Booster Pumps

The increasing sizes of modern boiler-feed pumps coupled with the practice of operating these pumps at speeds considerably higher than 3600 rpm have led to NPSH require ments as high as 150 to 250 ft (46 to 76 m). In most cases, it is not practical to install the direct-contact heaters from which the feed pumps take their suction high enough to meet such requirements. In such cases, it has become the practice to use boiler-feed booster pumps operating at lower speeds, such as 1750 rpm, to provide a greater available NPSH to the boiler-feed pumps than can be made available from strictly static elevation differences. Such booster pumps are generally of the single-stage, double-suction design.

Discharge Pressure and Total Head The discharge pressure is the sum of the maximum boiler drum pressure and the frictional and control losses between the boiler-feed pump and the boiler drum inlet. The required discharge pressure will generally vary from 115 to 125% of the boiler drum pressure. The net pressure to be generated by the boiler-feed pump is the difference between the required discharge pressure and the available suction pressure. This must be converted to a total head, using the formula in USCS units Total head, ft in SI units Total head, m net pressure, lb/in2 X 2.31 sp. gr.

Slope of the Head-Capacity Curve In the range of specific speeds normally encountered in multistage centrifugal boiler-feed pumps, the rise of head from the point of best efficiency to shutoff will vary from 10 to 25%. Furthermore, the shape of the head-capacity curve for these pumps is such that the drop in head is very slow at low capacities and accelerates as the capacity is increased.

If the pump is operated at constant speed, the difference in pressure between the pump head-capacity curve and the system-head curve must be throttled by the feedwater regulator. Thus the higher the rise of head toward shutoff, the more pressure must be throttled off and, theoretically, wasted. Also, the higher the rise, the greater the pressure to which the discharge piping and the closed heaters will be subjected. However, it is not advisable to select too low a rise to shutoff because too flat a curve is not conducive to stable control; a small change in pressure corresponds to a relatively great change in capacity, and a design that gives a very low rise to shutoff may result in an unstable head-capacity curve, difficult to use for parallel operation. When several boiler-feed pumps are to be operated in parallel, they must have stable curves and equal shutoff heads. Otherwise, the total flow will be divided unevenly and one of the pumps may actually be backed off the line after a change in required capacity occurs at light flows.

As feedwater flows increased in the 650 to 1300 MW fossil central stations and new construction of nuclear power plants occurred, the pump specific speed (NS) increased. [Refer to Section 2.1.] Specific speeds of 1200 to 1500 for typical feedwater pumps increased to 1600 to 2100. The performance curve characteristic for 1200 to 1500 NS pumps typically has a constantly rising curve slope. The performance characteristic for a pump with a NS of 1600 to 2100 often will exhibit a depression (change to a very low, or negative slope) at reduced flow rates (Figure 13).

NOTE: If a high specific-speed pump is operated at low loads and reduced flow rates, there is risk of entering a performance curve region that will result in flow instability and surge. High subsynchronous vibrations and possible vane pass energy is excited (Figure 14, NS = 1700). This operating condition is potentially damaging to the boiler feed pump. Attention to impeller and diffuser areas is critical to prevent this condition from occurring. Underfiled impellers (see Subsection 2.3.1) and high area ratios between the impeller and the diffuser or volute will tend to flatten the performance curve and can result in a depressing effect on the slope of the performance curve.

Driver Power A boiler-feed pump will generally not operate at any capacity beyond the design condition. In other words, a boiler-feed pump has a very definite maximum capacity because it operates on a system-head curve made up of the boiler drum pressure plus

Performance Curve Characteristic

Total Developed

Total Developed Head slope change

Ns 1600 -1700

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