A submersible pump carries two mechanical seals in series: a lower seal between the pumped liquid and a fluid-filled chamber, and an upper seal between that chamber and the motor. Pumped liquid has to get past both before it reaches the windings, and the chamber between them lubricates and cools the seals.

Because the two seals run in different conditions, each one’s faces and elastomers are chosen separately, and a replacement has to match the pump model rather than a standard seal size.

Which Seal Arrangement a Submersible Pump Uses

Almost every submersible pump uses two mechanical seals in series, with an unpressurized oil- or glycol-filled chamber between them. The lower (outer) seal keeps the pumped liquid out of the chamber, and the upper (inner) seal keeps the chamber fluid out of the motor.

The names for this layout vary. Wastewater pump specifications call it a tandem double or an independent double mechanical seal, and in submersible service tandem, dual and double all mean the same two seals in series.

Those specifications reject the conventional double seal, with a single spring between two rotating faces that needs a constant pressure difference to stay closed.

Why Submersible Pump Seals Fail

Submersible pump seals fail mostly at the lower seal, whose faces wear against the solids in the pumped liquid, and at the upper seal when the chamber is filled to the wrong level.

The causes to check on a failed seal:

  • Abrasive wear of the lower seal: grit and solids in the pumped liquid reach the lower seal’s faces and wear them.
  • Debris behind the impeller: plastic and other solids that lodge on the back of the impeller, next to the lower seal, can destroy it.
  • Dry running: a pump that draws the level down until it is running dry damages its seals, so set the stop float above the pump’s minimum level.
  • Chamber filled too low: the fluid must cover the upper seal while the pump runs; below that, the upper seal overheats.
  • Chamber overfilled: the fluid needs an air pocket above it to expand into, and a chamber filled to the top builds enough pressure to blow the seals out. Fill to the level and by the procedure in the manual for that pump.

How to Tell the Lower Seal Is Failing

The lower seal has failed when pumped liquid shows up in the seal chamber, and on a wastewater pump a moisture probe in that chamber reports it while the upper seal is still keeping the motor dry.

On a moisture alarm, plan to pull the pump and replace the lower seal, and the chamber fluid with it.

Many pumps also carry a moisture sensor in the stator cavity. Where the two sensors are wired separately, the seal-chamber probe is commonly wired to alarm only and the stator sensor to stop the pump.

Where one pair of dual-purpose probes or a series circuit serves both cavities, one alarm cannot tell you which cavity is wet, and a faulty sensor can trip the wrong fault.

On a pump without probes, drain a sample from the chamber when the pump is pulled. Oil that comes out milky white or with water in it means the oil, and probably the lower seal, needs changing.

What Fluid Fills the Oil Chamber

The chamber takes the fluid the pump’s O&M manual specifies, which is usually a paraffinic turbine oil and on some models a glycol solution.

When the specification cannot be found, an ISO VG 32 turbine oil suits the cool conditions most submersibles run in; for hot or cold pumpage, pick the viscosity grade from the oil vendor’s temperature data.

Grundfos fills the seal housing of its 9-30 kW SE and SL wastewater pumps with a propylene glycol motor liquid.

Seal Face Materials

The two seals take different face pairs because they run in different conditions: the lower seal meets the solids in the pumped liquid and usually needs hard faces on both sides, while the upper seal runs in the chamber fluid and can use a carbon face against a hard one. The pump maker specifies the pair for each position, and a replacement should keep it unless the duty has changed.

Lower Seal

For raw sewage and other liquids carrying grit or solids, the lower seal runs hard against hard. By duty:

  • Relatively clean effluent: carbon against ceramic is the general-purpose pair.
  • More viscous liquids: tungsten carbide faces, which are harder.
  • Raw sewage, slurry and high solids: tungsten carbide or silicon carbide on both faces.

Tungsten carbide and silicon carbide are both in service on lower seals; the choice between them is covered in silicon carbide vs tungsten carbide.

A hard pair does not tolerate dry running.

Upper Seal

The upper seal runs in the chamber fluid, a clean lubricant, so the conventional pair of a carbon-graphite face against a hard mating ring suits it. That pair is not universal: some municipal sewage specifications require tungsten carbide against tungsten carbide on the upper seal as well.

Elastomer Materials

Every elastomer that touches the chamber fluid must resist it, and in an oil-filled chamber that means NBR or FKM, never EPDM.

A pump whose chamber is switched from glycol to oil, or the reverse, needs its O-rings checked against the new fluid. On the pumped-liquid side, the lower seal’s elastomers are chosen for the liquid, as on any pump seal.

Component or Cartridge Seal

A submersible takes the seal format its seal housing was built for, and Flygt and Grundfos between them use both component and cartridge seals, so the replacement follows the original format for that model:

  • Flygt Plug-in seals: both seals in one ready-to-install unit, used on the 3000-series wastewater pumps 3153, 3171, 3202, 3301 and 3315 and on the 2600 dewatering range.
  • Flygt 3085: separate inner and outer seal units.
  • Grundfos SE and SL: a shaft seal cartridge, withdrawn from the shaft with a puller.

How to Match a Replacement Seal

Match a replacement by the pump’s make and model and the seal listed in the OEM parts list, since submersible pump seals are identified by the pump they fit. EN 12756, the standard for seal dimensions, is written for the seal cavities of standard end-suction centrifugal pumps, so do not assume a submersible’s seal is a standard size until you have checked it against the original.

A compatible replacement has to reproduce three things: the dimensions, the face pair of each seal, and an elastomer that suits the chamber fluid. With the old seal in hand and no parts list, measure the seal and note the face materials of both seals before ordering. Newer Flygt Plug-in units are interchangeable with earlier generations of the same family.