Constant level Type
A level-sensing device in each filter cell communicates with a control valve in the effluent line in this sort of filter control. Mechanical linkage, pneumatic pressure, electrical signal, or a combination of these can all be used as communication methods. A wide range of level-controlling devices are available. A butterfly valve with an appropriate actuator to position the valve disc is the most frequent.
Constant-level control works in the same way as effluent rate of flow control. The sensing system compares current conditions to desired set points and works with the control valve to align the two. The level-sensing device in a constant-level control system determines the actual water level and compares it to the set point. The sensor device, control valve actuator, and communicating controller all influence the comparison technique. When dirt head loss is low, the control valve opening is reduced to impart head loss to the filter system, similar to the effluent rate control system at the start of a filter run. The control valve is opened when the dirt head loss increases to balance the overall filter head loss. When the effluent valve is fully opened and the dirt head loss continues to increase, the filter water level rises, signaling the need for a backwash.
- Constant-level control systems have the advantage of preventing potential media surface scouring because the cell water level is constantly above the media surface.
- Constant-level control systems, like effluent rate-of-flow control systems, allow for negative head loss, or vacuum suction, over the filter system to extend filter operation time.
- A constant-level control system will maintain a constant rate of filtration if the influent flow is maintained at a constant rate.
- At hydraulic breakpoints, switching back and forth between flow control and rate control is possible.
- Flow stabilization concerns might arise with constant-level control systems.
- The operator is not able to visually see the head loss building in the filter.