Separating water from hydraulic oil is a difficult task. In industrial environments, several techniques are used including vacuum dehydration, centrifugal separation, air stripping or purging, polymeric absorption and droplet coalescing. The United States Navy needed a technique for restoring mineral hydraulic oil onboard submarines to a maximum saturation level of 0.05 percent (500 ppm) water. Selected hydraulic systems that have a seal interface or exposure to salt sea water experience easily degraded conditions related to oil contamination. The Navy has been unable to identify either government or commercial devices that perform this task with portable equipment onboard a submarine. Submarine hydraulic systems do not presently have continuous in-service dehydration capability. Studies have shown that traditional coalescing water separators (designed to separate water droplets from fuels) and centrifugal water separators are unsuccessful when used to remove emulsified water from hydraulic oil.1 There are two major reasons for this: the high viscosity of hydraulic oil relative to diesel fuel, and the additives commonly used in hydraulic oils.

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  • Figure 1. Interaction between additives and oil/water. The polar heads of the additives are represented by circles and nonpolar tails by zigzag lines.

    There are many different hydraulic fluids used in hydraulic systems, such as petroleum-based, synthetic, phosphate ester, silicone synthetic and water-based fluids. The U.S. Navy has used petroleum-based hydraulic fluids in its submarine fleets for a long time. A typical petroleum-based hydraulic fluid consists of refined mineral oil, additives and dye.2 The refined mineral oil can be either paraffinic or naphthenic. Both contain a high proportion of saturated hydrocarbons. The larger the hydrocarbons in the oil, the higher the viscosity of the oils.

    Viscosity is one of the most important properties of hydraulic oils, which must be thick enough to provide lubrication of the hydraulic pump and provide good sealing at the close internal clearances in pumps, motors, valves and cylinders. For instance, Navy submarines use 2075-TH (MIL-PRF-17672) hydraulic oil. The oil has a nominal viscosity of 32 centistokes (cSt) at 40°C, which is about 32 times thicker than water. The traditional coalescing water separator, designed to separate water droplets from fuel, is not a feasible mechanism for separating water droplets from hydraulic oil due to the high viscosity property of the hydraulic oil. The coalescing water separator employs gravity to separate water droplets from fuel, which can be described using Stokes Equation3:

    Where U is the settling rate of a water droplet, d is the diameter of the water drop, rw is the water density, rf is the hydrocarbon fluid density, g is the acceleration due to gravity, µ f is the hydrocarbon fluid viscosity.

    Because the suspended particle (for example, water drop) settling rate is inversely proportional to the hydrocarbon fluid viscosity, it will decrease as the hydrocarbon fluid viscosity increases. For example, the viscosities of diesel fuel (#2) and 2075-TH hydraulic oil are 2 cSt and 32 cSt at 40°C, respectively. The densities of the two fluids are 0.850 and 0.870 to 0.880 kg/L at 15°C (60°F), respectively. One can easily deduce that the water drop settling rate in the diesel fuel is about 20 times faster than that of the hydraulic oil by using Equation 1 for a water drop of the same size. This strongly indicates that the water droplets tend to suspend in the hydraulic oil under normal gravity force conditions. The centrifugal water separator should have a better performance compared to the natural settling process because the separator enacts a greater g-force on the water drops. However, the g-force required to obtain the same water separation performance with the hydraulic oil as is normally obtained with the diesel fuel would be much higher (at least 20 times). It is expected that the centrifugal separator will not have a good water separation efficiency due to the lack of significant density difference between the hydraulic oil (density: 0.889) and the water drops (density: 1.0 Kg/L at 15°C (60°F)).


    Figure 2. Surface Active Molecule

    Additives are used in hydraulic oils to improve performance under various conditions. Typical additives in hydraulic oils include antiwear agents, antioxidants, corrosion inhibitors, viscosity index improvers and antifoam agents.2 Most additives are high molecular weight hydrocarbon chemicals containing polar molecular functional portions or ionic portions. For example, 2,6-di-tert-butylphenol (antioxidant) and zinc alkyl dithiophosphate (antiwear agent) are commonly used as additives in hydraulic oils. The 2,6-di-tert-butylphenol molecule has a polar phenol function portion and the zinc alkyl dithiophosphate molecule has an ionic function group. It has been found that a chemical compound (for example, additives) containing a hydrocarbon portion/polar portion or a hydrocarbon portion/ionic portion will cause a surfactant effect when added to a mixture of oil and water4,5 (Figures 1 and 2).

    Figure 2 shows a chemical compound containing two chemical function portions, the head group and the tail group. The head group is the polar or ionic portion, which strongly interacts with water via dipole-dipole or ion-dipole interactions. Consequently, the head group is hydrophilic (water-liking). The tail group, however, is formed by the hydrocarbon chain portion, which interacts only weakly with water molecules. This hydrocarbon tail is usually called hydrophobic (water-hating). When the additive with hydrophilic heads and hydrophobic tails is added to the oil containing a small amount of water, the hydrophilic heads will attach to the water molecules under agitation conditions, forming water emulsion in the oil (Figure 1).

    As shown in Figure 1, the water emulsion consists of many fine water droplets (down to 2 to 10 microns), surrounded by the hydrophilic heads and hydrophobic tails of the additive. The hydrophilic heads position themselves on the interfacial surfaces of the water droplets and the oil, while the hydrophobic tails orient outward from the interfacial surfaces. As a result, this interfacial reaction between polar and nonpolar molecules forms a stable water emulsion in the oil. This water emulsion does not tend to break up using conventional separation methods (such as coalescing technology) because the coalescing method does not provide a stronger interaction force with the water than the polar groups of the additives.

    The goal of Phase I of this project was to demonstrate the feasibility of an innovative hydraulic oil filter design that combines a water-selective membrane, cross-flow filtration, and a high-capacity spiral-wound membrane element into an integrated portable filter system.

    The primary objective for the proposed project is the completion and demonstration of a 37 gal./hr. (based on 1.5 gal./hr.ft2-membrane area) portable prototype to prove the feasibility of the proposed oil filter design for removal of water and debris from hydraulic oil. Other specific objectives of this project included:

    1. Identify the most suitable filter media for the application.

    2. Design and construct a 37 gal./hr. prototype.

    3. Demonstrate that all four design features are attainable. These include: water-selective membrane, cross-flow filtration, high-capacity spiral-wound membrane elements, compact and portable features.

    From all the tests performed in Phase I, the following conclusions can be reached based on the test results:

    1. The concept of using a water-selective membrane for separating water from oil in combination with the spiral-wound filter cartridge design was proven feasible.

    2. The tested water-selective membranes showed high filtrate flow rate and high water removal efficiency for filtering 2075 TH and 2190 TEP (MIL-PRF-17331) oils.

    3. A filtrate flux rate of 1 to 2 gph/ft2 can be achieved based on 2075 and 2190 oils.

    4. 2075 TH and 2190 TEP oils can hold three to five percent emulsified water. This water level can be maintained through the water-settling chamber. Excess water in the feed can settle out in the water settling chamber. The filtered 2075 TH and 2190 TEP quality will surpass the requirement of 0.05 percent for water.

    5. The oil filtrate flux rate increases with the increase of oil temperature.

    6. The oil filtrate flux rate is not affected with the increase of the water content level in unfiltered oil.

    7. The oil filtration efficiency is not affected by seawater.

    8. The required cross-flow rate in the membrane cartridge is 85 percent of the feed rate.

    9. The prototype system meets the design aspect.

    10. A 2.5 gpm oil filtration system can be designed with one 8-inch OD X 20-inch long spiral-wound membrane cartridge.

    11. The overall dimensions of the 2.5 gpm oil filter housing are approximately 19-inch OD (base plate) X 44 inches high without prefilter, feed pump and other monitoring components.

    12. The weight of the 2.5 gpm filter housing is about 120 pounds.