Condenser Circulating Pumps

Types of Pumps Condenser circulating pumps may be of either horizontal or vertical construction. For many years, the low-speed, horizontal, double-suction volute centifugal pump (Figure 39) was the preferred type. This pump has a simple but rugged design that allows ready access to the interior for examination and rapid dismantling if repairs are required.

The larger central station and combined cycle power plants have switched to wet-pit vertical pumps that are either fully or partially submerged in the water pumped. Central stations also installed vertical dry-pit pumps in the 1950s and 1960s. These dry-pit designs are large vertical volute casing pumps surrounded by air.

Mechanical Considerations The dry-pit installation was a single-suction, medium-specific-speed, mixed-flow pump (Subsection 2.2.1, Figure 109). This design combined the high efficiency and low maintenance of the horizontal double-suction radial-flow centrifugal pump with lower cost and slightly higher rotative speeds.

Because of their suction and discharge nozzle arrangements, these pumps are ideally suited for vertical mounting in a dry pit, preferably at the lowest water level, so they are self-priming on starting. They are directly connected to solid-shaft induction or synchronous motors, either close-coupled or with intermediate shafting between the pump and the motor, which is then mounted well above the pump pit floor.

Like the horizontal double-suction pump, the vertical dry-pit mixed-flow pump is a compact and sturdy piece of equipment. Its rotor is supported by external oil-lubricated bearings

FIGURE 39 Horizontal double suction single stage pump, IDP model LN (Flowserve Corporation)

of optimum design. This construction requires the least attention, for the oil level can be easily inspected by means of an oil sight glass mounted at the side of the bearing or oil reservoir.

Because the rotor is readily removed through the top of the casing, facilitating maintenance and replacement, the pump does not have to be removed from its mounting and the suction and discharge connections do not have to be broken to make periodic inspections or repairs.

In recent years, power plant designers have shown a preference for the wet-pit column-type condensate circulating pump. The term wet-pit normally implies a casing diffusertype pump, employing a single open vane impeller. The wet-pit pump (Figure 40) employs a long column pipe that supports the submerged pumping element. It is available with open main shaft bearings lubricated by the water handled, or with enclosed shafting and bearings, lubricated by clean, fresh, filtered water from an external source. There is some danger of contamination of the lubricating water from seepage into the shaft enclosure tube during shut-downs.

Pulling up the column in a long pump requires special facilities and, in addition, the discharge flange must be disconnected when withdrawing the pump and column from the pit. This design has been designated a "non-pull-out" design (Figure 40). To avoid the necessity of lifting the entire pump when the internal parts require maintenance, some units are built so the impeller, impeller shroud, casing, and shaft assembly can be removed from the top without disturbing the column pipe assembly. (The driving motor must be removed.) These designs are commonly designated "pull-out" designs (Figure 41).

Condenser cooling water is often corrosive. Power plants are often located near salt or brackish bodies of water. Plants near rivers often encounter water contaminated with high silt levels. With such waters, selection of materials can be critical to long service life. Material selection for sea water applications must also consider the potential for electrolytic (galvanic) corrosion.

Performance Characteristics Condenser circulating pumps are normally required to work against low or moderate heads. Extreme care should be exercised in calculating the system frictional losses, which include losses from friction in the condenser. If more total

head is specified than is required, the resulting driver size may be unnecessarily increased. For instance, an excess of 1 or 2 ft (0.3 or 0.6 m) in an installation requiring only 20 ft (6 m) of head represents an increase of 5 to 10% in excess power costs.

The range of suction lift for dry-pit pumps must be determined very accurately and checked with the manufacturer to ensure that cavitation will be avoided in the installation. Priming facilities must be provided, or the pump must be installed in a dry pit at such an elevation that the water in the suction channel leading to the pump will be maintained at the level recommended by the manufacturer. This presents no problem in a wet-pit installation because the pump column can be made long enough to provide adequate submergence, even with minimum water levels in the suction well or pit. The dry-pit pump will generally have 3 to 4% higher efficiency than the wet-pit type and therefore 3 to 4% lower power consumption. The two types are available for the same specific speed range. When pumping total head is 25 ft (7.6 m) or less, an axial-flow propeller (approximately 10,000 specific speed in USCS units) can be used in either type of pump.

The low-specific-speed, double-suction pump has a very moderate rise in head with reducing capacities and a nonoverloading power curve with a reduction in head. The mixed-flow impeller with a higher specific speed has a steeper head-capacity curve and a reasonably flat power curve that is also nonoverloading. As the specific speed increases, the steepness of the head-capacity curve increases and the curvature of the power curve reverses itself, hitting a maximum at the lowest flow. Finally, the curve of a high-specific-speed propeller pump has the highest rise in both head-capacity and power-capacity curves toward zero flow. The head range developed by the mixed-flow pump is ideal for condenser service; this pump is usually furnished with an enclosed impeller, which produces a relatively flat head-capacity curve and a flat power characteristic.

Higher head circulating water pumps were developed in the 1970s as cooling towers were introduced to improve plant efficiency and environmental contamination. The cooling tower arrangement effectively increased the total system resistance head requirements.

System Hydraulics The dry-pit pump is not too sensitive to the suction well design because the inlet piping and the formed design of the suction passages into the pump normally ensure a uniform flow into the eye of the impeller. On the other hand, the higherspeed wet-pit pumps are more sensitive to departures from ideal inlet conditions than the low-speed centrifugal volute pump or the medium-speed mixed-flow pump. A discussion of the arrangements recommended for wet- and dry-pit pump installations is presented in Section 10.1.

Drivers Whether a dry-pit or a wet-pit pump is used, the axial thrust and weight of the pump rotor are normally carried by a thrust bearing in the motor, and the driver and driven shafts are connected through a rigid coupling. The higher rotative speeds of the wet-pit pumps reduce the cost of the electric motors somewhat. This difference may be offset, however, by the fact that the thrust load of the wet-pit pump is higher than that of the dry-pit pump.

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