Automation of Water Resource Recovery Facilities. Water Environment Federation

the lowest flow condition. At higher flow, with large pumps operating, the small pumps may not be able to contribute to the flow because they are at a dead-head condition on the pump curve. In this instance, they may be stopped during operation of the larger pumps if they are not adding to the flow.

5.4.2 Well Pumping—Variable-Speed Pumps

      With variable-speed pumps, the well level can be controlled to a narrow level setpoint of the well because its speeds can vary to better control the level. Even with variable-speed pumps, it may be desirable to allow the well level to vary over a range to dampen the incoming flow. This can be accomplished by adjusting the tuning parameters of the control loop. When setting the level setpoint, energy use of the pumps should also be considered. Maintaining the well level as high as possible reduces the head of the pumping system, which results in lower energy use. A high wet well level also avoids suction head issues that could lead to pump cavitation in some instances. Alternatively, a high well level may increase the risk of well flooding. In addition, a constantly high well may allow for materials to settle within the well. The well level setpoint must be a trade-off for these considerations.

      With variable-speed pumps, the pump speed is altered to maintain the level setpoint. The pump speed is increased as the well level increases above the setpoint and decreases as the well level decreases below the setpoint. If more than one variable-speed pump is available, all operating pumps are typically controlled to the same speed setpoint. This provides greater efficiency and better control. Each stage of pump (e.g., lead, Lag1, Lag2…Lagx) is started and stopped based on pump speed. If the pump speed setpoint is at its maximum and the well level is rising, the next highest stage pump is started (e.g., if only the lead pump is operating, then the Lag1 pump will start). If the pump speed setpoint is at its minimum and the well level is decreasing, then the running pump with the highest stage is stopped.

      If a combination of constant speed and variable-speed pumps exist, it is often beneficial to control the system similar to that with multiple variable-speed pumps. However, only the variable-speed pump will be able to modulate its speed. In this instance, the variable-speed pump remains as the lead pump whenever possible so that it is running to modulate the output and control the level to a setpoint. If a variable-speed pump is unavailable, the constant speed pump will become lead and the control will need to revert to the constant-speed strategy with the well controlled over a wider range.

5.4.3 Flow Pumping

      In a flow pumping system, the control is relatively simple in that the pump speed is varied to maintain the desired discharge flow or pressure for the system. If the flow is higher than the setpoint, the speed is reduced; if the flow is lower than the setpoint, the speed is increased.

5.4.4 Flow Splitting with Valves or Gates

      When splitting flow to various legs using valves or gates, the main control for each valve or gate is a simple flow control loop. The flow to each leg is used as the process variable and the valve or gate is opened or closed as needed to maintain that flow. The flow setpoint for each leg is typically a percentage of the total flow to the system. For example, if it is desired to equally split the flow to three basins and the total flow is X, then the flow setpoint to each of the three legs would be 1/3 × X. However, in a closed system with several valves or gates, the total flow into the system may not exactly match the flow for each leg, thereby causing instability in the control. For that reason, one of the legs is typically left in an uncontrolled state, with the valve in a static position.

      The leg with the static position will then account for any minor discrepancies between total flow and the sum of individual flows. This position should be intermediate to the other valve positions so that it is controllable. If the uncontrolled valve becomes the most-open or most-closed valve, either its position should change or another valve should be chosen as the uncontrolled valve. Of course, if a two-valve system is only used, it is not possible to have the uncontrolled valve in an intermediate position. In this instance, one of the valves will control the flow and the other will have a slower control loop to maintain both valves within a desirable range of operation. The desirable range of operation is to leave one of the two valves as open as possible while still achieving a controllable flow split.

      It is common to use MOV control as a higher order (and slower) control of the system. The MOV control will adjust the position of the MOV if it is not in a desirable range (e.g., as open as possible while still allowing control). This will cause flow in the other legs to decrease, and their flow control loops will increase the position of their control valves to maintain the system.

6.0 SCREENING, GRIT REMOVAL, AND CONVEYANCE CONTROL STRATEGIES

      6.1 Process Description

      Screenings and grit removal are used to remove large materials and abrasive grit to protect downstream equipment.

6.1.1 Bar Screens

      Bar screens stop material that is too large to fit through bar openings. Facilities typically have a number of bar screens in parallel. More than one bar screen is typically needed so that availability is maintained during maintenance. In addition, optimal velocity through the bar screen should be maintained between 0.4 and 0.9 m/s (1.25 to 3.0 ft/sec). This prevents grit settling in the chamber at low velocity and material being pushed through the screen at high velocity.

      Many bar screens have rakes that can operate intermittently. The rake must operate frequently enough to ensure that material does not build up on the rack and blind it; however, if the rake operates too frequently, it will increase wear on the equipment, which will increase the frequency of maintenance and likely reduce its availability.

6.1.2 Grit Removal

      There are several variations of grit removal equipment. Grit removal maintains settling velocity in a range that will allow materials with a high specific gravity to settle out while not allowing organic matter to settle. All grit removal equipment contains mechanical equipment that must operate to move grit from the units. Many modern systems consist only of grit pumps and cyclones to produce grit that can be disposed.

6.1.3 Conveyance Systems

      In most instances, bar screens and grit removal systems discharge to a conveyance system of some sort to transport screenings and grit to a container for removal from the system. These conveyors are typically some sort of belt system with a driver and rollers.

6.2 Process Variables

      The following are typical process variables needed for control:

      • Bar screen channel velocity,

      • Bar screen channel level,

      • Bar screen rake operating time,

      • Bar screen channel differential level,

      • Grit system transport equipment operating time, and

      • Conveyance system equipment operating time.

6.3 Controlled Variables

      The following are typical controlled variables used for automatic control:

      • Bar screen channel gates (open–close),

      • Bar screen rake (on–off),

      • Grit system channel gates (open–close),

      • Grit system transport equipment (on–off),

      • Aerated grit air supply (valve position),

      • Air or water jet valve (open–close), and

      • Conveyance system transport equipment (on–off).