水基液壓系統(tǒng)-外文文獻(xiàn)

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Water-based hydraulic systemsWater-based hydraulic systems traditionally have been used in hot-metal areas of steel mills. The obvious advantage of water systems in these industries is their fire resistance. Water-based hydraulic systems also have obvious cost advantages over oil-based fluid. First, non-toxic, biodegradable synthetic additives for water cost $5 to $6 per gallon. One gallon of concentrate can make 20 gallons of a 5% solution, so the cost of water-based hydraulic fluid actually can be less than 30 cents per gallon.Considering the costs associated with preventing and cleaning up environmental contamination, water-based hydraulic systems hold the potential for tremendous cost savings at the plant level. Oil that has leaked already becomes a very important problem. It must be collected, properly contained. Water containing synthetic additives, however, can by dumped into plant effluent systems.Cost savings at the plant level dont stop at the lower cost of the fluid and its disposal. Because water-based hydraulic fluid consists of 10 parts water and one part synthetic additive, 5 gallons of additive mixes with water to make 100 gallons of water-based fluid. A 50gallon container is certainly easier to handle than two 55-gallon drums, so warehousing is simpler, cleaner, and less cluttered. Transportation costs also are lower.5 o: p# a6 |% N! e! ]Other potential plant-wide savings include improved safety for workers because the water-based fluid is non-toxic as well as non-flammable. These attributes can reduce plant insurance rates. Spills cost less to clean up because granular absorbents or absorbent socks are unnecessary. Water is "hot" again# g# D" W* zThe oil embargo in the 1970s sparked interest in water-based fluids as a less-costly alternative to oils. Even the most expensive water additives became attractive when designers learned that one gallon of concentrate would make 20 gallons of fluid." Y9 y7 q& ?* d, rAs oil prices gradually dropped, so did interest in water-based hydraulics. In retrospect, interest in water-based fluids centered around their cost saving potential. Most designers lost interest when they discovered that they could not just change the fluid in their systems from oil to water without making other substantial changes. They then become reluctant to accept other "disadvantages" - read substantial changes - of switching over to water-based hydraulics.& [4 w: G, R) u# j7 e4 `+ p. _8 IWhat were viewed as disadvantages were really different rules that apply to water-based hydraulic systems? Designers probably resisted learning more about water-based hydraulics because they were intimated by all the work required to lean about how to design a new system or retrofit an older system. By closing their minds to this different technology, they missed the many other advantages of water-based fluid beyond initial cost. Now that environmental concerns have added disposal costs to the price of hydraulic fluids, water-based hydraulics has again become a hot topic.Fighting freeze9 W, y+ J4 j0 I8 o6 V- F9 yWater-based hydraulic systems do, of course, have limits to their applications. One limitation is the potential of freezing. This possibility is probably the most significant blockade to more widespread application of water-based systems, especially in the mobile equipment industry. Longwall mining is by far the largest sector of mobile equipment that has been able to take advantage of water-based systems. Temperatures underground do not approach the freezing point of water, and fire resistance is essential. Mobile and even marine equipment used in temperate climates could cash in one the advantages of water based systems, but there is no guarantee that such equipment always will be used in above-freezing temperatures.Nevertheless, adding an anti-freeze to a water-based fluid can depress its freezing temperature to well below 32°F. Ethylene glycol - used in automotive anti-freeze - is toxic and is not biodegradable, so its use for anti-freeze in water-based hydraulic fluid would defeat the environmental advantage water-based fluid has.There is an alternative. Propylene glycol is not toxic and is biodegradable. It costs more than ethylene glycol and is not quite as effective antifreeze, so it must be used in slightly higher concentrations. Two more techniques to reduce freezing potential are to keep fluid circulating continuously and use hose where practical.Sealing the system9 n$ i$ ~ a. V' i$ U$ u; JTwo more perceived problems with water hydraulic systems are bacterial infestation and difficulty in maintain proper concentrations. Sealing the system from atmosphere can hold bacterial growth in check. Addition of an anti-bacterial agent to the fluid can have a lasting effect on preventing bacterial buildup if air is excluded from the system.' t! h0 B8 P1 K: i& sA sealed reservoir eliminates another problem suffered by many hydraulic systems: water ingression. This addresses another misconception about water-based systems: water-based systems not sealed from the atmosphere must be closely monitored to ensure that the additive concentration stays within tolerance. That is because water evaporates from the reservoir more readily than the additive does. Consequently, water evaporation causes the additive concentration to increase. When new fluid is added to a system, samples of the existing fluid must be taken to determine the concentration of additive in solution. These results then reveal the ratio of additive to fluid that must be added so that fluid concentration is correct.. p' z- \& v' N: n$ ^With a system that seals fluid from the atmosphere, the evaporation problem is virtually eliminated. Fluid that escapes by leakage is a solution containing water and additive. Therefore, the quantity of fluid in the system changes, but concentration does not. System fluid is replenished simply by adding a pre-mixed solution of water and additive to the reservoir.3 s1 v6 Y; v* SSpecial considerationsWater-based hydraulic systems can be more prone to pump cavitation if they are not properly designed. Generous porting and other passageways should be provided to keep fluid velocities below 20 ft/sec - preferably, below 15 ft/sec in pressure lines. Velocity in suction lines, in general, should not exceed 2-3 ft/sec. Velocities in return lines should be held below 5-10ft/sec.Higher return velocities can promote foaming when fluid re-enters the reservoir. Components should also be carefully sized because rapid changes in fluid pressure and velocity can cause dissolved air to precipitate from solution and cause damage similar to that produced by cavitations.( m/ n- q+ u* `0 X# XAn important consideration for water-based systems is that major components should be designed specifically for use with water fluid, rather than modified from versions originally intended for oil service. Tubing, hose, and fittings usually can be identical to those for oil systems. Pumps, valves, and actuators for water service, however, exhibit some significant differences from components for oil systems. Pump gears, for example, should be made of super-hard alloys to resist wear. A pumps gear face should be wider than that of an oil pump because waters low viscosity requires a larger area to form an adequate lubricant film. Cylinders used in water systems should have bronze-clad pistons to minimize wear between pistons and cylinder walls. Spring- or O-ring-energized seals should be used to minimize leakage across the piston.2 d; Y- _. Z( Valves for waterValves for water-based fluid usually are packed with seals separating metal parts to prevent metal-to-metal contact. This is because water - even with lubricant additives - does not provide the full-film lubrication of oil. Metal surfaces in relative motion in valves for water-based fluid are separated by bearing-type materials. ; x# g' ]8 f0 u- ]Valves for water service also are slightly larger than those for oil. This may be another reason why water-based systems have not gained wide acceptance. Originally, the larger size of components for water-based fluid created a handicap when designing systems, and more costly construction inflated prices of valves for water-based fluid to three times or more that of valves for oil. Now, however, valve sizes are comparable to those for oil. Many valves are available with standard NFPA footprints. The price differential has also become less. Components for water-based fluid still may cost perhaps 3% more than those for oil systems, but this may be a bargain when you consider the cost-saving potential of water-based systems.$ x9 Y* b)Fluid leakageLeakage continues to be a nagging problem in many hydraulic systems. New seal materials and designs, and O-ring face-seal fittings are powerful weapons in the battle against leakage. But the battle is far from over because of misapplication, improper installation, or simple lack of understanding. Although theres no excuse for leakage in most systems, it still occurs. Assuming that leakage will not be eliminated in the near future, water-based fluid can dramatically reduce the costs associated with leakage. ! Q: P/ U( R4 a: hInternal leakage can be just as wasteful. This leakage can carburize the oil by generating heat. Internal leakage typically is routed back to tank, so this technique transforms mechanical energy into heat instead of useful work. Using a stainless steel spool with PTFE seals in a valve for water-based fluid eliminates the need for clearance between moving components. Because there is no clearance, there is no internal leakage., O, a; w, i4 c; ^But beyond the obvious and intangible costs of fluid leakage, disposing of the fluid that has leaked from a system becomes a concern. Allowing hydraulic oil to enter plant effluent systems becomes an expensive proposition when removal and disposal costs are considered. Realizing that cleanup and disposal costs will only go up, and that the price of oil is unstable suggests that water-based hydraulics can be an economical solution to environmental problems.