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.
Valves for water
Valves 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. 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, because of its much lower viscosity, 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. 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.
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.
Leakage continues to be a nagging problem in many hydraulic system. 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 there's 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.
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 water-based 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.
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.
Accepting water hydraulics
Even the most expensive water additives become attractive when designers realize that 1 gal of concentrate can make 20 gal of fluid. No wonder, then, that interest in water-based fluids often centers around cost saving potential. However, designers must also realize that they can't just change the fluid in their systems from oil to water without making other substantial changes.
What are viewed as disadvantages are really different rules that apply to water-based hydraulic systems. Designers probably resist learning more about water-based hydraulics because they are 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 may miss 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.
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 one the advantages of water based systems, but there is no guarantee that such equipment always will be used in above-freezing temperatures.
Most hydraulic systems are best served by a pressurized cylindrical reservoir. Additional cost is the main reason why they are not used more extensively. With a cylindrical reservoir, condensation forming inside the top runs down the sides and into the sump. By elevating one end of the reservoir, solid contaminants collect at its lowest point and can be removed through a cleanout box. The cylindrical shape also withstands internal pressure without depending on the struts and stiffeners that can provide areas for contaminants to congregate in a rectangular reservoir.
A sealed reservoir must allow the fluid level to rise and fall without allowing air to repeatedly enter and exit. Several methods can accommodate a variable fluid level, but a simple and inexpensive approach uses a breather and two check valves, each with a different spring rate.
With a sealed system, fluid level is highest at initial startup, before fluid has been pumped to the system. When the system is started initially, air enters the reservoir through a breather as fluid leaves the reservoir. After fluid has been circulated through the system and returns to the reservoir, air is not allowed to exit through the breather. Instead, the air pocket becomes pressurized. When the fluid level rises further, pressure of the air pocket eventually will reach 3 to 5 psi. At this pressure, air exits the reservoir through a check valve to avoid overpressurizing the reservoir.
Pressure in the reservoir serves the additional function of precharging the main pump. The positive pressure in the suction line prevents pump cavitation. When the fluid level drops, instead of drawing in more air, the air pocket expands, which lowers the precharge pressure. Over time, the only air in the system is that which entered initially.
Special considerations for water hydraulics
|Water-based hydraulic systems can be more prone to pump cavitation if they are not properly designed. Points to consider:|
|Porting and passageways should be provided to keep fluid velocities below 20 ft/sec — preferably, less than 15 ft/sec in pressure lines.||Velocity in suction lines should generally not exceed 2-3 ft/sec. Velocities in return lines should be held less than 5-10 ft/sec. Higher return velocities can promote foaming when fluid reenters the reservoir.||Components should be carefully sized: rapid changes in fluid pressure and velocity can cause dissolved air to precipitate from solution and cause damage similar to cavitation.||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.|