Pregrade
Fine screen operation
The operation of the Bormet screen is intermittent to minimise the wear and increase the SCR (Screening Capture Ratio). The screening material blind, the screen and the components smaller than the slot opening are also captured (cake filtration).
Intermittent operation will be triggered by measuring the water level (upstream and downstream) an using the difference for switching.
The concept of a continuous travelling filter band (like an escalator) enables the screen to collect all waste material including large or round solids, and to gently handle the screenings.
Hydraulic Properties
In order to guarantee correct operation of the screen, the optimum flow velocity between the elements is approx. 1.0 m/s. Higher flow velocities will press the screenings between the filter elements making the cleaning of the belt difficult. Lower velocities between the elements will enable the sand and grit to settle in the bottom of the channel. That is why Spaans Babcock always supply a flow control device (channel narrowing) to reach the optimum flow velocity.
Cleaning Performance
The Bormet fine screen is a self-cleaning screen. In addition to the cleaning by gravity and high speed rotating brush, the Bormet screen has a double row of water sprays (the flow of the spray system can be reduced).
Three cleaning steps:
- Gravity + Outer spray pipe – tangential to the filter belt.
- Inner spray pipe – tangential to the brush, washing through the filter belt. Will remove the particles that are stuck between the elements.
- High speed rotating brush, fully equipped with 360 degrees of bristles. The centrifugal force created by the high-speed brush will prevent screenings from sticking to the bristles. Cleaning of the brush with the inner spray system.