Operation Principles

The steam turbine is basically a steam expander, expanding the steam over a given pressure range along an isentropic path. Space does not permit a complete development of all the aspects of the turbine. However, a few fundamentals will be reviewed to help give an understanding of how turbines fall into different efficiency ranges.

Refer to Figure 7-12 for a diagram showing the various turbine stage losses. Efficiency, T|s, is defined as the actual enthalpy change divided by the isentropic enthalpy change or

Initial conditions

Initial conditions

Exhaust conditions

Pressure drop loss

Exhaust pressure

Windage and disk friction loss

Exhaust loss

Mechanical loss

Exhaust conditions

Pressure drop loss

Exhaust pressure

Entropy, s

Figure 7-12. Various turbine stage losses. (Courtesy of Elliott Company)

Without stretching the imagination too much, a high enthalpy drop, Ah. across a stage will raise the steam velocity, C, lowering the velocity ratio, and drop the efficiency. Of course, the wheel speed, u, could be increased but this is also limited by stress considerations. Two conclusions can be drawn from the relationships. One is that single-stage turbines tend not to be efficient unless steam conditions are quite correct, and the other is thai, if efficiency is to be maintained, a multistage is required.

A few other general comments can be made. A partial-admission iur bine is not as efficient as a full-admission turbine, as seen in Figure 7-14, In actual practice, full admission is not widely used in the small turbines. In many cases, the steam flow is small relative to the size of the turbine and only a partial arc of admission is required to get the area to support the needed steam velocity, as was just shown with the equations. Whenever there is a choice, another consideration must be weighed. The steam has to cross the horizontal joint, which may cause design and operational problems. If the casing is cored for internal passages, the cost is high compared to the alternative of taking the steam out of the casing above the joint and piping it in below the joint. This may not seem all that bad. However, due to thermal expansion problems, the crossover pipe has a tendency to crack during operation. Presumably, the larger turbines use the 360' admission more for the necessity of physical space for steam admission than for the efficiency.

For wide ranges of operation, multiple valves help keep the efficiencies high (see Figure 7-15). As power output is increased, the lower curve flow

Figure 7-14. Efficiencies of full-admission and partial-admission turbines.
Figure 7-15. In a multivalve turbine, power is increased by increasing the admission area to provide more flow. This results in a step effect as illustrated.

limit is encountered. To increase flow, more area must be provided, represented by the solid vertical line to the upper curve. More power is now available because more steam can be admitted. This stair-step approach may be done several times, until the full arc of admission (360°) is achieved. This action takes place in a multivalve turbine (see Figures 7-16 and 7-17). If a wide range of operation is anticipated, this type of turbine should be considered and its higher cost evaluated. With a multivalve turbine, when load drops, the upper curve dictates the flow until the solid vertical line, at which the lift automatically closes a valve dropping the turbine to the lower characteristic, and all is well. However, if cost was such that a single-valve turbine was chosen, for a nominal additional cost, a hand valve can be furnished, which can be manually opened and closed at the load point presented by the solid vertical line. When the operator becomes complacent and leaves the valve open, as indicated by the excess opening, the area indicated as wasted steam is in effect. Therefore, though the addition of hand valves is an economical method of widening the efficient operating range, it does present a problem.

The same curve may be used to represent a turbine specified for a greater power than needed, which would be the upper curve, when all that was needed was the lower curve. Again, the same effect, except with no recourse other than poor operation.

Figure 7-16, Muitivaive steam turbine. (Courtesy of Demag Delavat Turbomachinery

Figure 7-16, Muitivaive steam turbine. (Courtesy of Demag Delavat Turbomachinery

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