API 610 OH2 vs OH3: Which Pump Configuration is Right?

API 610 OH2 and OH3 are overhung pump configurations for refinery and chemical service. This guide compares API 610 OH2 (horizontal, centerline-mounted, separate bearing bracket) and OH3 (vertical in-line, separate bearing bracket) to help engineers select the right configuration for high-temperature and general process applications.

API 610 Overhung Pump Overview

Overhung Design

• Impeller overhung outside bearings (cantilever shaft)
• Advantages: simple, compact, easy maintenance (back pull-out)
• Use: general refinery, chemical, moderate flow/head
• Types: OH1 (foot-mounted, integral bearing bracket), OH2 (centerline-mounted, separate bearing bracket), OH3 (vertical in-line, separate bearing bracket), OH4 (vertical in-line, integral bearing bracket)
• Consider: temperature, pressure, flow, maintenance

API 610 OH Types

OH2 (Centerline-Mounted, Horizontal)

Design Features

• Centerline-mounted: casing supported at the shaft centerline, so thermal expansion stays symmetric (standard API 610 OH2 pump)
• Horizontal shaft: impeller overhung, separate bearing bracket at drive end
• Back pull-out: remove rotating assembly without disturbing piping
• Pressure: up to 4500 psi (310 bar)
• Temperature: up to 400°C (centerline mounting maintains alignment at high temperature)

OH2 Applications

• Refinery: general hydrocarbon, charge, reflux, transfer
• Chemical: process pumps, acid, caustic, solvent
• Power: boiler feed (moderate pressure), condensate
• Gas processing: amine, glycol, injection
• Typical: flow 10-1000 m³/h, head 50-300m

OH2 Advantages

• Simple: foot mounting, easy installation
• Maintenance: back pull-out (seal/impeller without piping removal)
• Cost: generally lower than OH3 (horizontal, simpler layout)
• Availability: most common API 610 type, many suppliers
• Versatile: wide range of sizes, materials, options

OH2 Limitations

• High temp: without proper centerline support, thermal growth can misalign the pump and driver
• Alignment: thermal growth changes alignment at operating temp
• Large size: overhung shaft deflection limits size
• Very high pressure: casing design limits (use BB for >300m head)
• Consider: centerline OH2 is the standard form for hot service (API 610)

OH3 (Vertical In-Line, Separate Bearing Bracket)

Design Features

• Vertical in-line: pump mounted in the vertical piping run, supported by pipe and base structure
• Vertical shaft: motor above, separate bearing bracket supports the shaft
• Advantages: compact footprint, direct in-line piping, good for high-temperature and limited-space layouts
• Pressure: up to 4500 psi (310 bar)
• Temperature: up to 450°C+ in suitable designs

OH3 Applications

• High temp: thermal oil, hot hydrocarbon, reactor circulation
• Vertical space: limited footprint, vertical installation
• Chemical: high-temp process, polymer, reactor
• Power: high-temp feed, circulation
• Typical: flow 10-500 m³/h, head 50-250m

OH3 Advantages

• High temp: compact vertical design suits hot, space-limited services
• Compact: vertical orientation, small footprint
• Compact: vertical in-line layout saves floor space
• Piping: vertical in-line or side-top (flexible)
• Alignment: less affected by thermal growth

OH3 Limitations

• Vertical: motor above pump (maintenance access, lifting)
• Cost: can be higher than OH2 for equivalent duty (vertical construction)
• Size: limited (vertical overhung, shaft deflection)
• Maintenance: motor removal for seal/impeller (more complex)
• Availability: fewer suppliers, longer lead time

OH2 vs OH3 Comparison

Selection Guide

When to Choose OH2

• Temperature: up to ~350°C for general service, higher with appropriate materials and centerline OH2
• Space: horizontal layout available
• Maintenance: prefer easy back pull-out
• Cost: budget-conscious
• General service: refinery, chemical, moderate conditions

When to Choose OH3

• Temperature: high-temperature service where vertical in-line layout is preferred
• Space: limited footprint, vertical installation
• Piping: vertical in-line configuration suits limited space
• Thermal cycling: frequent temp changes (centerline more stable)
• Consider: OH4 (inline) if truly compact inline needed

Consider OH4 (Inline)

• Vertical inline: pump in vertical piping run, motor above
• Advantages: very compact, no baseplate, easy piping
• Limitations: limited size, motor weight on pump, maintenance
• Use: compact installations, water, general chemical
• Consider: if footprint extremely limited

API 610-Specific Considerations

High Temperature Hydrocarbon

• Thermal growth: significant (pump expands more than motor)
• OH2 centerline: casing supported at centerline (handles thermal growth)
• OH3: vertical in-line (excellent for high temp, compact)
• Alignment: cold alignment with thermal offset
• Piping: expansion loops, spring hangers (reduce strain)

Thermal Oil / Hot Oil

• Temperature: 300-400°C (thermal oil systems)
• OH3: preferred (vertical in-line, compact, thermal stability)
• Seal: API 682, Plan 23 (cooled flush), metal bellows (high temp)
• Material: 12% Cr, alloy steel (high temp strength)
• Consider: thermal expansion, insulation, safety (hot surfaces)

General Refinery

• Moderate temp: 50-300°C (most refinery pumps)
• OH2: standard (centerline-mounted, horizontal, cost-effective)
• Seal: API 682, Plan 11 (clean) or Plan 23 (hot)
• Material: 12% Cr (standard), 316SS, duplex (corrosive)
• Consider: if high temp section, use centerline OH2 or OH3

Maintenance Comparison

OH2 Maintenance

• Back pull-out: remove rotating assembly without piping
• Seal: cartridge seal, spacer coupling (quick replacement)
• Time: 2-4 hours (seal replacement)
• Access: horizontal, easy (ground level)
• Consider: coupling alignment after reassembly

OH3 Maintenance

• Motor removal: lift motor (above pump) to access seal/impeller
• Seal: cartridge seal, but motor must be removed
• Time: 4-8 hours (motor removal, alignment)
• Access: vertical, may need lifting equipment
• Consider: spare motor, lifting beam, alignment tools

Common Issues

• OH2 for high temp: Thermal growth, misalignment, seal failure. Use centerline OH2 or OH3.
• OH3 for general: Higher cost, complex maintenance. Use OH2 if temp moderate.
• Ignoring thermal growth: Nozzle loads, misalignment. Centerline mount, cold alignment offset.
• Wrong seal for high temp: Elastomer failure, leakage. Use Plan 23, metal bellows, Kalrez/PTFE.
• No alignment after maintenance: Bearing/seal failure. Laser align, correct for thermal growth.
• Piping strain: Nozzle damage, misalignment. Support piping independently, align after piping.
• OH3 motor removal without lift: Safety hazard, damage. Use lifting equipment, proper procedures.
• No spare parts: Long downtime. Keep spare seal, bearing, motor (OH3).

Procurement Recommendations

1. Determine max operating temperature (≤350°C → OH2, >350°C → OH3 or centerline OH2)
2. For high temp: specify OH3 (vertical in-line) or centerline OH2
3. For compact footprint: specify OH3 or OH4 (inline)
4. Specify API 610 compliance (L10 bearing life, vibration, testing)
5. For high temp: specify API 682 seal with Plan 23 (cooled flush)
6. Specify material per fluid (12% Cr, 316SS, duplex, alloy)
7. For OH3: specify motor mounting, lifting provisions, coupling
8. Require back pull-out design (OH2) for easy maintenance
9. Include spare seal, bearing assembly (critical pumps)
10. For high temp: specify centerline mounting (thermal growth)
11. Verify supplier has OH2/OH3 design and application experience

Related Industries

API 610 OH2 vs OH3 selection applies to oil & gas (refinery, pipeline, upstream), petrochemical (ethylene, ammonia, thermal oil), chemical processing (high-temp reactors, polymer), power generation (boiler feed, circulation), and general industrial (process pumps). The core principle — selecting between OH2 (horizontal, centerline) and OH3 (vertical in-line) based on temperature, space, and maintenance requirements — is consistent across all critical pump applications.