Edited by Paul J. Heney
senior editor

Water hydraulics can also be used in mobile equipment by mixing antifreeze with the water. Adding propylene glycol to the water can depress its freezing point without sacrificing the environmental compatibility of the water. This refuse collection truck uses water as the primary fluid for all hydraulic functions.

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 oilbased 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 waterbased 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 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 gallons of additive mixes with water to make 100 gallons of water-based fluid. A 50-gallon container is certainly easier to handle than two 55-gallon 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 waterbased 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
The 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.

As 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.

What were viewed as disadvantages were really different rules that applied to water-based hydraulic systems. Designers probably resisted learning more about water-based hydraulics because they were intimidated by all the work required to learn 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 freeze
Water-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 on 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 antifreeze 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 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 maintaining 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 of 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 issue: 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.

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.

Valves for water
Valves for water-based fluid usually are packed with seals separating metal parts to prevent metal-tometal contact. This is because water — even with lubricant additives — does not provide the full-film lubrication of oil. In valves for oil service, lapped spools can be used because oil forms a film on metal components to keep surfaces separated. Metal surfaces in relative motion in valves for water-based fluid are separated by bearing-type materials. Moreover, water can readily leak through the clearances found in non-packed valves for oil service.

Valves for water service also are slightly larger than those for oil. 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.

Cartridge valves that fit into cast, ductile-iron bodies also are available, as are lapped-spool versions of interchangeable cartridges. Special materials are used instead of seals when proportional control is needed, because seals can promote unacceptable stick-slip operation.

The spool in a valve for oil service can ride directly in the valve body. Proportional valves for water-based fluid, though often have a spool that rides in a cast sleeve instead of in the valve body. The sleeve wears because it is softer than the spool. Both sleeve and spool are hardened to Rc 6-72 to reduce wear rates. Valves for water-based fluid also have longer lands to reduce leakage.

Fluid leakage
New seal materials and designs, and O-ring face-seal fittings are powerful weapons in the battle against leakage. The battle is far from over, however, because of misapplication, improper installation, or simple lack of understanding. Assuming that leakage will not be eliminated in the near future, waterbased fluid can dramatically reduce the costs associated with leakage.

Internal leakage can be just as wasteful. For example, lapped-spool valves are designed to leak because the leakage creates the oil film necessary to lubricant moving parts. 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 waterbased fluid eliminates the need for clearance between moving components. Because there is no clearance, there is no internal leakage.

Packed-spool valves eliminate leakage and the need for pilot-operated check valves. When the valve centers to an all-ports-blocked condition, pilot-operated checks are not needed to prevent cylinder drift. If there is no port-to-port leakage, the cylinder will not drift.

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.