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ASME SECTION VII-2008 pdf download

C1.230 ELECTRIC BOILERS Electric boilers have a lot in common with fired boilers. Both require close attention to water treatment for success- ful operation. They need deaerated soft water. Boiler water blowdown is necessary to keep the solids concentration within the Manufacturer’s recommended limits for good performance. Safety valves protect against overpressure. Steam generation and, therefore indirectly, pressure are controlled by varying power input. Electric boilers that use electric resistance heating coils immersed in water are very low capacity boilers and will not be addressed further. 2 Electrode-type boilers generate saturated steam by con- ducting current through the water itself. Boiler water con- ductivity must be monitored and controlled. If the conductivity is too low, the boiler will not reach full operating capacity. When the conductivity is too high, overcurrent protection will shut off the power. High boiler water solids may cause foaming and shorts to ground. It may be necessary to increase blowdown or add antifoam agents to control foaming. Very pure water will limit steam output and will require addition of addi- tives to increase the boilerwaterconductivity. The operator is advised to consult the boiler manufacturer and a water treatment consultant for specific advice. Solids from the saturated steam tend to accumulate slowly on the insulators supporting the electrodes from the grounded shell. The unit must be shut down periodically so the insulators can be washed off to prevent arcing. High voltages up to 16,000 V may be used. Protection is needed for ground faults, overcurrent, and, for three phase systems, loss of phase. The main electrical discon- nect switch must be locked out before performing mainte- nance on the boiler. C1.300 PACKAGE BOILERS Small sized power boilers can be completely assembled in the boiler manufacturer’s shop, including burners, fans, controls, sootblowers, etc. These boilers are shipped to the operating site by barge, rail, or over the road vehicles. They are called package boilers and can be designed to operate at high pressures and capacities. Operating pres- sures of1,000 psig (7 MPa) and capacities of300,000 lb/hr (140 000 kg/hr) and steam temperatures of 950°F (510°C) can be achieved. C1.400 FIELD-ASSEMBLED BOILERS Boilers that are too large to ship completely assembled must be field erected. Some are capable of producing up to several million pounds per hour of steam superheated to 1,100°F (595°C). Natural circulation boilers may be designed for pressures up to about 2,900 psig (20 MPa). Boilers that use a pump to control circulation may be designed for even higher pressures. Because these “forced circulation” boilers do not rely on density differences to establish and maintain circulation, they may even be designed at pressures exceeding the critical pressure, 3,208 psia (22 MPa). C1.500 COMBUSTION The three requirements for combustion are fuel, oxygen, and heat. The most common fuels used in boilers are coal,
Heat is required to bring the fuel up to its kindling or ignition temperature. There is a specific kindling tempera- ture for every combustible substance. A fuel will not start or continue to burn unless this temperature is reached; once ignited, the fuel will continue to burn as it supplies its own heat, provided enough oxygen is available to continue the reaction. The ignition temperature of coal is affected by the gas- eous constituents being distilled off and the fixed carbon in the coal. The temperature produced during the normal combustion process ranges from 2,500°F (1 400°C) to 3,000°F (1 600°C), far in excess of the ignition tempera- ture. Air must come in contact with the combustibles in the coal for combustion to continue. The more surface area exposed to the air, the faster the burning. The combustion process can be improved by crushing or grinding the coal into small particles, and by creating turbulence with the air supplied for combustion. As with coal, the more surface of oil exposed to air, the faster the burning. The combustion process can be improved by atomizing the oil into a very fine mist and by creating turbulencewiththeairsuppliedforcombustion. Heavy fuel oil must be heated to bring it down to the properviscosityforefficientatomization. Fuelcanbeatom- ized either mechanically with the high pressure drop across the oil gun tip or by using steam or air to create a shearing action.
C1.600 BOILER EFFICIENCY The efficiency of a boiler is dependent on how much of the heating value of the fuel is lost, as either sensible heat or by incomplete combustion. Sensible heat loss in the dry stack gases is by far the largest loss. An oversupply of excess air (over and above that required for complete com- bustion of the fuel) will increase this heat loss and also tends to cool the boiler. The amount of excess air can be determined by measuring the amount of oxygen in the 4 stack gases. This loss should be minimized by not supply- ing more excess air than required. Moisture is the next largest cause of heat loss. Moisture in the fuel becomes superheated steam when the fuel burns, and this steam is lost up the stack. The hydrogen content ofthe fuel combines with oxygen, forming additional water as superheated steam. The humidity of the air is another addition of moisture to the furnace and also causes a loss in efficiency. Incomplete combustion can be caused by poor mixing of fuel and air or lack of sufficient air supply to the fuel as it is being burned. When carbon is only partially oxidized, carbon monoxide gas is formed. Incomplete combustion therefore can be detected by the measurement of carbon monoxide in the stack gas. Incomplete combustion of fuel represents a serious loss and can be minimized by using sufficient air and a suitable means of mixing the fuel and air. It is impossible to burn all the carbon out of coal, but the amount remaining in the ash can be held to a minimum by controlling the factors that contribute to good com- bustion.

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