water hydraulics – theory and applications 2004krutz/papers/krutz_h2ohydraulics... · workshop on...

Workshop on Water Hydraulics, Agricultural Equipment Technology Conference (AETC ’04), February 8 -10, 2004, Louisville, Kentucky.

_______________________________________________________________________________________________________ 1 – Professor, Dept of Agricultural and Biological Engineering, Purdue University, West Lafayette, Indiana, IN 47906, U.S.A. ( e-mail : [email protected]) 2 – Associate Professor, School of Mechanical & Production Engineering, Nanyang Technological University, Nanyang Avenue, Singapore 639798. ( e-mail : [email protected] ) 1

Water Hydraulics – Theory and Applications 2004

by

Gary W. Krutz1 and Patrick S. K. Chua2 ABSTRACT

This objective of this paper is to introduce readers to the world of modern water hydraulics technology, commencing from its early development to its current applications. This paper will also present the factors that resulted in the decline of water hydraulic applications and the factors that have contributed to the re-emergence of water hydraulics today. The advantages of using water in hydraulic systems are discussed, various important properties of water are presented, and some of the recent research works done by various investigators in water hydraulics are also highlighted. Finally some of the many applications of modern water hydraulics today are shown. The authors hope that the list of references at the end of the paper, although non-exhaustive, will provide useful reading and direction to readers who wish to pursue greater insight into modern water hydraulics technology.

INTRODUCTION Tap water as the working medium is a totally new concept as many users have been accustomed to only mineral oil or other fluids in hydraulic machines. The application of pressurized water as the working medium is not new as its history actually went back more than two thousand years. Inventions utilizing water hydraulics were used by Archemedes, the Egyptians, the Romans etc. (Varandili, 1999). The first applications of transmitting power through a pressurized fluid medium utilised water in the late 1700’s and became a relatively accurate and economic means of transmitting power during the Industrial Revolution of the 1850’s. In fact, pressurized water had been used considerably to power machineries in the 19th Century (Fig. 1). However, the advancement of water as a pressure medium started to decline near the turn of the 20th century due to the following factors (Riipinen et al. 2003, Trostmann, 1996).

(a) the rapid development of electrical power which provided high voltage power supply over long distances and also providing a means of accurate machine control; and

(b) in 1906, the first oil hydraulic system was introduced.

The advancement of electrohydraulic controls and the subsequent invention of the electrohydraulic servovalve led to considerable advancements in technology for oil hydraulics. Oil hydraulic systems were able to compete with electric systems in terms of automatic control of machinery. Water hydraulics was no longer a competitive source of hydraulic power transmission and ever since was superseded by oil hydraulics except in applications which are environmentally sensitive,


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e.g. where any risk of fire or contamination cannot be tolerated. An interesting and detailed historical development of water hydraulics can be found in Hollingworth (1995) and Trostmann et al (2001).

Fig. 1 : The Historical Development of Oil and Water Hydraulics (Conrad 1998)

FACTORS AFFECTING THE DECLINE OF WATER AS THE PRESSURE MEDIUM During much of the 20th century, water could not compete with oil as the hydraulic fluid because of the following factors : (a) Corrosion : Water caused corrosion in the pipings and components of the water hydraulic system due

to dissolved gases in the water, e.g. oxygen, chlorine, carbon dioxide. High chlorine content in water can corrode even stainless steel. Due to the presence of micro-organisms in the water, microbial-induced corrosion adds to further problems. This leads to the use of more expensive corrosion resistance materials and even corrosion inhibiting chemicals. This raised the costs of water hydraulics systems.

(b) Freezing : Water freezes at 0 °C which is much higher than that of mineral oil. As water freezes, its

volume expands by about 9% and this can cause severe damage in a water hydraulic system. This limits applications in low temperatures unless water is heated or environmentally friendly antifreeze solutions are used.

However, freezing is not encountered in the equatorial countries as the lowest daily temperature does not normally fall below 20 °C. The noon temperature, which will be the highest day temperature, is usually in the region of 30 °C to 37 °C and this is within the maximum temperature (50 °C)


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recommended for water hydraulic applications. Therefore, freezing is not an issue in the application of water hydraulics in these countries and a wider usage of this technology is expected in these countries in the future, both indoor and outdoor.

(c) Lower viscosity : The viscosities of water and mineral oils are 1 cSt and 29 cSt respectively at 20 °C and

atmospheric pressure. Lower viscosity of water means less power loss due to less flow friction. However, it leads to more internal and external leakages for the same clearances as oil hydraulic system leading to additional power loss. Water’s low viscosity compared with other hydraulic media has more leakage if tolerance and clearances are kept at the same level as in traditional oil hydraulics components. In order to maintain the same leakage for water as for oil, the clearance, assuming laminar flow, should be at most a third of that of oil hydraulics, if not less (Varandili 1999). If the same clearance is used in both water and oil hydraulic components, the leakage rate for water is about 30 times that of oil (Varandili 1999, Backe 1999). Internal leakage in a water hydraulic spool valve has been studied by Lakkonen et al. (2003). Lower viscosity also means higher velocity and turbulence which increases the risk of erosion, but decrease the risk of scale and biofilm formation on surfaces. However, lower viscosity will lead to faster flow through valves for the same pressure difference. 50% more water flows through the same cross-sectin than is the case for oil (Backe 1999). This translates to higher speed and faster response of water hydraulic actuators compared to oil hydraulic actuators.

(d) Micro organisms : Microbial growth in water hydraulic systems increases fluid frictional resistance and energy consumption, reduces capacities and accelerates microbial-induced corrosion. Results are fouling, bad smell, clogging of components, thereby causing malfunctioning of the system and even stopping the operation altogether. Growth occurs on surfaces and a filter element has more surface area that offers a good environment for microbes. Therefore, a filter also collects microbes from water and growth occurs in it (Aaltonen et al. 1999, Hilbrecht 2000). Micro organisms can form biofilms which are matrix-enclosed bacterial communities adherent to each other and to surfaces or interfaces. Growth at interfaces is a part of the microbial survival strategy which finds the optimal substrate supply and protection from environmental stress. Adhesion onto surfaces enhances the availability of food and, thereby, promotes microbial growth. Biofilms also enhance the ability of microbes to resist biocides. Micro organisms exist due to the existence of nutrients in the water. Regular cleaning with approved detergent is essential to eliminate micro organisms’ nutrients such as oil and grease, from the water hydraulic system. A study by Frolund and Nielsen (1999) has shown the importance of good housekeeping for closed tap water hydraulic systems to reduce microbial growth. Other measures include ultraviolet light treatment, pasteurization, addition of biocides, preservatives into the water, de-ionised water or a sealed system, etc. Use of biocides to eliminate micro organisms is discouraged as biocides are harmful to the environment and are aggressive towards the materials used in the system. Various methods of reducing microbial growth in water hydraulic systems were investigated by Thomsen et al. (2003). They found that the used of P3-POLIXDES and ethanol were the most effective methods.

Details of microbial ecology of water hydraulic systems are presented and discussed by Riipinen et al. (2003). Microbial growth, its effect and control are discussed in detail by Trostmann et al. (2001).


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(e) Higher bulk modulus – The higher bulk modulus of water (2.05 x109 Pa) compared to that of mineral oil (1.3 x 109 Pa) at the same temperature increases the severity of water hammer effect even though it makes the water hydraulic system more responsive and accurate for control purposes.

(f) Cavitation is due to evolution of dissolved gases, entrained air, the higher vapor pressure of water and sudden change of fluid velocity in the hydraulic system. Water has a high vapor pressure 107 times greater than that of oil, so that water based systems are highly susceptible to cavitation which leads to erosion of internal surfaces and eventual failure of the component. Cavitation reduction measures include keeping the operating temperature of a water hydraulic system down via some form of cooling system. Cooling has the added advantage of reducing the evolution of dissolved gases in water, reducing lime scale deposits and minimizes bacteria growth. The recommended operating temperature range for water hydraulics is 3 °C – 50 °C (Trostmann 1996). Proper system design such as avoiding sharp bends, restrictions, etc will reduce the risk of cavitation.

(g) Formation of scale is affected by temperature rise and evaporation of water. Demineralised water eliminates formation of scale but increases risk of corrosion due to the dissolution of atmospheric carbon dioxide in water making it acidic. It will also cause more abrasion due to removal of calcium and magnesium ion which provide some lubricative effect. However, it has been reported that placing magnetic conditioner on the water pipes can prevent and even dissolve existing scales due to polarization caused by the magnetic field which causes the crystalline structure of the scale to become unstable (Varandili 1999). Maintaining turbulent flow is also another approach but it depends on the operating requirements.

(h) Poor lubrication – Compared with mineral oil used used in steel systems, water is a poor lubricant. The faster automation for industrialization demanded increasing component load and speed requirements which water was unable to offer. Technology was not that advanced during most part of the 20th Century to offer low friction coefficient or lubrication-free materials with the ability to withstand corrosion. Using special component materials, coatings and adding lubrication additives will compensate for water’s different lubricity. Oil as a fluid has a significant amount of additives that provide its inertness and lubricity. FACTORS CONTRIBUTING TO THE RE-EMERGENCE OF WATER AS THE PRESSURE MEDIUM The application of oil hydraulics has, since the early 20th Century, continued to grow as the world continued to industrialise and progress. In the 1970s, the industrialized countries started to become concerned with environmental, safety and health issues such as renewable energy, pollution, environmental contamination, bio-degradability, explosion, fire and so on. By the 1980s, environmental, health and safety concerns resulted in the search for bio-degradable fluids for use in hydraulic systems. Water was then identified as the viable alternative fluid as it was felt that the accompanying technology to support water as the pressurized medium in a hydraulic system was available. R & D work commenced in the late 1980s on what was to become modern tap water hydraulics and in the 1990s, the world saw the re-emergence of modern water hydraulics technology. By 1994, over 10 companies were producing modern water hydraulic components. International technical conferences on fluid power started devoting at least a section to modern water hydraulics R & D work. Water hydraulic applications started to increase gradually ever since.


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The ever growing concern in the industry and public on the use of mineral oil in hydraulic equipment for the manufacturing of products are mainly due to the following factors : (a) Increased Safety Awareness

Mineral oil used in oil hydraulic equipment poses a fire hazard in the event of a spill or leakage. This is especially critical in industries where furnaces are used e.g. iron and steel mills, or where the products are highly explosive and flammable such as ammunition and fireworks factories.

(b) Improved Environmental Protection

Recently, the adverse environmental effects of oil hydraulics have been acknowledged. Today, an acute awareness of the environmental impact of industrial operations exists in both the political as well as social arenas. Oil spills from the hydraulic systems constitute a severe environmental hazard. This has driven corporations to strive for a better “Green Image” and to look for more environmentally friendly means of production. A large source of pollution is due to the hydraulic systems. Leakage of the oil from oil hydraulic system and the indiscriminate dumping of used oil will contaminate the soil and ground water. Biodegradable hydraulic oils and water/oil emulsions were introduced to reduce the environmental impacts of spills from hydraulic systems but lubrication additives were still required. However, water/oil emulsions remain environmentally damaging and the bio-oils are only slowly degradable (Sørensen 2001, Trostmann 1996; Trostmann and Clausen 1995, Backé, 1999). The hydraulic oils and other pressure media (excluding tap water) used in the U.S. alone amount to several million tons annually and it is estimated that only 15% of these media are recovered with the other 85% ending up in our environment (air, soil and water). It is estimated that seven million barrels of hydraulic fluids are lost annually through leaks and line breaks (Varandili, 1999). This is one of the important factors that have led to the resurgence of water as a pressure-transmitting medium (Trostmann, 1996, Trostmann and Clausen, 1995)

(c) Health of Product Consumers

As the world becomes more developed, people become more educated and concerned of the effect of what they consume on their health. It is therefore important for industry to assure consumers that their products are safe or does not affect their health when consumed. Such contamination can occur in the form of oil leakage and oil vapor deposition on the products e.g. in the food industry (e.g. abattoir, dairy, meat-packaging industry), pharmaceutical industry, chemical industry, etc when oil hydraulic equipment is used in the manufacturing process (Backé, 1999).

(d) Long Term Manufacturing Costs which Affect Profits

The increasing competitiveness of the market demands that every facet of the manufacturing operation be looked into to reduce costs. One facet is power media. If there is a cheaper and suitable alternative to using mineral oil for hydraulic equipment, industry would be very interested to use the alternative.

(e) Advancement in technology in :

(i) highly corrosion resistant materials such as lower cost stainless steel (ii) precision machining which can produce very fine tolerances to reduce leakage


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(iii) lubrication free materials. (iv) reduction of microbial growth (v) environmental friendly anti-freeze

(f) Water has the following advantages over oil as pressure medium (Trostmann 1996, Trostmann

et al. 2001):

(i) Water does not pose any environmental hazard. This reduces the cost of purchase, storage and disposal compared to oil as water is environmentally-friendly and non-toxic and can be more readily and cheaply disposed of unlike mineral oil.

(ii) Environment means not only the environment of the earth, but also the product

environment, such as in clean rooms or food processing facilities. Any leaks or spills of a water hydraulic system will simply evaporate without leaving a sticky, oily, dirty, slippery residue. As much as 85% of all hydraulic fluids eventually leave their systems through slow leaks, catastrophic line breaks or failures of fittings and seals (Joseph 1996).

(iii) The cost of using water as the working medium in hydraulic machinery is considerably

lower than that of mineral oil. It will therefore be very cost-effective in the long term to use water instead of oil in hydraulic machinery.

(iv) Water does not deplete scarce resources as water is naturally recycled, found in

abundance and readily available. Salt water hydraulic components do exist. (v) Tap water in industrialized countries is hygienic. This means there will be negligible

product contamination in the event of a leakage. Product compatibility, especially in the food, pharmaceutical and chemical industries, offers an advantage to water hydraulics that is difficult to replace. This is important provided the system is well maintained and not contaminated with bacteria which could be spread to the surroundings by the hydraulic system. Pneumatics, which offers a possible alternative but needs oil mist lubricant, has a higher energy consumption, lower efficiency and lower power density. However, one still needs to be careful if the water hydraulic system is used in sensitive areas or near sensitive machinery where fluid leakage is not tolerated.

(vi) Non-flammable – water poses no fire risk. (vii) Using water in the production line means that workers do not breathe in harmful oil

vapors or suffer exposure of skin and eyes to the vapor. (viii) Lower insurance costs as a result of non-flammability and non-toxicity. (ix) The thermal conductivity of water is 4-5 times that of mineral oil (Trostmann and

Clausen 1995). This means that water systems tend to require less cooling capacity or none. This further saves costs.

(x) Lower solubility of air in water than in oil. Mineral oil contains as much as five times

the amount of air in solution compared to water. Higher air solubility in a pressure


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medium can adversely affect the control accuracy and rigidity of the oil hydraulics system and makes it spongy which can be a safety hazard in some applications.

(xi) Variable over head costs in manufacturing will be less dependent on petroleum market

fluctuation if water is used instead of oil in hydraulic systems. (xii) Stricter legal requirements relating to the use of fluids in machines.

(xiii) Since no oil is transported, there is no possibility of oil spillage and clean up. (xiv) No allergy risk for users, service personnel, distributors, etc. with the use of water.

(xv) Additional engineering and maintenance education is available for servicing water

hydraulic systems. PROPERTIES OF WATER AND THEIR EFFECTS IN A WATER HYDRAULIC SYSTEM Some of the properties of water have positive as well as negative effects on the water hydraulic system as described in the following sections. (a) Density The mass density, ρ, which is the mass, M, per unit volume, V, of a substance, is defined as :

VM

=ρ

Hydraulic energy losses in the system are affected by the density of the hydraulic fluid. The density of a hydraulic fluid is affected by changes in the temperature and pressure in the hydraulic system. It is important to keep the density of the hydraulic fluid as low as possible in order to minimize pressure losses and to reduce dynamic effects on the control valves. Fig. 2 shows how the mass density of water is affected by temperature and pressure and Fig. 3 compares the variation of the relative mass density of water with that of mineral oil for a given temperature as pressure varies. . The mass density of water is about 10% higher than that of mineral oil (Trostmann and Clausen 1995).

Fig. 2 : Variation of Mass Density of Pure

Water with Temperature and Pressure (Trostmann et al. 2001).

Fig. 3 : Comparison of the Mass Density of Water with that of Mineral Oil at a Given Temperature as Pressure Varies (Trostmann et al. 2001).


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(b) Specific Heat The specific heat capacity at constant pressure is defined as the amount of energy needed to change the temperature of one kg of a substance by 1 °C and it does not vary much with temperature and pressure as shown in Fig. 4. It is high for water compared to most substances and it is twice that of mineral oil (Trostmann and Clausen 1995). Typical average values at constant pressure and temperature of 40 °C are 4.180 kJ/kg °C for water and 1.90 kJ/kg °C for mineral oil. This means that water has a greater capacity (2.2 times) to absorb heat than mineral oil and, therefore, it would take a longer time for it to be heated up compared with the same quantity of mineral oil for the same capacity.

(c) Viscosity Perhaps the greatest difference between water and oil, lies in the viscosity of the two fluids. Viscosity is a measure of the internal friction in a fluid when one layer moves in relation to another layer, or in other words, viscosity is a fluid’s resistance to flow. Water, at atmospheric pressure and 20°C, has a viscosity of 1cSt. The viscosity of water is typically less than 1/30th of that of mineral oil at 50 °C (Trostmann and Clausen 1995). The almost constant low viscosity of water presents advantages. It results in very low line losses. These low pressure drops result in more efficient hydraulic systems requiring less power input, which means smaller pumps and smaller motors to turn the pumps. Oil systems require a trial and error simulation because oil’s viscosity changes with temperature. Glycol additives to water might only change viscosity to 3 - 5 cSt.

Fig. 4 : Specific Heat of Water as a Function of Temperature and Pressure (Trostmann et al. 2001)


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Much of water’s viscosity characteristics have to do with the viscosity relationship to temperature. The viscosity of water is less strongly related to temperature changes than that of oil. At a pressure range of 1-1000 bar and a temperature range of 3 °C –50 °C, the viscosity of water varies by a factor of approximately 3 while in the same pressure range but that of oil varies by a factor of about 10 in the temperature range of 20 °C –70 °C (Figs. 5 and 6). This provides another advantage for a water hydraulic system in that it is more stable, in terms of flow velocity and efficiency, over a wide range of operating temperatures than a system using oil. However, the disadvantage of having a low viscosity is that the tendency for internal leakage in water hydraulic systems is greater, requiring hydraulic components to be constructed with much tighter tolerances.

Fig. 5 : The Kinematic Viscosity of Water as a Function of Temperature and Pressure (Trostmann et al. 2001).

Fig. 6 : Variation of the Kinematic Viscosity of a Mineral Oil with Temperature and Pressure (Trostmann et al. 2001).


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(d) Thermal Conductivity Thermal conductivity of a substance is the property to transfer energy from temperature differences between adjacent parts of the substance. Typical values of the thermal conductivity of water at constant pressure and temperature of 20 °C is 4 – 5 times (Trostmann and Clausen 1995) that of mineral oil as shown in Table 1. Water’s higher conductivity makes it easier to transmit heat. This means that it is easier to control the fluid temperature in a water hydraulic system than in an oil hydraulic system. The thermal conductivity of water can be considered to vary linearly with temperature for a given pressure in the water hydraulic’s operating temperature range of 3 °C to 50 °C as shown in Fig. 7.

Water Mineral Oil Thermal Conductivity [W/m °C] 0.600 0.12

Table 1 : Typical Average Values of the Thermal Conductivity of

Water and Mineral Oil at Constant Pressure.

(e) Vapor Pressure The vapor pressure of water at 50 °C is approximately 0.12 bar while that of mineral oil at the same temperature is about 10-6 bar (Fig. 8). The high vapor pressure of water (typically 1.2 x 105 times that of mineral oil) limits its maximum working temperature to about 50 °C in order to avoid the risk of cavitation and erosion. Therefore, a water cooling system might be necessary. However, water’s higher specific heat and higher thermal conductivity compensate for its higher vapor pressure so that the risk of cavitation due to temperature rise under normal operating conditions is minimized.

Fig. 7 : Thermal Conductivity of Water as a Function of Temperature and Pressure (Trostmann et al. 2001).


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(f) Bulk Modulus One advantage water has over oil is in terms of the bulk modulus. Water has a bulk modulus nearly 50% greater than that of mineral oil, 2.4x104 bar compared to 1.6x104 bar. The air solubility for water is about 20% of that of mineral oil (Trostmann and Clausen 1995). The presence of air bubbles in water increases the compressibility of the working fluid. The velocity of a small air bubble is inversely proportional to the viscosity of the surrounding fluid. Thus, bubbles in water (lower viscosity) escape about 30 times faster than bubbles in oil and the variation in compressibility of water in a hydraulic system is small compared to that of oil in a similar hydraulic system (Urata 1999). This means that water is more rigid than oil with lesser variation in rigidity, providing a stiff and very responsive to pressure fluctuations and load sensing applications. This property provides the use of water in high frequency hydraulic control applications ideal. However, higher bulk modulus can have greater pressure wave propagation in water and therefore, special care must be given when designing water hydraulic systems in order to prevent water hammer which happens during sudden valve closures. Fig. 9 compares the air solubility of water with some other hydraulic pressure medium. Fig. 10 shows the effect of undissolved air on the bulk modulus of water. The elasticity of the fluid in a hydraulic system and the elasticity of the walls of the container enclosing the fluid (actuators, valves, pipes, fittings, sealings, flexible hoses, etc) as well as the presence of entrained air/gas contribute to the total compressibility of a hydraulic system. The reciprocal value of this compressibility is called the effective bulk modulus.

Fig. 8 : Comparison of the Vapor Pressure of Water with that of Mineral Oil as a Function of Temperature (Trostmann et al. 2001).


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Computing the effective bulk modulus βe,(Trostmann et al. 2001) : From the above, it should be noted that the effective bulk modulus is always less than the bulk modulus

of the gas, or that of the container or gtot

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13

(g) Cavitation Cavitation is the formation and collapse of bubbles due to a pressure drop induced by fluid with the pressure being lowered to the vapor pressure of the flowing fluid. In a hydraulic system, it may happen that the fluid at some time does not fill out the enclosed volume of a component, and cavities form in the fluid. Vapor cavitation occurs when the pressure drops lower than the vapor pressure of the fluid at a temperature. At this level the fluid boils and the resulting vapor fills the cavity. Damaging implosions occur at regions where the pressure suddenly rises. The vapor pressure of water increases with increasing temperature as shown in Fig. 8. It is important to keep the fluid temperature in a water hydraulic system to below 50 °C in order to avoid raising its vapor pressure which can result in cavitation, for example, in the pump inlet where the fluid velocity is high

Fig. 9 : Comparison of the Solubility of Air in Water with that of Other Hydraulic Pressure Media as a Function of Absolute Pressure (Trostmann et al. 2001).

Fig. 10 : Effect of Undissolved Air/Water Ratio on Bulk Modulus (Trostmann et al. 2001).


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and pressure is low. The low pressure may reach the vapor pressure level of water. The pressure drop in the suction line from the reservoir of a water hydraulic system can be significant as to reach the vapor pressure level of the water if this part of the hydraulic system is not designed correctly and this will cause cavitation at the pump inlet. Trostmann et al. (2001) recommends that the safety margin in this suction line of the water hydraulic system should be about 0.8 bar (Fig. 11). Trostmann et al (2001) further recommend that the fluid velocity in the suction line be kept below 1.5 m/s. Valves and actuators need redesign for usage with water. In oil, cavitation is mainly due to the vaporisation of dissolved air, i.e. gas cavitation. In water, cavitation is mainly due to the boiling of the water itself, i.e. vapor cavitation. Cavitation causes erosion, excessive noise in pumps, vibration in pipelines, cavitation choking and general degradation of performance of valves and actuators. Detail explanation of cavitation phenomena, cavitation mechanisms and methods for cavitation detection are discussed in Koivula (2000).

Fig. 11 : Pressure Losses in the Suction Line of a Water Hydraulic System (Trostmann et al. 2001).


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(h) Water Hammer When a fluid flows through a pipe and is suddenly blocked, a large pressure transient may occur, generating a loud noise. This phenomenon is called water hammer. It is the term used to express the resulting pressure shock caused by the sudden decrease of the fluid flow in a hydraulic system, an example is, when a valve suddenly closes or a piston hits the end of a cylinder at a considerable speed. The pressure peak in the pressure transient can be calculated approximately as (Trostmann 1996):

closurevalvetopriorvelocityfluidvfluidtheinsoundofspeedc

densityfluidwherevcpmac

:::

0

0

ρρ=

At atmospheric pressure, the speed of sound reaches the maximum at about 70 °C. At higher pressure levels, the temperature corresponding to the maximum speed of sound in water shifts slightly upwards (Fig. 12). Higher speed of sound in a fluid means faster propagation of water hammer effect and that increases the severity. The speed, and therefore water hammer effect, increases with increasing temperature. Table 2 shows that the water hammer effect is more severe in water (32% greater) than in oil for a given fluid velocity. The damping out of the pressure transients in water will also take a longer time due to the relatively low viscosity of water.

Fig. 12 : Variation of the Speed of Sound in Water as a Function of Temperature and Pressure (Trostmann et al. 2001).


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c(m/s) ρ (kg/m3) vo(m/s) pmax(bar)

Water 1,500 1,000 5 75.0

Mineral Oil

1,300 870 5 56.6

Table 2 : Comparison of the Peak Pressure Values of Water and Mineral Oil in the Event of a Water Hammer Effect for a Given Flow Velocity.


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A comparison of the characteristics of water with those of other hydraulic fluids is shown in Table 3 :

Table 3 : Comparison of the Characteristics of Water with Other Hydraulic Fluids (Trostmann 1996).


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RESEARCH AND DEVELOPMENT IN WATER HYDRAULICS TECHNOLOGY A great deal of research and development work has been done or is in progress moving the frontier of water hydraulics forward. In the area of component development, Hantke and Murrenhoff (2003) have been working on low power piezoelectric pilot stage valve for mining operations. Urata (2003) investigated the equilibrium position of the torque motor. Leino et al. (2003) studied CFD-modeling of a water hydraulic poppet valve. Tammisto et al.(1999) developed a prototype of an arc-cylinder pump and motor which has no reciprocating parts as the pistons do not move axially in order to reduce the inertia effect. The rotating parts are the inclined cylinder block and the piston block which is rigidly fixed to the shaft. Both rotate on different planes. Hollingworth (1999) developed a servovalve which consists effectively of reducer and relief valves forming a two-stage pressure control valve which effectively serves as a servovalve. The effect of water as a pressure medium in this position servo system is discussed based on a linear system model. A discussion of some of the advantages of proportional valve techniques for water hydraulic systems was presented by Koskinen et al. (1995). Mauer (1995) showed how a proportional valve can be designed and constructed. Urata et al. (1995) developed a water hydraulic servovalve with its spool supported by hydrostatic bearings. No sticking of the spool was observed when tested. A displacement sensor for non-metallic hydraulic cylinders for use in water hydraulic application was developed by Hartono et al. (2003). A description and discussion of the Danfoss water hydraulic motor and manual pressure compensated flow control valve was presented by Trostmann and Clausen (1995). Figs. 13 and 14 show a Danfoss water hydraulic axial piston motor and typical motor efficiencies.

Fig. 13 : Cross-sectional View of Danfoss Water Hydraulic Axial Piston Motor (Trostmann

1996).


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Cavitation in valves was investigated by Suzuki and Urata (2003) and Zhou and Yang (1999). Zhou and Yang found that once cavitation occurs, a particular low frequency fluctuation element of pump outlet pressure is much higher in amplitude than that in no-cavitation condition. Low frequency fluctuation can also be caused by inner leakage of pump. They also discussed how the two conditions can be differentiated. The effect of reference signal to the behaviour of the water hydraulic position servo was studied by Makinen et al. (1999) whereas Virvalo et al. (1999) studied force control of cylinder drive and the influence of an external force on the position accuracy of a position servo. Ceramics have been found to be a good material to reduce sliding friction. A polymer called polyeter-eterketon (PEEK), which has a friction coefficient of only 0.02 while sliding on steel with a water film, has been developed and it is now commonly used to coat sliding parts in water hydraulic components. Naturally, various investigators are also interested in low-pressure water hydraulics (10 and 40 bar) as at such a pressure range level problems related to tribology and leakage are eased (Laitinen et al. 2003). Low-pressure water hydraulics is a relatively new branch of fluid power technology between conventional pneumatics and oil hydraulics. How water hydraulics can exploit this niche area has been discussed by Kunttu et al. (1999). Urata (1999) has recommended that modern water hydraulics should cover the working pressure level between that of pneumatics (0.7 MPa) and oil hydraulics (7 – 21 MPa). He has given an excellent insight into the design aspects and challenges facing researchers/designers as well as users of water hydraulic systems. In this area, attempts to modify a pneumatic on/off spool valve to a proportional valve with reasonable characteristics have been made by Sairiala et al. (2003) whereas a low pressure (max. 3.5 MPa) water hydraulic motor with power density between that of pneumatics and oil hydraulics was developed by Shinoda et al. (1999). Investigators in low-pressure water hydraulics do not

Fig. 14: Efficiencies for a Danfoss Water Hydraulic Motor, Type MAH 12.5, at 140 bar (Trostmann 1996).


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appear to be in agreement as to what range of pressure should be considered as ‘low pressure’. Ruble (2000) recommended the following pressure ranges (Table 4) for high, medium and low pressure hydraulics :

High Pressure > 300 bar Medium Pressure 20 – 300 bar Low Pressure 3 – 20 bar

Table 4: The Three Pressure Ranges Proposed for Water Hydraulics

When ambient sea water is utilized as the hydraulic fluid in open hydraulic circuits, the system resembles pneumatics since return line, reservoirs and pressure compensators are no longer needed. Sea water hydraulics is therefore of great interest to many investigators as the cost savings in using sea water as the pressure medium can be significant. Terava et al. (1995) developed a machine for sea water hydraulics and a self-cleaning filter system for sea water hydraulic power pack whereas Nie and Zhuangyun (2000) developed a new type of sea water hydraulic axial piston pump/motor which had an eccentric valve plate. Sea water hydraulic motors/pumps were also designed and developed by Pohls et al. (1999) whereas Bech et al. (1999) went down scale by developing two pumps suitable for mini-power packs using sea water hydraulics. In the area of hazardous environments, Raneda et al. (1999) proposed a model-based teleoperation method that allows the control of machinery in situations where remote control of machinery is warranted, e.g. radioactive, highly toxic, highly explosive hazardous environments. An attempt to substitute the expensive water hydraulic servovalve with two on/off directional control valves was made by Laamanen et al. (2003). However, drawbacks in this kind of system are complex hydraulic circuit due to the large number of valves, noise caused by continuous switching of the valves and the complexity of the controller. Two hydraulic and water hydraulic position servo systems were compared experimentally by Mäkinen and Virvalo (2003) whereas Linjama et al. (2003) compared pneumatic and low-pressure water hydraulic position control systems. It is well known that water has different lubricating properties. Therefore special materials and designs of all components with moving parts are required. Materials with very low coefficient of friction and design features that reduce friction between sliding parts by reducing the contact forces play a very critical role in the performance and life of water hydraulic components. Jiao et al. (2003) studied tribological characteristics of some material combinations used in water hydraulics. Samland and Hollingworth (1995) discussed the importance of using ceramics and thermoplastics as materials in water hydraulic components. Brookes et al. (1995) studied different material pairs for piston slipper pad and swash plate for water hydraulic axial piston pump with sea water hydraulics in mind. Experiments on wear resistance of materials in water hydraulics for pressures up to 40 MPa were performed by Ramo et al. (1999). Analysis of hydrostatic bearing for water hydraulic servovalve was carried out by Suzuki and Urata (1999). The importance and design of fluid connections in water hydraulics were presented by Voigt (1999) whereas the water hydraulic components and systems, development and best practice in design, and industrial applications of water hydraulics were discussed by Conrad et al. (2000). Conrad et al. (2003) has also presented the modeling, simulation, analysis and design of water hydraulic actuators for motion control of machines such as lifts, cranes and robots.


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An electrically driven forklift was converted for water hydraulic lifting and tilting under a joint research project by Danfoss Graham of Milwaukee and the Dresden University of Technology in Germany. The forklift (Fig. 15) uses a joystick to regulate pump output and control lifting and tilting speeds. (ME Magazine 2001).

Water hydraulics research work has also been on going at Purdue University in West Lafayette, Indiana, [Krutz 2000, Krutz and Bystrom 2000]. A water hydraulic test stand (Fig. 16) consisting of two systems, capable of 9 gpm and 12 gpm and pressures of 2000 psi is available at Purdue University. This test stand gives Purdue the capability to conduct further research into the design of water hydraulic components. Some of the projects include water hydraulic control development and water-based transmission. A mobile water hydraulic vehicle was developed at Purdue University (Cassens et al. 2003). This system was designed and modified to operate on a lawn mower (Fig. 17) with a maximum speed of 3 mph. Existing oil hydraulic components were replaced by water hydraulic components. Water power was used to power all circuits including; steering, propulsion, reel drives, and reel lifts

Research had also encompassed aspects of port design to reduce the amount of cavitation in components during actuation. A transparent hydraulic cylinder (Fig. 18) was constructed to study the effects of cavitation on port design. The transparent cylinder allowed for visualisation of the cavitation that was

Fig. 17 : The Lawn Mower Incorporating Electrohydraulic Control and Water Hydraulics Developed at Purdue.

Fig. 15 :The Forklift with Working Medium Converted from Electrical to Water (ME magazine 2001).

Fig. 16 : Hydraulic Test Stand with Water and self-contained power pack that is Capable of a Flow Rate of 4.54 gpm with a Pressure Range of 25-140 bar.


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occurring during operation. Studies showed that a straight-through flow port was best for reducing bubble formation. Another research project involved the development of a gearbox with hydrostatic bearings (Fig. 19) in which the working fluid was water. The concept involves the use of a nonmetallic bearing material. The project had focused on the design of the bearing housings and races as well as the injection ports in order to achieve the complete suspension of the bearing on the thin film of water developed through the shaft rotation. Further details of water hydraulic facilities and research at Purdue University can be found in http://pasture.ecn.purdue.edu/%7Eehcenter/ and http://pasture.ecn.purdue.edu/~ehcenter/.

Water hydraulics research and development work has also been on-going at Nanyang Technological University, Singapore, since 1998, especially in the area of novel design and on-line condition monitoring of actuators (Yu et al. 1999, Tan et al. 2000, He et al. 2001). A mobile and compact water hydraulic power pack with associated electrical/electronic control has been used in the research (Fig. 20). Increasing internal leakage due to cylinder piston seal wear (Figs. 21 & 22), detection of hydraulic motor piston shoe wear and detection of piston crack (Fig. 23) have been the topics of interest. Other works include the modeling and simulation of water hydraulic components and novel design and development of a low-pressure tap water timer valve for sprinkler system for use in lawns and parks (Chua 2003).

Fig. 18: Transparent Cylinder for Cavitation Investigation.

Fig. 19 : Water Gearbox for Study in Developing Hydrostatic Bearing.


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Water hydraulic power pack

Fig. 21 : Internal Components of the Water Hydraulic Cylinder.

Fig. 22 : Cylinder Piston Close-up View.

Fig. 20 : Mobile Water Hydraulic Test System at NTU.

Control Panel

Directional Valve

Pressure Transducer

Flow Sensor

Proportional Flow Control Valve.

Heat Exchanger

Electric Motor


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APPLICATIONS OF WATER HYDRAULICS

List of Industries Suitable for Water Hydraulic Applications The following is an overview of existing and future application areas for water hydraulics. • Food processing and packaging.

In the food processing and packaging industry, pneumatics and electric motors with low output power are commonly used (Shinoda et al. 1999). It is expected that water hydraulics will substitute electrical and pneumatic power sources (Bech et al. 1999). Pneumatics, even though compact and light, has the disadvantages of low efficiency, polluting the working environments by oil mist and noise from air actuation and valve switching. Electric motor needs a cover for water proofing and this increases the size and weight and also leads to poor ventilation of the motors. Besides there is the risk of electrocution as the environment is normally moist or wet. Experts expect food processing to be one of the biggest new application areas for water hydraulics as well-maintained water systems present no potential for contamination of food and facilitate easy and convenient washing of equipments after production. Ultra low-pressure water hydraulics is suitable for the food industry (Brisland 1998).

• Civil defense (water mist fire-fighting) • Pharmaceutical industry • High-pressure water jet cleaners • Construction industry – with machinery working in nature reserves (environmentally sensitive areas) • Chemical industry • Water treatment plants and small scale sea water desalination plant • Automation in slaughterhouse • Fertilizer plants • Explosives and fire-works manufacturing plants • Heavy industries and in industrial operations where high temperatures are involved in the operation

which calls for non-flammable tap water hydraulics, e.g. iron and steel mills, injection moulding, glass production, die casting, etc.

Fig. 23 : Components of the Water Hydraulic Axial Piston Motor.


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• Military • Machine tools and robotics systems • Garbage trucks and road sweepers (Sorensen 1998) • Mining • Shipping (allowing ship to utilize the sea water around it as power medium instead of having to carry

oil out to sea as power medium, thus reducing oil spills at sea) • Offshore and marine industries (environmental protection)

[Example, the removal of algae and other contaminants from the pool surface of salt-water marine mammals for health of the animals (Ruble 2000)].

• Dairy industry • Agricultural industry including lawn care equipment (environmental protection and fire hazard

protection) • Mobile machines working in environmentally sensitive environments. Mobile hydraulic units have

considerable oil spill (Bech et al. 1999) • Paper mills and woodworking industries (non-flammability of water hydraulics system). (Hyvonen

et al. 2000 and Hyvonen 2000). • Nuclear industry. [Non-flammability, using pure water avoids contamination of reactor elements,

pure water cannot be radioactively charged, etc (Bakke 1999, Siuko et al 1995, Trostmann 2001)]. Some Typical Applications of Water Hydraulics Today Some typical applications of water hydraulics today are shown in Figs 24 - 31 :

Fig. 24 : Water Hydraulic-Driven Saw Used in Abattoir for Sawing through Ribs and Backs of Pork/Cattle Offers Ease of Cleaning and Hygienic Operation

(Conrad et al. 2000 and Trostmann 1996).


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Fig. 25 : Water Hydraulic Conveyor Belt in a Highly Hygienic Food Production Area (Trostmann 1996, Trostmann and Clausen 1995).

Fig. 26 : Swimming Pool with a Submerged Variable Floor Drive (Usher 1998).


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Fig. 27 : Tap Water Hydraulic-Driven Burger Machine. Two Water Cylinders are Used to Form the Five Meat Burgers in One Row (Conrad 1998).

Fig. 28 : Water Hydraulics used to Drive Rescue Tools without Fire Risk in the Presence of Sparks (Conrad 1998).

Fig. 29 :Water Mist Fire Fighting (Trostmann et al. 2001).


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More applications of water hydraulics can be found in Sorensen (1998, 1999), Hyvonen et al. (1999), Siuko et al. (1999), (Ruble 2000), Vecchiato et al. (2003), Conrad et al. (2000), Hilbrecht (2000), Trostmann et al. (2001) and Danfoss website (2003). CONCLUSION Engineers have seen from the foregoing that the challenge in developing water hydraulics to a level of technology comparable to that of traditional hydraulic systems lies in the physical properties and characteristics of water. Besides the obvious, water is significantly different from oil in many ways. Some of these differences give water a distinct advantage over oil in terms of performance as a pressure transmitting medium while others give rise to needs for further development of water hydraulic components. Current technological efforts for water hydraulics are far less than those for oil hydraulics. However, the experience gained from oil hydraulics is very important for future water hydraulics research. Engineering relationships between water and oil are significantly different and cannot be overcome or trivialized by simple modification of design parameters from oil hydraulics. The main differences in characteristics between water and other hydraulic fluids have earlier been summarized in Table 3. Water hydraulics should be regarded as a new evolving technology. It has certainly brought in challenges as well as opportunities for the engineers and manufacturers. When the current drawbacks are resolved through advances in technology, modern water hydraulics will have a wider spectrum of application fields than oil hydraulics presently has. Presently, water hydraulics is more expensive than its oil counterpart, due to the fact that it is an evolving or new technology and naturally comes with a lower volume and higher price. Moreover, the production technology is not widespread yet or well established and this resulted in higher unit production cost which the user has to consider. Moreover, the use of water hydraulic systems has not developed as fast as expected due to the lack of common goals and the components developed are scattered over a very large range of power levels, and missed sizes and types of components (Bech et al. 1999). The current market share of water hydraulics is believed to be less than 5% (Bakke 1999). As long as the quantity produced is small and one is still at the bottom of the learning curve, the price of the final component

Fig. 31 : Garbage Truck with its Waste Packing Unit’s Oil Hydraulics Replaced by Water Hydraulics, Saving 12,000 litres of Oil Waste Per Year in Goteborg (Sorensen 1998).

Fig. 30 : Road Sweeper with Water Hydraulic Operated Cleaning Units (Sorensen 1998).


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will be unfavorable. Hence, while water is cheap, water hydraulic components are not. This will be resolved when more engineers and companies join in the development and use of this new technology. Recent sales growths are 50 % to 100 % per year. It is expected that water and oil hydraulics will co-exist in the near future with water filling in the niches where environment, health, fire and safety, product compatibility, etc are important considerations. In the longer term, it is expected that water hydraulics will take over many of the existing oil hydraulic applications besides having new applications of its own. If a system is going to be utilized, it needs a driving force of which water hydraulics has one – the environmental lobby. A technology will not be used unless it offers an ultimate financial advantage over other technologies or unless it is required by law. National legislation on environmental protection will certainly increase the application of water hydraulics. Europe has oil spill legal penalties. Some nations are beginning to change not only their environmental regulations but also the enforcement and Sweden is one example (Ruble 2000). As the demand increases from both users and regulatory agencies, more manufacturers will begin putting the resources towards the development of water hydraulic components and systems. Water hydraulic actuators will become both smaller and larger – smaller in size and larger in power output due to the increasing pressure range that is being made possible. Just like oil hydraulics dominated power transmission and control in the last few decades, water hydraulics technology could well supercede oil hydraulics technology and revolutionize the power transmission industry in the years to come. One area not discussed was nano-technology level components for biosensors and biotechnology.

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