Part Load Performance

Steam turbines maintain relatively good performance during off-design or part-load operation. Figures 11-40 and 11-41 provide representative curves showing offdesign performance. As indicated, steam flow is roughly proportional to power output at most loads and efficiency is largely unchanged. Note that the minimum steam flow required to keep the machine operating is about 10%.

Figure 11-42 is a representative curve to approximate steam rates for turbines operating at part-load and -speed. Consider, for example, a turbine rated for full-load operation at 25,000 hp (18,640 kW) at 5,000 rpm. Off-design

Fig. 11-41 Steam Turbine Off-Design Performance, Efficiency vs. Steam Flow. Source: Cogen Designs, Inc.

operation is 20,000 hp (14,912 kW), which is at 80% of full load, and 4,500 rpm (90%). From the curve, the power correction is 1.04 and the rpm correction is 1.05. The total correction is the product of the two correction factors (1.04 x 1.05), which is 1.09. The part-load steam rate is the product of the full-load steam rate and the correction factor. If the full-load steam rate is 7.40 lbm/hp-h (4.50 kg/kWh), the part-load steam rate would be 7.40 lbm/hp-h x 1.09 = 8.06 lbm/hp-h (4.50 kg/kWh x 1.09 = 4.91 kg/kWh).

While the steam turbine itself operates slightly less efficiently at reduced speed, the driven equipment may operate at improved efficiency at reduced speed under part load. Centrifugal compressors, pumps, and fans show this characteristic, as illustrated in Figure 11-43.

Figure 11-44 shows steam flow versus load for various steam turbines. Notice that while multi-stage turbines are more efficient at full load than single-stage units, they lose

Fig. 11-42 Representative Part-Load/Speed Correction Curves. Source: Elliott Company

Fig. 11-43 Variable Speed Operation at Part-Load Conditions. Source: Tuthill Corp., Murray Turbomachinery Division turbines can be designed to improve efficiency at part load at the expense of full-load efficiency. Generally, the lowest operating costs will be achieved if the turbine is designed to optimize steam rate at the normal or most common load experienced, with allowances made for operation at other load points.

Fig. 11-44 Steam Flow vs. Load for Various Steam Turbines. Source: Tuthill Corp., Murray Turbomachinery Division

Fig. 11-43 Variable Speed Operation at Part-Load Conditions. Source: Tuthill Corp., Murray Turbomachinery Division efficiency more rapidly as load moves away from the design point. This occurs because the nozzle area of a single-stage turbine can be adjusted to change flow throughout the turbine without affecting the velocity ratio. This cannot be accomplished in a multi-stage turbine.

In multi-stage turbines, hand valves or sequentially opening valves can only adjust the nozzle area in the first turbine stage. When some first stage nozzles are closed under partial pressure, pressure in all stages except the last one goes down, causing the steam jet velocity to be higher in the first stage and lower in the last stage. This reduces first stage velocity ratio and increases last stage velocity ratio.

However, as shown in Figure 11-44, multi-stage

Fig. 11-44 Steam Flow vs. Load for Various Steam Turbines. Source: Tuthill Corp., Murray Turbomachinery Division

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