Turbines helper

In contrast to steam turbines, in which runaway overspeediug is always a problem, pump-turbines operating at design head go to zero torque at about 130 to 140 percent of design speed. Thus, overspeed protection may not be necessary if the pump-turbine can withstand 140 to 150 percent of design speed and it is the sole driver. When a steam-turbine helper is used, it should be provided with the usual overspeed trip-out mechanism. 

Figure 24.1 is a sketch of a centrifugal pump, driven by a three-phase motor, with a turbine helper. This particular pump was charging a light gas oil stream to a high-pressure hydrocracker. The pump was operating quite close to its design conditions of... 

Figure 24.1 A pump driven by a motor with a turbine helper.

What about integrating these two turbines The idea is to make better use of the steam turbine helper for start up in the PRT live-box train design in Figure 11.20. In other words, the steam turbine helper becomes a letdown turbine and the normal FCC process steam can be routed through this letdown turbine. Thus, the letdown turbine is integrated with the PRT as shown in Figure 11.22. The integrated power recovery system can produce electricity while the main air blower is run by motor. 

Gas turbine frame sizes are limited, and custom sizing is not done as it is with steam turbines therefore, the increment matching can only be done with a helper turbine. There is some benefit besides the power matching when used with a single-shaft turbine. The steam turbine can be used as the starter. For two-shaft turbines, a dedicated starter is needed on the gasifier turbine compressor package. 

The expander is somewhat difficult to govern, and is someluiics used with a helper steam turbine for use in starting and speed control. 

With high fuel costs, the search is on for processes with higher thermal efficiency and ways to improve efficiencies of existing processes. One process being discussed lately for its high efficiency is the gas turbine combined cycle. The gas turbine exhaust heat makes steam in a waste heater boiler. The steam drives turbines, perhaps even a gas turbine steam helper. References 10, 11, and 12 treat this subject and mention alternate equipment hookups, some in conjunction with coal gasification plants. 

Synchro-Self-Shifting ( SSS ) Clutches The automatic freewheel action of an SSS clutch simplifies plant startup and shutdown sequences. It is, of course, receiving its power input from a gas turbine or similar driver, while its output is connected to a driven machine. The drive disengages automatically when the driven machine speed exceeds that of the driver. Since large gas turbines are usually brought up to speed with either a startup gear motor or a helper turbine, thousands of SSS clutches (Fig. 29-77) are finding application here. 

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