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Top 9 Common Failures of Gear Pumps
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Top 9 Common Failures of Gear Pumps

  • Katarina Knafelj Jakovac

    January 27, 2024

Gear pumps are used for pumping working media with high viscosity, such as water emulsions, synthetic fluids, various types of oils, and the like.

Gear pumps are often employed as primary lubrication pumps for piston compressors, reducers, multipliers, or older-model marine diesel engines.

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Image: Gear pump (Source)

They are also commonly used in hydraulic drive systems in the position of hydraulic pumps, functioning as positive displacement pumps, transferring the working medium by increasing pressure and flow through a reduction in the volume of chambers in the pump. They are used to achieve relatively small flows with relatively high delivery heads.

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(Source)

A high-pressure reciprocating compressor features a gear pump directly connected to the crankshaft, and its motion dynamics control its operation.

Another variant is a gear pump driven by an electric motor when the gear pump serves as an auxiliary pump in a forced lubrication system.

The forced lubrication system schematic below illustrates a gear pump at position 1) driven by the compressor's crankshaft.

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Image: Forced lubrication system of the compressor (Source)

Other components in the lubrication system include 2) pressure regulation valve, 3) filter, 4) oil cooler, 5) temperature indicator, 6) pressure indicator, 7) low-pressure alarm and cut-off switch, 8) auxiliary pump start switch, 9) optional main pump start switch, 10) oil supply and return pipes, 11) oil reservoir, or daily tank, 12) auxiliary pump, 13) non-return valve, 14) oil level indicator, 15) relief valve, and 16) pre-lubrication pump.

API 676 standard defines the design characteristics of gear pumps and the criteria they must meet, such as a 3 mm corrosion allowance on components where the maximum allowable working pressure occurs during operation.

In the standard version, pump housing components are made of cast iron.

The housing and welded joints are inspected after production using non-destructive methods, often employing magnetic particle testing.

The gear pump pressurizes the working medium using gear elements – driving and driven gears seen in the image.

The gears are made of high-strength hardened steel mounted on the shaft with sliding bushings.

The gears can have external or internal teeth depending on the design. The gap between the inner side of the housing and the top of the teeth is sufficient for the gears to rotate freely in the housing and prevent the working medium from flowing between the housing and the teeth.

The special geometry of the gears results in extremely low noise levels and low pressure pulsation during pump operation.

Sealing must comply with the API 682 standard, so in many gear pump variants, the output of the drive shaft is sealed with a sealing braid.

The direction of shaft rotation determines which pump inlet is suction and which will be pressure. The inlet with the flange where the gears exit the mesh is the suction inlet.

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Image: Gear pump components (Source)

Gear pump components include: 1. Sealing braid housing, 2. Sealing braid, 3. Front cover, 4. Bushings, 5. Driving and driven gears, 6. Wedges, 7. Drive shaft, 8. Spring seat, 9. Seal, 10. Flange, 11. Washer, 12. Tension nut, 13. Adjustment screw, 14. Spring, 15. Relief valve, 16. Housing, 17. Flange, 18. Bushings, 19. Rear cover, 20. Housing gaskets, 21. Driven shaft.

The screws used to connect the housing and covers must be of high strength. Drilled holes with threads must have a minimum diameter equal to 1.5 times the diameter of the sleeve.

The legs for mounting the pump should be machined with a flatness tolerance of 25 µm. The upper surface is machined parallel to the base, and where the pump has common legs for the support and housing, the legs are machined flat and parallel in the same plane within 150 µm/m distance between mounting surfaces after pump assembly. Gear pumps are mostly reliable and rarely fail.

When a failure occurs, it is necessary to know the basic reasons for the failure and how to eliminate it.

1. The pump has no delivery at all. The causes of this failure are a lack of working medium, incorrect direction of rotation of the electric motor, or a faulty electric motor that cannot start the pump at all, excessive net positive suction head, and incorrectly connected suction and discharge pipes.

Troubleshooting: Check the openness of the suction valve, open it completely, check if the system is fully filled with the working medium, and ensure that the pump is fully submerged.

Check if the electric motor is connected and rotating in the correct direction (usually, an arrow on the housing indicates the direction of rotation).

Check if the delivery height matches the technical data in the machine's user manual and if the suction and discharge pipes are properly connected, especially considering that the pipes are often small in diameter.

2. The pump does not deliver enough working medium. This failure occurs when the pump is running, but certain causes such as air pockets in the housing, low rotation speed, incorrectly selected or partially closed pressure valve, mechanical damage to gears, or leakage through a damaged safety valve prevent 100% delivery.

Troubleshooting involves thorough venting of the pump, checking the rotation speed of the electric motor, checking the condition of the pressure valve, fully opening the pressure valve, or a combination of all the above.

If after these activities the delivery is still insufficient, it is necessary to stop the pump, disassemble and inspect the gears (perhaps the gearing is damaged), and check the backlash between the gears and the housing.

Sometimes it is necessary to service the safety valve.

3. The pump runs for a short time, then loses on the suction side. The possible cause is damage to the suction pipe, excessive delivery height, too many air bubbles in the working medium, or the working medium is overheated, leading to the appearance of a gaseous phase in the liquid phase.

These faults are eliminated by thoroughly inspecting the suction pipe for cracks and checking if all flange connections are in good condition, and their bolts are tightened to the appropriate torque.

Additionally, the temperature of the working medium should be checked, overheating should be prevented, and the pump should be thoroughly vented.

4. The pump requires too much power. Increased power demand of the drive unit also manifests as increased current intensity.

Potential causes may include excessive rotation speed of the electric motor, increased viscosity of the working medium, partial blockage of the suction pipe, mechanical damage such as a bent shaft of the drive or driven gear, and misalignment of the unit due to inadequately tightened pipes pulling the pump.

Fault troubleshooting involves an initial check of the rotation speed (the electric motor is disconnected and its rotation is checked unloaded) and checking the viscosity and chemical composition of the working medium through laboratory analysis.

The suction pipe is disconnected and flushed to check for flow, the discharge pipe is disconnected, and its tension on the pump is checked.

If these steps do not help, it is necessary to disassemble the pump, check the gear pair for damage to metal surfaces, gear engagement geometry, and meshing of teeth.

Finally, the unit is realigned if it is an auxiliary pump.

5. The pump produces excessive noise during operation. We usually do not pay attention to the noise level while walking through the facility surrounded by machines, each of which produces a certain level of noise during operation.

When you stand next to the pump and listen carefully, you will easily notice irregular or increased noise levels by the sound the pump produces, sometimes described as howling or buzzing.

Causes of such behavior are excessive rotation speed, a smaller diameter of the suction pipe than necessary, so the working medium of higher viscosity cannot completely fill the space between the two gears, unsupported suction and discharge pipes causing increased vibration during pump operation, and worn-out pump parts.

To eliminate these faults, it is necessary to examine the rotation speed, check if the suction pipe is of the appropriate diameter according to the technical documentation and user manual, and check the viscosity of the working medium.

It is essential to secure the suction and discharge pipes by adding rubber clamps to dampen vibrations.

If none of the above helps, the pump should be disassembled, and the parts checked for damage and replaced if necessary.

6. The pump leaks too much at the seal. The obvious cause is damage to the seal, braiding.

Leakage can also occur if the seal braiding is made of an unsuitable, too soft material, the shaft is bent or damaged, or if the housing of the seal braiding is not properly fastened.

The first step in eliminating these faults is to replace the seal braiding, replacing all rings of the sealing package, and tightening all housing screws.

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Image: Seal braiding marked in blue (Source)

If leakage persists, it is necessary to disassemble the pump and check the condition of the shaft and seal braiding, replacing them if damage is detected.

1. Pump parts wear excessively. Increased wear will occur when the working medium is contaminated with abrasive particles, the pump rotates at too high a speed, or the unit is misaligned.

Another cause may be excessive axial or radial load on the gear pair, if the pump operates dry without the presence of the working medium, or the medium lacks sufficient viscosity.

These listed faults are eliminated by regular laboratory testing of the working medium and the mandatory use of the pump exclusively with the working medium for which it is intended, checking the rotation speed as described in point 4, and aligning the unit.

Before starting, the pump must be completely filled with the working medium.

2. The electric motor overheats. When the drive unit operates at an elevated temperature, the cause is often an inappropriate voltage, the pump operates at an elevated temperature, dust coverage, improper phase connections, and a lack of natural ventilation.

Troubleshooting for electric motor faults is detailed in a separate article, and for starters, the drive unit should be regularly serviced and ensure it has adequate ventilation, and all pump operation faults should be resolved in advance.

3. The pump overheats. The cause of overheating may be the repeated circulation of a small amount of working medium within the pump housing, especially when the gear pair in engagement is damaged, the housing of the seal braiding is excessively tightened, pressing the shaft, the pump operates without the working medium (dry), and the unit is misaligned.

Faults are eliminated by re-aligning the unit, mandatory starting the pump only when fully submerged, checking the condition of the seal braiding, and appropriately securing the housing of the braiding.

A simple and transparent way to keep track of faults, repairs, and general services of gear pumps is to use asset management software.

The image shows a screenshot of such software where technical documentation, spare parts, all costs, work orders, and other resources needed for maintenance are recorded for the gear pump, along with all reports.

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Image: Tracking gear pump information in asset management software (Source: Metrikon application asset profile screenshot)

Asset management software allows for a structured maintenance process and monitoring the condition of the gear pump. Data is kept in one place in real-time and available to all employees involved in the maintenance process, reflecting the company's business processes. The equipment owner has insight into the costs of spare parts, work, and the reliability of the gear pump throughout its machine's life cycle. Data on faults and the costs of resolving them recorded through work orders are a key component of asset management software.

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Image: Display of a work order in Metrikon

The record of gear pump faults also provides data for detailed analysis and investigation of the causes of faults and future activities to prevent and completely eliminate faults.

Asset management software provides the equipment user with a complete analysis of maintenance processes and valuable output data for making future business decisions, such as investing in a new gear pump if it is shown that the existing one is no longer cost-effective for repairs.

Katarina Knafelj Jakovac
Katarina Knafelj Jakovac social media icon
January 27, 2024

Katarina Knafelj Jakovac holds Master degree in Mechanical engineering with long term work experience in Oil industry. She is Certified Reliability Leader specialized for mechanical equipment and operational excellence. Author of blog Strojarska Radionica (Mechanical Workshop) where she shares professional knowledge and personal experience in maintaining various rotating machines, machine systems and process equipment. Adores mechanics, thermal engineering and internal combustion engines. She is dedicated to the continuous improvement of machine maintenance and quality management of physical assets.