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newSchoolWorks.com › VoTechWorks › Building & Construction › Nuclear Power Plant Technician / Operator Prep › Nuclear Plant Maintenance: Procedures, Equipment, and Reliability › Lesson 6
BLD-NUCL84-08 · Lesson 6 · 28 clock hours · lab

Pump Maintenance: Alignment, Mechanical Seals, Bearing Inspection, and Lubrication

Objective. Students will perform a dial-indicator shaft alignment check on a motor-pump coupling, inspect and replace a mechanical seal, check bearing clearances, and lubricate bearings per the manufacturer's procedure.

⚠ Pending professional review

This lesson, its worked examples and its practice questions has not yet been verified by a licensed professional engineer or master tradesperson or by a certification or approval body. It is a study draft and may contain errors. Do not use it as instructions for work on a job site or on live equipment. Always follow your instructor, your supervising professional, and current official references and regulations.

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Pump Maintenance: Alignment, Mechanical Seals, Bearing Inspection, and Lubrication

Pumps are among the most critical pieces of rotating equipment in a nuclear plant. Reactor coolant pumps, feedwater pumps, and emergency core cooling system pumps must be reliable at all times. Proper shaft alignment, functional mechanical seals, correct bearing clearances, and scheduled lubrication are the four pillars that keep pumps running within design limits. This lab introduces each task using a small training centrifugal pump and motor set.

Safety Before You Touch Any Equipment

Before beginning any pump maintenance task, verify that the equipment has been properly isolated under the plant's lockout/tagout (LOTO) procedure in accordance with OSHA standards. Confirm that all energy sources — electrical, hydraulic, and stored mechanical — are de-energized and locked out. Review the work order and the manufacturer's manual before touching the pump. Wear the PPE specified for the job: safety glasses, chemical-resistant gloves when handling O-ring lubricant or grease, and steel-toed boots. If the pump has been in service in a radiologically controlled area, follow radiation protection (RP) requirements, including dose rate surveys and the use of appropriate dosimetry, before disassembly.

Shaft Alignment

When a motor and pump are coupled together, their shaft centerlines must be aligned within tight tolerances. Misalignment creates vibration, increases bearing wear, damages mechanical seals, and can cause premature coupling failure. Two types of misalignment are measured:

A dial indicator mounted on a magnetic base is the standard tool for this measurement. The indicator tip contacts a machined surface on the coupling hub or shaft. You rotate the shaft manually and record indicator readings at four positions: 12 o'clock (top), 3 o'clock, 6 o'clock (bottom), and 9 o'clock. The difference between opposing readings (12 vs. 6, 3 vs. 9) indicates the amount of misalignment. Manufacturer and plant procedures publish acceptance tolerances — typically expressed in thousandths of an inch (mils) — that define whether shims must be added or removed under the motor feet to correct the alignment. Always document raw readings and any shim changes on the alignment data sheet before closing out the work order.

Mechanical Seals

A mechanical seal prevents process fluid from leaking along the pump shaft where it exits the casing. A typical cartridge mechanical seal consists of a rotating face (attached to the shaft) pressed against a stationary face (held in the seal chamber), with O-rings providing secondary sealing. The faces are lapped to a very fine surface finish; contamination or improper installation destroys that finish and causes immediate leakage.

To replace a mechanical seal:

1. Disassemble the seal chamber following the procedure sequence.

2. Remove the old seal and inspect both faces for scoring or cracking.

3. Clean all mating surfaces carefully — lint-free wipes only.

4. Lubricate new O-rings lightly with the compatible O-ring lubricant specified by the manufacturer. Do not over-lubricate; excess lubricant can attract contaminants.

5. Install the cartridge seal, keeping faces clean and square. Avoid touching lapped faces with bare hands.

6. Torque all fasteners to the value specified in the procedure using a calibrated torque wrench. Proper compression of the seal faces is achieved through correct torque, not feel.

7. Visually verify no O-ring is pinched or rolled before final assembly.

Bearing Clearance and Lubrication

Rolling-element and sleeve bearings support the rotating shaft and absorb radial and axial loads. Excessive clearance indicates wear; insufficient clearance can cause overheating. A feeler gauge (also called a thickness gauge) is used to measure radial clearance in the bearing housing. Insert blades of increasing thickness until you feel light drag; the thickest blade that passes is the clearance measurement. Compare this value to the acceptance band in the manufacturer's manual. If clearance exceeds the maximum or falls below the minimum, the bearing must be replaced before the pump returns to service.

Correct lubrication is equally important. Using the wrong grease type can chemically degrade seals or create incompatible grease mixtures that reduce lubrication effectiveness. Using too much grease can overheat bearings by churning. The manufacturer's manual specifies:

Apply grease with a hand grease gun to the grease fitting on the bearing housing. Count strokes carefully. Record the grease type, quantity, date, and your initials on the PM (preventive maintenance) checklist. Incomplete records are a compliance finding under the plant's maintenance rule and quality assurance program.

Documentation and the PM Checklist

Every step in this lab corresponds to a sign-off on the PM checklist. Nuclear maintenance is controlled work: if it is not documented, it did not happen. Alignment readings, shim sizes, seal part numbers, bearing clearance measurements, grease type, and quantity must all be entered accurately and legibly. The instructor will verify your seal installation before you re-assemble the seal chamber. Failing any single required item means repeating that task — just as it would on the plant floor.

Key terms10

Angular misalignmentA condition in which two coupled shafts meet at an angle rather than along a common straight centerline, producing vibration and accelerated wear.
Parallel (offset) misalignmentA condition in which two shaft centerlines are parallel to each other but displaced laterally, so they do not share the same line.
Dial indicatorA precision measuring instrument with a plunger that displays small linear displacements on a graduated dial, used to quantify shaft runout and coupling misalignment.
Cartridge mechanical sealA pre-assembled, self-contained seal unit that prevents process fluid from leaking along a rotating shaft, consisting of lapped rotating and stationary faces, O-rings, and a gland plate.
Feeler gaugeA set of precision metal blades of known thicknesses used to measure small gaps or clearances, such as radial bearing clearance in a housing.
Radial bearing clearanceThe total measured gap between a bearing's rolling elements (or journal) and its race or housing bore, indicating the degree of wear and whether the bearing remains within service limits.
Lockout/Tagout (LOTO)An OSHA-required energy control procedure that uses physical locks and warning tags to de-energize equipment and prevent accidental re-energization during maintenance.
PM checklistA preventive maintenance document that lists each required inspection and service task with sign-off lines; in nuclear plants it serves as the formal record of work performed and is subject to quality assurance requirements.
ShimA thin metal plate inserted under a motor or pump foot to raise or reposition the machine and correct shaft misalignment.
NLGI gradeA numerical classification established by the National Lubricating Grease Institute that describes grease consistency (thickness); selecting the correct NLGI grade is required to ensure proper bearing lubrication.

Worked examples4

1. A dial indicator is mounted on the coupling and the shaft is rotated to take parallel misalignment readings. The readings are: 12 o'clock = +0.004 in, 6 o'clock = −0.008 in. What is the total indicator reading (TIR) and what is the actual offset misalignment?
  1. Step 1 – Calculate TIR: Subtract the algebraically smaller reading from the larger. TIR = (+0.004) − (−0.008) = 0.004 + 0.008 = 0.012 in.
  2. Step 2 – Convert TIR to actual offset: For a single dial-indicator setup on a coupling, the actual shaft offset is one-half of the TIR, because the indicator sweeps through both sides of the centerline. Actual offset = 0.012 ÷ 2 = 0.006 in.
  3. Step 3 – Compare to tolerance: If the manufacturer's acceptance tolerance for parallel offset is ≤ 0.003 in, then 0.006 in exceeds the limit and shim correction is required.
Answer: TIR = 0.012 in; actual parallel offset = 0.006 in, which exceeds a 0.003 in tolerance and requires correction.
2. The manufacturer's manual states that the bearing housing should receive 1.5 oz of grease per re-lubrication. Your hand grease gun delivers approximately 0.05 oz per stroke. How many strokes are needed?
  1. Step 1 – Identify the required quantity: 1.5 oz.
  2. Step 2 – Identify the delivery rate: 0.05 oz per stroke.
  3. Step 3 – Divide required quantity by delivery rate: 1.5 ÷ 0.05 = 30 strokes.
  4. Step 4 – Apply exactly 30 strokes and record the quantity (1.5 oz) and stroke count on the PM checklist.
Answer: 30 strokes of the grease gun are required to deliver 1.5 oz.
3. You measure radial bearing clearance with a feeler gauge. The 0.003 in blade passes freely; the 0.004 in blade will not enter. The manufacturer's acceptance band is 0.002 in to 0.005 in. Is the bearing within tolerance?
  1. Step 1 – Determine the measured clearance: The thickest blade that passes freely is 0.003 in, so clearance = 0.003 in.
  2. Step 2 – Compare to the acceptance band minimum: 0.003 in ≥ 0.002 in minimum. ✓
  3. Step 3 – Compare to the acceptance band maximum: 0.003 in ≤ 0.005 in maximum. ✓
  4. Step 4 – Conclusion: The measurement falls within the acceptable range; the bearing does not require replacement at this time. Document 0.003 in on the PM checklist.
Answer: 0.003 in is within the 0.002–0.005 in acceptance band; the bearing is serviceable.
4. Angular misalignment readings taken at the coupling face are: top (12 o'clock) = +0.005 in, bottom (6 o'clock) = +0.001 in. What is the angular TIR across the vertical plane?
  1. Step 1 – Calculate angular TIR for the vertical plane: TIR = larger reading − smaller reading = 0.005 − 0.001 = 0.004 in.
  2. Step 2 – Interpret the result: The TIR of 0.004 in represents the total face gap difference measured across the coupling diameter in the vertical plane.
  3. Step 3 – Compare to tolerance: If the manufacturer's angular TIR tolerance is ≤ 0.002 in per coupling diameter, then 0.004 in exceeds the limit and the angular position of the motor must be corrected by shimming one pair of feet relative to the other.
Answer: Angular TIR in the vertical plane = 0.004 in; this exceeds a 0.002 in tolerance and requires correction.

The math in this lesson1

Learners compute total indicator reading and halve it to find actual shaft offset in thousandths.
N-Q.3
Brush up in the math halls at newSchoolWorks.com:
In class: Using a small centrifugal training pump and motor set, students mount a dial indicator on the coupling, rotate the shaft to take angular and parallel misalignment readings at four positions (12, 3, 6, and 9 o'clock), record readings on an alignment data sheet, and determine whether shim corrections are needed based on published acceptance tolerances. · Students disassemble the pump seal chamber, remove the worn mechanical seal, clean all mating surfaces, install a replacement seal cartridge using correct O-ring lubrication and compression technique, and reassemble the seal chamber to procedure-specified torque. · Students use a feeler gauge to check radial bearing clearance on the pump bearing housing, compare the measurement to the acceptance band in the manufacturer's manual, add the correct type and quantity of grease to the bearing housing using a hand grease gun, and complete the lubrication record on the PM checklist.
Materials: Training centrifugal pump and motor set with flexible coupling, magnetic-base dial indicator set, feeler gauge set, mechanical seal cartridge (replacement), compatible O-ring lubricant, calibrated torque wrench, grease gun, compatible bearing grease, alignment data sheet, PM checklist, manufacturer's manual excerpts, PPE
Check: Alignment readings within acceptable tolerance and documented on data sheet; seal installation verified visually by instructor with no pinched O-ring; bearing clearance and lubrication quantity recorded correctly; all PM checklist sign-offs present; failing any single item requires the task to be repeated.

Practice check8

Answer each question, then check it. Nothing is recorded; this is your own practice.

OSHA 10 / Safety and Health Fundamentals
1. Before beginning disassembly of a pump in a nuclear plant, the first required safety action is to:
NUCP Uniform Curriculum / Plant Maintenance and Equipment
2. A dial indicator gives parallel misalignment readings of +0.006 in at 12 o'clock and −0.006 in at 6 o'clock. What is the actual shaft offset?
NUCP Uniform Curriculum / Plant Maintenance and Equipment
3. When installing a replacement cartridge mechanical seal, O-rings should be lubricated with:
NUCP Uniform Curriculum / Plant Maintenance and Equipment
4. Which of the following best describes angular misalignment in a motor-pump coupling?
NUCP Uniform Curriculum / Plant Maintenance and Equipment
5. A bearing housing requires 2.0 oz of grease. The grease gun delivers 0.04 oz per stroke. How many strokes must be applied?
NUCP Uniform Curriculum / Plant Maintenance and Equipment
6. A feeler gauge check shows that the 0.006 in blade passes freely and the 0.007 in blade will not enter. The manufacturer's acceptance band is 0.002–0.005 in. What is the correct action?
NRC Non-Licensed Operator Prep / Quality Assurance and Documentation
7. In nuclear plant maintenance, why is accurate completion of the PM checklist considered a compliance requirement rather than just a best practice?
NRRPT Radiation Protection Tech / Radiation Safety Practices
8. Which PPE consideration is specifically important when performing pump maintenance in a radiologically controlled area of a nuclear plant, in addition to standard industrial PPE?
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