title: "API 610 Pump Types: OH, BB, and VS Classification Explained"
description: "Identify the main components, understand what each one does, and connect the diagram to inspection and pump selection in real industrial systems."
Every API 610 pump type designation answers three questions: where is the impeller relative to the bearings, how does the casing split, and where is the driver? The classification codes OH (overhung), BB (between-bearings), and VS (vertically suspended) divide centrifugal process pumps by mechanical arrangement, not hydraulic duty. An OH2 pump mounts the impeller cantilevered from the bearing frame with a radial-split casing and coupled motor, while a BB3 pump places the impeller between two bearing housings with an axially split casing. Understanding the arrangement logic helps you match pump construction to the maintenance access, alignment tolerance, temperature limits, and piping layout your installation requires.
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- API 610 type codes describe mechanical configuration: OH codes place the impeller overhung from bearings, BB codes support the rotor between two bearing housings, and VS codes suspend the pump vertically.
- Casing split and driver location complete the designation: the second digit within each family specifies whether the casing is radially or axially split, and whether the driver is directly coupled, separately coupled, or integral.
- Arrangement determines maintenance access and thermal capability: axially split casings allow removal of the rotating assembly without disturbing piping, and between-bearings designs tolerate higher temperatures and pressures than overhung configurations.
- Hydraulic performance is independent of the type code: a BB3 pump and an OH2 pump can deliver identical flow and head if fitted with the same impeller diameter and speed, so always consult the certified pump curve.
- Edition matters for compliance and data-sheet format: API 610 11th, 12th, and 13th editions use the same type-code structure but differ in material requirements, test procedures, and purchaser options.
How the API 610 Type Codes Are Organized
API 610 divides centrifugal pumps into three mounting families based on rotor support geometry. Overhung pumps carry the impeller on a cantilevered shaft extension beyond the bearing housing. Between-bearings pumps position the impeller midway along the shaft with bearings on both sides. Vertically suspended pumps hang the driver and bearing assembly above a discharge head and submerge the impeller in the pumped liquid or a sump.
Within each family, a two-character code assigns a number to the primary mounting logic and a second digit to variations in casing split, stage count, and driver arrangement. OH1, OH2, OH3, OH4, OH5, and OH6 all share overhung impeller geometry but differ in radial versus axial casing splits, single versus multi-stage construction, and direct versus separately coupled drivers. The same pattern applies to BB and VS families.
The classification does not encode flow rate, head, temperature, pressure rating, materials of construction, or seal type. Two pumps with the same type code may serve entirely different process duties if one uses a larger impeller diameter, higher speed, or wear-resistant metallurgy. The arrangement code identifies the mechanical architecture you must accommodate during installation, alignment, and disassembly.
Overhung Types OH1 through OH6
Overhung pumps cantilever the impeller from a single bearing frame. The shaft extends through a stuffing box or mechanical seal chamber and ends at the impeller. All radial and axial hydraulic loads transfer back through the shaft to the bearing housing. This compact design simplifies the casing and reduces the axial footprint, but concentrates deflection and stress at the seal area.
OH1 mounts a single-stage, radially split casing with a foot support and couples the motor separately. The entire bearing frame and seal chamber sit on the suction casing foot. You remove the motor, coupling, and bearing frame as a unit without draining the casing, making OH1 suitable for clean liquids at moderate temperatures where seal and bearing inspection is frequent.
OH2 also uses a radially split casing but center-line supports the pump and motor on a common baseplate. The casing splits horizontally at the shaft centerline, eliminating pipe-strain sensitivity and thermal-growth mismatch between the casing and the driver. OH2 is the most common process-pump arrangement for hydrocarbon services where you expect thermal expansion and need to maintain alignment over a range of operating temperatures.
OH3 switches to an axially split casing, allowing you to remove the impeller, shaft, and seal assembly from the front without disturbing the suction or discharge flanges. Axial disassembly reduces maintenance time in applications with frequent seal or impeller replacement, but the cantilevered rotor still limits temperature and pressure capability compared to between-bearings designs.
OH4 y OH5 extend the overhung principle to vertical inline configurations. OH4 mounts the driver above a vertically oriented casing with a separately coupled shaft, while OH5 uses a close-coupled motor bolted directly to the casing. Both save floor space in tight piping layouts and eliminate baseplate alignment, but require overhead clearance for motor removal.
OH6 describes an integrally geared overhung pump where a speed-increasing or speed-reducing gearbox sits between the motor and pump. This arrangement appears in specialized high-speed or low-speed applications where direct coupling is impractical.
Between-Bearings Types BB1 through BB5
Between-bearings pumps support the rotor with one bearing housing on the suction side and another on the discharge side. The impeller or impellers sit midway along the shaft span. This geometry distributes radial loads more evenly, reduces shaft deflection at the seals, and tolerates higher temperatures and pressures than overhung designs.
BB1 uses a single-stage, radially split casing with bearings external to the casing. The rotor slides through the suction-side bearing housing, passes through the casing, and exits through the discharge-side bearing. You remove the upper casing half to access the impeller without disturbing the lower casing or piping. BB1 is common in boiler-feed, condensate, and high-energy services where radial-split casings are adequate but rotor stiffness and bearing life matter.
BB2 switches to an axially split casing while retaining external bearing housings. The casing splits perpendicular to the shaft, so you can withdraw the entire rotor assembly from one end without breaking the suction or discharge flanges. This configuration dominates high-temperature hydrocarbon services and hot-oil applications where casing thermal growth would distort a radial split joint.
BB3 keeps the axially split casing and adds multistage capability. Each stage consists of an impeller and diffuser, stacked along the shaft to generate higher head than a single impeller. The rotor still withdraws from one end for maintenance. BB3 is the standard choice for high-pressure injection, pipeline transfer, and high-head boiler feed where a single stage cannot achieve the required discharge pressure.
BB4 describes a two-stage, radially split design with opposed impellers. The first-stage impeller faces one direction and the second-stage impeller faces the opposite direction, balancing axial thrust and reducing the load on the thrust bearing. BB4 is used in medium-pressure services where the reduced axial load justifies the added complexity over BB1.
BB5 extends the opposed-impeller logic to multistage radially split construction. Multiple pairs of opposed impellers stack along the shaft, each pair balancing axial forces. BB5 appears in very high flow, medium head duties where the hydraulic and mechanical advantages of opposed impellers outweigh the manufacturing complexity.
Vertically Suspended Types VS1 through VS7
Vertically suspended pumps hang from a discharge head bolted to a foundation or mounting plate. The driver and bearing frame mount above the discharge head, and the pump shaft extends downward into a column or casing. The impeller or impellers sit at the bottom of the assembly, submerged in the pumped liquid. This configuration eliminates suction piping, reduces NPSH requirements, and places the driver above the liquid level for safety and access.
VS1 mounts a single-stage, radially split diffuser with a separately coupled vertical motor. The impeller discharges into a stationary diffuser bowl, and liquid rises through the column to the discharge head. VS1 is typical in sump-drainage and vertical-turbine-pump applications where the motor and bearing must remain accessible above the liquid surface.
VS2 replaces the radially split diffuser with an axially split design, allowing you to withdraw the bowl assembly without removing the column. This simplifies maintenance in deep installations where lifting the entire column is impractical.
VS3 uses a multistage diffuser stack with radial splits. Each stage adds head as liquid rises through the column. VS3 serves high-lift duties in water supply, mine dewatering, and vertical process pumping where a single stage cannot develop sufficient discharge pressure.
VS4 combines multistage construction with axially split diffusers, enabling removal of each stage independently. This arrangement is common in deep-well turbine pumps and vertical process pumps where stage-by-stage inspection is required.
VS5 describes a vertical inline design with a close-coupled motor integral to the pump head. The compact assembly saves space but requires motor removal for any internal inspection.
VS6 uses a vertical separate-motor configuration with a flexible or rigid shaft coupling. The motor mounts on a separate support above the pump, and the coupling accommodates minor misalignment and thermal growth.
VS7 covers vertical pumps with an integrally geared drive, typically a right-angle gearbox that converts a horizontal motor to a vertical pump shaft. This arrangement appears in specialized installations where a vertical pump is required but a horizontal motor is preferred.
Casing Split, Support, Stages, and Driver Arrangement
The second digit in each type code refines the basic OH, BB, or VS architecture. Radial splits open horizontally or vertically through the casing centerline, exposing the impeller and volute after you remove the upper half or front cover. Radial-split casings are simpler to manufacture and maintain at moderate temperatures, but thermal expansion can distort the split joint and cause leakage at high temperatures.
Axial splits separate the casing perpendicular to the shaft, allowing you to withdraw the rotor assembly without breaking the casing flanges. Axial-split designs require more machining and larger bolting but eliminate the thermal-distortion problem that limits radial splits. High-temperature services often specify axially split casings.
Support method determines how the pump transmits weight and piping loads to the foundation. Foot-mounted pumps bolt a casing foot or bearing-frame base to the foundation, and the driver mounts separately or on a common baseplate. Center-line-mounted pumps attach the casing at the shaft centerline, minimizing thermal-growth movement and maintaining alignment as the casing expands. Vertical pumps hang from a discharge head, and the driver weight transfers through the pump column to the foundation.
Single-stage pumps use one impeller per shaft. Multistage pumps stack multiple impellers to develop higher head, with each stage adding incremental pressure. Opposed-impeller designs balance axial thrust by facing impellers in opposite directions, reducing thrust-bearing loads in high-flow applications.
Driver coupling method separates designs with rigid or flexible couplings between the motor and pump shaft from close-coupled designs where the motor bolts directly to the pump casing or bearing frame. Separately coupled pumps allow independent motor removal and precise alignment, while close-coupled pumps save space and eliminate alignment steps at the cost of requiring motor removal for any internal work.
Hydraulic Duty Is Not Determined by the Code Alone
The API 610 type code specifies mechanical arrangement, not flow, head, power, efficiency, or materials. A BB3 pump and an OH2 pump can deliver identical hydraulic performance if both use the same impeller diameter, rotational speed, and volute design. The certified pump curve, not the type code, defines what the pump will deliver at a given operating point.
Selecting the correct arrangement requires matching mechanical capability to installation constraints and operating conditions. Overhung pumps fit tight spaces and simplify maintenance when temperatures and pressures remain moderate. Between-bearings pumps tolerate higher temperatures and pressures, distribute loads more evenly, and extend bearing life in continuous-duty services. Vertical pumps eliminate suction piping and NPSH concerns but demand overhead clearance and impose column-load limits on the discharge head.
Temperature and pressure ratings depend on casing material, wall thickness, flange class, and joint design. API 610 establishes minimum mechanical-design margins and material standards, but the actual pressure-temperature envelope comes from the manufacturer’s pressure vessel calculations and the applicable code. Always verify the pump nameplate and certified drawings against the process conditions before specifying or installing.
Seal type, bearing arrangement, auxiliary systems, and materials of construction are purchaser options within each type code. A BB2 pump may use a single mechanical seal, dual pressurized seals, or a seal-less magnetic coupling depending on the service. The type code tells you the rotor is supported between bearings and the casing splits axially, not what seal system is installed.
Selection Examples for Common Process Services
Clean hydrocarbon transfer at moderate temperature and pressure: An OH2 pump with a radially split casing, center-line support, and separately coupled motor provides adequate capability with simple maintenance access. The overhung configuration keeps the footprint compact, and the center-line mounting accommodates thermal expansion without pipe-strain sensitivity.
High-temperature crude oil service: A BB2 pump with an axially split casing and external bearings isolates the rotor from thermal distortion and allows full-load operation at temperatures that would distort a radial split joint. The between-bearings geometry reduces shaft deflection at the seals and extends seal life in high-temperature service.
Boiler feedwater at high pressure: A BB3 multistage pump develops the required head with moderate impeller speeds, reducing wear and cavitation risk compared to a high-speed single-stage design. The axial split allows rotor removal without disturbing the high-pressure piping, and the between-bearings support handles the radial loads from multistage diffusers.
Vertical sump drainage: A VS1 pump with a single-stage diffuser and separately coupled motor eliminates suction piping, places the driver above the liquid for safe access, and allows motor removal without draining the sump. The vertical configuration reduces the required NPSH and fits the available floor space.
Deep well water supply: A VS3 multistage vertical turbine pump generates high head with each stage adding incremental pressure, and the submerged impellers operate below the liquid surface without suction-lift limitations. The radially split diffusers allow stage-by-stage inspection during scheduled maintenance.
Verify the Applicable Edition and Data Sheet
API 610 has undergone multiple revisions. The 11th edition (2010), 12th edition (2017), and 13th edition (2023) share the same type-code structure but differ in material requirements, inspection procedures, hydrostatic test pressures, and acceptable deviations. Equipment specified to an older edition may not satisfy a newer edition’s mechanical-seal qualification, baseplate-design, or vibration-acceptance criteria unless explicitly re-certified.
Always confirm which edition the pump was manufactured to and which edition the project specification requires. Mixing editions creates compliance gaps. If the specification calls for 12th edition and the offered pump is 11th edition, the purchaser must explicitly accept the deviations or the vendor must upgrade materials, testing, and documentation to meet the newer standard.
The API 610 data sheet compiles process conditions, hydraulic requirements, materials, auxiliary systems, and testing witnessed by the purchaser. Section 1 covers process data (flow, head, temperature, pressure, liquid properties). Section 2 specifies the pump type code, materials of construction, and mechanical features. Section 3 defines the driver. Section 4 lists instrumentation and auxiliary piping. Completing the data sheet forces early decisions on seal type, bearing lubrication, flushing plans, materials upgrades, and spare-parts scope.
Certified drawings issued after purchase show the actual arrangement, piping connections, anchor-bolt locations, grouting requirements, and clearance envelopes. These drawings supersede catalog dimensions and must be checked against the foundation design, piping layout, and crane coverage before the pump ships. Baseplate dimensions, coupling-guard clearances, and auxiliary-piping orientations vary by frame size and cannot be assumed from the type code alone.
Preguntas frecuentes
Can I convert an OH2 pump to an OH3 configuration by changing the casing?
No. OH2 uses a radially split casing with the impeller accessed from above, while OH3 uses an axially split casing with front pull-out access. The shaft length, bearing-frame mounting, seal-chamber geometry, and casing bolt patterns differ between the two designs. Converting between radial and axial splits requires replacing the entire pump hydraulic assembly and often the bearing frame, making it equivalent to a new pump purchase.
Do all BB2 pumps use mechanical seals?
No. The BB2 code specifies an axially split casing and between-bearings rotor support but does not determine the seal type. BB2 pumps may use single mechanical seals, dual pressurized seals, dual unpressurized seals, or seal-less magnetic couplings depending on the process fluid and the purchaser’s specification. Seal selection is independent of the type code and appears separately on the API 610 data sheet.
Why do vertical pumps use VS codes instead of OH or BB?
Vertical pumps hang from a discharge head with the impeller submerged below the driver, creating a fundamentally different load path and maintenance sequence compared to horizontal pumps. The shaft transmits weight and hydraulic thrust downward instead of radially, and the pump must be lifted vertically for disassembly rather than separated horizontally. The VS classification captures these unique installation, alignment, and maintenance requirements that do not apply to overhung or between-bearings horizontal pumps.
Is a BB3 pump always more expensive than an OH2 for the same flow and head?
Usually, but not always. BB3 pumps use axially split casings, multistage construction, and heavier bearing frames, which increase manufacturing cost compared to OH2 radial-split single-stage designs. However, if the process conditions require high temperature, high pressure, or extended bearing life, the total installed cost including downtime, seal replacement frequency, and alignment sensitivity may favor BB3. Compare lifecycle cost and maintenance access, not just the initial equipment price.
Do I need to buy the API 610 standard to select a pump type?
You do not need to purchase the standard to understand the type-code structure or compare OH and BB configurations. However, if your project specification requires API 610 compliance, you must reference the applicable edition for material grades, test procedures, acceptance criteria, and documentation requirements that are not published in open sources. Manufacturers provide certified data sheets and compliance statements, but interpreting deviations and verifying compliance details requires access to the standard.
Conclusión
API 610 pump types organize centrifugal process pumps by mechanical arrangement, not hydraulic duty. The OH, BB, and VS codes answer where the impeller sits relative to the bearings, how the casing splits for maintenance, and where the driver mounts. Match the arrangement to your installation space, temperature limits, pressure rating, and maintenance access before comparing hydraulic performance on the certified pump curve. Verify the applicable edition and complete the API 610 data sheet with your process conditions, materials, and auxiliary-system requirements to ensure the selected pump meets both mechanical and hydraulic specifications.