face-seal

name	face-seal
description	Face seal design — mechanical face seal, contact pressure, PV limit, fluid film, seal face materials (SiC/carbon/tungsten carbide), leakage rate, API 682, seal flush plans, dual seal, balanced vs. unbalanced.
metadata	{"priority":7,"promptSignals":{"phrases":["face seal","mechanical seal","mechanical face seal","API 682","seal flush plan","dual mechanical seal"],"minScore":3}}

Mechanical Face Seal Design — Complete Skill

Face Seal Fundamentals

Mechanical face seal: two flat lapped faces (rotating + stationary) pressed together by spring force; thin fluid film between faces provides lubrication; seals fluid at shaft/housing interface

Primary sealing faces: typically carbon-graphite vs. silicon carbide (SiC) or tungsten carbide (WC) Secondary seals: O-rings, bellows, or V-rings provide sealing between face and shaft/housing

Balance ratio: B = A_HP / A_f [A_HP = area exposed to high pressure; A_f = face seal area; B < 1 = balanced; B = 1 = unbalanced] Unbalanced: B > 1 → high face load; suitable for low pressure Balanced (B < 0.85): lower closing force → lower face pressure → suitable for high pressure, better heat generation

API 682 balanced seal requirement: B ≤ 0.75 for process pressures > 1.7 MPa (typical)

Face Pressure and PV Limit

Net closing pressure: P_c = k × x + p_seal × B - p_vapor [MPa; k = spring rate; x = compression; p_seal = seal pressure; p_vapor = vapor pressure] Or: P_c = F_spring/A_f + p_seal × (B - 1) [for balanced seal; B < 1 reduces net closing pressure]

Face contact pressure: p_face = P_c = closing pressure [MPa]; target: 0.2–1.5 MPa for most applications

PV limit (product of contact pressure × sliding velocity): PV = p_face × V_face [MPa·m/s; V_face = π × D_mean × N] PV_limit: SiC/Carbon: 1–3 MPa·m/s; WC/WC: 2–5 MPa·m/s; SiC/SiC: 3–8 MPa·m/s

Allowable face temperature: T_face = T_fluid + P_generated / (heat_dissipation_coefficient) P_generated ≈ μ × p_face × V_face × A_face [W; μ = face friction coefficient 0.05–0.15] Maximum: T_face < T_boil_fluid (to prevent vaporization at face → unstable operation)

Seal Face Materials

Face material combinations:

Combination	PV limit	Chemical resistance	Cost
Carbon-graphite / SiC	2 MPa·m/s	Good for most	Standard
Carbon / WC	2 MPa·m/s	Good	Moderate
SiC / SiC (both faces hard)	5 MPa·m/s	Excellent	Higher
Carbon / Al₂O₃	1 MPa·m/s	Good	Lower
SiC / WC	3 MPa·m/s	Excellent	High

Carbon-graphite: self-lubricating; 5% graphite + 95% carbon; k_thermal = 8 W/(m·K); allows dry running briefly Silicon carbide (SiC): excellent wear; k = 120 W/(m·K) (excellent heat dissipation); Al-Si SiC (pressureless sintered) for process pumps Tungsten carbide (WC-Co): extreme wear; k = 80 W/(m·K); needs clean fluid (Co binder susceptible to chemicals)

Face lapping: flatness < 0.001 mm (1 μm) over face width; roughness Ra < 0.05 μm (mirror finish)

Leakage Rate

Theoretical leakage (hydrodynamic film): Q_leak = π × h³ × Δp / (6μ × ln(r₂/r₁)) [m³/s; h = film thickness; Δp = pressure difference; r₁, r₂ = inner/outer radius] Target film thickness h: 0.2–3 μm (operating on hydrodynamic film)

Vapor leakage (thin film on rotating vapor): Q_vapor = λ_leak × leakage_area × (P_seal - P_atm) [simplified; λ depends on fluid state]

Acceptable leakage rates: API 682 standard: < 5 mL/hr per ANSI/API test for gas seals; practically zero for liquid seals Fugitive emission (EPA): < 500 ppm VOC leakage per EPA Method 21

Seal Arrangements (API 682)

Arrangement 1 (Single Seal)

Configuration: one seal; no buffer fluid; most common; simplest Flush: ISO Plan 11 (hot, dirty fluid); Plan 21 (cooled flush); Plan 13 (vent to inlet)

Arrangement 2 (Double Seal — Tandem)

Configuration: two seals in series; same orientation; buffer fluid in between (atmosphere pressure) Purpose: capture leakage from inner seal; vent to safe location; outer seal retains inner seal fluid Buffer pressure: < seal pressure (not pressurized); fluid = lubricant or process-compatible Plan 52: buffer reservoir with circulation; for API Arrangement 2

Arrangement 3 (Double Seal — Double)

Configuration: two seals in opposition (back-to-back or face-to-face); barrier fluid at pressure > seal pressure Purpose: zero process leakage; barrier fluid must not contaminate process (food, pharmaceutical) Barrier pressure: P_barrier = P_seal + 0.17 MPa (25 psi; API 682 requirement) Plan 53A: pressurized barrier reservoir; Plan 53B: gas pressurized Bidirectional sealing: both seals face outward; positive containment regardless of pressure fluctuations

API 682 Flush Plans (Selected)

Plan	Description	Application
11	Recirculate from discharge to seal	Clean, cool service; most common
13	Recirculate from seal to suction	Self-flushing for vertical pumps
21	Recirculate from discharge through cooler	Hot service (T > 80°C)
23	Circulation using pumping ring + cooler	High-T with low boiling margin
31	Recirculate via cyclone separator	Dirty/abrasive service
32	External flush	Pure injection from external clean source
52	Unpressurized buffer (Arrangement 2)	Tandem seal with buffer reservoir
53A	Pressurized barrier (Arrangement 3)	Double seal, pressurized reservoir

Seal Selection Criteria

Temperature: < 80°C: O-ring secondary seals (NBR, FKM); straightforward 80–200°C: Plan 23 or 21 cooling; PTFE-encapsulated O-rings

200°C: bellows secondary seals (inconel, hastelloy); high-T elastomers

Hazardous/toxic service: Arrangement 3 (double seal) required per API 682 for toxic or flammable fluids (EPA regulations) Category 1 (severe): API 682 Arrangement 2 or 3; SiC primary faces; Plan 11, 52, or 53

Abrasive service: Plan 31 (cyclone separator); WC/SiC face pair; higher flush flow rate Abrasive particles < 0.5 mm for face protection; flush at pressure > process + 0.1 MPa

Standards

Standard	Scope
API 682	Pumps — shaft sealing systems for centrifugal and rotary pumps
ASME B73.2	Seal chambers for chemical centrifugal pumps
ISO 21049	Pumps — shaft sealing systems
EPA Method 21	Fugitive VOC leak measurement
ANSI/API 610	Centrifugal pumps (seal requirements)

Output

Provide: seal arrangement (1/2/3 — API 682), face pair (materials, PV limit [MPa·m/s]), balanced/unbalanced (balance ratio B), spring type, face pressure P_c [MPa], surface velocity V_face [m/s], PV product vs. limit, estimated power dissipation P_gen [W], face temperature T_face [°C] vs. limit, leakage rate [mL/hr or ppm], API flush plan (e.g., Plan 21 with cooler), flush flow rate [L/min], barrier/buffer pressure [MPa] (if Arrangement 2 or 3), secondary seal material (O-ring/bellows/PTFE), service category (API 682 Category 1/2/3), and applicable standard (API 682, ISO 21049).