Find more articles on: Variable Frequency Drives
From 50% to 90% of the electrical power consumed at a typical waste water treatment plant (WWTP) with a diffused aeration system is used to run blower motors. Any improvement in the efficiency of the blower drive system has a significant impact on energy use and operating costs.
Many electric utilities sponsor Demand Side Management (DSM) programs for new installations and retrofits to existing WWTP systems, and offer cash rebates or low-interest loans to assist in financing such programs. They also provide incentives if variable frequency drives (VFDs) are installed on process equipment that is normally constant speed. These include multistage centrifugal blowers commonly used for diffused aeration activated sludge processes. In many cases, however, VFDs are not implemented because engineers don't understand the technology involved and the sophisticated controls needed.
Designing with VFDs
Before designing a system using VFDs, a study is made in which the effects of static pressure, friction losses in piping and diffusers, blower performance, and motor data are analyzed. The system headloss curve is plotted, as shown in Fig. 1. Blower performance curves then are corrected to site conditions at full and reduced speed and superimposed on the system curve. This identifies the minimum blower speed that will provide adequate pressure to overcome static head. Then, the savings from using VFDs are calculated.
Blower energy consumption with VFDs is typically 10% to 20% lower than consumption using conventional controls. Utility billing rates for on-peak, off-peak, and demand are used to determine savings. Most projects show a two-yr payback.
Instead of a low-amperage switch for protecting against blower surge (damaging air flow pulsations), an air flow transmitter and low flow detection logic are used to prevent operation below the surge point. This technique provides more accurate equipment protection and extends the usable operating range of the blower.
Integrating air flow control logic for all basins into a unified strategy eliminates the need for discharge pressure control. Special Most-Open-Valve logic eliminates wasteful discharge throttling and distributes air flow proportionately in spite of upsets and equipment limitations.
The Uniontown, Pa. WWTP plant is an example of a successful retrofits system with ESCOR's automatic aeration control system. This plant has two blowers with VFDs and two constant-speed centrifugal blowers with inlet throttling. They serve six activated sludge diffused aeration basins and air lift RAS pumps. Built in 1990, the plant was expanded in capacity and upgraded in 1994 to receive additional sewage from the surrounding Uniontown community.
In the ESCOR system, blower speed is modulated to provide air flow based on the demand from the dissolved oxygen (DO) control logic. Schedule control is provided as a backup. If the variable-speed blowers can't provide enough air, the constant speed blowers are started, discharging into a common header with the variable-speed units. Layout of the system is seen in Fig. 2.
The two 350-hp blowers with VFDs and two 200-hp constant-speed centrifugal blowers send air to the six aeration basins. Sensors measure the DO level in the basins. If the level is outside a specified range, the blower air flow is modulated to correct the DO level. For constant speed blowers, motor-operated inlet butterfly valves are adjusted to restrict the air and change air flow. With VFDs, the blower speed is adjusted to correct the air flow. The parameters monitored by the ESCOR system can include blower amperage or air flow (for surge and motor overload protection), blower bearing temperature or vibration, and inlet filter pressure.
The DO and air flow signals are sent to the central control unit (CCU) over 4-20 mA current loops. Each basin may have a different operating DO level, and the operator may select average or worst case DO for control. The consulting engineering firm for the Uniontown WWTP was Chester Engineers, Pittsburgh, Pa.
Test data shows the power savings available from using VFDs in this application, as well as the increased operating range provided. [ILLUSTRATION FOR FIGURE 3 OMITTED]
Housed in the NEMA 12 enclosure of the CCU, a single-board industrial computer working in tandem with distributed I/O hardware is the core of the system. The system's boot and control files are stored on a solid-state (Flash EPROM) disk. This system is more reliable in hostile environments than floppy and hard disk drives - no matter how "ruggedized" they are.
High-level language programs are used in the CCU software to provide flexibility and convenient operator interface through a keypad (similar to a microwave oven's). This scheme also enables the ESCOR system to communicate with a customer's other systems.
RELATED ARTICLE: UNDERSTANDING DO CONTROL
The dissolved oxygen (DO) level in an aeration basin is the key factor to control. Because of fluctuations in loading, the oxygen demand in most aeration systems is constantly changing. To meet process requirements, manual controls are usually set to meet the maximum demand for any given period. In a plant with manual controls, equipment can only be adjusted on an occasional basis - daily at best, but commonly once a week. Excess aeration wastes energy and may also impede treatment. However, if air flow is reduced too far, the basin DO level will be low, and the resulting process problems far outweigh any energy savings.
Although they can save approximately 25% of the energy needed for aeration, automatic DO control systems have not been used in many plants in the past because of a variety of factors. One reason is that the older probes for measuring DO levels needed frequent maintenance. New sensor technology and self-cleaning probes have eliminated this drawback.
Traditional proportional-integral-derivative (PID) controllers (where "P" stands for proportional band or gain; "I" for integral time, usually called resets/minute; and "D" for the derivative time in seconds) aren't suitable for controlling aeration systems. (See "What To Know About PID Controllers" on page 20.) This is simply because of the long time lag between a change in sewage flow or air flow and the response in DO level. A PlD controller, in addition to being difficult for operators to tune, is often unable to achieve both stability and effective control during oxygen demand variations.
For example, when a controller makes an adjustment increasing air flaw, the change in a basin's DO level will lag behind by many minutes. The controller will consequently continue to increase air flow. As a result, the DO will eventually be too high. This is called overshoot.
Then, the controller starts decreasing air flow, but the basin's DO level will once more lag behind. The system will over-compensate again, and at that point, the cycle begins anew. Repeating this cycle is called hunting. Since this is not good for the process, an operator will usually switch to manual operation.
Hunting can be eliminated by tuning the system for a very slow response. However, if a sudden change in load from rainstorm run-off or an industrial discharge occurs, and the controller is tuned to react slowly, it won't properly correct for the sudden load, which is called a slug.
In older types of DO controls, it's common to vary air flow to basins independently according to their individual DO levels. A constant pressure is maintained in a common air header to prevent changes in one basin's valve from affecting air flow to the other basins. In this type of system, blower control partially relies on inefficient discharge throttling and typically wastes 10% of the blower power overcoming excess pressure.
RELATED ARTICLE: UNDERSTANDING AUTOMATIC CONTROLS
A basic knowledge of control system terms is essential to evaluating the benefits of automatic control. The control logic is handled by the central processing unit (CPU) or central control unit (CCU). Input signals from field devices and the control commands output to them are referred to as I/Os (inputs/outputs). Discrete or digital I/Os are on/off signals like switches or pilot lights. Analog I/O signals, such as a 4-20 mA flow transmitter signal, are proportional to the measured variable.
Programmable logic controllers (PLCs) were originally developed as relay replacements but have evolved to include analog I/O, PID control algorithms, math functions, and high-level language blocks. PLCs are generally programmed in a relay ladder logic (RLL) language proprietary to each PIC maker. They are the best choice when the control requires high-speed timing and logic functions and predominantly discrete I/O.
Process control computers were developed from the digital computers used for management information systems. Now, industrial computers that emulate a personal computer are available. They often include MS-DOS or Windows operating systems, a printer port, and communication ports in one device.
Solid-state data storage is frequently used instead of disk drives. Programming is usually in higher level languages such as C, Pascal, or BASIC. Many are available with built-in monitors and keyboards suitable for industrial atmospheres. Compatible I/O systems are available from many sources and are not usually proprietary.
Process controllers evolved from pneumatic and electronic proportional-integral-derivative (PLD) controllers. Originally, they were single loop dedicated devices with one analog input and one analog output per unit. Many now have multiple loop capability, alarm outputs, programming, and communications.
Various types of communications networks far PLCs, I/O systems, computers, and process controllers are available. Many have proprietary protocols, but the trend is toward open systems and networks. Some open data and I/O communications are based an formal industry standards (for example the IEEE-488 GPIB standard). Others are based on de facto standards, such as Optomux or ModBus serial communications.
Operator interface, or the access to the system for status and process information and tuning, varies from simple switches and pilot lights to sophisticated computer software. New systems generally use same type of personal computer software that displays trends, annunciates alarms, and transfers setpoints and tuning parameters to the control hardware. Several I/O drivers should be available simultaneously to allow mixing hardware from different vendors an one PC.
Graphics software uses a schematic or physical representation of the process to display status and performance, making most important information available at a glance. Most graphics interface programs can duplicate PID controller face plates and digital displays as well as annunciators and pilot lights. Trend displays show real time and historical data in a bar chart or an X-Y format, making it easy to monitor the process. The data can be printed in a variety of ways.
A distinction should be made between a control system, which modifies process operations automatically, and a monitoring system, which merely gathers data and presents it to an operator. Supervisory Control And Data Acquisition (SCADA) systems generally fall into the monitoring category. After reviewing the data, an operator must implement the changes. However, most vendors are adding control features to SCADA software.
Trends in hardware and software are leading to both increased control consolidation and more distributed control. While this may seem to be contradictory, it's a result of low-cost electronics and data communications systems being applied to the field devices (pumps, valves, sensors etc.) and to the operator interfaces.
For example, related control logic for separate pieces of equipment can be integrated into one unit instead of having independent logic for each unit. A single PLC or controller for a nutrient addition process can coordinate several chemical feed pumps, a pH sensor, influent flow sensors, and mechanical mixers. This represents consolidation. This PLC may be connected to a PC with a graphics interface program. The PC's graphics program may connect several independent controllers or PLCs, each responsible for a separate process.
A local PLC or process controller can be provided for each feed pump to implement control commands and provide equipment protection. The I/O may be separated into several remote units mounted close to the mechanical equipment and connected to the controller CPU by a communications network. This reduces wiring and maintenance problems. This distributed control reduces the impact on the entire facility of a singe controller failure.
All automatically controlled equipment should be provided with manual overrides. They are not only required in case of controller failure or system servicing, but are also useful for testing and troubleshooting. Note that the MANUAL setting on some controllers may not bypass the processor. Additional devices may be needed far manual operation if the controller incurs catastrophic failure.
Tom Jenkins, P.E., is General Manager, ESCOR, Milwaukee, Wis., an engineering/consulting firm specializing in process control systems for waste water treatment plants.