Supply 420 Ma Signal

120VAC—60Hz

FIGURE 28 Boiler feed pump shaft seal injection/leakage control system; electro-pneumatic control of constant drain temperature suctions toward the center, both boxes are kept under positive pressure to prevent leakage of air into the pump. For higher heads and larger capacities, a three-stage pump, as in Figure 34, may be used. The first-stage impeller is of the double-suction type and is located centrally in the pump. The remaining impellers are of the single-suction type and are also arranged so both stuffing boxes are under pressure. Two liberal vents connecting with the suction volute on each side of the first-stage double-suction impeller permit the escape of vapor.

Current plant construction utilizes vertical can-type condensate pumps (Figures 30 and 31). The chief advantage of these pumps is that ample submergence can be provided without the necessity of building a dry pit. The first stage of this pump is located at the bottom of the pumping element, and the available NPSH is the distance between the water level in the hot well and the centerline of the first-stage impeller.

Condensate pumps are located very close to the condenser hot well, and the suction piping is generally so short that the frictional losses in this piping are not significant. However, strainers are occasionally installed in this piping, and a great deal of attention must be paid to the frictional losses through them and to their frequent cleaning. Cases have been reported on occasion where the pressure drop across these strainers was sufficient to cause flashing at the suction nozzle of the condensate pumps.

FIGURE 29 Feed pump casing and discharge head thermocouple installation, to monitor and control uniform temperature distribution (Flowserve Corporation)
FIGURE 30 Vertical condensate or heater drain pump (Flowserve Corporation)

FIGURE 31 Vertical turbine condensate pump (Flowserve Corporation)

FIGURE 32 Single-stage horizontal condensate pump with axially split casing (Flowserve Corporation)
FIGURE 33 Two-stage horizontal condensate pump with axially split casing (Flowserve Corporation)

The increasing use of full-flow demineralizers in condensate systems and, in the 1960s, the increasing discharge pressures required from the condensate pumps resulted in the need to split condensate pumping into two parts. The condensate pumps proper thus develop only a small portion of the total head required. The balance of the required head was provided by separate condensate booster pumps, which have generally been of the conventional horizontal, axially split casing type. As larger plants were constructed, the vertical can-type multistage condensate pump became the standard.

To prevent air leakage at the stuffing boxes, condensate pumps equipped with packing are always provided with seal cages. The water used for gland sealing must be taken from

the condensate pump discharge manifold beyond all the check valves. Pumps fitted with mechanical seals also use injection of condensate at a pressure higher than atmospheric pressure to prevent air from being drawn into the pump and feedwater system across the seal faces.

Condensate pumps were supplied with cast iron casings and bronze internal parts, but the emergence of once-through boilers has created the need to eliminate all copper alloys in the condensate system to avoid deposition of copper on the boiler tubes. Stainless steel-fitted pumps have become the standard selection for this service.

Condensate Pump Regulation When a condensate pump operates in a closed cycle ahead of the boiler-feed pump, the two pumps can be considered as a combined unit insofar as their head-capacity curve is concerned. Variation in flow is accomplished either by throttling the boiler-feed pump discharge or by varying the speed of the boiler-feed pump.

In an open feedwater system, several means can be used to vary the condensate pump capacity with the load:

1. The condensate pump head-capacity curve can be changed by varying the pump speed.

2. Older plant condensate pump head-capacity curves are altered by allowing the pump to operate in the "break" (Figures 35 and 36).

3. The system-head curve can be artificially changed by throttling the pump discharge by means of a float control.

4. The pump can operate at the intersection of its head-capacity curve and the normal system-head curve. The net discharge is controlled by bypassing all excess condensate back to the condenser hot well.

5. Methods 3 and 4 can be combined so the discharge is throttled back to a predetermined minimum, but if the load, and consequently the flow of condensate to the hot well, are reduced below this minimum, the excess condensate handled by the pump is bypassed back to the hot well.

The impulse for the controls used in methods 1, 3, and 4 is taken from the deaerator level.

Operating in the break, or "submergence control" as it has often been called, was applied successfully in many installations before the 1960s. Condensate pumps designed for submergence control require specialized hydraulic design, correct selection of operating speeds, and limitation of stage pressures. The pump is operating in the break (that is, cavitates) at all capacities. However, this cavitation is not severely destructive because the energy level of the fluid at the point where the vapor bubbles collapse is insufficient to cre-

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Renewable Energy Eco Friendly

Renewable energy is energy that is generated from sunlight, rain, tides, geothermal heat and wind. These sources are naturally and constantly replenished, which is why they are deemed as renewable.

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