Principles of naval engineering - Historic Naval Ships Association

Chapter 11-BOILER FITTINGS AND CONTROLSUnder steady boiler loads, the output ofthe combustion air Standatrol is steady at someValue which will maintain combustion air to thefurnace at the rate required to maintain thesuperheater outlet steam pressure at 1200 psig.For each boiler, the combustion air demandsignal from C4b is applied, through a bias relay(C15a) and a rate relay (C15b), to the two forceddraft blower selector valves (C5b),The bias relay acts to maintain the minimumair flow demand signal at a value consistent withminimum blower speed and damper position. Therate relay acts in combination with the bias relayto accelerate any changes in the input steamdemand signal by providing an exaggerated loadingpressure. The rate relay may also be adjustedto decrease the effects of changes in thesteam demand signal. The exaggerated signalof the rate relay is slowly returned to normalthrough the action of a bleed valve within therate relay.Each blower selector valve (C5b) transmitsa penumatic pressure through the 3-way airtrapping valves (C6a and C7a) to the blowerspeed control drive (C6) and to the blowerdamper control drive (C7). Note that the pneumaticpressure transmitted by the selector valveis control pressure rather than loading pressure,since it goes to a control drive. The controlpressure causes the blower speed control driveand the blower damper control drive to be positionedin accordance with the demand forcombustion air. The blower selector valves(C5b) are provided with bias control knobs whichcan be used to equalize the distribution of combustionair when both blowers are in operation.The 3-way air trapping valves (C6a andC7a)function to close the forced draft blower dampersto their mechanical bottom stops and to reduceblower speed to the minimum required for stablecombustion, in the event of loss of control airsupply.The output from the air flow transmitter(C3) is also applied to the fuel limiting relay(C15). The output of the fuel limiting relay,representing fuel demand, is applied to the fuelflow-air flow Standatrol (C4a2). In the fuel flowairflow Standatrol, the signal from the fuellimiting relay (C15) is balanced against a signalrepresenting the amount of fuel burned; this"fuel burned" signal comes to the fuel flow-air,flow Standatrol (C4a2) from the fuel flow differentialrelay (C2c). The output signal of thefuel flow-air flow Standatrol (C4a2) is appliedto the fuel control valve (C8) in the return fuelline from the burners. This control pressurefrom the Standatrol C4a2 positions the fuel controlvalve so that the required amount of fuel willbe burned in order to maintain steam pressure at1200 psig at the superheater outlet. Notice thatthe amount of fuel burned is controlled by limitingthe return flow of fuel; the supply pressurein the line to the burners is fixed.Thus, far, we have been considering the combustioncontrol system as it operates when thesteam demand (steam flow from the boiler) remainsconstant. Now let us see what happenswhen there is an increase in steam demand.For simplicity, the various changes that occurare presented as a numbered list. Remember,however, that some changes may be occurringat the same time as others.1. Steam flow increases, so there is anincreased steam flow signal from the steamflow transmitters (F2) to the steam demandrelay (C4al).2. Steam pressure drops below 1200 psig,so there is a decrease in the steam pressuresignals from the steam pressure transmitters(Cla).3. The steam demand relay (C4al) is connectedso that an increased signal from thesteam flow transmitter and a decreased signalfrom the steam pressure transmitter result inan increased output loadingpressurefrom C4al.This increased loading pressure from C4al goesto the combustion air Standatrol (C4b) and to thefuel limiting relay (C15).4. The increased loading pressure fromC4al to the combustion air Standatrol (C4b)causes an increase in the output loading pressurefrom the combustion air Standatrol; theultimate effect of this increase is to increasethe control pressure to the blower damper controldrives and to the blower speed controldrives. The blowers speed up and the dampersopen wider. Actually, during this period in whichthe unbalance is just beginning to be corrected,the blowers speed up enough to allow a temporary"overfiring" rate so that the steam pressure canquickly be restored to normal.5. As the blowers begin to pick up speed, themeasured air flow signal from the air flowtransmitter (C3) to the combustion air Standatrol(C4b) and to the fuel limiting relay (C15)also increases.6. In the fuel limiting relay, the fuel demandsignal is held back to a value whichcorresponds to the value of the measured air315