APIRotating Equipment

API 685

Sealless Centrifugal Pumps

US / International
Rotating Equipment

Reference profile

Designation
API 685
Title
Sealless Centrifugal Pumps
Issuing body
American Petroleum Institute (API)
Current edition
3rd Edition (ISO 15783 Equivalent)
Status
Active; premier global standard for heavy-duty sealless centrifugal process pumps.
Category
Sealless Centrifugal Process Pump Standard
1

Overview

API Standard 685 (ISO 15783) covers the minimum requirements for sealless centrifugal pumps—specifically Magnetic Drive Pumps (MDP) and Canned Motor Pumps (CMP)—for use in petroleum, heavy chemical, and gas processing industries.

Sealless pumps eliminate dynamic shaft seals entirely, providing hermetically sealed, zero-emission fluid containment. They represent the ultimate safety standard for handling toxic, lethal, carcinogenic, or highly volatile hydrocarbons without dynamic seal leak paths.

2

Scope & Applicability

Covers

  • Sealless Pump Classifications: Magnetic Drive Pumps (MDP) with external motor driving an inner magnet rotor across a non-magnetic containment shell, and Canned Motor Pumps (CMP) with an integrated motor stators encased in a metallic can.
  • Primary Containment Integrity: Metallic (Hastelloy C-276, Titanium) or non-metallic containment shells engineered to match full pressure casing MAWP ratings.
  • Secondary Containment & Control Systems: Outer secondary pressure housings with secondary lip seals/packing and mandatory leak detection sensors to guarantee 100% backup containment upon primary shell breach.
  • Internal Product-Lubricated Bearings: Alpha-sintered Silicon Carbide (SiC) sleeve bushings and thrust bearings lubricated directly by the pumped process fluid.
  • Testing & Inspection: Hydrostatic pressure testing (1.5x MAWP), helium mass spectrometer leak testing of containment shells, and internal flush flow verification.

Does NOT cover (exclusions)

  • Centrifugal pumps equipped with dynamic shaft seals—covered by API 610 and API 682.
  • Commercial general-service sealless water or mild chemical pumps—refer to ISO 15783 or ASME B73.3.
  • Sealless positive displacement pumps—refer to API 674/675/676.
  • Field magnet re-magnetization and complex motor rewinding—refer to vendor specialized service centers.
3

Key Requirements

MDP vs CMP Structural Selection

  • Magnetic Drive Pump (MDP): Uses external electric motor driving outer magnets to transmit torque magnetically through a stationary containment shell to inner magnets. Permits standard C-face API motors and easy motor replacement.
  • Canned Motor Pump (CMP): Integrates motor and pump into a single hermetically sealed pressure casing. Eliminates external bearings and flexible couplings; highly compact and ideal for cryogenic LNG/LPG or high-pressure services.

Secondary Containment & Leak Detection

  • Secondary Containment: Mandatory secondary pressure housing surrounding the containment shell designed to hold full MAWP upon primary shell failure.
  • Leak Monitoring: Continuous pressure, optical, or liquid level leak switches mounted between primary and secondary containment for immediate control room alarm (DCS trip).

Product-Lubricated Internal Bearings

  • Silicon Carbide (SiC) Bearings: Internal sleeve and thrust bearings must be made from Alpha-sintered SiC with optional Diamond-Like Carbon (DLC) coating to withstand transient dry running.

Inspection & Testing Mandates

  • Hydrostatic Test: Pump pressure casing and containment shell must be hydrostatically tested at a minimum of 1.5x MAWP for at least 30 minutes.
  • Helium Leak Test: Containment shell/can must undergo a mandatory mass spectrometer helium leak test to verify absolute hermetic tightness.
4

Technical Details

Eddy Current Power Losses: Metallic containment shells (e.g. Hastelloy C) experience magnetic eddy current power losses (typically 5% to 15% of driver power). Motor sizing and thermal dissipation calculations must account for this heat input.

Demagnetization Safety Limits: Rare-earth permanent magnets (Samarium Cobalt SmCo or Neodymium NdFeB) must be operated well below Curie temperature limits to prevent irreversible demagnetization.

Helium Leak Rate Criterion: Containment shell helium leak rate must not exceed 1 x 10^-6 std cm^3/s to confirm zero emission status.

Where to find it in the codeConsult API 685 Section 6 for Basic Design (MDP/CMP types, Pressure casings, Containment shells, SiC bearings), Section 7 for Accessories (Motors, Outer magnets, Couplings), Section 8 for Inspection & Helium Leak Testing, and Annex D for Internal Flush Piping Plans.
5

Engineering Notes

  • Always select API 685 sealless pumps for highly lethal, carcinogenic, or ultra-volatile fluids (e.g., Benzene, Phosgene, Hydrocyanic acid, Liquefied Ethylene) where even minor API 682 mechanical seal emissions are unacceptable.
  • Install a fine magnetic suction strainer (< 100 microns) upstream of sealless pumps to trap ferrous scale and metallic particles before they enter the internal SiC bearings and magnet gap.
  • Never run sealless pumps dry; internal SiC bearings rely 100% on the process fluid for hydrodynamic lubrication and cooling.
6

Related Standards

  • API 610

    Centrifugal Pumps for Petroleum, Petrochemical and Natural Gas Industries (Sealed pumps).

  • API 682

    Pumps—Shaft Sealing Systems for Centrifugal and Rotary Pumps.

7

Source, Edition & Status

Publisher
API
Edition Year
N/A
Industries
Chemical, Petrochemical, Oil & Gas
Content Status
reviewed
Last Reviewed
2026-07-01
Last Updated
2026-07-01
Verified By
Engineering Reviewer
Official Link
www.api.org

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