PRINCIPLES OF NAVAL ENGINEERING 



transmitter (Fl). The difference in the two 

 signals put into the differential relay (F4-1) 

 causes a decreased output pressure to be 

 transmitted from the differential relay to the 

 feed water Standatrol (F4-2). The feed water 

 Standatrol therefore sends a decreased signal 

 through the feed water selector valve (F5) to 

 the feed water flow control valve, causing 

 the valve to begin to close. 



Let us examine this point more closely. 

 The steam demand has increased but the feed 

 water flow control valve is closing . Why? Be- 

 cause it is necessary to compensate for swell— 

 the momentary increase in the volume of the 

 water that occurs when the firing rate is in- 

 creased. As swell occurs, the pneumatic signal 

 from the drum water level indicating trans- 

 mitter (F3a) increases. As a result, the out- 

 put pressure of the feed water Standatrol (F4-2) 

 begins decreasing even more rapidly, closing 

 down on the feed water flow control valve (F6a) 

 and further restricting the flow of feed water 

 to the boiler. 



As the feed water flow decreases, there is 

 a proportional drop in the pneumatic pressure 

 from the feed water flow transmitter. The ef- 

 fects of this pressure decrease are felt slowly, 

 however, because of the restricting action of the 

 bleed valve in volume chamber Fla. 



As the steam drum water level begins to 

 drop, there is a proportional decrease in the 

 pneumatic pressure from the drum water level 

 indicating transmitter (F3a). At the same time, 

 the bleed valve in volume chamber F2c is de- 

 creasing the pneumatic signal between the steam 

 flow transmitter (F2) and the transient compen- 

 sating relay (C4a3) and increasing the pressure 

 in another chamber of the compensating relay. 

 The effect of this bleed valve action is to balance 

 the inputs to the two chambers of the compensa- 

 ting relay so that the compensating relay output 

 pressure is now equal to the pressure it is re- 

 ceiving from the steam flow transmitter. In 

 other words, the reversing action of the trans- 

 ient compensating relay (C4a3) has been stopped, 

 and the compensating relay is now transmitting 

 a pneumatic signal that is exactly the same as the 

 new (and higher) steam flow signal it receives. 



The increased loading pressure from the 

 transient compensating relay (C4a3), together 

 with the decreased loading pressure from the 

 feed water flow transmitter (Fl), increases the 

 output pressure of the steam flow-water flow 

 differential relay (F4-1). The increased output 

 pressure of F4-1 reverses the action of the feed 



water Standatrol (F4-2) and causes its output 

 to increase, thus opening the feed water flow 

 control valve wider and allowing more feed 

 water to flow to the boiler. 



When the feed water flow is equal to the 

 steam flow, and when the steam drum water 

 level has returned to normal, the system 

 stabilizes and the output of the feed water 

 Standatrol (F4-2) stays at the higher value 

 which will maintain the new and higher rate 

 of feed water flow. 



A similar (but of course reversed) series 

 of events occurs when there is a decrease in 

 steam demand. The first effect of the de- 

 creased steam demand is a wider opening of 

 the feed water flow control valve to compen- 

 sate for shrink— that is, the decrease in the 

 volume of the boiler water that occurs when 

 the firing rate is reduced. The final effect is 

 a smaller opening of the feed water flow con- 

 trol valve and a reduced flow of feed water to 

 the boiler. 



Maintenance 



To ensure trouble-free operation of the 

 control system, it is important that the sys- 

 tem be properly maintained and calibrated at 

 all times. Maintenance and calibration should 

 be conducted in accordance with the Planned 

 Maintenance Subsystem of the 3-M System. 

 An example of maintenance actions and tests 

 to be conducted on an Automatic Control Sys- 

 tem are shown in figure 11-42. Particular 

 emphasis should also be placed on the use of 

 maintenance, repair, calibration procedures 

 found in the applicable manufacturer's technical 

 manual. If each component is kept in a properly 

 maintained and adjusted condition, the need for 

 a general overhaul or major recalibrationof the 

 control system will be minimized. 



NOTE: When checking the adjustments and 

 calibration of any component of the control 

 system the settings should not be changed ex- 

 cept under the supervision of "QUALIFIED" 

 maintenance personnel. It is also extremely 

 important, when making adjustments to the 

 control system, that the person doing the work 

 know the effect adjustments have on the op- 

 eration of the entire control system. In other 

 words only "QUALIFIED" personnel should be 

 allowed to perform maintenance, repair, and 

 calibration of any automatic control system 

 components. 



318 



