Dewatering/Pumping Equipments in Construction

DEWATERING/PUMPING EQUIPMENTS

Dewatering equipment are used to perform dewatering on construction sites, which is defined as the process of separating water from another material like saturated soil or sludge. The separation of water is performed by using a force generated by vacuum or centrifugal motion. Dewatering equipment saves money by reducing solids handling and disposal expenses. It is an alternative and economical option compared with heat drying systems for water removal. The selection of dewatering equipment depends on the corrosion potential of the material removed and the contaminants present in the liquid. As the reactivity of the liquid increases, the equipment is constructed with more durable materials. The most common methods used for dewatering are discussed as follows:

Dewatering methods

* Sump pumping: In this method of dewatering water is disposed by adjusting the pump of the trench. Centrifugal pumps are generally more useful. This method is used for shallow bases in waterlogged areas. In this way ditches are dug on both sides of the trench. The ditches size is usually 20 cm in diameter of a semicircle. Sumps are made at a distance of 40 m to 60 m in the ditches. The size of the Sumps is kept as 1m x 1m xlm. The water flows into the ditches and collects in the Sumps. Water is pumped out from the Sumps continuously.

* Deep well construction: This method of dewatering is more suitable when digging operation (digging trench) is lower than water table or artesian water present in soil. This method is useful when the groundwater level needs to be lowered further. Wells are large-diameter (greater than 6 inches) holes, drilled relatively deep (greater than 10 feet), and contain slotted casings and down hole pumps. Wells work best in soils consisting of sand, or sand and gravel mixtures and can dewater large areas to great depths.

* Freezing methods of dewatering: In this method, large pipes of 10 to 15 cm diameter are laid in the ground around the area where excavation is to be done. The spacing between such pipes is 1 to 1.5 m. These pipes are closed at the lower end. Small pipes of 5 cm diameter are laid, which are open or perforated at the lower end. The upper ends of all small pipes are connected to the refrigeration plant by a single pipe. In these small pipes, 23°C to 30°C cold water or other liquid is circulated. Coldwater escapes from a small pipe and climbs up into a large pipe and returns to the refrigeration plant. This cold water freezes the moist soil and forms a wall of frozen clay.

* Well points: In this method of dewatering the ground water flow is te diverted into deep well-points in the ground and the part to be excavated is kept free from ground water. Well points are prepared at a distance of about 1m around the area to be excavated. Water is released at the rate of 20 to 25 liters per second at these well points, which causes the soil to be dug and the well point to sink deeper into the ground. The water edge is maintained for a short time even after the well point has reached the prescribed depth. This creates annular space around the well point as the rises. The water flow is then stopped and filtered material like sand and gravel is filled in the annular space around the well point which stops the filling of soil particles as well as debris at the well point. Well - points are attached to the header with riser, tee-piece, and Swinger arm. The header pipe is paired with a suction pump to create a vacuum in the well-point and riser, so that water from the surrounding land enters the header through the well-point and descends to the ground water table. Well point systems may be single stage or multiple stage:

Single stage system: Water can be lifted from a depth of 5 m with a suction pump. Therefore this method is used where the depth of excavation does not exceed 5 m. The plant is not disturbed until the excavation work is completely completed.

Multiple stage system: When excavation depth is more than 5 m below ground level (W.T.), well-point system is done in phases. In the first stage excavation is carried out to a depth of 5m by arranging the required well-points. In the second stage additional well points are dug into the ground and excavated to a further depth of 5m. This is a way, how well-points are arranged and excavated to a greater depth. The sides of the excavation are given a proper slope.

* Cement grouting: Cement grout is a mixture of cement, sand and, water. In this process several holes are made in the ground. Cement grout is forcibly inserted into each of these holes. The grout is filled in the hall until the grout comes out of the hall. Cement grout freezes in stone cracks and clay cavities, making the stone or clay water-tight and monolithic. Minimal channels of resistance are prepared before starting grouting in soft soil. For this at a short distance two bottom perforated pipes are pushed into the soil and water is forcibly inserted in one of them and water is forcibly inserted in the first pipe till water appears in the other pipe. In this way, minimum barrier channel is formed between the two pipes. In this way many other minimal barrier channels are formed in the soil. Then the cement slurry is forcibly inserted in the pipe through which the water has inserted. It is inserted by force and the cement slurry is continued to be inserted until cement slurry appears in the second, perforated pipe. Hence a waterproof layer is formed by forcibly inserting grout into the soil around the channel. And that way the springs of water in the base trenches can be stopped. He or

* Chemical consolidation of soils: In this method of dewatering, the soil around the area to be excavated is hardened with a solution of chemical compounds like silicate of soda and calcium chloride. In this method also pipes are lowered into the ground. When a pipe is lowered into the ground, a chemical is forced into it. The first chemical insertion is done after the pipe reaches the appropriate depth. The pipe is then slowly pulled out and at the same time another chemical is forcibly inserted into the pipe. The chemical reaction between these two chemicals makes the soil hard. This method is very costly.

Dewatering equipments

The type of dewatering equipment to be used will depend on the corrosion potential of the material, such as sludge, to be pumped, hazardous contaminants, and so on. Equipment may need to be constructed with durable materials. There are a number of different types of dewatering equipment:

* Centrifuges separate solids from liquids through sedimentation and centrifugal force. A bowl, spinning at high speed, separates the water from the solids which are compacted against the bowl walls. Centrifuges remove solids from liquids through the process of sedimentation and centrifugal force. The solids sludge is fed through the stationary feed tube. The sludge moves with acceleration through the ports in the conveyor shaft, which is then distributed to the periphery of the bowl.

* Vacuum filters involve creating a vacuum to draw out water from solids. They can dry solids enough to eliminate the need for subsequent steps such as digestion or heat treatment before disposal, incineration, or usage. A vacuum filter is an equipment that creates a vacuum to draw water from the solids. The filter consists of a drum submerged on a cake or sludge. A filtering medium is placed over the drum. The whole arrangement of valves and pipes is such that vacuum is applied to the inner side of the filter medium when the drum rotates. The rotation of the drum draws water from the sludge. When the drum carries the sludge to the atmosphere, the cake layer formed is chipped by a knife blade.

* Filter presses use a porous press to separate solids from liquids. Solids are captured in pores between two or more porous plates, and built up on the surface. Water is forced through the pores either from plate pressure by pushing the plates together or from a build-up of solids pressure by continuously pushing solids into the cavities. This equipment uses a filter medium to separate solids from the liquids. A filter press captures the solids in the pores between two or more porous plates. bom

* Drying beds consist of perforated or open joint drainage pipes laid within a gravel base. Sludge is placed on top of a sand layer and allowed to dry. Water is removed through natural evaporation and by gravity draining from the sludge mass through the supporting sand to the drainage piping. Cracks develop as the sludge dries, allowing evaporation to occur from the lower layers which accelerates the drying process.

* Sludge lagoons are excavated areas in which digested sludge can be deposited and dried for several months to a year or more. Depths may range from 2-6 ft. After the solid dries, it can be removed for lagoon re-use or levelled to be developed into landscape material or soil. Since lagoons require only the necessary land area and excavation equipment, they are operationally inexpensive; however, they have very little versatility. They are limited to applications with no drying time constraints and solids that contain no hazardous materials that could contaminate groundwater.

Pumps simply remove liquid from a volume of liquid, whereas dewatering equipment separates water from another material such as soil or sludge. They can be used for a number of different operations, including:

❖ To keep water out of foundations, pits, tunnels, and other excavations.

❖To lower the water table below excavation level.

❖To pump water out of cofferdams.

❖To supply water for jetting, sluicing and other general purposes.

❖To drying solids and foundation grouting.

The choice of pumping equipment depends on various factors, such as:

❖ The type of liquid to be pumped

❖ The amount of liquid to be moved

❖ The rate at which the liquid needs to be moved

❖ The height of the suction lift - distance from the water to the pump

The different types of pump used for dewatering are discussed below:

* Centrifugal pumps contain a rotating impeller which creates a vortex that sucks air out of the hose. Water rises to the pump as a result of atmospheric pressure. Priming involves filling the pump casing with liquid before the pump is started. This is done to prevent the casing becoming filled with vapours or gases that inhibit pumping. Self-priming pumps have a reserve supply of water in the impeller chamber. Air-operated centrifugal pumps, often known as 'sump pumps', consist of a small centrifugal pump fixed to an air motor. These are often used in tunnels and foundation pits to handle sewage, oil or sludge

* Displacement pumps can be either reciprocating or diaphragm pumps. Reciprocating pumps work by the action of a piston or ram moving in a cylinder. Larger pumps may have two or three cylinders. Water is drawn into the cylinder when the piston moves in one direction, and pushed out at the rear. These have high efficiency and reliability, and are able to pump against varying heads at a uniform rate. However, they are better for low- flow conditions. Diaphragm pumps work by drawing water into a cylinder in which a flexible diaphragm is raised and lowered. The downward motion of the diaphragm pushes the water out through the delivery pipe. They can pump liquids containing 10-15% solids, and are suitable for work where the flow of water varies greatly.

* Submersible pumps can be used for lowering groundwater or removing water from a deep sump. The pump unit is suspended from the rising main or, if a flexible hose is used, from a wire cable. The pump consists of a centrifugal unit and motor mounted in a single cylindrical unit with a space between pump and casing which allows the water to move upwards to the rising main. They are intended for heavy duty work that involves lifting gritty water.

* Air lift pumps consist, not of moving parts, but of a long vertical pipe connected to a supply of compressed air. The air carries the water up the pipe to the discharge area. Air lift pumps are often used for moving silt from the base of a cofferdam.