Chapter 11-BOILER FITTINGS AND CONTROLS 



appropriate loading pressures or control pres- 

 sures; the air pressure suppliedfor this purpose 

 is called supply pressure . 



It is not necessary to take up the operating 

 principles of the various pneumatic units, pro- 

 vided we remember their basic function: to 

 develop, transmit, and receive pneumatic signals 

 in the form of variable air pressures. We should 

 also have some idea of the specific functions 

 served by the various kinds of pneumatic units 

 listed below. 



Transmitters . In general, a transmitter may 

 be defined as an instrument that produces a pneu- 

 matic signal (in the form of variable air pres- 

 sure) proportional to one of the basic variables 

 in the controlled process. 



Relays . A relay is a pneumatic device that 

 receives one or more pneumatic signals, alters 

 or combines signals in various ways, and pro- 

 duces an output signal which goes to one or 

 more other pneumatic units. There are several 

 different kinds of relays: ratio relays, Standa- 

 trols, rate relays, selective relays, and limiting 

 relays. The specific functions of these units will 

 become apparent later, as we trace the sequence 

 of events in the boiler control system. 



Control Drives . A power unit that mechan- 

 ically positions valves or dampers in accord- 

 ance with the amount of control pressure re- 

 ceived is called a control drive. 



Control Valve . A control valve is a valve 

 used to control the flow of fluid in a line. The 

 control valve is positioned by a control drive 

 in accordance with control pressure. In other 

 words, a control valve is the final control 

 element. 



Selector Valves . A selector valve is a pneu- 

 matic instrument that provides selection of 

 manual or automatic control of the system com- 

 ponents that follow it. A selector valve also 

 provides a means for manual control of the sys- 

 tem. 



Figure 11-40 shows the control relationships 

 in the combustion control system and the feed 

 water control system. The relationship of the 

 major components is illustrated schematically 

 in figure 11-41. Using this schematic diagram 

 as a guide, we will trace the sequence of events 

 in order to arrive at an understanding of the 

 basic control relationships. Notice that each unit 

 in figure 11-41 is identified by a Bailey number 

 (and in some cases by a name). The numbers 

 and names are given in the legend for figure 

 11-41 and are used in the following discussion. 



It is important to remember that a pneu- 

 matic unit may have more than one pneumatic 

 signal coming into it and that it may transmit 

 a penumatic signal to more than one unit. In 

 describing the sequence of events, it is some- 

 times necessary to ignore some signals while 

 following others through to their final conclusion. 

 But the system functions as a whole, not as a 

 series of isolated or separate events. This 

 means that a great many signals are being 

 transmitted and received at any given time 

 and that a number of actions are taking place 

 simultaneously. 



Combusion Control System.— The combustion 

 control system maintains the energy input to 

 the boiler equal to the energy output by regu- 

 lating combustion air flow and fuel flow so 

 that the main steam line pressure is main- 

 tained at 1200 psig. In other words, the con- 

 trolled variable is steam pressure, the desired 

 value is 1200 psig, and the manipulated vari- 

 ables are fuel flow and combustion air flow. 

 Combustion air flow and fuel flow are read- 

 justed in accordance with steam demand, as 

 indicated by the measurement of steam flow. 

 The actual measured steam flow thus provides 

 the system with an additional feedback signal. 



There are five initial signals in the com- 

 bustion control system: steam pressure, fuel 

 supply flow, fuel return flow, combustion air 

 flow, and steam flow. Each of these variables 

 is measured, and pneumatic transmitters de- 

 velop loading pressures that correspond to the 

 measured values of the variables. The two fuel 

 flow signals are combined in a fuel flow dif- 

 ferential relay, as described later; in one 

 sense, therefore, it is possible to say that this 

 system has four basic signals instead of five. 



The combustion control system is set to 

 maintain the superheater outlet steam pressure 

 at 1200 psig, with variations not exceeding 

 ±0.25 percent of the set pressure at all steam- 

 ing rates. Steam pressure transmitters (Cla) 

 measure steam pressure from the superheater 

 outlet of each of the two boilers and establish 

 output loading pressure signals that are directly 

 proportional to the measured steam pressure. 

 For the range of steam pressures being meas- 

 ured (900 to 1500 psig), the output loadingpres- 

 sure of the steam pressure transmitter is 3 to 

 27 psig; for the set steam pressure of 1200 psig, 

 the steam pressure transmitter develops and 

 transmits a pneumatic loading pressure of 15 

 psig. In other words, the loadingpressure varies 

 directly with the applied steam pressure between 



313 



