API Pump Types, Defined

Designed to handle various hydrocarbons in accordance with the high standards of the American Petroleum Institute, API pumps ranging from end suction to vertical turbine models are commonly used in the petroleum, petrochemical, and natural gas industries.

API Pump

Published September 01, 2021   |   6 minute read

Here’s a brief primer to get you started learning about API standards and API pumps:

What Are API Pumps?

API Pumps are built in accordance with the relevant API specifications, which provide comprehensive guidelines governing safe and reliable pump design and construction for use in the oil and gas industry. API pumps frequently must convey fluids at high pressures and temperatures and are manufactured to handle hydrocarbons like crude oil, natural gas, refined fuels, and aromatic hydrocarbons; non-hydrocarbon fluids used in the industry such as sulfuric and hydrofluoric acids, caustics, and solvents; and even water services such as boiler feed, sea water, condensate, and fresh water. 

The most commonly used API pump standard is API-610 for General Refinery Service, which specifies requirements for centrifugal pumps.

There are numerous other important API specifications to be aware of related to pumps, mechanical seals, and process gas compressors, such as:

API-685 (Sealless Pumps)

API-676 (Rotary Positive Displacement Pumps)

API-675 (Metering Pumps)

API-674 (Reciprocating Positive Displacement Pumps)

API-681 (Liquid Ring Vacuum Pumps and Compressors)

API-682 (Mechanical Seals)

API-617 (Axial and Centrifugal Process Gas Compressors)

API-618 (Reciprocating Compressors)


The API-610 standard provides detailed guidelines for:

  • Pump classifications
  • Centerline-support (in most cases)
  • Pressure casings
  • Nozzle loadings
  • Gasketing
  • Bearings and bearing housings
  • Lubrication
  • Mechanical seal chamber standard dimensions and tolerances; note that specification API-682 covers mechanical seals and seal system requirements in detail
  • Closed and keyed impellers 
  • Wear rings and running clearances
  • Shafting and rotor dynamics
  • Baseplates 
  • Couplings and guards
  • Materials of construction
  • Testing and inspections
  • Reverse operation as hydraulic power recovery turbines

API pumps are usually installed in classified locations, where hazardous or explosive vapors are likely to be present. The motor will often be TEFC (Totally Enclosed Fan Cooled) and rated for a Class 1, Division 2 area (explosive vapors might be present); but they are also sometimes equipped with explosion-proof motors that have a more stringent area classification of Class 1, Division 1 (explosive vapors likely to be present). The purchaser should inform their vendor of the hazard classification for the area where the equipment will be installed.

API vs. ANSI Standards

API and ANSI (American National Standards Institute) are frequently confused and conflated. API standards provide guidelines focused on safety and reliability, while ANSI primarily provides a dimensional standard that manufacturers must comply with for their ANSI pumps.

API pumps are manufactured to be more durable and reliable than ANSI pumps. Design differences focusing more on the capability to handle greater temperatures and pressures enable API pumps to better control emissions, and be more efficient and reliable, overall. There is no API industry pump dimensional standard.

ANSI pumps are ideal for chemical, food, and general manufacturing processes. They tend to be horizontal, foot-mounted, end-suction, single-stage, centerline-discharge centrifugal pumps with an overhung impeller mounted on the end of the shaft. One manufacturer’s ANSI pump of a given size will have the same dimensions as another manufacturer’s pump of the same size.

Different Types of API Pumps

API-610 categorizes centrifugal pump types as either Overhung (OH), Between Bearings (BB) also known as split-case, or Vertically Suspended (VS).

Overhung (OH) style pumps include: 

  • OH1 – horizontal foot-mounted, single-stage, flexibly-coupled, one bearing housing, this pump type is rarely used compared to the ubiquitous OH2 type
  • OH2 – horizontal centerline-mounted, single-stage, flexibly-coupled, one bearing housing
  • OH3 – vertical inline, single-stage, flexibly-coupled, one bearing housing
  • OH4 – vertical inline, single-stage, pump and driver shafts are rigidly coupled
  • OH5 – vertical inline, single-stage, close-coupled with impeller mounted directly on driver shaft
  • OH6 – usually vertical inline but can also be horizontal, integrally-geared speed increasing gearbox, impeller mounted on gearbox output shaft, gearbox input can be close- or flexibly-coupled

Between Bearings (BB) style pumps include:

  • BB1 – axially-split, one- and two-stage
  • BB2 – radially-split, one- and two-stage
  • BB3 – axially-split, multi-stage
  • BB4 – radially-split, multi-stage, single-casing (also known as ring-section, segmental- or segmented-ring, or tie-rod pumps)
  • BB5 – radially-split, multi-stage, double-casing

Vertical-Suspended (VS) style pumps include

  • VS1 – wet pit, single-casing diffuser style, discharge through the column
  • VS2 – wet pit, single-casing volute, discharge through the column
  • VS3 – wet pit, single-casing axial-flow, discharge through the column
  • VS4 – wet pit sump, line-shaft driven, discharge through separate discharge pipe
  • VS5 – wet pit sump, cantilever line shaft driven, discharge through separate discharge pipe
  • VS6 – double-casing diffuser style, discharge through the column, essentially a VS1 mounted in a “suction can” or “suction barrel”
  • VS7 – double-casing volute style, discharge through column, essentially a VS2 mounted in a “suction can” or “suction barrel”

Overhung pumps usually are “end suction top discharge” type with the suction located on the end of the casing and the discharge facing vertically up at a 90-degree angle. They can sometimes also be “top suction top discharge.” They’re some of the most common and versatile types of pumps used worldwide, suitable for high and low flows and pressures and temperatures. They usually have only a single stage, but there are multi-stage variations available.

Vertical inline pumps are usually single-stage and are designed for installation in-line with the process piping. The vertical or upright design means they have a smaller footprint than their horizontal counterparts, which can be important where space is at a premium.

Horizontal split case pumps are centrifugal pumps with a casing split into two separate components. The split can be either radial or axial, depending on the particular type of pump.

Split-case pumps are also often multi-stage pumps, featuring multiple stages connected in series. Fluid enters the first chamber at suction line pressure and moves on to the next stage, where pressure is further elevated. The more stages the pump has, the higher the final discharge pressure. These pumps are known for producing higher pressures with the addition of every stage, while maintaining a constant flow range.

Vertical suspended pumps are designed to pump fluid from an underground source like a tank or sump. These pumps’ impellers would normally remain submerged in the liquid and so remain “primed.”

Vertical turbine pumps use diffuser-style impellers and bowl assemblies, with the liquid discharge flowing up the pump column and out the discharge head. They sometimes are installed in a suction “barrel” or “can,” when no liquid sump is present, in order to increase the amount of NPSHA in a given application. Vertical turbines are also often multi-stage, and are sometimes known as deep well pumps or line shaft turbine pumps. 

Sump pumps are volute-style pumps with a cutwater and (usually) single-stage impeller and with a separate discharge pipe supplied by the pump manufacturer and terminating with a flange above the pump mounting plate. 

Vertical pumps use vertical motors that are mounted above ground level and out of the liquid, connected via a vertical shaft to impellers at the bottom. In many vertical turbine installations, the pump thrust is supported by the motor bearings, and the mechanical seal can be removed and replaced by removing a rigid coupling without having to remove the motor.

Some types of pumps require specific end user approval to use because they do not meet all API-610 requirements, such as OH1, VS4, VS5, and BB4. 

How to Choose the Right API Pump for Your Application

With so many different types of API pumps available for such a wide range of industrial applications — all with different installation and maintenance needs — choosing the right one can seem like a daunting task. This is where Sunair Co. is ready to assist you.

Sunair Co. has provided customers with high-quality commercial and industrial equipment, to organizations large and small, for all industries and applications, since 1974. With an experienced and knowledgeable team of highly trained experts, Sunair Co. can help you find the perfect pump for your application.

For more information or to get started on choosing the right API pump for your system, contact us today.

What Is API?

The largest U.S. trade association for the oil and natural gas industry, the American Petroleum Institute (API) maintains more than 500 standards and recommended practices related to various segments of industry operations. The group’s stated mission is “to enhance operational safety, environmental protection and sustainability across the industry, especially through these standards being adopted globally.”

Leave a comment

Get updates to your inbox

Receive the latest industry news, once a month. Unsubscribe anytime.

Other Popular Articles

How to Measure Component Mechanical Seal Size

September 09, 2021

To determine the size of a component mechanical seal, measure the inside diameter of the seal face, the length of the spring, and diameter and thickness of the stationary seat.

Read Now

Impactful Opportunities for Automation in Food Manufacturing

April 07, 2022

Pairing Sunair Co.’s high-quality equipment with the most impactful automation opportunities within the food manufacturing industry—across the packaging, production, sanitation, product tracing, and quality control phases—enables companies to realize significant operational efficiencies and cost savings.

Read Now

Leading Food & Beverage Applications of Screw Pumps 

September 02, 2022

Sunair Co. provides top-quality sanitary single and twin screw pumps from the most trusted brands throughout the industry to transfer a wide range of food materials and ingredients safely and effectively. 

Read Now

How to Select Mechanical Seals for Pumps

June 28, 2021

Carefully consider applications, pertinent safety requirements, liquid to be transferred, pressure and temperature, and environmental concerns when selecting mechanical seals for pumps.

Read Now