Selecting the Right Grease Condition-monitoring Tests

In response to changing process requirements in power stations, process control valves need to operate remotely and/or automatically. In the case of large steam and water systems, these are typically motor-operated valves (MOV) that have an AC or DC electric motor to provide mechanical power. These motors are mated to a gearbox turning a stem nut that moves the valve stem.

One manufacturer of these units utilizes a bronze worm gear working with a steel pinion gear. The stem nuts are typically bronze as well, but the valve stem can be constructed of 410 stainless steel. These parts, as well as the bearings and a limit switch gearbox, all require grease lubrication (Figure 1).

Figure 1. One Type of Motor-operated Valve Actuator

The Benefits of Testing
Testing of grease from MOVs is necessary to identify problems and schedule grease changes. In most applications, the grease can provide quality service for years without any problems; but in the case of nuclear power plants, it is important to be able to quantify the condition of the grease. This is especially true for safety-related valves which must perform as expected when required. Also, valves may be located in areas that can be accessed only during reactor outages that occur approximately every 18 months.

Grease testing is necessary to both verify performance and to prevent unnecessary grease changes. While topping-off grease using inspection plugs is generally a straightforward practice, if a total grease change is required, the MOV is often removed from service and replaced. As with oil, unnecessary grease changes can be expensive.

Figure 2. Sampling Motor-operated Valve Actuators

Methods of Testing
Quantifying the condition of the grease has traditionally been accomplished by measuring the stiffness using ASTM penetration tests to determine the NLGI grade or by tactile methods to compare with samples or past experience. Both have limitations. In addition, full-scale penetration testing to ASTM D217 requires a large amount of grease; and while other variations are available that require less, the amount is still large compared to what can easily be obtained.

In-service sampling is often performed using a piece of Tygon tubing inserted into the area close to the actuator worm gears. A thumb is first held over the end of the tube until the tube is close to the area of interest and then, using plunge or jabbing motions, grease is captured in the tube.

The factory-fill grease used in some gearboxes is usually an NLGI grade #0 grease. Presumably, a soft grease was preferred because it has sufficient mobility to compensate for the age hardening of the previously used calcium complex-thickened grease. Some power stations prefer a grade #1 grease, while grade #2 greases or oils have been used in MOV actuators gearboxes by other suppliers. Stiffer greases can be an advantage on the stems because of an increased resistance to oil bleeding and water/steam washoff and better wear protection.

Figure 4. Blotter Test Samples

New Grease Equivalent
Traditionally, the grease used in the Figure 2 actuator gearboxes was a calcium complex grease with a naphthenic base oil. This had inherent extreme pressure (EP) characteristics but suffered from age hardening, which is typical of these thickeners. Some power stations used alternatives and, starting in 1993, users considered types of a newer calcium sulfonate-thickened grease.

Calcium sulfonate greases also have inherent EP characteristics plus the added advantages of being resistant to age hardening as well as offering outstanding stability and corrosion protection. Plus, they were being considered as a common grease for the main gearbox, the limit switch gearbox and the stems. However, for safety-related equipment, the older calcium complex grease remained the only approved product.

This matter was brought to a head in 2001 when the only approved calcium complex grease was taken out of production. While several calcium sulfonate greases were considered as the replacement, with the expressed need to obtain a long service life, the grease selected contained a Group II hydrotreated base oil with an enhanced oxidation resistance.

In 2002, the new grease formulation was approved by the equipment supplier and power generation industry groups. In addition to the other advantages, these greases are compatible with the obsolete grease. Because of the importance of the application, considerable testing was conducted by a number of industry groups.

This testing considered not only normal applications, but extreme applications that might be the result of a steam line break. EPRI testing first aged the grease at 150°C (302°F) for 300 hours and then irradiated the grease to 220 MRAD, followed by environmental qualification testing with steam.

The evaluation testing included the following: worked penetration (¼ and ½ scale), weight loss after aging, dropping point (ASTM D2265), infrared (FTIR) traces, differential scanning calorimetry (HPDSC) and rheometer studies including yield stress, pin-on-disc (POD), and friction and wear studies. The new grease performed well, exceeding the OEM requirements and surpassing the previous grease in almost every regard.

To obtain an even better understanding of the performance improvement, the aging equivalent time interval was doubled to twice the standard Electric Power Research Institute (EPRI) conditions. The calcium sulfonate grease was aged up to 600 hours at 150°C (302°F).

The 600-hour aged grease was subjected to tests that could be used on the small samples taken from in-service gearboxes. These included high-pressure differential scanning calorimetry (HPDSC), Fourier transform infrared (FTIR), blotter chromatography, dropping point, four-ball wear, base number (BN), penetration by three methods, rheological testing and RULER voltammetric testing.

As with oils and fluids, at power stations a number of tests are available, but these were selected based on the degradation that might be expected and what characteristics are important for this application. Other grease testing is covered in the article by B. Herguth.

Table 1. HPDSC OIT Time for Aged Grease Samples

Oxidation Stability (HPDSC ASTM D942)
High-pressure differential calorimetry determines the oxidation induction time (OIT) of lubricating greases subjected to oxygen at test conditions of 3.5 MPA (500 psig) and temperatures between 155°C and 210°C. The longer this OIT, the higher the oxidative stability will be, as shown by the results in Table 1. However, it is unusual that the OIT of the aged greases increased. The reason for this is unknown, but it is possible that the aged greases were less alkaline and, hence, less reactive with the aluminum trays used in this test. In any case, the OIT is very good.

FTIR Testing
FTIR testing is often used with oils to determine oxidation peaks (1,800 to 1,670 cm-1) and to assess the depletion of additives. In the case of Figure 3, the new and used greases showed no significant differences.

Figure 3. FTIR Graph for New and Used Grease

Blotter Test
This test is often used to assess the soot level and remaining detergency of motor oils by observing the color, appearance and spreading of the oil on the blotter paper. This can also be a useful field test for greases. In Figure 4, the aged greases had darkened considerably, but the stability of the grease with respect to the oil bleeding did not change significantly. More importantly, the grease remained grease-like and did not harden with aging.

Table 2. Dropping Point Values for Series of Aged Grease Samples

Dropping Points (ASTM D2265)
When greases are aged, their stability can be affected. One OEM requirement for this application is that the grease must have a minimum dropping point. This was tested on the new and aged greases with no significant decrease, as shown in Table 2. The results were all above the maximum that could be tested and well above the required minimum.

Table 3. Four-ball Wear Test Values for Series of Aged Grease Samples

Four-ball Wear (ASTM D2266)
Because of the sliding motion between the worm gear teeth, the bearings and the stem nut, it is important that the wear resistance performance will not be affected in service. One measure of this is the four-ball wear, in which a center ball is rotated under load against three stationary lower balls. Less wear (smaller scar) is better. As shown in Table 3, aging had no effect on the wear resistance.

Table 4. BN Values for Series of New and Aged Grease Samples

Base Number (ASTM D974)
Calcium sulfonate greases are different from the traditional fiber-type greases, and one characteristic difference is that they are overbased. This alkalinity reserve can counteract oxidation and the buildup of acids. Base number (BN) typically decreases with oxidation and aging from a higher new oil (grease) value.

Table 4 shows that when the 7 pH endpoint is used, there is no significant decrease in the BN and, thus, no indication that the grease has oxidized. When the higher 9.4 pH endpoint is used, it may indicate a slight decrease in oxidative life for the aged batch (K-7-23) from 7.25 to 6.92. There seems to be some batch-to-batch variation in the new grease values, from 7.08 to 10.72.

Table 5. Rheometer Penetration Testing Values for Series of Fresh and Aged Greases

Penetration Test
The stiffness of a grease is often measured using the ASTM cone plunger (penetration) methods, but other methods are available to measure grease mobility including rheological testing using equipment that puts a variable load on the grease samples.

These other methods can provide additional data such as the yield point that can be correlated with the ASTM penetration with the added benefit of requiring small sample volumes. The yield point (or yield stress) is the minimum force required to produce flow or when the grease changes from an elastic material to more of a fluid.

More traditional ASTM penetration tests were also performed using two different penetration methods run by different labs (Table 5). Penetration is used by many stations as one of the measurements of grease life (a condemning limit).

When the grease hardens into the penetration range of an NLGI #3 grade (220 to 250 mm), the grease is considered to be at the end of its life and needs to be replaced. With the previous calcium complex grease, it was common for the grease to harden, which did occur, to more than a NLGI #3 grade after the 300-hour EPRI aging.

The test results in Table 5 show a decrease in the yield stress with aging, good batch-to-batch variation and also a slight softening with aging in the penetration testing. The bounds for the full-scale penetration are within the NLGI limits for a grade #1 (310 to 340 mm) grease. Therefore, even after aging for 600 hours, the grease stayed within its original grade.

This indicates that the new-technology grease will likely not harden in service and, thus, provide a longer useful service life. Hardening is considered to be more detrimental than a softening because a softer grease can still provide lubrication, unlike if the grease cakes and tunnels, then the gear lubrication can be compromised.

Table 6. RULER Test Data for Fresh and Aged Grease Samples (0 to 600 hours)

RULER Voltammetric Test Method
The RULER test method is typically used for oils (ASTM D6971, D6810), but the same technique can be applicable to greases. This test quantifies the remaining antioxidant levels that can be related to the remaining service life and/or the oxidation resistance of the formulation.

Table 6 shows all the RULER results for the 300-, 450- and 600-hour aged samples, respectively. A moderate depletion of the antioxidants is occurring on the aged greases, and the sample at 600 hours indicates a 82 percent remaining grease life.

Summary
The calcium sulfonate-thickened grease with a Group II hydrotreated base offers a number of performance advantages over the previous calcium complex grease. Reports from various power stations indicate that double and even triple the service lives are being realized in motor-operated valve applications.

Conclusions

1. Even when aged twice as long as the EPRI criteria, the Group II calcium sulfonate grease did not show significant deterioration in performance-based lab tests. This indicates long service lives and considerable cost savings.

2. When aged, the Group II calcium sulfonate grease tends to slightly soften rather than harden like the previous calcium complex grease.

3. The tests that indicate some differences after severe aging of the grease are color, blotter chromatography, yield stress and RULER.

4. The tests that detected change require a small sample volume, suggesting that they can be suitable for in-service condition monitoring when only a small grease sample is available.

References

This article with complete references can be found online at www.practicingoilanalysis.com.

About the Authors

Ken Brown is principal. Kevan Slater is a reliability consultant. Wayne Mackwood is a grease technology leader, and Troy Olmsted is a technical customer service representative.