Fluid Transport

Pump Type Selection: Choosing Between API 610 OH/BB/VS, ASME B73 & Positive Displacement

Work down the pump family tree from kinetic vs positive displacement, apply the ASME B73 vs API 610 decision, and land on the right API 610 §4.2.2 / Table 1 type family for your service.

Launch Calculator B73 vs API 610

First fork

Kinetic vs PD

Viscosity above ~300 cP or precise metering pushes you to positive displacement.

B73 envelope

≤275 psig / ≤300°F

Exceed any B73 limit, or run flammable hydrocarbons, and you move to API 610.

API 610 §4.2.2 / Table 1

OH / BB / VS

Overhung, between-bearings, and vertically suspended type families.

Contents

Use this guide when you need to:

  • Decide between centrifugal and positive displacement.
  • Tell whether ASME B73 or API 610 applies.
  • Pick the right OH, BB or VS type for the service.

1. The Pump Family Tree

Selecting a pump type is a top-down decision. You start at the broadest fork — how the machine adds energy to the fluid — and work down to a specific configuration. Every industrial pump falls into one of two top-level families: kinetic (dynamic) pumps that add velocity which is then converted to pressure, and positive displacement (PD) pumps that trap a fixed volume of fluid and force it into the discharge.

The Two Top-Level Families

Family How it adds energy Behavior
Kinetic (dynamic) Impeller imparts velocity; the casing/diffuser converts it to pressure Head varies with flow (a pump curve); flow is sensitive to system resistance
Positive displacement A moving boundary traps a fixed volume and displaces it each cycle/revolution Near-constant flow regardless of discharge pressure; pressure set by the system — needs relief protection

The Family Tree

Each top-level family branches into the configurations you actually specify on a datasheet:

KINETIC / DYNAMIC • Centrifugal – Overhung (impeller cantilevered off the bearings) – Between-bearings (shaft supported on both ends) – Vertically suspended (column/can) • Regenerative turbine (peripheral) — low flow, high head, low NPSH • Special-effect (jet/eductor, electromagnetic) — niche services POSITIVE DISPLACEMENT • Reciprocating – Piston / plunger (power pumps) – Diaphragm (sealless, controlled-volume metering) • Rotary – Gear (external / internal) – Lobe – Screw (twin/triple) – Vane – Progressive cavity (PC)
Why the family matters first: the family fixes how the machine behaves on a P&ID. A centrifugal pump throttles gracefully but loses flow as backpressure rises; a PD pump holds flow but will overpressure a closed discharge, so it always needs a relief valve. Get the family wrong and no amount of detail design recovers it.

2. Centrifugal vs Positive Displacement

The first real decision is which top-level family fits the duty. Centrifugal (kinetic) pumps dominate process and pipeline service because they are simple, compact, and economical across a huge range of flow and head. Positive displacement pumps win in specific corners where centrifugals struggle. The routing below is engineering guidance, not a code mandate — but it captures where each family is the right tool.

When Positive Displacement Wins

Condition Favored family Why
Viscosity above ~300 cP Rotary PD (gear, screw, lobe, PC) Centrifugal efficiency and head fall off sharply with viscosity; PD performance is largely unaffected
Precise metering / dosing Controlled-volume PD (diaphragm/plunger) Delivered volume is set by stroke and speed, independent of discharge pressure — repeatable to a fraction of a percent
Very low flow at very high head Reciprocating (plunger) PD A tiny centrifugal at high head runs at very low specific speed and poor efficiency; a plunger pump delivers high pressure efficiently at low flow
Broad mid-range flow & head Centrifugal Lowest cost, smallest footprint, simple control by throttling or VFD
Rule of thumb: if viscosity is high, the metering must be precise, or the duty is very-low-flow / very-high-head, look hard at positive displacement. Otherwise the broad mid-range belongs to centrifugal pumps, and the rest of this guide focuses on choosing among centrifugal types.

Behavioral Contrasts That Drive Selection

Attribute Centrifugal Positive Displacement
Flow vs pressure Flow drops as discharge pressure rises (follows the curve) Flow nearly constant; pressure rises until something gives
Closed discharge Survives briefly at shutoff head (but heats up) Overpressures — relief valve is mandatory
Viscous fluids Performance degrades quickly Handles viscous fluids well
Shear-sensitive fluids High shear at the impeller Gentle (lobe, PC) options available
Flow turndown / metering Throttle or VFD; not precise dosing Stroke/speed gives precise volumetric control

3. ASME B73 vs API 610

Once you have settled on a centrifugal pump, the next fork is which design specification governs it. For clean, general-service duties there is a lighter, lower-cost class of pump built to ASME B73 (B73.1 horizontal end-suction, B73.2 vertical in-line). For severe or hydrocarbon service, the heavy-duty API 610 class applies. The dividing line is an application envelope: B73 pumps are dimensionally interchangeable, foot-mounted chemical-process pumps suited to a bounded set of conditions.

The ASME B73 Application Envelope

A B73 chemical-process pump is generally appropriate when all of these limits are satisfied:

Parameter B73 typical limit
Discharge pressure ≤ 275 psig
Pumping temperature ≤ 300 °F
Speed ≤ 3600 rpm
Driver power ≤ 150 HP
Total head ≤ 450 ft
Suction pressure ≤ 75 psig

Exceed any single limit, or run flammable hydrocarbon service, and you move up to API 610. API 610 pumps are heavy-duty and centerline-mounted (the casing is supported on its horizontal centerline so it stays aligned as it heats up), with stricter requirements for nozzle loads, shaft stiffness, sealing, and materials. The B73 envelope is a screening tool — the actual choice also weighs site standards, reliability targets, and the owner's specification.

Why Centerline Mounting Matters for API 610

Foot-mounted (B73) casings grow upward off the baseplate as temperature rises, shifting the shaft out of alignment. Centerline-mounted (API 610) casings expand symmetrically about the shaft axis, holding alignment through thermal transients. That is one of several reasons hot and hydrocarbon services demand the API 610 class rather than B73.

Quick Comparison

Attribute ASME B73 API 610
Service Clean, general / chemical process Hydrocarbon, severe, high-energy
Mounting Foot-mounted Centerline-mounted
Interchangeability Dimensionally standardized across makers Application-engineered to the datasheet
Cost / robustness Lower cost, lighter duty Higher cost, heavy duty, long life
Configurations Horizontal end-suction (B73.1), vertical in-line (B73.2) OH / BB / VS type families (§4.2.2)

4. API 610 OH/BB/VS Type Families

API 610 §4.2.2 / Table 1 (normative) sorts centrifugal pumps into three families by mechanical configuration — and within each, into numbered types. The family letters describe how the rotor is supported:

  • OH — Overhung: the impeller is cantilevered off the end of a shaft supported by a bearing housing on one side only.
  • BB — Between bearings: the shaft is supported by bearings at both ends, with the impeller(s) in between.
  • VS — Vertically suspended: the pump hangs vertically from a mounting plate, with the rotating element suspended in a column, can, or pit.

Overhung (OH) Types

Type Description Typical service
OH2 Horizontal, overhung, single-stage, centerline-mounted, separate bearing bracket — the workhorse of API 610 process pumps Tower reflux, propane and light-ends, tank-farm transfer, the broad bulk of process duties
OH3 Vertical in-line, overhung, separately coupled — installs directly in the pipe run Space-limited plots and pipe racks where a horizontal baseplate won't fit
OH6 High-speed, integral-gear, overhung (the "Sundyne" configuration) — gearbox steps the impeller up to high speed High head at low flow, where a conventional pump would need many stages

Between-Bearings (BB) Types

Type Description Typical service
BB1 Axially (horizontally) split, single-stage, double-suction High flow at low-to-moderate head — cooling water, fire water, large transfer
BB2 Radially split, single- or two-stage, between bearings Reactor charge, amine circulation, moderate-head hydrocarbon service
BB3 Axially split, multistage Boiler feed, pipeline mainline service — high head, lower pressure class than barrel
BB5 Radially split, barrel (double-casing), multistage Very high pressure — high-pressure injection, charge, and pipeline duties

Vertically Suspended (VS) Types

Type Description Typical service
VS1 Vertical turbine, wet-pit, diffuser, single casing Cooling-water and fire-water lift from open pits and basins
VS4 Vertical, line-shaft, volute, sump-mounted Sump and drainage service
VS6 Vertical, double-casing (can/suction-can), diffuser Low NPSH-available duties — LPG, propane, and other light hydrocarbons near bubble point
How head and flow map to the families: OH covers the broad single-stage middle; BB1 takes high flow/low head, while BB3/BB5 climb to high head and high pressure; VS earns its place when suction conditions (a pit, or very low NPSH-available) drive the geometry. OH6 is the low-flow/high-head specialist.

5. PD Standards & Selection Workflow

If the first fork sent you toward positive displacement, a parallel set of API standards governs those machines. Each covers a different PD configuration:

Standard Scope When it applies
API 674 Positive displacement pumps — reciprocating (power pumps: piston/plunger) High-pressure, low-flow service driven through a crank; e.g. high-head injection
API 675 Positive displacement pumps — controlled-volume (metering) Precise, repeatable dosing where delivered volume is set by stroke and speed (diaphragm/packed-plunger)
API 676 Positive displacement pumps — rotary (gear, screw) Viscous and high-viscosity service, continuous smooth flow

Selection Workflow: Family → Spec → Type

Putting the whole guide together, type selection runs in three steps:

STEP 1 — FAMILY Is viscosity > ~300 cP, is precise metering needed, or is it very-low-flow / very-high-head? YES → Positive displacement → go to Step 2a NO → Centrifugal → go to Step 2b STEP 2a — PD SPEC Reciprocating power pump (high P, low Q) → API 674 Controlled-volume / metering → API 675 Rotary gear or screw (viscous, smooth) → API 676 → then select gear / lobe / screw / vane / PC by fluid STEP 2b — CENTRIFUGAL SPEC Within the B73 envelope (≤275 psig, ≤300°F, ≤3600 rpm, ≤150 HP, ≤450 ft head, ≤75 psig suction) AND clean, non-flammable? YES → ASME B73.1 / B73.2 NO (exceeds a limit or flammable HC) → API 610 → go to Step 3 STEP 3 — API 610 TYPE (§4.2.2) Match head / flow / suction conditions to a type family: High flow, low head → BB1 Broad single-stage middle → OH2 (or OH3 if space-limited) High head, low flow → OH6 (integral-gear) High head, multistage → BB3 → BB5 as pressure climbs Pit / very low NPSHa → VS1 / VS4 / VS6
Work it top-down. The family decision constrains everything that follows; the specification (B73 vs API 610, or which API 6xx) sets the rulebook; only then do you pick the specific type. Reversing the order — picking a familiar type first and back-fitting the duty — is how mis-selections happen.

Type Selection Cheat Sheet

Service / condition Type to consider
General process, mid-range flow & head OH2 (horizontal overhung)
Space-limited, install in the line OH3 (vertical in-line)
High head, low flow OH6 (high-speed integral-gear)
High flow, low head (cooling/fire water) BB1 (double-suction axially split)
Moderate-head hydrocarbon (amine, charge) BB2 (radially split)
High head, multistage (boiler feed, pipeline) BB3 → BB5 (barrel) at higher pressure
Low NPSH available (LPG, propane) VS6 (vertical can / suction can)
Wet-pit lift (cooling/fire water) VS1 (vertical turbine)
Sump / drainage VS4 (vertical sump)
Viscosity > ~300 cP Rotary PD per API 676 (gear/screw)
Precise metering / dosing Controlled-volume PD per API 675
Very low flow, very high head Reciprocating PD per API 674

Common Mistakes to Avoid

  • ❌ Choosing a familiar pump type first, then back-fitting the duty to it
  • ❌ Forcing a centrifugal onto viscous (>300 cP) or precise-metering service
  • ❌ Specifying a B73 pump for flammable hydrocarbon or hot (>300°F) service
  • ❌ Treating the B73 envelope numbers as the only screen — site standards and owner specs also decide
  • ❌ Forgetting that any PD pump needs discharge relief protection
  • ❌ Confusing the OH/BB/VS letters (rotor support) with stage count or split type

Key Standards & References

  • API 610 (12th Ed) §4.2.2 – Centrifugal Pumps for Petroleum, Petrochemical and Natural Gas Industries (normative source for the OH/BB/VS type families)
  • ASME B73.1 – Specification for Horizontal End Suction Centrifugal Pumps for Chemical Process
  • ASME B73.2 – Specification for Vertical In-Line Centrifugal Pumps for Chemical Process
  • API 674 – Positive Displacement Pumps — Reciprocating
  • API 675 – Positive Displacement Pumps — Controlled Volume (metering)
  • API 676 – Positive Displacement Pumps — Rotary

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Frequently Asked Questions

When should I specify API 610 instead of ASME B73?

Use an ASME B73.1/B73.2 chemical-process pump for general clean service within its application envelope (discharge up to 275 psig, temperature up to 300°F, speed up to 3600 rpm, driver up to 150 HP, total head up to 450 ft, suction up to 75 psig). If the duty exceeds any of those limits, or the service is flammable hydrocarbon, specify an API 610 heavy-duty, centerline-mounted pump.

What do the API 610 codes OH, BB and VS mean?

API 610 §4.2.2 groups centrifugal pumps into three families by configuration: OH = overhung (impeller cantilevered off the bearing housing, e.g. OH2 horizontal, OH3 vertical in-line, OH6 high-speed integral-gear); BB = between-bearings (shaft supported on both ends, e.g. BB1 axially split double-suction, BB2 radially split, BB3 multistage axially split, BB5 barrel multistage); VS = vertically suspended (e.g. VS1 wet-pit turbine, VS4 sump, VS6 vertical can for low NPSH available).

When do I choose positive displacement instead of a centrifugal pump?

Favor positive displacement when viscosity is above roughly 300 cP (centrifugal efficiency falls off sharply), when you need precise metering or dosing (controlled-volume PD per API 675), or when you need very low flow at very high head (reciprocating per API 674). Centrifugal pumps handle the broad mid-range of flow and head most economically.