Chemical reactor sealing gland: packed stuffing box for top-entry agitator shafts

01

Four rings or six rings with a lantern ring: what sits inside an agitator stuffing box

Chemineer's installation manuals for its QED Plus and 50DTA agitators describe two packed arrangements. The 50DTA stuffing box holds four rings of self-lubricated packing and needs no added lubrication for the life of the packing. [2] The QED auxiliary stuffing box, bolted to the vessel nozzle with a gasket supplied by others, takes three rings of packing, a lantern ring, further packing rings and a gland plate with a split gland; the six-ring box is supplied with six rings and a lantern ring. [1]

When the rings are installed, RS Components' packing leaflet asks for each ring to be tapped home separately with the joints of successive rings staggered, preferably at 90°, and Chemineer likewise asks for the packing split to be staggered and each ring seated as it is installed. [5] [1] Freudenberg's datasheets add that the cut ends should be distributed symmetrically around the circumference and the cap nuts tightened evenly. [3]

Ask for the drawing of the gland housing with ring count and cross-section, lantern ring position, lubrication connection, and whether the gland follower is split.

02

Ratings: 50 psig and 100 psig agitator boxes against the pressure and speed guidelines of packing yarns

An agitator maker rates the assembled box; a packing maker gives guideline values for the braid alone. The two sets of numbers are far apart and are not interchangeable.

Editorial calculation: 50 psig is about 3.4 bar and 100 psig about 6.9 bar, while 2.5 MPa is 25 bar, so the yarn guideline is about 3.6 times the 6.9 bar rating of the QED-A box and about 7 times the 3.4 bar rating of the 50DTA box. Freudenberg states its values come from a limited number of laboratory tests and are non-binding guidelines whose suitability depends on the actual operating conditions. [3] [4] Chemineer tells operators not to run shaft seals at temperatures or pressures higher than those the unit was designed for. [1] Use the rating of the assembled box, not the yarn.

Editorial calculation, with an assumed 80 mm agitator shaft at 125 rpm: surface speed = π × 0.08 m × 125 / 60 ≈ 0.52 m/s, about one fortieth of the 20 m/s guideline for the Kombilon braid. [4] On slow agitators the pressure, temperature and shaft movement decide the packing, not the speed. Freudenberg lists low leakage rates even with slight shaft deflection among the advantages of the Kombilon 6742 braid. [4]

State vessel design pressure and vacuum, the vapour or liquid at the seal, temperature at the nozzle, shaft diameter and speed, and whether the packing may be wetted by product or must be self-lubricating.

03

Housing bore, shaft finish and alignment the gland needs

Packing does not tolerate a shaft that wanders in the bore. RS Components' leaflet says the stuffing box must not be used as a guide bearing and that bearings should be provided as close to the box as possible. [5]

RS Components also recommends square faces at the base of the box and on the leading edge of the follower, because bevelled faces force packing onto the shaft at each end and promote extrusion through the clearances. [5] The leaflet's 0.1 mm runout limit is general guidance for pumps and valves; Chemineer's 0.127 mm figure is an alignment setting for the housing during agitator installation. Check both against the drawing for a long overhung agitator shaft. [5] [1]

04

Running in the packing: one flat on the gland nuts every 15 minutes, and when to repack

Both Chemineer manuals give the same start-up routine: tighten the gland nuts finger tight, start the agitator, then take up the nuts gradually, one flat at a time, allowing 15 minutes between each take-up for the packing to reseat itself, until the leakage is acceptable. [1] [2]

For the QED-A box, Chemineer asks for the packing to sit for five or ten minutes after the first tightening so that it can cold flow and adjust to gland pressure, then for the nuts to be loosened back to finger tight before start-up. Once run in, the gland nuts on the auxiliary box are retightened on a schedule that can vary from weekly to monthly depending on the sealing wanted, and over-tightening wears packing prematurely and scores the shaft. [1]

RS Components says the follower should be long enough to enter to 40% of the available packing space and that the packing should be replaced once that much take-up has occurred. [5] Editorial calculation, with an assumed six rings of 10 mm section: the packing space is 60 mm deep, so repacking is due after about 24 mm of follower travel. Its failure table links no leakage at start-up to an over-tightened gland, and a charred or blackened bore of used packing to lubrication failure. [5]

Ask the supplier for the run-in procedure, maximum gland take-up and replacement packing specification with the box.

05

Data to send for a reactor agitator stuffing box

Paste it into your enquiry below or into an email to a maker, then replace every bracket.

Vessel: [design pressure and vacuum; temperature at nozzle; vapour or liquid at seal]
Shaft: [diameter; speed; overhang; steady bearing; runout]
Nozzle: [flange standard and size; bolt circle; gasket]
Packing: [ring count and section; yarn; self-lubricating or greased]
Lantern ring: [yes/no; grease or flush connection]
Gland: [split follower; studs and nuts; material]
Documents: [drawing; rating of assembled box; run-in procedure]

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