Imagine standing in an industrial pump room, surrounded by various models and configurations of centrifugal pumps. The technical terminology—particularly abbreviations like "OH pump" and "BB pump"—might seem overwhelming at first glance. Today, we'll clarify two common horizontal centrifugal pump types in industrial applications: Overhung (OH) pumps and Between Bearings (BB) pumps.

Before examining OH and BB pumps, it's essential to understand the API 610 standard—the defining specification for centrifugal pumps. This standard identifies three primary pump configurations:

• OH (Overhung): The impeller mounts on a single shaft end, extending cantilever-style beyond the bearing housing.
• BB (Between Bearings): The impeller positions between two bearings, providing superior balance and support.
• VS (Vertically Suspended): The impeller and pump shaft hang vertically within the pumped fluid.

As OH and BB pumps are most prevalent in horizontal installations, we'll focus exclusively on these configurations.

OH pumps feature single-bearing support, with the impeller extending beyond the bearing housing—analogous to a weightlifter holding a barbell with one arm. This design keeps bearings isolated from process fluids.

The OH family includes several configurations:

• OH1: Base-mounted, end-suction pumps with separate bearing brackets. This common design offers simplicity and easy maintenance.
• OH2: Centerline-mounted pump casings with independent bearing housings, ideal for high-temperature applications by minimizing thermal expansion effects.
• OH3/OH4/OH5: Vertically coupled designs with close-coupled or separate drive units, typically used in space-constrained installations.
• OH6: Gear-driven high-speed units for light hydrocarbon or gas compression applications, characterized by compact dimensions and elevated rotational speeds.

• Space efficiency: Compact footprint suits confined spaces.
• Simplified maintenance: Easy mechanical seal inspection and replacement reduces downtime.
• Mid-pressure capability: Performs reliably in stable hydraulic conditions at moderate pressures.

• Shaft deflection risk: Large impellers or high pressures may cause shaft bending.
• Flow/temperature constraints: Less suitable for high-flow or high-temperature systems.

• Water supply systems
• Chemical dosing
• Light hydrocarbon transfer

BB pumps position the impeller between two bearings—like a weightlifter using both arms—providing symmetrical support that reduces dynamic shaft loads in high-pressure conditions.

• BB1: Axially split casing, single- or double-suction for high-flow efficiency.
• BB2: Radially split, single-stage units for high-pressure service.
• BB3: Axially split, multistage configurations for elevated head requirements.
• BB4: Radially split, segmented multistage pumps for ultrahigh pressures.
• BB5: Barrel-type, radially split multistage pumps offering compact high-pressure capability.

• Enhanced shaft stability: Dual-bearing support minimizes deflection across load conditions.
• High-pressure/temperature tolerance: Engineered for demanding process requirements.
• Reduced vibration: Symmetrical rotor design decreases axial thrust and vibration.

• Space requirements: Larger footprint and heavier construction.
• Maintenance complexity: Multistage designs require extensive disassembly for internal inspection.

• Boiler feed systems
• Pipeline transportation
• Oilfield water injection

When choosing between OH and BB configurations:

• Space constraints exist
• Handling clean, low-viscosity fluids at moderate pressures
• Maintenance accessibility is prioritized

• High differential pressures or temperatures are present
• Flow stability under variable conditions is required
• Multistage operation or extended continuous service is needed

Ultimately, pump selection requires evaluating process conditions, operational demands, and lifecycle costs. This analysis provides the technical foundation for making informed decisions between these two fundamental centrifugal pump designs.