Turbine speeds are limited by the centrifugal stress that can be applied to the blades and blade roots. For a 3,000-hp turbine, a speed of about 14,000 rpm can be expected, but a speed of only 8,000 rpm can be expected from a 10,000-hp turbine. Higher horsepowers and speeds can be obtained in special turbines, and are frequently specified for large centrifugal compressors. These turbines need very careful checking for lateral critical speeds, torsional critical speeds, radial bearing stabilities, thrust bearings, balance, and permissible flange loadings.

A Campbell diagram is frequently used to determine the effect of multiple excitation frequencies in high-speed steam turbines. Figure 7-11 shows a Campbell diagram for a condensing steam turbine. If this particular- turbine operates at a speed of 8,750 rpm, the turbine blades would not be excited. But, if the turbine speed is reduced to 7,500 rpm, the turbine blades would be excited at four times running speed. If the turbine were operated at 10,000 rpm, a three-times running speed excitation would be encountered. What this means is that any vibration in the

Figure 7-11. Typical Campbell diagram. {Courtesy of Elliott Company)

Figure 7-11. Typical Campbell diagram. {Courtesy of Elliott Company)

twelfth-stage turbine blades would be reinforced every third or fourth vibration of the blade. In other words, severe blade vibration could be expected at 7,500 rpm and 10,000 rpm, in this particular turbine.

Sometimes, because of process requirements, it is impossible to avoid some excitation frequencies. If the Campbell diagram shows this will occur, then the blade in question must be carefully designed to keep stresses low. When properly addressed in design, operation can take place in an area of excitation. The major variables affecting turbine selection may be listed as follows:

1. Horsepower and speed of the driven machine.

2. Steam pressure and temperature available or to be decided.

3. Steam needed for process, so that a back-pressure turbine can be considered.

4. Steam cost, and the value of turbine efficiency. Should it be singlestage or multistage? Should it be single-valve or multivalve? Is the steam an inexpensive process by-product, or is the entire cost of generating the steam chargeable to the driver?

5. Should extraction for feedwater heating be considered?

6. Should a condensing turbine with extraction for process be considered?

7. Control system, speed control, pressure control, process control. What speed or pressure variation can be tolerated and how fast must the system respond?

8. Safety features such as overspeed trip, low-oil trip, remote-solenoid tnp, vibration monitor, or other special monitoring of temperature, temperature changes, and casing and rotor expansion.

The price range from the minimum single-stage turbine to the most efficient multistage is quite wide.

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