PRINCIPLES OF NAVAL ENGINEERING 



flow signal. Even if the steam demand signal 

 from C4al is higher than the measured air flow 

 signal from C3, the output of C15 cannot exceed 

 the air flow signal during the period in which the 

 firing rate is increasing. 



7. The output signal of the fuel limiting re- 

 lay (C15) is applied to the fuel flow-air flow 

 Standatrol (C4a2). The fuel control signal— that 

 is, the output of the fuel flow-air flow Standa- 

 trol— begins to increase, thus closing the fuel 

 control valve and increasing the fuel supply to 

 the burners. Since the rate of increase in fuel 

 flow is caused by steam demand but limited by 

 the measured air flow, the system can never 

 supply too much fuel to the burners for the 

 amount of combustion air being supplied. 



8. As the steam pressure at the superheater 

 outlet returns to the set pressure of 1200 psig, 

 the steam pressure transmitter signal also 

 rises. Increasing signals from the steam pres- 

 sure transmitters result in a decreasing loading 

 pressure from the steam demand relay (C4al) 

 and decreasing control signal from the com- 

 bustion air Standatrol (C4b) to the forced draft 

 blower and damper drives (C6 and C7, respec- 

 tively). This reduces the temporary "over- 

 firing" rate previously mentioned. When the 

 measured air flow signal from the air flow 

 transmitter (C3) reaches a value which returns 

 the combustion air Standatrol to balance, the 

 output pressure of C4b stabilizes at a value 

 which will maintain this air flow. The output 

 pressure of the fuel flow-air flow Standatrol 

 (C4a2) stabilizes in a similar way to maintain 

 the same rate of fuel flow to the burners. At 

 this time, the main line steam pressure has 

 returned to 1200 psig and the air flow and fuel 

 flow are adjusted so as to maintain this pres- 

 sure under the new (and higher) steam demand 

 conditions. 



When there is a decrease in steam demand, 

 the system functions to slow down the forced 

 draft blowers, partially close the blower damp- 

 ers, and open the fuel control valve so that the 

 supply of fuel to the burners will be decreased. 

 After seeing how the system operates when the 

 steam demand is constant and when the steam 

 demand is increasing, it should not be too dif- 

 ficult to trace the signals and events that occur 

 when the steam demand is decreasing. 



Feed Water Control System.— While the com- 

 bustion control system is functioning to control 

 combustion air and fuel, the feed water system 



is functioning to control the amount of feed 

 water going to the boiler. 



There are three elements in the feed water 

 control system: steam flow, feed water flow, and 

 boiler drum water level. The feed water flow 

 transmitter (Fl) and the steam flow transmitter 

 (F2) act together to provide a proportioning 

 control— that is, to provide a flow of feed water 

 that is proportional to the flow of steam. The 

 drum water level indicating transmitter (F3a) 

 introduces a secondary signal that continuously 

 adjusts the position of the feed water flow con- 

 trol valve (F6a) in order to maintain the de- 

 sired water level in the boiler steam drum. 



The feed water flow transmitter (Fl) de- 

 velops a pneumatic signal that is proportional 

 to feed water flow. This signal is applied, 

 through a volume chamber (Fla), to the steam 

 flow-water flow differential relay (F4-1). The 

 other input to relay F4-1 is the output pressure 

 from the steam flow transmitter (F2). The out- 

 put pressure from F2 is applied to F4-1 through 

 the transient compensating relay (C4a3); under 

 conditions of steady steam demand, the output 

 signal of C4a3 exactly duplicates the output 

 signal of the steam flow transmitter (F2), but 

 when there is a change in steam demand the 

 output signal of C4a3 is not the same as the 

 output signal of F2. 



The output from the steam flow-water flow 

 differential relay (F4-1) is applied to the feed 

 water Standatrol (F4-2), where it is balanced 

 against a signal from the drum water level 

 indicating transmitter (F3a). When the two in- 

 puts to the feed water Standatrol (F4-2) are at 

 their set point values, a constant pneumatic 

 output pressure is transmitted from the Standa- 

 trol through the feed water selector valve (F5) 

 to the feed water flow control valve (F6a). A 

 spring adjustment in the feed water Standatrol 

 (F4-2) maintains the steam drum water level 

 at a set height. 



When steam demand increases, there is a 

 proportional increase in the loading pressure 

 output of the steam flow transmitter (F2) which 

 is transmitted to the transient compensating 

 relay (C4a3). At the compensating relay, the 

 signal is temporarily reversed— that is, the in- 

 put signal representing an increase in steam 

 flow becomes an output signal representing a 

 decrease in steam flow. This output signal from 

 C4a3 is applied to the steam flow-water flow 

 differential relay (F4-1), which also receives 

 an input signal from the feed water flow 



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