3. Mechanical seals and shaft sealing
Mechanical seal
| PT | EN | ES |
|---|---|---|
| Vedação mecânica / Selo mecânico | Mechanical seal | Sello mecánico |
A precision dynamic seal between a rotating shaft and a stationary casing, consisting of:
- A rotating face (mounted on the shaft, often silicon carbide or carbon)
- A stationary face (mounted in the casing, typically silicon carbide or tungsten carbide)
- Spring loading to maintain face contact
- Secondary seals (O-rings, bellows) to seal against the shaft and casing
Gotcha: ABNT NBR 13929 prefers “vedação mecânica”; vendor catalogs in Brazil more often use “selo mecânico”. Both are understood; in a formal contract, follow the standard or pick one and be consistent. ES practice strongly prefers “sello mecánico”.
Single mechanical seal
| PT | EN | ES |
|---|---|---|
| Selo mecânico simples | Single mechanical seal | Sello mecánico simple / sencillo |
One pair of seal faces. Adequate for benign fluids at moderate temperature and pressure. Fluid-side face is exposed to process fluid; the back side is open to atmosphere.
Double mechanical seal (dual seal)
| PT | EN | ES |
|---|---|---|
| Selo mecânico duplo / Vedação dupla | Double mechanical seal / Dual seal | Sello mecánico doble |
Two pairs of seal faces in series, separated by a barrier or buffer fluid. Used when:
- Process fluid is hazardous, toxic, flammable, or otherwise must not leak
- Process fluid is a poor lubricant (low viscosity, dirty, or shear-sensitive)
- Vapor-pressure considerations require positive face cooling
Configurations per API 682:
- Arrangement 2 (PT: Arranjo 2; ES: Disposición 2): Two seals with buffer fluid at lower pressure than process — leakage flows inward to be safely vented
- Arrangement 3 (PT: Arranjo 3; ES: Disposición 3): Two seals with barrier fluid at higher pressure than process — barrier fluid leaks slowly into process; no process leak to atmosphere
API 682 plan numbers (e.g., Plan 53A, Plan 54) describe the support-system piping; the seal arrangement is separate.
Packing (gland packing)
| PT | EN | ES |
|---|---|---|
| Gaxeta / Empanque | Packing / Gland packing | Empaquetadura |
Older shaft-sealing technology: rings of compressible material (graphite fibers, PTFE-impregnated yarns) packed into a stuffing box around the shaft and compressed by a follower (the gland).
Gotcha: packing requires continuous slow leakage to lubricate the shaft surface and dissipate friction heat (typical 30-60 drops per minute). A “dry” packing is overcompressed and will burn out. Many inexperienced operators tighten until leakage stops — the next failure is shaft-sleeve scoring within hours.
Packing is largely obsolete for new industrial pumps but remains appropriate for:
- Slurry / abrasive duty where mechanical seals fail rapidly
- Service where periodic re-tightening is acceptable
- Equipment retrofitted with packing-style stuffing boxes
PT term “empanque” appears occasionally and is acceptable; “gaxeta” is standard. ES “empaquetadura” is uniformly used across Latin America.
API 682 plan numbers
| Plan | Function | Common use |
|---|---|---|
| Plan 11 | Recirculation from discharge to seal | Standard for clean fluids |
| Plan 13 | Recirculation from seal back to suction | High-pressure pumps where Plan 11 over-pressurizes seal |
| Plan 21 | External flush from clean source through cooler | Hot fluids requiring seal cooling |
| Plan 23 | Internal recirculation through cooler | Hot fluids where external flush adds dilution |
| Plan 32 | External flush, no return | Slurry / abrasive duty (seal flush dilutes process) |
| Plan 52 | Buffer-fluid system at low pressure (Arrangement 2) | Hazardous fluids with allowable inward leakage |
| Plan 53A | Pressurized barrier-fluid system (Arrangement 3) | Toxic fluids; barrier pressurized above process |
| Plan 54 | Externally pressurized barrier system | High-reliability barrier for critical service |
Gotcha: when bidding asks for “Plan 11”, that is shorthand for “single mechanical seal with recirculation from discharge”. The seal arrangement (single vs double) and the support-system plan are separate specifications. Both must be in the RFQ.
Stuffing box
| PT | EN | ES |
|---|---|---|
| Caixa de gaxeta / Caixa de vedação | Stuffing box | Caja de empaquetadura |
The cavity around the shaft where the seal or packing sits. In modern mechanical-seal pumps, the stuffing box is sized large enough to accommodate cartridge seals; in older packing-style pumps, it is sized for packing rings only.
Big-bore stuffing box (PT: caixa de vedação ampliada; ES: caja ampliada): an oversized cavity that accommodates double mechanical seals without modifying the casing. Now common in API 610 service.
Cartridge seal
| PT | EN | ES |
|---|---|---|
| Selo cartucho / Cartucho de vedação | Cartridge seal | Sello cartucho |
A pre-assembled mechanical seal supplied as a single unit on its own sleeve. Eliminates field assembly (and field-assembly errors) — the sleeve mounts on the shaft, the cartridge bolts to the stuffing-box flange. Typical for modern API 682 installations.
Why it matters: cartridge seals reduce mean-time-between-failure significantly because face installation and spring loading are factory-set.
Seal flush
| PT | EN | ES |
|---|---|---|
| Fluido de selagem / lavagem do selo | Seal flush | Fluido de sellado / Lavado del sello |
A clean fluid injected into the seal cavity to:
- Cool the seal faces
- Lubricate the running surfaces
- Flush away particulates that would damage faces
- Dilute corrosive species at the face
Flush sources include process fluid (Plan 11), external clean fluid (Plan 32), or recirculated buffer fluid (Plan 52).