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.]
Dewatering Definitions
Dewatering: The separation of liquids from solids or the removal of liquid (typically water) from one location and the transfer of the liquid to another location.
Unwatering: The transfer of surface water, such as stormwater runoff, from one location to another.
Slurry: A liquid/solids mixture of varying concentrations. Light to medium slurries are in the range of 5 percent to around 15 percent suspended solids concentration by weight.
Pumpage: The product of dewatering.
Total Dynamic Head (TDH): The theoretical height that a specific pump can raise a specified column of liquid at optimum output.
Performance Curve: The plotted relationship between the height of a column of liquid and the relative volume of that liquid pumped.
Disposable Pumps: Pumps built with a primary focus on initial purchase price with a minimum regard to pump life.
Rebuildable Pumps: Pumps engineered and built with materials and workmanship designed to optimize life cycle and long-term value (i.e., pumps of such operational value worth rebuilding).
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.
Groundwater Removal Applications
In some cases, the groundwater is removed so that it does not seep as rapidly into a hole or other excavated area. Groundwater is removed for other reasons-such as for the sake of analysis, safety and environmental reasons and other special situations. Groundwater removal may also be a means to provide a clean water source.
Borehole pumps are often used for groundwater removal in applications that require the pump to fit within a pipe diameter and remove any water that enters that pipe.
In this type of application, the diameter of the hole is a limiting flow factor. The elements of borehole pumps can be staged to develop higher head, as required. The motors in all borehole pumps are cooled by the pumpage. The units can have bottom suction orientations or the motor can be located on the bottom. The clearances are tighter in this design, and several methods are available to limit silt intrusion. Lighter designs suitable for clean potable water applications of composite construction may not be suitable for long-term, more rigorous duties.
Water Management Applications
Ballast in offshore rigs or levels in tailings ponds must be managed to a certain level. Over time, a space fills and water must be transferred to a new location or decanted to achieve process goals. While the solids content is not high, these applications are not considered clean and often see corrosion from the chloride content of saltwater. Ballast is equalized to lend stability to the structure. In tailings ponds and other waste applications, water is removed to create capacity for additional waste.
Dewatering Pump Selection
Depending on the procurement/maintenance philosophy of the owner/operators-as well as the size and pump design chosen for particular applications-the dewatering pumps are considered to be either disposable or rebuildable.
The discharge connection is often at the top of the pump to limit the outer diameter of the pump, allowing placement in tighter spaces. The top discharge design also functions as a cooling jacket but care must be exercised to remove solids from that jacket. The accumulation of solids in the cooling jacket, can clog the pump and/or limit heat dissipation. Units with air-filled motors-which depend on being submersed to dissipate heat-cannot be exposed to the atmosphere for long periods, which effectively limits the level to which they can pump, especially under load. More sophisticated dewatering pumps with isolated positive cooling systems can run with dry motors under load.
In dewatering applications, as with most pumping applications, mechanical seals must maintain their integrity against discharge pressure, keep solids out of the motor and withstand the loads from moving around and pumping solids. Seals for slurry service are engineered and constructed differently than clean water seals.
Conclusion
There is a wide variety of pump types, materials of construction (including coatings) and designs available in today's marketplace. When selecting a pump for any application-dewatering or otherwise-it is important to consider the actual operating parameters within the context of Total Life Cycle Costs. Less expensive disposables should be evaluated against application specific units-designed to last for long cycles and then be rebuilt for extended use-can often prove to be the best overall value in the long run.
Pumps & Systems, March 2008