directing the o & m of motors
TRANSCRIPT
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Note: The source of the technical material in this volume is the ProfessionalEngineering Development Program (PEDP) of Engineering Services.
Warning: The material contained in this document was developed for SaudiAramco and is intended for the exclusive use of Saudi Aramcos
employees. Any material contained in this document which is notalready in the public domain may not be copied, reproduced, sold, given,
or disclosed to third parties, or otherwise used in whole, or in part,
without the written permission of the Vice President, Engineering
Services, Saudi Aramco.
Chapter : Electrical For additional information on this subject, contact
File Reference: EEX20305 W.A Roussel on 874-1320
Engineering EncyclopediaSaudi Aramco DeskTop Standards
Directing The Operation And Maintenance
Of Electric Motors
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CONTENTS PAGES
PREVENTIVE MAINTENANCE REQUIREMENTS....................................................1
Minimum Annual Requirements................................................................................2Visual Inspection..................................................................................................2
Bearing Inspection ...............................................................................................4
Vibration Level Testing........................................................................................9
Insulation Resistance (IR) and Polarization Index (PI) Testing .........................11
Additional Requirements .........................................................................................13
Motor Alignment................................................................................................13
Motor Lubrication ..............................................................................................19
Oil Lubricant Testing .........................................................................................20
Inspection of RTDs ............................................................................................22
Insulation Cleaning and Drying .........................................................................22
Measure Bearing Insulation ...............................................................................25DETERMINING WHETHER MOTORS ARE FUNCTIONING PROPERLY ............29
Motor Maintenance Record and Interpretation ........................................................29
Insulation Resistance Data and Data Interpretation............................................31
Vibration Level Data and Data Interpretation ....................................................32
Maintenance History of Motors .........................................................................35
DETERMINING THE CORRECTIVE ACTIONS FOR COMMON MOTOR
PROBLEMS ..................................................................................................................37
Vibration Alarms......................................................................................................37
Temperature Alarms ................................................................................................39
Winding Alarms.......................................................................................................40Bearing Alarms ..................................................................................................41
Motor Trips ..............................................................................................................42
Faults..................................................................................................................42
Process Interlocks...............................................................................................43
WORK AID 1: PROCEDURE AND ACCEPTABLE TEST VALUES
(PERFORMED DURING MOTOR MAINTENANCE AND COMPILED
FROM SADP-P-113, NFPA 70B, AND ESTABLISHED ENGINEERING
PRACTICES) FOR DETERMINING WHETHER MOTORS ARE
FUNCTIONING PROPERLY .......................................................................................44
WORK AID 2: PROCEDURE FOR DETERMINING CORRECTIVE
ACTIONS FOR COMMON MOTOR PROBLEMS (BASED ONESTABLISHED ENGINEERING PRACTICES) .........................................................48
ADDENDUM ................................................................................................................56
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PREVENTIVE MAINTENANCE REQUIREMENTS
Electrical preventive maintenance consists of routine inspections, tests, and service on
electrical equipment. The purposes of electrical preventive maintenance are:
To reduce the hazards to life and property that can result from a failure of
electrical equipment.
To detect impending equipment trouble and to reduce or to eliminate
unscheduled downtime of equipment and systems.
The performance and the extent of an EPM program must be determined through a cost
analysis of the performance of the EPM program versus the cost of nonperformance of the
EPM program. An EPM program that will collectively cost more than the replacement of the
equipment would not be cost-effective. The determination of the EPM programs content andof the frequency of program performance must consider the following items:
The impact of the program on personnel safety.
The potential for equipment loss or damage.
The impact of the maintenance schedule on production.
This section of the Module will present information on the following preventive maintenance
requirements:
Minimum Annual Requirements
Additional Requirements
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The first part of the annual visual inspection is performed while the motor is in operation.
The inspection should be performed through use of a look-listen-feel approach. The
maintenance person should look at the motor to ensure that there is no physical damage to the
motor or to the connected equipment. The operational parameters of the motor (amperes,
voltage, and power factors) also should be observed to be within the established limits that are
listed on the motors nameplate. The maintenance person should listen carefully to the
sounds that the motor makes while it is in operation. A motor that is operating correctly will
make a smooth, steady sound. Oscillations or unusual sounds can indicate a pending
electrical or mechanical failure. The maintenance personnel also should feel the motor for
excessive heat in the vicinity of the motor stator and the motor bearings (e.g., feel the stator
case and end bells).
The second part of the annual visual inspection is performed while the motor is deenergized.
The following is a list of the general items to inspect while the motor is deenergized. (The
manufacturers technical information should be consulted for specific requirements.)
Inspect for water and for condensation on or in the motor.
Inspect for rust and for corrosion on the connection boxes and seal points.
Inspect for dirt, for dust, and for foreign objects on or around the motors
ventilation ports.
Inspect for proper anchors, mounts, grounds, and ground connections.
Check the air gap at eight radial locations or at all poles of a synchronousmotor. Record this information. Excessive variation of the air gaps may
suggest misalignment or excessive wear of the motors bearings.
Inspect for signs of excessive heat such as charred or cracked insulation and
discolored or blistered paint and varnish.
Inspect the integrity of the electrical terminals.
Inspect for frayed or worn insulation.
Inspect the stator and the rotor coil insulation for thermal aging, cleanliness,and tightness of bracing.
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Ensure that all fasteners are properly secured.
Inspect for clogged rotor and stator ventilation ducts.
If possible, inspect the end windings of the motor for dirt, dust, grease, oil, or
other foreign material.
DC motors require the following additional inspection items:
Inspect the commutator surface for indications of faulty commutation and for
high mica insulation between commutator segments..
Inspect all brushes for satisfactory operation, for proper length, and for proper
brush holder compression.
Inspect for cleanliness in the commutator area. An excessive runout or an
irregular commutator surface can generate excessive amounts of carbon dust in
the commutator area. An excessive accumulation of carbon dust can contribute
to flashovers.
Inspect the brush fit in the brush box to ensure ease of movement of the
brushes in the brushbox.
The value that is obtained through performance of an annual visual inspection often is
dependent on the mental approach that the inspector uses to perform the inspection. If the
annual visual inspection is performed under the mental assumption that no problems exist, theinspector often will miss the subtle signs of potential problems; however, if the annual visual
inspection is performed under the mental assumption that a problem does exist and that the
purpose of the inspection is to identify the problem, the inspector is more likely to locate the
signs of potential and existing problems.
Bearing Inspection
The complexity of the bearing inspection will depend upon the actual type of bearing and
lubrication system that is installed. Anti-friction bearings that use a grease lubrication system
require the least amount of maintenance of all the types of bearings that are used in SaudiAramco installations. Sleeve bearings that use an oil transfer system require the greatest
amount of maintenance. The exact bearing inspection requirements should be in accordance
with the manufacturers technical manual.
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The first portion of an anti-friction bearing inspection is performed while the motor is in
operation. This portion of the inspection involves listening for sounds such as squeals,
screeches, and clicks that emanate from the bearings. Such sounds are usually a sign of
improper lubrication or of a dirty bearing.
After the motor is shut down, the bearing housing should be opened, and the bearing should
be checked for proper lubrication (grease), signs of excessive heat, and indications of high
currents across the bearing. On relatively small motors, the shaft should be checked for
excessive endplay or for freeplay while the shaft is manipulated by hand. On motors that
have shafts that are less than five inches in diameter, the shafts also should be rotated by
hand. While the shafts on the motors are rotated, the inspector should feel for any hard points
on the bearings.
Anti-friction bearings that use grease for lubrication should have the bearing cleaned and
should have the grease changed during the annual bearing inspection. Anti-friction bearingsthat use oil for lubrication should be inspected to ensure that the oil is clean and that the oil is
at the proper level. If necessary, oil should be added to the bearing. The manufacturers
technical manual should be consulted to ensure that the proper amount and the correct type of
grease or oil is used.
Sleeve bearings require more maintenance than anti-friction bearings because these bearings
must be disassembled to allow an adequate inspection. The exact procedure for disassembly
and for inspection of motor sleeve bearings should be in accordance with the motor
manufacturers technical manual.
After the bearing is disassembled, the bearing should be checked for unusual signs of wearand for signs of excessive heat. An exact check of bearing wear can be performed through
measurement of the air gap around the rotor and through comparison of these measurements
to the manufacturers specifications and to previous air gap measurements. The bearing also
should be checked for signs of circulating currents that are evidenced by localized pitting or
heating on the soft metal inner surface of the bearing. Large motors are provided with
bearing pedestal insulation to prevent circulating currents; therefore, excessive bearing current
would indicate a failure of the bearing insulation. The oil lubrication system also should be
inspected to ensure that there is adequate oil flow to the bearing and to ensure that bearing
temperatures are within specifications.
The Electrical Engineer should be able to inspect the bearing and to identify the type ofbearing damage. This identification of damage is especially useful in cases of chronic
(repetitive) bearing failures.
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The babbitt surfaces must be examined for evidence of the following unusual conditions:
Rub marks in the top half of the bearing usually are a result of machine
misalignment.
Rub marks on the babbitted thrust faces are an indication of axial thrust loads.
These rub marks usually result from improper axial alignment or from
excessive shaft end play.
The bearing should be checked for evidence of wiping or of pulling of the
babbitt metal. This bearing wiping generally is the result of the bearing
overload due to machine misalignment or due to machine vibration.
The wear pattern on the babbitt surface of the bottom half of the bearing should
be noted. The wear pattern should extend axially along the lower half of thebearing and should be centered on the bottom of the babbitt surface. The width
of the wear pattern should be uniform from one end of the bearing to the other.
Uneven wear patterns typically are due to improperly fitted bearings or are due
to a bent shaft.
The lower bearing babbitt should be checked for circumferential scratches.
These scratches run perpendicular to the bearing wear marks and commonly
are caused by foreign particles that pass through the oil film.
The upper and lower halves of the bearing should be checked for general
surface roughness. This roughness may be caused by abrasive particles in theoil.
The journal surface should be checked for protruding sharp edges.
The bearing surface should be checked for pitting. This pitting is normally due
to corrosion, to careless handling, or to bearing currents.
After the bearing surface inspection has been conducted, the bearing oil reservoirs should be
drained and flushed.
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Split Sections of a Sleeve Bearing
Figure 1
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Vibration Level Testing
Vibration level tests are performed to check for excessive vibration or for changes in vibration
that are above the established limits. All equipment that rotates will vibrate to some degree;however, changes in the vibration levels can be a sign of a pending malfunction. These
abnormal vibration levels, if left unattended, can cause damage to the motor shaft, to the
motor rotor, to the motor endbells, to the motor bearings, and to the other equipment that is
connected to the motor.
Different types of vibration must be considered when a vibration level test is performed.
Because motors rotate in the radial plane and the axial plane, a complete vibration analysis
must consider both vibrational planes. The number of test points that should be checked on a
specific motor application should be based on the manufacturers recommendations, motor
history, and instructions for technical services.
The source of vibration can be identified through performance of a frequency analysis. The
frequency analysis will identify the frequency at which the excessive vibrations occur. This
frequency can then be used to determine the cause of the excessive vibration. The amplitude
of the frequency will indicate the severity of the problem. The following is a list of the
typical sources of vibration:
Imbalance
Misalignment
Resonance
Bearings
Gears
Vane Passing
Fans
Air gap eccentricity
Cavitation
Oil whirl
Pipig
Bent shaft and bowed rotor
Looseness
Belts and pulleys
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Figure 2 shows a sketch of a motor and illustrates where the vibration measurements should
be made. The axial plane of vibration is measured at two locations, the front end (AF) of the
motor and the rear end (AR) of the motor. The radial plane of vibration is divided into two
components: vertical and horizontal. The vertical component consists of VRand VFand thehorizontal component consists of HR and HF. Four vibration level measurements must be
made in the radial plane: one at VR, one at VF, one at HR, and one at HF.
Motor Vibration Measurements
Figure 2
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The following steps can be used as a guide to insulation resistance testing:
All leads of the motor should be connected together, and the insulation
resistance should be recorded from the leads to ground. If the individual
winding phase connections are accessible, each phase winding should be
disconnected, and phase-to-phase measurements also should be taken.
The insulation resistance check should be performed as soon as the motor is
turned off and the motor windings are still hot.
The insulation resistance test voltage should be determined by the voltage of
the motor. A 500 volt handcrank instrument can be used for motors that are
less than 600 volts. A 1000 volt or 2500 volt motor driven or rectifier type
instrument should be used for motors of 2300 and 4000 volts. A 5000 volt
megger should be used for 13.2 kV motors.
Spot measurements should be conducted for 60 seconds.
Ambient temperature and moisture should be recorded.
All resistance values should be corrected to 50oC.
The results of the 60 second spot measurement should be recorded on the
maintenance form and should be compared to previous measurements.
The Polarization Index (PI) is a ratio of the ten minute insulation resistance to the one minuteinsulation resistance. The following equation can be used to determine the Polarization Index
(PI) of a motor.
The value of the PI should be equal to or greater than two to be considered satisfactory.
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Additional Requirements
In some instances, the Electrical Engineer may determine that the minimum annual
requirements do not provide sufficient data to determine if a motor is operating properly.This section provides additional tests that can be performed to obtain more specific data on
motors. Prior to the performance of these preventive maintenance items, the Electrical
Engineer must consider if the benefits that are derived from this maintenance are worth the
additional cost. Sound engineering practices dictate that the following are advisable
preventive maintenance items to be considered when extra maintenance data are desired:
Motor Alignment
Motor Lubrication
Oil Lubricant Testing
Inspection of RTDs
Insulation Cleaning and Drying Measure Bearing Insulation
Motor Alignment
Many failures of motors and motor bearings can be attributed to a motor alignment problem.
Motor misalignments often appear as other motor problems, such as bearing overheating,
excessive bearing wear, motor overheating, excessive noise, and excessive motor vibration.
Motor misalignment also can appear as problems with the connected load such as overheating
of connected equipment and connected equipment bearing damage.
The motor alignment should be checked whenever the following indications are present:
The bearing temperature (of the motor or of the connected equipment)
increases with no lubrication system problems.
The motor air gap increases/decreases.
The noise that is generated by the bearing (motor or connected equipment)
increases.
Motor or connected equipment vibration increases.
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A straight edge, a feeler gauge, or a dial indicator can be used to check motor alignment
through comparison of the motor shaft position to the load shaft position. Each instrument
will check for a different type of motor misalignment. The straight edge will ensure that the
motor shaft and the load shaft line up axially and will ensure that no lateral displacement has
occurred. This method is especially useful for alignment of belt drives because the straight
edge will contact the motor sheave and load sheave squarely when the motor and the load are
properly aligned. Figure 3 illustrates the use of a straight edge for alignment of belt drives.
Figure 3 shows one correct alignment of a belt drive and two incorrect alignments of belt
drives. In the correct alignment, the straight edge is used to line up the sheaves of the motor
and of the load. In the first incorrect alignment, the motor and the motor sheave are cocked in
relation to the load sheave. The installation that is shown would produce rapid belt wear and
would place an unnecessary combination load on the shaft of the motor.
The second incorrect alignment shows that the motor and the load are not lined up in the axial
direction. This type of installation would also produce rapid belt wear and would develop anunnecessary combination load on the shaft of the motor and the shaft of the load.
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Alignment of Belt DrivesFigure 3
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A feeler gauge can be used to check for an angular displacement between the motor shaft and
the load shaft (the face of both shafts should meet). The dial indicator can be used to check
for any type of rotational misalignment of the motor shaft and the load shaft. A rotational
misalignment will usually be caused by a bend in one of the shafts.
The most accurate method to use to check motor alignment is the dial indicator method. The
dial indicator can be used to check angular misalignment and to check run out. Figure 4
shows a motor shaft and a load shaft with the coupling hubs installed.
The following steps can be used as a guide to check angular misalignment:
The alignment of the motor and the load should be checked after the coupling
hubs have been installed.
The dial indictor base should be mounted to the side of one coupling hub (Hub1).
The button of the dial indicator should be placed against the finished face of the
other hub (Hub 2).
A reference mark should be inscribed on Hub 2 to mark the position of the dial
indicator button.
Both shafts should be rotated simultaneously while the indicator button is kept
on the reference mark on Hub 2.
The dial indicator reading should be noted at each quarter revolution.
The angular misalignment of the shafts will be indicated by a deflection of the
dial indicator dial. The misalignment of the shafts should not exceed a total
dial indicator reading of .001 for each one inch of radius of the coupling hub.
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Angular Alignment
Figure 4
After the shafts have been checked for angular misalignment and are parallel within the limits
that are specified, the shafts should be checked for run out to ensure concentricity of the
shafts.
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The following steps, in reference to Figure 5, can be used as a guide to check run out of the
shafts:
The dial indicator base should be mounted to one coupling hub (Hub 1).
The dial indicator button should be placed on the machined diameter of the
other hub (Hub 2). A reference mark (not shown in Figure 5) should be scribed
on Hub 2 to mark the location of the indicator button.
Both shafts should be rotated simultaneously while the indicator button is kept
at the reference mark on Hub 2.
The dial indicator reading should be noted at each quarter revolution.
The run out between the hubs will be indicated by a deflection of the dialindicator dial. The total run out between the hubs should not exceed .002 inch.
Run-Out
Figure 5
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The lubrication of antifriction bearings and sleeve bearings with oil is a continuous function;
therefore, only a periodic inspection of the oil system is required. The inspection of the oil
system depends on the complexity of the oil system. As a minimum, however, the following
items should be checked:
That the oil level in the oil level indicators is in the normal range. If the oil is
out of the normal range, the proper type of oil should be added.
That the oil flow indicators on circulating oil systems indicate proper oil flow.
If improper oil flow is noticed, the circulating oil system should be investigated
to determine the cause of the lack of oil or of the loss of oil flow.
Oil pressure indicators on forced-flood oil lubrication systems should be
checked to verify that the oil pressure to the bearing is in the normal range.
Under normal operating conditions, the lubricating oil will not need to be changed over a two
year period. However, if the annual oil lubricant tests identify foreign particles in the oil or
contamination of the oil, the oil system needs to be drained, flushed, and refilled with the
proper lubricant.
Oil Lubricant Testing
Oil lubricant testing is performed to ensure that the oil is free from contamination and that the
oil still performs as a lubrication agent. Over time, all lubricants will chemically break down
and will eventually cease to effectively lubricate. A test of the oil will help to identify any
potential lubrication problems prior to failure.
Two types of tests are performed on oil lubricants: a simple foreign material contamination
test and a complex chemical analysis of the lubricant. The foreign material contamination test
is performed to ensure that contaminants such as water, sand, or other foreign material have
not entered the lubricant. The presence of any foreign material could cause damage to the
bearing. The chemical analysis checks for a breakdown of the lubricant on a molecular level
and includes an analysis of viscosity and of viscosity index to determine if there have been
any changes in the performance of the lubricant. The chemical analysis should be performed
in a laboratory.
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The foreign material contamination test is easy to perform in the field. A sample is drawn
from the oil lubrication system and then the lubricant is allowed to settle. Once the oil has
settled, it should be inspected for any foreign material. This test should also include a check
of the level of the oil in the oil reservoir or in the bearing to verify that the level is correct.
Because the chemical analysis is performed in a laboratory environment, the exact procedures
for the chemical analysis are beyond the scope of this module. The oil lubricant sample
should be obtained from the motor and then the sample should be sent to a laboratory facility
for testing.
The foreign material contamination test should be done on a quarterly basis or more often if
experience shows that the motor has a history of contamination problems. The oil lubrication
chemical analysis is done less often but should be done at least every six months. The oil in a
oil lubrication system should be changed every one or two years.
The periodicities of the oil tests can be changed based on the history of the motor. Prior toany change in oil lubrication test frequencies, the following items should be considered:
The average temperature of the lubricant.
The likelihood and the severity of bearing overloads.
The likelihood that contaminants will enter the system.
If the temperature of the bearing lubricant has been operating in the upper portion of the
lubricants temperature range, the lubricant should be tested more often. If the temperature of
the lubricant has exceeded its normal operational range, the lubricant should be immediately
tested. Follow-up testing of the oil should occur weekly for the next month.
If the bearing is subjected to periodic overloads or to cyclic overloads, the periodicity of the
oil tests should be reduced.
If there is a high likelihood that contaminants will enter the oil system, or if the oil system has
a history of contamination, the oil tests should be scheduled on a more frequent basis.
Decreased oil test periodicity does not remove the responsibility for determination of the
cause of contamination or for elimination of the source of the contaminants.
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Several methods can be used to clean motor insulation. The most effective method depends
upon the type of dirt and upon the amount of dirt that are lodged on the insulation. The
methods that can be used to clean motor insulation are listed below in the order of preference:
Dry wiping
Brush or suction cleaning
Blowing
Solvent cleaning
Water, emulsion, and alkali cleaning
Shell blasting
Dry wiping should be used to remove dry dirt from the surfaces of insulation that are located
in accessible areas. Dry wiping is performed through use of clean, dry, lint free cloths. Non-
lint free cloths should not be used because the lint will adhere to the insulation and will
increase the dirt collection. Lint is particularly objectionable on high-voltage insulationbecause the lint tends to cause a concentration of corona discharge.
Brush or suction cleaning also should be used to remove dry dust and dirt from the surfaces of
insulation that are located in accessible areas. The dry dust and dirt should be removed by
brushing with bristle brushes and then vacuum suction cleaning. Wire brushes should not be
used. Brush or suction cleaning is a desirable method to clean insulation because the dirt is
not scattered and the dirt does not settle on other apparatus. The use of this cleaning method
is limited to accessible areas that can be reached by the brush and the vacuum.
Blowing out dirt with a jet of air only should be done to remove dirt from inaccessible
crevices and only when the motor is dry. Blowing is performed through use of drycompressed air (30 psi or less) and vacuum suction. A vacuum suction is connected to one
end of the motor, and the compressed air is directed into the other end of the motor. The air
should be directed in a manner that will dislodge the dirt from the insulation and will allow
the vacuum suction to draw the dirt out of the motor.
Solvent cleaning is particularly effective for removal of tar, grease, wax, and oil from
electrical apparatus. The surfaces should be wiped with a cloth that is wet with the solvent,
and then the surfaces should be wiped with a dry cloth. To avoid lint deposits on the
insulation, non lint free rags should not be used. If solvents are used on windings with
silicone rubber or with an abrasion resistant coating, severe damage to the coating can occur.
The manufacturers technical manual should be consulted for the solvent that should be usedfor insulation cleaning.
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Motors can be cleaned by hose washing or by pressure spray from a steam generator. Steam
from a shop line or a spray of hot water and compressed air may be used. When insulation is
cleaned through use of this method, the jet pressure and temperature should not exceed 30 psi
and 80C (176F), respectively. To remove tar, wax, grease, or oil from insulation, a
nonconductive detergent compound must be added to the water. These detergent compounds
contain non-ionic emulsifying agents.
Emulsion cleaners also contain solvents to soften hard deposits so that these deposits can
more easily be removed. These detergent compounds are not electrical conductors and are
safe for use on insulation. After the deposits have been removed, the motor windings should
be thoroughly rinsed with water to remove all traces of the cleaning compound. The water
should then be promptly removed from the motor windings through use of air pressure and
lint free rags.
Shell blasting is the process of air blasting with ground nut shells to remove hard dirt depositsfrom insulation. Shell blasting may abrade the insulation and should only be performed under
the direction and supervision of the manufacturer.
In Saudi Aramco, solvent cleaning, water/emulsion/alkali cleaning, and shell blasting are not
recommended for field maintenance. Motor windings with heavy oil and dirt contamination
must be sent to Dhahran shops for cleaning.
The best method for use in cleaning a given motors insulation should achieve the following
objectives:
The method should be able to remove the type of dirt that is present.
The method should cause the least amount of insulation damage.
The manufacturer should be consulted when there is a doubt as to the best method of
insulation cleaning.
The motor windings can be dried after they are cleaned through the use of external heat or
through the circulation of current through the windings. The use of resistance heaters or
steam coils is advisable when external heat is to be applied to the motor. Because the space
heaters that are located inside of the motor are not of sufficient capacity for use in drying the
motor windings, additional heaters are required. The additional heaters should be placed nearthe bottom of the motor, and care must be taken to protect the windings against direct heat
radiation from the heaters.
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A dc welding set can be used to heat the motor windings through use of the circulation of
current in the windings. The current that is circulated in any part of the winding should not
exceed the rated current value of the motor.
Motor windings cannot effectively be dried unless means are provided to circulate air through
the motor to remove the moisture. Openings must be provided near the bottom of the motor
for the entry of fresh air, and openings must be provided near the top for the discharge of the
heated moist air. A small fan might be necessary to provide assistance in the circulation of
air.
Regardless of the method that is chosen to dry the motor windings, the heat to dry the motor
windings must be applied gradually to allow the water vapor that is produced to travel out
through the insulation. A rapid application of heat could cause steam pressure to rupture the
insulation; such as a rupture would permanently damage the insulation.
Measure Bearing Insulation
The bearing insulation is tested to ensure that the bearing pedestal has sufficient dielectric
strength to resist the flow of current through the bearing to ground. Bearing insulation that
does not have sufficient dielectric strength will allow current to flow in the bearing; such
current flow has a destructive effect on the shaft journals and the bearings.
Bearing insulation consists of placement of a non-conductive sheet barrier between the
bottom of the bearing pedestals and the sole plates. Care must be taken to ensure that all
hold-down bolts, dowels, and oil piping are insulated from the pedestal. Figure 6 shows an
insulated bearing pedestal. The bearing pedestal rests on the sole plate and is insulated from
the sole plate through use of a micarta insulation strip. All dowel pins and hold-down bolts
are insulated with micarta tubes to prevent a short circuit between the pedestal and the sole
plate.
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Ball-type bearings also can be insulated through placement of an insulating material between
the outer race of the motor bearing and the motor housing end shield. Figure 7 shows an
expanded view of an end-shield type bearing with a sleeve insulator. When the bearing is
constructed, a non-conductive insulating sleeve is molded to the motor bearing. After the
insulating sleeve is molded, the sleeve is machined into the motor housing end shield. The
combination bearing/insulating sleeve is press-fitted into the motor housing end shield. The
insulating sleeve that is bonded to the motor bearing electrically isolates the rotor
shaft/bearing from the motor housing end shield. Such electrical isolation prevents the flow
of current through the bearing to ground.
Bearing insulation is tested through use of a megger. The megger is connected between the
motor shaft and ground. After the megger is connected, a test voltage of 500V is applied
between the motor shaft and ground. The test voltage is applied for one minute, and at the
end of one minute, the bearing insulation resistance is read from the meter that is on the face
of the megger.
End Shield Type Bearing with Sleeve Insulator
Figure 7
The bearing insulation test is performed to ensure that the insulation material has not been
damaged or short circuited. The test will also verify that no external or control device has
bridged the bearing insulation. It is possible to bridge the insulation gap when external
equipment such as metal oil ports or supports are attached to the motor.
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The following are the general steps that are necessary to verify the integrity of the bearing
insulation:
Uncouple the shaft from connected equipment.
Disconnect the ground strap at the base of the pedestal.
Disconnect all temperature monitoring equipment.
Megger the insulation that is between the bearing housing and ground.
The test results are considered unsatisfactory if the insulation measures less than 200k ohms;
however, the desired insulation value is one megohm. Before the pedestal insulation is
considered defective, all other sources of a short circuit must be eliminated. These sources
include but are not limited to the following items:
Uninsulated pipes that touch both the pedestal and sole plate.
Guard rails in contact with the pedestal.
Tools, ladders, or other equipment in contact with the pedestal.
Pumps or other equipment that are geared or coupled to the motor.
Other items that should not be overlooked are good housekeeping measures. Tools or other
miscellaneous items are often the cause of low insulation measurements.
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DETERMINING WHETHER MOTORS ARE FUNCTIONING PROPERLY
Determining whether motors are functioning properly is one of the major tasks involved in
equipment management. The Electrical Engineer must be able to evaluate motor maintenancerecords to determine if the motor is functioning properly. To determine if the motor is
functioning properly, the Electrical Engineer must compare motor maintenance values against
the acceptable limits and against the normal operational values. More importantly, the
Engineer must be able to recognize trends that indicate a deterioration of the motors
operation. Many times a motor will show early signs of a pending failure through trends, and
if the Electrical Engineer learns to recognize these trends, major equipment failures can be
averted.
Motor Maintenance Record and Interpretation
Motor maintenance record forms contain the information and the data that are obtained from
motor testing. The Motor Maintenance Record is a two-part form. Part I of this form
contains the motor identification data and a record of the insulation resistance test data and the
vibration test data. Part II of this form is a record of the work that was performed on the
motor over the motors life. As Figure 8 shows, Part I of the Motor Maintenance Record is
divided into the following four areas:
The Motor Identification Data
The 60 Second Insulation Resistance Graph
The Insulation Resistance Data
The Vibration Level Data
The first section of the Record Motor Maintenance Record contains the following data for
motor identification:
Make
Type
Serial No.
Voltage
Amps
Frequency
Speed Rating (hp/kW)
S.F.
Insulation Class
Location
Date Installed
Description of Duty
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Motor Maintenance Record (Part I)
Figure 8
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The temperature of the motor windings also should be measured and recorded in the
appropriate block on the form. The temperature of the motor windings can be measured
through use of the RTDs that are installed in the motor If the motor does not have installed
RTDs, temporary temperature probes that are inserted into the motor housing are used. The
temperature of the motor windings is then used to calculate the temperature corrected, 60
second phase-to-ground IR measurement. The equation for use in temperature correction of
IR measurements is located in Work Aid 1.
The temperature corrected, 60 second phase-to-ground IR measurement and the calculated
phase-to-ground polarization index should be compared to the minimum acceptable values
and to the previously recorded values to determine whether these values are acceptable.
Work Aid 1 contains the minimum acceptable values for IR and for PI.
On motors that have accessible phase connections, the phase-to-phase insulation resistance
should be measured and should be recorded for all possible phase combinations. A one-minute and a ten-minute insulation resistance measurement should be recorded for each phase
combination. Through use of the one-minute and the ten-minute phase-to-phase insulation
resistance measurements, the phase-to-phase Polarization Index (PI) can be calculated. The
one-minute, the ten-minute, and the PI measurements should be compared to the minimum
acceptable values and to the previous readings to ensure that the values are acceptable. Work
Aid 1 provides the minimum acceptable values for IR and for PI.
Vibration Level Data and Data Interpretation
The vibration level of the motor should be determined through use of installed probes or
through use of temporary vibration probes, and the vibration level data should be recorded in
the Vibration Levels Data section of the form. The vibration level for both the drive end
bearing and for the non-drive end bearing should be recorded. The level of vibration should
be compared to the minimum acceptable values and to the previous values. Any changes in
the vibration level could be a signal of pending trouble and should be investigated. A trend of
an increase vibration could indicate wear on the motor bearings and would warrant further
investigation.
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Figure 9 is a typical vibration level trend chart that shows a graph of vibration level (in in/sec)
with time (in months). This graph shows a relatively constant vibration level from January of
1986 to November of 1986. Such a graph is consistent with normal motor operation. The
graph also shows a sharp rise in the vibration levels for two consecutive months: December
of 1986 and January of 1987. Such a rise indicates that a problem has developed and that the
problem is getting worse. If this problem is not corrected, it eventually will lead to a
breakdown as shown by the dotted line on the graph. Warning time and repair level, also
shown in Figure 9, illustrate the use of trend analysis in directing the operation and
maintenance of electric motors. Through establishment of a repair level that is below the
breakdown level, a sufficient amount of warning can be provided to schedule corrective
maintenance before a failure occurs.
The point at which a breakdown will occur can be predicted from experience with a similar
machine or from vibration standards. Work Aid 1 contains the maximum acceptable vibration
levels for Saudi Aramco motors. The value that is displayed on the graph in Figure 9 is anoverall value that represents the energy content of all the vibration frequencies.
Trend Chart
Figure 9
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Vibration level measurements indicate the magnitude of the vibration, but these
measurements do not indicate the source or the cause of the vibration. A narrow band
frequency analysis should be performed through use of a vibration spectrum, shown in Figure
10, to determine the exact source of the vibration. Figure 10 shows a frequency spectrum
graph of acceleration (in gs) versus frequency. The graph shows low values of acceleration
at the lower frequency ranges and higher values of acceleration at the higher frequency
ranges. These higher values indicate problem areas, and these frequencies can be pin-pointed
to specific sources of vibration.
The analysis of the vibration spectrum is beyond the scope of this Module. If a problem is
identified through use of trend analysis, a narrow band frequency analysis should be
performed by the appropriate personnel.
Typical Vibration Spectrum
Figure 10
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Maintenance History of Motors
As Figure 11 shows, Part II of the Motor Maintenance Record is divided into the following
sections:
Motor Identification Data
Description of Preventive and Corrective Maintenance Performed
The Motor Identification Data for Part II of the Motor Maintenance Record are the same as
previously discussed for Part I of the Motor Maintenance Record. The identification data are
duplicated so that the motor to which the information pertains can be identified if the forms
become separated. Part II of the Motor Maintenance Record is a narrative of the preventive
and corrective maintenance that is performed on the motor. The results of all inspections and
tests that are performed on the motor should be recorded in this section.
The information that is recorded in this section will allow others to better understand themaintenance history of the motor. The following are examples of the types of information
that are placed in the Description of Preventive and Corrective Maintenance Performed
section:
Results of the visual inspection.
Results of the bearing inspection.
Results of the RTD inspection.
Motor alignment data.
Dates and methods of insulation cleaning and drying.
Signs of abnormal operation.
Corrective troubleshooting.
Corrective maintenance.
A list of parts that were replaced or that were refurbished.
Any pertinent data that will help in future maintenance activities.
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Motor Maintenance Record (Part II)
Figure 11
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DETERMINING THE CORRECTIVE ACTIONS FOR COMMON MOTORPROBLEMS
In addition to being able to identify motor problems, the Electrical Engineer must be able todetermine the proper corrective actions for the identified problems. If the wrong corrective
actions were taken, the problem could be compounded, and the damage to the motor could
escalate. This section of the module will discuss common motor problems and the corrective
action for each problem. The corrective actions that are presented are by no means the only
method to solve the problem. Determination of the appropriate corrective actions for a
specific problem must be accomplished through use of manufacturers troubleshooting guides
and the Electrical Engineers knowledge and experience. The following common motor
problems and their corrective actions are discussed in this section:
Vibration Alarms
Temperature Alarms Motor Trips
Vibration Alarms
A sudden and significant increase in vibration amplitude is a very apparent indicator that
something is wrong with a motor. A gradual increase in vibration amplitude may not be
noted until damage occurs. Vibration monitoring equipment is installed on motors above 185
kW
(250 Hp) to alert personnel to abnormal vibration levels so that action can be taken to correct
the problem before damage occurs. The vibration limits of motors are set so that the motoralarms or shuts down prior to serious damage to the motor. Motors below 185 kW (250 hp)
are not equipped with vibration monitoring equipment and must be monitored by experienced
personnel to judge changes in vibration levels.
The vibration alarm and shutdown setpoints depend on the speed of the motor and the type of
vibration probe that is installed. Figure 12 shows the recommended vibration alarm and
shutdown setpoints for motors with various speeds and vibration monitoring probes.
The variations in alarm and shutdown levels occur because different speed motors will
naturally vibrate at different levels. The changes in alarm and shutdown levels for different
types of vibration probes occur because of the difference in the accuracy of the probes.
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Vibration Alarm and Shutdown SetpointsFigure 12
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Vibration alarms are symptoms for a variety of motor problems. The following are examples
of problems that can cause vibration alarms:
Loose mounting bolts
Loose coupling
Motor and load misalignment
Worn motor bearings
Mechanically unbalanced load
Mechanically unbalanced rotor
Bent or cracked shaft
Excessively pulsating load
Out of synchronism (synchronous motor only)
The corrective action for a vibration alarm will depend on the root cause of the vibration
alarm. In the case of an alarm, the motor should be stopped immediately to prevent excessivedamage to the motor or to the surrounding equipment. The possible causes of a vibration
alarm should be investigated one at a time. The most common problems should be
investigated first. Common problems can be found through a review of the Motor
Maintenance Record form. Items such as loose bolts, loose couplings, or misalignments also
are considered common problems. Work Aid 2 contains a procedure for troubleshooting a
vibration alarm. Possible corrective actions for some of the possible causes also are provided
in Work Aid 2.
Temperature Alarms
The goal of a temperature alarm is to stop a motor when a motor-related high temperature
condition exists. When properly applied, the high temperature alarm will trip a motor before
the high temperature condition can cause damage. The following types of temperature alarms
are associated with motors:
Winding Alarms
Bearing Alarms
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The temperature alarm or the trip settings will be dictated by the insulation class of the motor.
The alarm or the trip setting will always be lower than the maximum allowable insulation
temperature to ensure that no damage occurs to the winding insulation. The motor alarm/trip
setpoints should be in accordance with the following:
Motor Alarm or Maximum Allowable
Insulation Class Trip Setting Insulation Temperature
degrees C degrees C
B 120 130
F 145 155
H 170 180
Work Aid 2 contains common winding temperature problems and a course of corrective
actions for each problem. The corrective actions for each of the possible causes of windingtemperature alarms are different. The proper action to be taken to correct the problem will
depend upon the root cause of the winding temperature alarm.
Bearing Alarms
Bearing alarms are provided to protect the bearings from damage that can be caused by
excessive temperatures. Excessive heating of a bearing will cause a rapid deterioration of the
bearing and the bearing lubrication, and such deterioration can lead to a rapid failure of the
motor.
Excessive bearing temperatures, although detrimental to the bearings, are generally only a
symptom of a larger problem. An investigation must be performed to correct the root cause
of the bearing alarm as well as to repair the bearing, if necessary. The following are some of
the causes of bearing alarms:
Loss of lubrication flow
Motor out of alignment
Dirt in lubrication system
Overlubrication of grease lubricated bearings
Overload on motor
Old bearings Incorrect bearings
Bearing alarms must be set to respond at a temperature that is lower than the temperature at
which bearing damage will occur. The maximum temperature that a bearing can withstand
without damage will depend on the type of bearing that is installed.
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The maximum temperature and bearing alarm setpoints should be as follows:
Maximum Allowable Bearing
Alarm and Temperature ino
C based onBearing Type Trip Setting in oC a 40oC rise above ambient
Anti-Friction 82 92
Sleeve 80 90
The proper corrective action for bearing alarms will depend on the root cause of the bearing
alarm. In general, the first priority is to stop further heating of the bearing. The next step is to
cool the bearing. The bearings will start to cool as soon as the motor is tripped or as soon as
the load is removed from the motor. Motors will automatically trip on a bearing alarm to help
prevent further heating of the bearings. After the immediate corrective actions have been
taken, the next step is to determine the cause of the problem and to correct the situation. Afterthe root cause of the problem has been corrected, the bearing and motor shaft should be
inspected. Work Aid 2 presents some of the possible causes of bearing alarms and a course of
corrective action for each possible cause.
Motor Trips
Motor trips are installed to prevent damage to the motor or to the system due to undesirable
motor or system conditions. Motor trips are divided into two main categories:
Faults Process Interlocks
Faults
Motor trips are installed to protect against electrical system faults and against mechanical
faults. The type of motor trip that is provided depends on the specific motor installation. The
following is a list of the possible motor trips:
Motor overload
Phase overcurrent
Ground fault Current unbalanced
Vibration
Temperature alarm
Phase-to-phase fault
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If a motor trip occurs, the Electrical Engineer must use all available indications to analyze the
cause of the motor trip. On very large motors, alarm panels are provided that make the
troubleshooting procedure much easier. Smaller motors or motors without alarm panels
require more thought and troubleshooting skills. Ground faults and phase-to-phase faults
quickly can result in motor winding insulation break down. Lockout relays (86 devices) are
used in conjunction with the relays that detect ground faults and phase-to-phase faults to limit
the motor damage that can be caused by these faults. Because lockout relays must be
manually reset after they operate, these relays force maintenance personnel to investigate the
cause of the problem prior to re-energization of the motor. Correct determination of the cause
of the motor trip is vital in the recovery of the motor. An Electrical Engineer must use his
knowledge and common sense to analyze the cause of a motor trip. Many times, the obvious
signs can be overlooked because they seem too simple. Work Aid 2 provides a procedure
to guide the Engineer in troubleshooting. Work Aid 2 also provides a list of the common
motor problems and their possible causes. In addition to the troubleshooting guides, the
Engineer should also obtain the maintenance records and the manufacturers troubleshootingguides for the motor.
The corrective actions that must be taken for a motor trip will depend on what causes the relay
to actuate. Different corrective actions must be performed for the different causes of relay
trips. The wrong actions could cause more harm than good. Each set of corrective actions is
different, but all corrective actions have the same goal. The goal of corrective actions is to
prevent further damage to the motor until the motor fault is cleared and the motor is repaired.
The first step in any motor trip corrective action will be to turn off the motor to ensure that the
motor will not accidentally restart if the relay resets. Work Aid 2 contains the possible
courses of action for each possible cause of a motor trip.
Process Interlocks
The second category of motor trips is process interlocks. Process interlocks are provided to
control a system and are not provided to protect the motor; therefore, when a motor trips or
fails to start due a process interlock, the motor control circuit is operating properly. In some
instances, these process interlocks are confused with a motor problem and delay the recovery
process. The Engineer must be fully aware of the motors start and stop permissives prior to
troubleshooting the motor circuit.
Process interlock trips are varied and depend on the different systems in which motors areinstalled. Process interlocks can be associated with system pressure, temperature, level, or a
multitude of other parameters. When a motor trips or fails to start, the process interlocks
should be the first step of the troubleshooting process. Indications of all these parameters
should be readily available. An example of a process interlock that operates during normal
operation is a motor that trips after the pump that is driven by the motor fills a storage tank.
Although the motor tripped, the trip was an expected event, and no further investigation is
required.
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WORK AID 1: PROCEDURE AND ACCEPTABLE TEST VALUES (PERFORMEDDURING MOTOR MAINTENANCE AND COMPILED FROMSADP-P-113, NFPA 70B, AND ESTABLISHED ENGINEERING
PRACTICES) FOR DETERMINING WHETHER MOTORS AREFUNCTIONING PROPERLY
Use this Work Ai d to complete Exercise 1.
Procedure
Perform the following steps to determine if the motor is operating properly.
1. Temperature correct the insulation resistance values.
2. Compare the insulation resistance values to the minimum acceptable insulationresistance values and to the previously measured insulation resistance values to
determine whether the current insulation resistance values are acceptable.
3. Calculate the polarization index (PI) and compare this PI to the minimum acceptable
PI to determine whether the current value is acceptable.
4. Compare the current vibration level measurements to the maximum allowable
vibration levels to determine whether the current motor vibration levels are acceptable.
5. Review the narrative portion of the Motor Maintenance Record to determine whether
the motor has other discrepancies that require corrective action.
Technical Requirements
Insulation Resistance Test - If IR is below the minimum acceptable level, the motor insulation
should be cleaned and dried.
The minimum acceptable insulation resistance value is determined from the following
equation:
RM = 1 megohm per kV of motor rated voltage plus 1 megohm
where: RM = Minimum insulation resistance in megohms at 50oC
kV = Rated voltage of the motor in kilovolts
Any rapid drop in the IR value, even if the IR is still above the minimum value, is considered
unacceptable.
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Polar ization Index
Minimum acceptable polarization index - 2:
Temperature Correction of IR
Use the following equation to temperature correct insulation resistance values:
RC= KTx RT
where: R C = Insulation resistance in megohms, corrected to 40oC
RT = Measured insulation resistance, in megohms, at winding temperature T
KT = Insulation resistance temperature correction factor
To find KT, use Figure 14, Insulation Resistance Variation with Temperature.
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Insulation Resistance Variation with TemperatureFigure 14
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Vibration Level Measurement
The maximum allowable vibration levels for motors with seismic velocity transducers is 4.6
mm/s (0.18 in/s) zero to peak.
The maximum allowable vibration levels for horizontal motors with proximity probes are as
follows:
3600 rpm - 2 mils
1800 rpm - 2.5 mils
1200 rpm - 3 mils
or less
If the maximum vibration level is exceeded, the cause should be investigated and should be
corrected.
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Troubleshooting Chart for Common Motor Problems
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anti friction bearing A bearing that employs an assembly of balls or rollers forrotation
babbitt Soft metallic lining material of a sleeve bearing.
bearing race The portion of an antifriction bearing that is connected to theshaft or bearing housing and that allows the ball bearing to
rotate.
corrective maintenance The maintenance that is carried out after a failure has occurred.This maintenance is intended to restore an item to a state in
which it can perform its required function.
endplay The amount by which the shaft of a motor can move in the axial
direction.
freeplay The amount by which the shaft of a motor can move in the radialdirection.
gs Acceleration that is due to the force of gravity.
Insulation Resistance (IR) The resistance that is offered by an insulation to the flow ofcurrent that results from an impressed direct voltage.
Polarization Index (PI) The ratio of the ten-minute insulation resistance measurement to
the one-minute insulation resistance measurement.
preventive maintenance The maintenance that is intended to prevent or to reduce theprobability of failure or the performance degradation of an item.
This maintenance is carried out at predetermined intervals,
according to prescribed criteria.
resistance temperature A temperature monitoring device that works on the principle ofa change in resistance as the temperature changes.
service factor (SF) A multiplier that, when applied to the rated power, indicates thepermissible power loads that can be carried by a motor.
sleeve bearing A bearing with a cylindrical inner surface in which the journal ofa rotor shaft rotates
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ADDENDUM
Topographic Map of McCamey North, Texas
Topographic Map of Mc Elroy Ranch, Texas
Topographic Map of Tatum, New Mexico - Texas
Design Basis Scoping Paper