Chapter 11- BOILER FITTINGS AND CONTROLS 



V 



(CLOSED LOOP) 





Figure 11-38.— Closed control loop. 



98.100 



(2) the rate of air flow from the nozzle 

 determines the intensity of the pneumatic pres- 

 sure imposed upon the valve motor operator. 



If the vane and the nozzle are relatively far 

 apart, a good deal of air will flow out of the 

 nozzle and there will be relatively low air 

 pressure acting on the motor operator. If the 

 vane and the nozzle are closer together, the flow 

 of air from the nozzle will be retarded and 

 higher pressure will act upon the motor oper- 

 ator. The level of air pressure acting on the 

 motor operator determines the position of the 

 steam valve, since the motor operator positions 

 the valve in accordance with the air pressure 

 received from the transmitter. The motor oper- 

 ator and the steam valve together thus form the 

 controlling means of this system. 



As previously noted, practically all boiler 

 control systems aboard ship are of the pneu- 

 matic type, depending upon compressed air for 

 their operation. Compressed air is used as the 

 controlling or balancing force for the operation 

 of the many pneumatic transmitters and relays 

 in the system and, at a higher pressure, as the 

 source of power to operate some or all of the 

 control drives and control valves that control 

 the flow of fuel, combustion air, and feed water. 

 In some older ships, the compressed air supply 

 is obtained from the ship's service compressed 

 air system, through reducing valves. In newer 



ships, a separate combustion control compress- 

 ed air system, with its own air compressor, is 

 installed for the boiler controls. 



In any pneumatic boiler control system, a 

 great many units are used to develop, transmit, 

 and receive pneumatic "messages" or "sig- 

 nals" in the form of variable air pressures. 

 The pneumatic units are interconnected by cop- 

 per tubing. A typical pneumatic unit in a boiler 

 control system operates by receiving one or 

 more pneumatic pressures from one or more 

 sources (frequently from other pneumatic units), 

 altering or combining the pressure or pressures, 

 and then sending a new pneumatic pressure to 

 another pneumatic unit in the system. 



The actual mechanisms which develop, trans- 

 mit, and receive pneumatic signals vary, depend- 

 ing upon the manufacturer and upon the function 

 of the units. The nozzle-and-vane assembly 

 shown in figure 11-39 is only one of a number 

 of pneumatic devices that could be used to ac- 

 complish the functions of comparison and com- 

 pution. Bellows, escapement valves, and various 

 other devices are used in pneumatic control 

 systems to compare the measured value with 

 the desired value and to compute the amount 

 of correction required. Similarly, the measur- 

 ing means and the controlling means shown in 

 figure 11-39 are commonly used in pneumatic 

 control systems, but they are not the only 

 possible devices for such applications. 



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