Back-Raked
Screens
Applications
These
screens are suitable for seawater installations for power and desalination
projects, for river water intakes where weeds, leaves, logs and branches
need to be removed, on sewage stations for inlet works, storm over-flow
sites, or for industrial complexes where effluents have to be treated
prior to discharge.
Back
Raking
The
screen is specifically designed for heavy duty removal of weed, debris
and sewage. The device incorporates patented back-raking features.
The specific advantages of back-raking can be summarised as follows
:
The
raking tines plunge into the weed or debris mass at the base of the
intake channel. They do not have to descend through the contaminated
water as is the case with for front-raked machines.
All
working parts are protected behind the static grid and consequently
function on the clean side of the bar screen.
All
debris, rags or weeds are removed from the bar screen in the first
raking motion. The tines moves upwards from the base of the channel
and therefore clean the entire bar screen in one movement.
Positive
discharge. The raking tines move upward through the bar screen
forming an angle of 90 degrees to the bars. At the top section
the raking tines describe an arc until they are almost vertical.
At this point they start the descending motion through the
curved section of bars at the top of the unit. A scraper plate
or discharge chute ensures all debris is removed. Once arrived
at the bottom of the device, the raking tines return gradually
to their initial position, being 90 degrees in relation to
the static grid, by turning counter-clockwise.
All
machines have a bar section of 48 mm. The raking tine length
is 600 mm. in front of the bars.
The
raking tine protrudes through the screen by 600mm in the screening
mode.
Discharge
The
aesthetically pleasing screen is particularly inconspicuous when compared
to the conventional type of cable-hauled raking machine. The top section
appears as a curved set of bars protruding as low as 1,6 m above deck
level. The height of the curved section above deck level can be varied
depending on the method of discharge adopted, i.e. either onto a conveyor
or into a bin.
Installation
The
back-raked machines are completely assembled in our works and run
'dry' for a period of time to ensure all settings and adjustments
are correct. The units are usually dispatched to site in one piece
where the final installation takes 24-48 hours. The units are free-standing
and require only grooves in the channel walls for positive location.
Fixing is only required at deck level and the machines can be removed
in one piece at any time for inspection and maintenance.
Materials
The
unit is designed to suit to a broad selection of materials determined
by the location and type of duty for which the screen is employed.
The
standard machine incorporates carbon steel bars and carbon steel side
channels and guides.
The
raking tines can be of stainless steel.
The
unit can also be manufactured entirely in stainless steel if necessary
or a combination of materials as specified by the contractor, consultant
or engineer.
Patented
motion
The
drawings illustrate the motion of the two raking bars as they operate
in relation to the bar screen. This unique feature ensures very high
debris removal with positive discharge at deck level.
Special
features
The
unique patented motion of the raking tines in relation to the bars
enables this machine to overcome all standard objections that could
possibly apply to back-raked machines :
You
should note the considerable tine penetration that this unit incorporates.
This is due to the unique motion of the raking tines moving at right
angles to the bars and then progressively into the vertical position
before descending to the base of the intake channel. This motion is
indicated on the illustration. The basic machine utilises two raking
bars. As one is discharging the other at the foot of the intake is
plunging into the contaminated water.
On
deep sumps, or where weed loading is particularly difficult, four
raking tines can be applied or in exceptional circumstances six or
even eight. It is very easy to attach another set of raking bars at
any time during the station's life. Therefore, complete flexibility
is maintained regarding rate of discharge and speed of raking.
Positive
bar location. In addition to the bars being fixed at the foot of the
machine, they are also firmly located at the discharge position.
On
all screens there are an even number of tine beams and an even number
of bar support beams. The distance between the tine beams and the
bar support beams is determined by the stress calculations in function
of the allowed differential. The differential in the screen is absorbed
by the nylon rollers behind every bar on the screen. These are fitted
on the tine beams and on the bar support beams. The rollers do not
only support the bars on differential, but keep the bars also in their
exact position. They give a continuos support to the bars on the height
where the differential in the screen can occur.
