Dewatering is the process of removing groundwater or surface water from a site, typically on mining and construction sites, to lower the water table to ensure operations can be carried out safely and efficiently.
On mine sites, dewatering is usually an ongoing process throughout the life of the mine, and is crucial to allow the safe and productive access to minerals. While on construction sites, dewatering works will often be required before works such as digging and drilling can start on sub-surface excavations, including cellars, foundations, shoring and underground car parks. Therefore, having a reliable pump that is able to remove water from deep underground is imperative to efficient dewatering.
There are a number of pumps available for dewatering and the best one for your application will depend on a number of factors including the type and depth of the groundwater, the surrounding geology and environment, and how far away the water will be discharged.
Here, we look at some of the most common types of dewatering pumps, including submersible, end suction, well-pointing piston and trash pumps.
As the name suggests, submersible pumps can be either fully or partially submerged in water. This is due to the motor being hermetically sealed and close-coupled to the body, allowing the pump to be submerged in fluids without the motor being compromised.
Instead of pulling the water to the surface, as the pump is positioned in the water, it pushes water to the surface via a discharge pipe by converting rotary energy into kinetic energy into pressure energy.
Submersible pumps for dewatering applications are designed to handle smaller abrasives like sand, small stones and clay. Depending on the solids that will be present in the fluid being moved, an open or closed impeller can be used. An open impeller consists of two parts that must be maintained as they will wear, causing a decrease in pump performance. On the other hand, a closed impeller is used for the toughest applications where materials completely pass through the impeller and where high-chrome materials are present. This makes them ideal for heavy duty work where gritty material is present in the water.
Submersible pumps have a number of advantages:
- Requires less operating space
- Does not need to be primed as it is already submerged in the fluid
- The pump is positioned at the same level as the water, making them very efficient as additional energy is not needed to move water into the unit
- They are quiet as they are positioned underwater
- Cavitation is less of an issue as there is no spike in pressure when the water flows through the pump
- The selection of submersible pumps is versatile, with some able to easily handle solids and others better for liquids only
They also have a few disadvantages:
- The seals can corrode with time, allowing water to seep into the motor and making it unusable until it is repaired
- The seal makes it hard to get access to the motor for repairs
End suction pumps
End suction pumps are one of the most common types of centrifugal pumps and come in a variety of sizes, materials and construction to suit a wide range of applications.
These pumps have a casing with the suction coming in one end and the discharge coming out on top. As these pumps are positioned on the surface, a long suction pipe (or inlet hose) needs to be lowered into the water. A rotating impeller creates a vortex that sucks air out of the hose to bring the water up to the surface through the pipe and discharges it via an outlet pipe to a location away from the pumping site.
These pumps require priming before the pump is started to prevent the casing filling with vapours or gases that will inhibit pumping. However, self-priming versions are also available.
The disadvantage of self-priming centrifugal pumps is that any small vacuum leak can prevent it from priming, causing it to continuously pull air from the leak instead of the air in the suction line, resulting in the priming cycle never being completed.
Priming time can also be affected by the diameter and length of the suction hose or pipe due to the volume of air that needs to be removed. The longer it takes for the pump to prime, the more heat that is added to the liquid, further extending priming time.
Furthermore, self-priming pumps need to be located as close to the water source as possible, which can be a problem depending on location of dewatering works. Once primed, it will need to maintain adequate net positive suction head (NPSH) to remain operational without suction limitation. If sufficient NPSH cannot be maintained, the pump can cavitate and lose prime.
End suction pumps are best for dewatering applications when suction lift is below 25 feet, if it is higher than this a submersible pump should be used instead.
Well-point piston pumps
Well-point piston pumps are a type of positive displacement pump. These pumps offer self-priming capabilities and are able to run dry without the unit being damaged.
These pumps are used in well-point dewatering systems as they are suited to moving thick fluids and slurries, as well as abrasives, and are primarily used on construction sites where the groundwater level is close to the surface.
There are a number of advantages of well-point piston pumps, including:
- Wide range of pressure
- Manage the force without moving the flow rate
There are also some disadvantages, including:
- As these pumps are usually bulky and heavy, maintenance and operation costs are high
- Only handle lower flow rates
- Uses a pulsating flow
Trash pumps are usually portable, making them easy to move around a site, and as they are able to handle dirty water that is clogged with mud, sand or other debris, they are ideal for dewatering applications. They can run on diesel, petrol or electricity.
These pumps work by sucking in the dirty water and separating the contaminant before ejecting the filtered water – the method of this separation process will depend on the type of trash pump being used.
Compared to standard pumps, trash pumps are designed with deeper impeller veins and larger discharge openings, allowing them to move fluids containing solids that would normally clog a standard centrifugal pump, and at a higher speed. Furthermore, unlike many other pumps, trash pumps don’t grind up the solids entering the pump, and feature a full pump housing.
There are three types of trash pumps, and the most appropriate one will be determined on the type of water that will be moved. These are:
- Semi-trash pump: This type is suitable for use in applications where the water contains minimal mud or sand smaller than 15mm as they are designed with a smaller discharge opening. Common applications include removing water from flooded basements and other emergency situations, moving fluid from one tank to another, and removing water from manholes, utility vaults and swimming pools
- Centrifugal trash pump: This is the most common type of trash pump. They come in different sizes depending on the debris that will be present in the water – smaller models can handle solids up to 25mm, while larger models can handle solids up to 100mm
- Diaphragm trash pump: This type is larger than their centrifugal counterpart. Air diaphragms are available in sizes 25-75mm and air-driven submersible pumps are also available. These trash pumps are usually used for draining ponds, and moving sewage and chemical-contaminated water