| name | mechanical-seal |
| description | Mechanical face seals — seal components, balance ratio, closing/opening forces, PV limit, seal selection (API 682), flush plans, dual seals, material pairs. |
| metadata | {"priority":7,"promptSignals":{"phrases":["mechanical seal","face seal","seal balance","API 682","seal flush","PV limit","pump seal"],"minScore":3}} |
Mechanical Face Seals — Complete Skill
Seal Components
Rotating face (mating ring): hard material (silicon carbide, tungsten carbide) — attached to shaft
Stationary face (primary ring): softer material (carbon graphite) — spring-loaded against rotating face
Secondary seals: O-rings or bellows preventing leakage behind faces
Drive: pins, slots, or bellows transmit torque to rotating face
Spring: maintains closing force (single coil, multiple springs, or bellows)
Force Balance
Closing Force (from pressure)
F_close = p × A_effective = p × (π/4)(d_b² - d_1²) [for unbalanced seal]
p = sealed fluid pressure [Pa]
d_b = balance diameter (O-ring contact diameter on gland)
d_1 = seal face ID
Opening Force (hydraulic)
F_open = p × A_face × K_o
A_face = π/4(d_2² - d_1²) where d_2 = face OD
K_o = opening force factor (0.5-0.8 for typical flat face)
Spring Force
F_spring = k × δ (spring force)
Usually: k×δ ≈ 0.2-0.5 MPa equivalent face pressure
Net Closing Pressure
p_c = (F_close + F_spring - F_open) / A_face
Minimum: p_c,min ≈ 0.1 MPa (keeps faces in contact)
Balance Ratio
B = A_close / A_face = (d_b² - d_1²) / (d_2² - d_1²)
Unbalanced seal: B > 1.0 (closing force increases with pressure — limits pressure)
Balanced seal: B = 0.65-0.85 (reduced net closing force — handles higher pressure)
Typical balance: B = 0.75 (standard balanced mechanical seal)
Max pressure — unbalanced seal: ~10-20 bar (face distortion limit)
Max pressure — balanced seal: 40-100 bar (with proper design)
PV (Pressure × Velocity) Limit
Face PV
PV = p_face × V_face [Pa × m/s = W/m²]
p_face = net face closing pressure [MPa]
V_face = mean face sliding speed = π × d_mean × N/60 [m/s]
PV limits:
Carbon/Silicon Carbide: PV < 35-50 W/mm² (dry); 300-500 W/mm² (lubricated, flooded)
Carbon/Tungsten Carbide: PV < 40-60 W/mm² (dry)
Operating PV should be < 50% PV_limit for reliability
Material Pairs
| Rotating (Hard) | Stationary | Application |
|---|
| Silicon carbide (SSiC) | Carbon graphite | Water, hydrocarbons (standard) |
| Tungsten carbide (WC-Co) | Carbon graphite | Abrasive fluids |
| Silicon carbide | Silicon carbide | High-abrasion, hot water, chemicals |
| Stainless hardface | Carbon graphite | Low-cost, clean fluids |
| Alumina | Carbon graphite | Water (low cost) |
API 682 — Seal Selection
Arrangement Types
Arrangement 1: Single seal (most common: pumps with low-toxicity fluids)
Arrangement 2: Dual seal pressurized (buffer fluid between seals) — for hazardous fluids
Arrangement 3: Dual seal unpressurized (containment) — zero emission requirement
Seal Types
Type A: pusher seal (spring-loaded; secondary O-ring translates)
Type B: bellows seal (spring and secondary seal integrated as bellows; no O-ring hang-up)
Type C: bellows (for high-solids, cryogenic, high-temperature)
Flush Plans (API 682)
Plan 11: recirculation from pump discharge to seal chamber (most common)
Plan 13: recirculation from seal chamber to pump suction
Plan 23: seal cooler with circulating pump (high-temperature services)
Plan 32: external flush (clean external fluid injected at higher pressure than product)
Plan 52: external buffer vessel (Arrangement 2 unpressurized)
Plan 53A/B/C: pressurized external reservoir (Arrangement 3)
Plan 54: external pressurized supply (hazardous fluids)
Seal Failure Modes
Dry running: instant face damage (graphite sublimation, scoring)
Contamination: abrasives from fluid damage faces
Cavitation damage: pitting on hard face
Chemical attack: wrong material vs. fluid chemistry
Thermal cracking: insufficient cooling, thermal shock on start-up
Spring fatigue: corrosion + cycling → spring failure → loss of contact pressure
Output
Provide: balance ratio B, net closing pressure p_c [MPa], mean face velocity V [m/s], PV [W/mm²] vs. limit, material pair recommendation, API 682 arrangement and type, flush plan selection, maximum allowable pressure [bar].
