A cartridge mechanical seal is a complete seal — rotating and stationary faces, springs, secondary seals — already assembled onto its own shaft sleeve and gland plate at the factory, set to the correct face load, and fitted to the pump as one piece. Nothing about the seal is measured or set on the shaft during installation.
Cartridge describes how the seal is built. The same seal can be single or double, balanced or unbalanced, pusher or bellows, and still be built as a cartridge; the faces do the sealing exactly as they do in a loose component seal.
Cartridges are the default on process duties where a failed seal stops production — chemical, refinery, power, pharmaceutical and food plants, and increasingly on water and wastewater pumps.
What Is Inside a Cartridge Mechanical Seal
The sealing elements inside a cartridge are the ones you would find in any mechanical seal — a rotating face, a stationary face, springs and the secondary seals behind them. What the cartridge adds is the hardware that holds those elements in position relative to each other before the unit ever reaches the pump.
| Part | What it does | What it removes from the field job |
|---|---|---|
| Shaft sleeve | Carries the rotating half of the seal and slides over the pump shaft | Setting the rotary face directly on the shaft; the shaft itself no longer wears or frets under the seal |
| Gland plate | Bolts to the seal chamber face, holds the stationary face, carries the chamber gasket and the flush, quench and drain ports | Aligning a separate gland and piping it after the seal is already on the shaft |
| Drive collar | Clamps the sleeve to the shaft with set screws or a friction clamp, outside the chamber | Reaching inside the chamber to lock the drive; the impeller clearance can be reset after the seal is clamped |
| Sleeve O-rings | Seal between sleeve and shaft, and between gland and chamber face | Finding and fitting the right static seals loose at the bench |
| Setting device | Holds the faces at their set length in transit and during fitting — clips that are removed before startup, or an internal spacer that stays in | Measuring a set height from the chamber face and hoping the collar does not move while it is tightened |
A worn cartridge seal comes off the pump complete and goes back to the bench as a unit — faces re-lapped, elastomers replaced, the assembly re-set and re-tested.
How a Cartridge Seal Differs From a Component Seal
A cartridge arrives already set and, in most cases, air pressure tested by the manufacturer, so face load and alignment are established before it leaves the factory. A component seal arrives as loose parts and is set on the pump: the fitter measures from a reference point on the pump, positions the rotary unit at that dimension and locks it there.
What it does not buy is a lower-leaking face. The European Sealing Association’s own field emission survey found no large improvement for cartridge over non-cartridge designs in single-seal installations. The gain to expect is fewer installation failures, not a better seal.
The cost is room and money. A cartridge is physically larger and needs a seal chamber that accommodates it, and it costs more than the equivalent component seal — so component seals still make sense on very small pumps and in shops that stock seal parts rather than whole assemblies. If that trade-off is the decision in front of you, the component versus cartridge comparison works it through.
Installation itself is short: slide the unit on, bolt the gland to the chamber, set the impeller clearance, tighten the drive collar, then pull the setting clips before the pump turns — the full cartridge seal installation procedure has the sequence and the checks.
Types of Cartridge Mechanical Seals
Cartridges divide first by how many pairs of faces are inside: one pair (single) or two (double). Either can be a pusher design or a metal bellows design — the pusher or non-pusher choice is independent of the cartridge build.
Every seal type defined by API 682 — Types A, B and C — is cartridge mounted. For shafts that cannot be dismantled to slide a seal on, such as large vertical or split-case pumps, split cartridge seals assemble around the shaft in halves.
Single Cartridge Mechanical Seal
A single cartridge has one pair of faces, lubricated by the pumped fluid itself, and it is the normal choice.
Specify a single cartridge seal unless the fluid gives you a reason to do otherwise. Two reasons override it: the fluid must not reach atmosphere, or it cannot lubricate a face pair on its own.
Double Cartridge Mechanical Seal
A double cartridge holds two pairs of faces in one unit with a ported cavity between them, filled with an external fluid that lubricates the inboard faces and keeps the pumped fluid off atmosphere. It is specified when the application demands containment or an outside lubricant.
The fluid between the faces defines the arrangement: a barrier fluid runs above seal chamber pressure, a buffer fluid below it. The orientation of the two face pairs varies as well — see double mechanical seals for the fluid choice and double seal arrangements for the layouts, or the double cartridge range for what we build.
Which Pumps a Cartridge Seal Fits
Any pump with a seal chamber built to a seal standard will take a standard cartridge: ASME B73.1 and B73.2 chemical process pumps, and European pumps whose cavity is to ISO 3069 with seal dimensions to EN 12756. Chambers built for soft packing are the ones to check before ordering.
A packing box that looks big enough may still be short of radial clearance or of the axial depth the cartridge and its gland need. Converting from gland packing is usually a matter of fitting the right chamber rather than forcing a cartridge into the existing one.
Before ordering, have four things in hand: which chamber family the pump has, the chamber bore and depth, the shaft diameter under the seal, and the gland bolt pattern. Take them off the pump itself — our guides to B73.1 chamber bores and fitment, measuring a stuffing box and telling a seal chamber from a stuffing box cover what to read and where.