API 685 is the standard for seal-less centrifugal pumps in petroleum, petrochemical, and gas industry services. This article covers API 685 requirements, magnetic drive vs canned motor pump design, compliance testing, and application guidance for hazardous and zero-emission services.
API 685 is the international standard for seal-less centrifugal pumps (magnetic drive and canned motor pumps) used in petroleum, chemical, and gas industries. It defines design, materials, testing, and documentation requirements for zero-leakage pumps handling hazardous, toxic, or expensive fluids. This guide covers API 685 requirements, pump types, application criteria, and compliance verification.
API 685 Overview
| Aspect | API 685 Requirement | Notes |
|---|
| Scope | Seal-less centrifugal pumps (magnetic drive + canned motor) | Petroleum, chemical, gas industry services |
| Pump types | Magnetic drive (overhung, between-bearing) + canned motor | Both types covered by same standard |
| Pressure rating | Up to 250 bar (design pressure) | Higher by agreement |
| Temperature | -40ยฐC to +400ยฐC (standard) | Cryogenic/higher by agreement |
| Design life | 20 years minimum (continuous operation) | Except wear parts |
| Testing | Hydrotest, performance test, NPSH test, noise, vibration | Witnessed test optional |
| Documentation | Data sheets, curves, drawings, test reports, manuals | Specific list in standard |
| Materials | Material classes (A-1 through A-8, D-1, D-2) | Based on service severity |
Critical: Magnetic Drive vs Canned Motor Pumps
| Feature | Magnetic Drive Pump | Canned Motor Pump |
|---|
| Design | External magnets on motor shaft, internal magnets on pump shaft, containment shell | Motor rotor immersed in process fluid, stator canned with thin liner |
| Motor | Standard motor (separate, air-cooled) | Integral motor (process-fluid cooled) |
| Bearing | Process-lubricated sleeve bearings (carbon/PTFE) | Process-lubricated sleeve bearings (same) |
| Efficiency | Lower (magnetic coupling losses ~3-5%) | Higher (no coupling loss, but motor windage) |
| Max power | Up to ~1000 kW | Up to ~500 kW (typically smaller) |
| Temperature | -40 to +400ยฐC (with cooling) | -40 to +350ยฐC (motor insulation limit) |
| Maintenance | Motor can be serviced separately; containment shell accessible | Motor and pump integrated; more complex service |
| Cost | Lower | Higher (integral motor) |
| Best for | General hazardous service, wide range | Very hazardous (zero leakage critical), small-medium |
Critical: API 685 covers both magnetic drive and canned motor pumps. Magnetic drive pumps (like our magnetic drive pump) use an external magnetic coupling through a stationary containment shell โ standard motor, easier maintenance. Canned motor pumps integrate the motor into the pump (rotor in process fluid) โ truly zero leakage (no dynamic seal at all), but more expensive and complex. For most chemical/hazardous service, magnetic drive is preferred (cost-effective, reliable). Our magnetic drive pumps meet API 685 design principles.
Critical: API 685 Material Classes
| Class | Wetted Parts | Service |
|---|
| A-1 | Carbon steel (cast) | Non-corrosive, low temp (water, oil) |
| A-2 | 316 stainless steel | General corrosive, moderate |
| A-3 | 316L stainless steel | General corrosive, weldable |
| A-4 | Duplex stainless (2205) | Seawater, chloride, high strength |
| A-5 | Alloy 20 (20Cb-3) | Sulfuric acid, general corrosive |
| A-6 | Hastelloy C-276 | Severe corrosive (HCl, oxidizing) |
| A-7 | Titanium Gr. 2 | Severe corrosive, seawater hot |
| A-8 | Nickel 200/201 | Caustic (high temp), specific |
| D-1 | Duplex (high strength) | High pressure, corrosive |
| D-2 | Super duplex (2507) | Very severe chloride, high pressure |
Critical: API 685 material classes are based on service severity. For general chemical service, A-2/A-3 (316/316L) is standard. For seawater/chloride, A-4 (duplex 2205) or D-2 (super duplex). For severe corrosive (HCl, sulfuric acid), A-5 (Alloy 20) or A-6 (Hastelloy C). Our fluoroplastic magnetic drive pumps use PTFE/PFA lining for extreme corrosion (beyond metal classes).
Critical: Containment Shell Design (Magnetic Drive)
- Function: Stationary shell between external and internal magnets โ seals process fluid inside pump, allows magnetic flux to pass through. Must withstand full discharge pressure and process corrosion.
- Materials: (1) Non-metallic (PTFE, PFA, carbon fiber composite) โ no eddy current losses, but lower pressure/temp, (2) Metallic (316SS, Hastelloy C, titanium) โ high pressure/temp, but eddy current losses (heat generation), (3) Ceramic (silicon carbide) โ high performance, expensive, brittle. Our magnetic drive pumps use appropriate containment shell materials.
- Eddy current loss: Metallic containment shells generate eddy currents from rotating magnetic field โ heat (3-5% power loss). This heat must be removed by process fluid circulation. For low-specific-heat or high-temp fluids, use non-metallic shell or add cooling. Our designs account for eddy current heating.
- Pressure rating: Containment shell is a pressure boundary โ must be designed for full discharge pressure (not suction). API 685 requires hydrotest of shell at 1.5ร design pressure. Our pumps include shell pressure testing.
Critical: Bearing and Lubrication
- Process-lubricated bearings: API 685 seal-less pumps use sleeve bearings lubricated by the process fluid itself (no oil). Bearing materials: carbon graphite, silicon carbide (SiC), PTFE. SiC is preferred for corrosive/abrasive service (hard, corrosion resistant).
- Bearing wear monitoring: API 685 requires bearing wear monitoring (RTD or proximity probe) to detect bearing wear before failure. Our magnetic drive pumps can include bearing wear monitors.
- Minimum flow: Bearings rely on process fluid for lubrication and cooling. Running below minimum flow โ bearing overheating/failure. API 685 requires minimum flow protection (bypass or instrument). Our pumps specify minimum flow.
- Dry run protection: Dry running (no fluid) โ bearing seizure โ catastrophic failure. API 685 requires dry run protection (level switch, flow switch, or power monitoring). Our systems include dry run protection.
Critical: When to Specify API 685
| Service | API 685 Required? | Why |
|---|
| Hazardous/toxic fluid (HCl, phosgene, HF) | Yes | Zero leakage = safety |
| Carcinogenic/mutagenic (benzene, etc.) | Yes | Environmental/health regulation |
| Expensive fluid (pharmaceutical, specialty) | Yes | Prevent product loss |
| High temperature (๏ผ200ยฐC) hazardous | Yes | Seal failure risk high |
| Vacuum service | Yes | Seals leak air inward |
| General water/non-hazardous | No | Sealed pump sufficient, lower cost |
| Solids ๏ผ2% (abrasive) | Caution | Bearing wear; use hardened bearings |
| Very high viscosity (๏ผ500 cP) | Caution | Magnetic coupling slip; use gear/screw pump |
Critical: Common Problems
- Problem 1: Containment shell failure. Shell cracks/corrodes โ process fluid leaks to atmosphere โ hazard. Solution: proper material selection, hydrotest,ๅฎๆ inspection, eddy current monitoring, pressure relief. Our pumps use high-quality shells.
- Problem 2: Bearing failure (dry run/low flow). Running dry or below minimum flow โ bearing overheat โ seizure โ magnet coupling damage. Solution: minimum flow protection, dry run protection, bearing wear monitor, proper operation. Our pumps include protection.
- Problem 3: Magnet demagnetization (high temp). High temp (๏ผ150ยฐC for NdFeB, ๏ผ250ยฐC for SmCo) โ magnets lose magnetism โ torque loss โ pump stops. Solution: select magnet material for temp (SmCo for high temp), cooling, temp monitoring. Our pumps use appropriate magnets.
- Problem 4: Eddy current overheating (metallic shell). Metallic shell generates heat โ fluid temp rises โ vaporization โ cavitation/bearing failure. Solution: non-metallic shell (if pressure allows), cooling jacket, proper fluid circulation. Our designs manage eddy current heat.
- Problem 5: Canned motor insulation failure. Process fluid contacts motor windings (through can) โ insulation degradation โ motor failure. Solution: proper can material, insulation class,ๅฎๆ resistance testing, clean fluid. Our canned motor partners use proper insulation.
- Problem 6: Non-API pump misapplied. Using general magnetic drive pump for API 685 service โ doesn't meet testing/materials โ failure. Solution: specify API 685, verify data sheet, witness test, check material certificates. Our API 685 pumps meet all requirements.
Selection Summary
- General hazardous chemical service: Magnetic drive pump (316SS/PTFE, API 685 design, zero leakage, cost-effective)
- Extreme corrosion (acid, solvent): Fluoroplastic magnetic drive pump (PTFE/PFA lined, beyond metal classes)
- High temp (๏ผ200ยฐC): Jacketed magnetic drive pump (cooling jacket, SmCo magnets, high-temp containment shell)
- Seawater/chloride: Magnetic drive with duplex 2205 wetted parts (API 685 A-4 class)
- Very hazardous (zero leakage critical): Canned motor pump (integral motor, truly no seal, but higher cost)
- High pressure (๏ผ100 bar): API 685 magnetic drive with metallic containment shell (Hastelloy/titanium), hydrotested
- Critical: API 685 = SEAL-LESS CENTRIFUGAL STANDARD (magnetic drive + canned motor, zero leakage) + MATERIAL CLASSES (A-1 to A-8, D-1/D-2 based on service) + CONTAINMENT SHELL (pressure boundary, eddy current loss, material selection) + PROCESS-LUBRICATED BEARINGS (SiC, wear monitoring, minimum flow, dry run protection) + TESTING (hydrotest, performance, NPSH, vibration) + DOCUMENTATION, all = safety for hazardous fluids + regulatory compliance + reliable zero-leakage operation
- Our magnetic drive pumps, fluoroplastic magnetic drive, and jacketed magnetic drive meet API 685 design principles. For API 685 pump selection, talk to our chemical engineers with your fluid, pressure, temperature, hazard classification, and required material class.