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1、Instrumentation IIPart B1 Chapter 8Boiler Control SystemsOn-Off Boiler Feedwater Control On-Off Boiler Feedwater ControlIn this type of control,water is supplied intermittently to the boiler,either by fully opening or fully closing a valve in the feedwater line,or by starting and stopping the feedwa
2、ter pump.The two-wire switch is used to energize the boiler feedwater pump.The three-wire switch is used for cutting off the fuel to the boiler in case of low drum level.See Cutaway ModelElectro-Mechanical Float SwitchElectro-Mechanical Float SwitchSingle Element Feedwater Control System Fig.3 illus
3、trates a pneumatic single element feedwater control system,although electronic control systems are designed in a similar fashion.Single Element Feedwater Control System Single Element Feedwater Control System The principle of level measurement is similar to that used for a remote level indicator,exc
4、ept that a differential pressure transmitter is used instead of an indicator.A constant head of water is maintained on one side of a diaphragm within a transmitter,while the other side is exposed to the varying head caused by the drum level in the boiler.Single Element Feedwater Control System A cha
5、nge in drum level causes a proportional change in output from the transmitter,which is compared with the set point signal in the level controller(LRC).If there is any deviation of level from the set point,the controller output will reposition the feedwater control valve to correct the drum level.Thi
6、s is known as feedback control,as it responds only to changes in boiler water level.Single Element Feedwater Control System An automatic-manual transfer station is usually located at the controller to allow the operator to start the boiler with the level control on manual.When the boiler is under lo
7、ad and the drum level is stabilized,the operator can switch the control system to automatic control.A recorder can be located in the controller so plant personnel can have a second means of verifying the drum level in the boiler.This type of boiler level control is used when the boiler control panel
8、 is located in a remote area away from the boiler.The low-water fuel cutoff must receive the following attention to keep it in properoperating condition1.It should be tested frequently.With the burner in operation,the water supply is shut off and the boiler is slowly drained.If the cutoff is working
9、 properly,the fuel valve should close when the water level is approximately 25 mm(1 inch)above the bottom of the gage glass.2.It is essential to thoroughly blow out the chamber every shift by means of the drain valve provided.This will prevent sediment from collecting in the float chamber and impedi
10、ng the action of the float.3.The mercury switches should be inspected at least once per month.If the glass tube becomes cracked,air will enter and cause oxidization of the mercury.This will cause the mercury not to flow properly when the tube is tilted.A film on the inside of the tube is an indicati
11、on of oxidation and when this occurs,the switch should be replaced.4.The electrical wiring to the switch should be checked at the same time as the switch.Connections should be checked and wiring should be replaced to prevent short circuits when bare spots or brittle insulation is evident.5.Every few
12、 months the float should be dismantled and checked for scale,sediment deposits,and signs of corrosion.Types of Single Element Proportional FW ControlWe are going to look at:ThermohydraulicThermoexpansionDifferential Pressure(head type)Single Element Thermohydraulic Regulator The generator consists o
13、f two tubes,one within the other with the outer tube connected to a bellows/diaphragm of a regulating valve.The ends of the inner tube are connected to the steam and water spaces of the drum so the level in this inner tube will be the same as in the drum.BAILEY”Thermohydraulic Feedwater RegulatorWhe
14、n the drum level drops more of the water in the generator tube is exposed to steam so its temperature and pressure go up and exert more force on the bellows which must open the feedwater valve.BAILEY”Thermohydraulic Feedwater RegulatorWhen the drum level rises less of the water in the generator tube
15、 is exposed to steam so some of the steam in the generator condenses and therefore there is less pressure on the bellows so the feedwater valve closes.BAILEY”Thermohydraulic Feedwater RegulatorSimple and reliable but slow.There is one of these on Boiler 1 over in the RRC Powerhouse.BAILEY”Thermohydraulic Feedwater RegulatorSingle Element Thermoexpansion RegulatorThis regulator consists of a tube“A”Single Element Thermoexpansion Regulatormounted on a rigid beam