Pump selection — API 610 / ISO 13709 — pump type classification
API 610 sorts centrifugal pumps into three families by how the rotor is supported. In an overhung (OH) pump the impeller sits on the end of the shaft, beyond its bearings. In a between-bearings (BB) pump the impeller or impellers run on a shaft carried by a bearing at each end. In a vertically suspended (VS) pump the pumping element hangs below the driver, in the liquid or in a can. A number after the letters identifies the layout inside each family, which gives 18 type codes from OH1 to VS7; the current 12th edition splits BB1 into BB1-A and BB1-B, making 19. A code describes construction, not compliance: a pump is an API 610 pump only when it is designed, built and tested to the standard and to the purchaser’s datasheet.
What an API 610 type code tells you, and what it does not
The code is shorthand for a layout. OH2, for example, means a horizontal, single-stage overhung pump whose casing is supported at the shaft centreline and driven through a flexible coupling. Datasheets, plant equipment lists and maintenance records use the codes because a short code carries what otherwise takes a sentence: whether the pump stands horizontal or hangs vertical, whether the casing splits along the shaft or across it, whether the rotor is carried at one end or at both, and whether the discharge leaves through the column or through a separate pipe.
What the code leaves out matters as much. It says nothing about materials, the seal arrangement and its flush plan, the hydraulic size or the duty point. And it is not a statement of compliance. API 610 sets requirements that a layout alone does not meet, on pressure casings, allowable nozzle loads, bearings and lubrication, vibration, and inspection and testing. A foot-mounted chemical process pump has the OH1 layout without being an API 610 pump, and a vertical sump pump can have the VS4 layout without being one either. When a project asks for API 610, the edition and the datasheet named in the enquiry govern. The process pump standards comparison sets out how API 610, ISO 13709, ASME B73 and ISO 5199 relate.
All API 610 pump types in one table
The table follows the order of the API 610 classification. The third column is what can be seen on an installed pump; the last column gives typical duties, not limits.
| Type | Layout | Recognise it by | Typical services |
|---|---|---|---|
| OH1 | Horizontal, foot-mounted, single stage, flexibly coupled. | Feet under the casing bolted to the baseplate; one bearing housing behind the casing. | General process duty at moderate temperature and pressure. Most ASME B73.1 and ISO 2858 end-suction pumps have this layout. |
| OH2 | Horizontal, centreline-mounted, single stage, flexibly coupled, one bearing housing. | The casing rests on supports at shaft-centreline height instead of on feet below it. | Hot and high-pressure process duty, where the casing must grow evenly with temperature and take higher nozzle loads. |
| OH3 | Vertical in-line, single stage, with its own bearing bracket; flexibly coupled. | Suction and discharge on one line; the pump has its own bearings and the motor sits on a bracket above. | Process units short of floor space; the pump can be installed in the pipework. |
| OH4 | Vertical in-line, single stage, rigidly coupled. | A rigid coupling joins the pump shaft to the motor shaft; the motor bearings normally take the thrust. | Compact in-line duty. |
| OH5 | Vertical in-line, single stage, close-coupled. | The impeller is mounted on the motor shaft; there is no separate pump shaft and no coupling. | Light, compact in-line duty such as utilities and circulation. |
| OH6 | High-speed, integral gear-driven, single stage. | A speed-increasing gearbox built into the pump carries the impeller on its output shaft, with no coupling between them. | Low flow at high head, for example light hydrocarbon service. |
| BB1 | Axially split, one or two stages. The 12th edition splits it into BB1-A (foot-mounted) and BB1-B (near-centreline-mounted). | A horizontal joint along the shaft centreline; the upper casing half lifts off with the nozzles left in the lower half. | Large flows at moderate head: water supply, cooling water, transfer and pipeline duty. |
| BB2 | Radially split, one or two stages. | The casing splits across the shaft, with a cover bolted to the casing end instead of a horizontal joint; normally centreline-supported. | Hot or high-pressure duty on one or two stages, where a round gasketed joint is preferred to a long flat one. |
| BB3 | Axially split, multistage. | A horizontal joint along a multistage rotor; every stage is exposed when the upper half is lifted. | High-head duty such as pipelines, water injection and boiler feed. |
| BB4 | Radially split, single-casing multistage (ring-section or segmental). | Stage casings stacked between a suction casing and a discharge casing and clamped by tie bolts. | Boiler feed, pressure boosting, reverse osmosis and other high-head water services. |
| BB5 | Radially split, double-casing multistage (barrel). | The stage bundle sits inside a heavy outer barrel closed by a bolted end cover. | The highest pressures: boiler feed on large units, water injection, high-pressure hydrocarbon service. |
| VS1 | Wet pit, single-casing diffuser (bowl) pump; discharge through the column. | Diffuser bowls at the foot of the column; the discharge head stands above the floor. | Wet pits and sumps, cooling-water intakes, raw water. |
| VS2 | Wet pit, single-casing volute pump; discharge through the column. | A volute casing at the foot of the column instead of diffuser bowls. | Moderate flow and head from pits and sumps. |
| VS3 | Wet pit, single-casing axial-flow pump; discharge through the column. | A propeller in a bowl: large diameter, low head. | Very large flows at low head: cooling water, flood control, intakes. |
| VS4 | Single-casing volute sump pump, line-shaft driven; discharge through a separate pipe. | A line shaft in a column with steady bearings along it, and a discharge pipe running beside the column. | Process sumps, drain pits and tanks. |
| VS5 | Cantilever sump pump; discharge through a separate pipe. | A stiff shaft with no bearings below the support plate. | Sumps where bearings running in the liquid would suffer, for example liquids carrying solids. |
| VS6 | Double-casing diffuser pump (can pump); discharge through the column. | The bowl assembly hangs inside a suction can set below the floor; both nozzles are on the head. | Low NPSH available: condensate, liquefied gases, pipeline boosting. |
| VS7 | Double-casing volute pump; discharge through the column. | A volute pump inside a suction can. | As VS6, where a volute stage suits the duty. |
Overhung pumps: OH1 to OH6
An overhung pump carries its impeller on the end of the shaft, outboard of the bearings. The rotating assembly can usually be drawn out from the back of the casing without disturbing the pipework, which is why the layout dominates small and medium process pumps. The price is that the impeller’s hydraulic load acts on a cantilever, so shaft stiffness and bearing design decide how far from its best efficiency point the pump should run.
OH1: horizontal, foot-mounted
The casing stands on its own feet, bolted to the baseplate, and a separate bearing frame carries the shaft. It is the simplest horizontal layout and the usual one for ASME B73.1 and ISO 2858 chemical process pumps, whose high-temperature versions can be centreline-mounted instead. Because the feet sit below the shaft, the casing grows upward as it heats, lifting the shaft relative to the driver and loading the coupling alignment; that is the reason hot duty moves to OH2.
YSM’s G196 and D Mark III ranges are ASME B73.1 pumps with this layout: horizontal, end suction, foot-mounted and back pull-out. They are B73.1 pumps, not API 610 pumps.
OH2: horizontal, centreline-mounted
The casing is supported at the height of the shaft centreline. When it heats up it grows equally above and below the supports, so the shaft stays where the coupling alignment put it, and the casing can take higher nozzle loads from hot pipework. A single bearing housing carries both the radial and the thrust loads. OH2 is the most common horizontal overhung layout for refinery and petrochemical process duty.
YSM’s KZ end-suction pump follows ISO 13709 / API 610 type OH2 conventions: centreline-mounted end suction with back pull-out, for hot and cold process duties to 425 °C and 100 bar, with selections reviewed against the project’s API 610 datasheet.
OH3, OH4 and OH5: vertical in-line
The three vertical in-line types put suction and discharge on one line, so the pump can be installed in the pipework like a valve, with the motor above. They differ in how the motor and the pump share the load. An OH3 has its own bearing bracket and a flexible coupling, so pump and motor bearings are independent and the motor can be changed without disturbing the pump. An OH4 joins the pump shaft to the motor shaft with a rigid coupling, so the motor bearings normally take the pump’s thrust, often with a product-lubricated guide bearing in the pump. An OH5 mounts the impeller directly on the motor shaft. Along that sequence the unit gets more compact and depends more on the motor’s bearings. ASME B73.2 vertical in-line chemical pumps sit in this part of the family, typically with their own bearings and a flexible coupling: the OH3 layout.
OH6: high-speed integral gear
An OH6 pump carries a small impeller on the output shaft of a speed-increasing gearbox built into the pump, with no coupling between them, so the impeller runs at high speed. It reaches heads that would otherwise need a multistage pump, at low flow, in a compact unit. It is a specialist layout for low-flow, high-head duties, often on light hydrocarbons.
Between-bearings pumps: BB1 to BB5
A between-bearings pump carries its impeller or impellers on a shaft supported by a bearing at each end. The rotor is stiffer than an overhung rotor of the same size, which suits large flows, high heads and multistage construction. Two questions sort the five types: does the casing split along the shaft (axially) or across it (radially), and does the pump have one or two stages or many? BB5 adds a third: one casing or two.
BB1: axially split, one or two stages
The casing is divided on a horizontal joint through the shaft centreline, with both nozzles in the lower half. Lifting the upper half exposes the whole rotor for inspection while the suction and discharge pipes stay connected. Most BB1 pumps are single stage with a double-suction impeller, which keeps the NPSH required low at large flows: the familiar split case pump of water works, cooling water and transfer duty. The 12th edition of API 610 splits the type in two: BB1-A for foot-mounted casings and BB1-B for casings supported near the shaft centreline.
YSM’s S series split case pump is a single-stage, double-suction, axially split volute pump with bearing housings at both shaft ends: a BB1-type layout. It is built to GB/T 5657, not to API 610.
BB2: radially split, one or two stages
The casing splits across the shaft instead of along it, and the joint is a round gasketed face instead of a long flat one. That joint is easier to keep tight at high pressure and temperature, so BB2 pumps are common on hot, high-pressure single- and two-stage duties, normally centreline-supported like an OH2.
BB3: axially split, multistage
The BB3 applies the axial split to a multistage rotor. Lifting the upper casing exposes every stage, which keeps maintenance straightforward, but the long flat joint has to hold full discharge pressure. It is used for high-head duties such as pipelines, water injection and boiler feed.
BB4: radially split, single casing, multistage
A BB4, also called a ring-section or segmental pump, is built up from stage casings stacked between a suction casing and a discharge casing and held together by tie bolts. The construction is modular and economical at high head. Each stage joint is a radial joint that faces the outside of the pump; for the highest pressures and the most hazardous liquids the BB5 barrel is the usual choice.
YSM’s KM multistage casing pump is a ring-section design, a BB4-type layout, for boiler feed, reverse osmosis and pressure boosting with heads to 1,000 m.
BB5: radially split, double casing (barrel)
In a BB5 the complete stage bundle sits inside a heavy outer barrel closed by a bolted end cover. The barrel holds the pressure, the inner casing is designed for the differential pressure across it, and the bundle can be withdrawn as one cartridge. It is the layout for the highest pressures: boiler feed on large units, water injection and high-pressure hydrocarbon service.
Vertically suspended pumps: VS1 to VS7
A vertically suspended pump hangs below its driver: the motor stands on a head or a support plate and the pumping element sits in the liquid, in a pit, a sump, a tank or a can. Four things sort the seven types: whether the pump hangs in an open wet pit or inside its own can, whether the hydraulic end is a diffuser bowl, a volute or an axial-flow propeller, whether the discharge comes up through the column or through a separate pipe, and, for sump pumps, whether the shaft runs in bearings along the column or is a cantilever.
VS1, VS2 and VS3: wet pit, discharge through the column
All three hang in an open pit and discharge up through the column to a head above the floor. A VS1 uses diffuser bowls and can stack several stages; a VS2 uses a volute casing; a VS3 uses an axial-flow propeller for very large flows at low head.
YSM’s KVT vertical turbine pump is built in the VS1 wet-pit configuration, with single or multistage closed impellers and side discharge through the pump head; an API 610 version is available on request.
VS4 and VS5: sump pumps with a separate discharge
Both hang from a support plate into a sump and bring the liquid up through a discharge pipe beside the column. A VS4 drives the impeller through a line shaft supported by steady bearings along the column, which allows long settings. A VS5 is a cantilever design: a stiff shaft with no bearings below the plate, so nothing has to run in the pumped liquid, at the price of a limited length.
YSM’s G171 vertical sump pump has the VS4-type layout: the pump hangs from a support plate, a lineshaft runs in steady bearings inside the column and the discharge pipe is separate. It is a vertical sump design, not an API 610 pump.
VS6 and VS7: double casing (can pumps)
A can pump sets the pumping element inside its own suction barrel, sunk below the floor, with the suction and discharge nozzles on the head. Because the first stage sits well below the suction nozzle, the liquid gains static head on its way down, so VS6 and VS7 pumps suit low-NPSH duties such as condensate and liquefied gases. A VS6 uses diffuser bowls; a VS7 uses a volute. The KVT is also built in the VS6 can configuration; which of VS1 and VS6 fits is decided by the installation during the selection review.
OH1 vs OH2, BB4 vs BB5 and the other comparisons engineers ask about
| Comparison | The difference | Why it matters |
|---|---|---|
| Overhung vs between-bearings | Impeller on the end of the shaft, outboard of the bearings, vs a shaft supported at both ends. | Overhung pumps are compact and pull out from the back of the casing; between-bearings rotors are stiffer and carry large flows, high heads and many stages. |
| OH1 vs OH2 | Casing supported on feet below the shaft vs at the shaft centreline. | Centreline support holds alignment as the casing heats and accepts higher nozzle loads, so hot duty moves to OH2. |
| OH2 vs OH3 | Horizontal on a baseplate vs vertical in the pipework. | OH3 saves floor space and baseplate work; OH2 is easier to reach for maintenance. |
| BB1 vs BB2 | Casing split along the shaft vs across it. | The axial split gives easy access to the rotor; the radial joint holds higher pressure and temperature. |
| BB1 vs BB3 | One or two stages vs multistage, both axially split. | The stage count follows the head. |
| BB4 vs BB5 | Stage casings clamped by tie bolts vs a stage bundle inside a pressure barrel. | The barrel keeps every internal joint inside the pressure boundary, which is why it is chosen for the highest pressures and the most hazardous liquids. |
| VS1 vs VS6 | Open wet pit vs inside a suction can. | The can sets the first stage well below the suction nozzle and closes the suction; the wet pit needs a sump. |
| VS4 vs VS5 | Line shaft with steady bearings along the column vs a cantilever shaft with no bearings in the liquid. | VS4 allows long settings; VS5 keeps bearings out of the pumped liquid. |
Recognising an installed pump’s type on site
When a pump arrives without its datasheet, the construction usually settles the family and often the type. Work through it in this order:
- Orientation. Horizontal on a baseplate, vertical in the pipework, or hanging from a plate or head into a pit, a sump or a can.
- Bearings. One bearing housing behind the casing means overhung; a bearing housing at each end of the casing means between-bearings.
- Casing support. Feet below the casing point to OH1; supports level with the shaft centreline point to OH2 or a centreline-mounted BB pump.
- Casing split. A flat horizontal joint along the shaft is an axial split (BB1, BB3); round joints across the shaft are a radial split (BB2, BB4, BB5). Tie bolts running the length of the casing mark a ring-section BB4; a heavy barrel with an end cover marks a BB5.
- Vertical pumps. Discharge from the head above the floor means the column carries it (VS1, VS2, VS3, VS6, VS7); a separate discharge pipe beside the column means a sump pump (VS4, VS5). Both nozzles on the head with the pump below floor level means a can (VS6, VS7).
- Coupling, on vertical in-line pumps. A bearing bracket and a flexible coupling is OH3, a rigid coupling is OH4, and an impeller on the motor shaft is OH5.
The nameplate, the serial number and the installed drawing then confirm the model. The nameplate and drawing guide lists the fields to record before a replacement is quoted.
Where ASME B73.1 and ISO 5199 pumps sit on the API 610 map
Several pump standards share the layouts in the API 610 table without sharing its requirements. An ASME B73.1 chemical process pump is normally a horizontal, end-suction, foot-mounted overhung pump: the OH1 layout, with centreline-mounted versions for high temperature. ASME B73.2 pumps are vertical in-line overhung pumps, typically with their own bearings and a flexible coupling: the OH3 layout. ISO 2858 end-suction pumps, usually specified with ISO 5199, are normally foot-mounted overhung pumps as well.
The layout is shared; the requirements are not. ASME B73.1 fixes the installation dimensions for each size designation. ISO 5199 is a technical specification for general application. API 610 is written for petroleum, petrochemical and natural gas industry services, with heavier requirements on pressure casings, nozzle loads, bearings, vibration and testing. An OH1 chemical pump and an OH2 refinery pump can have the same impeller diameter and still be different machines. The what is an ANSI pump guide, the ASME B73.1 explainer and the standards comparison set out the differences.
YSM pump ranges by API 610 layout
The table maps each YSM range to the layout it has. A layout match is where a selection review starts, not a statement of compliance. API 610 is referenced only where the range page says so: the KZ follows type OH2 conventions, and the KVT offers an API 610 version on request.
| Range | Layout | What the range page states |
|---|---|---|
| G196 | OH1 layout: horizontal, end suction, foot-mounted, back pull-out | ASME B73.1 horizontal end-suction chemical process pump |
| D Mark III | OH1 layout: horizontal, end suction, foot-mounted, back pull-out | ASME B73.1 horizontal end-suction chemical process pump |
| KZ | OH2: centreline-mounted end suction, back pull-out | ISO 13709 / API 610 type OH2 conventions; selections reviewed against the project’s API 610 datasheet |
| S series | BB1-type layout: single stage, double suction, axially split, bearing housings at both shaft ends | Axially split double-suction volute pump built to GB/T 5657 |
| KM | BB4-type layout: ring-section multistage | Multistage casing pump for boiler feed, reverse osmosis and pressure boosting, heads to 1,000 m |
| KVT | VS1 (wet pit) and VS6 (can) configurations | Vertical turbine pump; an API 610 version is available on request |
| G171 | VS4-type layout: line shaft in steady bearings, separate discharge pipe | Vertical sump and process pump; not an ASME B73.1, ISO 2858 or API 610 pump |
Where a project calls for API 610, the pump is reviewed against the datasheet and the edition named on it.
API 610 pump type FAQ
What are the API 610 pump types?
API 610 classifies centrifugal pumps into 18 types in three families: overhung (OH1 to OH6), between-bearings (BB1 to BB5) and vertically suspended (VS1 to VS7). The family says how the rotor is supported; the number identifies the layout within the family, such as how the casing is mounted, how it is split, and how many stages and casings the pump has. The current 12th edition splits BB1 into BB1-A (foot-mounted) and BB1-B (near-centreline-mounted), which makes 19 designations.
What is an overhung pump?
An overhung pump carries its impeller on the end of the shaft, beyond the bearings, so the impeller overhangs the bearing housing. Most small and medium process pumps are overhung, including ASME B73.1 chemical process pumps (normally OH1) and refinery pumps of type OH2. The layout allows back pull-out maintenance: the rotating assembly comes out of the back of the casing while the pipework stays connected.
What is the difference between a between-bearings pump and an overhung pump?
In an overhung pump the impeller sits outboard of the bearings on the end of the shaft; in a between-bearings pump the impeller or impellers sit between two bearings, one at each end of the shaft. The between-bearings rotor is stiffer and carries large flows, high heads and multiple stages; the overhung pump is more compact and simpler to maintain. API 610 codes the two families OH and BB.
Where is the impeller mounted in an overhung pump?
Inside the casing, on the free end of the shaft, outboard of the bearing housing. The bearings sit behind the casing, in a bearing frame or bracket, and the shaft passes through the seal chamber to the impeller. Because the impeller is cantilevered, shaft stiffness and the bearing arrangement limit how far from its best efficiency point the pump should run for long periods.
What is the difference between OH1 and OH2 pumps?
Both are horizontal, single-stage overhung pumps. An OH1 casing stands on feet below the shaft; an OH2 casing is supported at the shaft centreline. When a foot-mounted casing heats up it grows upward and moves the shaft relative to the driver, while a centreline-mounted casing grows evenly about the shaft, holds alignment and takes higher nozzle loads. That is why hot, high-pressure process duty uses OH2.
What is a BB5 pump?
A BB5 is a radially split, double-casing multistage pump, usually called a barrel pump. The stage bundle sits inside an outer barrel that holds the pressure and is closed by a bolted end cover, and the bundle can be withdrawn as one unit. It is used for the highest pressures, such as boiler feed on large units, water injection and high-pressure hydrocarbon service.
What is the difference between VS1 and VS6 pumps?
Both are vertical diffuser pumps that discharge up through the column. A VS1 hangs in an open wet pit; a VS6 hangs inside its own suction can below the floor, with both nozzles on the head. Because the first stage sits well below the suction nozzle, a VS6 suits services with little NPSH available.
Is an ANSI pump an API 610 pump?
No. An ANSI pump is built to ASME B73.1, the chemical process pump standard that fixes the installation dimensions for each size. It normally has the OH1 layout, but not the API 610 requirements for pressure casings, nozzle loads, bearings, vibration and testing. A project that specifies API 610 needs a pump designed, built and tested to it.
Does an API 610 type code mean a pump complies with API 610?
No. The code describes the layout only, and many pumps outside API 610 service share the OH1, BB1 or VS4 layouts. Compliance comes from designing, building and testing the pump to the edition of API 610 and the datasheet named in the purchase order, and the manufacturer states it for the specific pump.
Related reading
- Process pump standards compared — ASME B73, ISO 2858, ISO 5199 and API 610 side by side.
- What is an ANSI pump? — the OH1 layout of the chemical process pump in detail.
- ASME B73.1 explained — what the standard fixes and what it leaves to the purchaser.
- API seal flush plans — the numbered sealing plans used on process pumps.
Type definitions on this page paraphrase the pump classification of API 610 and ISO 13709; the standard, in the edition named on the purchase order, governs.