Principles of naval engineering - Historic Naval Ships Association

PRINCIPLES OF NAVAL ENGINEERINGpump discharge pressure. When the boosterpump discharge pressure is inadequate, the auxiliarypilot valve will not deliver steam to thecontrolling valve of the governor. Thus inadequatefeed booster pump pressure allows the mainvalve in the governor to close, shutting off theflow of steam to the main feed pump turbine.SAFETY DEVICESSafety devices used on auxiliary turbines includespeed-limiting governors and severalkinds of trips. Safety devices differ from speedcontrol devices in that they have no control overthe turbine under normal operating conditions. Itis only when some abnormal condition occurs thatthe safety devices come into use to stop the unitor to control its speed.Speed- Limiting GovernorsThe speed-limiting governor is essentiallya safety device for variable speed units. Itallows the turbine to operate under all conditionsfrom no-load to overload, up to the speed forwhich the governor is set, but it does not allowoperation in excessof 107 percent of rated speed.It is important to note that this type of governoris adjusted to the maximum operating speed ofthe turbine and therefore has no control over theadmission of steam until the upper limit of safeoperating speed is reached.One common type of speed- limiting governoris shown in figure 16-12. This governor is usedon main condensate pumps, feed booster pumps,lube oil service pumps, and other auxiliaries inthe engineering plant. The particular speed-limitinggovernor shown here is designed for use ona main condensate pump with a vertical shaft;speed-limiting governors that operate on verymuch the same principle are used on main feedpumps and other auxiliaries that have horizontalshafts.The governor shaft is driven directly by anauxiliary shaft in the reduction gear, and rotatesat the same speed as the pump shaft. This speedis proportional to— although lower than— thespeed of the driving turbine. Two flyweights arepivoted to a yoke on the governor shaft and carryWEIGHTSFigure16-12. — Speed-limiting governor.43447.54