A split mechanical seal is a mechanical seal whose faces and O-rings, and in a fully split design every other part as well, are made in halves, so the seal assembles around the shaft instead of sliding on over the shaft end. Nothing but the seal comes off the equipment: the pump or mixer stays coupled and aligned while the seal is changed.

Split seals are used mainly on large pumps handling water-based fluids, on between-bearing and double-ended pumps, and on mixers and agitators, in industries such as power generation, water treatment, pulp and paper and desalination.

How a Split Seal Works

Once its halves are bolted together on the shaft, a split seal seals the same way a solid mechanical seal does: a rotating face runs against a stationary face, one of them spring-loaded, and the seal is made axially between those two faces. The splits add a static joint at each split line, closed off by the O-rings and gaskets that sit across it.

Most split seals mount outside the stuffing box or seal chamber.

A split seal is arranged so the sealed fluid acts on the outside diameter of the split rings, where its pressure presses each ring’s halves together.

How a Split Seal Is Built

A split seal is built from the same parts as a solid seal, each divided into halves that close back together on the shaft, and each kind of part is split in its own way.

The faces are usually made whole, then scored and snapped in two, so the halves meet on an irregular fracture surface that fits only its own partner. Face halves therefore come as matched pairs, and halves from two different seals will not seal together.

Split faces are typically carbon or silicon carbide. Tungsten carbide, the usual choice for arduous duty in a solid seal, is a poor choice here.

The O-rings are cut through once so they can be wrapped around the shaft, and the two ends are butted or snapped together on assembly.

The gland and rotating holder are machined in halves that pin and bolt together around the shaft, and the gland then bolts to the face of the stuffing box. A band or clip around the outside of each split face ring holds its two halves together and in line.

Types of Split Seals

Split seals differ in how much of the seal is split, whether it arrives assembled, and whether it is a single or a double seal:

  • Fully split or semi-split: In a fully split seal every part comes in halves. In a semi-split seal only the wearing parts, the faces and their O-rings, are split; the seal can be fitted unsplit as original equipment, with split faces and O-rings going in at the first repair.
  • Cartridge or component: A cartridge split seal arrives as pre-assembled, self-aligning subassembly halves that install by bolting together around the shaft and to the equipment, with no measuring or setting by the fitter. A component split seal arrives as gland halves, holder halves and loose face halves that the fitter assembles on the shaft.
  • Single or double: Split seals are mostly single seals. Split double seals, with a pressurized barrier fluid between two sets of faces, also exist.

Limits of a Split Seal

A current split seal performs comparably to a cartridge seal on water-based duty, and falls short on hazardous fluids, pressure, startup leakage, material choice and first cost; it also needs a pump in sound condition.

Hazardous fluids are the firm limit. The Fluid Sealing Association does not recommend split seals for hazardous services.

Split seals as a class are available for pressures from slight vacuum to 450 psi (31 bar) and for shaft diameters from 1 in (25 mm) to 36 in (915 mm). Take the rating for your shaft size and speed from the model’s own data sheet.

A newly installed split seal may leak up to 1 drop per minute per millimetre of shaft size for its first 48 to 72 operating hours. That startup leakage typically subsides or stops after 200 operating hours.

Material choice is narrower than for a solid seal. With the faces already typically limited to carbon and silicon carbide, the elastomers are typically offered only in FKM, EPDM and FEPM; if the fluid needs another compound, a split seal is usually not available for it.

A split seal costs slightly more to buy than a solid component or cartridge seal. On single-stage centrifugal pumps, a split seal’s return on investment typically turns positive above a 2.5 in (65 mm) shaft; on double-ended pumps and equipment that is hard to dismantle, it does so at smaller shaft sizes.

A split seal is not a repair for a worn, misaligned or out-of-balance rotating assembly. Because it seals outside the stuffing box, it can go onto a previously packed pump without a new sleeve or shaft.