Synthetic submarine hydraulic oil

Dr. Neil Canter, Contributing Editor | TLT Tech Beat October 2009

A newly developed PAO-based fluid handles stressful operating conditions better than a mineral oil-based lubricant.

KEY CONCEPTS
• The current submarine lubricating oil is a mineral oil-based fluid that has not adequately handled stressful operating conditions and has failed too quickly, leading to a potentially hazardous environment and higher disposal costs.
• A newly developed PAO-based synthetic fluid exhibits a superior oxidation induction time, lower wear scars in four-ball testing and a much higher viscosity index.
• The PAO-based fluid is undergoing additional testing and a submarine trial is anticipated in the near future.

Lubricant fluids used in submarine propulsion systems must meet very challenging operating conditions. Submarines function very independently, which means their operating systems need to perform reliably over long time periods. If a problem occurs with a submarine at sea, the chances for outside assistance are remote.

A lubricating fluid in a submarine must therefore be very reliable. STLE members Dr. Richard Sapienza, CEO of METSS Corp., Westerville, Ohio, and William Ricks, senior research scientist for METSS Corp., say, “Space constraints in a submarine mean that it cannot carry too many materials. Lubricants must not only be reliable but also multifunctional.”

The current lubricating oil used in submarines (known as 2190-TEP) is a mineral oil-based fluid that meets the military specification MIL-PRF-17331. Sapienza says, “This oil has been used in submarines for the past 40 years without any difficulties. It is prepared from well-refined petroleum basestocks (Group I or II) and contains a mild extreme pressure additive package.”

But the U.S. Navy has increased the severity of the operating conditions in its fleet. Sapienza comments, “The U.S. Navy has the goal of faster, hotter and longer with respect to its submarines. Faster refers to the new drive systems that have much higher gear-to-fluid volume interactions. Hotter corresponds to the higher operating temperatures which lead to more thermal efficiency.” In addition to submarines, newer surface ships with controllable pitch-propeller systems have placed additional demands on the 2190-TEP lubricant.

Another significant application for the lubricating oil is in air compressors. Sapienza says, “The submarine compressor oil is the most severe application for the mineral base hydraulic oil. The oil is stressed by temperatures as high as 200 C and pressures as high as 4,500 psi.” 

The net result of these more stressful operating conditions is that the existing mineral oil-based hydraulic fluid is failing more quickly, leading to high oil replacement and disposal costs. Three of the problems seen are (1.) High depletion of antioxidants in the mineral oil-based fluid, (2.) Sharp increases in total acid number and (3.) Severe off-gassing events. Degradation is leading to the formation of components such as formaldehyde and carbon monoxide that can be particularly hazardous in the close operating conditions of the submarine. Estimated disposal costs of $28/gallon have also been incurred, according to Sapienza.

The more rigorous performance conditions that cannot be handled by a mineral oil-based lubricant necessitate the switch to a synthetic basestock. Such a product has been developed and is in the process of being evaluated.

PAO-BASED FLUID
METSS has developed a synthetic-based fluid that exhibits superior performance as compared to the existing mineral oil-based oil. The new fluid, designated Nye 2190-S, is now available commercially through specialty lubricant manufacturer Nye Lubricants, Inc., of Fairhaven, Mass. Sapienza says, “We have been able to develop a synthetic fluid that displays improved thermal stability and oxidation resistance, better lubricity and an improved viscosity index.” The synthetic fluid is formulated with a PAO basestock and a complementary additive package. This fluid is compatible with the existing mineral oil, which minimizes any difficulty during changeover.

Oxidation testing using the Rotating Pressure Vessel Oxidation Test (RPVOT – ASTM D2272) shows a substantial improvement with the synthetic fluid. Ricks says, “The oxidation induction time (OIT) for our PAO fluid is over 2,500 minutes, which is far better than the 330 minutes value obtained from the mineral oil product. We also found that addition of the synthetic fluid to the mineral oil-based product also improves performance. In a 50:50 blend of the two fluids, the properties of the PAO fluid dominate and OIT values in excess of 1,000 minutes are observed.” 

Lubricity testing was conducted using the four-ball wear test (ASTM D4172). Testing using both 52100 steel under a 40 kg load and 440 C steel under a 20 kg steel load leads to lower wear scars for the PAO-based synthetic fluid.

The viscosity index for the PAO fluid is 138, which is much higher than the value of 94 for the mineral oil-based product. Ricks adds, “This result is very significant because it means that the synthetic fluid can better tolerate the higher operating temperatures now seen in submarine propulsion systems.” This is particularly important to ensure ease of valve operations.

A series of additional performance tests was conducted by a major supplier of hydraulic components to the U.S. Navy. Samples of the mineral oil- and PAO-based fluids were taken before and after this testing and are shown in Figure 1. The former fluid has significantly darkened after testing while the PAO fluid remained unchanged.


Figure 1. Performance testing conducted on the current mineral oil-based submarine fluid and a potential replacement based on PAO shows that the former fluid on the left significantly darkens while the latter fluid on the right remains unchanged. (Courtesy of METSS Corp.)

Sapienza says, “We did additional testing on these samples and found that the RPVOT results for the mineral oil-based fluid had dropped from 330 to 65. In contrast, the synthetic fluid had not seen any reduction in performance.” The concentration of antioxidants in the mineral oil-based fluid had also declined while no change was observed with the synthetic.

Two other important characteristics are degassing and microbial contamination. Degassing is a test that assesses the volatile degradation products that can be formed after the fluid is heated for a specific period of time. Sapienza says, “Typically, a fluid is heated at 85 C for 24 hours and then the head space is analyzed for degradation components. This test is particularly important for any fluid that is being used in the closed environment of a submarine. The PAO fluid passed the test.” 

Algae and bacteria are two microbes that have been found in the hydraulic fluid systems of submarines using the mineral oil-based fluid. Sapienza says, “These microbes can attack and degrade the fluid. Testing of the PAO fluid has shown that it is so pure that no microbes grow in it and, as a result, we do not need to include any biocides in the formulation.” 

Further testing of the PAO-based fluid is ongoing. Sapienza hopes that the U.S. Navy will set up a trial in a submarine in the near future. He adds, “We are also looking to evaluate the PAO fluid in the wind turbine industry. Many of the issues faced in submarines are also seen in wind turbines, which need to operate over long operating time intervals in remote locations.”

Additional information can be obtained at the METSS Corp. Web site: www.metss.com or by contacting Sapienza at rsapienza@metss.com.

Neil Canter heads his own consulting company, Chemical Solutions, in Willow Grove, Pa. Ideas for Tech Beat items can be sent to him at neilcanter@comcast.net.