1. What Is a Gear Pump?

A gear pump is a positive displacement pump that uses the meshing of gears to move a fluid.

In oil and fuel transfer applications, gear pumps deliver a fixed volume of liquid with each

revolution, providing a predictable, pulse-free flow even at varying pressures.

A typical oil or fuel gear pump consists of:

• Two or more gears (driving and driven gears)
• A close-fitting pump housing or casing
• Suction and discharge ports
• Bearings, shafts, and seals

Because the internal clearances in a gear pump are small, these pumps are especially suitable

for clean, lubricating liquids such as lube oil, hydraulic oil, diesel, and light fuel oils.

2. Working Principle of Gear Pumps

Gear pumps operate on a simple mechanical principle. The rotation of the gears traps the liquid

in the spaces between the teeth and the casing and transports it from the suction side to the

discharge side.

2.1 Step-by-Step Operation

1. The driving gear is rotated by an electric motor, engine, or other prime mover.
2. As the gears unmesh on the suction side, a low-pressure region is created, drawing oil or fuel into the pump.
3. The liquid is trapped in the spaces between gear teeth and the pump housing.
4. The gears rotate, carrying the trapped liquid around the outside of the gears toward the discharge port.
5. On the discharge side, the gears mesh again, forcing the liquid out of the spaces and into the discharge line.
6. This continuous trapping and displacement of fluid results in a steady, non-pulsating flow.

2.2 Key Characteristics of Gear Pump Operation

• Flow is directly proportional to speed (rpm) and gear size.
• Discharge pressure is determined by the system resistance, not the pump itself.
• Internal leakage (slip) increases with higher differential pressure and lower viscosity.
• Efficiency is strongly influenced by fluid viscosity and operating speed.

3. Types of Gear Pumps for Oil and Fuel Transfer

Several types of gear pumps are used for transferring oil and fuel. Each type has distinct

performance characteristics that suit different applications and fluid properties.

3.1 External Gear Pumps

External gear pumps are the most common design for oil and fuel transfer. They use two identically

sized, externally toothed gears mounted side by side in a tight-fitting housing.

• One gear is the driving gear; the other is the driven gear.
• Both gears rotate in opposite directions.
• Fluid is carried around the outside of the gears and discharged where the gears mesh.

External gear pumps are widely used for:

• Diesel fuel transfer
• Lubricating oil circulation
• Light fuel oil and heating oil transfer
• Hydraulic oil pumping in low to medium pressure systems

3.2 Internal Gear Pumps

Internal gear pumps use an inner gear (rotor) that drives an outer gear (idler) with internal teeth.

A crescent-shaped partition separates the suction and discharge sides.

Key features of internal gear pumps:

• Capable of handling a wide viscosity range, from thin fuels to heavy oils.
• Smoother, quieter operation compared to external gear pumps.
• Good suction capability for tank stripping and low NPSH conditions.

They are commonly used in:

• Heavy fuel oil and bunker fuel transfer
• Bitumen and viscous lube oils (when jacketed heating is provided)
• General refinery and terminal fuel handling systems

3.3 Gerotor (Eccentric Internal Gear) Pumps

Gerotor pumps are a special type of internal gear pump with an inner and outer rotor having different

numbers of teeth. They are compact and often used in automotive and mobile equipment.

Typical uses include:

• Engine lubricating oil pumps
• Automatic transmission fluid pumps
• Small diesel and gasoline fuel supply pumps

3.4 Comparison Table: External vs Internal Gear Pumps

4. Benefits of Gear Pumps in Oil and Fuel Transfer

Gear pumps offer several advantages that make them the preferred choice for many oil and fuel

transfer duties.

4.1 Positive Displacement and Accurate Flow

Because gear pumps are positive displacement machines, they deliver a fixed volume of liquid per

revolution. This provides:

• Accurate, repeatable flow rates for metering and dosing fuels and oils.
• Stable flow independent of discharge pressure changes (within pump limits).
• Excellent control characteristics when used with variable speed drives.

4.2 Ability to Handle a Wide Range of Viscosities

Gear pumps can transfer liquids from very light fuels to highly viscous lubricants:

• Low-viscosity fluids like gasoline, kerosene, and diesel fuel.
• Medium-viscosity oils like hydraulic oil, gear oil, and engine oil.
• High-viscosity lube oils, polymer oils, and some heavy fuels (with heating).

4.3 Compact, Simple, and Reliable Design

The construction of gear pumps is straightforward with few moving parts:

• Compact footprint suitable for skid-mounted fuel transfer systems.
• High mechanical reliability and long service life with proper lubrication.
• Low maintenance requirements compared to more complex pump types.

4.4 Self-Priming Capability

Most gear pumps are self-priming, meaning they can evacuate air from the suction line and create

enough vacuum to draw liquid into the pump:

• Useful for transfer from underground storage tanks.
• Essential for mobile refueling units and truck-mounted fuel systems.

4.5 Smooth, Low-Pulsation Flow

Gear pumps provide a nearly steady flow with minimal pulsation, which is important for:

• Fuel burner supply on boilers and heaters.
• Engine fuel injection pre-supply systems.
• Precision lube oil and hydraulic systems.

5. Limitations and Key Design Considerations

While gear pumps are versatile for oil and fuel transfer, there are important limitations and

design considerations to respect.

5.1 Sensitivity to Solids and Contamination

Gear pumps rely on tight clearances between gears and housing. Solid particles can cause:

• Wear of gear teeth and housing.
• Seizure or jamming of rotating parts.
• Loss of volumetric efficiency and leakage.

For this reason, proper filtration on oil and fuel transfer lines is essential.

5.2 Pressure Limits

Gear pumps generate flow, not pressure. Excessive downstream restriction will cause:

• Rapid pressure buildup.
• Potential damage to the pump or system components.

Relief valves or internal bypass arrangements are mandatory in gear pump systems to protect

against overpressure.

5.3 Temperature and Thermal Expansion

Oils and fuels may be pumped at elevated or low temperatures. Temperature affects:

• Viscosity, which in turn affects slip and efficiency.
• Thermal expansion of metal components, influencing clearances.
• Seal material compatibility and service life.

Gear pump selection should consider minimum and maximum fluid temperatures and appropriate

materials of construction.

5.4 Cavitation Risk

Inadequate Net Positive Suction Head Available (NPSHa) can result in cavitation, causing:

• Noise and vibration.
• Damage to gear teeth and housing surfaces.
• Loss of capacity and efficiency.

Proper suction line design and fluid level control are required to prevent cavitation.

6. Common Gear Pump Applications in Oil and Fuel Transfer

Gear pumps are applied across many sectors of the oil, gas, power generation, marine, and

transportation industries.

6.1 Fuel Transfer and Fuel Handling Systems

• Diesel fuel transfer:

  Moving diesel from storage tanks to day tanks, generators, and industrial burners.

• Light fuel oil transfer:

  Feeding boilers, furnaces, and process heaters with light fuel oil.

• Gasoline and kerosene transfer:

  Used in properly designed systems with explosion-proof motors and compliant seals.

6.2 Lube Oil Circulation and Transfer

• Lubricating oil transfer between storage and equipment reservoirs.
• Lube oil circulation for compressors, turbines, and large rotating machinery.
• Oil filtration, conditioning, and flushing systems.

6.3 Hydraulic Oil and Power Transmission

• Low to medium pressure hydraulic systems in industrial and mobile machinery.
• Hydrostatic transmissions in vehicles and construction equipment.
• Auxiliary hydraulic power units for marine and offshore equipment.

6.4 Marine and Offshore Applications

• Fuel transfer on ships and offshore platforms.
• Heavy fuel oil and diesel supply to main and auxiliary engines.
• Bilge and sludge oil transfer (when fluids are properly conditioned).

6.5 Power Generation and Industrial Boilers

• Fuel forwarding pumps feeding boiler burner systems.
• Recirculation of fuel oil in heated storage tanks.
• Start-up and emergency diesel generator fuel supply.

7. Gear Pump Selection Criteria for Oil and Fuel Transfer

Proper selection of a gear pump is critical for reliable oil and fuel transfer. The following

criteria should be evaluated carefully.

7.1 Flow Rate and Differential Pressure

• Required flow rate:

  Usually specified in L/min, m³/h, or gpm. Consider both nominal and peak demands.

• Differential pressure:

  Difference between discharge pressure and suction pressure, including line losses, filters, valves, and static head.

7.2 Fluid Properties

• Viscosity:

  Measured in cSt or SSU. Gear pumps are sensitive to viscosity changes.

  Specify viscosity at minimum, normal, and maximum operating temperatures.

• Density/specific gravity:

  Influences power requirements and NPSH calculations.

• Lubricity:

  Non-lubricating fluids require special materials and may reduce pump life.

• Chemical compatibility:

  Ensure wetted materials (metals, seals, gaskets) are compatible with the oil or fuel type.

7.3 Operating Conditions

• Minimum and maximum operating temperatures of fluid and ambient environment.
• Start-stop frequency and duty cycle (continuous or intermittent operation).
• Installation location (indoor, outdoor, marine, hazardous area).

7.4 Drive and Speed Considerations

• Motor or engine power rating and speed.
• Direct drive vs. geared or belt drive to achieve optimal pump rpm.
• Use of variable frequency drives (VFD) for flow control.

7.5 Example: Basic Gear Pump Sizing Approach

For preliminary selection, the required pump displacement can be estimated from:

Q = D × n × ηv

Where:

• Q = flow rate (e.g., L/min)
• D = displacement per revolution (e.g., L/rev)
• n = rotational speed (rev/min)
• ηv = volumetric efficiency (typically 0.8–0.95 depending on viscosity and pressure)

Rearranging:

D = Q / (n × ηv)

This gives an approximate displacement to compare with available gear pump sizes for the specified

oil or fuel application.

8. Typical Gear Pump Specifications for Oil and Fuel Service

Gear pump specifications vary with model and manufacturer, but typical ranges for oil and fuel

transfer applications are summarized below.

8.1 General Performance Ranges

8.2 Sample Specification Table: Small Diesel Transfer Gear Pump

8.3 Sample Specification Table: Lube Oil Circulation Gear Pump

9. Installation and Piping Guidelines for Oil and Fuel Gear Pumps

Correct installation is essential for the long-term reliability of gear pumps used in oil and

fuel transfer.

9.1 Pump Location and Mounting

• Install as close as practical to the oil or fuel source to minimize suction line length.
• Ensure the pump is accessible for maintenance and inspection.
• Mount on a rigid baseplate with proper alignment to the driver.
• Use flexible couplings to accommodate slight misalignments and thermal expansion.

9.2 Suction Piping Design

• Use adequately sized suction piping to limit pressure drop and maintain NPSHa.
• Keep suction lines short, straight, and gently sloped toward the pump.
• Avoid high points that can trap air and cause priming problems.
• Install a suction strainer with appropriate mesh size to protect the pump from solids.
• Ensure all suction connections are airtight to prevent air entrainment.

9.3 Discharge Piping and Relief Valve Arrangement

• Size discharge piping for the design flow and acceptable velocity.
• Install an external relief valve set below the pump’s maximum allowable working pressure, if the pump does not have an internal bypass.
• Route relief valve discharge back to the suction tank or to the suction line (with caution and proper design).
• Provide isolation valves on suction and discharge sides for maintenance, but never close both with the pump running.

9.4 Alignment and Foundation

• Align pump and driver shafts according to acceptable tolerances.
• Use shims for accurate leveling of the baseplate.
• Grout the baseplate after final alignment to minimize vibration.
• Recheck alignment after the foundation grout has cured and the system reaches operating temperature.

10. Operating and Maintenance Best Practices

Following good operating and maintenance practices extends pump life and maintains performance

in oil and fuel transfer systems.

10.1 Start-Up Procedure

• Verify correct rotation direction before connecting the pump to the system (brief jog with no liquid or coupling removed).
• Check that the system is filled with fluid and suction piping is fully primed.
• Open all relevant suction and discharge valves.
• Start the pump and monitor suction and discharge pressures, flow, and noise.
• Check for leaks at seals and connections.

10.2 Routine Inspection and Maintenance

• Inspect seals, gaskets, and connections regularly for signs of leakage.
• Monitor bearing condition and lubrication where applicable.
• Check relief valve function and verify settings.
• Change or clean suction strainers and filters at recommended intervals.
• Periodically verify flow rate and pressure to detect wear or slip increases.

10.3 Managing Wear and Internal Clearances

Over time, wear increases internal clearances and slip. Indications include:

• Reduced flow at a given speed.
• Higher discharge pressures required to achieve the same flow.
• Increased noise or vibration.

When these symptoms appear, inspection and potential overhaul of gears, bushings, and housing

may be necessary.

10.4 Handling Viscosity and Temperature Changes

Oils and fuels can vary in viscosity with temperature. Operators should:

• Avoid running the pump at high speed with cold, highly viscous oil.
• Allow adequate warm-up time for heated fuel oil systems before full load operation.
• Adjust pump speed or system backpressure to stay within allowable operating limits.

11. Troubleshooting Gear Pumps in Oil and Fuel Service

The table below summarizes common problems with gear pumps in oil and fuel transfer and suggests

possible causes and corrective actions.

12. Safety and Environmental Considerations

Oil and fuel transfer systems must be designed and operated with appropriate safety and

environmental protections.

12.1 Flammability and Explosion Risk

• Gasoline, kerosene, and some light fuels are highly flammable and may require explosion-proof motors and electrical components.
• Bonding and grounding of equipment help prevent static electricity build-up and ignition risks.
• Ventilation and leak detection are important in enclosed spaces.

12.2 Environmental Protection

• Secondary containment around pumps and tanks to contain leaks or spills.
• Drip trays under pump assemblies to collect small leaks or seal failures.
• Proper disposal or recycling of used oils, filters, and contaminated cleanup materials.

12.3 Regulatory Compliance

• Compliance with local, national, and international regulations for fuel storage and transfer.
• Adherence to relevant industry standards for pump design, installation, and operation.
• Regular inspection and documentation as required by authorities or internal safety policies.

13. Summary and Key Takeaways

Gear pumps are a cornerstone technology in oil and fuel transfer systems, from small lubrication

circuits to large-scale fuel handling operations. Their positive displacement nature, ability to

handle a wide viscosity range, and simple, robust design make them a highly effective solution

for transferring diesel, lube oil, light fuel oil, hydraulic oil, and many other petroleum-based fluids.

When selecting and applying gear pumps in oil and fuel transfer:

• Define the required flow, pressure, viscosity, and temperature ranges accurately.
• Choose the appropriate type of gear pump (external, internal, or gerotor) for the fluid and duty.
• Design suction and discharge piping carefully to avoid cavitation and overpressure.
• Use proper filtration and ensure materials are compatible with the oil or fuel.
• Implement sound installation, operation, and maintenance practices to maximize reliability and service life.

With correct engineering and operation, gear pumps provide reliable, efficient, and safe performance

in a wide variety of oil and fuel transfer applications across industrial, marine, and mobile environments.