Wastewater Treatment in Turbo Mode

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Courtesy of Sulzer

Sulzer turbocompressors type ABS HST are successful in wastewater treatment. Today, 2000 units are installed around the world—for example, in municipal plants like in the city of Sögel, Germany. Here, the turbocompressors not only reduced the operating costs but also helped the plant to cope with load fluctuations. The secret behind this success is an advanced design with proven magnetic bearing technology and a high-speed motor.

The wastewater treatment plant of Sögel, a joint municipality with approximately 16000 inhabitants located in the Emsland region of Lower Saxony, has received a series of upgrades to achieve a capacity of 66500 PE (population equivalent) in recent years. The plant treats not only domestic wastewater, but also industrial wastewater from nearby food industry factories. The plant used an aeration system with up to seven rotary lobe blowers that provided oxygen to the biological sewage treatment. However, process-related increases in pressure were causing single-unit failures with rising frequency. In addition, the relatively high share of industrial wastewater (up to 75%) led to large load fluctuations within a few hours. These fluctuations required a rapid response in air supply.

Following its assessment of a range of technological solutions, the treatment plant of Sögel decided to purchase Sulzer HST turbocompressors. The decision was made partially because of recommendations from a neighboring municipality, which has already been operating Sulzer’s turbocompressors for several years.

The supply of oxygen to four aeration tanks is now handled by five centrally controlled turbocompressors. While between one and three units run during normal operation, additional turbocompressors can be switched on during peak load times to meet considerable increases in the polluting load.

High-speed technology

HST is an electric-motor-driven, low-pressure centrifugal air compressor. The impeller of the radial compressor, the electric motor, and the cooling fan are integrated to one common shaft, which runs at a high speed. A frequency converter generates the required high supply frequency, and the bearing challenge is solved by using contactless active magnetic bearings (AMB). In addition to low friction losses, AMB allows a totally oil-free design.

Sulzer designed the HST compressor almost entirely in house. The AMB, the motor, the control system, the cooling, the turbo unit, and the packaging are all Sulzer’s own design. Only the frequency converter was sourced externally. The high-speed unit of the HST compressor is a highly integrated and challenging device. The challenges include high mechanical stresses, rotor dynamics, cooling, and efficiency optimization. The goal was to achieve a well-designed, high-speed unit with high efficiency in a wide operating range and with sufficient margins to all technical limits, such as stresses, critical speed, maximum temperatures, etc. A lot of experience and powerful modeling tools, especially finite element modeling, were required to meet the design goals. Such system integration is one of the key strengths of the Sulzer R&D team.

There are several HST compressor models. Their power ranges from 69 to 400 kW and their speed from 47800 to 16200 1/min. The pressure ratio ranges from 1.6 to 2.2. The figure shows the cross-section of the high-speed unit of the HST-20-6000 compressor.

There are several HST compressor models. Their power ranges from 69 to 400 kW and their speed from 47800 to 16200 1/min. The pressure ratio ranges from 1.6 to 2.2. The figure shows the cross-section of the high-speed unit of the HST-20-6000 compressor.

Active magnetic bearings

Sulzer has used AMBs in HST compressors ever since it began producing them. In an AMB, the rotating part is levitated using actively controlled electromagnets. Even though the basic operating principle is straightforward and the AMB system is simple from the end-user viewpoint, the system is technically complex. In a typical application, like an HST compressor, there are radial actuators consisting of four horseshoe electromagnets at both ends of the machine. In addition, there are typically two axial magnets to keep the rotor in place axially. The control system needs to know the rotor position. Therefore, five position measurements are needed: Two radial directions at both ends and the axial direction. Often, the axial direction is measured from both ends of the machine. This way, the relative thermal expansion between the rotor and the stator can be monitored.

Important features are the backup battery system and the touchdown bearing. The backup battery will levitate the rotor in case of a power outage. The function of the touchdown bearing is to ensure safe stopping of the machine in case of a very rare AMB failure.
An AMB can be basically controlled by conventional proportional-integral-derivative (PID) regulators. However, in demanding industrial applications, a higher-order multi-input-multi-output control scheme is often needed. In addition, so-called force rejection control is a necessary feature. Force rejection control eliminates the rotation-synchronous bearing force component caused by rotor unbalance. Consequently, during normal operation, the observed vibrations in the stator side are very low. Another important consequence is that a very high level of rotor unbalance is tolerated, which improves the reliability of the system.

In order to levitate the rotor, the magnetic bearing controller (MBC) measures the rotor positions and makes control decisions 10 000 times per second. In addition to the instant position signals, the MBC monitors approximately a hundred different variables: whirling orbits, thermal expansion, static loads, coil currents, coil condition, temperatures, internal voltages, etc. Therefore, the AMB is not only a bearing, but also an extensive condition-monitoring system. It reports deviations from the normal conditions to the compressor control unit, which shows the alarm messages to the end user in the graphical screen.

Development of motor and control system

Sulzer uses two types of high-speed motors in the HST compressor: copper-coated induction motors and permanent magnet (PM) motors. Compared with a copper-coated induction motor, a PM motor has less electromagnetic loss and is smaller. Less electromagnetic loss reduces the power needed for cooling, and the smaller size reduces the air friction. Therefore, the difference in total efficiency is larger than the difference in electromagnetic efficiency only. Another advantage of the PM motor is the better power factor. This means that, for a given power, the current of the PM motor is smaller than the current in the induction motor. This reduces the losses in the frequency converter and enables the use of a frequency converter with lower current rating.

Because of the advantages of PM motors, Sulzer has put a lot of effort into developing a suitable PM motor construction for HST compressors. The motor it has developed uses neodymium-iron-boron magnets and carbon fiber sleeve to provide the mechanical strength. Sulzer recently introduced a new control system Sulzer ICE (intelligent control equipment). It is responsible for controlling the compressor and communicating with automation system and users. It complies with today’s demands considering graphical user interface, remote connectivity, and extended I/O capabilities.

The Sulzer ICE user interface depicts the operating range of the compressor, the limits, and the current operating point.

The Sulzer ICE user interface depicts the operating range of the compressor, the limits, and the current operating point.

Satisfied customer

By converting to HST turbocompressors, installing new aeration units, and optimizing the controls, the specific energy consumption of the Sögel wastewater treatment plant has dropped by more than 50% in the past six years. More than 80% of this reduction is due to the installed turbocompressors. In addition, the maintenance costs were reduced. The earlier rotary piston blowers required checking of oil levels and belt tension on a daily basis, replacing of filters, oil, and bearings by internal and external specialists, and demounting and dismantling of units to enable maintenance. The HST compressors are nearly maintenance free. Only the filters need to be replaced. According to a conservative estimate by the operator, the conversion to Sulzer equipment translates into yearly savings of EUR 10000 in maintenance costs. This example demonstrates how Sulzer equipment enhances the performance of wastewater treatment and drives down operating costs.

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