Work log — Process plant, seal selection and groove design week
Engineer: K. Yarrow (static equipment)
Project: MV-9 mixing vessel agitator shaft seal
Log period: Day 01 to Day 06
Document ref: WL-2026-0911-SL09
Day 01 — morning stand-up, and a drip that became a decision
Stand-up 07:55. The MV-9 mixing vessel’s agitator shaft seal has started to weep at the gland, a steady drip of the process slurry, roughly one drop every twenty seconds at the end of a batch, and the environmental officer has put a stop-batch note on it because the slurry is a 9.5 pH dispersion that the plant does not want on the floor or in the drain. The incumbent seal is a gland-packed stuffing box with a PTFE rope packing, and it has served eight years, which is a good life for that packing in this duty, but the shaft runs at 48 rpm with a 2.5 bar vessel pressure and a 75 mm shaft, and the packing has simply finished its life, the log from last service flagged a wearing gland follower.
Morning I took the seal apart on paper before I took it apart on the plant: the stuffing box has four rings of 12 mm square PTFE rope, a lantern ring at the middle ring position with a lube feed, and a gland follower that has worn 1.4 mm into the hard surfacing on its running face, so the follower is pressed harder than it should be, closing the braid unevenly and letting the slurry sneak past the last ring. The decision at 10:00, with the process engineer on the phone, is to replace the worn follower and the packing, and while the box is open I am taking the chance to re-substantiate the seal’s groove geometry against the actual shaft and box before ordering any more square rope, because a seal that leaks through a worn follower will leak again through a wrong-sized ring.
Evening note: the honest part of the work starts now, because a stuffing box that has run eight years has probably drifted from its drawing, and ordering the same square rope off the same drawing is how a plant relives its own leak.
Day 02 — the tape measure on the groove, and the drift on the drawing
Tuesday I measured the actual geometry, because the drawing had sixty percent trust. The shaft at the sealing area is not 75.00 mm but 74.86 mm at the lowest wear zone, a 0.14 mm loss over eight years of rope packing, and the box bore at 100.20 mm against the drawing 100.10, a 0.10 mm ovality across the two punches of the dial bore gauge. None of this is catastrophic, but it is exactly why the square rope chart now means nothing until I re-base the ring number and the squeeze on the worn shaft. The stuffing box depth is 76 mm and the lantern position is 38 mm from the face, both to drawing, so the box geometry is intact and the drift lives in the shaft surface and the bore ovality.
The 10:15 decision, after the process engineer and I walked the options: replace the PTFE square rope with a four-ring set of carbon-yarn packing with a PTFE-impregnated outer, sized for the 74.86 mm worn shaft rather than the nominal 75 mm, and refurbish the shaft with a 40 mm long chromium sleeve over the seal area so the new packing rides a true, hard, 75 mm surface instead of an egg-shell of wear. The sleeve restores the diameter the packing chart wants, and the crushed carbon yarn with its PTFE core handles the 9.5 pH slurry better than the old rope ever did, with a lower running friction and a gland that can be re-tightened in quarter turns.
Afternoon I checked the alternate route the maintenance manager floated, a mechanical single seal drop-in, and I priced it honestly in the log: the mechanical seal would remove the daily gland-tightening ritual but demand a flush system, a face flatness better than 0.4 microns, and a shaft sleeve anyway, so on this 48 rpm, 2.5 bar, batch-duty vessel the refined stuffing box wins on first cost, on fail-soft behaviour, and on a crew that already knows how to service it. The mechanical seal is the right answer for a continuous dryer, not for a batch agitator that stops and starts twelve times a day.
Evening note: the sleeve order is a forty millimetre chromium cylinder and four turns of carbon yarn, and the whole job is cheaper than one mechanical seal cartridge, which is the kind of arithmetic a plant manager remembers. Tomorrow the box comes open, the drift gets measured by hand, and the new stack goes in.
Day 03 — the box open, and the packing that told its own story
Wednesday the vessel was empty and I opened the box at 08:30. The four old rings came out with their corners worn flat, the PTFE rope deformed into a square-edged slug at the gland end, and the lantern ring was gummed with the dried slurry where the lube feed had been starved, so the middle ring was running nearly dry for who knows how long. The mating faces tell the quiet part of the history: the gland follower’s wear face is scored with a 0.4 mm deep groove where the rope’s hard edge rode on the soft worn follower, and the shaft’s 0.14 mm loss sits exactly opposite the damaged ring, so the leak was a stack of small honest failures, worn rope, worn follower, starved lantern, and a shaft skinny at the seal.
The shaft sleeve came at 10:00 and I fit it in the morning: a 75.00 mm chromium sleeve, 40 mm long, pressed and pinned over the 74.86 mm journal, with a 0.02 mm interference chosen so the sleeve seats without opening the balance, and the run-out on the sleeve OD measured at 18 microns total, clean enough for a stuffing box that does not even ask for a bearing-class surface. Then the packing went in ring by ring: four rings of the carbon yarn, each cut to a 44 degree scarf with a PTFE core, the first three rings covering the 40 mm sleeve, the lantern ring seated at the 38 mm feed position with the lube line un-gummed and re-blown, and the final ring closed by a new chrome-plated follower that sits flat on the fresh face.
Afternoon I set the gland preload the honest way: hand-tight, then one eighth of a turn, and I ran the agitator at 48 rpm for twenty minutes with the box at 2.5 bar nitrogen to prove the seal before the slurry ever sees it. The gland face stayed dry at one eighth, no weeping, and the shaft temperature at the stuffing box sat at 34 C, cool for a box that has just been re-packed. I logged the preload detail because a freshly packed gland that is torqued like the old one will overheat in an hour and leak by nightfall.
Evening note: the shrink-fit arithmetic above is in the fix record, 74.86 mm running journal, 75.00 mm sleeve, 0.02 mm interference, 18 micron OD run-out, and it is the difference between a slick repair and a repair that wobbles the next gland tightness out of spec.
Day 04 to Day 05 — slurry back in the vessel, and the groove design question that kept the log honest
Day 04 the vessel went back into service with the full batch, and the re-packed gland held dry through the entire 6 hour batch, no drip on the pan, the environmental officer’s stop-batch note cancelled at 14:30. The gland needed one more eighth turn at the three-hour mark as the carbon yarn settled, a normal settling adjustment for a new pack, and the shaft stayed at 35 C across the batch, so the fresh sleeve is running cool and the packing is seating the way the chart promised.
Day 05 I used the quiet hour to re-substantiate the groove design maths that the week’s earlier tape measure opened up, because a seal that leaks once will leak again if the groove detail is wrong. The four ring stack at 10 mm per ring with the lantern at 38 mm gives a 40 percent annular fill on a 75 mm shaft in a 100 mm bore, within the recommended 35 to 45 percent band for carbon yarn, and the gland follower travel before bottoming is 9 mm against the 6 mm minimum the chart demands, so the re-tightening philosophy has enough stroke to survive a couple of years before the next re-pack. I checked the numbers against the sleeve run-out too, since a thick sleeve that wobbles would have eaten a new follower: 18 microns on the OD is comfortably below the one-touch figure the packing can absorb, and the box bore’s 0.1 mm ovality is honest but does not pinch the last ring because the yarn is compliant across that scale.
Evening note: the groove and the geometric detail do the talking, not the seal’s brand, and if the next refurbishment is a tenth as careful with the tape measure, this vessel will not meet the environmental officer’s pan again for a good long while.
Day 06 — handover, and the seal that passes inspection
Handover with the plant inspector at 10:00. He watched the box at running pressure for twenty minutes, wiped the gland with a paper towel that stayed clean, and signed the seal integrity check with the note drips per minute, zero. The new packing and sleeve are logged against the vessel’s PM card as a scheduled refurbishment, not an unauthorised repair, and the four future re-pack dates, every 18 months on this duty, are in the maintenance calendar. The plant saves the difference between the refined stuffing box and the mechanical seal, roughly one third of the cartridge cost, and walks away with a seal its own crew can service blindfolded.
Closing the week with the numbers: shaft drift measured at 74.86 mm and sleeved back to 75.00 mm, bore ovality 0.10 mm and accepted, four rings of carbon yarn at 44 degree scarfs, lantern ring un-gummed and re-blown, gland set at one eighth of a turn cold and one settling turn in service, shaft at 34 to 35 C across the batch, and drips per minute at zero through the inspector’s whole unblinking watch.
Attachment A — stuffing box geometry record
| Parameter | Measured | After repair |
|---|---|---|
| Shaft at seal | 74.86 mm | 75.00 mm (sleeved) |
| Box bore ovality | 0.10 mm | accepted |
| Packing fill | unknown | 40 percent |
| Follower travel | worn 1.4 mm | 9 mm before bottoming |
| Sleeve OD run-out | n/a | 18 microns |
| Drips per minute | one per 20 s | zero |
Attachment B — seal and groove design checklist
1. Measure the worn shaft and bore before ordering any packing. 2. Refurbish the shaft surface a seal can actually ride on. 3. Check lantern ring position and blow the lube line clear. 4. Confirm packing fill sits between 35 and 45 percent. 5. Confirm follower travel exceeds the chart minimum. 6. Set gland preload cold with small turns, then settle in service. 7. Prove the seal on gas or water before the process fluid returns.
Glossary
Stuffing box: the annular chamber that holds compression packing around a shaft. Lantern ring: a spacer ring in the packing that admits lube or flush to the middle of the stack. Scarf joint: an angled cut on a packing ring end that lets the ring close without a step. Gland follower: the flange and sleeve that compresses the packing. Annular fill: the percentage of the box cross-section occupied by packing.
Week-end reflection
Friday evening. Three bullets for the file. First, a leaking seal is rarely one dead part, the MV-9 was four small honest failures stacked together, and fixing only the rope would have shipped a leak rerouted through the worn follower. Second, geometry beats brand, the tape-measured 74.86 millimetre shaft and the re-substantiated 40 percent fill are worth more than any premium packing name, and the chromium sleeve paid for itself in one avoided re-leak. Third, the cheap answer stayed the right answer, the refined stuffing box beat the mechanical seal on this batch duty, and the crew already knows how to make it behave, which is an asset no datasheet prices. The vessel now runs dry at the gland, and the environmental officer’s stop-batch note has a date stamp on its back.
