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  • 1. TOTAL COST OF OWNERSHIP A Financial Approach to the Process Industry TCO-Maintenance & Operations / October 1, 2006 A Complete System Approach to Non-Energy Operating Costs Abstract: Much attention is paid to the energy efficiency of Variable Speed Drive (VSD) technologies. However, energy efficiency is only the “tip of the iceberg” when considering the Total Cost of Ownership of VSD alternatives. End users of VSD’s need to consider the costs associated with maintaining, repairing, and operating the variable speed control technology they decide to use in their plant. The differences in the costs of replacing parts, repairing vibration caused damage, correcting misalignment, and replacing worn out or obsolete VSD components can be considerably higher than the energy cost differences between the same technologies. The methods available to adjust the speed Total Cost of Ownership Analysis: of a driven system are numerous. Pulley Systems, Variable Frequency Drives, Eddy It is an accepted fact that in variable load Current Drives, Fluid Drives, and Permanent process applications, the use of speed Magnet Adjustable Speed Drives top the list control on motor driven systems offers of options. But which one is the best significant energy savings. A quick look at option? the Affinity Laws that govern centrifugal pump applications helps to explain and When making this decision it is important to confirm this fact. In summary, the Affinity look at the Total Cost of Ownership Laws are: associated with each of these options. Total Cost of Ownership (TCO) analysis has to 1. Flow - Q1/Q2 = N1/N2 include all costs that go along with each P1/P2 = (N1/N2)2 2. Head - option. Initial purchase price typically is only 10 to 25% of TCO. Drive system energy 3 3. Horsepower - HP1/HP2 = (N1/N2) efficiency, non-energy system operating costs (such as long term maintenance Where Q = Flow, N = Speed, P = Pressure, and requirements), drive system life, and the HP = Power. cost of process downtime all need to play a part in the purchase decision. Some of What the Affinity Laws tell us is that flow these, such as initial price, are easy to get a changes are proportional to changes in handle on. Non-Energy Operating Costs pump speed but that the power required to however, tend to be a little more difficult to drive the flow is proportional to the cube of quantify when comparing VSD options. the pump speed. So, a 50% reduction in flow requires a 50% reduction in pump Most manufacturers of VSD’s pay very speed. However, the power required to careful attention to the reliability and produce this lower flow will only be 12.5% of maintainability of their VSD as a stand-alone the original level! piece of equipment. All too often, the reliability and operation of the Process It is this relationship between flow, speed, System that the VSD is being used in is and power that makes adjustable speed neglected. The reason for this is simple. control such an attractive option in the Until recently, the VSD technologies process world. available to the industrial user were all very
  • 2. similar in the impact they had on the The potential for seal damage and complete system’s maintenance and subsequent leakage is very real. In fact, in a operation. Breakthroughs in variable speed government publication that covers process power transmission, specifically VSD’s that leaks and their impact on air quality, the utilize permanent magnet eddy currents to EPA proposes that process operators plan transmit torque across an air gap, have to replace 20% of the seals in their system changed the VSD landscape significantly. annually as a standard leak prevention This paper will examine the Non-Energy program. In an average oil refinery using Operating Costs associated with traditional 3000 pumps this equates to 600 seal VSD’s. Specific attention will be paid to the replacements annually… two seals every differences and advantages of Permanent day! The maintenance demand created by Magnet Adjustable Speed Drives. A this task is enormous and ties up talent that summary and comparison of the actual could be used on process optimization system Non-Energy Operating Costs for the instead. different VSD options will also be provided. Pulley Systems: Traditional VSD Technology: Pulley systems (or Friction Drives) are the For the purpose of this discussion, the term oldest of the VSD technologies having been “Traditional VSD’s” will refer to: Pulley in use even before the introduction of motors Systems, Variable Frequency Drives to industry. Pulley Systems are easy to set (VFD’s), Eddy Current Drives, and Fluid up, require no specialized training, and offer Drives. a small degree of overload protection. There are several hundred suppliers in All traditional VSD technologies offer North America and the technology has seen significant advantages when compared to very little innovation in the past few Control Valves and Dampers. Because of decades. the Affinity Laws, the potential for energy savings is substantial. Also, because VSD’s In a pulley system, speed of the driven load control flow by controlling the driver speed is adjusted by moving a drive belt to a and, therefore, the head of the system, the different size pulley or by using an potential for cavitation and the damage that adjustable pulley, or sheave. If the drive comes with it is greatly reduced. An pulley is kept at a constant diameter then a advantage of Control Valve systems is that doubling of the driven pulley diameter will they appear to be extremely simple and result in a halving of the speed. Unless a easy to understand. The additional energy sheave is being used, this type of system and non-energy costs of Control Valves, requires that the process be stopped in however, can not be ignored. order to change the speed and provides a limited set of speed choices to the operator. Traditional VSD’s either control the load This is not an appropriate choice for speed speed by changing the driver speed or by control in a true process control loop. changing the amount of power being However, in a system requiring a one-time, transferred to the load. In both cases, the semi-permanent adjustment this seems, on driver is physically connected to the driven the surface, to be a fairly viable option. load. Because of this physical connection, any misalignment between the driver and Adjustable pulley drive systems, or sheaves, the load will result in vibration leading to operate using the same principles and shortened seal and bearing life in the function by moving the sides of an system. Generally, realignment of the adjustable drive pulley towards or away from system occurs at least once per year. Even each other. This changes the effective pitch when this preventative maintenance occurs, diameter of the pulley, and therefore, the vibration remains one of the biggest output speed. While capable of continuously concerns for rotating equipment. In severe modulating speeds in response to a process cases, the vibration can be enough to loop, these systems tend to be hard on belts actually damage the driven equipment. and produce relatively high mechanical losses.
  • 3. In both of these systems, proper operation radial loads greatly reduce the life of depends on friction between the belt and the bearings and will cause premature leakage pulleys. Because of this necessary friction, in pump seals. there are numerous opportunities for wear which, in turn, leads to increased costs for Pulley Systems offer a simple and easy system maintenance and repair. to understand approach to varying the speed of a driven load. However, in addition to the vibration-related maintenance costs common to all traditional VSD technologies, Pulley Systems utilize belts that have maximum life expectancies of 3 years. On average, the use of Pulley Systems in variable speed applications can result in maintenance costs more than twice as high as other traditional VSD options. Figure 1: Pulley System Variable Frequency Drives: Belts typically are constructed in a manner similar to an automobile tire. An elastic Variable Frequency Drives (VFD’s) were compound is formed into a loop. introduced to industry in the 1980’s. Major Strengthening belts made of fiberglass, suppliers in North America include: Rockwell steel, or another cord material are Automation; ABB; General Electric (GE); embedded into the loop and the cross and Siemens. VFD technology in general section of the material is given a specific has seen significant amounts of innovation shape. Typical life expectancy of a generally focused on getting more power out properly installed and tensioned belt is of a smaller package. The adoption rate of approximately 25,000 hours, or slightly less VFD’s by industry in the 1990’s was quite than three years of continuous operation. high. This adoption rate has slowed dramatically in recent years as more users As is the case with automobile tires, these become aware of the problems associated belts wear with use. Side wall friction and with VFD’s. The biggest issues identified by belt/pulley slippage wears away belt material users are: during normal operation. In the case of a load seizure, these systems are designed to • Reliability of drives either slip or to break the belt. In either • Need to replace or upgrade drives case, the belt is typically rendered unusable every 4 – 7 years and must be replaced. • Harmful impacts to other equipment on same electrical service. In addition to friction, the belts tend to deteriorate as a result of oxygen in the air, Due to their apparent low initial cost, many heat, and lubricants in the system. Small industrial users think of VFD’s as the de- pulley diameters will also put extra strain on facto standard for process speed control. belts resulting in shorter life expectancies. It This technology has a fairly long history and has been shown that belt life can be cut in the market price continues to decline. half simply due to a 35º F increase in These factors make the VFD appear, on the ambient operating temperatures. surface, to be a good value. However, VFD’s also carry with them several issues To maintain the proper amount of friction that negatively affect not only the efficiency between the belts and pulleys, tensioning of of the process they are used in, but also the the belt is required. A belt that is too loose reliability of those processes. will rapidly wear out due to friction generated heat. In fact, one of the primary causes of VFDs control the speed of the motor by belt failure is improper tensioning. Properly changing the sine wave AC supply voltage tensioned systems lead to longer belt life but to DC voltage, which is then converted into a also place high radial loads on the motor pulse width modulated (PWM) AC voltage of and load bearings and/or seals. These variable frequency. The frequency of these
  • 4. for leaks, checking exterior paint condition and touching up when necessary, checking the radiator surfaces for foreign materials, confirmation of fan and cooling system operation. Figure 2: Small Variable Frequency Drives Figure 4: Example of Bearing Fluting Air conditioning systems also require annual maintenance schedules. When considering the maintenance impact of an air conditioning system, purchasers of VFD’s need to consider the following activities as a base minimum: checking of refrigerant level and quality, measurement of condenser coil airflow, cleaning and inspection of the system’s motor, compressor, ducting, and filters. VFD’s often appear to be the most Figure 3: Large Variable Frequency Drive System economical solution to adjustable speed applications. However, the general pulses ranges from 1 kHz up to 20 kHz. As maintenance costs associated with these pulses are sent to the motor, a system vibration combined with the capacitive charge is created between the additional costs of maintaining the motor’s stator and rotor, which induces a system’s ancillary equipment typically voltage on the rotor shaft. As this induced results in annual non-energy operating voltage builds, it can discharge to ground expenses that are, at a minimum, equal through the motor’s bearings. This to 20% of the system’s initial total phenomenon repeats on a continuous basis. installed costs. This repeating cycle of voltage build-up and discharge causes pitting of the motor’s Eddy Current Drives: bearings and bearing race in a manner similar to the process of Electric Discharge Eddy Current Drives were first introduced in Machining (EDM). This pitting can become the 1930’s for use on railcars. Their use quite severe over time – creating a pattern quickly spread throughout industry such that of grooves in the bearing race. This pattern the leading supplier, Eaton, was selling over of wear is called “fluting”. $100 million per year of these drives during the 1970’s. Since that time, there has been When considering a VFD solution for an very little product innovation in this category adjustable speed application, many users of VSD’s and relative market share has neglect to consider the ancillary equipment fallen. Two of the largest remaining often needed when comparing maintenance suppliers in North America of Eddy Current and operating costs. Air conditioning Drive technology are TECO/Westinghouse systems and transformers both require and Dynamatic. regularly scheduled maintenance. At a minimum, preventative maintenance for a Eddy Current Drives use an electromagnetic transformer includes: power factor testing, coupling between the motor and the driven oil level checks, oil sampling and testing, load. The motor runs at its rated speed at max and min temperature checks, checking
  • 5. all times while the speed of the load is these drives every two years which may adjusted. There are two basic types of Eddy require a full dismantling and rebuild of the Current Drives currently on the market – Eddy Current Drive system. foot-mounted (Figure 5) and shaft-mounted (Figure 6). Figure 7: Eddy Current Drive Schematic In addition to the above maintenance, proper Eddy Current Drive operation is Figure 5: Foot Mounted Eddy Current Drive dependent on the dissipation of heat within the drive’s components. Annual inspections of the cooling air passages within the drive are critical, especially in dusty environments. End users who are considering the installation of Shaft Mounted Eddy Current Drives also need to include the maintenance and operating costs associated with the use of belts and pulleys. As mentioned earlier, the non-energy operating costs associated Figure 6: Shaft Mounted Eddy Current Drive with these systems can be substantial when compared to other VSD system options. The basic principle of operation of an Eddy Current Drive is that an armature, typically a A recent case study published by a North steel drum that may have another American municipality determined that conductive material as a lining, is attached there are significant costs associated to the drive motor. This armature assembly with maintaining and operating Eddy turns at the nameplate speed of the motor at Current Drives in their process! On all times. Attached to the load shaft is a average, the non-energy operating costs multi-pole electromagnet. A variable DC associated with the use of Eddy Current current is supplied to the electromagnet. As Drives can be as high as 18% of the the current to the electromagnet is system’s initial total installed cost. increased, the magnetic field increases. As the magnetic field moves relative to the Fluid Drives: armature, eddy currents are created, magnetically coupling the two elements and transferring torque from the motor to the The principle of using fluid to transmit power load. Alignment between the armature and was introduced in 1905 by the Vulcan the electromagnet is maintained with Engineering Company and was originally supporting bearings within the drive unit applied to driving a low-speed ship’s (Figure 7). propeller with a high-speed steam turbine. The advantage of this new Fluid Drive was While Eddy Current Drives use magnetically that it allowed for speed reduction without induced forces to transfer torque, they still the use of complicated and temperamental depend on a series of bearing surfaces to gear systems. In 1930 the first variable keep their rotating components aligned. In speed Fluid Drive was installed in England fact, Eddy Current Drives typically have up with the first U.S. installation following to four more bearings than the motor they shortly in 1932. At the time of introduction, are attached to. As the number of bearing Fluid Drives offered the benefits of smooth surfaces increases, so to does the likelihood and continuous control of speed using a of bearing failure. Also, end users need to constant speed motor. Compared to the replace brushes and dress the slip rings in options available at the time (gears and
  • 6. pulley systems), the Fluid Drive was an Fluid Drives have been used in industrial attractive technology. applications for over 100 years. Smooth load speed control has been the While the technology has continued to be advertised benefit of this technology. available to this time, the operating However, when long-term non-energy principles and designs of Fluid Drives have operating costs are considered the remained rather stagnant. The largest Fluid choice of a Fluid Drive for adjustable Drive suppliers today, (Falk and Voith), have speed applications quickly becomes a introduced small changes to their drives, but questionable decision. The age of this overall the technology is the same as it was technology’s installed base combined in the 1930’s. with the industry’s maintenance experience produce annual non-energy When considering the installation of a Fluid operating costs in excess of 15% of Drive, a process owner needs to consider initial installed costs. the extensive maintenance activities required of this VSD technology. Proper Permanent Magnet Adjustable operation of industrial Fluid Drives is Speed Drives dependent on the volume and pressure of oil flow through the system. Heat dissipation, The introduction of Permanent Magnet transmitted power, and component Adjustable Speed Drives (PMASD’s) by lubrication are all dependent on proper oil MagnaDrive Corporation, a U.S. company flow. Excessive system vibration (such as based in Bellevue, Washington, has given that caused by shaft misalignment) can the process industry another option to result in damage to the bearings, shafts and consider when reviewing adjustable speed sealing components of a fluid drive. When applications. The company’s technology vibration is excessive, damage to the drive’s has one of the fastest adoption rates for a internal rotating components is also a very new introduction in its targeted industries real possibility. with over 5,000 installations currently operating. A major industrial fluid drive maintenance shop in the U.S. lists the following PMASD‘s consist of two primary maintenance trends and concerns components. The first component, a set of associated with fluid drives: copper conductor plates, is connected to the • Ball or Roller Bearing Wear motor shaft; the second component of the • Internal oil pump impeller wear permanent magnet drive is a rigid assembly • Internal oil pump reversing valves of permanent, rare earth magnets which is • Shaft sealing surfaces connected to the load. During operation, • Scoop tube assembly and seals relative motion between the parts creates an • Gaskets and o-ring condition interwoven, eddy current field that transmits • Oil quality/contamination torque across the air gap. To adjust the • Vibration speed of the driven load, the amount of • Oil cooing system torque transmitted from the motor to its load • Wear of the input shaft thrust bearing is controlled by changing the distance • Fatigue cracking of input rotor vanes between the conductor plates and the magnet assembly. During operation and • Fatigue cracking of lower bearing support throughout the entire speed range, no physical connection exists between the In addition, it is suggested in industrial motor and the load (Figure 8). This maintenance bulletins that the stresses “disconnected connection” has experienced in fluid drive system subjected demonstrated vibration reductions up to to repeated start/stop cycles can increase 85%. the loads experienced by the internal rotors. These increased loads can cause damage that shows up as either increased vibration or unstable load speed.
  • 7. across all industries. In comparison to available baseline energy information, the use of a permanent magnet drive can result in significant non-energy operational savings with annual non- energy operating costs as low as 3% of initial installed costs. References: Figure 8: Operation of Permanent Magnet ASD 1. AC Drive Worldwide Outlook. ARC Advisory Group Market Study. The advantage of PMASD’s is found in the 2. California Energy Commission. Energy fact that they operate without any physical Solutions For California Industry: Ways connection between the driver and the load. To Improve Operations And Profitability. By completely isolating these components 2001. from each other, PMASD’s eliminate the 3. Coyote Electronics, Inc. Fort Worth, TX. alignment-related issues experienced by all 4. Dabbs, Tom, The True Cost of traditional VSD technologies. The result is Maintenance, Pumps & Systems increased seal and bearing life in the overall Magazine, April 2004, pump-zone.com system and a reduction in the cost of 5. DSI – Dynamatic, Inc., Sturtevant, WI. operating and maintaining that system over 6. Emerson Electric, St. Louis, MO. its life. 7. Frost & Sullivan. European Variable Also, because most PMASD’s require no Speed Drives Market Report. 2005. ancillary equipment, the cost of maintaining 8. Frost & Sullivan. North American and operating these additional components Variable Speed Drives Market Report. 2005. is completely avoided. Larger horsepower PMASD’s may require water cooling to 9. Hydraulic Institute, Pump Life Cycle Costs, 2005. prevent excessive heat build-up in the conductor rotors. In these applications, the 10. Iowa Energy Center website. minimal cost of maintaining a heat www.energy.iastate.edu/news/pr/pr- acmaintenance.html exchanger skid does need to be considered. 11. Jones, Garr. Pumping Station Design, Butterworth-Heinemann; 3rd ed. Woburn, However, the costs associated with these MA, 2005. systems are historically low, comparing very favorably to compressed refrigerant cooling 12. MagnaDrive Corp., Bellevue, WA. systems. 13. Maintenance Resource.com website. www.maintenanceresource.com. PMASAD construction is, from both a 14. PowerFlow Engineering, Inc. Westland, mechanical and electrical design point of MI. PowerFlow Engineering Maintenance Bulletins. www.power- view, identical to the construction of the flowengineer.com/bulletins.html. electrical motors they are typically attached to. PMASD manufacturer’s maintenance and 15. Progress Energy. Electrical Maintenance Workshop: Transformer Maintenance & operations manuals suggest that the only Testing. 2003. http://www.progress- required maintenance is to lubricate any energy.com/custservice/carcig/resourcec bearing surfaces on the same schedule as is tr/presentations/ElectricalTestingWorksh opk.pdf used for the motor bearings. One manufacturer’s maintenance recommenda- 16. Reliance Electric. Maintenance and tion for motor lubrication in standard duty Troubleshooting of Electric Motors website. www.reliance.com. applications is to check and re-lubricate every 1 to 7 years depending on the size of 17. Stock Equipment Company. Chagrin Falls, OH. the motor. www.stockequipment.com/solution1.asp The potential value of permanent magnet 18. US Department of Energy. Industrial Electric Motor Systems Market Study. technology is clear. In site evaluations, the permanent magnet coupling concept 19. Voith Turbo GmbH & Co. KG. Crailsheim, Germany for speed control has been proven
  • 8. Total System Non-Energy Operating Cost Comparison of Various Technologies Annual Cost of Technology Typical Failure Maintenance and Repair as Modes a Percentage of Installed Cost • System penetration leaks • Mechanical linkage hysteresis & Restrictive Flow deadband • Cavitation / Flashing damage Control 20% • Misalignment between driver and (Valves or Dampers) load causing premature seal & bearing wear • Radial loads on bearings & pump seals causing premature wear • Belt wear • Environmental degradation of belt Pulley Systems 30% materials • Improper tensioning causing loss of power • Misalignment between driver and load causing premature seal & bearing wear • Electronic component failure • Bearing fluting VFD’s 20% • Failure of ancillary equipment • Overheating of circuitry • Motor winding damage due to insufficient cooling air flow • Misalignment between driver and load causing premature seal & bearing wear • Mechanical weakening of Eddy Current Drives components due to rapidly changing 18% electrical currents • Complex and aging controls • Replacing brushes • Misalignment between driver and load causing premature seal & bearing wear • Fluid leakage leading to loss of Fluid Drives 15% power • Inadequate cooling of torque transfer fluids • Demagnetization caused by operation outside of specified range. PMASD’s 3% • Failure of actuator