Mechanical seals are critical components in high temperature pumps, preventing leakage of hot, hazardous, or expensive fluids at the rotating shaft. This article covers mechanical seal types for high temperature service (single, double, tandem, metal bellows), seal face materials, elastomer selection, API 682 flush plans, and selection guidelines for boiler feed, thermal oil, and hot water applications.
Mechanical seals are critical components in high-temperature pumps, preventing fluid leakage while withstanding elevated temperatures, pressures, and corrosive fluids. Proper seal selection and cooling system design are essential for reliability. This guide covers mechanical seal types, materials, API 682 seal plans, and selection for high-temperature pumps.
Mechanical Seal Fundamentals
- Two lapped faces (rotating and stationary) form a seal
- Spring maintains face contact (compensates for wear)
- Secondary seals (O-rings, gaskets) prevent leakage around components
- Fluid film between faces provides lubrication and cooling
- Leakage: typically <5 drops/min (acceptable), <1 drop/min (good)
- For high-temp: face cooling and material selection are critical
Seal Types for High-Temp Pumps
| Type | Temp Limit | Pressure | Leakage | Best For |
|---|
| Single spring (pusher) | <200°C | <50 bar | Low | General, water, oil |
| Multiple spring (pusher) | <200°C | <100 bar | Low | General, higher pressure |
| Metal bellows | <400°C | <50 bar | Very low | High temp, no elastomer |
| Single cartridge | <200°C | <100 bar | Low | Easy installation, general |
| Double cartridge | <250°C | <200 bar | Zero | Hazardous, toxic, high-value |
| Canned motor | <450°C | <200 bar | Zero | Zero leakage, critical |
| Magnetic drive | <350°C | <150 bar | Zero | Zero leakage, hazardous |
API 682 Seal Categories
Category 1
- General purpose, non-cartridge or cartridge
- Temp: -40 to 200°C, Pressure: <20 bar
- For non-hazardous, general service
- Not recommended for high-temp process pumps
Category 2
- Cartridge seal, API 682 compliant
- Temp: -40 to 250°C, Pressure: <100 bar
- For general refinery/chemical service
- Most common for high-temp pumps (with cooling)
Category 3
- Heavy-duty cartridge, for critical/hazardous service
- Temp: -40 to 400°C, Pressure: <200 bar
- Enhanced features: robust design, multiple springs, special materials
- For hazardous, high-temp, high-pressure, or zero-emission service
Seal Arrangements (API 682)
Arrangement 1 (Single Seal)
- One seal set, product lubricates faces
- For non-hazardous, non-toxic, general service
- Leakage to atmosphere (small amount acceptable)
- Common plans: Plan 11, 21, 32
- For high-temp: requires cooling (Plan 21 or 32)
Arrangement 2 (Double Seal, Unpressurized)
- Two seals, buffer fluid between (lower pressure than product)
- Product leaks to buffer fluid (not atmosphere)
- For hazardous, toxic, but not zero-emission required
- Common plan: Plan 52 (buffer fluid reservoir)
- For high-temp: buffer fluid must be compatible, cooled
Arrangement 3 (Double Seal, Pressurized)
- Two seals, barrier fluid between (higher pressure than product)
- Barrier fluid leaks to product (zero product to atmosphere)
- For hazardous, toxic, zero-emission required
- Common plans: Plan 53A (gas pressurized), 53B (piston), 53C (accumulator)
- For high-temp: barrier fluid cooled, compatible with product
API 682 Seal Plans for High-Temp
Plan 11 (Recirculation)
- Product from discharge through orifice to seal chamber
- Simple, no external system
- For general service, temp <150°C
- Limitations: no cooling (product temp — seal temp)
- Not recommended for >150°C (seal overheats)
Plan 21 (Recirculation + Cooler)
- Product from discharge through cooler and orifice to seal
- Cools seal fluid (typically 50-100°C below product temp)
- For 150-300°C service
- Components: cooler (water or air), orifice, valves, pressure gauge
- Most common for high-temp single seals
Plan 32 (External Flush)
- Clean, cool fluid from external source injected to seal
- Best cooling (seal temp — flush temp)
- For >300°C, dirty product, or where product not suitable for seal
- Flush fluid: water, compatible oil, clean product from another source
- Limitations: requires external source, flush mixes with product
- For thermal oil: use cool thermal oil from expansion tank
Plan 52 (Buffer Fluid)
- Double seal (Arrangement 2), buffer fluid reservoir (unpressurized)
- Buffer fluid: water, glycol, compatible oil
- Reservoir: vented to flare or safe location
- For hazardous, toxic (but not zero-emission required)
- For high-temp: buffer fluid cooled, compatible with product
Plan 53A/B/C (Barrier Fluid)
- Double seal (Arrangement 3), barrier fluid pressurized above product
- 53A: gas pressurized reservoir (nitrogen)
- 53B: piston pressurized (self-contained)
- 53C: accumulator pressurized (gas over liquid)
- Barrier fluid: water, glycol, compatible oil (clean, filtered)
- Zero product leakage to atmosphere
- For hazardous, toxic, zero-emission required
- For high-temp: barrier fluid cooled (heat exchanger), compatible
Plan 62 (Quench)
- Steam or water quench on atmospheric side of seal
- Prevents coking/crystallization of product at seal face
- For high-temp product that solidifies at ambient (thermal oil, sulfur, polymers)
- Often combined with Plan 21 or 32
- For thermal oil: steam quench prevents coking at seal face
Seal Materials for High-Temp
Seal Faces
| Material | Hardness | Temp | Wear | Best For |
|---|
| Carbon graphite | Soft | <250°C | Good | General, water, oil |
| Silicon carbide (SiC) | Very hard | <400°C | Excellent | High temp, abrasive, general |
| Tungsten carbide (WC) | Very hard | <300°C | Excellent | Abrasive, high pressure |
| Alumina (Al₂O₃) | Hard | <300°C | Good | Corrosive, cost-effective |
| Stellite | Hard | <500°C | Good | Very high temp, metal-to-metal |
- Common pairings: SiC vs carbon (general), SiC vs SiC (high temp/abrasive), WC vs carbon (high pressure)
- For high-temp: SiC vs SiC (both hard, high temp capability)
Springs and Metal Components
- 316 stainless: general, <400°C
- 17-7PH: high strength, <350°C
- Inconel 718: high temp/corrosion, <650°C
- Hastelloy C: severe corrosion, <500°C
- For high-temp: Inconel 718 springs (resists fatigue and corrosion)
- Metal bellows: Inconel 718, Hastelloy C (for >200°C, no elastomer)
Secondary Seals (Elastomers)
| Material | Temp | Oil | Acid | Best For |
|---|
| NBR (Nitrile) | -30 to 120°C | Excellent | Fair | Oil, fuel, general |
| EPDM | -40 to 150°C | Poor | Good | Water, steam, acid |
| Viton (FKM) | -20 to 200°C | Excellent | Fair | High temp, oil, chemical |
| FFKM (Kalrez) | -20 to 300°C | Excellent | Excellent | Severe chemical, high temp |
| PTFE | -200 to 260°C | Excellent | Excellent | Corrosive, wedge design |
- For >200°C: elastomers fail — use metal bellows or PTFE wedge
- For thermal oil: Viton or FFKM (NBR may swell)
- For corrosive: FFKM or PTFE (verify compatibility)
Gaskets
- Spiral wound (316/graphite): general, <500°C
- Camprofile (316/graphite): high pressure, <500°C
- RTJ (ring type joint): very high pressure, <600°C
- PTFE: corrosive, <260°C
- For high-temp: spiral wound or camprofile (graphite filler)
Metal Bellows Seals (High-Temp Solution)
Advantages
- No elastomer secondary seal (metal bellows replaces spring and O-ring)
- Temp capability: up to 400°C (depending on bellows material)
- No hang-up (bellows flexes axially, no O-ring friction)
- Good for high temp, coking fluids, polymers
Limitations
- Pressure limit: typically <50 bar (bellows pressure rating)
- Bellows fatigue: cyclic loading (pressure, temperature changes)
- Higher cost than pusher seals
- Not for high pressure (use double pusher for >50 bar)
- Bellows material: Inconel 718 (most common), Hastelloy C (corrosive)
Applications
- Thermal oil circulation (250-400°C)
- Hot water/boiler feed (150-250°C)
- Polymer processing (200-300°C, coking)
- Sulfur pumping (120-150°C, solidifies at ambient)
- Hot hydrocarbon (200-400°C)
Zero-Leakage Solutions
Double Mechanical Seal (Arrangement 3)
- Two seals, barrier fluid between (pressurized above product)
- Zero product leakage to atmosphere
- Barrier fluid: water, glycol, compatible oil (clean, filtered, cooled)
- Plan 53A/B/C system
- For hazardous, toxic, high-value fluids
- Temp: up to 250°C (with barrier cooling)
Canned Motor Pump
- No shaft seal — motor rotor immersed in product
- Zero leakage, no seal maintenance
- Temp: up to 450°C (with proper design)
- Limitations: lower efficiency (can losses), high cost, limited power
Magnetic Drive Pump
- No shaft seal — magnetic coupling transmits torque
- Zero leakage, no seal maintenance
- Temp: up to 350°C (magnet material limit)
- Limitations: temp limit, torque limit, eddy current losses
Seal Selection Process
- Define service: fluid, temp, pressure, hazardous classification
- Determine leakage requirement: general (single), hazardous (double), zero (canned/magnetic)
- Select seal type: pusher (<200°C), metal bellows (>200°C), double (hazardous)
- Select seal plan: Plan 11 (<150°C), Plan 21 (150-300°C), Plan 32 (>300°C), Plan 53 (double)
- Select materials: faces (SiC), springs (Inconel 718), elastomers (Viton/FFKM or metal bellows)
- Verify compatibility: fluid + materials + temp
- Specify API 682 Category 2 or 3 (for process pumps)
- Include seal system: cooler, reservoir, instrumentation
- Require seal datasheet and installation drawing
- Verify supplier has high-temp seal references
Common Seal Failures
- Face cracking: Thermal shock, dry running. Use SiC, avoid dry run, warm-up.
- Elastomer hardening/cracking: High temp, chemical attack. Use Viton/FFKM or metal bellows.
- Bellows fatigue: Thermal cycling, pressure cycling. Use Inconel 718, minimize transients.
- Coking at faces: Product solidifies at ambient. Use Plan 62 quench, metal bellows.
- Seal overheating: No cooling, Plan 11 for >150°C. Use Plan 21/32, verify cooler.
- Barrier fluid loss: Leakage, reservoir level low. Monitor level, use 53B/C.
- Face wear: Abrasive product, dry running. Use SiC/SiC, flush, filter.
- Spring hang-up: Coking, corrosion. Use metal bellows, clean flush.
Procurement Recommendations
- Specify API 682 (4th Edition) for all process pump seals
- For <200°C: single cartridge seal, Category 2, Plan 11 or 21
- For 200-400°C: metal bellows seal, Plan 21 or 32, Plan 62 quench if coking
- For hazardous: double seal (Arrangement 3), Plan 53A/B/C
- For zero leakage: canned motor or magnetic drive
- Specify SiC seal faces for high-temp/abrasive
- Specify Inconel 718 springs and metal bellows
- For >200°C: no elastomers (use metal bellows or PTFE wedge)
- Include seal system (cooler, reservoir, instrumentation)
- Require seal datasheet with all materials and dimensions
- Include spare seal cartridge (1 for critical pumps)
- Require seal installation and operation manual
- Verify supplier has high-temp seal references
Related Industries
Mechanical seal principles apply to oil and gas (refinery, upstream, pipeline), chemical processing (high-temp, corrosive), power generation (fossil, nuclear), pulp and paper (digester, black liquor), pharmaceutical (sterile, hazardous), and food and beverage (sanitary, CIP). The core principle — proper seal type, material, and cooling system for reliable leak-free operation — is consistent across all pump applications.