Sealless pumps delete the mechanical seal — the pump's most common failure point. Mag-drive pumps couple the impeller through a magnetic field across a containment shell; canned-motor pumps put the motor rotor inside the process fluid. Both give zero external leakage — ideal for toxic, flammable or valuable fluids — but they must never run dry and have their own failure modes.
Vertical pumps hang the impeller(s) down into the liquid: vertical turbine (line-shaft, multistage) for deep wells and high head with low NPSH available; vertical sump (cantilever) pumps for pits and tanks. The first stage is submerged, so the dreaded NPSH problem largely disappears.
Choose by the constraint that rules: leakage/hazard pushes you sealless; geometry/NPSH pushes you vertical; otherwise the standard sealed horizontal pump wins on cost and simplicity.
1 · Sealless pumps
Across Part 5 we saw that the mechanical seal is where most pumps fail and where hazardous fluid escapes. The radical answer is to remove it: with no rotating shaft penetrating the casing, there is nothing to leak and nothing to wear out as a seal. Two designs achieve it.
Magnetic drive (mag-drive)
The motor turns an outer ring of magnets; an inner magnet ring fixed to the impeller follows it through a stationary, sealed containment shell. Torque crosses the gap magnetically — no physical connection, no seal. The impeller is carried on product-lubricated bearings (often silicon carbide) running in the process fluid. Clean and simple, but: the fluid must be clean enough to lubricate the bearings, it must never run dry (the bearings cook in seconds), magnets can de-couple under shock load, and a metal containment shell generates eddy-current losses (heat, lost efficiency).
Canned motor
Goes further: the motor's rotor is sealed in a can and runs inside the process fluid, which also cools the motor and lubricates the bearings. Fully hermetic, compact, and the standard for the most hazardous, high-pressure or high-temperature duties (API 685). The same rules apply — clean fluid, never dry-run — and monitoring the internal bearings is harder, so many use bearing-wear sensors.
| Sealed (mechanical seal) | Mag-drive | Canned motor | |
|---|---|---|---|
| External leakage | Tiny (vapour) | Zero | Zero (hermetic) |
| Best for | General service | Hazardous / clean fluids | Most hazardous / high P&T |
| Dry-running | Damages seal | Destroys bearings fast | Destroys bearings fast |
| Efficiency | Highest | Lower (eddy losses) | Lower (eddy losses) |
| Maintenance | Seal swaps | No seal; watch bearings | No seal; sealed unit |
Sealless doesn't mean maintenance-free. You trade seal failures for a hard rule: never run dry, never run dead-headed, keep the fluid clean. Loss of flow that a sealed pump would survive for a while will wreck a sealless pump's product-lubricated bearings in seconds — so low-flow and dry-run protection is essential.
2 · Vertical pumps
When the liquid is below the pump floor — a deep well, a sump, a wet pit, a tank farm with poor suction — a horizontal pump struggles or cavitates because it can't get enough NPSH. Vertical pumps solve it by lowering the pumping element into the liquid.
Vertical turbine (line-shaft)
A stack of impeller stages (a "bowl assembly") sits submerged at the bottom, driven by a long line-shaft from a motor on the surface. Because the first stage is underwater, suction is flooded and NPSH is rarely an issue; stacking stages gives high head. The workhorse for deep wells, river/sea intakes, firewater and cooling-water lift.
Vertical sump / cantilever
A shorter vertical pump mounted on a sump or pit, with the impeller hung at the bottom of a column. Cantilever designs have no submerged bearings (the shaft is supported only above the liquid), which suits dirty or abrasive sump duty where a submerged bearing would wear out.
3 · Which pump? A selector
The choice is driven by whichever constraint dominates — containment, or geometry/NPSH. Set the service and see what it points to.
Interactive — Pump-type selector
Decision toolKey takeaways
- Sealless (mag-drive, canned-motor) deletes the seal for zero-leakage hazardous service — but never run dry or dead-headed, and keep the fluid clean.
- Vertical (turbine, sump) reaches deep liquid and sidesteps NPSH because the first stage is submerged.
- The standard sealed horizontal pump still wins on cost, efficiency and simplicity when containment and geometry allow.
- Choose by the dominant constraint — containment → sealless; geometry/NPSH → vertical; otherwise standard.