problems. The two most common casualties that youcan identify just by noting variations in the systempressure are loss of pitch control and loss of hydraulicoil pressure.Loss of Pitch ControlA loss of pitch control can be caused by either amechanical or an electrical failure. Mechanical failurestend to occur more frequently. You should be aware,however, that electrical problems can occur that willoccasionally produce a loss of control. Normally, anelectrical problem, such as a broken or loose cannonplug or loss of feedback position, will require the systemto be shifted to manual control. In DDG-51 class ships,however, the CPP system has both a normal and analternate system by which the pitch position can bemonitored. If the normal system fails to providecommand or feedback data, the CPP system can beshifted to the alternate system. When investigating aloss of pitch control on any gas turbine-powered ship,you must be aware of the components in the CRP/CPPsystems that are most likely to fail. The following aresome of the components you will have to monitor mostfrequently.ELECTROHYDRAULIC SERVO VALVE.—The most common component failure is theelectrohydraulic servo valve. This valve is the primarycomponent for remote operation and control. If thisvalve were not installed, all operations would requirepersonnel to be stationed at the OD box at all times.You can easily identify a faulty electrohydraulicservo valve. Any of the following symptoms shouldalert you as to the possible failure of this valve:Pitch fails to respond to a desired integratedthrottle control (ITC) change.Pitch changes (fluctuations) occur without apitch change command.Hub servo pressure increases steadily without achange in system demands.AUXILIARY RELIEF VALVE.— A faultyauxiliary relief valve also will cause a loss of pitchcontrol. If the valve fails in the open position, all of thecontrol oil will be ported back to the sump. Pitch cannotbe changed without control oil to position the auxiliaryservo piston in the OD box. In addition to the loss ofpitch control, you should investigate any loss ofpressure. You should be able to spot a pressure loss bychecking the HOPM pressure gauges. This should beone of your first steps in checking the system.REDUCING VALVE.— A faulty reducing valve isanother cause for a loss of pitch control. If the reducingvalve fails in the closed position, the flow of control oilwill be cut off to the auxiliary servo piston, and pitchwill fail to respond. Like the auxiliary relief valve, thisloss of pressure will have to be viewed at the HOPMduring the initial system investigation.Loss of Hydraulic Oil PressureUsually, a loss of hydraulic oil pressure will causean alarm to be generated at the PACC/PCC. Thegeneration of this alarm, of course, will immediatelyalert the operator to a problem. The alarm will soundwhen casualties occur either to the main relief valve orto a sequencing valve. The alarm may or may not sound,however, when a major leak occurs. Look at the threemain causes of hydraulic oil pressure loss and theresulting alarms.MAIN RELIEF VALVE.— A faulty main reliefvalve can be identified by a low-pressure alarm at theconsole, but the actual answers can be found at theHOPM. This component failure can be easily identifiedby the operator. The operator simply looks at the HOPMpressure gauges and notes that all pressures areextremely low or nonexistent.SEQUENCING VALVE.— You may suspect that asequencing valve is faulty after a loss of pitch control aswell as a loss of hydraulic oil pressure, depending onhow the valve fails. For instance, if the valve fails in theopen position, then all the oil would becomehigh-pressure oil and the low-pressure alarm would notsound. In this case, the auxiliary servo supply (controloil) pressure would be drastically low and the systemcontrol would fade. On the other hand, if the sequencingvalve fails in the closed position, a low-pressure alarmwould sound and alert the operator. In this instance, theoperator would also be able to see an extremely sluggishpitch response time.MAJOR LEAK.— A major leak can provide thesame symptoms as a loss of hydraulic oil pressure,depending on the location of the leak.We have just told you about some of the componentsyou should check when you detect a loss of pitch controlor a loss of hydraulic oil pressure. Now, we will discusssome of the most common maintenance procedures youas a supervisor will expect your personnel to perform.3-17
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