Mechanical Seals Explained
Mechanical seals are one of the most misunderstood components on any pump. A seal that lasts six years on one duty can fail in six weeks on another that looks identical on paper. This guide sets out how seals actually work, why they fail, and how to specify them correctly.
1. How a mechanical seal works
A mechanical seal consists of two flat faces — one rotating with the shaft, one stationary — pressed together by springs and hydraulic pressure. A microscopic film of process fluid lubricates the faces; without it the faces overheat and destroy themselves.
The seal doesn't stop leakage — it controls it. A properly running seal loses a very small amount of fluid at the faces, invisible under normal operation.
2. Why seals fail
- Dry running — the fluid film breaks down, faces overheat, faces crack or blister.
- Abrasive service — solid particles damage the sealing faces mechanically.
- Wrong elastomer — O-rings swell, harden or dissolve in the process fluid.
- Poor installation — shaft misalignment, damaged sleeves, wrong compression.
- Cavitation upstream — vapour bubbles collapse against the seal faces.
3. Single seals
A single mechanical seal — one face pair — is standard on clean, moderate-temperature, non-hazardous fluids. Simple, cheap, effective for utility water, cooling water and similar duty.
4. Double seals (tandem and back-to-back)
A double seal uses two face pairs with a barrier or buffer fluid between them. Essential for hazardous, toxic, or highly abrasive service — the barrier fluid protects the primary seal and provides a monitored failure indicator.
5. API 682 seal plans
API 682 codifies the flush and support arrangements around mechanical seals. Common plans in Paragon's service:
- Plan 11 — recirculation from pump discharge back to seal chamber (basic cooling on clean service).
- Plan 32 — external clean flush from a separate source (dirty or abrasive service).
- Plan 53A/B — pressurised barrier fluid on a double seal (hazardous or toxic fluids).
- Plan 62 — quench with steam or water on the atmospheric side (crystallising or hot fluids).
6. Selecting the arrangement
The right seal arrangement depends on the fluid, temperature, pressure, hazard classification and expected life. Under-specifying for cost saves money once and pays for it repeatedly in downtime and unplanned outages.
Field Insight
If a centrifugal pump keeps eating mechanical seals, the answer is almost never a better seal. Fix NPSH, alignment, flush plan or the wrong elastomer first — then the seal you have will last.
- Seals control leakage, not eliminate it.
- The vast majority of seal failures come from dry running or wrong flush plan.
- Double seals with barrier fluid are essential on hazardous or toxic service.
- API 682 plans exist to codify what flush an application needs.
- Fix the root cause before swapping the seal for something more expensive.