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newSchoolWorks.com › VoTechWorks › Building & Construction › Nuclear Power Plant Technician / Operator Prep › Nuclear Plant Maintenance: Procedures, Equipment, and Reliability › Lesson 5
BLD-NUCL84-08 · Lesson 5 · 28 clock hours · lab

Valve Maintenance: Types, Disassembly, Inspection, Packing, and Reassembly

Objective. Students will disassemble, inspect, repack, and reassemble a gate valve and a globe valve using a written maintenance procedure, applying correct torque values and documenting all steps on a maintenance data sheet.

⚠ 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.

VoTechWorks program approval and accreditation are still in progress. Credentials are issued only by the certifying bodies named in each program, after their own exams.

Why Valve Maintenance Matters in a Nuclear Plant

Valves control the flow of reactor coolant, steam, feedwater, and dozens of other process fluids. A leaking or seized valve can force a plant shutdown, create a radiological hazard, or cause a pressure boundary failure. That is why every valve maintenance task in a nuclear facility is governed by a written procedure, every measurement is recorded, and every fastener must be torqued and documented before the job is considered complete. The maintenance data sheet is your legal record—if it is not written down, it did not happen.

Gate Valves vs. Globe Valves: Know the Difference

A gate valve uses a wedge-shaped or parallel disc (the gate) that slides perpendicular to the flow path. It is designed for fully open or fully closed service, not throttling. When open, flow resistance is very low because the gate retracts completely out of the flow path. The bonnet houses the stem and packing and is bolted to the valve body.

A globe valve uses a disc that moves parallel to the flow axis, pressing against a circular seat inside the body. Globe valves are designed for throttling as well as shutoff. The internal geometry creates higher pressure drop than a gate valve of the same size, but the globe valve gives finer flow control.

Knowing the type matters during disassembly because the internal geometry, seat arrangement, and stem thread direction differ between the two.

Disassembly: Follow the Procedure Exactly

Before touching any valve, complete your pre-job brief, verify the system is de-energized and isolated, and confirm lockout/tagout (LOTO) is in place per OSHA 29 CFR 1910.147. Wear safety glasses and gloves throughout the lab.

To remove a gate valve bonnet:

1. Record the valve tag number and current position on the data sheet.

2. Loosen bonnet bolts in a cross-pattern (also called a star pattern) to relieve pressure evenly and avoid warping the flange. Never loosen bolts in a circular sequence.

3. Remove bolts, lift the bonnet straight up, and place it on a clean work surface.

4. Remove the stem and disc assembly; note the orientation before lifting.

Inspection and Acceptance Criteria

Once disassembled, inspect the following:

Record every finding using precise language on the condition assessment form: describe location, size, and character of each defect. Check each item against the acceptance criteria table in the procedure and mark each finding as acceptable (A) or rejectable (R).

Packing Removal, Sizing, and Installation

The stuffing box is the cylindrical cavity in the bonnet that holds the packing around the stem. Old packing must be removed completely using a packing hook or pick—leave no fragments behind because debris under new packing causes uneven loading and early leakage.

Measuring packing cross-section: Use calipers to measure the stuffing box bore diameter and the stem diameter. The required packing cross-section equals half the difference: (bore − stem diameter) ÷ 2. Select pre-formed graphite packing rings that match this cross-section.

Cutting rings: When cutting from bulk packing, wrap the packing around the stem, mark the circumference, and cut at a 45-degree diagonal (bias cut). The diagonal cut allows the joint ends to mate without a gap when the ring is seated, reducing leak paths.

Installing rings: Install rings one at a time. Rotate each ring so that the cut joint is offset (staggered) approximately 90 degrees from the previous ring—never stack cuts on top of each other. Seat each ring fully with the packing follower before installing the next. After all rings are installed, thread on the packing gland and tighten to the initial torque specified in the procedure.

Reassembly and Torquing

Replace the bonnet gasket with a new one of the correct material and dimension—never reuse a compressed gasket. Lower the bonnet carefully, aligning bolt holes without dragging the gasket. Hand-tighten all bolts before using a torque wrench.

Tighten bonnet bolts in two or three passes using a cross-pattern sequence, reaching the final torque value only on the last pass. Using a calibrated torque wrench ensures accurate loading. Record the torque value and the wrench calibration sticker number on the data sheet.

Hydrostatic Bench Leak Test

After reassembly, connect the valve to the hydrostatic bench test rig. Fill the valve body with water, vent all air, and raise pressure to the test pressure specified in the procedure. Hold for the required time while observing the bonnet joint, stem packing, and body for visible leaks or pressure drop. Any visible weeping at the packing is a failure—depressurize, re-examine gland torque, and retest. Record the test pressure, hold time, and result on the data sheet.

Documentation: The Final Step

Every sign-off block on the maintenance data sheet must be completed before the work order is closed. Unsigned blocks mean incomplete work. Your instructor will review the data sheet, verify torque entries, and witness the hydrostatic test. An untorqued fastener or any missing documentation entry is an automatic fail of this lab.

Key terms9

stuffing boxThe cylindrical cavity in the valve bonnet that surrounds the stem and holds the packing material to prevent process fluid from leaking along the stem.
packing cross-sectionThe thickness of packing material required to fill the annular space between the stem and the stuffing box bore; calculated as half the difference of bore diameter and stem diameter.
bias (45-degree diagonal) cutThe method of cutting packing rings at a 45-degree angle so the cut ends mate flush around the stem, minimizing leak paths at the joint.
cross-pattern torque sequenceA bolt-tightening order in which fasteners are tightened in a star or cross pattern rather than in a circle, ensuring even load distribution across the gasket or flange.
wire-drawingA narrow erosion groove cut into a valve seat by high-velocity fluid leaking past a partially closed valve; a common rejectable seat defect.
hydrostatic bench testA pressure test in which the assembled valve is filled with water, pressurized to a specified value, and examined for visible leakage to verify the integrity of the pressure boundary.
acceptance criteriaThe specific dimensional and visual limits stated in the maintenance procedure that determine whether a component is serviceable or must be repaired or replaced.
maintenance data sheetThe official written record accompanying a maintenance work order on which technicians document all measurements, inspection findings, torque values, and test results; each step requires a sign-off.
lockout/tagout (LOTO)An energy-control procedure required by OSHA that isolates and de-energizes equipment before maintenance begins, preventing unexpected startup or release of stored energy.

Worked examples4

1. The stuffing box bore of a gate valve measures 1.250 inches. The stem diameter measures 0.750 inches. What packing cross-section should you select?
  1. Write the formula: packing cross-section = (bore diameter − stem diameter) ÷ 2
  2. Substitute values: (1.250 in − 0.750 in) ÷ 2
  3. Subtract: 1.250 − 0.750 = 0.500 in
  4. Divide: 0.500 ÷ 2 = 0.250 in
Answer: Select packing with a 0.250-inch (1/4-inch) cross-section.
2. You are installing 5 graphite packing rings in a stuffing box. At what rotational offset should each ring's cut joint be placed relative to the previous ring?
  1. The standard practice is to stagger ring joints so no two cuts are aligned.
  2. With 5 rings and a 360-degree circle, divide 360 by 5: 360 ÷ 5 = 72 degrees.
  3. However, the practical field rule stated in most procedures is to offset each ring approximately 90 degrees from the previous one when 4 or fewer rings are used, or to distribute evenly when more rings are used.
  4. For 5 rings, distribute joints at approximately 72-degree intervals around the stem circumference; record the actual orientation used on the data sheet.
Answer: Stagger each ring joint approximately 72 degrees from the previous ring (360 ÷ 5) so no two cuts align; joints must never be stacked.
3. A valve has 8 bonnet bolts. You are performing a two-pass cross-pattern torque. The final torque is 40 ft-lb. In the first pass, you apply 50 percent of final torque. What is the first-pass torque value, and how do you determine the tightening sequence for an 8-bolt pattern?
  1. Calculate first-pass torque: 40 ft-lb × 0.50 = 20 ft-lb.
  2. Number the bolts 1 through 8 clockwise around the flange.
  3. For a cross-pattern on 8 bolts, tighten in the sequence: 1, 5, 3, 7, 2, 6, 4, 8 (opposite pairs, then filling in).
  4. First pass: tighten each bolt to 20 ft-lb in that sequence.
  5. Second pass: tighten each bolt to 40 ft-lb in the same sequence.
  6. Record the wrench calibration number and final torque for each bolt on the data sheet.
Answer: First-pass torque is 20 ft-lb. Tighten the 8 bolts in a cross-pattern sequence (e.g., 1-5-3-7-2-6-4-8) to ensure even gasket loading, then repeat at 40 ft-lb on the final pass.
4. During inspection, you measure a stem diameter of 0.748 inches. The procedure acceptance criterion states the stem must be within 0.002 inches of its nominal diameter of 0.750 inches. Is this stem serviceable?
  1. Calculate the deviation: nominal − measured = 0.750 − 0.748 = 0.002 inches.
  2. The acceptance criterion allows up to 0.002 inches of deviation.
  3. 0.002 in deviation equals the limit exactly; it is not greater than 0.002 in.
  4. Mark the finding on the condition assessment form: stem diameter 0.748 in, deviation 0.002 in, result: Acceptable (A).
Answer: The stem is serviceable; its 0.002-inch deviation equals but does not exceed the acceptance limit. Document the measured value and mark it Acceptable on the form.

The math in this lesson1

Learners compute packing cross-section as half the bore minus stem diameter to the thousandth inch.
N-Q.3
Brush up in the math halls at newSchoolWorks.com:
In class: Following a written disassembly procedure, students remove the bonnet from a 2-inch gate valve using the correct wrench sequence, inspect the disc, seats, and stem for wear and scoring, record findings on a condition assessment form, and determine whether the component is serviceable based on provided acceptance criteria. · Students remove old packing from the gate valve stuffing box, measure the correct packing cross-section using calipers, cut new graphite packing rings to the correct length using the 45-degree diagonal cut method, install rings in the correct staggered orientation, and set initial packing gland torque per the procedure. · Students reassemble the valve, torque bonnet bolts to the specified value in a cross-pattern sequence using a calibrated torque wrench, perform a hydrostatic bench leak test using the training test rig, and complete all sign-off blocks on the maintenance data sheet before presenting the finished work order package to the instructor.
Materials: 2-inch gate valve (training use), 2-inch globe valve (training use), graphite packing rings, calibrated torque wrench, calipers, maintenance procedure (valve specific, training version), maintenance data sheet, hydrostatic bench test rig, pipe wrenches, assorted hand tools, PPE (safety glasses, gloves)
Check: Instructor inspects final valve assembly, reviews torque values on data sheet, and observes hydrostatic test; no visible packing leak at test pressure and all data sheet sign-offs complete; any untorqued fastener or undocumented step is a fail.

Practice check8

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

NUCP Uniform Curriculum – Maintenance Procedures
1. What is the primary purpose of removing bonnet bolts in a cross-pattern sequence rather than in a circular sequence?
NUCP Uniform Curriculum – Valve Maintenance
2. A technician is installing graphite packing rings and places the cut joint of each new ring directly on top of the cut joint of the previous ring. What problem does this create?
NRRPT / NUCP – Component Inspection and Defect Identification
3. During inspection of a gate valve seat, a technician observes a narrow groove running across the seating surface caused by historical fluid leakage at partial closure. This defect is called:
OSHA 10 – Hazardous Energy Control
4. Before beginning any valve disassembly on a nuclear plant system, which control is required by OSHA to prevent unexpected re-energization of the system?
NUCP Uniform Curriculum – Measurements and Calculations
5. A stuffing box bore measures 1.500 inches and the stem measures 0.750 inches. What is the correct packing cross-section?
NUCP Uniform Curriculum – Valve Testing and Acceptance
6. After reassembling a gate valve and completing the hydrostatic bench test, the technician observes a slow weep of water at the packing gland. What is the correct next action?
NUCP Uniform Curriculum – Maintenance Best Practices
7. Why must a compressed bonnet gasket never be reused during valve reassembly?
NRC Non-Licensed Operator Prep – Quality Records and Documentation
8. On a maintenance data sheet, a technician completes all repair steps but does not sign off the final torque verification block before returning the work order. Under the pass/fail criteria for this lab, what is the outcome?
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