Edwards Engineering Corporation

Transcription

1 SUGAR MILL DRIVE CONSIDERATIONS One of the most important tasks of a sugar factory manager is deciding on how best to invest limited resources on capital expenditures such as sugar mill drives. In an attempt to provide some assistance in this regard we have prepared the following document outlining and explaining the different parameters that should be considered prior to making a decision. PARAMETERS ELECTRIC DRIVES HYDRAULIC DRIVES MECHANICAL DRIVES REMARKS Low Rotating Inertia No Yes No See Note 1 Shock Absorbing Capability No Yes No See Note 2 Instant, Accurate, Repeatable Torque Limiting No Yes No See Note 3 Easily Reversible Yes, but $$ Yes Yes, but $$ See Note 4 Impervious To Ambient (Hot, Dusty, Damp) Conditions Load Sharing Between Two Drive Units On Same Shaft No Yes Yes See Note 5 Yes, but $$ Yes No See Note 6 Low Maintenance No Yes Yes See Note 7 Harmful System Harmonics Yes No No See Note 8 Need For Special Rooms To Contain Switchgear, Etc. Yes No No See Note 9 Ability To Start Under Full-Load Yes & No Yes No See Note 10 Ability To Mount To The Rolls Directly Eliminating Costly Foundations, Couplings, Shafts, Etc. Yes, but not practical Yes No See Note 11 Flexibility In Drive Layout No Yes No See Note 12 Versatility Possibly Yes No See Note 13 Power To Size Ratio Good Excellent Better See Note 14 Smooth Step-Less Speed Control From 0 To Max. Speed Ability To Drive Each Roll Individually No Yes No See Note 15 Possibly Yes No See Note 16 Reliability, Durability Good Better Best See Note 17 Overall Efficiency??? See Note 18 Costs??? See Note 19

2 Notes: Edwards Engineering Corporation 1. Low Rotating Inertia: This is very important parameter because rotating inertia is the major cause of broken components in mechanical transmission in sugar factories. When a steam turbine or electric motor and gear reducers as well as the associated couplings, pinions, gears, etc. are spinning at several thousand RPM, or even 1500 RPM, and the mill jams reducing the speed to zero in the blink of an eye, something is going to break because the rotating inertia will not allow the transmission to stop that fast. Not so with low-speed hydraulic motors. One of the hidden benefits of using a hydraulic drive for a sugar mill is the absence of broken shafts, couplings, gears, etc., typically found with mills driven by electric or mechanical means. This represents not only a savings in the replacement cost of the broken or damaged items, but also a savings in reducing down time. 2. Shock Absorbing Capability: In addition to inertia, shock is another frequent cause of equipment failure. Electro-mechanical or thermo-mechanical mill drives are not good shock absorbers, but hydraulic drives on the otherhand have an inherent ability to absorb shock because the hydraulic fluid used to transmit the power. Shock loading is one of the reasons that electro-mechanical and thermo-mechanical sugar mill drives are normally required to have a 2.0 service factor, but hydraulic mill drives can be selected with a 1.5 service factor. 3. Fast, Accurate and Dependable Torque Limiting to Eliminate Damage to Mill Rolls, Shafts, and Crown Gears, etc.: Because hydraulic drives using low-speed, high-torque (LSHT) hydraulic motors have very low rotating inertia the torque can be controlled very accurately within + / - 3% and quicker than one can blink their eyes. Torque limiting is a misnomer when using electro-mechanical or thermo-mechanical mill drives. There is simply too much inertia in these types of transmissions to be able to accurate and reliably control the torque. Mill house floors, at sugar factories around the world, are littered with broken parts to attest to this statement but at EEC we are so confident in the ability to limit the 2

3 torque with our drives that we offer to replace any gears, couplings or shafts that break. We haven t had any takers in the twenty years we have been offering hydraulic mill drives. 4. Fast and Easy Reversing: A seldom needed but a greatly appreciated feature inherently available with all hydraulic transmissions. Also available with electric transmissions, but at a significantly higher cost. 5. Unaffected by Ambient Conditions such as Dust, High Humidity, Heat, etc.: A frequently needed and greatly appreciated feature inherently available with all hydraulic transmissions, but not with electro-mechanical transmissions. See note Load sharing between multiple drive units on same shaft: A little known benefit of hydraulic drives that is very desirable if ever the need for driving both ends a of mill roll. For example; if driving both ends of the top roll as in an Edwards Engineering Assist Drive. This can be done with electric drives too, but very complicated and costly. With mechanical transmissions it is virtual impossible! 7. Low Maintenance Requirement: Depending on the type of variable speed electric drive the maintenance will be either high or very high. DC motors in a sugar factory environment will need frequent servicing (monthly). AC variable frequency motors will also require servicing, but less frequently. Hydraulic and thermo-mechanical transmissions on the otherhand will require very minimal maintenance. Hydraulic mill drives from EEC are practically maintenance-free. Typically, only the filters and oil need occasional attention. The maintenance is so low that we frequently have offered to perform all needed maintenance for the first season or two. 8. Harmful system harmonics: An often overlooked but potentially a big problem is system harmonics that can be generated by variable speed electric drives containing 3

4 semiconductors. These harmonics if fed back into the grid can cause severe damage to computers and other electric equipment on the grid and may interfere with process controls. Some countries and municipalities have recently enacted laws requiring harmonic filters to be utilized to counter this potential problem. These filters involve considerable capital outlay. Hydraulic drives from EEC use standard AC motors (or steam turbines) to drive the hydraulic pumps. Therefore, harmful harmonics cannot be produced and fed back into the grid. 9. Need for special rooms to contain switchgear, etc.: DC and variable frequency AC drive systems often require a specially designed room to house the switchgear. Depending on the type of variable speed electric drive utilized, this room will have to be air-conditioned and the air will need to be filtered before entering it to protect the equipment from normal atmospheric conditions found in sugar factories. 10. Ability to Start Under Load: The hydraulic drives are capable of starting fully loaded. This is helpful if a stall occurs while milling. A variable frequency electric motor can only produce approximately 40% of full load torque at start up. However, a variable speed DC drive has the ability to start fully loaded. 11. Ability to mount to the rolls directly eliminating costly foundations, couplings, shafts, etc.: Since hydraulic drives are smaller, lighter and more compact than electro-mechanical or thermo-mechanical drives they are often shaft mounted directly to the mill rolls. This is possible but not as practical with electro-mechanical drives and virtually impossible with thermo-mechanical drives. This feature is very desirable for new installations because there is a substantial savings to be had in eliminating the expensive foundation, shafts, couplings, gears, etc. 4

5 12. Flexibility in Drive Layout: With electro-mechanical or thermo-mechanical transmissions there is seldom any options to choose from when laying out the transmission but because the power source of a hydraulic transmission is connected to the hydraulic drive motor(s) via flexible hoses and / piping design engineers are free to layout the equipment according what is most practical and not forced to live with compromises. 13. Versatility: The options available to the design engineer when driving a sugar mill with an electro-mechanical or thermo-mechanical transmission are very limited. They consist mainly of connecting the transmission to the top roll of the mill through a series of couplings, shafts, and reducers. Hydraulic transmissions on the otherhand can be connected to the mill in a similar way or directly to the mill top roll from one end or from both ends or connected to each roll individually allowing the crown gears to be eliminated and thus allowing the individual rolls to be driven at differential speeds. Only hydraulic transmissions offer this versatility. Please see the attached application drawings for more information. Edwards Engineering Assist Drives are another example of the versatility of a hydraulic transmission. 14. Power to Size Ratio: This may not seem like an important parameter to consider but it is and I ll explain why. Sugar factories are always trying to cram more equipment into smaller spaces. A good example is the sugar factory that wants to expand production (again) and there is little or no room for larger mill drives. That is where hydraulic transmissions are again the preferred choice because nothing packs more power into a limited amount of space than a hydraulic transmission. Typically hydraulic transmissions will be1/3 the size of a comparable thermo-mechanical transmission and 1/2 the size of a comparable electromechanical transmission. In addition, the power unit can located anywhere there is a suitable space, even it that space is nowhere near the milling floor. Another example of the versatility of hydraulic transmissions. 5

6 15. Smooth, Step-less Speed Control from zero to maximum speed. Variable speed electric and mechanical drives offer step-less speed control but not down to zero RPM. However, inching the mill during off-season maintenance is not a problem with a hydraulic transmission but it could be with an electro-mechanical solution. Therefore, a separate inching drive will need to be considered when using an electromechanical or thermomechanical mill drive. 16. Ability to drive each mill roll individually, This is something in vogue at the moment because of the possibility of greater extraction and higher efficiency. Some studies have been done and are being done to see if in fact differential roll speeds will result in high extraction. Greater efficiency is a reality and simple to prove because the tail bar and box couplings along with the crown gears can be eliminated when driving each roll directly. Efficiency was never a big concern at sugar factories in the past because they made their own electricity and always had enough to allow wasting it but greater efficiency is the goal now, especially of at co-generation sugar factories because a kilowatt saved is a kilowatt that can be sold to the grid. Driving each mill roll individually is not possible with a thermomechanical drive solution and complicated & impractical with electro-mechanical drives. See note Reliability, Durability. Hydraulic mill drives have been in use as sugar mill drives for approximately twenty years now and their reliability and durability has been proven. This evident by their increasing popularity and by the fact that most mill operators are placing repeat orders. If hydraulic drives were not all they are purported to be then there would not be as many repeat orders as there are. The reliability and durability of hydraulic mill drives is only surpassed by the reliability of mechanical transmissions. Few mechanical devices or more reliable on this planet than a properly maintain steam turbine. On the otherhand, the reliability of variable speed electro-mechanical mill drives is still to be fully determined. This 6

7 matter was recently mentioned in a technical paper delivered by the renowned Raymond Ravilland at a sugar factory engineering workshop held in Germany last October The major limiting factor with such installations is the reliability of the frequency converters electronic circuits cards, which require a number of (expensive) spare cards to be kept in stock Efficiency: All variable speed drives (electrical, hydraulic or mechanical) have a variable efficiency. Therefore, it is difficult to say precisely what the efficiency is without knowing all the different variables. However, we have produced a chart to help explain the typical efficiencies of the seven most popular mill drive solutions. This chart (attached) should be used as a guideline only. 19. Costs: Of all of the parameters to be considered this is the one that gets the most attention and unfortunately it will probably have the most influence on the final decision. When one considers that a mill drive is expected to last 20 to 25 years minimum then why is it that the least expensive solution is the only one that most factory managers are interested in? Usually, little or no value is assigned to the other eighteen parameters. Ideally factory managers should be purchasing the drive system with the best value, and not the one with the lowest price. A drive system that has the lowest price but the highest maintenance cost or lowest efficiency, etc. is not necessarily the best choice but too often it is the final choice. Often the advantages of a higher priced drive systems will result in a savings elsewhere and be recouped in a matter of months if only a proper cost / benefit analysis is performed. To give an exact figure on the costs for each different drive solution would be impossible due to the many different variables. For example, how does one compare the cost of a roll mounted hydraulic drive that eliminates the need for the foundation, shafts and couplings to a mechanical drive that cannot possibly be mounted in the same way. Furthermore, price comparisons should only between comparable drive solutions. For example, compare 7

8 hydraulic against hydraulic or electric against electric, etc. Otherwise it is like comparing an apple to an orange. RECOMMENDATIONS: A. Since there are three basic ways to drive a sugar mill (electrically, hydraulically and mechanically) all three should be considered when shopping for a new or replacement mill drive. No having competent hydraulic engineers on staff should not be an excuse. B. Always take into consideration the various drive parameters and if possible assign a value to each. C. Electro-mechanical and thermo-mechanical mill drives are frequently purchased based a written specification but seldom are hydraulic mill drives. Sugar factory engineers are usually comfortable and knowledgeable in electro-mechanical and / or thermo-mechanical drives and will eagerly produce a written specification for all vendors to bid to, but not so with hydraulic drives and the result often is dissatisfaction with the drive system when it arrives and begins to work. Therefore, always make sure that all drive suppliers, including hydraulic, are required to bid to a written specification. If your sugar factory does not have a corporate specification for hydraulic drives then produce one, or have one produced. The same goes for a hydraulic piping specification. We can help here, if needed. D. Choose the drive system with the best value, not the lowest price. The sweet taste of a low price is long forgotten when the bitter taste of dissatisfaction arises. E. If you decide on a hydraulic mill drive make sure that training of your engineers and technicians is a part of the equipment purchase. Hydraulic mill drives do not have to be intimidating! ### 8