Understanding the distinct benefits — and challenges — of water hydraulics can help you decide if this technology is feasible for your applications.
Danfoss High Pressure
To most people, hydraulics means oil hydraulics. This should come as no surprise, considering the overwhelming majority of hydraulic systems actually use oil as the hydraulic fluid. Oil came to dominate hydraulics technology because its higher viscosity and lubricity pose less of a challenge to design and manufacture components that achieve long life and exhibit high volumetric efficiency.
Water hydraulics, therefore, generally had been relegated to applications requiring highly fire-resistant or even fireproof hydraulic characteristics — foundries and hot rolling mills, for example. Even these systems, though, almost always employ additives to impart some degree of lubricity to the water. These solutions most often are composed of 95% water with 5% additives.
Tapping into newer technology
More than a decade ago, Danfoss introduced the first line of components designed to operate on pure water (containing no additives). Brand named Nessie, these components use water straight from the tap, which not only makes hydraulic systems that are fireproof, but completely compatible with the environment as well.
Today, the product range contains all the components required to create a complete water-hydraulic system:
- Directional, flow, and pressurecontrol valves,
- 2/2-way solenoid valves for high pressures and temperatures,
- axial-piston pumps rated for maximum flows of 1 to 35 gpm and
- pressures to 2300 psi,
- motors capable of speeds to 4,000 rpm and torques to 100 N-m,
- cylinders specially designed for water hydraulic applications,
- power units — complete with filter, level gage and valves — in a variety of sizes to deliver flows 60 gpm, and
- a complete range of accessories, such as fittings, hoses, filters, heat exchangers, and pressure gages.
These components are used not only in applications with hydraulic systems, but anywhere water is used as a pressure medium. In many cases, equipment can take full advantage of water's many benefits by powering the hydraulic system and spraying water for cleaning or fire suppression.
Benefits of water
The main advantages of water are obvious. In contrast to oil, water is not flammable and is absolutely environment friendly. These properties make water hydraulics ideal for applications in which fire prevention and environmental protection are top priorities. However, other characteristics of water give it distinct advantages over oil.
Water has a lower viscosity than oil. Therefore, when either fluid flows though the same size orifice at the same flow rate, water will undergo a substantially lower pressure drop. This means — with all other factors equal — water can transmit power through a system more efficiently than oil can. Or water can transmit the same power as oil, but in a smaller space. This means water-hydraulic systems hold potential for higher power density than oilbased systems.
Water also transfers heat more effectively than oil does. This means a water-hydraulic system with the same heat generation as an oil-based system would require a smaller heat exchanger. Finally, water exhibits a lower film strength than oil, so water is less likely to retain bubbles. Therefore, problems from foaming are less likely to occur with water hydraulics.
Challenges with water
On the other hand, water's lower viscosity poses a greater challenge to sealing. Water leaking from a rod or shaft seal may not pose as serious a problem as oil leaks do. However, lower volumetric efficiency results from the higher internal leakage with water hydraulic components unless countermeasures are designed in. These countermeasures generally encompass nonmetallic sealing surfaces.
Perhaps more importantly, water's low viscosity hinders the formation of full-film lubrication found in oil-hydraulic systems. To compensate, moving surfaces generally are very hard with ultrasmooth finishes.
Water's lower film strength exacerbates the problem because it is much less likely to form a protective coating on moving parts to prevent abrasive wear.
Water also has a higher vapor pressure than oil. This makes water more susceptible to cavitation when it is drawn into a pump. To prevent cavitation, suction lines to water hydraulic pumps should always be pressurized. Also, water-hydraulic systems must operate within a narrower range of temperatures than oil. This is because water can freeze at temperatures below 0°C.
Extensive range of components
Long service life and safe operation of water-hydraulic systems require the use of high-quality, corrosion-resistant materials. Through the use of polymer coatings on sliding surfaces and specially developed seals, tribological challenges have been conquered. This means that waterhydraulic systems with fixeddisplacement, axial-piston pumps can achieve good volumetric and mechanical efficiency typically at pressures to 2,300 psi.
All of these examples show that in individual cases and based on the actual requirements of an application, the decision must be made in favor of water hydraulics. But they also show that, subject to technical feasibility, the often higher costs of design should not put off potential users, as the operating and consequential costs will usually make all the difference over the service life of a machine.
With its distinct advantages over other technologies, water hydraulics often is not used as an alternative to oil hydraulics. The strengths lie in applications that demand high power density, safe, fireproof operation, and environmental compatibility. Its use must be technically feasible and commercially sound, which usually occur when these conditions are top priorities.
Contact the author by calling 414/371-8468 or e-mail [email protected]. Portions of this article were submitted by Danfoss, Offenbach, Germany.