Overview
Mechanical seals prevent pumped fluid from leaking along the pump shaft. They replace traditional packing where leakage control, emissions reduction, or long service life is required. Selection involves seal arrangement (single, double, tandem), face materials, elastomers, and API 682 flush plan, matched to fluid properties, pressure, temperature, and emissions requirements.
Seal face pressure balance ratio B = (A_h - A_f) / A_f — balanced seals (B ~0.7) handle high pressure; unbalanced (B ~1.3) for low pressure.
Why Mechanical Seals Instead of Packing
| Factor | Packing | Mechanical Seal |
|---|---|---|
| Leakage | 10-100 drops/min | ≤1 drop/min (single); zero to atmosphere (double) |
| Power loss | High friction | Low friction (1/3 to 1/5 of packing) |
| Shaft wear | Grooves shaft | No shaft wear |
| Maintenance | Requires periodic adjustment | Long service life (1-5 years) |
| Initial cost | Low | High |
| Emissions control | Poor | Excellent |
| Best for | Low-value fluids, dirty service | Clean fluids, emissions-critical, continuous duty |
Seal Components
Every mechanical seal has five essential elements:
- Rotating face: hard material (SiC, WC, ceramic)
- Stationary face: soft material (carbon, SiC)
- Secondary seals: O-rings, gaskets, wedges (elastomers or PTFE)
- Spring/bellows: provides closing force to keep faces in contact
- Drive mechanism: transmits torque to rotating face
Seal Arrangements
Single Seal (API Arrangement 1)
- One set of seal faces; pumped fluid is on one side, atmosphere on the other
- Leakage: small amount of process fluid seeps to atmosphere (a few drops per minute)
- For non-hazardous, non-toxic, non-volatile fluids: water, cooling water, mild chemicals
- Lowest cost
- Do not use for: toxic, carcinogenic, flammable, volatile, or valuable fluids
- Typical flush: API Plan 11 (product recirculation from discharge through orifice to seal chamber) or Plan 13 (recirculation to suction for high suction pressure)
Double Seal / Dual Pressurized (API Arrangement 3)
- Two seals facing each other with a pressurized barrier fluid between them at pressure HIGHER than process pressure
- Process fluid cannot leak to atmosphere — barrier fluid leaks inward (small) and outward (to atmosphere)
- For toxic, carcinogenic, volatile, flammable, or zero-emissions-required services
- Barrier fluid pressure must be 1-2 bar above seal chamber pressure
- API Plans: Plan 53 (pressurized external barrier reservoir), Plan 54 (pressurized external barrier circulation system)
- Can detect barrier pressure loss as a leak indication
Tandem Seal / Dual Unpressurized (API Arrangement 2)
- Two seals in series; inner seal sees process pressure, outer seal vents to a buffer fluid at atmospheric pressure (typically to flare or drain)
- For hazardous, flammable, toxic fluids where leakage past the primary seal must be contained but zero-process-emission is not strictly required
- API Plan 52 (unpressurized buffer fluid reservoir connected to flare)
- Primary seal leaks process fluid into buffer; secondary seal contains it
- Less costly than double seal but not zero-emission to atmosphere
Cartridge Seals
All seal components pre-assembled on a sleeve, factory-set to correct spring compression.
- Advantages: no installation errors; no measurement needed; swap in under 30 minutes; consistent face loading
- Disadvantage: higher initial cost
- API 682 (4th edition) and modern projects mandate cartridge seals for all process pumps — do not use component seals.
Face Material Selection
Face pair must be one hard and one soft (or hard/hard for abrasive service).
| Stationary (soft) | Rotating (hard) | Best For |
|---|---|---|
| Carbon graphite (resin-impregnated) | SiC (Silicon Carbide) | General water/chemical — most common |
| Carbon graphite | Tungsten Carbide (WC) | Dirty/abrasive fluids, higher pressure |
| Carbon graphite | Al₂O₃ ceramic | Low-cost water, low pressure |
| SiC | SiC (hard/hard) | Abrasive slurries, poor lubricity fluids, high PV (pressure × velocity) |
| Carbon graphite (antimony-impregnated) | SiC | High temperature; chemically aggressive |
- SiC vs WC: SiC has better thermal conductivity and chemical resistance but is brittle; WC is tougher and better for shock loads but heavier and more expensive. Most common modern choice: carbon/SiC.
Secondary Seals (Elastomers)
| Elastomer | Temperature | Best For | Avoid |
|---|---|---|---|
| NBR (Buna-N) | -40 to 120°C | Oil, water, general | Aromatics, ketones, strong acids |
| FKM (Viton) | -20 to 200°C | Chemical, high temperature, oil/gas | Amines, hot water/steam, ketones |
| EPDM | -50 to 150°C | Steam, water, caustic, polar fluids | Petroleum oils, hydrocarbons |
| FFKM (Kalrez, Chemraz) | -20 to 320°C | Most chemicals, high temperature — premium | Cost (10× FKM) |
| PTFE | -200 to 260°C | Universal chemical resistance | Cold flow; needs careful design |
API 682 Flush Plans (Common)
| Plan | Description | Application |
|---|---|---|
| Plan 02 | Dead-ended seal chamber (no flush) | Clean, low-temperature fluids; small pumps |
| Plan 11 | Product from discharge through orifice → seal → suction | Most common single seal plan; clean fluids |
| Plan 13 | Seal chamber → suction (for high suction pressure) | Vertical pumps, high suction pressure |
| Plan 21 | Plan 11 + cooler | Hot fluids; reduces seal temperature |
| Plan 23 | Internal circulation through cooler (closed loop) | Hot water/boiler feed; best for high T efficiency |
| Plan 31 | Cyclone separator — clean to seal, dirty to suction | Dirty fluids with solids; abrasives |
| Plan 32 | External clean flush from external source | Dirty/abrasive/solids service; critical |
| Plan 52 | Unpressurized buffer (tandem seal to flare) | Volatile/hazardous fluids |
| Plan 53 | Pressurized barrier (double seal) | Toxic/carcinogenic/zero-emission |
| Plan 54 | External pressurized barrier circulation system | High pressure, critical service |
| Plan 62 | Quench (steam/water) on atmospheric side | High T, crystallization-prone fluids |
| Plan 65 | Leakage detection/collection | Drain with float switch for alarm |
Selection by Application
Clean Water / Cooling Water
- Single cartridge seal
- Carbon / SiC faces
- NBR or EPDM elastomers
- API Plan 11
- Low cost; 3-5 year life expected
Hot Water / Boiler Feed (>100°C)
- Single cartridge seal, high-temperature elastomer (FKM or EPDM)
- Plan 23 (cooling loop through cooler)
- SiC/SiC faces for poor lubricity
- External cooling if >150°C
Hydrocarbons / Oil
- Single or tandem seal depending on vapor pressure and regulations
- FKM elastomers; carbon/SiC
- Plan 11/13 (single), Plan 52 (tandem)
- Fire-safe qualification per API 682
Chemicals / Acids / Corrosive
- Double pressurized seal (Plan 53)
- FFKM (Kalrez) secondary seals or PTFE wedges
- SiC/SiC faces
- External barrier fluid compatible with both process and atmosphere
- Hastelloy or Alloy C-276 metal parts for severe corrosion
Slurry / Solids
- Single seal with Plan 32 external clean flush (most reliable)
- SiC/SiC hard/hard faces
- Plan 31 cyclone separator (if solids are heavier than liquid)
- Consider a dynamic seal or expeller for high solids (>5%)
Cryogenic (<-40°C)
- Special low-temperature secondary seals (PTFE, special elastomers)
- Metal bellows seals (no elastomer spring)
- Plan 11 with adequate vapor margin
Vacuum Service
- Double pressurized seal (Plan 53) — prevents air ingestion
- Barrier fluid pressure always above atmospheric
Seal Life Expectancy (typical)
| Service | Life (hours) |
|---|---|
| Clean water, ambient | 20,000-40,000 (3-5 years) |
| Hydrocarbon process | 8,000-25,000 |
| High-temperature water | 8,000-15,000 |
| Abrasive slurry | 1,000-8,000 |
| Chemical / corrosive | 4,000-12,000 |
Summary
Mechanical seal selection starts with fluid hazard classification: single seals for non-hazardous clean fluids (water, mild chemicals), double pressurized seals for toxic/flammable/zero-emissions service, tandem seals for VOC-containment. Cartridge seals are the modern standard. Face pair defaults to carbon/SiC; SiC/SiC for abrasive or high-PV service. Elastomer must match fluid chemistry (FKM for hydrocarbons, EPDM for steam/water, FFKM for universal chemical resistance, NBR for oil/general). Flush plan is critical: Plan 11 for general single seals; Plan 53 for double seals; Plan 32 external flush for dirty service. API 682 standardizes all these choices for refinery and process pumps.