Pumping Conditions And Pump Types

Pumping conditions in mining services can be determined from a consideration of the types of mines and the material being mined. For example, the broadest category would be a division between open-pit and underground or deep mines. Open pit mines seldom exceed 600 ft (183 m) in depth so the pumping heads generally are not much greater than this unless very long discharge lines are required. For many open-pit mines, the greatest pumping load does not result from groundwater but from rainfall. Unless a certain increase in water level can be tolerated at the bottom of the pit, the maximum rainfall rate and the drainage area involved will determine the required pumping capacity.

As mining progresses, the pumps generally must move with the operator. This suggests the use of barge-mounted vertical pumps. A practical cost-effective way of achieving dewa-tering under this scenario is the use of a pond system with barge-mounted vertical can-tilevered shaft pumps. Barge mounting adds a great deal of flexibility. Simple barges, as in Figure 1, illustrate the simplicity of this mounting. Except for occasional greasing of the bearings, there is little maintenance, as these pumps do not require a stuffing box or

FIGURE 1 A simple barge (Hazleton Pumps, Inc.)

mechanical seal. Almost any size can be barge mounted. Figure 2 shows one of three 1500 HP (1119 kW) vertical cantilevered shaft pumps that are installed on one barge. Each pump is rated at 14,000 gpm (3180 m3/h).

Where there is significant rainfall, the flow rates are generally large and the combination of large capacities with moderate heads may suggest a double suction pump. The selection of a double suction pump may permit the use of a higher speed pump; however, the selection may not be based on hydraulic considerations alone. An examination of hydraulic design for a double suction pump may show it is ideal for a barge-mounted vertical pump, providing there is ample water level in the pit. On the other hand, minimum water levels may be required to facilitate mining operations. If these conditions exist, which commonly occurs, selection of a top inlet single stage pump may be better. Top inlet barge mounted vertical pumps can pump the water level down without drawing the mud from the bottom. This is the conflict, as the desirable features of the double inlet pump are lost.

Comparative features can be readily seen from an examination of the net positive suction head (NPSH) requirements. High-capacity moderate head pumps have a specific speed (NS) which may require a greater NPSH; that is, a greater submergence. This may alter the pump length and the ability to utilize a cantilevered shaft pump. Such pumps are ideal for barge mounting if the NPSH (required submergence) conditions exist.

Figures 3 and 4 are reproductions of charts published by the Hydraulic Institute. An example will highlight the available design choices. Calculations may show that some combinations of flow, head, and pump speed are not feasible. This is an important consideration before proceeding with design. Assuming we have a possible floating pump station with a pumping requirement of 7500 gpm (1700 m3/h) and a calculated discharge head of 300 ft (91m), Table 1 shows the calculation of the required absolute suction conditions. Converting from absolute pressure to submergence from the water level to the impeller centerline, under standard atmospheric conditions, the table shows how much submergence is required for a given operating speed. The table shows that a single inlet pump at 1800 rpm is not practical, and a pump designed for 1200 rpm is barely acceptable. A dou-

FIGURE 2 Vertical cantilevered shaft pumps used for barge mounting (Hazleton Pumps, Inc.)

ble suction pump at 1800 rpm is also barely acceptable if there is to be a cushion for low barometer days. A double suction pump at 1200 prm is fully acceptable. We are not locked into this selection, as a lower flow rate with more pumps may be fully acceptable at a higher speed.

The calculation for NPSH shows that the 1800 rpm double suction pump and the 1200 rpm single suction pump have about the same NPSH requirement. Although both are for the same hydraulic conditions, they are substantially different pumps. If the NPSH requirements can be met with a simple single inlet higher-speed barge mounted pump installation, a substantial reduction in cost and weight is possible.

Technically, other solutions are possible when the NPSH is marginal. If the NPSH requirement is only a few additional feet, it may be possible to lengthen the pump to increase the submerged depth. In comparing available NPSH and required NPSH, the altitude of the installation and the temperature of the liquid should be considered. Altitude correction can be simply applied as 1 ft (0.3 m) for each 1000 ft (305 m) of elevation in the calculation of NPSH available.

The development of large overhung vertical shaft pumps has been a boon to mining operations. Figure 5 shows the installation of four floats, each with a 400 HP (300 kW) top inlet cantilevered shaft pump. The type of mining operation frequently dictates the pump design and particularly the installation. Figure 6 shows float mounted pumps designed to

Capacity U.S. GPM

FIGURE 3 Recommended maximum operating speeds for single suction pumps (Hydraulic Institute ANSI/HI 2000 Edition Pump Standards, Reference 1)

Capacity U.S. GPM

FIGURE 3 Recommended maximum operating speeds for single suction pumps (Hydraulic Institute ANSI/HI 2000 Edition Pump Standards, Reference 1)

Capacity U.S. GPM (total pump capacity)

FIGURE 4 Recommended maximum operating speeds for double suction pumps (Hydraulic Institute ANSI/HI 2000 Edition Pump Standards, Reference 1)

Capacity U.S. GPM (total pump capacity)

FIGURE 4 Recommended maximum operating speeds for double suction pumps (Hydraulic Institute ANSI/HI 2000 Edition Pump Standards, Reference 1)

skim the clean liquid in a settling pond. The float construction acts as a weir so only the cleanest liquid is returned to the plant. The float mounts 250 HP (185 kW) top inlet vertical cantilevered shaft pumps. An interesting feature is the hydraulic thrust balancing by the use of opposed flexible hose discharge lines. Note that these forces also exist in steel pipeline and must be adequately restrained.

TABLE 1 Calculation of required absolute suction conditions

Pump Type

Operating Speedā€”rpm

Specific Speed

NPSH Required Ft (m)

Submergence Ft (m)

Single Suction*

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