Chapter 11-BOILER FITTINGS AND CONTROLS 



BOILER CUTOUT VALVES 



SETTLING POTS, 

 WITH MAGNETS - 



EQUALIZING 

 VALVE 



'INDICATOR 

 CUTOUT VALVES 



98.97 

 Figure 11-33.— General arrangement of Yarway 

 remote water level indicator. 



Cooling fins attached to the pipe that carries 

 water from the steam drum to the inner tube 

 ensure that the water in the inner tube will be 

 at a slightly lower temperature than the steam 

 in the inner tube. Cooling fins on the generator 

 ensure that the water and steam in the generator 

 will be cooler than the water and steam in the 

 inner tube. For a number of reasons, the trans- 

 fer of heat is more rapid from the steam in 

 the inner tube to the steam in the generator 

 than it is from the water in the inner tube to 

 the water in the generator. 



As the water level in the steam drum drops, 

 causing a corresponding drop in the water level 

 in the inner tube, more of the generator is ex- 

 posed to the steam in the inner tube. This causes 

 more water in the generator to flash into steam, 



thus increasing the pressure on the water in 

 the closed regulator system and expanding the 

 bellows, which are normally compressed by 

 spring pressure. Expansion of the bellows opens 

 the feed-regulating valve in the feed line and 

 allows more water to flow to the boiler. When 

 the water level in the steam drum rises, the 

 reverse process occurs and the feed-regulating 

 valve tends to close. 



BOILER CONTROLS 



Automatic boiler controls consisting of in- 

 dependent combustion control and feed water 

 control systems have been installed on a num- 

 ber of naval ships. All indications point to an 

 increasing use of automatic controls, particular- 

 ly as boilers are designed for higher operating 

 pressures and temperatures. Many high pres- 

 sure boilers require such rapid and sensitive 

 response to feed water, fuel, and combustion 

 air demands that the use of automatic controls 

 is almost a necessity. 



The function of an automatic combustion con- 

 trol system is to maintain the fuel input and 

 the combustion air input to the boiler in accord- 

 ance with the demand for steam and to proportion 

 the amount of air to the amount of fuel in such 

 a way as to provide maximum combustion ef- 

 ficiency. The feed water control system func- 

 tions to provide the required boiler feed and 

 maintain the steam drum water level at or near 

 normal position (middle of the water gage glass 

 at all steaming rates. 



An installed control system is quite often 

 simpler in theory than one would suppose when 

 first viewing the complex assortment of com- 

 ponents and tubing. To begin with, then, let us 

 look at the basic principles of automatic control 

 and see how they apply to a very simple control 

 system. 



Any control system, simple or complex, must 

 perform four functions. It must: 



Measure something on the output side of a 

 process; 



Compare the measured value with the desired 

 value; 



Compute the amount and direction of change 

 required to bring the measured output value 

 back to the desired output value; 



Correct something on the input side of the 

 process so that the output side of the process 

 will be brought back to the desired value. 



Measurement, comparison, computation, and 

 correction— these are the basic operations 



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