What is in this article?:
- Water Hydraulics: Benefits and Limitations
- Valves for water
Water hydraulics combines the high power density of hydraulics with the clean operation. But water's inherent physical properties present design challenges.
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Water-based hydraulic systems traditionally have been used in longwall mining applications and 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 not-so-obvious cost advantages over oil based fluid. First, non-toxic, biodegradable synthetic additives cost much less per-gallon than oil based fluids do. One gallon of concentrate can make 20 gal of a solution containing 95% water and 5% additive..
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 or been drained from a system can't just be dumped down the drain. It must be collected, properly contained, and hauled away by a certified carrier — an expensive proposition. Water containing synthetic additives, however, can by dumped into plant effluent systems.
Cost savings at the plant level don't 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 gal of additive mixes with water to make 100 gallons of water-based fluid. A 50-gal container is certainly easier to handle than two 55-gal drums, so warehousing is simpler, cleaner, and less cluttered. Transportation costs also are lower.
Other potential plant-wide savings include improved safety for workers because the wate rbased 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.
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. Propylene glycol is an alternative, which is not toxic and is biodegradable. It costs more than ethylene glycol and is not quite as effective an 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. Hose insulates fluid from exterior temperatures; metal tubing provides little insulation.
Sealing the system
Two 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. Sealing the system from the atmosphere also keeps out most airborne contaminants — a common cause of component failure.
A sealed reservoir eliminates another problem suffered by many hydraulic systems: water ingression. Dissolved suspended water contaminates hydraulic oil. The only detriment water ingression has in a water-based system, though, is that is alters the concentration of additive. Water ingression is still undesirable, but its occurrence is far less detrimental in a water-based system than in one using oil.
This addresses another misconception about water-based systems: water-based systems 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.
With a system that sealed 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.
Water-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 less than 20 ft/sec — preferably, below 15 ft/sec in pressure lines. Velocity in suction lines, in general, should not exceed 2 to 3 ft/sec. Velocities in return lines should be held less than 5 to10 ft/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 cavitation.
An 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. An oil valve retrofitted for water service may work, but its compromise in performance will be obvious when compared to a valve designed for water 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 pump's gear face should be wider than that of an oil pump because water's 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 minimized leakage across the piston.