Last edited 10 Aug 2016

Pumps and dewatering equipment


[edit] Introduction

Pumps and dewatering equipment are used to remove water from a volume of liquid, solid material or soil. 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:

  • Keeping water out of foundations, pits, tunnels, and other excavations.
  • Lowering the water table below excavation level.
  • Pumping water out of cofferdams.
  • Supplying water for jetting, sluicing and other general purposes.
  • Foundation grouting.
  • Drying solids.

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

  • The project complexity.
  • 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 loss incurred due to friction.
  • The size of the pump.
  • The type of liquid.

[edit] Types of pump

The different types of pump in general use include:

[edit] Centrifugal pumps

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.

[edit] Displacement pumps

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.

[edit] Submersible pumps

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.

[edit] Air lift pumps

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.

[edit] Dewatering equipment

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:

[edit] Centrifuges

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.

[edit] Vacuum filters

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.

[edit] Filter presses

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.

[edit] Drying beds

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.

[edit] Sludge lagoons

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.

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