HYDROCYCLONES

Description

Hydrocyclones are used for the separation of solid particles, such as sand, from liquid. They can be made of plastic, ceramic, iron or steel and have no moving parts. They are arranged singly or in banks around a central manifold.

Hydrocyclones are conical in shape. Inlet water enters the hydrocyclone tangentially and the spiral movement down the cone promotes a high centrifugal force. Particles with a size or specific gravity greater than the cut point or threshold value of the hydrocyclone are forced outwards by this centrifugal force. They impinge on the internal walls of the hydrocyclone and run down to be deposited at its base or removed as the underflow. Finer particles and the carrier liquid travel in a vortex back up the hydrocyclone and exit through the vortex finder as overflow. The underflow volume is designed to be a small fraction of the overflow.

The cut point size of a hydrocyclone is related to its internal diameter and the diameter of the vortex finder - the smaller these diameters, the finer the particles that can be separated. The angle of the cone also influences cut point - the smaller the angle, the finer the particles separated. However, as separation of finer particles requires reduced throughput and increases the risk of blockage, parallel banks of hydrocyclones are used and screening is a prerequisite.

Stub hydrocyclones, which are squat in shape, are used to separate particles of similar sizes but of different specific gravities.

Application

Hydrocyclones are used for the removal of sand, silt, suspended solids and other particles larger than 5-50µm from various liquids. Most applications occur in drilling water, borehole water, recycled water, cooling fluids and process and effluent streams in industry. Cyclones can be used in primary or secondary effluent treatment or even to recover raw product, such as starch, from an effluent stream.

Large stub hydrocyclones are used in stormwater overlows to prevent urban pollution. In storm conditions the most pollutive material is retained as underflow and the cleaner material by-passes the treatment system. In this way, 60% of the BOD and 70% of the suspended solids in stormwater can be kept within a sewerage system.

Sizing

A hydrocyclone installation treating a flow of about 50 m3/hour would require a feed pump of about 7kW. Typically, hydrocyclones are used to treat flows of 20-3000 m3/hour. The stub hydrocylones used in storm overflows consume no external power.