API 610 defines three main pump configurations: OH (overhung), BB (between bearings), and VS (vertical suspended). Within these, OH2 and BB2 are the two most common for refinery and petrochemical service. They look different, perform differently, and cost differently. Choosing the right configuration is one of the first decisions in API 610 pump selection.
This guide compares OH2 and BB2 side by side, so you can specify the right one for your application.
How Each Configuration Works
OH2 (Overhung, type 2): The impeller hangs off the end of the shaft, outside the bearing housing. The shaft is supported by bearings on one side only. The pump casing is end-suction, top-discharge. The impeller is accessible by removing the casing cover — no need to disconnect piping.
BB2 (Between bearings, type 2): The impeller sits between two bearing housings. The shaft is supported on both sides. The BB2 pump casing is radially split (bolt circle perpendicular to the shaft axis); the end covers are removed and the rotor assembly is withdrawn axially for maintenance. The impeller is double-suction (fluid enters from both sides), balancing axial thrust.
The fundamental difference: OH2 has one bearing support (overhung). BB2 has two bearing supports (between bearings). This determines everything that follows.
Performance Comparison
| Factor | OH2 | BB2 |
|---|---|---|
| Flow range | 10–300 m³/h | 100–3,000 m³/h |
| Head range | 50–200 m | 50–250 m |
| Impeller type | Single suction | Double suction |
| Axial thrust | High (needs balance drum or thrust bearing) | Balanced (double suction cancels thrust) |
| Shaft deflection | Higher (overhung shaft) | Lower (supported on both sides) |
| Efficiency at BEP | 70–80% | 75–85% |
| NPSHr | 3–6 m | 2–4 m (double suction) |
OH2 dominates the small-to-medium flow range. For 50–200 m³/h at 50–150 m head, OH2 is compact, efficient, and cost-effective. This is the most common API 610 pump in refineries.
BB2 dominates the large flow range. For 300–2,000 m³/h at 50–150 m head, BB2 is the only practical option. The double-suction impeller handles large flow without excessive NPSHr.
Maintenance Comparison
OH2 maintenance: Back-pull-out design. Remove the casing cover, pull the rotor assembly (shaft + impeller + bearings) straight back. The casing stays bolted to the piping. 4–8 hours for a seal replacement.
BB2 uses a radially split casing (bolt circle perpendicular to the shaft), suitable for high-pressure and high-temperature service. Remove the end cover, unbolt the bearing housings, and withdraw the rotor assembly axially; the casing stays bolted to the piping. 8–16 hours for a seal replacement (larger pump, heavier parts).
| Maintenance Factor | OH2 | BB2 |
|---|---|---|
| Seal replacement time | 4–8 hours | 8–16 hours |
| Bearing replacement time | 4 hours | 8 hours |
| Need to disconnect piping | No (back-pull-out) | No (split casing) |
| Lifting equipment needed | Small hoist (500 kg) | Large crane (2–5 tons) |
| Spare parts cost | Lower | Higher (larger parts) |
OH2 is easier and cheaper to maintain. BB2 is more complex — but it handles flows that OH2 cannot.
Cost Comparison
| Duty Point | OH2 Price | BB2 Price |
|---|---|---|
| 100 m³/h at 100 m | $15,000–$25,000 | $25,000–$40,000 (oversized for this duty) |
| 300 m³/h at 100 m | $30,000–$45,000 (at upper limit) | $35,000–$55,000 |
| 1,000 m³/h at 80 m | Not available (OH2 max ~300 m³/h) | $60,000–$100,000 |
At small flow (below 200 m³/h), OH2 is cheaper. At large flow (above 300 m³/h), BB2 is the only option. In the overlap zone (200–300 m³/h), both work — compare total cost including maintenance.
When to Choose OH2
Choose OH2 when:
- Flow is below 300 m³/h
- Head is 50–200 m
- Space is limited (compact footprint)
- Easy maintenance is important (back-pull-out)
- The pump is in a general process service (charge pump, transfer pump, injection pump)
Typical applications: Refinery charge pumps, chemical injection pumps, product transfer pumps, small boiler feed pumps.
Our API 610 OH2 pump range covers 10–300 m³/h at 50–200 m head for refinery and petrochemical service.
When to Choose BB2
Choose BB2 when:
- Flow is above 300 m³/h
- Head is 50–250 m
- Low NPSHr is required (double suction impeller)
- High reliability is critical (balanced thrust, lower shaft deflection)
- The pump is in a large-volume process service (main fractionator pump, large circulation pump)
Typical applications: Main fractionator reflux pumps, large crude oil transfer pumps, cooling water circulation pumps, pipeline booster pumps.
Common Selection Mistakes
Mistake 1: OH2 for large flow. Engineer specifies OH2 for 500 m³/h. The pump is at the extreme end of its range. Shaft deflection is high. Seal fails within months. Right answer: BB2.
Mistake 2: BB2 for small flow. Engineer specifies BB2 for 50 m³/h. The pump is oversized. Runs far off-BEP. Wastes energy. Right answer: OH2.
Mistake 3: Ignoring NPSHr. OH2 has higher NPSHr than BB2 (single suction vs double suction). If NPSHa is marginal, BB2 may be required even at moderate flow. Always check NPSH before selecting configuration.
Mistake 4: No maintenance space for BB2. BB2 needs overhead crane access for the top casing half. If the pump room has low ceiling, BB2 maintenance is impossible. OH2 back-pull-out works in tight spaces.
Decision Matrix
| If your duty is... | Choose... |
|---|---|
| 100 m³/h at 100 m, general service | OH2 |
| 500 m³/h at 80 m, large circulation | BB2 |
| 250 m³/h at 150 m, NPSHa = 4 m | BB2 (lower NPSHr) |
| 80 m³/h at 120 m, tight space | OH2 |
| 1,000 m³/h at 60 m, cooling water | BB2 |
For API 610 pump configuration selection, our API 610 OH2 pump range covers small-to-medium flow service. For BB2 and other configurations, talk to our engineers with your flow, head, and NPSH data.