What's the Difference Between OH3 and OH4 Centrifugal Pumps?

In the API 610 standard, OH (Overhung) pumps are a category of centrifugal pumps with the impeller mounted on the end of a shaft that is supported by bearings on only one side. Among them, OH3 and OH4 pumps are both horizontal overhung centrifugal pumps, but they differ significantly in design, maintenance requirements, and application suitability.

Understanding these differences can help engineers and end users select the most appropriate pump for their operating conditions.

OH3 Centrifugal Pump Design

An OH3 pump is an overhung centrifugal pump equipped with an independent bearing frame.

Its key features include:

• Separate pump shaft and motor shaft
• Coupling connection between the pump and motor
• Independent bearing housing and lubrication system
• Higher shaft rigidity and load-carrying capability
• Easier maintenance and component replacement

Although the OH3 is still classified as an overhung pump, meaning the impeller is mounted on the overhung end of the shaft, the independent bearing frame provides improved rotor support and operational stability compared with more compact designs.

OH4 Centrifugal Pump Design

An OH4 pump is a close-coupled overhung centrifugal pump.

Its main characteristics include:

• Compact, integrated pump and motor assembly
• No independent bearing frame
• Motor shaft supports the pump rotor load
• No flexible coupling required
• Reduced installation space and overall weight

Because the motor and pump are closely integrated, OH4 pumps offer a simpler design with fewer components, making them a cost-effective solution for many industrial applications.

Performance and Application Comparison
Typical Applications of OH3 Pumps

Due to their robust construction and independent bearing support, OH3 pumps are commonly used in:

• Petrochemical processing
• Oil and gas facilities
• Power generation plants
• High-temperature liquid transfer
• Continuous-duty industrial services
• Applications requiring high reliability

OH3 pumps are often selected for demanding process conditions where long service life and operational stability are critical.

Typical Applications of OH4 Pumps

OH4 pumps are generally suitable for:

• Industrial water supply systems
• Cooling water circulation
• Utility services
• Light to medium-duty industrial processes
• Installations with limited space

Their compact design and lower installation costs make them an attractive choice for applications where operating conditions are less severe.

OH3 vs OH4: Key Differences

How to Choose Between OH3 and OH4 Pumps

The choice between an OH3 and an OH4 pump depends on the specific operating requirements of the application.

An OH3 pump is generally the better option when:

• High reliability is required
• The system operates continuously
• Process temperatures are elevated
• Mechanical loads are significant
• Maintenance accessibility is important

An OH4 pump is often preferred when:

• Space is limited
• Installation simplicity is a priority
• Budget considerations are important
• Operating conditions are relatively moderate

When selecting a pump, engineers should evaluate factors such as flow rate, discharge pressure, fluid characteristics, operating temperature, maintenance requirements, and lifecycle costs rather than relying solely on pump classification.

Conclusion

Both OH3 and OH4 pumps are horizontal overhung centrifugal pumps defined by API 610, but they are designed with different priorities in mind.

OH3 pumps emphasize reliability, durability, and suitability for demanding process applications, while OH4 pumps focus on compactness, simplicity, and cost efficiency.

By understanding the strengths and limitations of each design, users can select the pump type that best matches their operational needs and achieve optimal system performance.