Water and Effluent Treatment Magazine
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12 WET NEWS february 2015 Main Picture: The original design of the aeration system to be provided centred on a fine bubble Diffused aeration plant to be supplied by four positive displacement blowers. right: four Turbocompressor Tupe abS HST 20 from Sulzer's HST range provided the best whole-life cost over 20 years Wire to air evaluation • When it came to selecting new blowers for Taunton sewage treatment works, a two- pronged evaluation exercise was implemented. Nomenca's Trevor farrow explains. TECHKNOW • The HST range of blowers are equipped with magnetic bearings to support the rotor • The system uses a number of sensors at either end of the rotor to determine its position in both the radial and axial plane • The battery back-up system is continuously monitored to ensure that it can provide the necessary protection for the rotor • rotating at speeds in excess of 40,000rpm NEED TO KNOW 1 The evaluation considered a comprehensive selection of current blower technologies and looked at not only capital expenditure and energy usage but also the projected maintenance costs 2 During the selection process, Wessex Water and Nomenca visited some existing HST installations to gather feedback from the users 3 Desktop evaluation of 12 alternative machine types clearly identified the Sulzer turbocompressor as the optimum machine for the proposed application H igh-efficiency turbo blow- ers have been installed at Wessex Water's Taunton sewage treatment works to re- duce operational costs by signifi- cantly reducing energy use. This follows an evaluation of forecast costs associated with the aera- tion system for the new activated sludge plant at the works in Som- erset, under the DWF improve- ments scheme. The originally proposed conventional positive displacement blowers have been changed to high-efficiency blow- ers. The new blowers were installed a er a trial at the com- pany's Swanage sewage treat- ment works which validated the potential savings with this type of machine against an existing posi- tive displacement machine. The original design of the aer- ation system to be provided at Taunton centred on a Fine Bub- ble Diffused Aeration plant to be supplied by four positive dis- placement blowers configured on a duty/assist/assist/standby basis; this design being in accord- ance with Wessex Water Design Standards. In association with Wessex Water's Energy Team, considera- tion was given to the evaluation of a selection of turbo blowers as opposed to positive displacement machines; and two separate evaluation exercises were undertaken: The first involved an onsite trial using a high-efficiency turbo blower used to demonstrate and validate the potential opex sav- ings when directly compared with an existing positive dis- placement machine The second took the form of a desktop study using information provided by three FBDA suppli- ers, with each supplier putting forward their options for three different types of machine to meet the duty requirement across the treatment range. THE VERDICT "We have been able to review and implement changes in a relatively short space of time, which was important as we were already on site and couldn't afford to have any undue delays" Garry Orford, Wessex Water's project manager ONSITE SluDGe TreaTMeNT & DiSPOSal Each supplier was asked to offer a positive displacement machine, a hybrid machine and a high- efficiency turbo blower. In all a total of 12 machines were evalu- ated to determine the best whole- life cost taking account of the range of duty conditions specified For each machine considered the respective FBDA supplier was required to submit the "Wire to Air"* power figures at maximum, average and minimum SOR, tak- ing account of the losses within the overall system. Each was asked to take into account the tolerance(s) on flow and power, VSD efficiency and system pres- sure drop across the aeration sys- tem of pipework and diffusers. Energy consumption on an annual basis was determined using operating scenarios previ- ously considered at detail design stage. The evaluation considered a comprehensive selection of cur- rent blower technologies from a number of manufacturers and looked at not only capital expend- iture and energy usage but also the projected maintenance costs. The conclusion of this evaluation was that four Turbocompressor Tupe ABS HST 20 from Sulzer's HST range provided the best whole-life cost over 20 years. Sulzer's HST range of blowers are equipped with magnetic bear- ings to support the rotor, meaning there is never any contact between the rotor and the static elements of the unit, even during start up or shut down (which is the case with alternative technol- ogies). This feature eliminates all possibility of wear and vibration in the machine and also takes away any requirement for routine maintenance tasks such as moni- toring and replenishing oil levels. Nigel Jackson, Sulzer's UK sales manager, says: "The use of magnetic bearings in turbocom- pressors has now been with us for nearly 20 years and they have proven to be reliable, efficient and cost effective. We employ some of the world's leading engi- neers in the use of this technology and have been able to refine the process to such a degree that movement of the rotor is con- trolled to incredibly fine toler- ances. This in turn allows us to close clearances and reduce losses making the machine more efficient." Power The system uses a number of sen- sors at either end of the rotor to determine its position in both the radial and axial plane. This posi- tional information is then relayed to the Magnetic Bearing Control- ler (MBC) which then decides whether to increase or decrease power to the relevant sector of the bearing which repels the rotor. As all of the many sensors measure the position of the rotor every revolution, which in some models can be rotating at speeds in excess of 40,000rpm, the quality of control and stability achieved is extremely high. One obvious question from potential users is what happens if the power to the machine is lost, does the rotor fall? This was natu- rally a prime consideration for the designers, well aware of the potential consequences of a rotor spinning at very high speed com- ing into contact with other static components, and their solution has been proven to be extremely robust. Under normal operation, power for the MBC (which levi- tates the rotor) is derived from the integral frequency inverter that controls the output flow of the machine. If an outage occurs the machine's inbuilt instrumenta- tion immediately detects the volt- age loss and instigates a shut- down using a purpose designed battery back-up system to main- tain levitation of the rotor until it comes to a controlled stop, with the rotor then resting on conven- tional mechanical safety bear- ings installed in the machine. When power is restored, the rotor li s from the safety bearings before beginning rotation. The battery back-up system is also continuously monitored to ensure that it can provide the necessary protection for the rotor. With machines installed all around the world, many in areas of uncertain power supplies, this protection system has proven to be robust and well up to the task. During the selection process, Wessex Water engineers together with representatives of Nomenca (Wessex Water's contract M&E partner) visited some existing HST installations to gather feed- back from the users. This not only gave them a clear picture of the benefits of the machines but also provided a platform to question the users about the per- formance and reliability of the units. For me, desktop evaluation of 12 alternative machine types clearly identified the Sulzer tur- bocompressor as the optimum machine for the application being proposed. The machines were ultimately selected on the grounds of the most favourable NPV over a 20-year lifespan, tak- ing account of all parameters associated with energy calcula- tion, capital cost and upkeep cost during their life cycle. A fully integrated design for the aeration system has been achieved by bringing together Sulzer as the turbocompressor supplier with Xylem, the supplier of the FBDA system. Although each supplier has been appointed under separate contracts, the overall performance and guaran- tees to be provided for the aera- tion system as a whole rests jointly with each. Garry Orford, Wessex Water's project manager for the scheme, says the company was committed to operating efficiently. He adds: "When it comes to designing schemes we are mindful of doing everything we can to reduce energy consumption, so the use of turbocompressors in these cir- cumstances would appear to be ideal. By all involved working closely together, we have been able to review and implement changes in a relatively short space of time, which was impor- tant as we were already on site and couldn't afford to have any undue delays." n Trevor Farrow is project manager at Nomenca. * "Wire to Air": a term used to describe the total energy used to produce the required flow and pressure for any particular application. PROJECT SPECS • install high-efficiency turbo blowers at Wessex Water's Taunton sewage treat- ment works • evaluate a selection of current turbo blower technologies • undertake an onsite trial using a high-efficiency turbo blower to demon- strate and validate the potential opex savings • undertake a desktop study using information provided by three fbDa suppliers