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newSchoolWorks.com › VoTechWorks › Manufacturing & Marine › Industrial Maintenance › Pumps: Selection, Installation, Operation, and Maintenance › Lesson 6
MFG-INDU16-04 · Lesson 6 · 15 clock hours · lab

Mechanical Seals and Packing: Inspection and Replacement

Objective. Students will remove a worn mechanical seal and packing from pump shafts, identify failure mode from wear patterns, install replacement components to manufacturer specification, and adjust packing gland to the correct leak rate.

⚠ Pending professional review

This lesson, its worked examples and its practice questions has not yet been verified by a licensed engineer or certified journey-level 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 Shaft Sealing Matters

Every centrifugal pump must prevent the pumped fluid from leaking out along the rotating shaft. Two technologies do this job: mechanical seals and compression packing. Each has a place in industrial and marine maintenance. Choosing the wrong one—or installing either one incorrectly—leads to premature failure, product loss, environmental violations, and shaft or casing damage. This lab teaches you to read failure evidence, replace both seal types correctly, and set packing to the exact leak rate the equipment requires.

Mechanical Seals: How They Work

A mechanical seal creates a running face between two precision-lapped surfaces—a rotating face attached to the shaft and a stationary face held in the gland or stuffing box. A spring (or wave spring in cartridge designs) holds the faces together with a controlled closing force. A thin film of pumped fluid or flush liquid lubricates and cools the faces. If that film disappears, the faces run dry and fail quickly.

Cartridge mechanical seals are pre-assembled units that slide onto the shaft sleeve as one piece. Setting tabs or clips hold the spring compression at the factory dimension; you break or remove them after the gland is bolted down. This eliminates the measurement errors common with component seals.

Recognizing Mechanical Seal Failure Modes

Reading the wear pattern tells you why the seal failed, not just that it failed. Repair the root cause or the replacement will fail the same way.

Shaft Sleeve Inspection Before Installation

A scored shaft sleeve damages the new seal's O-ring as it slides into position. Before installing any seal, clean the sleeve and run a surface comparator (also called a surface roughness comparator or Rugotest) across the sleeve surface. The manufacturer's installation sheet specifies a maximum surface finish (commonly expressed as a roughness value). If the sleeve is scored beyond that limit, replace or polish it before proceeding. Never install a new seal on a damaged sleeve.

Cartridge Mechanical Seal Installation Sequence

1. Lock out / tag out the pump per your facility's energy-control program (OSHA 29 CFR 1910.147 in a general industry setting) before any disassembly.

2. Drain and flush the pump casing; wear the PPE specified for the pumped fluid—at minimum, safety glasses and nitrile gloves.

3. Remove the old seal and clean the seal chamber and gland plate thoroughly. Remove all elastomer fragments.

4. Inspect the shaft sleeve with the surface comparator; note findings on the checklist.

5. Lubricate the new cartridge O-rings with the fluid recommended by the manufacturer (often clean water or a compatible grease—never petroleum grease on EPDM elastomers).

6. Slide the cartridge onto the sleeve, aligning anti-rotation pins or drive lugs.

7. Torque the gland bolts in a cross pattern to the manufacturer's specification.

8. Remove the setting tabs or clips after the gland is fully bolted.

9. Record the spring compression measurement if required by the work order; on cartridge seals this is factory-set, but verify the tabs came free and the faces are seated.

10. Restore energy, prime the pump, and check for leakage at start-up.

Compression Packing: How It Works

Compression packing fills the stuffing box with rings of braided fiber (graphite-impregnated, PTFE, or aramid, depending on the service). The gland follower compresses the rings against the shaft, creating a seal. Unlike a mechanical seal, compression packing is designed to leak slightly—a slow drip rate lubricates and cools the packing and prevents shaft scoring. Zero leakage means the packing is overtightened, the shaft will score, and the packing will burn.

Installing and Adjusting Compression Packing

1. LOTO first. Remove old packing with hook picks; never use a sharp tool that could scratch the shaft.

2. Clean and measure the stuffing-box bore and shaft diameter to confirm correct packing cross-section size.

3. Cut each ring to length by wrapping around the shaft and cutting at a 45-degree butt joint—not square. Use a sharp knife on a flat surface.

4. Install rings one at a time, seating each fully before adding the next. Stagger the cuts 90° apart for successive rings (or as specified—90° or 120° patterns are common) so the gaps never align.

5. Install the gland follower finger-tight, then snug evenly.

6. Start the pump. Allow break-in leakage for several minutes, then tighten the gland follower in small, equal increments on each side until you achieve the specified drip rate—typically a slow, steady drip rather than a stream or zero leakage.

7. Let the packing run and re-check; expect to make a final adjustment as the packing seats.

Safety Summary

Lockout/tagout is mandatory before any pump is opened. Wear safety glasses at all times in the lab; add a face shield when working with corrosive process fluids on the job. Nitrile gloves protect against oils and mild chemicals; select gloves rated for the actual fluid in industrial service. Never bypass the slow-drip requirement to stop all leakage—doing so on a real pump causes shaft and stuffing-box damage and is a maintenance error, not a fix.

Key terms9

Mechanical sealA device that uses two precision-lapped running faces—one rotating with the shaft and one stationary—held in contact by a spring to prevent fluid from leaking along the shaft.
Cartridge mechanical sealA self-contained, pre-assembled mechanical seal unit in which the spring compression is set at the factory; setting tabs or clips are removed after the seal is bolted in place.
Stuffing boxThe cavity in a pump casing that surrounds the shaft and houses either compression packing or the stationary element of a mechanical seal.
Compression packingRings of braided fiber material (such as graphite-impregnated or PTFE braid) compressed in a stuffing box by a gland follower to restrict—but not eliminate—shaft leakage.
Gland followerThe adjustable plate or nut assembly that compresses the packing rings in the stuffing box; tightening it increases sealing force and reduces leak rate.
Surface comparatorA reference tool with known surface-finish samples used to visually or tactilely compare the shaft sleeve surface finish to the acceptable specification before installing a new seal.
Failure modeThe specific way a component fails—such as dry run, misalignment, or chemical attack—identifiable by the physical wear or damage pattern left on the part.
Setting tabs (clips)Temporary tabs or clips on a cartridge mechanical seal that hold the spring at the correct compression distance during installation; they must be removed before start-up.
Lockout/tagout (LOTO)An energy-control procedure that isolates and de-energizes equipment and applies a physical lock and tag so it cannot be started while maintenance work is being performed.

Worked examples4

1. A cartridge mechanical seal manufacturer states the seal faces must be set at a spring compression of 0.500 inch. The setting tabs hold the faces at that distance. After removing the old seal, a technician measures that the old seal's spring was compressed only 0.350 inch. By how much was the old seal under-compressed, and what is the likely consequence?
  1. Identify the specified compression: 0.500 in.
  2. Identify the measured (actual) compression of the failed seal: 0.350 in.
  3. Calculate the difference: 0.500 in − 0.350 in = 0.150 in under-compressed.
  4. Assess consequence: insufficient spring force means the faces cannot maintain full contact, allowing fluid to bypass the seal faces and causing early leakage failure.
Answer: The seal was under-compressed by 0.150 inch. Insufficient closing force allowed the seal faces to separate, permitting fluid leakage and accelerated face wear.
2. A stuffing box holds five rings of compression packing. You must stagger the ring cuts at 90° intervals around the shaft. Starting the first ring cut at the 12 o'clock position (0°), at what angular position should each of the remaining four ring cuts be placed?
  1. Ring 1 cut position: 0° (12 o'clock).
  2. Ring 2: 0° + 90° = 90° (3 o'clock).
  3. Ring 3: 90° + 90° = 180° (6 o'clock).
  4. Ring 4: 180° + 90° = 270° (9 o'clock).
  5. Ring 5: 270° + 90° = 360° = 0°, but the pattern has cycled; in practice the fifth ring returns to 0° since the sequence repeats every four rings.
Answer: Ring 1 at 0°, Ring 2 at 90°, Ring 3 at 180°, Ring 4 at 270°, Ring 5 at 0° (pattern repeats). No two adjacent rings share the same cut position, preventing a straight-through leak path.
3. You are cutting packing rings for a 1.50-inch diameter shaft. To find the correct cut length for each ring, you wrap the packing around the shaft and mark the overlap point. Using the formula Circumference = π × diameter, what is the minimum cut length for each ring (round to two decimal places)?
  1. Identify shaft diameter: 1.50 in.
  2. Apply formula: Circumference = π × d = 3.14159 × 1.50 in.
  3. Calculate: 3.14159 × 1.50 = 4.71239 in.
  4. Round to two decimal places: 4.71 in.
  5. Note: this is the wrap-around length; a 45-degree butt joint means the cut ends meet at an angle on the shaft, but the measured wrap length remains 4.71 in.
Answer: Each packing ring should be cut to approximately 4.71 inches in length to fit a 1.50-inch diameter shaft.
4. After starting the pump with new packing, the technician counts the drip rate and observes 1 drop every 2 seconds. The specification calls for a drip rate of 1 drop every 3–5 seconds (i.e., 12–20 drops per minute). Is the current leak rate within specification, and what adjustment—if any—is needed?
  1. Convert observed rate to drops per minute: 1 drop per 2 seconds × 60 seconds/min = 30 drops per minute.
  2. Convert specification to drops per minute: 1 drop per 3 s = 20 drops/min; 1 drop per 5 s = 12 drops/min. Spec range: 12–20 drops per minute.
  3. Compare: 30 drops/min is above the maximum of 20 drops/min.
  4. Conclusion: the leak rate is too high; the gland follower must be tightened slightly—in small, equal increments on each side—and the rate re-checked.
Answer: The observed rate of 30 drops per minute exceeds the specified maximum of 20 drops per minute. The gland follower should be tightened in small equal increments until the drip rate falls into the 12–20 drops-per-minute range.

The math in this lesson2

Learners compare drip rates in drops per second with a specification.
6.RP.37.G.4
In class: Instructor presents a failure-mode display of five used mechanical seals and five sets of used packing, explaining the wear pattern that indicates each failure cause (dry run, excessive heat, misalignment, wrong material, improper installation); students label a photograph handout identifying each failure mode. · Students remove mechanical seals from training pump shafts, clean the seal chamber, inspect the shaft sleeve for scoring using a surface comparator, and install new cartridge mechanical seals following the manufacturer step sequence, recording spring compression measurement. · Students install compression packing in a packing-gland trainer, stagger the packing ring cuts per procedure, install and tighten the gland follower, then operate the pump trainer under supervision and adjust the gland to achieve the specified slow-drip leak rate rather than zero leakage.
Materials: Training pump shafts with seal chambers, mechanical seal display set (failed examples), new cartridge mechanical seals, compression packing material (braided), packing removal tools (hook picks), surface comparator, packing gland trainers, torque wrench, manufacturer installation instructions, safety glasses, nitrile gloves
Check: Instructor inspects completed mechanical seal installation (spring compression within spec, no cocked seal faces) and observes packing gland adjustment; student must achieve proper drip rate within two adjustments and sign a completed inspection checklist.

Practice check8

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

NCCER Industrial Maintenance – Pumps and Sealing Systems
1. A mechanical seal face shows a crescent-shaped wear pattern concentrated on one side of the face. This wear pattern most likely indicates which failure cause?
NCCER Industrial Maintenance – Pumps and Sealing Systems
2. Why is it essential to remove the setting tabs on a cartridge mechanical seal only AFTER the gland bolts have been fully torqued?
NCCER Industrial Maintenance – Pumps and Sealing Systems
3. When installing compression packing rings, cuts on successive rings must be staggered. What is the PRIMARY reason for staggering the cuts?
MSSC CPT – Maintenance Awareness
4. A technician tightens the packing gland until there is absolutely no drip from the stuffing box on a centrifugal pump handling water. What is the most likely result of this adjustment over time?
NCCER Industrial Maintenance – Pumps and Sealing Systems
5. Before installing a new mechanical seal on a pump shaft sleeve, a technician uses a surface comparator. What is the purpose of this check?
NCCER Industrial Maintenance – Pumps and Sealing Systems
6. A used mechanical seal shows blistered and cracked elastomer O-rings, heat discoloration on both seal faces, and coking deposits on the spring. Which failure mode does this evidence indicate?
MSSC CPT – Safety
7. Which of the following actions is the FIRST step required before opening a pump for mechanical seal replacement in an industrial facility?
NCCER Industrial Maintenance – Pumps and Sealing Systems
8. A pump uses EPDM elastomer O-rings in its mechanical seal. A technician lubricates the O-rings with petroleum-based grease during installation. What is the likely result?
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