to several destinations. This may be to distribute flow evenly to several units or proportionally between several units. For example, consider sludge flow to four dewatering devices. You may want each of the devices to receive one-quarter of the total flow; this would be an even distribution of flow. Alternately, perhaps one of the dewatering devices has some mechanical difficulty that limits its capacity or you have multiple units with different capacities. In these instances, you may want to disproportionately provide a lower flowrate to that unit. In that case, you may want only 20% of the flow to go to the problem unit and the rest to evenly distribute what is left (e.g., 27% each).
Flow splitting can occur by using individual pumps (or compressors in the case of gases) for each flow stream. Alternatively, valves or gates can be used to split each stream. Variable-output pumps or compressors can better achieve a good flow split than valves or gates because they can act independently of the other pumps or compressors. However, using individual pumps or compressors is typically more expensive, particularly for capital costs.
5.2 Process Variables
The following are typical process variables that are needed for control:
• Source (well) level,
• Discharge flow,
• Discharge pressure, and
• Individual flow (each leg).
5.3 Controlled Variables
The following are typical controlled variables that are used for automatic control:
• Individual pumps (on–off and speed) and
• Valve position of each leg (% open or increase and decrease).
5.4 Control Strategies
5.4.1 Well Pumping—Constant-Speed Pumps
With constant-speed pumps, the well level is controlled over a range to provide for lead–lag operation of the pumps. Start and stop setpoints for each stage (e.g. lead, Lag1, Lag2…Lagx) are typically used to start and stop the pumps, with the lead pump stop setpoint at the lowest well level and the highest lag pump start setpoint at the highest well level. Each stage will have a start level higher than a stop level setpoint and the range will overlap but be at a higher well level than the next lowest stage. The start and stop setpoint for each stage must be far enough apart in well level to ensure that the pump is allowed to run and be off for a sufficient amount of time.
If the pumps are different sizes, it is important to consider pump curves for each pump. If the pump system consists of both small and large pumps, the smaller pumps serve as the lead and lowest lag pumps because they are operating at 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
