The Issue of Old Grease
EP Editorial Staff | March 26, 2019
Aging grease gets no respect in plants, and little or no attention until it fails.
By Randall Noon, P.E.
Old grease can be a sneaky failure mechanism, especially in infrequently operated equipment. When grease fails to do its intended function, it can mimic other failure modes, such as misalignment, binding, or wear, while the grease itself may appear to be just fine. Old grease can cause switchgear to open too slowly or fail to open at all. In motor-operated valves and similar equipment, old grease can cause the valve to open or close too slowly, and may cause the motor to draw higher-than-expected current—neither of which is good.
What’s particularly insidious and frustrating about the failure behavior of old grease in a piece of equipment is that when the machine is operated a second time soon after the first, it may perform just fine. Thus, a motor-operated valve that opened or closed too slowly the first time may operate well the second time, the third time, and beyond.
Because of this apparent healing effect that often occurs the second time equipment is operated, and for a few other reasons, the Nuclear Regulatory Commission, Washington (nrc.gov) doesn’t allow required periodic safety-related tests to be performed a short time after the equipment has been exercised or otherwise operated. The practice of doing this, ostensibly to ensure a good test result, is called “pre-conditioning,” and it can result in fines and punishment by the NRC for the plant and the individuals involved.
Grease is not a single substance, although it may look and feel like one. Industrial lubricating grease, as opposed to the bear or goose grease used by our great-grandmothers, is an emulsion of several ingredients that are usually divided into three categories: a base lubricant or oil, a thickener, and various additives. Typically, grease performs its lubrication function in the following way.
Initially, the grease has a relatively high viscosity. When shear force is applied to the grease, as occurs when a valve is actuated or switchgear begins to open, the viscosity quickly drops in response to the motion between the greased parts. As the shear rate increases, the viscosity of the grease decreases. The resulting viscosity approaches that of the base oil in the grease.
The quick drop in viscosity due to the application of shear is called shear thinning or thixotropy. This is a primary characteristic that differentiates grease from other semi-solid materials such as mentholated petroleum jelly or old-fashioned hair products from the 1950s, despite the fact that these types of materials may sometimes be described in common parlance as greasy. The graph (above) shows how grease typically functions.
As depicted in the graph, some greases will briefly resist a reduction in viscosity if the shear rate, or motion between the contacting parts, is less than a certain shear-rate threshold. This brief level spot is called the Newtonian Plateau. Newtonian fluids, as opposed to thixotropic fluids, do not change viscosity as a function of shear rate. By definition, they have constant viscosity. A Newtonian fluid would be represented graphically by a straight, horizontal line.
EFFECTS OF AGING
When grease has become too old, especially in equipment that is infrequently operated, the initial viscosity, i.e., the initial resistance of the grease to relative motion between the greased parts, is significantly higher than before, and the Newtonian Plateau may extend further to the right. In other words, the parts will not initially slide past each other as easily as before. More initial force is needed to overcome the higher initial viscosity and the extended Newtonian Plateau that is now present.
Consider this example: A small, solenoid-operated valve in a plant is supposed to open when a solenoid is energized. The solenoid pushes a rectangular plug through a guide that uncovers the “open” port. The valve may then stay open for a prolonged period of time. (The solenoid is outside the valve body and does not affect the grease that’s internal to the valve.) When the valve requires closure, the solenoid is de-energized and the valve plug is re-positioned by a return spring, thus covering the port. To be re-positioned, the plug is pushed through guides lubricated by grease. The plug and guides are positioned horizontally, so gravity is not a factor.
When new, the valve operates as designed. After a time, however, it ceases to close. When operated a second time after an initial failure, however, the valve again appears to be working appropriately. Examination of the valve internals finds the lubricating grease has hardened to the point that the return spring can’t overcome the greater resistance, causing the unit to “hang up” in the open position.
The same model valve is also used in various other places in the plant and has been operating well in all of those locations. After a bit more investigation, however, it is noted that the failed valve was installed in a hotter environment than the others.
Tests of the grease in a laboratory oven, heated to the same temperature as the environment in which the failed unit operated, finds that the grease had become stiff and provided more resistance than the return spring could provide. Over time, some of the other valves in the plant also begin to fail in a similar fashion. The cooler environmental temperatures of those units had simply slowed the hardening process in the grease.
The change that occurred to the grease in the preceding example is sometimes called grease dry-out, hardening, or caking. One or several factors acting together may be causing this. One factor is oxidation, in which the outer surface of the grease reacts with air over time to form an outer, thicker “skin” that does not have the same properties as the original grease.
The effect is often noticed in a can of grease where the lid has been left open for a while and the contents haven’t been disturbed. A layer of skin forms at the top of the grease in contact with air. This skin is generally thicker in consistency than the rest of the grease, and may have a slightly different, perhaps darker, color. It also may not be as shiny-wet in appearance as the grease was when new.
Often, re-mixing the grease with a stick to bring the “good” grease to the surface and break up the skin appears to take care of the problem, i.e., eliminate the skin. However, the oxidized material has simply been mixed into the rest of the grease—it hasn’t really disappeared. Since the oxidized grease, i.e., the skin, is usually a relatively small volume, compared to the rest of the grease in the can, the original properties of the product may appear to have been restored, but in actuality, they haven’t. In this case, the can of grease should be discarded.
Oil separation can also cause grease to harden. Sometimes the oil base will separate from the emulsified mixture and “bleed out” of the grease. It is then a different mixture with different properties; there is more thickener and less oil, and perhaps a lesser amount of additives. Often the effect is visually detectable by observing oil spots below where the grease has been applied. Sometimes, small pools of oil or beads of oil on the grease may be observed.
In a can of grease that has been undisturbed for a long time and stored in a warm or hot environment, a pool of oil may be noticed on top of the grease. As before, the separated grease might seem to be restored by re-mixing with a stick. But, mixing with a stick isn’t comparable to the emulsification process at the manufacturing plant. Given a bit of time, the grease will likely separate again. It should be discarded.
A third effect is out-gassing of volatiles. Even at modestly elevated temperatures, the more volatile, i.e, lighter-molecular-weight, hydrocarbons in the base oil may slowly evaporate out of the mixture, leaving behind less volatile, heavier-molecular-weight hydrocarbons that typically are more viscous.
A similar effect occurs in tar and gravel roofs. When the tar is first put down, it is pliant and plastic. After it has been exposed to the ultraviolet rays of the sun, which drives out the lighter volatiles, the tar becomes hard and brittle over time.
All of these effects are functions of time, temperature, and environmental conditions. It is worth noting that these three factors are not linear. Like a can of grease that is left undisturbed, infrequently used equipment, wherein re-mixing of the grease isn’t done by the mechanism action, provides more opportunity for grease to form a skin or lose oil.
For these reasons, the same grease in a valve or breaker that is frequently operated will seem to last longer without hardening or sticking than grease that is lubricating an infrequently operated unit, possibly one that is only used for a safety or standby function.
This last point also leads to the issue of service life versus shelf life. Most industrial greases will list a shelf life on the side of the can or tube. Along with the recommended shelf life will be the conditions under which the shelf-life warranty will apply, such as ambient temperature during storage. Shelf life is not the same as service life. Recall that grease in a can or tube is not exposed to much air, is certainly not supposed to be exposed to contaminants, and should be stored in a controlled-temperature environment that reduces oxidation and oil separation.
To be clear, shelf life is the maximum amount of time, if the storage conditions on the label are complied with, that the grease can be stored before its properties begin to degrade. Service life, on the other hand, is the minimum amount of time that the grease can be used at a particular location and be expected to perform as specified. Service life often may not be specified on a general-purpose grease label due to the many different applications and reliability levels for which the grease might be used.
Service life is also not to be confused with mean time between failure (MTBF), maintenance-free operating time (MFOT), or the predicted reliable life of the grease. This is an important distinction.
For example, a grease with a shelf life of 20 years may have a service life of five years, an active MTBF life of three years, and a dormant MTBF of two years. For 99.999% reliability, it may then have a predicted reliability life of just six months. It all depends upon the temperature, environment, time, and, especially, the required reliability. Consequently, equating shelf life to service life and entering the value for shelf life into your predictive-maintenance CMMS program may portend some unexpected equipment failures in the future.
Randall Noon is a registered professional engineer and author of several books and articles about failure analysis. He has conducted root-cause investigations for four decades, in nuclear and non-nuclear power facilities. Contact him at firstname.lastname@example.org.