Principles of naval engineering - Historic Naval Ships Association

Chapter 11-BOILER FITTINGS AND CONTROLSappropriate loading pressures or control pressures;the air pressure suppliedfor this purposeis called supply pressure .It is not necessary to take up the operatingprinciples of the various pneumatic units, providedwe remember their basic function: todevelop, transmit, and receive pneumatic signalsin the form of variable air pressures. We shouldalso have some idea of the specific functionsserved by the various kinds of pneumatic unitslisted below.Transmitters . In general, a transmitter maybe defined as an instrument that produces a pneumaticsignal (in the form of variable air pressure)proportional to one of the basic variablesin the controlled process.Relays . A relay is a pneumatic device thatreceives one or more pneumatic signals, altersor combines signals in various ways, and producesan output signal which goes to one ormore other pneumatic units. There are severaldifferent kinds of relays: ratio relays, Standatrols,rate relays, selective relays, and limitingrelays. The specific functions of these units willbecome apparent later, as we trace the sequenceof events in the boiler control system.Control Drives . A power unit that mechanicallypositions valves or dampers in accordancewith the amount of control pressure receivedis called a control drive.Control Valve . A control valve is a valveused to control the flow of fluid in a line. Thecontrol valve is positioned by a control drivein accordance with control pressure. In otherwords, a control valve is the final controlelement.Selector Valves . A selector valve is a pneumaticinstrument that provides selection ofmanual or automatic control of the system componentsthat follow it. A selector valve alsoprovides a means for manual control of the system.Figure 11-40 shows the control relationshipsin the combustion control system and the feedwater control system. The relationship of themajor components is illustrated schematicallyin figure 11-41. Using this schematic diagramas a guide, we will trace the sequence of eventsin order to arrive at an understanding of thebasic control relationships. Notice that each unitin figure 11-41 is identified by a Bailey number(and in some cases by a name). The numbersand names are given in the legend for figure11-41 and are used in the following discussion.It is important to remember that a pneumaticunit may have more than one pneumaticsignal coming into it and that it may transmita penumatic signal to more than one unit. Indescribing the sequence of events, it is sometimesnecessary to ignore some signals whilefollowing others through to their final conclusion.But the system functions as a whole, not as aseries of isolated or separate events. Thismeans that a great many signals are beingtransmitted and received at any given timeand that a number of actions are taking placesimultaneously.Combusion Control System.— The combustioncontrol system maintains the energy input tothe boiler equal to the energy output by regulatingcombustion air flow and fuel flow sothat the main steam line pressure is maintainedat 1200 psig. In other words, the controlledvariable is steam pressure, the desiredvalue is 1200 psig, and the manipulated variablesare fuel flow and combustion air flow.Combustion air flow and fuel flow are readjustedin accordance with steam demand, asindicated by the measurement of steam flow.The actual measured steam flow thus providesthe system with an additional feedback signal.There are five initial signals in the combustioncontrol system: steam pressure, fuelsupply flow, fuel return flow, combustion airflow, and steam flow. Each of these variablesis measured, and pneumatic transmitters developloading pressures that correspond to themeasured values of the variables. The two fuelflow signals are combined in a fuel flow differentialrelay, as described later; in onesense, therefore, it is possible to say that thissystem has four basic signals instead of five.The combustion control system is set tomaintain the superheater outlet steam pressureat 1200 psig, with variations not exceeding±0.25 percent of the set pressure at all steamingrates. Steam pressure transmitters (Cla)measure steam pressure from the superheateroutlet of each of the two boilers and establishoutput loading pressure signals that are directlyproportional to the measured steam pressure.For the range of steam pressures being measured(900 to 1500 psig), the output loadingpressureof the steam pressure transmitter is 3 to27 psig; for the set steam pressure of 1200 psig,the steam pressure transmitter develops andtransmits a pneumatic loading pressure of 15psig. In other words, the loadingpressure variesdirectly with the applied steam pressure between313