Preventing Turbomachinery "Cholesterol": The Story of Varnish

By Greg Livingstone and Sanya Mathura

Preventing Turbomachinery “Cholesterol” was written to address the misconceptions related to varnish, help readers understand its composition and evolution, dispel some of the myths surrounding varnish and deposits, detect its presence, discover the impact of varnish as it relates to flow restriction, and shed some light on the effect on efficiency, heat transfer, and lubricant properties.
It provides tangible and actionable steps to alleviate varnish obstacles, particularly on bearing operations. There is also one chapter solely devoted to achieving sustainable lubrication, as this is the future of the industry.
 
Unique Features

• Discusses the processes of oxidation, thermolysis, contamination, microdieseling, electrostatic discharge, hot spots, additive drop out, hydrolysis, and other varnish mechanisms.
• Focuses on proactive methods that can be used to minimize oil breakdown, including managing temperature, the benefits of water removal, and ways to manage antioxidant depletion.
• Covers technologies that can help prevent deposits, such as the implementation of depth media filtration, centrifugal separation, electrostatic precipitation and agglomeration, solubility enhancers and chemical flushes.

Includes two sample tests for the ICML’s VPR and VIM certifications, along with answers and explanations.

• Language: English
• Publisher: Industrial Press, Inc.
• Release date: May 30, 2023
• ISBN: 9780831196325

Greg Livingstone

Greg Livingstone is the Chief Innovation Officer for Fluitec. He has three decades of industrial lubrication experience focused on how lubricants degrade and how to mitigate the risks associated with oil failure. He has helped to develop oil analysis tests, filtration technologies, solubility enhancers, and other technologies used to increase the life and performance of industrial lubricants. And he has volunteered in multiple industry organizations, such as STLE, ICML, and ASTM. Along his journey, he has published dozens of papers on these subjects and helped hundreds of clients in over 50 countries.

Book preview

1

THE BASICS OF VARNISH

VARNISH IS A TERM that has been used loosely throughout the industry to describe any form of deposit that adheres to the inside of equipment. As per ASTM, the official definition of varnish is a thin, hard, lustrous insoluble deposit that consists of a wide range of colors and is resistant to removal with saturated solvents. Quite often, the terms varnish and sludge are used interchangeably to describe these deposits. However, for the purposes in this book, we will define varnish as oil-derived deposits that adhere to the insides of the equipment and cannot be easily removed, while sludge will be identified as wipeable deposits. Color should not be used to define a difference between these two terms, as it is indicative of the chemistries and operating conditions used to create the deposits and can widely vary.

Typically, varnish is formed when the lubricant undergoes some form of degradation or breakdown. This occurs when there is a physical or chemical change of the fluid to adversely affect its performance. Although oxidation is one of the most prevalent forms of degradation, other forms, such as thermolysis, hydrolysis, additive depletion, and contamination, can also contribute to the formation of varnish. These methods will be described in further detail in subsequent chapters.

In turbomachinery, it is often helpful to distinguish varnish deposits by the conditions in which they form. Varnish, being composed of degraded oil components, can transition in and out of solution. The conditions upon which the degraded oil molecules, or solute, can stay in solution in the oil, or solvent, are governed by the oil’s solubility. The biggest factor affecting the solubility of oil degradation products is the temperature of the oil; degradation products will stay in solution in warm oil more readily than when the oil cools.

The most common type of varnish, and the one most practitioners think of, is this type of varnish. Some refer to it as degradation product precipitate, or cold varnish. Cold varnish refers to an accumulation of degradation products that reach the saturation point in the oil, precipitate out of the fluid, and find places to accumulate in the internals of the equipment. The saturation point is reached either because there is a high concentration of degradation products that are no longer stable in solution or because the temperature of the oil has dropped, which is why it is referred to as cold varnish. Cold varnish often forms bathtub rings in reservoirs, as the warm oil splashes on the wall of the reservoir and deposits form as the oil cools. Servo valves, especially in cooler spots of the system, are another common place for cold varnish to form. This is a particular problem in inlet guide vane (IGV) valves of peaking or cycling gas turbines.

Cold varnish is generally associated with bulk oil degradation. This makes it detectable through oil analysis methods, such as the membrane patch colorimetry (MPC) test.

Another type of varnish occurs due to localized conditions creating deposits directly at the heat source. This is also referred to as hot varnish. It is common in turbomachinery applications where there is a combination of high speeds and high loads, resulting in shear-stress and extremely high localized temperatures. Unlike cold varnish, hot varnish is not necessarily associated with bulk oil oxidation. It is common for localized deposits to form without any evidence in oil analysis tests. As such, the key detection methods for hot varnish are system operating parameters such as vibration and temperature, which measure the mechanical conditions where the deposits are forming. Hot varnish is also an important distinction when considering a preventive solution. There are multiple methods of managing oil degradation products responsible for cold varnish, such as some types of kidney loop filters. These are ineffective, however, at resolving hot varnish problems. Typically, a chemical solution is required, such as using a solubility enhancer or moving to a more thermally stable formulation, often employing synthesized hydrocarbon