Transformer oil pumps have evolved dramatically over the past several decades. Once considered to be merely a replaceable routine maintenance item, pumps are now almost universally recognized as a critical component of“forced oil cooled” transformers — components that require sophisticated engineering, high quality construction and systematic preventative maintenance.
When a transformer oil pump performs properly, it ensures maximum cooling to maintain the transformer’s peak load capacity. However, impairments to a pump can result in costly breakdowns and potentially catastrophic damage to the transformer. Such impairments are notoriously difficult to detect and prevent in pumps that are designed and/or constructed inadequately.
Transformer oil pump manufacturers in the U.S. have provided worldwide leadership in addressing these problems by introducing design improvements and innovations such as ultrasonic sensors that monitor the condition of bearings. Major utility companies have also driven the development of high performance transformer oil pumps by requiring thermal, mechanical, sealing, electrical and fluid systems that provide dependable operation. “We established a database of bearing system designs versus problems beginning in 1979,” says Jack Harley, of First Power Group LLC. “It was clear that ball bearings were not a good solution for long life in this application.
Pumping transformer oil is a demanding application. The pump must operate continuously, year after year, pumping high-temperature oil and remain hermetically sealed, in harsh outdoor environments.
The transformer oil also functions as the pump’s lubricant in transformer oil pump designs. The problem is 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 and for its ability to maintain stability and good dielectric properties at high temperature. Highly refined mineral oil is a poor lubricant for the ball bearing systems in many types of transformer oil pumps. “Based on decades of experience, Reliant Energy specifies the Cardinal/Harley bronze sleeve bearing system exclusively for both new and remanufactured transformer oil pumps,” says C.R. Bell, of Reliant Energy P.E., who is also a licensed master electrician.
Wear of the bearing system and impeller can lead to the release of metal particles into the oil circulating through the pump, cooler and the transformer. As a result, the dielectric properties of the oil and insulation can degrade, potentially causing hazardous arcing. This can also cause a reduction in pump flow and discharge pressure which causes 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.
State-of-the-art pumps mitigate these risks by improving the bearing design, ultrasonic monitoring of bearing condition and high quality construction.
The single most important design improvement is the replacement of ball bearing systems with bronze sleeve bearings.
Transformer oil provides a poor lubricant for ball bearings. In fact, ball bearings are a viable solution only when lubricated by heavier oil or grease. They fail prematurely when lubricated by lightweight, low-viscosity transformer oil.
Ball bearing pumps that are not operated continuously will commonly fail as a result of false brinelling of the bearings caused by transformer vibration or slight flow caused by convection. 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, since the bearing is rolling only small distances, there is no movement that replaces the displaced lubricant. The resulting wear debris oxidizes to form an abrasive compound, further accelerating wear. All U.S. manufacturers have discontinued using ball bearings in transformer pump designs. North America’s largest manufacturer and remanufacturer of transformer oil pumps, Cardinal Pumps & Exchangers in Salem, OH, a division of Unifin International, retrofits all ball bearing pumps with pump-specific bronze sleeve type radial/thrust bearings and hardened steel thrust collars.
The bearings need to have proper surface finish and precisely positioned grooves to pass the oil and 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. A patented ultrasonic bearing wear monitoring system was developed in 1984 by J.W. Harley/TecSonics Inc. (a recent acquisition of Cardinal Pumps & Exchangers Inc.) to overcome the shortcomings of conventional transformer pump bearing wear detection methods based on sound, vibration and oil contamination.
The ultrasonic bearing wear system, TecSonics™, provides advance warning. By tracking data over time, the monitoring system provides rate-of-wear information that enable informed decisions about selective preventive maintenance, to protect equipment, avoid breakdowns and optimize maintenance effort and expense.
Six precision ultrasonic sensors are mounted in both thrust and radial bearings at strategic points. A permanently mounted piezoelectric transducer emits a high-frequency sound wave and precisely measures the echo time to determine the distance between the sensor and the bearing surface, to an accuracy of 0.0002 inch. 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, whether the pump is operating or not. And the shaft rotation sight plug facilitates checking for proper shaft/impeller rotation.
Pumps should be constructed of rugged 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/high quality 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 assure long-term, vibration-free operation.
Durable electric supply power cords withstand ultraviolet rays, oil, water and extreme weather conditions, ensuring reliable transformer pump performance.
The Economics of Transformer Oil Pump Investment
Investment in high-quality new and remanufactured transformer oil pumps has a high economic return. A good pump will typically cost much less than 1 percent of the cost of the transformer it supports and yet it provides long-term insurance against breakdown, damage or failure of the transformer. A failure or major outage of this equipment can cause severe upheaval to the well-being of their electrical power distribution system.
Properly designed sleeve bearing pumps reliably perform for more than 15 years — more than three to four times the typical useful life of ball bearing pumps. In addition, pumps with ultrasonic monitoring systems are less costly to operate because preventative maintenance can be efficiently planned.
|Cardinal pumps offer rugged split casing design with heavy-duty Class 30 cast iron used for the pump casting, motor enclosures, impellers and volutes.