Structural characteristics of API685 pump

The core structure of API 685 pump lies in "completely eliminating dynamic shaft seals" and achieving zero leakage through two completely different technical paths: magnetic drive or shielded motor. All of its structural features revolve around the fundamental proposition of "how to transmit power and ensure reliability without a rotating shaft penetrating through the pump casing".

Based strictly on the search results, we will provide a systematic analysis from four dimensions: common core structure, structural differences between the two major technical schools (magnetic pump/shielded pump), key component design, and performance boundaries.

1、 Common structural features of API 685 pumps (essential differences from API 610)
The essential differences between the mandatory requirements of API 685 pumps and API 610 pumps in terms of structural dimensions
Shaft seal form with zero dynamic sealing - no rotating shaft penetrating pump casing API 610 must be equipped with mechanical seals (dynamic seals) [1] [5] [7]
The pump casing must be divided into radial sections according to API 610, and axial sections are optional [8] [10]
The use of cast iron is strictly prohibited for pressure bearing materials. API 610 allows the use of cast iron under specific working conditions [8] [10]
The rotor supports sliding bearings, with self-lubricating rolling bearings as the main medium and oil lubrication [3] [8] [9]
Dry operation taboo absolutely prohibited - no medium, no lubrication, mechanical seal damaged within seconds. The pump can operate dry for a short time [8] [9] [10]
Monitoring system mandatory or strongly recommended (bearing wear, temperature, vibration) optional configuration [3] [8]
Core conclusion: API 685 completely eliminates the weakest link of the pump in terms of structure - mechanical seal. This is the fundamental difference between it and API 610, from which all subsequent structural features are derived.

2、 Detailed explanation of the structural characteristics of the two major technological schools
API 685 covers two completely different technical routes, magnetic pumps and shielded motor pumps, with structural differences far greater than commonalities.

✅  2.1 Structural characteristics of Magnetic Drive Pump
Structural essence: standard motor+magnetic coupling+pump head, two axis structure, non-contact torque transmission.

Core component composition (according to power transmission path):

Component structure features, engineering functions, and information sources
The external magnetic rotor is connected to the motor shaft, with embedded permanent magnets (neodymium iron boron/samarium cobalt) and active magnetic poles, generating a rotating magnetic field
Thick walled isolation sleeve (0.5-1.5mm), nickel based alloy (C4/Hastelloy), fully welded static sealed primary pressure boundary, zero leakage barrier for the medium; Thickness is more than three times that of the shielding sleeve, with a large corrosion resistance margin [5] [7] [9]
The inner magnetic rotor is connected to the pump shaft/impeller, and is covered with a corrosion-resistant non-metallic sheath driven magnetic pole for synchronous rotation
Sliding bearing medium self-lubricating, silicon carbide (SiC)/graphite supported rotor, oil-free lubrication [5] [7] [8]
Secondary containment API 685 mandatory requirement - secondary pressure boundary of outer shell+backup sealing ring, temporary contact in case of main isolation sleeve failure [2] [5] [7]
Key design innovations (taking Flowserve HPX Mag as an example):

Dynamic thrust balance system: automatically balances axial force, limits vibration, and ensures smooth operation over a wide flow range

Internal flushing path: The high-pressure area behind the impeller is drained to the magnetic coupling, and the self-cleaning filter is anti clogging, without the need for an external API 682 flushing system [citation [5] [citation [7]]

Back pull out design: During maintenance, the pump casing and pipeline remain stationary, the drive end is fully withdrawn, the medium is completely enclosed, and personnel have zero contact [citation [2] [citation [5] [citation [7]]]

⚠️  Special structural variants:

Multi stage magnetic pump (MDCX type): impellers are symmetrically arranged back-to-back, with self balancing axial force, designed according to API 685 [citation]

✅  2.2 Structural characteristics of Canned Motor Pump
Structural essence: The pump and motor are integrated coaxially, the rotor is immersed in the medium, and the single axis structure is driven directly by electromagnetic force.

Core component composition:

Component structure features, engineering functions, and information sources
The stator shielding sleeve is extremely thin (0.1-0.3mm), with a non-magnetic corrosion-resistant alloy (316L/Hastelloy) static sealing boundary, isolating the stator winding from the medium; Limited thickness and small corrosion resistance margin [3] [9]
The rotor shielding sleeve is wrapped around the surface of the motor rotor to prevent medium corrosion of the rotor core/magnetic steel [3] [9]
Circulating circuit in shaft/out of shaft/high-temperature separation type motor cooling+bearing lubrication - the medium is diverted from the impeller outlet, passes through the motor chamber and bearings, and returns to the impeller inlet [3]
Graphite bearing medium lubrication, wear monitor comes standard with radial/thrust support [3] [9]
When the shielding sleeve of the secondary contact motor casing ruptures, it temporarily blocks the medium, but the medium will come into contact with the stator winding, causing the motor to burn out [9]
Key structural variants (for different operating conditions):

Structural type, cyclic path characteristics, applicable operating conditions, and signal sources
Axial circulating medium → stator/rotor clearance → rotor shaft center hole → impeller inlet under normal operating conditions (-50~120 ℃, not easily vaporized, clean) [3]
High temperature separation type pump chamber and motor chamber are separated, and the external cooling circuit has a high temperature of>120 ℃ [3]
Mud sealed pump chamber and motor chamber are mechanically sealed and isolated, and external cleaning fluid is injected into the motor chamber containing solid particle media [3]
Reverse circulation type medium with opposite flow direction to the main flow is prone to vaporization/low NPSHa operating conditions [3]
3、 Structural Comparison of Two Major Technical Schools (Core Basis for Selection)
Comparison Dimension Magnetic Pump (API 685) Shielded Pump (API 685) Source
Power transmission magnetic coupling (two axes), motor standard component electromagnetic direct drive (single axis), motor specific [1] [9]
The thickness of the isolation/shielding sleeve is thick (0.5-1.5mm), with a large corrosion resistance margin and extremely thin (0.1-0.3mm), which poses a high risk of corrosion resistance [9]
The consequence of secondary contact is that the medium is temporarily blocked by the secondary shell, and there is no leakage. The shielding sleeve is broken → the medium invades the stator → the motor burns out [9]
Easy maintenance and repair. Back suction design, pump head independently repairable, extremely difficult to handle on site. Integrated packaging, requiring return to factory/professional workshop for major repairs [2] [5] [7] [9]
The coupling has (magnetic coupling), which needs to be aligned, and the motor pump is coaxial [1] [9]
Long and compact axial length [9]
Slightly loud noise (motor fan) and low noise (no fan) [9]
Bearing wear monitoring is in the trial period and is not a standard mature standard [9]
Suitable for strongly corrosive media ✅  Advantage (thick isolation sleeve) ❌  Not applicable (thin shielding cover) [9]
Suitable for solid particles ❌  Not applicable to ferromagnetic particles; Non iron particles are restricted ⚠️  There is a dedicated mud seal type (requiring external flushing fluid) [3] [9]
Suitable for high temperature ≤ 300 ℃ (Flowserve data) ≤ 400 ℃ (special design) [2] [9]
Suitable for high-power limited (<400kW) up to 1000kW+[9]
Key conclusion: Magnetic pumps are "adding magnetic actuators to standard pumps", with friendly maintenance and strong corrosion resistance; Shielded pump is a compact and silent pump that is suitable for high temperature, high pressure, and high power by inserting the motor into the pump casing. However, it is difficult to maintain and not resistant to strong corrosion.  

4、 API 685 standard imposes quantitative requirements on structures (heavy load)
The search results clearly indicate the difference in structural accuracy between heavy-duty unsealed pumps (compliant with API 685) and medium to light load pumps:

API 685 Heavy Load Requirements Engineering Significance in Structural Parameters/Light Load Requirements
Axial radial runout (TIR) ≤ 0.05mm ≤ 0.025mm, with twice the accuracy, ensuring a micrometer level oil film for sliding bearings
The rotor dynamic balance level ISO 1940 G6.3 ISO 1940 G2.5 improves the balance accuracy by 2.5 times, and the magnetic coupling components also need to be dynamically balanced
Vibration limit (filtering) ≤ 4.5mm/s ≤ 3.0mm/s (priority working area) with stricter stability requirements
The manufacturer's standard for flange load capacity complies with API 610/685 clause 6.5 and is consistent with API 610 heavy-duty pumps
These quantitative indicators are the essential differences between API 685 pumps and "ordinary unsealed pumps designed according to API 685", and they are also the acceptance clauses that you must check item by item in the technical bidding.

5、 Information gap statement for current search results
This search cannot provide the following key structural information, which needs to be obtained by purchasing the full text of the genuine API 685 standard or requesting technical specifications from the manufacturer:

Specific design criteria for sliding bearings: bearing aspect ratio, specific pressure limit, PV value limit, allowable material wear - only mentioning "silicon carbide/graphite" [citation [8] [citation [9] [citation [10], without engineering calculation formulas.

Detailed design of axial force balance structure: Flowserve mentioned a "dynamic thrust balance system" [citation], while Hangzhou Dalu mentioned a "back-to-back symmetrical arrangement of impellers" [citation], but without balance hole calculation or balance drum/balance plate design criteria.

The core technical parameters of magnetic circuit design include air gap magnetic density, working point of magnetic steel, demagnetization temperature curve, eddy current loss density - only mentioning "neodymium iron boron/samarium cobalt" [citation [6] [citation [9], without specific design limits.

Hydraulic calculation of circulation loop: API 685 Annex K involves "pressure temperature distribution of circulation loop" [citation], but this result does not provide any calculation formula or design curve.

The design pressure and leakage test acceptance criteria for the secondary containment vessel only mention the existence of a secondary safety system, without specific technical parameters.

6、 Summary: "Decision Maker Summary" of API 685 Pump Structural Features
✅  You can be 100% certain of the structural facts:
API 685 pump=unsealed pump=magnetic pump+shielded pump, absolutely prohibit dry operation.

Common structure: Radial split pump casing, sliding bearings (self-lubricating medium), strictly prohibited cast iron, no dynamic shaft seal.

Structural features of magnetic pump: thick isolation sleeve (corrosion-resistant), secondary safety shell, back suction maintenance, with coupling.

Structural features of shielded pump: thin shielding sleeve (not resistant to strong corrosion), integrated without coupling, circulating cooling circuit, and need to return to the factory for maintenance.

❌  The structural information you need to obtain on your own:
Accurate bearing design criteria, axial force balance calculation methods, and magnetic circuit design parameters - these are the core technical secrets of manufacturers and are not included in public promotional materials.

If you are conducting engineering selection or bidding for API 685 pumps, please provide specific operating parameters (medium, temperature, pressure, flow rate, head, solid particle situation), and I can:

To clarify whether "magnetic pump" or "shielded pump" is more suitable in terms of structure for you;

Draft technical inquiry letters for Flowserve, Sulzer, Hermetic, Nikkiso and other manufacturers, requiring them to provide calculation sheets/test reports for the aforementioned information gaps.