Hands On Water and Wastewater Equipment Maintenance

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title:Hands-on Water/wastewater Equipment Maintenanceauthor:Renner, Don.publisher:Taylor & Francis Routledgeisbn10 | asin:1566764289print isbn13:9781566764285ebook isbn13:9780585226118language:EnglishsubjectWater treatment plants--Equipment and supplies--Maintenance and repair, Sewage disposal plants--Equipment and supplies--Maintenance and repair.publication date:1998lcc:TD434.R45 1998ebddc:628.1/028/8subject:Water treatment plants--Equipment and supplies--Maintenance and repair, Sewage disposal plants--Equipment and supplies--Maintenance and repair.

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Chapter 1

Introduction to Maintenance

The Need for Maintenance

1.01 Maintenance is a part of everyday life, although little thought is given to some of the more routine ''chores," such as car and home repairs, lawn and shubbery care, painting, and many other items. However, even though these chores are considered the preservation of property or equipment, they are a form of maintenance. And when you look at the bottom line, maintenance really is the preservation of property.

1.02 The importance of water/wastewater plant maintenance has been discussed and written about for many years. It is a well-known fact that many plants suffer severe operating problems because of poor maintenance. When the topic is discussed, much attention is given to maintenance concepts and the overall program, but the basic fundamentals and details are overlooked or ignored. Following good basic maintenance procedures is like keeping your vehicle filled with fuel. Without fuel the vehicle will stop. Without proper maintenance, the equipment and your plant will shut down.

1.03 The maintenance program for water/wastewater plants should include not only the items that make the plant run efficiently but look good as well. Your plant is often judged by its appearance and not by how you treat the water. Maintenance procedures should be given a high priority in the daily work schedule, because making sure that the plant equipment functions properly is an important factor in achieving the proper water discharge quality. If the equipment does not operate properly, water quality standards cannot be met.

1.04 The need for a good maintenance program and the knowledge of how each component functions and should be maintained cannot be emphasized enough. By properly maintaining plant equipment, an operator can extend its operating life by at least 25 percent. The extended equipment life means less capital replacement expenses, better plant operation, and even better public relations. It should be a source of pride for both the employees and management.

The Keys to Good Maintenance

1.05 The performance of maintenance duties or tasks is only a small part of the maintenance function. Although it is important that you understand how to repair a piece of equipment, it is also important that you understand how the component performs its operation as well as some of its design, engineering, and construction features. Understanding the more "technical" aspects of various equipment components should help you gain knowledge of why breakdowns and failures occur. This extra knowledge should help you improve your maintenance program, increase equipment life, and make your job easier.

1.06 Waste or wastewater operators are usually very professional in the manner in which they run their plants. Setting up a maintenance program and performing routine maintenance are just other steps in operating the plant. There is nothing special that is required to set up a maintenance program or to see that the one you have works well. If you do not have the time to develop your own program, there are a number of "packaged" programs available, including manual and computer-driven types. chapter 2 explains more on establishing a maintenance program.

Scope of the Maintenance Program

1.07 All water or wastewater plants have different maintenance requirements. Location, size, staff, and funding all determine how a maintenance program should be set up. Although many design engineers recommend maintenance procedures, only plant personnel can determine what sort of program best suits their individual needs. However, to be effective and efficient, the program should be all-inclusive and not be limited to only the major plant components. The items listed in Figure 1.1 should be used as a guide to establishing a complete maintenance program.

1.08 You must consider not only how broad your mainte-

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planning is especially important if outside contractors are involved. Although outside contractors are generally cooperative, they sometimes have other commitments. Having a good plan and schedule is necessary to ensure that the work is done on a timely basis and that the repair costs and the outage time are kept to a minimum.

2.51 If you have only one well pump and it requires an overhaul or other preventive maintenance, some alternate means must be provided to keep you pumping water while the pump is not in service. The same condition applies if a motor control center or other major electrical system must be taken out of service. Except for extreme emergencies (and even these should be limited), no operation should require a total plant shutdown. Even a power failure should be planned for by having some sort of emergency power supply.

2.52 The key to a smooth operating preventive maintenance program is a good schedule. Planning your work and following the schedule is much like watching the fuel gauge on your vehicle. When the gauge shows the tank is less than one-half full, it is time to start planning on how much further you can drive before you have to fill the tank. Your preventive maintenance schedule performs the same function as your fuel gauge. As you approach a period of maintenance, that is the time to organize the work that is to be accomplished and get all of the necessary parts, tools, and other items together so the work can be performed in an efficient manner.


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Figure7.26.

Mountabletimers.

7.97 For example, if the discharge flow from a booster station was 2000 gpm maximum, three pumps each rated at 1000 gpm would be installed. This would permit two pumps to meet the demand, while permitting the third pump to act as a standby unit when one of the other two failed or was taken out of service. By alternating all three pumps, the wear that takes place would be more balanced and the total life expectancy of the pumps would improve. Remember that under these operating conditions all three pumps would wear out at approximately the same time.

7.98 If equipment is alternated evenly, some thought must be given as to how the equipment will be maintained to prevent one from breaking down while the other might be out of service for other maintenance. Because not all equipment wears out at the same rate, some provision should be made to perform a major overhaul of one unit ahead of the other if both are scheduled for the same time. A year early would be a suggested time frame. Also, scheduling a preventive maintenance inspection (by disassembling the unit) would help identify the amount of wear that was taking place and how soon an overhaul would be required.

7.99 There are two general methods used to alternate pumps and other devices. One is by the use of alternating relays (Figure 7.28), where only one unit is required at any one time (such as air compressors). For these applications, the alternating relay shifts the electrical power from one motor to the other each time a motor runs. The power transfer actually takes place after the first motor shuts off and the control circuit is deenergized.

7.100 The second method requires both alternating and duplexing of the pumps, such as in lift stations where both pumps may be required to run at the same time (paragraphs 7.51, 7.52, and Figure 7.15). For these applications, a single alternating relay can be modified with other controls, or a set of three relays can be used (Figure 7.28) to provide an alternating/duplexing operation.

7.101 Alternating relays might also be set up for a lead and lag arrangement where one pump performs most of the work, whereas the second (or third) pump only runs when the demand is high. Whichever arrangement is used, a good preventive maintenance program that includes not only the pumps or motors but also the controller is important to maintaining an efficient operating plant.

Other Relays and Switches

7.102 Water or wastewater treatment plants use many other types of control relays and switches besides those that have already been described. Pressure, flow, liquid-float level, chemical content, limit switches, surge suppressors, position indicators, push-button stations, proximity, photoelectric, programmable controllers, and temperature sensor or relays are just a few. Whatever type or style switch, relay, or sensing device is installed, it usually operates with either a set of normally open (NO) or normally closed (NC) contacts that are placed in the electrical control circuit.

7.103 Control devices are important to the proper and safe operation of any and all equipment. Therefore, when


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Figure7.27.

Pneumatictimingrelay.


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Figure7.28.

Alternatingrelays.

equipment fails to operate, check out not only the main electrical components but also the control devices. A malfunctioning relay can shut down the motor easier than a bearing failure.

Motor Control Enclosures

7.104 Virtually all treatment plant electrical equipment is placed in some style of housing. The housings provide not only a mounting support for the various electrical wires and components but also protection of the equipment from the environment in which they are placed. The enclosures range in size from a small single manual switch mounted on the wall to switchgear and motor control centers. All housing styles and designs are based on, and made in accordance with, standards that have been established by NEMA, NEC, UL (Underwriters Laboratories), and other electrical and testing associations.

7.105 Enclosure construction is divided into two general location classificationsnonhazardous and hazardous. Each general classification is further divided into subgroups that more clearly define their use and application (Table 7.4). A detailed description of some of the various enclosures is given in Figure 7.29.

7.106 Remember that these enclosures all provide specific levels of protection. If additional equipment protection is desired, it can be specified. For example, a type 4 enclosure could be specified in place of a type 1 enclosure if it was to be located near a dusty or moist environment, or a heavier enclosure might be used in an area that might be subjected to vandalism.

Motor Controller Maintenance

7.107 Maintaining electrical components requires a number of different procedures and practices, all of which are equally important. A few of the procedures that should be followed include:

(1) Establishing and following a good preventive maintenance schedule.

(2) Keeping a record of all tests and repairs.

(3) A good understanding of the electrical system and its components.

(4) Having a set of ladder and schematic drawings of the system and all controls.

(5) Following safe working habits.

(6) A file of manufacturers instruction manuals.

(7) A supply of necessary spare parts.

(8) Proper test equipment.

(9) Proper tools to perform the tests and repairs.

(10) Following an analytical and systematic approach to troubleshooting and solving a problem.


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TABLE 7.4. Enclosure Classifications.


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TYPE 1

Type 1 enclosures are intended for indoor use primarily to provide a degree of protection against contact with the enclosed equipment in locations where unusual service conditions do not exist. The enclosures shall meet the rod entry and rust resistance design tests.

TYPE 3

Type 3 enclosures are intended for outdoor use primarily to provide a degree of protection against windblown dust, rain and sleet; and to be undamaged by the formation of ice on the enclosure. They shall meet rain, external icing, dust, and rust resistance design tests. They are not intended to provide protection against conditions such as internal condensation or internal icing.

TYPE 3R

Type 3R enclosures are intended for outdoor use primarily to provide a degree of protection against falling rain; and to be undamaged by the formation of ice on the enclosure. They shall meet rod entry, rain, external icing, and rust resistance design tests. They are not intended to provide protection against conditions such as dust, internal condensation, or internal icing.

TYPE 4

Type 4 enclosures are intended for indoor or outdoor use primarily to provide a degree of protection against windblown dust and rain, splashing water, and hose directed water; and to be undamaged by the formation of ice on the enclosure. They shall meet hosedown, external icing, and rust resistance designtests. They are not intended to provide protection against conditions such as internal condensation or internal icing.

TYPE 4X

Type 4X enclosures are intended for indoor or outdoor use primarily to provide a degree of protection against corrosion, windblown dust and rain, splashing water, and hose-directed water; and to be undamaged by the formation of ice on the enclosure. They shall meet hosedown, externalicing, and corrosion-resistance design tests. They are not intended to provide protection against conditions such as internal condensation or internal icing.

Shall be manufactured of American Iron and Steel Institute Type 304 Stainless steel, polymerics, or materials with equivalent corrosion resistance, to provide a degree of protection against specific corrosive agents.

TYPE 6

Type 6 enclosures are intended for indoor or outdoor use primarily to provide a degree of protection against the entry of water during occasional temporary submersion at a limited depth.

Type 6P enclosures are intended for indoor or outdoor use primarily to provide a degree of protection against the entry of water during prolonged submersion at a limited depth.

TYPE 7

Type 7 enclosures are for indoor use in locations classified as Class I, Groups A, B, C or D, as defined in the National Electrical Code.

Type 7 enclosures shall be capable of withstanding the pressures resulting from an internal explosion of specified gases, and contain such an explosion sufficiently that anexplosive gas-air mixture existing in the atmosphere surrounding the enclosure will not be ignited. Enclosed heat generating devices shall not cause external surfaces to reach temperatures capable of igniting explosive gas-air mixtures in the surrounding atmosphere. Enclosures shall meet explosion, hydrostatic, and temperature design tests.

TYPE 9

Type 9 enclosures are intended for indoor use in locations classified as Class II Groups E or G, as defined in the National Electrical Code.

Type 9 enclosures shall be capable of preventing the entrance of dust. Enclosed heat generating devices shall not cause external surfaces to reach temperatures capable of igniting or discoloring dust on the enclosure or igniting dust-air mixtures in the surroundingatmosphere. Enclosures shall meet dust penetration and temperature design tests, and aging of gaskets (if used).

Class I - Flammable gases or vapors.

Class II - Combustible dust.

Class III - Ignitable fibers or flyings.

Division I - Normal situation; the hazard would be expected to be present in everyday repair and maintenance.

Division II - Abnormal situation; the material is expected to be confined within closed containers or closed systems and will be present only during accidental rupture, breakage or unusual faulty operation.

Groups

Class I - Gases and vapors are broken into 4 groups A, B, C, and D, depending on the ignition temperature of the substance, its explosion pressure and other flammable characteristics.

Class II - Dust locations are broken into groups E, F and G according to the ignition temperature and conductivity of the hazardous substance.

TYPE 12

Type 12 enclosures are intended for indoor use primarily to provide a degree of protection against dust, falling dirt, and dripping non corrosive liquids. They shall meet drip, dust, and rust resistance design tests. They are not intended to provide protection against conditions such as internal condensation.

Fumas NEMA 12 may be field modified for outdoor use. NEMA 3 requires the use of watertight conduit hubs. NEMA 3R requires the use of watertight conduit hubs at a level above the lowest live part and drain holes of diameter shall be added at the bottom of the enclosure.

TYPE 13

Type 13 enclosures are intended for indoor use primarily to provide a degree of protection against dust, spraying of water, oil and non corrosive coolant. They shall meet oil explosion and rust resistance design tests. They are not intended to provide protection against conditions such as internal condensation.FURNAS ELECTRIC COMPANY

Figure 7.29. Control enclosure description.


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TABLE 7.5. Controller Maintenance Schedule.

Task

Frequency

Inspect all controller components. Check for heat damage, wear, fatigue, loose connections, pitting of contacts, etc.

Quarterly

Vacuum dust and dirt from components.Quarterly

Vacuum or wipe out cabinets.Quarterly

Check blowers and cooling fans.Quarterly

Replace/clean air filters.Quarterly

Test and check all batteries.Quarterly

Electrically test all circuits for grounds, shorts, and continuity.

Semi-annually

Check and tighten all electrical terminals and connections (including the main bus bar)

Annuallly

7.108 Before attempting to work on any electrical components, make sure that the power is turned off and locked out. Any testing that is performed on "hot" circuits should be done with extreme caution and then very carefully. Also, if there is any question about the electrical system that cannot be explained, call in an outside contractor to do the work. It is more economical to have someone else perform the work than to have an accident and/or damage equipment.

7.109 Having the proper test equipment is also important. A clamp-on multimeter (volts, amps, and ohms) capable of handling up to 600 volts is a basic necessity to check out controls and control circuitry. The multimeter should also be capable of testing for continuity and have a set of test leads. In addition, a megohmmeter might be helpful when testing insulation and resistance. There are many more fancy types of test equipment (including infrared scanners) that are available for specific applications, but they can often be rented at a more economical price when they are needed.

7.110 There is nothing magical about performing routine scheduled maintenance of electrical equipment. All components should be inspected on a 3- to 4-month cycle (Table 7.5) although it can be extended to 6-month intervals if the components show no signs of fatigue, wear, or heat damage. Part of the procedures requires a thorough inspection (and cleaning if necessary) of the control cabinet. If dust or dirt is present, it should be removed by vacuuming or wiping. Blowing out the cabinet will only spread the dirt to other components. Wet or dirty components that cannot be cleaned must be replaced.

7.111 Check all blowers and cooling fans in enclosures that have them. If the enclosure has air filters, replace them at least quarterly, more frequently if the environment surrounding the building has high levels of dust and pollutants.

7.112 Controller contacts should be checked for both wear and dirt accumulations. The use of spray cleaners on the contacts frequently leaves a residue on other surfaces that collects dirt. Contact surfaces that are discolored are normal and should be left alone. Even slight pitting of the contacts is acceptable, because filing and dressing of the surfaces remove metal and will shorten the contact's life.

7.113 Contact should be replaced when the surfaces become badly worn or pitted. Also, it is recommended that the entire contact set (including the springs) be replaced at the same time. If they are not replaced as a set, old or worn contacts will create irregular contact within the set, resulting in hot spots and possible voltage imbalance within the controller.

7.114 All terminals should be checked for tightness on an annual basis. After the power has been disconnected, check the bus bar, main line, and control wiring connections for tightness. Loose bus bar and main line connections cause arcing, voltage imbalance, and frequent tripping of motors during starting. Loose control wiring connections increase the hazard of electrical shock, improper equipment operation, and electromagnetic interference with other electronic controls.

7.115 Batteries, both dry and wet cell, are also a maintenance problem, especially those that are on a trickle charge for their entire life. Often, when they are most needed, they cannot sustain their design operating life because they have become weakened. Batteries should be dated when installed and then tested at 3-month intervals to determine the condition of the terminals, cell voltage (under load), and any charging boards or circuits.

7.116 In some cases, high-voltage tests are performed on cables to check their insulation quality. These are not routine tests and, therefore, should only be performed when necessary and only by qualified personnel using the proper test equipment. Also, disconnect all solid-state devices before the test is performed to protect them from damage.

7.117 If a short circuit or other fault occurs within any part of the electrical or control system, make sure that the problem is thoroughly investigated and that all damaged components are repaired or replaced before the power is restored. In fact, this is a good time to perform a routine maintenance check on the entire circuit, placing emphasis on loose or burnt connections.

Checking Out A Circuit

7.118 Regardless of the amount of maintenance that is performed, there will always be some time when some part of the electrical system will fail. When a failure does occur, it is important to troubleshoot the affected system and its components in a systematic manner. Even if the breakdown points to a particular item, it is still advantageous to check out the remainder of the system to ensure that no other damage or problems exist.

7.119 If a schematic drawing is not available, make a sketch of the points that have been checked. This ensures that nothing is missed and that everything checked out satisfactorily. As an example, the circuit diagram in Figure 7.30 shows a hypothetical system. To determine why the motor won't run requires checking more than one or two items.

7.120 The first thing to do would be to use a voltage tester and determine the presence of power at the fuses FU1, FU2, and FU3 if a disconnect switch was used. If a circuit breaker was used as a disconnect device, then the power supply


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Figure7.30.

Troubleshootingacircuit.

should be checked ahead of the breaker. Then check the circuit breaker to make sure it was not tripped. If it was tripped, then problems exist farther along in the circuit that require checking.

7.121 After checking the circuit breaker, the next step would be to check the transformer connections to make sure that low-voltage power (110 volts) was available to the control circuit. Next, test the control fuses FU4 and FU5 to make sure that there is power in the control circuit. If power is present, check the under voltage relay (UV) and contacts, and then the overload relays (OL) and contacts. Note: When checking contacts, make sure that they are in their proper positionnormally open or normally closed.

7.122 This particular circuit has a backspin timer (BT) placed in it. The BT contacts are normally open, so the timer must complete its timing out cycle before the contacts will close, completing the control circuit. Both the timer and the contacts should be checked for proper position. Next, check the hand/off/auto switch and the stop button. (The stop button is a normally closed switch and is important to the continuity of the circuit.) The final item to check would be the pilot relay and contacts.

7.123 This example has presented only the progression of testing. No attempt was made to locate a trouble spot. If one would have been found, a repair would have been made. Then the remainder of the circuit should have been tested to make sure no additional problems existed.

7.124 So far, all of the testing has been performed with a voltage tester. However, a continuity tester would also be helpful in checking out the circuit. If the circuit checked out properly, a clamp-on ammeter might be used to check the current draw and make sure that no imbalance exists among the three phases.

7.125 Remember: Even low voltage causes shocks. When testing circuits, be careful that you are not grounded to any equipment. Also, keep clear of all bare wires and test leads. And, as previously mentioned, call in outside help if a problem cannot be solved. Manufacturers and their distributors can often provide a valuable source of information and assistance.

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Figure8.1.

ControllingaD-CmotorpoweredbyA-Cvoltage.

amount of current flowing to the motor and, therefore, the motor speed. Most of the old rheostats have been replaced by more modern controls.

8.08 Today, almost all water or wastewater treatment plants operate on alternating current (A-C). Although D-C power is not available from power companies, plants that have a need for D-C power can obtain it by using an A-C/D-C motor generator set if the demand is large, or by using an A-C/D-C convertor placed in the drive cabinet.

8.09 The use of D-C motors in plants is generally limited, so the use of A-C/D-C convertors is the preferred way of supplying power to a D-C motor. Generally, the power conversion is accomplished by a series of rectifiers. However, to control the motor speed, the D-C power leaving the cabinet must be regulated. A typical example is shown in Figure 8.1.

8.10 Using a preset speed command to the motor controller allows the motor to start to rotate. As the motor rotates, the tachometer placed within the motor cases senses the speed of the rotating shaft. The tachometer sends a signal to the controller, which, in turn, compares the tachometer signal to the speed setting. When the motor exceeds the speed setting, the controller reduces the current flowing to the motor, slowing it down. As the motor slows down below the speed setting, the controller increases the current flow to the motor.

8.11 A number of different types of controllers are used to regulate or control the speed of a D-C motor. Phase controllers, SCR (silicon-controlled rectifiers), PWM (pulse width modulation), and field supply bridges are some of the more common methods presently being used. The selection of a controller will be based on the application, the type of motor with which it is used, and the preference of the design engineer.

8.12 A number of different D-C motors are presently being manufactured. They range from fractional horsepower to over 500 horsepower in size. The smaller motors, which are often used for motion control, are generally fitted with permanent magnets for the stator fields. Larger size motors have stators that are shunt, compound, and series wound. The type selected for any given application will be based on the desired operating capabilities of the motor.

8.13 Small D-C motors have construction features that are very similar to small A-C motors (Figure 8.2). The larger horsepower motors often have more of a square appearance and are fitted with blowers that provide cooling air to the armature and windings. Each type of motor has its own starting and running torque characteristics, depending on the type of the windings that are used. Additionally, there are many brushless and single-phase D-C motors that are manufactured.

Eddy-Current Drives

8.14 An alternate selection for a variable speed drive in place of a D-C drive is the eddy-current drive. Although eddy-current drives are not as well known as other types of variable speed drives, they have been manufactured for many years. They consist of an A-C constant speed motor that is connected to an eddy-current drive with both units placed within the same housing (Figure 8.3). Their controls are very simple and compact because they do not handle high voltages or amperage.

8.15 The principle of operation of the eddy-current drive is very simple. A high-strength iron drum (driven coupling) is mounted on the rotor shaft of the A-C motor. A second segmented iron drum (output clutch rotor) that


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Figure8.2.

D-Cmotors.


Page 11

Chapter 3

Lubrication

3.01 Lubrication is probably the most important and most frequently performed of all maintenance functions. While developing a preventive maintenance schedule, you will discover that lubrication is the foundation of every preventive maintenance program. Without proper lubrication, equipment will not only operate less efficiently but also will break down or require repairs more frequently.

3.02 A good lubrication program also provides a bonus because potential problems frequently can be detected before they become breakdowns. As routine lubrication is being performed, listen for noises, check for hot equipment, and look for things that are unusual (rust, discoloration, water stains, scoring, etc.). They are signs of future problems.

3.03 This chapter provides many facts about lubricants. The intent is to make you aware of some of the variety of lubricants that are available to water or wastewater treatment plants, how they are formulated, and some of their applications. The reason for lubrication and its benefits are also explained. Not all of this information may apply to your plant. However, keep in mind that the more knowledge gained about products and equipment, the easier your job becomes and the better the plant and equipment will be maintained.

Reasons for Lubrication

3.04 Everyone knows that it is important to lubricate machinery, however, not everyone understands the reasons behind lubrication. All contacting and moving surfaces create friction and cause wear on both contacting surfaces. Even nonmetallic surfaces, which are highly praised as being wearless, do wear over time. Lubricants minimize the amount of friction and wear that occurs between moving parts and also provide other benefits as listed in Figure 3.1. The following paragraphs (3.5-3.16) describe these conditions in more detail.

3.05 Separates surfaces: Although not readily noticeable to the naked eye, all surfaces, regardless of how highly polished they are, have minor surface irregularities (Figure 3.2). These irregularities continually rub or contact each other, and the resulting resistance to movement or drag is termed friction.

3.06 The separation of any two contacting surfaces is important if friction is to be reduced. All lubricants, regardless of their formulation, have the ability to separate to surfaces. How much or how little separation takes place is determined by the lubricant's formulation. This lubricant can be a solid or semisolid (grease), liquid (oil), or gas (compressed air or other gas).

3.07 Prevents wear: At the same time that a lubricant reduces friction by separating two surfaces, it also reduces the amount of wear that takes place as the surfaces rub against one another. This reduced wear increases the life expectancy of the parts and limits the amount of clearance that occurs as the wear takes place.

3.08 The actual amount of wear that takes place depends on a number of factorsspeed of the machine, whether it's rolling or sliding motion, how rough the surfaces are, and how many contaminants are present in the environment surrounding the parts.

3.09 For example, the main chain of a primary collector is subject to wear from the sludge and other gritty debris in which it operates. However, the amount of wear that takes place between joints and links is negligible because of their relatively slow movement. Wear on the drive chain, on the other hand, occurs at a faster rate because of the higher speed.

3.10 Cushions shock: Lubricants also provide a cushion between moving parts that will dampen shock. It is important to remember that the lubricant's ability to resist or dampen shock depends on the lubricant's characteristics and the surfaces it must separate. Usually, the heavier the lubricant, the better it can separate the surfaces and absorb shock. Grease is far superior than oil for this application.

3.11 Remember, however, that there is a trade-off for everything. If a lubricant is to have good shock resistance, it


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Figure8.3.

Eddy-currentdrive.

contains alternating magnetic poles is placed within the first drum. The clutch rotor is mounted on the output shaft of the drive and rotates independently of the iron drum. A stationary field coil that creates a magnetic field is placed within the clutch rotor.

8.16 When the A-C motor and driven coupling are running without any load, the rotor clutch of the drive does not rotate. Before the clutch rotor can move, the field coil must be energized. Energizing the field coil produces magnetic lines of force (flux) that are picked up by the iron in the driven coupling that is rotating outside of the clutch rotor. As the driven coupling is rotating, it generates eddy currents on the drum and sets up a secondary magnetic field between the driven coupling and the clutch rotor.

8.17 This secondary magnetic eddy-current field causes the clutch rotor to follow the rotation of the drive coupling. The output speed of the clutch rotor is determined by the controller that regulates the electric current fed into the stationary field coils. The speed of the output clutch rotor is maintained by a tachometer generator located at the rear of the drive. The tachometer feeds the shaft speed back to the controller, which compares the output speed to the set point. The controller then adjusts the strength of the field coil to maintain the desired speed.

Alternating Current Drives

8.18 A number of different methods are presently being used to regulate or change the speed of A-C motors. All are successful and all have some advantages and disadvantages. Some drives have simple controls and others are complex. The complexity of the control does not always mean that it is a better controller, even though it appears to have certain benefits. From a maintenance standpoint, the more simple the drive, the fewer number of problems that can occur, and the easier it is to maintain.

8.19 A-C motors all rotate at a constant speed (3600, 1800, 1200 rpm, etc.). To change the constant speed to a variable speed requires some sort of change to the electrical power that is supplied to the motor. However, the nature of A-C power cannot easily be modified.

8.20 As explained in Chapter 6 (paragraphs 6.46-6.60), A-C motors operate by induced magnetic fields in the armature windings. A magnetic field produced in the stator windings creates magnetic lines of flux that induce magnetic fields in the rotor windings and cause rotation to occur. In addition, the three phase voltage that powers the motor also has an effect on motor speed and performance.

8.21 Understanding the principles of motor operation makes it easier to understand the different methods that are used to control motor speed. This text does not have sufficient space to describe in detail all of the various components that make up the different types of controllers. Instead, the text will describe how the different controllers act on the A-C power to regulate the motor speed. For specific information regarding a particular drive, contact the manufacturer.


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Converting A-C Power

8.22 It is not possible to convert standard A-C constant power directly to A-C variable power. Instead, the A-C constant power must be converted to D-C power, which can be regulated or modulated. The regulated D-C power is then converted back to a lesser form of A-C power that is supplied to the motor (Figure 8.4). When the A-C power is converted to D-C, a square sine wave is formed. Regulating the power of the D-C square sine wave results in the output of a stepped A-C sine wave that is supplied to the motor. Because the stepped A-C sine wave power occurs on each of the three electrical phases at the rate of 60 cycles per second, the output power flow is very smooth.

8.23 The conversion from A-C to D-C originally was accomplished with the use of a SCR controller. Basically, it simply turned the power to the motor off and on. A tachometer detected the motor speed and gave that signal back to the controller that regulated the power flow. Since that time, there have been many changes in the methods of control and the electronic components that are used. Diodes, thyristors, transistors, varistors, and other components are just a few of these components. Solid-state components, including computer style circuit boards and IGBT (Insulated Gate Bipolar Transistor) are also frequently used.

8.24 Each drive manufacturer has a different name for its specific controller, but all regulate the motor speed by changing the electrical power before or within the motor. The most common changes are to the voltage and/or the frequency of the A-C power supply. The variations in the power supplied to the stator windings change the magnetic fields in the rotor windings, which affects output speed.

8.25 Other methods of motor control include modulating the vector and flux characteristics of the stator windings. Changing the magnetic lines of flux that are given off by the stator windings affects the amount of energy (electromotive force) that the motor develops. On the other hand, vectors are the direction of the lines of magnetic force within the windings. Modulating the strength of these lines of magnetic force affects motor current and, therefore, motor output speed.

8.26 Changing or modulating the frequency, vector, or flux in the electrical power is usually accomplished electronically with an inverter or other device that has PWM (pulse width modulation). Most of these controllers are relatively small in size compared with voltage-regulating drives that are made with larger components that generate more heat and require more cooling area.

8.27 The newer controllers are also equipped with electronic devices that perform other functions. A few of these functions include dynamic braking (by control of the electric current), pressure or speed sensing, soft start, torque sensing, and overtemperature. In addition, the controllers frequently are connected to other external devices, such as PLCs (programmable logic controls), remote system computers, and microprocessors, that are a part of the control circuit.

Variable Speed Motors

8.28 One of the problems that plague variable speed drives is heat. The constant changing of voltage and frequency of the power supply creates heat not only in the controllers as was previously mentioned (although the newer electronic devices operate cooler), but within the motor itself. To compensate for this additional heat, the motors used for variable drive applications must be designed to meet the conditions.

8.29 Generally, motors used for variable speed drives are constructed with heavier frames and windings and have insulation that is more heat resistant. Additionally, they often have ribbed housings (to expose more surface area) and have external fan or blower cooling devices (FC or BC motor designations) (Figure 8.5). Standard motors can be used for emergency conditions but will not provide the service life that would be gotten from a fully rated motor.

8.30 The maintenance of variable speed drives must have a high priority when establishing a plant maintenance program. Not only does variable speed drive maintenance require specialized monitoring equipment but also it requires highly trained personnel who have an understanding of electronic as well as electrical circuitry. In addition, the frequency and thoroughness of the routine inspection are probably two to three times the level of that performed on standard electrical equipment.

8.31 Heat, as was mentioned previously, creates many problems in electrical or electronic components. Heat not only stresses the physical properties and construction of a component, but also it affects the quality and life of the insulation. Proper cooling of the equipment, even when it is new, is extremely important. Inadequate cooling leads to prema-

Figure8.4.

A-Cpowerconversion.


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Figure8.5.

A-Cvariablespeedmotor.

ture component failures. Therefore, proper cooling cannot be overstressed.

8.32 It is also important to regularly monitor the levels of the interior electrical circuits and components of the controller. These levels should be monitored and recorded when the units are first installed. This record of current flow and resistance will provide a base against which to compare future measurements. Routine monitoring of these electrical levels should be performed at least at 6-month intervals during the first 2 years and then increased to quarterly intervals when the readings start to show signs of decline.

8.33 As soon as the electrical or electronic components start to show signs of fatigue (the drive trips offline for no apparent reason or the resistance readings drop off), increase the frequency of the inspection and start ordering any necessary spare parts. Controllers that have replaceable circuit boards are just as liable to fail as the other components. Keeping an extra set of the necessary circuit boards is advisable. When a board fails, check to see if the manufacturer or some other shop can repair the board for a lesser expense. The initial spare board is still necessary, but the spare will provide more time to have the damaged board repaired.

8.34 If the plant has more than one variable speed drive made by the same manufacturer, there might be some similar components within each controller. An inventory list of all internal components should be provided with the machine, but it may not be accurate. Take the time to double-check the manufacturer's inventory list against what is installed in the controller, or make one of your own. Make sure that all of the part numbers agree or are the most up to date.

Mechanical Drives

8.35 Several different types of mechanical variable (or adjustable) speed drives are manufactured. Some types are similar in concept but have different internal components, whereas others are unique in their design and construction. The horsepower ratings vary from type to type but are generally limited to low or moderate horsepower. In addition, most units can be supplied with or without a drive motor. However, if the motor is attached directly to the variable speed drive, it is supplied as a ''package."

8.36 One type of mechanical variable speed drive uses two separate drive cones (discs) that are connected to the input and output shafts of the drive (Figure 8.6). Power is transmitted from the input drive cone to the output drive cone by the adjustable position drive balls. The variable speed range is 3 to 1 and can be applied as either a speed decrease or an increase.


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8.37 The position of the drive balls is controlled by a moveable adjusting ring. The drive ball spindle or axle is inserted into a socket in the adjusting ring and pivots the drive ball position, producing the speed change. Conical springs exert constant pressure on the drive cones to ensure positive power transmission in proportion to the torque demands. The adjusting ring can be actuated manually at the drive or from a remote location.

8.38 Other types of mechanical variable speed drive use one or more sets of dry contact discs to transfer power and provide speed change. Generally, disc type drives are limited to a maximum of 10 horsepower and moderate duty applications, such as mixer and chemical feed devices. They usually have a 5:1 speed change ratio.

8.39 Originally, disc drives used a combination of fixed and moveable discs to accomplish the speed change. One style featured a pivoting spring loaded center section that contacted the fixed input and output discs. The angle of contact between the discs determined the speed change output discs. The discs of the centerpiece are spring loaded to ensure constant pressure and positive contact between all rotating components while the drive is operating.

8.40 Newer disc drives have a moveable input disc placed on an angle that contacts a fixed output disc (Figure 8.6). Raising or lowering the input driving disc changes the point of contact with the output disc and the output speed of the drive. The output disc is spring loaded to ensure proper contact pressure between the components.

8.41 Another style of mechanical adjustable speed drive that has limited water or wastewater applications utilizes a set of eccentric cranks and overrunning clutches that connect the input and output shafts. The unit, shown in Figure 8.7, has a 4:1 speed ratio but is limited to 1.5 horsepower. Laboratories or pilot test plants might provide the most common application for these units.

8.42 The input drive rotary motion is changed to linear motion by the eccentrics. The eccentrics, in turn, actuate a series of crank bars that are connected to overrunning clutches mounted on the output shaft. The forward movement of the crank bar causes the overrunning clutch to rotate the output shaft. The position of the crank bars determines the amount of stroke that is applied to the overrunning clutch and the resulting output speed of the drive.

Figure8.6.

Discstylemechanicalspeeddrives.


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Figure8.7.

Overrunningclutchdrive.

8.43 Maintenance requirements of mechanical variable speed drives generally consists of periodic lubrication of the bearings and inspection of any drive couplings. The internal components are sealed from the atmosphere and only require periodic inspection and cleaning. Because the units all are designed to operate dry, the housings should be protected from the environment.

Belt Drives

8.44 Some simple adjustable belt drives are described in Chapter 10 (paragraphs 10.13, 10.14, and 10.28-10.30). These were externally mounted units and were a part of the belt drive connection between the motor and another mechanism. The belt drives covered in this section are different in design and construction than those discussed in Chapter 10. In fact, these are more of a "package drive" because they are often self-contained.

8.45 The adjustable speed belt drives shown in Figure 8.8 are enclosed within their own housing and are directly connected to the drive motor. In many other instances, they are also directly connected to some sort of a speed reducer. The speed reducer is necessary to accomplish the final speed reduction of the unit, whereas the belt drive provides the primary or initial speed reduction as well as the speed variation.

8.46 Belt drives most commonly have a 3 to 1 adjustable speed range although, speed ranges of 5 to 1 and even as high as 10 to 1 are available. Most belt drives are designed to accommodate drive motors of 5-15 horsepower, but units that can handle 50 horsepower motors are manufactured.

8.47 The belt drive consists of two adjustable sheaves and a wide drive belt. Motion is transmitted from the input to the output shaft by the contact between the edge of the drive belt and the adjustable sheaves. Because this area of contact is relatively small, it is important that the belt and sheaves be kept dry and clean.

8.48 Most manufacturers accomplish the variable speed adjustment by changing the position of the input sheave flanges (closer together or farther apart). This change must be made when the unit is in operation. It can be accomplished manually at the unit, or it can be accomplished by an electrical, pneumatic, or hydraulic actuator that is remotely controlled. Selection of the type of applicator used on a particular drive is usually determined by the application.

8.49 Changing the spacing of the sheave flanges causes the belt to ride at different positions with a corresponding change in the rate of travel in feet per minute. When the belt moves, the position of the output sheave flanges must also change. In most drives, the output sheaves are spring loaded, and the movement of the belt position forces the sheave flanges farther apart or allows them to come closer together.

8.50 In other units the output sheaves are connected by a linkage system that is a part of the adjusting mechanism. In these units, both sets of sheave flanges move at the same time, although in opposite directions, whenever an adjustment is made. The advantages of this design is a more positive and controlled speed change, as well as a reduction in belt stress and a longer belt life.

8.51 It is important to remember that, although a belt drive has a 3 to 1 adjustable speed range, it does not occur in only one direction. Instead, the drive has a theoretical neutral (1:1 ratio) point at the center of the sheaves, with an approximate 1.5 to 1 increase and decrease (top and bottom positions), as shown in Figure 8.9. Since the speed adjustment is made on the output shaft of the motor, the final speed of the drive must be accomplished by using an additional speed reducer connected to the output shaft of the belt drive.

8.52 For example, say a mixer required a drive with an output speed between 5 and 15 rpm (3:1 ratio) and was powered by an 1800 rpm motor. A speed reducer would first be selected to provide the proper speed reduction at the neutral point of the belt drive to achieve an output speed of approximately 10 rpm. With the midrange speed established, the belt drive would then provide the remainder of the speed changes necessary to achieve the 15 and 5 rpm desired output speeds.

8.53 Belt-driven variable speed drives require more maintenance attention than mechanical disc units. However, the focus of the maintenance activity should be directed to more frequent inspections of the drive belt with only regular periodic attention to lubrication and other mechanical and electrical components. Making sure that the drive belt is in


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Figure8.8.

Adjustablebeltvariablespeeddrives.

good condition is important because it is the key element in the proper operation of the unit.

8.54 Additionally, the proper lubrication of the sheave sliding surfaces (on the shafts) and other rotating components within the drive is also important. However, keep in mind that the amount of lubricant used within the drive around the belts should be kept to a minimum. Although the drive belts might have a certain tolerance to lubricants, any lubricant that gets on the belts will cause slippage of the belts, resulting in inconsistent drive output speed.

Figure8.9.

Sheaveflangepositionandratio.


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Hydraulic Drives

8.55 A number of different adjustable speed hydraulic drives are used to power various pieces of water/wastewater plant equipment. Most of these are used for pump applications, but mixers, conveyors, inclined screws, collectors, and trash rakes also are good applications.

8.56 The most simple variable speed hydraulic drive consists of a hydraulic pump that is powered by a constant speed electric motor. Hydraulic piping delivers the hydraulic fluid to hydraulic motor (Figure 8.10). The variable speed of the unit is obtained by controlling the amount of fluid delivered to the hydraulic motor.

8.57 Although these units have excellent speed control and high-torque capacity at the output shaft, they are not as efficient as other types of hydraulic drives. They also generate heat within the hydraulic fluid, especially if the output shaft runs at low speeds for extended periods of time. The low speed requires large volumes of fluid to be recirculated back to the pump which results in the heat generation.

8.58 Another style of hydraulic variable speed drive utilizes a fluid coupling similar to those used in vehicle automatic transmissions. These drives are composed of two separate shafts with one-half of an impeller coupling mounted on each shaft (Figure 8.11). The impellers are placed within a common housing or inner casing (sometimes called a torus). The inner casing is made with a number of openings or ports around the outer edge that permit hydraulic fluid to slowly escape from the coupling device when it is in operation. This inner casing mechanism is mounted within a larger case that acts as a reservoir for the hydraulic fluid and also protects and supports the rotating components.

8.59 The operation of the unit is not complex. When the unit is first started, the coupling halves are empty, and the input half is driven by a constant speed motor. Oil is pumped from the reservoir into the coupling halves. As the fluid starts to fill the inner casing, the motion and energy of the driven shaft are transferred from the input coupling half to the output coupling half by the hydraulic fluid. The more fluid that enters the coupling, the more energy (but only up to 97 percent) that will be transmitted.

8.60 However, during starting, the amount of fluid that is directed to the coupling determines the manner in which the drive delivers output energy. If only a small amount of hydraulic fluid is directed into the coupling, only small amounts of output energy will be delivered. This results in a "soft start" for the driven unit. If a fast start was desired, large amounts of fluid could be pumped into the coupling. Even so, with a fast start the coupling does offer a cushioned start rather than a sudden jolt.

8.61 After the unit has reached its desired speed, the flow of hydraulic fluid into the couplings is restricted. With the fluid being constantly discharged from the ports, the coupling will start to empty, and the output speed will reduce. The speed sensors detect this drop in speed and allow more hydraulic fluid to be pumped into the coupling. In reality, a small amount of hydraulic oil is pumped continuously into the coupling, and a speed balance is maintained.

8.62 When deceleration is required, the flow of hydraulic oil to the coupling is stopped, and the discharge ports in the inner casing allow the remaining hydraulic fluid to be discharged back to the reservoir. The loss of fluid permits the output shaft to slow down and eventually stop when all of the fluid is out. Again, this stopping cycle can be controlled to provide a "slow stop" if desired.

8.63 These units are available in several different styles from packaged units complete with the electric motor, hydraulic variable speed drive, and the oil pump, or they can be purchased as individual components and mounted on a common base. Regardless of how they are assembled, proper alignment between the components is crucial to proper operation.

8.64 There is a different type of variable speed drive that uses hydraulic fluid for power transmission, but not in the same manner as a fluid coupling. It consists of an input shaft, a "transmission pack," and an output shaft drive assembly (Figure 8.12). The input shaft is driven by a constant speed electric motor and extends into the housing. The housing supports all of the rotating components and also acts as a reservoir for the hydraulic fluid.

8.65 The transmission pack consists of a number of friction discs that are mounted on an interior end of the input shaft. The discs are alternately separated by a drive plate and are made with a gear tooth configuration cut into the inside diameter. This gear tooth configuration mates with the spline (lengthwise grooves that resemble gear teeth) cut into the input shaft. The drive plates float free on the inside diameter but are grooved on the outside diameter to match a set of fingers or spider arms that are a part of the output shaft.

8.66 A small oil pump connected to the input shaft supplies hydraulic oil to the transmission pack and a spring loaded power piston located in the housing. In a neutral position, the power piston is retracted from the transmission pack, allowing a free flow of oil between the discs and plates and back to the reservoir. Even though the input shaft is rotating, the unrestricted flow of oil within the transmission pack prevents any movement from occurring on the output shaft.

8.67 During operation, hydraulic oil is directed to the power piston. As the power piston moves forward, it decreases the amount of space between the discs and plates. The restricted space causes the oil that is passing between the discs to transmit motion from the constant speed input shaft to the drive plates and to the output shaft. All of the energy transmission is controlled by the hydraulic oil, and there is never any physical contact between the discs and plates.

8.68 Decreasing the speed of the output shaft is accomplished by simply relieving the oil on the power piston. This permits the springs to move the piston back, which, in turn, opens up the space between the discs and plate and permits the output shaft to stop. The power piston is spring loaded so


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Figure8.10.

Hydraulicmotorvariablespeeddrives.


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Figure8.11.

Variablespeeddriveusingahydrauliccoupling.


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Figure8.12.

Discpackhydraulicdrive.


Page 12

Separates two surfaces

Prevents wear

Cushions shock

Transfers heat

Protects against corrosion

Acts as a seal

Figure 3.1. Benefits of lubrication.

cannot have fluid characteristics that would enable the lubricant to be used at high speeds or would easily flow into spaces that had close tolerances. For example, greases that are used to lubricate open gear sets or other slow moving applications cannot be used in speed reducers. Speed reducer gears run at relatively high speeds and require an oil bath to ensure proper lubrication of the contacting gear tooth surfaces.

3.12 Transfers heat: A lubricant should only be thought of as a heat transfer medium, because it only has a limited ability to absorb heat. Because of its chemical and physical composition, a lubricant has little ability to absorb heat compared with other substances such as water. Even specially formulated high-temperature greases have heat-absorbing limitations before the oils in them break down.

3.13 However, a lubricant can remove heat from a surface or area by absorbing it and then giving it up to a cooler surface, but it has no ability to retain heat without the heat having an adverse effect on the lubricant's characteristics. To explain this, consider an oil lubricant in a gear case. If the gear case is located in a heated atmosphere and no provision is made to cool the oil, it will break down. Water, on the other hand, would absorb heat until it turned to steam and then it would still continue to absorb heat. Water can absorb heat regardless of its state, and that makes it a cooling agent.

3.14 Corrosion protection: It is a natural occurrence that as a lubricant coats a surface it automatically reduces corrosion. However, if a lubricant is washed away or removed from the surface, corrosion will start to take place once the air contacts the surface. Even if a lubricant is left undisturbed on a surface, the film coating will dry up in time and expose the surface to the elements and subsequent corrosion.

3.15 Corrosion, however, does not take place very quickly on parts that have been lubricated. Most lubricants are formulated with corrosion inhibitors and other chemical additives that improve their film consistency and reduce or limit corrosion from occurring.

Figure3.2.

Crosssectionofapolishedsurface.

3.16 Acts as a seal: The ability of a lubricant to act as a seal is directly related to the consistency of the lubricant and its application. When grease is used in open (sleeve) bearings or in any other kind of application that requires thick lubricants, it seals out dirt and moisture from the bearing area as it lubricates (Figure 3.3). However, if a positive seal is required to prevent contaminants from entering the bearing, a contacting type seal should be considered for the application.

Types of Lubricants

3.17 All lubricants can be grouped into four separate types: liquids (oils), semisolid (soft grease), solid (hard grease), and gas (compressed air or other gas). Although a gas is not often considered a lubricant, it is. For example, the compressed air that actuates a pneumatic controlled device acts as a lubricant by separating the moving surfaces, thereby reducing friction and permitting easy movement. (Note: Air cylinders and other larger pneumatically actuated devices do require small amounts of a lubricant for p

Hands On Water and Wastewater Equipment Maintenance

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