In the complex network of oil refineries, countless pipelines connect various equipment like veins, with centrifugal pumps serving as the heart that continuously circulates fluids throughout the system. Selecting the appropriate centrifugal pump is crucial for ensuring production safety and operational efficiency. The API 610 standard, recognized as the gold standard for centrifugal pump design in the petroleum industry, provides detailed classification of pump types. This article offers an in-depth analysis of the three main categories of centrifugal pumps defined by API 610: Overhung (OH), Between Bearing (BB), and Vertically Suspended (VS) designs, helping professionals make informed decisions for practical applications.

API 610 is a standard published by the American Petroleum Institute for centrifugal pumps used in petroleum, heavy chemical, and natural gas industries. The standard covers all aspects of pump design, materials, manufacturing, testing, and installation, aiming to ensure reliability, safety, and performance. Based on structural characteristics, API 610 classifies centrifugal pumps into three main categories: Overhung (OH), Between Bearing (BB), and Vertically Suspended (VS).

Overhung centrifugal pumps feature an impeller mounted on one end of the shaft, with the other end supported by bearings. This compact design facilitates maintenance and is suitable for medium-to-low flow and head applications. API 610 further divides overhung pumps into six subtypes (OH1 through OH6) based on specific structural details.

OH1 pumps are single-stage overhung designs with foot-mounted support and horizontal installation. They connect to motors through flexible couplings, with the pump casing typically centered on the baseplate for easy installation and maintenance. These pumps offer simple construction and lower costs, making them suitable for general fluid transfer applications.

The primary distinction of OH2 pumps lies in their centerline support design, where the casing is supported by brackets along the centerline rather than through feet. This configuration minimizes the impact of thermal expansion, enhancing operational stability for high-temperature or fluctuating temperature conditions. OH2 pumps typically employ a single bearing housing that absorbs axial forces and maintains proper impeller positioning during operation.

OH3 pumps are single-stage, direct-coupled, in-line designs with overhung impellers and independent bearing supports. The motor mounts directly on the pump for compact installation, while the separate bearing housing absorbs operational loads. This configuration proves particularly useful in space-constrained applications.

OH4 pumps share identical construction with OH3 models, differing only in their use of rigid couplings instead of flexible couplings. While rigid couplings transmit greater torque, they demand higher installation precision.

OH5 pumps feature close-coupled designs with impellers mounted directly on motor shafts. These vertical, in-line, single-stage pumps offer extremely compact construction and simplified maintenance, making them widely applicable across various industries.

OH6 pumps represent high-speed designs incorporating speed-increasing gearboxes. The impeller mounts directly on the gear shaft, which connects to the motor through flexible couplings. Available in both vertical and horizontal configurations, these pumps excel in high-head, low-flow applications.

Between bearing centrifugal pumps position impellers between two bearings supported by the pump casing. This arrangement provides superior shaft support, reduces deflection, and enhances operational stability for high-flow, high-head applications. API 610 categorizes BB pumps into five types (BB1 through BB5) based on casing construction.

BB1 pumps feature horizontally split casings for single or two-stage horizontal installations. This design facilitates quick disassembly and maintenance, allowing rapid replacement of internal components. BB1 pumps serve well in general fluid transfer applications.

BB2 pumps differ from BB1 models primarily in their radially split casing construction. This design withstands higher pressures, making it suitable for demanding applications.

BB3 pumps represent multistage horizontal designs with horizontally split casings, typically connecting to motors through flexible couplings. These pumps perform exceptionally in high-head applications.

BB4 pumps employ multistage, segmented construction where each stage forms a ring connected by tie rods. Also known as ring-section pumps, each radially split stage mounts on a shaft supported by bearings at both ends.

BB5 pumps feature barrel-shaped outer casings designed for high-pressure operation. These multistage designs incorporate double-casing, radially split construction and flexible couplings.

Vertically suspended centrifugal pumps submerge their casings in liquid while suspending from a mounting plate via columns. This configuration minimizes cavitation risk, making it ideal for low-liquid-level or self-priming applications. API 610 classifies VS pumps into seven types (VS1 through VS7) based on structural variations.

VS1 pumps feature single-casing vertical suspended designs, serving as wet-pit or diffuser pumps. The column functions as the discharge conduit.

VS2 pumps employ volute casing designs while maintaining other characteristics similar to VS1 models.

VS3 pumps represent axial-flow, single-casing vertical suspended designs where the pump column serves as the discharge conduit.

VS4 designs incorporate separate discharge columns with volute casings. The vertically suspended shaft may include intermediate bearing support.

VS5 pumps also feature separate discharge columns but differ by eliminating intermediate bearings. The pump shaft remains overhung, supported solely by top bearings.

VS6 pumps incorporate double-casing construction, earning the nickname "can pumps." These vertical suspended designs discharge through pump columns. Underground installation sometimes enhances NPSH (Net Positive Suction Head) availability.

VS7 pumps complete the API 610 classification with volute casing designs similar to VS6 models while maintaining other comparable characteristics.

The API 610 standard provides comprehensive centrifugal pump classification, with each type offering distinct advantages for specific applications. Proper pump selection requires careful consideration of flow rate, head, fluid characteristics, and operating environment in accordance with API 610 guidelines. This ensures reliable operation, enhanced efficiency, and production safety. As technology advances, new pump designs and smart monitoring systems continue to emerge, warranting ongoing evaluation of industry developments alongside fundamental API 610 principles.