Principles of naval engineering - Historic Naval Ships Association

Chapter 16. -AUXILIARY STEAM TURBINESarms which bear on a push rod assembly. Thepush rod assembly is held down by a strongspring.Because of centrifugal force, the position ofthe flyweights is at all times a function of turbinespeed. As the turbine speed increases, theflyweights move outward and lift the arms. Asthe speed of the turbine approaches the speedfor which the governor is set, the arms liftagainst the spring tension. If the turbine speedbegins to exceed the speed for which the governoris set, the flyweights move even farther out,thereby causing the governor valve to throttledown on the steam.When the turbine slows down, as from an increasein load, the centrifugal force on the flyweightsis diminished and the governor push rodspring acts to pull the flyweights inward. Thisaction rotates the lever about its pivot and opensthe governor valve, thus admitting more steamto the turbine. The turbine speed increases untilnormal operating speed is reached.The speed-limiting governor acts as aconstant- speed governor when the turbine is operatingat or near rated speed, although itis designed only as a safety device to preventoverspeeding. The governor has no effect on thespeed of the turbine at speeds below about 95 percentof rated speed.TripsSeveral kinds of trips are used as safety deviceson auxiliary turbines.Overspeed trips are used on turbines thathave constant- speed governors. The overspeedtrip shuts off the supply of steam to the turbineand thus stops the unit when a predeterminedspeed has been reached. Overspeed trips areusually set to trip out at about 110 percent ofnormal operating speed. In the past overspeedtrips were used primarily on constant- speed turbinesand on some commercial-type variablespeedunits. Recent specifications require overspeedtrips onallnavalauxiliary turbines of over100 horsepower. Figure 16-13 shows the constructionof an overspeed trip used on turbogenerator.Back-pressure trips are installed on turbogeneratorsto protect the turbine by closing thethrottle automatically when the back pressure(exhaust pressure) becomes too high. A backpressuretrip is shown in figure 16-14.Emergency hand trips are installed on turbogeneratorsto provide a means for closing thethrottle quickly, by hand, in case of damage toEMERGENCY GOVERNORDETAILOIL PIPEBELL CRANK HANDLETRIP RODPLUNGER38.93Figure 16-13. — Overspeed trip for turbogenerator.ADJUSTINGBUSHINGLOCKNUTFigure 16-14.— Back-pressure trip.96.25435