Often, design of the system allows for one of the devices to be out of service. For example, with a three-pump system, only two of the three pumps might be needed for automatic operation and, therefore, one of the three pumps will not operate automatically.
Typically, selecting which device is assigned to the lead or lag level is left to an operator. Sometimes, automatic switching of the lead device is provided to ensure that all devices receive roughly equivalent use; however, in practice, there is often a device that is less efficient or has some mechanical issues that require its use to be minimized. In that instance, allowing an operator to manually assign it a lower lag rating will ensure that its use is minimized.
While an operator may choose which device has a lead or a lag value, there is often still a need for automatic reassigning of lead–lag assignment in the event of a failure or loss of availability of any specific device. For example, if the lead pump fails, automatically moving the Lag1 pump to the lead pump position and the Lag2 pump to the Lag1 position will keep the control system operating correctly in automatic mode until an operator is available.
4.10.2 Most-Open Valve Control
Most-open valve (MOV) control is commonly used to control flow or pressure to several areas via multiple valves or gates. The fluid division between each path is determined by the pressure loss through each valve and the remainder of the path. Each valve must be at an operational position (% open) that is not too high so that it can control, by opening or closing, a finite amount. However, if all of the valves’ positions are too low, it will result in inefficient energy use as the system uses energy to overcome pressure losses across each valve.
For several valves, the position of each valve will typically be different. The critical position is MOV, which needs to be controlled. In a well-designed system with good control valves, this MOV position setpoint may be as high as 95% open. If it were any higher than this, it could not open further to obtain more flow. Some valves or gates have an operational range much smaller. In some instances, the valve may achieve full flow at about 50% open and opening further will not increase flow or decrease pressure. In that instance, the MOV position setpoint might be 45% open.
The MOV position setpoint is typically a secondary or cascade control strategy. The primary strategy is to control the flow or pressure for each path. Typically, the MOV position setpoint is controlled to maintain it within an acceptable range and no further control occurs if the position remains within this range. For example, the MOV setpoint may result in no control actions if it is maintained between 80 to 90% open. In instances where there are several valves, the MOV may not be one specific valve but may represent the average of two or more of the MOVs.
5.0 PUMPING AND FLOW CONTROL STRATEGIES
5.1 Process Description
Pumping and flow control is used throughout wastewater treatment. There are two main types of pumping systems: well pumping and flow pumping. Flow control can be achieved by flow pumping and/or combined with valve or gate flow splitting.
5.1.1 Well Pumping
Well pumping is commonly used to move material away from its source. The most common forms of well pumping in WRRFs are pump stations, either within the collection system or within the facility, that are used to move wastewater to a higher elevation. Well pumping can also occur at several locations within a WRRF to move material such as waste sludge, facility drainage, groundwater, basins, or other areas where fluid flows into a well to be pumped. The main purpose of well pumping is to remove material from the source (well). Therefore, the well level is typically the process variable used for control of these systems.
5.1.2 Flow Pumping
The main purpose of a flow pumping system is to control flow from the source to its destination. In these pumping systems, source water is independently controlled and is not dependent on the pumping system for its control. Flow pumping systems include return sludge pumping from clarifiers, mixed liquor internal recycle flows within activated sludge reactors, and polymer and other chemical additive flows.
The main purpose of flow pumping is to control the amount of material discharged from the pumps. Therefore, downstream flow, or pressure, is typically the process variable used to control these systems
5.1.3 Flow Splitting
Often, flow of material needs to be split 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