Nothing in nature is more closely associated with cleanliness,
freshness, and purity. Water also plays an important role as a
power medium. Prior to the 20th century, it served as the
medium for hydraulic motion. Now, economic and environmental
forces have had an impact in helping water regain its
prominence as a hydraulic fluid. Its inherent cleanliness,
combined with its long history as a power source, has pushed
water back to the hydraulic forefront. Whether used in its
pure state, or mixed with oil or additives, water is
increasingly seen as a desirable alternative to conventional
hydraulic fluids. As a result, engineers are viewing water
hydraulics with renewed interest. Many now recognize that it
is environmentally friendly, nonflammable, inexpensive, clean,
readily available, and easily disposable.
Who uses water hydraulics today?
Water hydraulics is used by companies that, for a variety of
reasons, must:
Advantages of Water Hydraulics
Water costs less
Few things are more abundant or readily available than water.
As a result, water's costs are just a fraction of those of
conventional hydraulic fluids. Operating costs are lower, too.
Transportation and storage costs are nonexistent. And when
additives and concentrates are needed, their costs are minimal
too, because they are used in small proportions.
Safety hazards reduced
Water-based fluids are fire-resistant. And pure water is
fireproof. When applications require additives, the resulting
mixtures generally share the same, safe properties of pure
water. Water also promotes safety in other ways: workers do
not breathe harmful oil vapors or risk exposure to skin and
eyes.
Lower insurance costs
Water is nontoxic and nonflammable. So water hydraulic systems
can actually lower industrial insurance premiums. Chances of
fire, environmental clean-up and industrial-related health
claims are lessened when water is the fluid choice.
Easy availability
Water is widely available throughout the industrialized world.
In contrast, conventional hydraulic fluids, including
biodegradables and nontoxics such as vegetable oils, are
dependent upon manufacturers' capacities. As a result, costs
can rise in response to the market's supply and demand.
Disposal costs less
Recent environmental legislation has changed disposal
practices. Some hydraulic fluids must be collected and
disposed of in a prescribed manner; even those manufactured as
"biodegradable and nontoxic." Disposal costs can be
substantial; made even worse when considering necessary
transportation charges. Often, disposal costs are about equal
to fluid purchase costs. But with water hydraulic fluids,
disposal costs are reduced or, in some cases, eliminated.
Environmental compliance costs less
Legislation will continue to hold companies liable for the
burdens they place on the environment. For companies that do
not comply, environmental taxes, clean-up requirements and
other responsibilities are likely to increase. But for
companies that comply with environmental regulations, costs
will be lessened. The use of water hydraulics makes compliance
easier and less costly.
Reduced product contamination
Product contamination is a concern for many industries.
Contamination prevents textiles and wood veneers from taking
dye or stain. It also discolors paper, alters pharmaceuticals
and causes foods to acquire undesirable tastes. Contamination
can also render some products unsaleable. Use of approved
fluids, even those designated nontoxic or environmentally
friendly, does not necessarily solve these problems. However,
if water seeps into products, it is considered far less
harmful.
Green image enhanced
Use of water hydraulics helps foster images of good corporate
citizenry. Users of water hydraulics live up to these images:
they typically are cost- conscious, sensitive to the
environment and caring about safety issues.
Water Hydraulics Meets its Challenges
Technological advancements form the cornerstone of today's
new water hydraulic systems. The main technical challenges,
including leakage and erosion, have been solved
cost-effectively. With a new breed of valves, seals and pumps,
engineers have harnessed all of the inherent advantages of
water as a hydraulic medium.
Leakage
Technical challenge:
Water's low viscosity makes control of external and internal
leakage more difficult.
Solution:
Systems specifically designed for water use close-tolerance
parts and erosion-resistant materials to minimize leakage.
With the introduction of new materials, such as ceramics and
polymers, components have been engineered to minimize internal
leakage by reducing component clearances and wear.
Some of these new materials allow self-lubrication of
moving pump surfaces to reduce the wear that results in
leakage. Part tolerances also have been reduced.
Erosion
Technical challenge:
Water's lower viscosity and higher specific gravity can result
in higher velocity and more turbulent flow. High velocity and
turbulence can erode metal surfaces if left unchecked.
Solution:
Today's water hydraulic systems are designed for water's lower
viscosities. Components with programmed slots, holes or
notches, when coupled with controlled velocities (for
instance, through spool-type valves), can minimize shock and
smooth a system's operation. Seated valves, special materials
and coatings, and newly-designed seals combat erosion.
Corrosion
Technical challenge:
Oxygen and sulfurous by-products from bacteria can corrode
some component materials.
Solution:
Noncorroding metals, such as stainless steel, brass or
specially-treated aluminum, can eliminate corrosion. Engineers
also eliminate corrosion by employing fiberglass, ceramics,
plastics, PVC and other synthetics. Additives also contribute
to the fight against corrosion by coating surfaces that have
the potential to corrode.
Temperature
Technical challenge:
Operating temperatures for water hydraulic systems are limited
to a range of 2 degrees C to about 50 degrees C, depending on
the components used.
Solution:
In instances of potential freezing, low energy trace heating
maintains minimum temperatures and keeps systems operational.
For applications subject to freezing, temperature ranges can
be extended through the use of antifreeze. Because the
definition of biodegradability is not yet universal, the
choice of an antifreeze additive is very important. A wise
selection will insure that disposal does not defeat the
primary reasons for using water.
Bacterial Growth
Technical challenge:
When a reservoir contacts the outside environment and is
maintained within normal operating temperatures, it can
provide a breeding ground for all kinds of microorganisms.
This can result in clogged filters and foul smells.
Solution:
Control of bacteria can be accomplished by:
- exposing water to ultraviolet light
- pasteurizing water
- adding bacteriacides/biocides
- using a tank air breather with a fine micron rating
- filtration
Sealed reservoirs can also prevent bacterial growth.
Protected from outside air and contamination, these systems
can run for many years without fluid changes.
Shock
Technical challenge:
Shock in a water hydraulic system can occur when fluid flow is
stopped abruptly, typically caused by rapidly closing a valve.
Solution:
Through proper system design and component selection, shock or
"water hammer" can be virtually eliminated. Valves
designed for controlled shifting can provide precise
acceleration or deceleration. Use of an accumulator can reduce
system stiffness.
The size of the accumulator depends on the system's
operating pressure and pressure limit. Since many water
systems are fed by an accumulator supply circuit, good design
is particularly important because over-sizing of valves can
lead to system shock.
Lubrication
Technical challenge:
Water's low lubricity can lead to increased contact friction
and wear of components.
Solution:
Conventional components require lubrication. When using
components that incorporate particular coatings or are made
from special materials, water itself acts as a lubricant.
Additives also can play a role in providing lubrication.
Water Hydraulic Systems and
Components
|
| Water
hydraulic systems employ all the basic types of
components seen in conventional systems-pumps, motors,
cylinders, valves, filters and reservoirs. However,
these components are designed to endure the demanding
conditions wrought by the introduction of water.
Special materials-stainless steel, ceramics, and
various synthetics-combine with tighter component
tolerances to produce systems that match the
performance of conventional hydraulics.
|
Components:
1) Reservoir
2) Pump with electric motor
3) Unloader and safety relief valve
4) Check valve
5) Accumulator
6) Valve manifold
7) Electronic control card
8) Cylinder
9) Hydraulic motor
10) Return line filter
Systems may include additional optional equipment |
Cylinders
Cylinders convert fluid power into linear motion and force.
Cylinders used in water hydraulics are made from special
materials, such as stainless steel, or materials with special
coatings and treatments, to resist the erosive and corrosive
nature of water.
Filters
Filters remove foreign elements from flowing fluid. Filtration
requirements of water hydraulic systems are similar to those
of oil hydraulics, but filtration materials and media must be
specifically compatible with water. Fiberglass is often chosen
for the filter itself, while filter housings, heat exchangers,
and heating elements typically use stainless steel.
Motors
A hydraulic motor produces rotary torque when pressurized
fluid is fed into it. Design innovations continue to improve
the motors used for water hydraulics. Some, specially designed
to function with tap water, use the water itself to lubricate
their moving parts. To minimize leakage against the inherent
low viscosity of water, they incorporate small clearances
between moving parts and longer leakage paths.
Pumps
Pumps convert mechanical power to fluid power. New
technological developments have expanded the types of water
hydraulic pumps used today. Axial piston and radial pumps are
now available, along with the more traditional plunger,
centrifugal, and piston-type pumps.
Reservoirs
Reservoirs store a system's fluid. In water hydraulic systems,
reservoirs are typically made from stainless steel, plastic,
or metal coated with corrosion-resistant materials. Water
hydraulic reservoirs generally need to be larger than those in
conventional hydraulic systems.
Seals
Seals prevent leakage. On water hydraulic systems, they
provide their own lubrication, since water's lubricating
properties are low. Self-lubricating seals are available along
with various filled-type seal elastomers.
Valves
Valves control the direction and amount of fluid flow.
Conventional spool and seated check valves are available in a
wide range of configurations. To resist corrosion and reduce
wear, valves typically are made from stainless steel or
ceramics. Valve housings can be made from stainless steel or
anodized aluminum
| Water
hydraulic components are produced with high
manufacturing standards and quality materials. In
today's market, they are made in limited quantities,
and therefore may cost more than those used in
comparable oil-based systems. But even today, initial
costs can be justified by factoring in savings from
operating costs over time. As the number of
applications grow and production costs come down,
users will realize even greater savings. |
Working with Water: Design
Considerations
Proper system design
Proper system design offers long-term benefits and is
essential to achieving the best performance, reliability, and
operating costs. System specifications must be determined,
then matched with the performance characteristics of the
components.
Because water has different properties than oil, several
factors must be considered when designing for use with water
hydraulics.
Let's begin with a look at water's basic properties along
with the benefits and challenges they bring to system design.
Compressibility:
Water is stiff and responsive, making it an ideal power medium
for high frequency actuators. Its compressibility is half that
of oil's. And it exhibits less decompression shock, resulting
in less system fatigue.
Viscosity:
Water's low viscosity allows it to flow more readily. This can
generate more flow energy which, in some cases, can cause
erosion. Water's low viscosity also permits it to flow more
easily through very small spaces.
Other properties:
Water has relatively high specific heat and thermal
conductivity, so less cooling capacity is required for
water-to-water heat exchange than for other fluids.
Water has a higher vapor pressure so the risk of cavitation
may be higher. But using a well planned system which follows a
few basic rules, together with proper component design, can
help prevent potential cavitation damage.
Because water is also an excellent conductor of
electricity, special attention is required to prevent the
possibility of electric shock. Electronic connections to
valves and other components should apply appropriate hardware.
Components must be designed with proper materials,
coatings, heat treatments, and seals to prevent erosion,
corrosion, leakage, and cavitation. Reservoirs should
typically be sized larger to accommodate the longer time
required for suspended air bubbles to dissipate and for
contaminants, which are more buoyant in water, to settle.
Pumps must be supplied with positive inlet pressure. Ports and
lines must be sized to reduce velocity to prevent turbulent
flow, cavitation, and erosion.
Fluids are selected to insure safe interaction with the
product, so as to prevent staining, freezing, boiling,
vaporizing, or growing bacteria.
Correct material selection
Correct selection of material in a water hydraulic system is
essential. For seawater, which is a particularly aggressive
fluid, proper material selection is especially important.
Some water hydraulic systems use inert, coated, and/or
corrosion resistant materials, such as stainless steel,
plastics, ceramics and metal coatings. Because galvanic action
may occur when using water with certain metals, attention
should be given when selecting metals for components.
Water's erosive properties can, in some cases, make
metal-to-metal contact unfeasible. The low lubricity and high
velocity of water also contribute to the need to use ceramics
and plastics in contacting surfaces. Use of ceramic-to-ceramic
contacts can enhance performance because of ceramic's
resistance to erosion, corrosion, cavitation, and friction.
Maintenance
As with all hydraulic systems, good maintenance procedures are
important.
When using water or water-based mediums, fluids should be
monitored periodically. Easily attainable records should be
maintained regarding appearance, dilution, pH, microbiological
levels, and water hardness.
Types of Water Fluids
Because each industry has its own requirements for
hydraulic fluids, the nature of the application will dictate
the type of water to use. Today, water can refer to several
types of water-based fluids.
| Natural
Waters |
Treated
natural waters |
Water
with additives |
| Tap
water |
De-mineralized
water |
HFA
= HWCF = HWBF: |
| Processed |
De-ionized
water |
 HFA-E
(oil in water emulsion) |
| Seawater |
De-salted
water |
HFA-M
(micro emulsion) |
| |
|
HFA-S
(synthetic solutions) |
| |
|
HFB
(water in oil or invert emulsion) |
| |
|
HFC
(water in glycol) |
Using additives
Although some of today's state-of-the-art components can
operate on pure water, many systems use a small percentage of
additives. The additive usually consists of mineral oil
together with emulsifiers and various inhibitors (corrosion,
vapor phase, and antifoaming agents, etc.). Bacteriacides and
antifreeze also are used in certain applications. Additives
are selected based on environmental and hygienic impact so
that water hydraulics' inherent benefits are not lost.
Water's Performance
Capabilities
Speed
In theory, circuit applications requiring fast response can
benefit from using water as a power medium. Why? Water is less
compressible. That means less time is needed to compress the
fluid which, in turn, reduces cycle time. Lower
compressibility also means that load changes are transmitted
to the controls more rapidly and decompression shock is
reduced.
Because the majority of applications operate within typical
hydraulic pressure ranges, speed gains may be negligible.
Still, the inherent speed of water hydraulics makes it a
viable candidate for any high frequency application that can
be handled by hydraulics. Water hydraulics can be used in
demanding, high frequency applications, such as robotics,
animatronics, and flight simulation.
Energy Savings
Water's lower viscosity means that it offers less flow
resistance. That leads to less heat generation and a reduction
in energy losses.
Water's higher energy efficiency results in lower
horsepower requirements. Valve and line sizes also can be
smaller, thus requiring less component materials.
Proportional control
The first in a wave of proportionally controlled water
hydraulic systems is already available. These systems enable
users to accurately control pressure and force. To
successfully implement proportional control, engineers have
worked to resolve problems associated with water's electrical
conductivity, thus enabling water hydraulics to successfully
employ proportional valves and servovalves. Ceramic spools in
valves that provide proportional control have been developed.
These valves combine the performance characteristics of
oil-based systems with the wear and corrosion resistance of
ceramics. Proportional control is achieved through the use of
closed-loop controllers in conjunction with either
proportional valves or servovalves.
Pressures
Water hydraulic systems typically operate in the same pressure
ranges as other hydraulic systems. They also offer favorable
pressure densities, which is why they have long been used in
such applications as large presses and rolling mills. The high
forces produced by water hydraulic systems have enabled them
to be used in the cutting of diamond crystals and other hard
materials.
Advances in Water Hydraulics
The introduction of new design techniques and new
materials-including ceramics, plastics, metal coatings, and
sealing compounds-has changed the face of water hydraulics.
Fluoropolymer coatings now provide corrosion protection
without porosity. These coatings also provide mechanical
toughness, abrasion resistance, and self-lubrication. In
addition, they can be applied in heavier thicknesses and, as a
result, do not wear quickly.
New fluids and additives also have played a key role in the
re-emergence of water hydraulics. Today's fluids provide
lubricity, thus reducing wear, friction, turbulence, and heat.
System Designs
As the use of water hydraulics increases, costs are expected
to decrease. Lowering of initial costs will be a breakthrough
for water hydraulics, enabling the inherent advantages of
water to be incorporated into even more machinery.
Component Designs
In the past five years, engineers have developed new designs
that even use seawater as both the hydraulic medium and the
lubricant. In rotating components, these designs allow high
speeds with greater mechanical efficiency. Newly developed
axial piston pumps and motors offer better power-to-weight
ratios by comparison to oil lubricated units.
A host of new valve designs includes full ceramic spools
and housings. The low thermal expansion of ceramic materials
allows for tighter tolerances, making the valves almost leak
free.
Servovalves, designed to accurately control pressure of
water hydraulic fluids, provide greater control over force.
Their unique design provides response and performance
characteristics comparable to oil servovalves.
Control Electronics
Water hydraulic proportional control systems, introduced in
the past two decades, have found application in steel and
aluminum mills where large presses are used. The system valves
typically are driven by electronic control cards based on
analog techniques.
More recently, engineers have incorporated
microprocessor-controlled, digital cards in water hydraulic
control systems. The sophisticated digital cards are expected
to supplant analog techniques in the control of position,
pressure, and velocity in water hydraulic systems.
The Future is Now for Water
Hydraulics
Growing concern about environmental issues has led to
renewed interest in water hydraulics. The inherent cleanliness
and safety of water frees design engineers from concerns over
oil disposal, clean-up, flammability, and worker safety. At
the same time, water hydraulics' impressive performance
characteristics have brought the technology to a greater
breadth of applications.
To meet the demand for technological advances, engineers
are designing a host of newer, faster, more precise, and more
durable components. Valve and control technology continues to
improve. Manufacturers of water hydraulic systems also have
embarked upon efforts to cut costs and develop a new breed of
pumps that would open up applications in smaller systems.
Even now, many users are finding they do not have to wait
to realize the advantages of water hydraulics. The inherent
cleanliness and low operating costs of water make water
hydraulics a viable power medium today.
Affordable, reliable, clean, safe and cost effective-water
hydraulics is playing an ever expanding role in the world's
industrial scene.
