- Buyer's Guide
There are many occasions where an oil designed to lubricate a machine surface will not stay in place long enough to efficiently perform its key responsibility. For those occasions, manufacturers rely on the use of grease. A grease is formed by thickening the oil with one of several types of gelling or thickening agents that make the oil stiff. A grease must do two things effectively to be useful. First, the grease must stay in place. Secondly, following staying put, the grease must gradually release the reservoir of oil into the loaded components so that the oil and additives can do their respective jobs.
Thickness, or stiffness, is an important property of grease. It relates to one of the two main functions of the lubricant: its ability to remain in a given place.
Grease stiffness is measured in the laboratory by a test such as ASTM D217. This requires equipment and experience that are not generally at hand in a plant environment. The same is true for measuring an oil’s consistency or viscosity (see ASTM D445). Measuring viscosity in a plant is made simple by using a viscosity gauge (see the EPRI NMAC Lubrication Guide, Rev. 3, page 5-4). Measuring grease thickness in-plant also uses simple equipment, though a bit more involved than the viscosity gauge because greases are more complex in nature. This article discusses how to set up and run these simple grease tests.
Limitorque motor-operated valve actuators are used widely in nuclear power generation. Certain greases tend to harden in use and, if this proceeds far enough, can interfere with equipment function. For this reason, maintenance procedures specify the grease hardness that dictates lubricant change out. This change out is not time-dependent and should be made only when demonstrated that it is required. It is necessary to monitor the thickness of greases in service in the main gearbox in order to determine when the lubricant has to be changed.
The inspection method was developed primarily for that application, and to provide a simple yet adequately accurate method through which this can be accomplished onsite. The method can also be useful to perform quality checks on new greases and those that have been in storage for a prolonged period of time.
The National Lubricating Grease Institute (NLGI) defines grease consistency grades based on the ASTM D217 worked penetrations. For example, Grade 1 has the range of 310 to 340 mm/10. There is a 15-number “no man’s land” between each new grade and then another 30 points for the new grade, for example, Grade 2 is 265 to 295, etc. This is similar to the ISO grading system for oils; for example, the 32 grade runs from 28 to 36 centistokes at 40°C. The in-plant system for estimating grease consistency involves collecting a series of greases of known grades and then comparing these to the unknown material. A knife or spatula is used to work the greases around. It is easy to match the known with the unknown.
Figure 1. TXU Grease Test Kit
Figure 1 shows the kit assembled at TXU Energy. This kit consists of six reference samples: Grades 0, 1, 1½, 2, 3 and 4. The 1½ was included because it is Mobilgrease 28, the grease widely used in the limit-switch gearbox of Limitorque motor-operated valve (MOV) actuators. (In fact, the kit is aimed at testing actuator greases.) The other reference samples are Nebula EP 0, EP 1, Marfak EP 2, Alvania EP 3 and Andok C (4-grade). Reference greases that are not prone to change in thickness over time should be selected. In situations where this is not the case, such as with Nebula EP 0 and EP 1, the reference samples are replaced with fresh grease once per year.
Figure 2. Testing Grease with TXU Kit
Figure 3. Sampling Tube Positioned Near
MOV Actuator Worm in Main Gearbox
The syringe and tubing in the kit were designed to allow representative samples to be obtained for the test. Figure 3 shows the tube in an MOV actuator in a position to pull a sample from near the worm drive of the main gearbox. The tube extends through a port to the outside where the syringe is attached. The test sample is pulled out by applying a vacuum to the tube. This vacuum is created by withdrawing the syringe plunger. Sampling the actuator from near the worm drive is important because that is the area where the grease is most stressed when the actuator is operating. The tube and syringe can be used to take samples from other areas and equipment as well.
The kit in Figure 1 also includes a plate on which to spread the known and unknown greases, and a small metal spatula to work the grease. Figure 2 shows the plate in use. Note that the greases are being worked with the spatula. Working the grease is desirable for a subjective test of this nature as it helps the technician simulate the working of the grease that occurs prior to laboratory testing and to compare the consistency of the used grease sample. Generally, Grade 4 is the thickness limit for successful operation of moving machinery. A more conservative view has been substantiated that would make this limit a Grade 3, particularly when dealing with high-speed (greater than or equal to 3600 rpm).
This simple grease test is an important predictive maintenance tool, particularly with MOV actuators. It can be performed in the plant to obtain quick answers that will help detect problems and direct necessary maintenance.
The prime advantage of this approach is that the method can be used quickly and at the equipment containing the grease to be tested. Samples need not be removed to a remote site for tests. A decision can be made whether change out is required due to grease thickness and if need be, right at the equipment itself. The disadvantage of the method is that one does not get an actual penetration number. Instead, one gets a subjective grade number. This, however, is as accurate as is needed in some maintenance work. Another advantage is the smaller sample size required especially when retrieving from difficult-to-reach locations.
Adopting this process for grease sampling and evaluation will significantly help prevent MOV-related failures, because a representative grease sample can be obtained and provides a proven method to determine the quality of the grease sampled.
Improved data and an improved trending database provide key ingredients to monitor/trend grease degradation allowing for a more predictive or condition-based means for scheduling refurbishment on a valve-by-valve basis.