TRAVELLING BAND SCREENS
DUAL-FLOW FROM OUTSIDE TO INSIDE
PURPOSE AND APPLICATION
Travelling screens are used to arrest the small debris contained in the water so that the downstream users remain unobstructed and clean (condensers, exchanges, spray-water circuits, membranes etc). Debris can be of all types including fish, shrimps, jellyfish, grass, man-made refuse, plastics, seaweed, etc. They are especially suited to deep pits.
They are used in thermal and nuclear power plants, LNG terminals, desalination or fertilizer plants, drinking water and irrigation plants.
DESCRIPTION
Installed in a channel in which flows the water to be cleaned, the screen consists of a set of rectangular screening mesh panels carried by two endless chains. The panels travel up and down again between an upstream wall plate and a downstream partition with a central, outlet aperture set across the channel.
The water enters along both sides of the upstream wall-plate, flows through the upstream face of the panels on each side, where the debris are arrested. The water then flows out through the back, partition aperture.
The debris-laden panels travel up above deck level, around the top and down again. One or two spray pipes with fantail jet nozzles set on the down stream side of the panels within the screen’s head structure, back-wash the panels, remove the debris which are projected into a collection trough. They travel in a deck flume to the debris disposal system (basket, separator, etc.)
The screens are normally stopped and are washed periodically when obstructed by debris. Permanent rotation can be provided.
ADVANTAGES
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Large debris-handling capacity
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Capable of high-speed rotation
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Compact civil structure
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Fully automatic
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Fish-friendly option
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Reduced maintenance
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Cost effective
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No debris carry-over
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Easy access to external debris trough
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No internal debris interference
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Over 100 years of experience
NECESSARY ANCILLARIES
Spray-water supply circuit
Head-loss monitoring system
Electrical control cabinet
Upstream bar rack (25 to 75 mm spacing (1 to 3”)
Pit dewatering stoplogs
Trash collecting system (basket, etc).
SIZES AND DATA
Mesh aperture
From 1x1 to 10x10 mm (0.04 to 3/8”)Flow rates
Up to 70000 m3/h (300000 gpm)Tidal variation
No practical limitPanel frame useful width
0.6 to 4 m (2 to 13 ft)
Wider machines on special order
OPTIONAL FEATURES
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Two or three-speed operation
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Seismic qualification
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Special high travel speed design for handling heavy debris loads
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Jellyfish lifting trays
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Screening medium resistant to fiber build-up
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Screening medium preventing jellyfish adherence
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Low pressure water-life protection system
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Atex compliance
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“Scoop-a-fishTM” total fish survival system
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Wall-mounted chain guides on full carrying structure*
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By-pass valves
*With full carrying structure, installation time and costs are substantially reduced as the screen is lowered into the pit in a single operation.
MATERIALS
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Main structure, panels, frames, guide rails:
For fresh-water applications: painted carbon steel or AISI 304L stainless steel
For seawater applications: AISI 316L stainless stee, duplex or super-duplex stainless steel
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Head-shaft, sprockets, chain links and pins, tensioning capstan screws:
For fresh-water applications: High-tensile steel
For seawater applications: Duplex or super-duplex stainless steel
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Bolts, nuts, washers:
For fresh-water applications: A2 or A4 stainless steel
For seawater application: A4, duplex or super-duplex stainless steel
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Small fittings, spray-water pipes:
For fresh-water applications: AISI 304L stainless steel
For seawater applications: AISI 316L, duplex or super-duplex stainless steel
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Screening mesh frames, sprocket teeth:
In all cases: composite materials
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Covers:
GRP or stainless steel on special request
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Anodes:
None required for fresh-water applications
For seawater applications: required for use with AISI 316L stainless steel and sometimes for duplex stainless steel.
