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Analyzing hydraulic-fluid purification schemes: to spin or not to spin?

With 70 percent of hydraulic-system failures attributable to poor fluid condition, the reasons for cleaning up your act are compelling: reduce equipment damage, downtime, and wear; reduce fluid-caused workpiece scrappage; lengthen fluid life and reduce its cost; and avoid environmental and EPA compliance problems. Fluid cleanliness is particularly critical in NC machines where servovalve clearances can be impeded by dirty fluid.

But how clean must hydraulic fluid be? The ideal fluid carefully balances viscosity, lubricity, compatibility, wear protection, rust inhibition, foaming resistance, and temperature-range characteristics. But, as your system acquires dirt, water, rust, chips, and other contaminants, this delicate compromise can be quickly negated. Purification, thus, must be thorough enough to regain and maintain the fluid characteristics you chose when originally selecting the fluid. Tanks or filters

The first of three basic purification approaches is a simple settling system where fluid is pumped from a reservoir into a holding tank. The tank may have steam-heated coils to raise fluid temperature enought to decrease viscosity and help break up emulsions.

The advantage of a settling tank is its simplicity. It does a fairly good job of removing solid contaminants if there is enough time and enough room to store an entire change of oil for the system. If the oil is badly emulsified, gravity may be insufficient and small particles will not settle out without help. This is an approximate technique and some solids will be left n the fluid. Most importantly, it will not effectively remove water from the fluid.

A second basic approach is filtration. Filters can be vacuum, coalescing, or mechanical types. Like settling tanks, they also have few moving parts. With coalescing features, they are effective in removing free water without the need for excessive agitation. Their highest efficiency is with fluids with little or no solids and less than 1 percent water.

Different filters offer different degrees of success. They rarely remove all the water or all the suspended solids, and may have to be combined with a presettling tank or expensive filter-dispenser cartridge. Some filters have a much higher operating and maintenance cost than alternative equipment. Coaslescing features

These filters use special cartridges to combine small, disperse water droplets into larger ones, Figure 1. The first phase, coalescence, occurs when hydraulic fluid is forced through a porous medium with many obstructions and passages. Clean, water-free fluid will pass with little resistance, but dispersed water will not pass. A small droplet may be carried along with the hydraulic fluid through the pores of the medium until it is stopped by an obstructing fiber or narrow passage.

the fibers are resin-coated for water repellence and are preferentially wetted--oil slides thorugh while water is held back. Droplets join and form larger droplets, and the water droplet grows, remaining stationary while the oil flows around it and through the filter medium.

As the water obstruction enlarges, it increases the diffential pressure, forcing the water through the downstream side of the filter to a water-repellent, second-stage spearator that stops the drops and directs them to a water sump.

The advantages of the coalescing filter are: (1) no moving parts (other than pumps), (2) free-water removal without excessive agitation that aerates the fluid, (3) removal of solids down to 2 microns, and (4) no noise or vibration.

The key disadvantages of these filters are: (1) They function best wth low-viscosity fluids, and as viscosity increases, their efficiency drops and several passes may be required. (2) Due to fine pore size, they can handle only a small amount of solids before cartridge replacement is necessary. (3) In early stages of filtration, large particles pass through the filter element. (4) They cannot be used with surfactants (extreme-pressure additives, rust and oxidation inhibitors, for example) that lower interfacial tension and prevent coalescence. (5) Similary, they cannot handle fluids with detergents since these additives disperse contaminants and keep them suspended.

Ther are three basic types of mechanical filters, all of which, Figure 2, can be cleaned or replaced when they become clogged with contaminants:

Metallic filters contain closely woven metal screens or discs as filtering elements. They remove coarse solids but not soluble oxidized material, water, or finely divided contaminants. They can be used safely with inhibited oils since they will not remove additives.

Absorbent (inactive) filters contain materials such as cotton waste, flannel, or paper as filtering elements. They remove both coarse and fine particles, water, and water-soluble impurities. They do not remove soluble oxidation products or additives from inhibited hydraulic fluids.

Absorbent (active) filters remove impurities and contaminants by chemical attraction in addition to mechanical means. Examples include bone-black, charcoal, chemically treated paper, Fuller's earth and active clays. In addition to coarse and fine insoluble particles, they remove practically all insoluble sludge, water, and soluble oxidized materials. They also remove most additives used in inhibited hydraulic fluids, so care should be used in their application. Centrifuge features

The centrifugal separator, Figure 3, removes both liquid and solid contaminants, including water, by an accelerated form of gravity settling. With forces of 1000 g or more, they can separate hydraulic fluids quickly, continuously, and completely.

Their advantages are: (1) automatic operation, (2) faster operating cycles, (3) minimal maintenance or need for cleaning, (4) continuous discharge of separated water, (5) no need for replaceable cartridges, (6) self-cleaning units can automatically discharge solids, (7) less floor-space requirements, and (8) the ability to selectively remove additives and surfactants.

The disadvantages of centrifugal separation include: (1) major periodic maintenance to service mechanical bearings, gears, etc, (2) higher noise levels than pumps, and (3) the inability to separate out particles significantly lighter than the fluid, such as fly ash. (This would require a separate polishing stage with a micronic cloth or paper filter.)

Centrifuges can be eithe portable or stationary. In some cases, the same unit can be used for hydraulic fluid, coolant, lubricating oil, or boiler oil.

Whe should you use centrifugal purification of hydraulic fluids?

* When fluid costs, treatment costs, or disposal costs are too high.

* When fluid supply is a problem.

* When manpower or downtime requirements are too costly.

* When fluids are heavily contaiminated with solids.

* When you need to avoid stripping away important additives or stabilizers from the fluid.

* When you need portability.

Although settling tanks and filtering systems cost less initially than centrifugal systems, they require much more floor space, frequent filter replacement, and substantial servicing time and cost. Centrifuges need little floor space, operate long hours iwhtout attention, and need much less cleaning and maintenance. Taking these factors into account, centrifuges often will pay for themselves in one to two years or less.
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Copyright 1984 Gale, Cengage Learning. All rights reserved.

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Publication:Tooling & Production
Date:Apr 1, 1984
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