Dry-running mechanical seal assembly for agitators: contacting carbon faces under nitrogen instead of a barrier liquid

01

What going dry costs: pressure and speed limits next to the liquid-lubricated version

John Crane sells the same dual cartridge to DIN 28138 in two face versions, and its published limits show the trade directly: the liquid-lubricated CK725 runs from vacuum to 16 bar(g) at up to 2 m/s, while the dry-running CK725D is limited to 6 bar(g) and 1.5 m/s; the data sheet adds 0–570 rpm overall and up to 430 rpm for dry running, with –40 to +200 °C. John Crane states that exchanging the seal faces converts the wet seal into a contacting dry-running seal. [6]

EagleBurgmann notes on each data sheet that the extreme values of the operating parameters cannot be applied at the same time because they interact, and that the figures are guide values to be confirmed for the exact application. [1] The Saisi listing, offered as an equivalent to the SeccoMix design, gives a lower temperature ceiling without cooling flange (150 °C against 200 °C) and a smaller shaft range than the EagleBurgmann sheet, so an equivalent design should be checked against its own data sheet, not the original's. [7] [2]

The face pair on the EagleBurgmann and Saisi sheets is FDA-conform carbon graphite running against silicon carbide. [1] [7] John Crane lists the advantages of contacting dry-running faces as freedom from contamination by barrier liquid, low face wear and a simple gas supply system. [6]

The image shows a stainless cartridge seal assembly with flange, shaft sleeve and stacked seal rings on a bench fixture. Ask whether a quoted cartridge has wet or dry faces: on the CK725 the housing is shared and the face version sets the limits.

02

Turning agitator rpm into face speed before choosing a dry seal

Dry-running limits are given as sliding velocity, and on large shafts that, not rpm, sets the ceiling. Editorial calculation (shaft surface speed = π × d × n ÷ 60): at John Crane's 430 rpm, a 40 mm shaft runs at about 0.90 m/s and a 160 mm shaft at about 3.6 m/s, above the 1.5 m/s dry-running limit; 1.5 m/s on 160 mm corresponds to about 179 rpm, and the two limits meet at a shaft of about 67 mm. [6]

Editorial calculation for the EagleBurgmann range: 2 m/s on a 220 mm shaft is about 174 rpm, and on a 40 mm shaft about 955 rpm. [1] The face diameter is larger than the shaft, so the real sliding speed is somewhat higher than these shaft-surface figures; ask the supplier for the speed at the face.

EagleBurgmann's paddle-dryer case gives a working point well inside these envelopes: a 200 mm shaft at 20 min⁻¹, vacuum to 1 barg, –20 to +90 °C in operation and up to 160 °C for short periods during cleaning or sterilisation; the case states 0.1 m/s sliding speed, while the editorial calculation from 200 mm and 20 rpm gives about 0.21 m/s at the shaft surface. Either figure is far below 2 m/s; confirm the basis with the supplier. [4]

A Hosokawa Micron conical dryer at F. Hoffmann-La Roche, described by EagleBurgmann, ran at 5 min⁻¹ and 0.5 to 5 mbar absolute at the seal, 50 °C, where lip seals had leaked under shaft wobble and could not hold high vacuum; a single dry-running seal with nitrogen quench, fitted in 2004 inside the existing space, cut nitrogen consumption against the lip seals. [5]

03

Nitrogen pressure, axial shaft growth and explosion-protection temperature

In the paddle-dryer case, the dual SeccoMix R seals are fed with a barrier gas, named as nitrogen, to keep product from leaking to atmosphere; the seal's process side protrudes into the vessel for cleaning, and product-contact materials are FDA-approved with NSF H1 bearing grease. [4] EagleBurgmann names its GSS4015 gas supply for dual SeccoMix 481 seals and states that the 481 tolerates pressure reversal, for example if barrier pressure fails. [1]

Long dryer shafts grow with temperature. At the non-drive end of the same dryers, thermal expansion moved the shaft further than the seal could absorb, so that seal was fitted with a fixed bearing and wipers that let the shaft slide in the seal sleeve; after this change EagleBurgmann reports three years or more of service life at the site. [4] The SeccoMix R sheet allows ±1.5 mm axial offset on 35–60 mm shafts and ±2.0 mm above 60 mm, and SeccoMix 481 offers an axial expansion joint and wiper ring as options. [3] [1]

The dryers handled solvents, so the seal had to suit ATEX category 2 (zone 1) and temperature class T3, which EagleBurgmann gives as a maximum product-side surface temperature of 200 °C; T4 would limit it to 135 °C and needs the operating conditions checked. [4] John Crane notes that explosion-protection requirements can narrow the published limits and may call for extra monitoring. [6]

04

DIN 28138 cartridges on a DIN 28141 flange: swapping wet for dry

Both the SeccoMix 481 and the CK725 are built to DIN 28138 for steel vessels to DIN 28136, with the mounting flange to DIN 28141 and the shaft end to DIN 28154. Their published flange tables match for the smaller sizes: for a 40 mm shaft both give a 175 mm flange diameter, 145 mm bolt circle and 4 × 18 mm holes, and for 100 mm both give 305 mm, 270 mm and 8 × 22 mm. [1] [6]

Editorial note: matching flange data means a dry-running cartridge can be specified for an existing DIN nozzle, but the cartridge length dimensions in the two tables are given under different designations and values, so check the space between flange and gear unit or bearing lantern before ordering. [1] [6] SeccoMix 481 is offered with or without an integrated floating bearing (481L) and as SeccoMix 451 for unstepped shafts; the CK726 adds a radial bearing to the CK725 cartridge. [1] [6]

05

Shaft and duty data for a dry-running agitator seal

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

Machine: [agitator, dryer, mixer; top, side or bottom entry]
Shaft: [diameter; stepped or unstepped; speed in rpm; axial and radial movement]
Vessel: [pressure range incl. vacuum; temperature incl. cleaning/sterilisation; nozzle standard]
Seal: [single or nitrogen-pressurised dual; bearing; cooling flange; wear trap]
Gas: [nitrogen pressure available; supply panel]
Approvals: [FDA/USP; ATEX category and temperature class]

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