Chapter 11-BOILER FITTINGS AND CONTROLS 



Under steady boiler loads, the output of 

 the combustion air Standatrol is steady at some 

 Value which will maintain combustion air to the 

 furnace at the rate required to maintain the 

 superheater outlet steam pressure at 1200 psig. 

 For each boiler, the combustion air demand 

 signal from C4b is applied, through a bias relay 

 (C15a) and a rate relay (C15b), to the two forced 

 draft blower selector valves (C5b), 



The bias relay acts to maintain the minimum 

 air flow demand signal at a value consistent with 

 minimum blower speed and damper position. The 

 rate relay acts in combination with the bias re- 

 lay to accelerate any changes in the input steam 

 demand signal by providing an exaggerated load- 

 ing pressure. The rate relay may also be ad- 

 justed to decrease the effects of changes in the 

 steam demand signal. The exaggerated signal 

 of the rate relay is slowly returned to normal 

 through the action of a bleed valve within the 

 rate relay. 



Each blower selector valve (C5b) transmits 

 a penumatic pressure through the 3-way air 

 trapping valves (C6a and C7a) to the blower 

 speed control drive (C6) and to the blower 

 damper control drive (C7). Note that the pneu- 

 matic pressure transmitted by the selector valve 

 is control pressure rather than loading pressure, 

 since it goes to a control drive. The control 

 pressure causes the blower speed control drive 

 and the blower damper control drive to be posi- 

 tioned in accordance with the demand for 

 combustion air. The blower selector valves 

 (C5b) are provided with bias control knobs which 

 can be used to equalize the distribution of com- 

 bustion air when both blowers are in operation. 



The 3-way air trapping valves (C6a andC7a) 

 function to close the forced draft blower dampers 

 to their mechanical bottom stops and to reduce 

 blower speed to the minimum required for stable 

 combustion, in the event of loss of control air 

 supply. 



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 fuel 

 flow-air flow Standatrol (C4a2). In the fuel flow- 

 air flow Standatrol, the signal from the fuel 

 limiting relay (C15) is balanced against a signal 

 representing the amount of fuel burned; this 

 "fuel burned" signal comes to the fuel flow-air, 

 flow Standatrol (C4a2) from the fuel flow dif- 

 ferential relay (C2c). The output signal of the 

 fuel flow-air flow Standatrol (C4a2) is applied 

 to the fuel control valve (C8) in the return fuel 



line from the burners. This control pressure 

 from the Standatrol C4a2 positions the fuel con- 

 trol valve so that the required amount of fuel will 

 be burned in order to maintain steam pressure at 

 1200 psig at the superheater outlet. Notice that 

 the amount of fuel burned is controlled by limit- 

 ing the return flow of fuel; the supply pressure 

 in the line to the burners is fixed. 



Thus, far, we have been considering the com- 

 bustion control system as it operates when the 

 steam demand (steam flow from the boiler) re- 

 mains constant. Now let us see what happens 

 when there is an increase in steam demand. 

 For simplicity, the various changes that occur 

 are presented as a numbered list. Remember, 

 however, that some changes may be occurring 

 at the same time as others. 



1. Steam flow increases, so there is an 

 increased steam flow signal from the steam 

 flow transmitters (F2) to the steam demand 

 relay (C4al). 



2. Steam pressure drops below 1200 psig, 

 so there is a decrease in the steam pressure 

 signals from the steam pressure transmitters 

 (Cla). 



3. The steam demand relay (C4al) is con- 

 nected so that an increased signal from the 

 steam flow transmitter and a decreased signal 

 from the steam pressure transmitter result in 

 an increased output loadingpressurefrom C4al. 

 This increased loading pressure from C4al goes 

 to the combustion air Standatrol (C4b) and to the 

 fuel limiting relay (C15). 



4. The increased loading pressure from 

 C4al to the combustion air Standatrol (C4b) 

 causes an increase in the output loading pres- 

 sure from the combustion air Standatrol; the 

 ultimate effect of this increase is to increase 

 the control pressure to the blower damper con- 

 trol drives and to the blower speed control 

 drives. The blowers speed up and the dampers 

 open wider. Actually, during this period in which 

 the unbalance is just beginning to be corrected, 

 the blowers speed up enough to allow a temporary 

 "overfiring" rate so that the steam pressure can 

 quickly be restored to normal. 



5. As the blowers begin to pick up speed, the 

 measured air flow signal from the air flow 

 transmitter (C3) to the combustion air Standa- 

 trol (C4b) and to the fuel limiting relay (C15) 

 also increases. 



6. In the fuel limiting relay, the fuel de- 

 mand signal is held back to a value which 

 corresponds to the value of the measured air 



315 



