Governors

Turbine shaft speed depends on the driven load and the amount of steam flowing into the steam chest. If the load on the shaft increases, at constant steam flow, the turbine speed decreases. Alternatively, if a constant load is maintained as steam flow increases, shaft speed will also increase. Mechanical energy output from a steam turbine is controlled by a governor, which operates a control valve to admit the correct amount of steam.

The mass flow rate of steam through a turbine is determined by the product of the absolute pressure of the steam entering the first stage nozzles and the throat area of the first stage nozzles. Figure 11-17 shows the effect of throttling on steam turbine operation in enthalpy vs. entropy (h-s). As throttling is increased, the steam energy available to do work in the turbine is reduced.

Steam turbines can operate at speeds ranging from 1,800 rpm to more than 14,000 rpm. Optimal speed for most industrial turbines is commonly in the range of 3,600 to 5,600 rpm. Typically, single-stage turbines will operate down to 50% of rated speed. Multi-staged turbines are often limited to 70% or above due to the length and weight of the shaft.

1-2 Minor throttling

2-3 Energy available to do work in turbine 1-4 Major throttling

4-5 Energy available to do work in turbine

1-2 Minor throttling

2-3 Energy available to do work in turbine 1-4 Major throttling

4-5 Energy available to do work in turbine

Fig. 11-17 Effect of Throttling on Steam Turbine Operation. Source: Tuthill Corp., Murray Turbomachinery Div.

Entropy

Fig. 11-17 Effect of Throttling on Steam Turbine Operation. Source: Tuthill Corp., Murray Turbomachinery Div.

As turbines move away from design speed, they lose efficiency. Figure 11-18 shows steam flow versus hp for the same representative steam turbine at various turbine speeds. Indications are provided as to when hand valves should be opened and closed to provide maximum efficiency.

Mechanical governors are used in the most basic control systems. In mechanical governors, shaft speed is sensed by a fly-ball mechanism and hydraulic relays provide input to the control valves. The speeder spring exerts a force on the thrust bearing, while the flyweights exert an opposing force proportional to the rotational speed of the turbine. Depending on the speeder spring tension and the speed of the turbine, an equilibrium point is established. Figure 11-19 is a cutaway illustration detailing a steam turbine governor valve.

Electronic governor systems are somewhat more sophisticated than mechanical governors and use speed pick-ups and electronic circuit boards instead of fly-balls

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