Selecting Optimum Turbine Speed

An ordinary American-type turbine is designed to run at 3600 rpm. Its overspeed trip will cut off the motive steam flow at about 3750 rpm. But the turbine can be run at any lower speed. There is usually a small knob on the left side of the governor-valve assembly, that is used to 

This means that if we drop the speed of the turbine by about 3 percent, motive-steam flow will decline by about 9 percent. However, there is an additional benefit. 

Slowing a turbine closes the governor valve. This may now permit us to close an additional horsepower valve, without losing our flexibility to control the speed of the turbine. Closing that final horsepower valve will save us another 10 percent of steam. 

Our overall objective is to wind up with both the process-control valve downstream of the pump and the governor speed-control valve in a mostly wide-open, but still controllable, position. To achieve this dual objective, we have to simultaneously  

do we need process-control valves, on the discharge of variable-speed-driven pumps Why, indeed  

It is rather like solving two equations with two unknowns. 

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For the preliminary estimate of the expected efficiency of expansion turbines, in most cases it is sufficient to neglect Reynolds number effects (Rg > 10 ) and use the efficiency and specific speed correlations shown in Figure 2-12 for partial admission axial impulse, reaction radial inflow and full admission impulse and reaction axial turbines. Due to the economic advantage of the radial turbine, die radial inflow turbine is die best selection when operating in die specific speed range 20 < Nj < 140, whereby die optimum efficiency will be achieved at N, = 80. 

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