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Pumps & Systems, March 2008

Water, unimpeded, flows to the lowest possible point. (No rocket science here.) However, in many situations, this is not only a point where the water is not desirable, but it is also a point where the presence of notable volumes of water markedly interfere with human activity and even create specific hazards (as in excavations at construction sites, mining operations and numerous other activities in low-lying areas.). For example, the movement and accumulation of water to an excavation can create unstable conditions in the surrounding soils and-at the very least-impede necessary activities. In practice, the removal of unwanted water, in most situations, is complicated by the accumulation of suspended solids like mud and sand, as well as a wide variety of other particulate matter collected by moving water. The resulting slurry creates a tough, abrasive and sometimes corrosive environment for any type of pumping equipment.

[For simplicity, this article will use the term dewatering to describe removing or moving water from its natural gravitational location to an alternate location-although in practice, it has many other meanings (see sidebar). Also, this discussion will be limited to dewatering applications that involve slurries (mixtures of solids and water) of 15 percent, or less. In addition, this article does not address the numerous and highly engineered permanent mine and other dewatering installations where permanent, dry pit, high-pressure slurry pumps continuously remove waters and solids. Further, self-priming, skid-mounted, diesel-driven units are not included.]

Pumps used for dewatering typically utilize integral submersible motors and are built tough enough to endure rugged conditions. The pump motors-because they are hermetically sealed to protect the motor windings from water intrusion-must dissipate heat and have watertight cable entry. The shaft seals of dewatering pumps must be designed, manufactured and assembled to exclude solids, and the materials of construction have to be suitable for the corrosive and/or abrasive nature of the pumped fluid.

In all pumping situations, optimizing and matching flow to the system head is key. However, in some dewatering applications, precise flow may not matter as much as achieving the actual TDH. In others, pumped flow has to meet or exceed seepage rates. Pumps with steep performance curves are better suited for the former; flatter curves for the latter.

Three of the most common dewatering applications are pit dewatering, ground water removal and water management.

Pit Dewatering Applications

In short-term projects like construction and similar longer-term activities like mining, excavation must occur to remove unwanted material and get to the ore bearing layers or to the layer suitable for the construction intended. Surface water and groundwater-seeking the lowest possible point-seeps or rushes into the hole and needs to be removed. The pumps used for dewatering in these applications are generally portable and capable of being transported and placed in different locations. Dewatering pumps are designed to be light in weight, but robust enough to handle light slurries. They must, of course, withstand abrasion while operating at relatively high speeds to achieve high head.

Depending on the soil and the digging process employed solids size and content are prime variables to address during pump selection. For example, high efficiencies are not achievable as compared to pumps in clean water service because more open clearances are required to handle the solids effectively.

Where possible, sumps are recommended in dewatering applications to optimize suction conditions for the pumps. The sumps can be designed to collect drainage and to facilitate further water/solids removal with heavy duty "mops." The sumps can utilize primitive screens to protect the pumps from construction blasting and tramp material. Usually, at the early stages of excavation, the pumps are either suspended into the slurry being removed or simply placed on the freshly dug, often uneven bottom. Pumps in this service need a suitable pedestal and require strainers to restrict entrance of solids too large for the pump to pass hydraulically. Special designs employ agitators that connect to the pump shaft and serve to put settled solids in suspension so that they can be more readily pumped.