its function is to shut off the boiler burner system if boiler water level drops to a predetermined unsafe level. This prevents overheating and weakening of the pressure-retaining parts of the boiler, which are normally cooled by water. All boilers require low water fuel cut-offs, except for: • Electrode-type electric boilers. • Coil tube and fin tube boilers that require forced circulation to prevent overheating. • Those under continuous operator attendance. Each boiler design has a unique lowest permissible water level (LPWL), identified by the boiler manufacturer. This is the lowest water level at which a boiler can be safely operated. The LWCO turns off the burner system before the water level drops to the LPWL. Because of this, it is a very important boiler safety control. Side Track Like all boiler fittings, low water cut-offs have maximum allowable working pressures (MAWP). When repairing or replacing LWCOs, make sure the components are suitable for the boiler MAWP. Page 3 of 45 FLOAT OPERATED LOW WATER FUEL CUT-OFF The low water fuel cut-off device shown in Figure 1 is often used on low capacity cast iron sectional heating boilers. This device consists of a float chamber connected to the same boiler openings as the directly connected water gauge glass. Therefore, the water level in the float chamber is the same as that in the boiler and the gauge glass. The float inside the chamber, which follows the changes in water level in the boiler, is directly connected to an electric switch, attached to the float chamber. The switch is separated from the water chamber by a flexible diaphragm, which acts as a seal. Figure 1 – Low Water Fuel Cut-Off Device When the water in the boiler is above the lowest permissible level, and is still visible in the gauge glass, the float keeps the electrical switch in the closed position. This allows power to energize the safety shut-off valve (SSOV). If the boiler water drops below the minimum safe level, the float switch opens, which causes the SSOV to close. This stops the fuel supply to the burner. Figure 2 shows a sketch of a LWCO design more commonly used for power boilers or larger heating boilers. The float chamber is connected directly to the steam and water space, independent of the gauge glass. The water level in this chamber will follow the level in the boiler very closely. A float, which follows the fluctuations in water level, is connected to a rod that pivots in the upper part of the chamber. Movement of the short end of the rod is transmitted through the bellows assembly Page 4 of 45 to a lever system that tilts the mercury switch. The bellows assembly forms a flexible steam and watertight seal between the float housing and the electrical junction box. Figure 2 – Low Water Fuel Cut-Off Device The mercury switch, used in the low water fuel cut-off and many other controls, consists of a small glass bulb. Electrical contacts that form part of an external wiring circuit are mounted inside the bulb at either one end or both. The bulb also contains a small quantity of mercury, and is used to open or close the electrical contacts inside the bulb. The interior of the bulb is under a high vacuum to prevent oxidation of the mercury and corrosion of the electrical contacts. By tilting the bulb to one side or the other, the mercury opens or closes the circuit. Caution Mercury switches eventually fail. Small cracks in the glass permit oxygen to enter the glass and oxidize the mercury. When this occurs, the mercury loses its lustre, and may appear rusty. The mercury will not flow properly and will not close or open the switch contacts reliably. When this occurs, the control must be replaced. When replacing any control that contains mercury, the mercury must be treated as a hazardous material. Wear correct personal protective equipment. Dispose of the mercury according to environmental regulations. Page 5 of 45 Figure 3(a) shows a single-pole mercury switch in the closed position. This is the position of the switch when the boiler water is above the LPWL. Figure 3 – Operation of Mercury Switch As the boiler water level drops, the float follows the changing level in the float housing and tilts the mercury switch to the opposite side by means of the rod and lever system. At a predetermined level, the bulb tilts enough so that it shifts the mercury to the opposite end of the bulb, and the circuit will be opened as shown in Figure 3(b). Some mercury switches, like in Figure 3(c), use three wires instead of two. When there is a drop in water level, the mercury moves to the opposite end of the bulb. This opens the electrical circuit to the SSOV, and closes alarm circuit contacts at the opposite warns end. This the operator that the burner has shut down due to low water. Figure 4 – Combined Pump Control and Low Water Cut-Off Figure 4 shows a float arrangement combines a that feedwater pump control switch with a low water fuel cut-off and alarm. In this design, one bulb contains a two-wire Page 6 of 45 switch that controls the feedwater pump circuit. The bulb with the three-wire switch controls a low water fuel cut-off and an alarm. As the boiler water level drops, the float closes the two-wire switch. This starts the feedwater pump when the water is still above the lowest permissible level. If the pump fails to start, or some other problem causes the drum level to continue dropping when the feedwater pump is running, the bulb with the three-wire switch acts as a boiler emergency shutdown. The electrical circuit to the SSOV opens, and it shuts off the burner before the water in the boiler reaches the lowest permissible level. An alarm circuit will also energize. Figure 5 shows a float controlled make-up water valve combined with a low water fuel cut-off. If the boiler drum level Figure 5 – Combined Feeder/Cut-Off Control drops, the float in the chamber moves to a lower position, which opens the make-up valve further to admit more water into the boiler. As the water level rises, the float moves up and decreases the valve opening. If the water level continues to drop when the make-up water valve opens, the switch will shut down the burner. Even though the low water fuel cut-off opens the SSOV, there is always a possibility that the fuel valve will stick in the open position and firing will continue. The boiler temperature or pressure will continue to rise until the pressure relief valve opens. The water level could drop to the point where heat transfer surfaces could overheat. To safeguard against this a combination low water cutoff and feeder valve offers the best protection for low-pressure steam heating boilers. Page 7 of 45 Some boilers have the float chamber of the low water fuel cut-off combined with the water column, as shown in Figure 6(a). Its operation is similar to the LWCO controls already discussed. Figure 6(b) is an exterior view of the Figure 6 – Column with Low Water Fuel Cut-Off water column and float chamber shown in Figure 6(a). Level A is the highest normal operating level in the boiler. At level A, the feedwater supply switch is open, and the boiler feed pump is off. When the water level drops to level B, the control switch closes, the feedwater pump starts, and feedwater supply resumes. During normal operation, the water level alternates between level A and level B. If the supply of make-up water fails or is insufficient, the water level will continue to drop. When the water level drops to level C, the cut-off switch opens the burner circuit before the level drops to the lowest permissible water level. At the same time, the alarm switch closes, and energizes the alarm circuit. The boiler cannot restart until the water level is restored to above level C. In some cases, the switch may also have to be manually reset. Page 8 of 45 Magnetic Low Water Fuel Cut-Off Figure 7 – Magnetic Low Water Fuel Cut-Off Figure 7 shows the operation of a low water fuel cutoff actuated by a float and magnet. At normal water levels, the permanent magnet attached to the pivoted mercury switch is drawn toward the magnetic plunger. The mercury switch tilts, which closes the contacts and keeps the burner in operation. As the water level drops, the float is lowered together with the plunger. At the cut-off point, the plunger drops entirely outside of the magnetic field (Figure 7(b)). Then, the tension spring pulls the permanent magnet away, and the three-wire mercury switch tilts to the opposite position. The contacts then close, which opens the burner circuit, and shuts off the fuel supply. This energizes the alarm circuit. Figure 8 – Electric Probe Type Low Water Fuel Cut-Off Electric Probe Type Low Water Fuel CutOff This cut-off usually consists of two electric probes, or electrodes, immersed in the water. These probes may be mounted directly on the Page 9 of 45 boiler shell, on the water column, or in a special probe housing. The schematic of one design is shown in Figure 8. As long as water covers both probes, a small current will flow through the water between the probes. This closes the electric circuit that energizes an electro-magnetic coil, or solenoid switch, in the power circuit to the fuel valve, and keeps the switch in the closed position. This allows the fuel valve to be in an open position. When the water level drops below the upper probe, the flow to the solenoid will stop, and the switch opens. This interrupts the power supply to the fuel valve, and shuts off the fuel. Another design of the probe type low water fuel cut-off has Figure 9 – Electric Probe Type Low Water Fuel Cut-Off only a single probe. The boiler shell or probe housing acts as a second probe (see Figure 9). The advantage of the electric probe type cut-off is its simplicity. It contains no moving parts, so the possibility of mechanical failure is eliminated. It is necessary to check and clean the probes at regular intervals. Scale or sediment on the probes can reduce or completely stop the current flow, which results in a boiler shutdown. Page 10 of 45 Low Water Fuel Cut-Off for Hot Water Heating Boilers Hot water boilers are completely filled with water, and are usually equipped with an automatic fill valve connected to the water supply. This keeps the system filled at all times. Low water conditions can develop in hot water heating boilers, as well as in steam boilers. In this case, the condition is usually due to human error or mechanical failure. The construction of a hot water boiler is not much different from that of a steam boiler. So, the effects of a low water condition can be just as disastrous. The same type of cut-off device used on steam boilers can be applied to hot water boilers, provided the pressure rating of the device is high enough. Hot water boilers may operate at a higher pressure than steam heating boilers. Figure 10 shows a float operated low water fuel cut-off device installed on a hot water heating boiler. Figure 10 – Low Water Fuel Cut-Off on a Hot Water Boiler There is considerable freedom in the arrangement of the low water fuel cut-off on hot water heating boilers. Since no normal water level has to be maintained, the control may be installed at any point above the lowest permissible water level specified by the manufacturer. Page 11 of 45 POSSIBLE CAUSES OF LOW WATER LEVEL A rapidly falling water level in the boiler may be caused by: • A faulty feedwater level controller. • Feedwater or condensate pump failure. • Interruption of the water supply to the feedwater pump. • Leakage from the boiler, due to ruptured tubes or open blowoff valves. Other causes may produce a gradual reduction in boiler water level. Each of these only occurs if the make-up water or condensate return systems are not working properly. These causes include: • Leaks in hot water supply or return piping. • Damaged condensate return piping. • Defective condensate return pumps. • Excessive blowoff or blowdown. • Relief valve discharge caused by full expansion tanks. Normally, when the level drops to the low water cut-off point, the boiler will shut down automatically. However, if the cut-off fails to shut the boiler down, the water level may drop to a dangerous level. It is essential for the plant operator to conduct regular tests of the low water cut-off. Side Track Procedures for testing low water cut-offs, and emergency response to low water conditions, are covered in Part B, Unit 4, Chapter 3 Boiler Operation. Page 12 of 45