ENERGY EFFICIENT WATER HYDRAULIC SYSTEMS

Karl-Erik RydbergDivision of Fluid and Mechanical Eng. Systems, Dept. of Mechanical Eng., Linköping University, Sweden

karry@ikp.liu.se

ABSTRACT

Water plays an important role in power transmissionsystems. The inherent cleanliness of water, combinedwith its long history as a power source, has pushedwater back to the hydraulic forefront. Obviously,economic and environmental forces have had an impactin helping water regain its prominence as a hydraulicfluid. Today, engineers are viewing water hydraulics asan interesting technology that can offer significantadvantages to solve power transmission and motioncontrol tasks.

In order to make use of the advantage of waterhydraulics the component and system design must beadapted, especially to the physical behaviour of pressu-rised water. The materials used in the hydrauliccomponents must have high corrosion and cavitationresistance. Poor lubricating properties requires specialdesign of all components with moving parts, such aspumps, motors and valves.

The major aim is to discuss and high light new ideasuseful for the design of water hydraulic systems. Thispaper also will consider the effects on the developmentof components and systems by using water as ahydraulic fluid. The fluid power technology havepromoted advances in operation capabilities, reducedcosts, increased reliability and protected theenvironment.

Key words: Water hydraulics, physical properties,cavitation erosion, hydraulic losses, system efficiency.

INTRODUCTION

The history of water used in hydrostatic systems goesback to the year of 1795. This year Joseph Bramahendorsed water as pressure medium in his newinvention, a hydraulic press. Then, water was used aspower transmission fluid until the end of the 19th

century. In the beginning of the 20th century oilhydraulics was developed as a new technology thatresolves the drawbacks of water hydraulics, such as badlubrication properties, corrosion, cavitation erosion andlow efficiency.

Oil hydraulics is still superior to water hydraulics inengineering properties such as system overall efficiencyand service life. Due to the specific properties of oil andcontinuously improved additives, the development of oilhydraulics has been shaped to the present days, seereference [1]. May be a more important factor is that thetechnological efforts accumulated for water hydraulicsare still far less that those for oil hydraulics. To improvewater hydraulics the properties of water and compa-tibility with industrial materials must be considered.

Today, the new water hydraulics is a real frontier offluid power technology. Production of water hydrauliccomponents is widely spread over the world and newapplications are introduced continuously. Some typicalapplications for water hydraulics are - food processing,nuclear power plants, fire fighting, steel mills, off shore,rock drilling, street sweepers and waste packer lorries.

When asking for a fluid that does not have any negativeimpact on the environment and does not create firehazards and safe for life the only fluid to be find iswater. The most important requirements for modernindustrial processes are safeties and harmless to theenvironment. Water hydraulics satisfies both of theserequirements.

In the development of energy efficient fluid powersystems the working principle of the system and therebythe behaviour of the fluid and the losses it will cause inthe system is of great importance. When it comes towater hydraulics the physical properties of the fluid, ismuch more critical then with use of oil.

In fact, water as a pressure medium is beset by certainproblems, due to its specific properties. The physicaland chemical behaviour of pressurised water requiresspecial materials in the hydraulic components and also aspecial design to overcome cavitation erosion, corrosionand lubricating problems. However, development anduse of water hydraulics is a natural approach to preventecological damage in many industrial areas. Today, it ispossible to manufacture water hydraulic system withhigh efficiency and an acceptable service life time.Through advanced of technology it will be possible toincrease the application fields of water hydraulics to awider spectrum than oil hydraulics presently have, [2].

PHYSICAL PROPERTIES OF IMPOR-TANCE IN SYSTEM DESIGN

The low viscosity and high bulk modulus of watermeans that applications requiring fast response andenergy efficient operation can benefits from using wateras pressure medium. Water’s lower viscosity means lessflow resistance. That leads to low heat generation andreduction in energy losses. High bulk modulus meansfast response, which in turn reduces cycle times.However, as will be shown below, low compressibilitycan also cause problems in form of high-pressuretransients and cavitation compared to systems withmore compressible fluids, such as mineral oil.(1) Effects of low viscosityViscosity is one of the most important properties of ahydraulic fluid and it has a great influence on theperformance of a hydraulic system. Typical viscosityvalues for oil and water are shown in Fig. 1.

Fig. 1 Comparison between viscosity of mineral oil and waterat different pressures and temperatures

Compared to mineral oil, water has a very low viscosity.During working conditions the relation is normallybetween 1:30 to 1:40. The effect of the low viscosity isthat turbulent flow will appear mainly in allcomponents. Not only in flow through valve orifices,also in pipes and in sealing gaps for pumps and motorsturbulent flow will be dominant.

Turbulent flow in pipes. Flow in pipes is normallyturbulent if water is used. In Fig. 2 can be seen that theflow conditions for water will change from laminar toturbulent at a velocity of about 0,14 m/s (Reynolds no. Re

=2300 and the pipe inner diameter is 16 mm). For mineraloil with a kinematic viscosity of 40 cSt the criticalvelocity occurs at a 40 times higher value, 5,6 m/s.

Fig. 2 Pressure drop versus velocity for laminar and turbulentflow in pipes (pipe diameter = 16 mm)

The pipe losses of pressure drop per meter, illustrated inFig 2 follows the equation,

dv

p f ⋅⋅

=∆2

2ρλ (1)

where the friction factor for laminar flow isRe/64=lλ , and for turbulent flow the factor is,

4 Re/316,0=tλ .

where v - flow velocity, m/sd - diameter of the pipe, mρ - density of fluid, kg/m3

From equation (1) it can be seen that, less pipe diameter(d) gives higher velocity for changing from laminar toturbulent flow. But, even with a diameter of 5 mm thecritical velocity for water is considerably low.Consequently, during normal conditions, nearly all pipeflows are turbulent in water hydraulic systems.

Leakage flow in sealing gaps will increase due to thelow viscosity of water. That means reduced volumetricefficiency for pumps, motors and valves in a hydraulicsystem. Assuming a gap with the same length, widthand pressure difference, the leakage flow ratio betweenwater and oil will be a function of the dynamic viscosity(η) and the height of the gap (h), according to theexpression,

3

3

,

,

oil

water

water

oil

oill

waterl

h

h

q

q⋅=

ηη

(2)

In order to reduce the leakage flow in a water hydrauliccomponent to the same level achieved when mineral oilis used the height of the gap must be reduced to 30 –35% of the gap in a oil hydraulic component. In the caseof the radial gap in a piston pump/motor this can causedifficulties due to thermal expansion induced by friction

between solid bodies. Therefore, higher leakage flowhave to be accepted in water hydraulic components. Ingeneral, water hydraulic equipment has to be producedby lower tolerances, which make the manufacturingmore expensive, [3].

(2) Bulk modulusThe compressibility of the fluid and elasticity of thesurrounding for the fluid adds up to a total compres-sibility, for each separate component in a hydraulicsystem. The inverse of the compressibility defines thebulk modulus. The effective bulk modulus (βe) for thefluid and its surroundings can be expressed as,

gsurroundinfluid

gsurroundinfluide ββ

βββ

+

⋅= (3)

From equation (3) it can be seen that βe will be close tothe lowest β-value of the two on the right hand. Even ifthe hydraulic fluid gives a dominant contribution to theeffective bulk modulus, the contribution from othersystem elements may be of significance as well. This isthe case if, for example, hoses are used as pipes in thehydraulic system.

Both βfluid and βsurrounding have a strong connection to thepressure level. There is also an influence fromtemperature, but not so strong as from the pressure. Ifthe fluid is a mixture of liquid and air bubbles (or othergas bubbles), the fluid bulk modulus (βfluid) will bedrastically reduced by the volumetric amount ofbubbles. The influence is strongest at low pressurelevels, let say below 50 bar.

From theory it is possible to calculate the effective bulkmodulus analytically, but this is rather complex to carryout, which has been discussed above. In practice, thumbrules based on experiences (measurements) is used toestimate a value or a function for the effective bulkmodulus, see Fig. 3.

Fig. 3 Typical values for effective bulk modulus with waterand mineral oil as fluid

(3) Heat capacity and heat transferHeat capacity is an other property, which has influenceon cavitation, especially in water hydraulic systembecause of the low boiling temperature. In a hydraulicsystem all pressure losses in pumps, pipes, valves andother components results in an increased fluidtemperature. This temperature increase is mainlydependent on the specific heat capacity of the fluid, cp.The thermal process can be assumed as isentropic andthe temperature difference, ∆T is a function of thepressure drop, ∆p, following the relation,

( )p

p cp

TT⋅

∆−=∆

ρα1 (4)

where αp - fluid thermal expansion coefficient, -ρ - density of fluid, kg/m3

Due to the difference in specific heat capacity, cpwater =4180 J/kgK and cpoil = 1890 J/kgK (at a temp. of 40oC),the temperature increase versus pressure drop for wateris about 50% of that for mineral oil. There is also adifference in the thermal expansion coefficientsαpwater=.0002 1/K, αpoil=.0007 1/K respectively.

In practice that implies that it is easier to control thefluid temperature in a water hydraulic system than in anoil system. In order to avoid vaporisation occurring inthe pump inlet, of a water hydraulic system it is impor-tant to hold the temperature of water in the tank below45oC.

Under the condition of turbulent flow in a pipe, it is alsonotable that water will have a much higher heat transfercoefficient than oil. Therefore, heat transfer by watercan be more that 20 times higher of that by oil. This isalso an important contribution to the low temperaturerise in water hydraulic systems.

(4) FrictionDue to the poor lubricating properties of water, specialmaterials and designs of all components with movingparts, such as pumps, motors and valves are required.The main wear in component seals and bearings can berelated to friction. When using water as lubricating fluidthe selection of materials and design features thatreduces the contact forces and thereby the frictionbetween sliding parts are of highest importance.

In order to reduce the wear on a surface, materials withlow friction coefficient must be used. For manymaterials the steady state friction coefficient follow theStribeck curve, [2]. The friction of seal materials for apump at constant speed may be explained by this curve.However, unsteady motion shows friction characte-ristics deviating from the Stribeck curve. For example,the static friction is time dependent due to the squeeze

effect between the surfaces, and this is not shown in aStribeck curve. It is hard to expect this squeeze filmeffect for water, since its viscosity is very low.

To reduce the friction and thereby the fatigue wear asuitable combination of polymers and metals orceramics have been found to be a good solution. Arelatively new polymer, PEEK (polyeter-eterketon)shows a very low friction coefficient, about 0,02, incontact to steel with a water film. This frictioncoefficient is comparable with 0,05-0,07 for steel tosteel with mineral oil. Today, PEEK, has turn to becommonly used for sliding parts in water hydraulicpumps and motors, [4].

(5) Cavitation – cavitation erosionCavitation is a common phenomenon in fluid powersystems. It will cause effects like noise, vibrations,reduced efficiency and erosion, see [5]. Cavitationcomes from gas bubble formation due to pressure dropinduced by the fluid flow. If cavitation occurs near amaterial surface and is violent enough, rapid wear of thecomponent will take place. Besides, lubricatingproperties and corrosion, cavitation erosion is the majorproblem in water hydraulic systems.

The main initiator for cavitation is the vapour pressure.Since cavitation remains from implosion of gas bubblesin the fluid, its vapor pressure is an important initiatorfor cavitation. At a temperature of 50oC the vaporpressure for water is about 0,12 bar and for mineral oilis the vapor pressure considerably lower, approximately1,0⋅10-8 bar. In other words, the risk for cavitation ismuch higher in water than in oil.

The amount of air in the fluid will create different stagesfor the initiation of cavitation. In mineral oil, cavitationis mainly due to vaporisation of dissolved air (gascavitation), whereas cavitation observed in water mainlycomes from boiling of water itself (vapor cavitation).The reason is that the air solubility is about five timeshigher for mineral oil (about 10 volume % at atmos-pheric pressure) then for water.

During normal conditions, cavitation (cavitationchoking) downstream a valve orifice will take placewhen the downstream pressure is becomes lower thanabout 50% of the upstream pressure. The erosion effectfrom the jet increases exponentially with the upstreampressure and can be 100 times higher for water then formineral oil. The amount of material removed bycavitation is lower the harder the material is. Metallicmaterial hardness is the most important variable in thiscase. This is the reason why high quality steel alloys(Cr, Ni, Mo, Mg), heat treated steels and ceramics areparticularly resistant to cavitation erosion, [5].

The implication on valve design is that the maximumpressure drop over one separate resistor must be reducedto, let say less than 70 bar. For higher pressuredifferences a multi-resistor valve is the most effectivesolution, but also more expensive. Fig. 4 shows diffe-rent designs of pressure valves with two resistors, [5].

Fig. 4 Examples of two-resistor pressure valves, [5]

In this case, the greater part of the pressure difference isdecompressed within the first resistor. With a suffi-ciently high pressure after the first resistor an absorptionof the vapour bubbles will take place, which eliminatecavitation erosion. At the second resistor, the cavitationerosion is much less severe due to the low pressuredifference.

(6) Pressure transientsPressure transients can be observed in pipes and othercomponents all the times when the velocity of flow ischanged. High-pressure peeks in a pipe can typically becaused by rapidly closing valves. The pressureamplitude ∆p [Pa] caused by a rapid change in flow ∆q[m3/s] will follow Joukowskis equation, which is

qA

qZp ec ∆⋅⋅=∆⋅=∆ ρβ1

(5)

where Zc - characteristic impedance of pipe, Ns/m5

A - cross sectional area of the pipe, m2

In systems with the same pipe diameter and the sameflow disturbances, the ratio of pressure amplitudesbetween water and oil is ∆pwater/∆poil ≈ 1.5, which isshown in Fig. 5.

Titel:PressureTranWaterSkapad av:TrycktransientFörhandsgranska:Den här EPS-bilden sparades intemed en inkluderad förhandlsgranskning.Beskrivning:Den här EPS-bilden kan skrivas ut på enPostScript-skrivare, men inte påandra typer av skrivare.

Titel:PressureTranOilSkapad av:TrycktransientFörhandsgranska:Den här EPS-bilden sparades intemed en inkluderad förhandlsgranskning.Beskrivning:Den här EPS-bilden kan skrivas ut på enPostScript-skrivare, men inte påandra typer av skrivare.

Fig. 5 Simulated pressure transients in water (the leftdiagram) and oil respectively

As can be seen from the figure, the simulated systemconsists of a constant pressure source, a 10 metre longline and a valve with a return connection to tank. Theplots in Fig. 5 shows pressure transients caused whenthe valve is closed instantaneously. The flow velocitybefore valve closing is 5 m/s for both simulations. Theresults shows that pressure transients are more severe inwater than in oil hydraulic system, because of theirhigher amplitude and a higher frequency. The amplitudeof pressure transients will be reduced by decreasing theinitial flow velocity and/or by increasing the valveswitching time.

ENERGY EFFICIENT SYSTEM DESIGN

(1) Pump efficiencyThe viscosity, bulk modulus and friction forces haveimplications on the pump overall efficiency. Highleakage flow caused by low viscosity of water meansthat the overall efficiency will be low, especially in thelow speed range. The higher bulk modulus of water willreduce the compression losses, but they are not sodominant at a pressure level of 15 MPa. In the highspeed range the friction losses will be more and moredominant. However, due to low viscosity the frictionforces will be reduced, which means higher efficiency.The pump overall efficiency calculated from anefficiency model developed in [6] is shown in Fig. 6.

Fig. 6 Pump overall efficiency versus shaft speed for oil andwater respectively

(2) System efficiencyThe most promising idea in the design of high efficientwater hydraulic system is to adapt both pressure andflow, as good as possible, to the actual load require-ments. In practice that means minimization of thehydraulic losses in the system.

As shown in Fig. 7, this can be realized by speed controlof the pump or by displacement control. The losses inthe hydraulic systems, depicted in the figure, are highlydependent on the application and the loading cycle. Themain question is whether the losses of the system will

stay inside a reasonable range during operation, withoutdamaging the system.

Fig. 7 System overall efficiency with speed and displacementcontrolled pump respectively and different fluids

Fig 7 shows clearly that the speed controlled pumpgives higher efficiency, especially in the low flowrange. When the pump speed is controlled, water (1 cSt)will shown a higher efficiency at maximum power.However, this depends mainly on the higher pumpspeed (3000 rpm) instead of 1500 rpm for displacementcontrolled pump.

(3) Pump pulsationsA specific problem related to hydraulic pumps is theflow pulsations (flow ripple) due to periodic variationsin displacement volume and compression effects in thefluid. When the flow ripple interacts with the impedanceof the system connected to the pump, pressurepulsations will be created. If the impedance of thesystem changes, part of the energy in the propagatingwaves will be reflected and standing waves occur. If afrequency in the pump pulsations is the same as aresonance frequency of the pipe connected to the pumpthe pulsation amplitude can be very large. The incomingamplitude can be gained up to 1000 times. Especially in

the low frequency range the pulsations can generatecritical vibrations in the hole system because ofinteraction with different mechanical resonancefrequencies.

In Fig. 8 the source flow from an axial piston pump (in-line pump with 7 pistons) is shown at two differentspeeds, 300 and 3000 rpm and for two different fluids,water and oil. From the flow curves it can be seen thatthe maximum pulsation amplitude is approximately50% higher for mineral oil than for water. Theamplitude is dominated by the compression effects andthe bulk modulus in the simulation models is just twotimes higher for water than for mineral oil (βwater=2,0GPa, βoil=1,0 GPa). Therefore, the flow needed toincrease the cylinder pressure to the system pressurelevel with oil as a fluid will be about twice of water. Inthe actual pump the leakage flow has some influence onthe pulsation amplitude, but much less than thecompressibility of the fluid.

Water, 300 rpm Oil, 300 rpmTitel:vatten300.epsSkapad av:VattenhydraulikvolFörhandsgranska:Den här EPS-bilden sparades intemed en inkluderad förhandlsgranskning.Beskrivning:Den här EPS-bilden kan skrivas ut på enPostScript-skrivare, men inte påandra typer av skrivare.

Titel:olja300.epsSkapad av:VattenhydraulikvolFörhandsgranska:Den här EPS-bilden sparades intemed en inkluderad förhandlsgranskning.Beskrivning:Den här EPS-bilden kan skrivas ut på enPostScript-skrivare, men inte påandra typer av skrivare.

Water, 3000 rpm Oil, 3000 rpmTitel:vatten3000.epsSkapad av:VattenhydraulikvolFörhandsgranska:Den här EPS-bilden sparades intemed en inkluderad förhandlsgranskning.Beskrivning:Den här EPS-bilden kan skrivas ut på enPostScript-skrivare, men inte påandra typer av skrivare.

Titel:olja3000.epsSkapad av:VattenhydraulikvolFörhandsgranska:Den här EPS-bilden sparades intemed en inkluderad förhandlsgranskning.Beskrivning:Den här EPS-bilden kan skrivas ut på enPostScript-skrivare, men inte påandra typer av skrivare.

Fig. 8 Pump source flow for water and oil at different shaftspeeds

The results confirm that the pressure pulsations in agiven pipe connected to a given pump will be nearly thesame for water and mineral oil (∆poil≈∆pwater=Zc∆q) aslong as the pump ripple amplitude is dominated bycompression effects and no resonance “collisions” in thepipe occur.

In fact, the pulsation amplitude is very sensitive topump and system design and the operating condition.However, the main problem related to pump speedcontrol is that the pump pulsation frequencies areproportional to the shaft speed. For some speedinteraction between pump pulsation and pipe resonancefrequencies can occur and give large pressureamplitudes. In such applications a promising idea is touse some form of fluid power attenuators, [7]. Thus canbe considered as an analogue of a band pass filter. The

attenuator will reduce the pressure amplitudes in acertain frequency range.

APPLICATIONS

Water hydraulic components and systems are moreexpensive than oil hydraulics, due to the requirement ofspecial materials and designs. Because of the high levelof monitoring required the operating cost is not muchlower than for oil hydraulics. However, in all industrialapplication areas where safeties, cleanliness and envi-ronmental harmless are important requirements, waterhydraulics can be a real competitor to oil hydraulics.Today, most of the components available for oilhydraulics have also entered the market for waterhydraulics. Even sophisticated components, such asservo valves are available, see [8], [9], [10] and [11].

In order to increase the overall efficiency of a hydraulicsystem variable displacement pumps and motors for oilare commonly used. However, there are no variablepump or motor for water hydraulics offered yet. In thepump case the problem can be solved, by controlling thepump shaft speed. In this paper it has been shown thatspeed control is more efficient than displacementcontrol, especially at partial loading of the system.However, it is important that the pulsation problem isconsidered.

Some typical applications for water hydraulics are -food processing, chemical industries, paper industries,steel mills, nuclear power plants, fire fighting, off shoretechnology, rock drilling, street sweepers and wastepacker lorries.

Due to environmental protection the mobile applicationsare of special importance. These applications are mainlyin vehicles, which are to be used in environmentalsensitive surroundings, such as cities, coastal waters,parks etc. In Sweden there are two interesting mobileapplications operated with water hydraulics. The firstone is a waste packer lorry and the second a streetsweeper. These vehicles are, owned by the wastehandling company Renova, the City Council ofGothenburg.

The waste packer lorry is a daily operated vehicle andthe hydraulic system must be designed for outdoortemperatures of –10 to +40 oC. Therefore, the tap wateris frost protected by 35% food grade propylene glycol.This fluid is classified as a non-hazardous to humansand the environment.

The application mentioned above makes clear that waterhydraulics is no longer restricted to stationary applica-tions. Good use can also be carried out in many mobilemachines. Among the characteristics offered by waterhydraulics are excellent functionality and reliability,

energy saving design and, last but not least, a veryenvironment-friendly pressure medium.

This is just the brief conclusion from the EuropeanEureka-project "Water-hydraulic lifting systems withvariable speed" in which Danfoss, in cooperation withthe "Institut für Fluidtechnik" at the Dresden Universityof Technology, converted an ordinary electrically drivenforklift truck for operation with a water-hydraulic liftingand tilting system, [12]. In this project it has beenproved that solely from the point of view of function,water-hydraulic systems lend themselves incrediblywell to mobile machines, even though units such asforklift trucks do impose high demands on waterhydraulics.

CONCLUSIONS

From being a technology for enthusiastic engineers andpioneers, the water hydraulic technology has becomemore commercial and considered as a competitivealternative to oil hydraulics. The advantages of waterhydraulics as clean and environmental friendlytechniques have been utilised. At present, pure waterhydraulics has a market share of less than 5% of thetotal market for hydraulic equipment, [3]. However,modern water hydraulic technology is still new and a lotof problems must be solved to make the technique morewidely available for power transmission.

It is quite clear that at present, oil hydraulics is stillsuperior to water hydraulics in engineering propertiessuch as lubrication, power density and service life time.The causes of superiority are the physical and chemicalproperties of oil. However, when the drawbacks aresolved through advances of technology the new waterhydraulics will be used in a number of new applicationsareas. To recognise what needs to be done to improvewater hydraulics, physical and chemical properties ofwater and its compatibility to industrial materials mustbe put into focus.

Now there is an increasing demand for water hydraulicsequipment. In part, the demand is environmentallydriven but, in many cases, there is both a clear technicaland an economic motive. Also, the ability of the waterhydraulics industry to deliver the components neededdepends primarily on the materials available to makethem. New materials suitable for water hydrauliccomponents are being offered to the engineer almostdaily.

At present, purchasing a water hydraulic system can bea more expensive proposition than buying a comparableoil system, based simply on the purchase price. But thenumber of applications where this premium is worthpaying and where the medium term cost is actually infavor of water continues to improve. So, in the future,

many more hydraulic systems will use water-basedfluids, raw or tap water, or possibly even sea-water.More manufacturers of components for oil hydraulicsystems will enter this market as it now moves into amaturing process, bringing the benefit of wider productranges and greater market understanding. Specifyingwater hydraulics for the correct application, in a cost-effective and reliable manner, will become ever easier.

REFERENCES[1] Trostman E: Water Hydraulics ControlTecknology. Marcel Dekker Inc, New York 1996, ISBN:0-8247-9680-2

[2] Urata E: Technological Aspects on the New WaterHydraulics. The Sixth Scandinavian Int. Conference onFluid Power, Tampere, Finland, May 26-28, 1999:21~34

[3] Backé W: Water or Oil-hydraulics in the Future,The Sixth Scandinavian Int Conf on Fluid Power,SICFP’99, Tampere, Finland, May 26-28 1999: 51~64

[4] Inoue K, Teraoka T, Urata E: Development of aNovel Water Hydraulic Pump, 1999

[5] Berger J: Kavitationserosion und Massnahmen zuihrer Vermeidung in Hydraulikanlagen für HFA-flüssigkeiten, Doctoral Dissertation, University ofAachen (RWTH), 1983

[6] Rydberg K-E: On performance optimization anddigital control of hydrostatic drives for vehicle appli-cations. Ph.D. thesis no 99, Linköping University,Sweden, 1983

[7] Weddfelt K: On Modelling, Simulation and Measure-ment of Fluid Power Pumps and Pipelines. Ph.D. thesis no268, Linköping University, Sweden, 1992

[8] Koivula T et al: Water as a Pressure Medium inPosition Servo Systems, The Sixth Scandinavian IntConf on Fluid Power, SICFP’99, Tampere, Finland,May 26-28 1999

[9] Takashima M et al: Development of HighPerformance Components for Pollution Free WaterHydraulic System, Third JHPS Int Symp on FluidPower, Yokohama ´96, 4-6 Nov 1996

[10] Koskinen K, Uusi-Heikkilä J, Vilenius M:Simulation and Control of Proportional Water HydraulicCeramic Spool Valve, 9th Bath International FluidPower Workshop, Bath UK, 9-11 Sept 1996

[11] Yamashina C, Miyakawa S, Urata E: Developmentof Water Hydraulic Cylinder Position Control System,Third JHPS Int Symp on Fluid Power, Yokohama ´96,4-6 Nov 1996

[12] Nessie water hydraulics – also suitable for mobilemachines- Sep. 2000, http://www.danfoss.com/Nessie/HydraulicSystem/Articles/index.asp