Pumping transformer oil is a demanding application. Understanding how to maximize pump life in this challenging operating environment can help you minimize breakdowns and unplanned maintenance.

Bearing system and impeller wear can lead to the release of metal particles into the oil circulating through the pump.

Once considered to be merely a replaceable routine maintenance item, comparable to say, a valve, transformer oil pumps are now almost universally recognized as a critical component of “forced oil-cooled” transformers -- a component that requires sophisticated engineering, high-quality construction and systematic preventive maintenance.

When a transformer oil pump performs properly, it ensures maximum cooling to maintain a transformer’s peak load capacity. However, impairments to a pump can result in breakdowns and potentially catastrophic damage to the transformer. Unfortunately, such impairments are notoriously difficult to detect and prevent in pumps that are designed or constructed inadequately.

Transformer oil pump manufacturers have addressed these problems with improvements such as ultrasonic sensors that monitor the bearings’ condition. Major North American utility companies also have influenced the development of transformer oil pumps by requiring thermal, mechanical, sealing, electrical and fluid systems that provide dependable operation.

Pumping transformer oil is a demanding application. Year after year, the pump must continuously pump high-temperature oil and remain hermetically sealed, even in harsh outdoor environments. One of the most challenging aspects of transformer oil pump design is the fact that the transformer oil also functions as the pump’s lubricant. The problem is that transformer oil is selected, not for its lubricating performance, but rather for its ability to function as an insulator to suppress corona and arcing within the transformer, as well as for its ability to maintain stability and good dielectric properties at high temperatures. Highly refined mineral oil works well inside the transformer, but it is a poor lubricant for the ball bearing systems found in many types of transformer oil pumps.

Bearing system and impeller wear can lead to the release of metal particles into the oil circulating through the pump, cooler and, ultimately, the transformer. As a result, the dielectric properties of the oil and insulation can degrade, potentially causing hazardous arcing. Degradation of the bearing system and impellers, as well as impairments of motor windings, also can cause a reduction in pump flow and discharge pressure, causing reduced cooling capacity.

Leaking electrical connectors and gasketed surfaces can impair pump performance and allow the ingress of moisture into the oil as well as oil leaks into the environment. Newer pump designs mitigate these risks in a number of ways, including improvements to bearing design and ultrasonic monitoring of bearing condition. Properly designed new or remanufactured pumps can take advantage of many of these advancements in transformer oil pump technology.

Bearing Design

One design improvement for transformer oil pumps is the replacement of ball bearing systems with bronze-sleeve bearings. As previously mentioned, transformer oil provides a poor lubricant for ball bearings. Ball bearings work well when lubricated by heavier oil or grease, but they fail prematurely when lubricated by lightweight, low-viscosity transformer oil.

Additionally, ball bearing pumps that are not operated continuously commonly can fail as a result of false brinelling of the bearings, caused by transformer vibration or slight flow caused by convection. False brinelling occurs when vibration pushes the lubricant away from a region that it is intended to protect. In a situation when a mostly stationary bearing is subjected only to oscillating or vibrating load, the lubricant may be pushed out of the loaded area. However, because the bearing is rolling only small distances, there is no action or movement that replaces the displaced lubricant. The resulting wear debris oxidizes to form an abrasive compound that accelerates wear.

All U.S. manufacturers, and some foreign suppliers, have discontinued using ball bearings in transformer pump designs. Some firms retrofit ball bearing pumps with pump-specific bronze-sleeve type radial/thrust bearings and hardened steel thrust collars. The key to the design of thrust and radial bearings for transformer oil applications is large thrust face sleeve bearings for long life and minimum wear. The bearings need to have proper surface finish and precisely positioned grooves to pass the oil and to maintain an adequate lubricant film under all conditions.

Monitoring Bearing Wear

Reliable long-term performance of transformer oil pumps depends not only on the bearing and hydraulic design systems but also on the ability to proactively detect wear, to ensure effective and energy-efficient cooling performance, and to protect the pump and transformer from damage and breakdowns.

An ultrasonic bearing wear system can provide advance warning. By tracking data over time, the monitoring system provides rate-of-wear information that enables users to make informed decisions about selective, preventive maintenance, which helps to protect equipment, avoid breakdowns and optimize maintenance efforts and expense.

The operating principle of ultrasonic monitoring systems is not complicated. On new or remanufactured pumps, ultrasonic sensors are mounted in both thrust and radial bearings at strategic points. A permanently mounted piezoelectric transducer emits a high frequency sound wave. The monitoring system precisely measures the echo time to determine the distance between the sensor and the bearing surface. Some systems provide an accuracy of 0.0002". Measurements are compared to baseline readings to determine if any bearing wear has occurred.

The temperature-compensated readings can be taken while the pump is under any operating condition, without disassembling the pump, and while the pump is operating or not. The sensors do not affect the pump performance. In addition to the ultrasonic system for monitoring bearing wear, it is also useful to have a shaft-rotation sight plug to facilitate checking for proper shaft/impeller rotation.


Beyond the design of the bearing system, the overall quality of construction, both in terms of the quality of materials and the quality of manufacturing, is of importance. Pumps should be constructed of cast-iron material for the pump castings (casings, motor enclosures and impellers) to provide long life in the field. To protect the exterior surfaces from corrosion, high-performance coatings (primer and top coats) should be applied.

All sleeve bearing pumps should have the bearing journals and thrust surfaces ground between centers to ensure alignment and surface finish. All pump shaft, impeller and motor assemblies should be dynamically balanced to ensure long-term vibration-free operation. Durable electric supply power cords also help ensure reliable transformer pump performance. They should be capable of withstanding ultraviolet rays, oil, water and extreme weather conditions.

Investment in high quality new and remanufactured transformer oil pumps has a high economic return. A good pump typically will cost much less than 1 percent of the cost of the transformer that it supports, yet it provides long-term insurance against breakdown, damage or failure of the transformer. And as all owners of large critical transformers will attest, a failure or major outage of this equipment can cause severe upheaval to the well-being of their electrical power distribution system.

High-quality pumps also pay for themselves in reduced maintenance and replacement costs. Properly designed sleeve bearing pumps reliably perform 15 or more years -- more than three to four times the typical useful life of ball bearing pumps.