Basis Hydraulics Hand Book

Introduction to Hydraulics Introduction to Filter Technology Introduction to Accumulator Technology

Introduction to Hydraulics

SERVICEENTRE  

In ndex   

             

SERVICEENTRE    

                         

SERVICEENTRE    

                                           

Introduction to Hydraulics Introduction to Filter Technology Introduction to Accumulator Technology

Introduction to Hydraulics

Preface HYDAC operates worldwide, offering an extensive product range to cover all areas of fluid technology. The products range from components and sub-systems, through to complex controlled and regulated drive units for ­mobile and industrial machines and systems. In addition we offer our customers a comprehensive package of technical services within the framework of HYDAC Fluid Engineering, for media such as hydraulic oils, lubrication oils, cooling/cutting fluids and water. Our objectives are exclusively to increase machine and system availability and to reduce our customers’ operational costs. HYDAC has at its disposal a worldwide network of expertise, high quality standards and customer knowledge and is therefore best placed to fulfil the exacting demands of the international market. The continuous expansion of our global presence with strong local focus enables us to respond to the needs of our ­customers in almost every part of the world. With 10 sales offices in Germany, over 40 overseas companies, some with their own production or assembly facilities, more than 500 sales and service partners and over 5000 employees, HYDAC is always close to its customers. To bring our staff, service partners and costumers in the position to reach the continuously growing needs in there ­business environment, we offer trainings, seminars and practical trainings in our Training Center. Doing so, the concept of lifetime learning gets more and more important. Learning after school, apprenticeship or ­university never stops, because learning is the most important tool to achieve education and so for the creation of individual lifeand working chances. Lifetime learning brakes through the borders of the conventional education structures and the classification in strict ­arranged parts of the education way, which ends often with school or university degree. It also includes education as a way to more self dependence in life to identify and use. The ability to lifetime learning will be a fundamental key for ­personal, economic and social success in the future. For these innovative forms of learning, we have decided on a learning concept, which combines the multimedia­ possibilities of E-Learning with the advantages of the in-house training and practical training. For this concept we are developing a book collection, which includes the basic topics of hydraulics and continuative ­hydraulic systems. This book can be used attendant to our seminars and trainings and also as a reference book for your business ­experience.

Jürgen Ringle Manager HYDAC Training Center

11

I

Introduction to hydraulics

1

Introduction

„Hydraulics“ is not a recent invention. Already Archimedes (285 - 212 BC) experimented with water power. That is when he discovered buoyancy forces and some hydrostatic laws. Heron of Alexandria (approx. 100 BC) used air and water pressure for sundry technical gimmicks and shenanigans, which at the time were believed to be witchcraft and wizardry. Among other things he developed an opening mechanism for temple gates. Function: When a fire was lit on the altar in front of the temple, the air in a subterrainean vessel, which also contained water, was heated up. The air expanded and displaced part of the water via an ascending pipe into a second vessel, which was suspended and lowered with increasing weight. Since it was connected to a mechanical gear system, the temple gates opened, when the vessel descended. Once the fire was extinguished, the whole process started in reverse. To the ­people at the time it seemed as if the gods themselves opened and closed the temple gates. As of the 16th. century Bernoulli, Pascal and Torricelli successfully occupied themselves with hydraulics and formulated the essential and fundamental laws of hydraulics. In the late 18th. century, due to the invention of the steam engine, the very first methods were developed, which are still being used today.

temple

temple gates

altar

gear

water supply tank

Fig. Door mechanism of Heron of Alexandria

13

1.1 Hydor

Greek word for water

Hydrology

Science of the element „water“ and its various shapes and properties

Hydraulic

Formerly: The science of fluid flow through pipes, channels and basins.



Modern definition: Transmission and control of forces and movement induced by fluids.

Fluids

Fluids, gas and steam

Liquids

Substances for the transfer of energy, like water, emulsions (water and oils), mineral oils, bio-oils and synthetic fluids.

Fluid Technology

Technology of mechanical properties of fluids (hydromechanics, aero-mechanics)

Hydromechanics

Technology of mechanical and physical properties and reactions of fluids in both static (hydraulicstatic) or in motion (hydraulickinetic).

Hydrodynamics

Generic term for hydraulicstatic’s and hydraulickinetics

Hydrostatics

Mechanics of static fluids (equilibrium in fluids), also: science of generation and transmission of forces and performance through static pressure of a given liquid.

Hydrokinetics

Science of mechanical and physical laws of fluids in motion and the resulting forces and performance.

1.2

14

Important notions

Typical applications of hydraulics

Industry

• Machine tools



• Injection moulding machines



• Presses



• Iron- and steel factories, rolling mills



• Nuclear and other power plants



• Mining industry

Mobile Technology

• Excavators and cranes



• Construction works, agriculture, forest industry



• Cars, trucks, railway

Ship Building Industry

• Rudder blade adjustments



• On-board cranes



• Bow-gates



• Bulkhead slides

Off-Shore Technology

• Hydraulic rams



• Sea floor mills



• Wave compensators

Public Water Ways

• Locks and weirs



• Bridges



• Ship lifting systems

Custom Made Machines

• Custom made machines



• Pilot operated aerials



• Robotics and handling technology



• Testing machines



• Aeronautics, astronautics

Aerospace Industry

Special requirements due to highly specialized technology

1.3

Types of energy transformation (comparison) Hydraulics



Pneumatics

Electricity

Mechanics

Source of energy E-Motor E-Motor Mains (Drive / Motor) Combustion engine Combustion engine Battery Hydraulic accumulators Pressure container

E-Motor Combustion engine Load Spring tension

Elements for Pipes, hoses Pipes, hoses Electric wires or cables, energy transmission Magnetic fields

Mechanical parts, levers or cranks, shafts

Source of energy Fluids Air Electrones

Solid and elastic bodies

Performance High pressures Low pressures Small forces, (E-motor to great forces Small forces hydraulic motor small design Medium design relation 1:10) tall design

Large pressure, tall design (quite often more advantageous compared to hydraulic solutions)

Dynamic Excellent Moderate Good variable adjustments by means of pressure by means of pressure electric control system (acceleration, and volume flow and volume flow deceleration)

Good

Power output Linear and rotational movements via hydraulic cylinders and hydraulic motors

1.4

Linear and rotational movements via pneumatic cylinders and motors achievable

Mainly rotational move- Linear and ments, linear movements rotational through magnets: movements Small forces, short lifts, prob. linear motors

Advantages and disadvantages of hydraulics

Like with other kinds of drive units, hydraulic systems have their advantages and disadvantages: Advantages: • tremendous forces (torque) can be easily transferred even with relatively small hydraulic units. • full load movement is possible right from the beginning (starting point). • stepless control of velocity, torque and lifting power. • equally suitable for quick and rapid movements and extremely slow precision movements. • simple overload protection and relatively easy energy storage by means of accumulator technology. • high economic efficiency due to simple centralized drive systems in combination with decentralized transformation of hydraulic energy into mechanical energy. Disadvantages: • due to compressibility of fluids caused by air contamination in the hydraulic circuit, pressure shocks and uneven movements in the system may occur. • temperature changes have an influence on the viscosity. This can cause among other things an increase of losses due to leakage and orifice blockage. • loss of efficiency due to friction in fluids. • high precision in production of hydraulic units is essential.

15

1.5

SI-Basic units

Physical description

Unit



Name

Symbol

Length

Metre

m

Mass

Kilogram

kg

Time

Seconds

s

Electric current

Ampere

A

Celvin

K

Substance

Mol

mol

Brigthness

Candela

Cd

Pascal

Pa

Temperature*

Pressure

* The iron and steel industry maintains the Celcius-temperature scale. Prefix

16

Abbreviation

Power -12

Pico

p

10

Nano

n

10

Micro

m

10

Milli

m

10

Centi

c

10

Deci

d

10

Deca

da

10

Hecto

h

10

Kilo

k

10

Mega

M

10

Giga

G

10

Tera

T

10

-9 -6 -3 -2 -1

2 3 6 9

12

2

Physical basics

Hydromechanics is the basic principle of hydraulics. Hydromechanics is the study of mechanical and physical properties plus the behaviour of static and dynamic fluids. Hydrostatics: Study of static fluids and their equilibrium in a hydraulic cylinder or a press. Significant property: Pressure [p] Pa Hydrodynamics: Study of dynamic fluids, like the conversion and translation of energy flows in turbines of hydropower plants or the ­ hysical behaviour of pressure fluids in valves and pipes.­ p Significant property: Volumetric flow / volume flow [Q] l/min

��������������

������������

�

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�

� Fig. Overview: hydrodynamics

17

2.1

Hydrostatics (physical properties of „pressure“)

As mentioned before, hydrostatics deals with static fluids and gases. Other sources refer to hydrostatics as the study of the state of equilibrium. Imagine a static cuboid, which is exposed to a certain pressure force Fn. A state of equilibrium is obtained due to the counter pressure (back pressure) p of the level surface on which the cuboid is resting. Hydrostatics is an important part of the vast field of hydraulics since it also deals with required forces or pressures. One example is a hydraulic cylinder. A certain force is generated inside the cylinder acting on the piston surface, which has to override an external resisting force. As a result an operation is carried out, like the pressing (moulding) of an ­component. According to DIN 24312 pressure p is the quotient of standard force Fn, which acts vertically on a given surface, and surface A.

Pressure[ p ] =

Newton [ F ] = Pascal Meter 2 [ A ]

In hydraulics, a pressure designation in Pascal [Pa] is not common:

N = 0.00001 bar m2 1 bar = 10 5 Pa 1 Pa = 1

pressure force F

cuboid level surface

back pressure p Fig. Hydrostatics

18

2.1.1

Hydrostatic pressure

Pressure is not only created by external forces. The mass of a body can generate a weight force, which in turn generates a gravitational pressure. Inside a column of fluids pressure is generated by the weight of the fluid mass above a given surface. The pressure depends on: • height of column: [h] = m • density of fluid: [r] = kg/m3 • gravitational acceleration: [g] = m/s2 This is the formula for hydrostatic pressure:

p = h  r  g [ Pa ]

Example: Which pressure is generated by a fluid column of hydraulic oil with a height of 10 m (r = 0.85 kg/dm3)?

p = 10  0.85  1000  9.81 = 83385 1 bar = 100 000 Pa 83385 Pa = 0.83385 bar

N = 83385 Pa m2

Solution: The hydrostatic pressure is a function of height, not of the shape of the vessel. If you fill vessels of different shape but with equal floor size and the same fluid and filling height, the resulting forces on the floor are equal. This is also called hydrostatic paradox. That is:

if

A1 = A2 = A3

then

F1 = F2 = F3

�

��

��

��

��

��

��

Fig. Hydrostatic paradoxon

19

2.1.2

Further properties of pressure

In addition to „pressure“ other terms are used in physics and will be explained in the following. Absolute pressure: The absolute pressure indicates the pressure compared to a vacuum. Pressures are examined and added, like: ­absolute pressure = relative pressure + atmosheric pressure. In a vaccuum the absolute pressure = 0. Relative pressure: The relative pressure signifies a relative pressure relationship. It describes the pressure difference between two different actual states. Atmospheric pressure: Atmospheric pressure exists all over the world. It is generated by the masses of air above us and differs from place to place. This is usually due to the geographical altitude of a location in relation to the surface of the oceans. The higher you are, the less air mass is above you and consequently the atmospheric pressure decreases with increasing height. Thus the atmospheric pressure is equal to the gravitational pressure, generated by earth‘s atmosphere.

atmosphere

upper fluid level

relative pressure (gravitational pressure)

absolute pressure

atmospheric pressure (gravitational pressure)

This effect is used in altimeters by measuring the air pressure. Subsequently the altitude can be calculated.

lower fluid level Fig. Overview: pressure terms

20

2.1.3

The law of Pascal

Due to the comparatively high pressures, which are used in modern hydraulic units, the gravitational pressure can be neglected. Thus it follows that the pressure must be equally high at all places. The system pressure can be calculated with Pascal‘s law. Pascal‘s law states: „That a confined fluid transmits externally applied pressure uniformly in all directions. More exactly, in a static fluid, force is transmitted at the velocity of sound throughout the fluid. The force acts normal and vertically on all surfaces“. Two more principles can be derived from that: • the principle of the translation of forces • the principle of the translation of pressure Example: What pressure p is created in a container if a force F = 1 to acts on a piston with a surface A = 20 cm2? F: Force in daN A: Surface in cm2 p: Pressure in bar Formula:

p=

F A

p = bar F = daN A = cm2 p = 1.000 daN/20cm2 p = 50 bar

force F

rod cylinder road area A pressure tank

pressure p Forces act vertically on the internal walls of a vessel

Fig. The law of pascal

21

2.1.3.1 Power conversion Let us have a look at two pistons with different active areas, which are connected with each other. A force F1 acting upon area A1 causes a uniform and even dispersion of pressure within the fluid chamber (Pascal‘s law). Consequently a ­resulting force F2 acts upon the active area of the piston A2. Generically:

p=

F A

In our case:

p1 =

F1 A1

and

p2 =

F2 A2

whereas p1 = p2 according to Pascal´ss law Consequently:

F1 F2 A = or : F2 = F1  ( 2 ) A1 A2 A1 The ratio of the forces is equal to the ratio of the active areas. This means that the larger force acts upon the larger ­surface. The path length of the piston is reciprocal to the active piston areas. With a piston ratio of 2:1, the smaller piston moves twice the distance s1 than the larger piston (s2). In practice we can find the conversion of power in: • manually operated lifting platforms • hydraulic lifting platforms for cars • presses Example: Please determine which piston generates a greater force? Surface A1 with a diameter of 30 mm, surface A2 with a ­diameter of 50 mm and a constant pressure of 50 bar. Formula:

F1 = p  A1

F2 = p  A2

F1 =

F2 =

��

�� �� ��

��

��

��

�� 22

Fig. Power gear ratio

2.1.3.2 Pressure conversion In analogy to power conversion there is also the possibility of pressure conversion. Let´s have a look at the figure further down. Two pistons with different active areas are connected solidly to each other by a rod. If pressurep acts upon active piston area A we get force F: the force F.

p=

F => F = p  A A

The force F generates with the ring surface A2 the pressure p2:

F = p1  A1 = p2  A2 => p2 = p1 

A1 A2

Thus we have a pressure conversion where the pressure ratios are reciprocal to the surface (area) ratios. This means that the higher pressure can be found on the side of the smaller area (surface). In practice we can find pressure conversion in: • pneumatic-hydraulic pressure intensifiers • sequential differential cylinders • brake power assist unit (brake force booster)

��

�� �� ��

��

��

� ��

��

� Fig. Pressure conversion

23

2.2

Volumetric flow / volume flow (hydrokinetics)

In hydraulics not only power provided by a cylinder is of great importance but also the velocity and the efficiency of the control of the fluid flow. Therefore we should also look at the laws of hydrodynamics, also known as the science of flowing and moving fluids. Flow in a hydraulic unit is called volume flow Q (technical term). It indicates the volume of fluids, which flows through the system in a certain time unit (sec). A Frenchman Joseph Michel Montgolfier gained first results in the area of hydrodynamics already in 1796. Based on his findings he developed a hydraulic water ram, a pump, with which it was possible to transport water to a higher altitude. Water from a reservoir flows through a pipe into the hydraulic ram. Initially the buffer valve is open and the shutter valve closed. The flow velocity increases, when the water passes through the buffer valve. Once a certain velocity has been reached the valve shuts suddenly. Thus the pressure before the valve is increased, which causes the shutter valve to open. The pressure surge pushes water into a boiler (pressure vessel). The air inside the vessel is compressed and now can push the water via a standpipe in a reservoir on a higher level. After the water has accumulated in the vessel, the pressure inside the pipe leading to the ram decreases again. The buffer valve opens and the shutter valve closes. Since all valves are back in the starting position, the whole process can start from the beginning. The name of the pump (in german: widder = ram ) is derived from its characteristic noises made by the opening and closing of valves. People at the time were reminded of the noises made by rams, hence the name of that pump.

plug / stopper

flatter valve / shutter valve

boiler

buffer valve

hydraulic water ram Fig. Principle of a hydraulic water ram

24

2.2.1

The law of volumetric flow

V t V=As As Q= t s v= t Q = A  v = A1  v1 = A2  v1 Q=

Volume flow Q is given by the volume of fluid V divided by time t. Liquid volume V is itself given by area A times length s. If A • s is substituted for V, Q is then given by: Distance s divided by time t is velocity v. Flow Q hence equals the cross-sectional area of the pipe A multiplied by the velocity of the liquid v.

��

��

��

�

��

�� ��

������� Fig. The law of volumetric flow

2.2.2

Flow phenomena and flow patterns

Inside pipes two different types of fluid flows occur: • laminar flow • turbulent flows The flow type depend on: • the cross-section of the pipe • the velocity of the fluid flow (volume flow) • the viscosity of the fluid The change from laminar to turbulent flow occurs at the so-called Reynold‘s Number:

Re =

vd mm 2 / s u

Formula:

Re = 2320 This number is only valid for round and smoothe pipes with even surface.

25

2.2.2.1 Laminar flow If the flow velocity is low, the fluid particles flow in „lines“ (hence „laminar flow“). The fluids generate „flow levels“, which move with different velocities. If the velocity is at it‘s peak in the center, the „outer“ level sticks to the interrnal surface of the pipe. There is a certan ­friction, which causes loss of pressure, which in turn can be calculated by means of the Bernoulli-equation.

��

���� Fig. Laminar flow

2.2.2.2 Turbulent flow If the velocity increases beyond a certain level, the flow gets turbulent. Turbulences occur as well as fluctuations of v­ elocities. The Brit Osbone Reynolds was the first to point out that this change from laminar to turbulent flow could be described by a parameter (Reynold‘s Number). There exists a certain value, which is responsible for a particular behaviour of fluid flow.

Fig. Turbulent flow

26

2.2.3

Fluid friction and pressure loss

Hydraulic energy cannot be transferred without loss through pipes and other componets like valves and filters. On the insides of pipes, valves and filters friction is generated, which in turn creates heat, which – of course – is a loss of ­pressure (pressure difference between inlet and outlet of a component). Pressure differnces are usually indicated by Dp. The quantity of pressure loss usually depends on: • the viscosity of the fluid • the length of the pipes • the coss-section of pipes • the „roughness“ of the inside of the pipes • number and design of pipe bends • the velocity of flow and • design and number of valves and filters

��

��

� �� �� Fig. Pressure loss due to friction inside pipes

27

2.3

Hydraulic energy

Like a combustion engine in a vehicle a hydraulic aggregate is usually called a drive unit. The aggregate provides the energy, which is used by the actuator (like a cylinder) to do its job. The operations are controlled by valves or the energy is led to individual actuators. Together with control valves and the actuators we can call this assembly a hydraulic system. The design of a hydraulic aggregate depends on the supposed efficiency, which in turn is a result of pressure and volume flow. The energy losses of the aggregate are described by ηges (efficiency). Thus we get an overall efficiency Pan of an aggregate of:

Pan =

pQ [ kW ] 600  h ges

Fig. HYDAC aggregate solution

28

2.3.1

The law of conservation of energy

The law of conservation of energy is one of the core issues in physics. It states, that the total energy in a closed system always remains constant. A closed system is a system without any interdependancy to the outside world. There is no exchange of energy, matter and information. Within a closed system one form of energy can be transformed into another, like electric energy to warmth. Wheras it is impossible to generate or destroy energy within a closed system. Thus the equation:

Eges before = Eges after For example in a hydraulic unit on one side you have the electric energy of a motor, which drives the unit. On the other side you use the energy to move loads. Inside the unit electric energy is transformed into hydraulic energy, which is ­transformed into mechanical energy. During these transformations part of the energy is transformed to warmth. However during these transformations the law of conservation of energy is always valid.

������

��������

���������

�������

Fig. Closed system

29

2.3.2

The Bernoulli Principle

The total energy of a flow of liquid does not change, as long as energy is not supplied from the outside or drained to the outside. Neglecting the types of energy, which do not change during flow, the total amount of energy is made up of ­potential energy, kinetic energy and pressure energy. The potential energy depends on the height of head of liquid and on static pressure and kinetic pressure. The kinetic pressure depends on the flow velocity and back pressure. Considering both the continuity equation and the Bernouilli equation the following may be deduced: If the velocity increases as the cross-section decreases, movement energy increases. The following might help as an explanation: Since the total energy remains constant, the potential energy and/or the pressure energy must become smaller, if the cross-section area is reduced. There is no measurable change in potential energy. The static pressure, however, ­changes, dependent upon the dynamic pressure, i. e. dependent on the velocity of flow.

g h +

pst v2 + = constant p 2

pges = pst + r  g  h + v 2 g

r  g  h = Pr essure caused by hight offluid column

r 2 v2 

pst = static pressure

r = back pressure ( dynamic ) 2

Fig. The Bernoulli Principle The hight of the fluid columns is a measure for the pressure existing at precisely this point.

30

5

Basics of hydraulic symbols

5.1

General remarks

For hydraulic circuit diagrams graphic symbols are required. They are standardized in DIN ISO 1219. In order to read and understand circuit diagrams it is necessary to learn the symbols and their function. The symbols don‘t tell you anything about the design and construction of the components, they only illustrate their function. An overview of the symbols of DIN 1219 you find in the appendix.

5.2

Symbols

With the help of the following basic symbols you can draw a major part of circuit diagrams.

5.2.1

Basic symbols (excerpt)



Lines, main line, electrical line



Internal or external control line, drain line, leakage line



To group two or more components in a sub-assembly



Mechanical connection (shaft, lever, piston rod)



Pump, motor (circle ∅), energy transfer unit



Measuring device (circle ∅ ¾ )



Check valves, rotary connection, mechanical pivots, rollers (circle ∅ ⅓)



Control elements, drive unit



Preparation devices (filters, separators, lubrication devices, heat exchangers)



Cylinders, valves

55

5.2.2

56

Function symbols (excerpt)



Shows direction of flow and operating medium (filled = hydraulic, open = pneumatic)



Arrows, (straight), linear movement, path and direction of flow through a valve, direction of heat flow



Arrows (curved), rotational movement, direction of rotation viewed on shaft end



Adjustability in pumps, motors, springs, selenoids



Closed path or connection



Linear electrical positioning elements acting in opposition



Spring



Throttle

5.2.3

Operation symbols (excerpt)



Push button



Push button, pull-out knob



Spring repositioning



Roller shaft (two directions)



Electrical, 1 winding



Internal control channel



External control channel



Hydraulic operation (1-stage, 2-stage)



Pneumatic operation (1-stage)

57

5.3

Symbols for hydraulic motors and hydraulic pumps

Hydraulic motors and pumps are represented in a circuit diagram by a circle. The triangles inside the circle tell you whether it is a pump or a motor, how many connectors and the direction of flow. For inlet and outlet you need two connectors. For the transfer of the energy a shaft is drawn to the motor or pump.

5.3.1

Hydraulic motors

Hydraulic motors transform hydraulic into mechanical energy. The turning movements are illustrated by triangles (they point to the inside g Motor) ����������

�������

�����

Fig. Hydraulic motor with a single flow direction

5.3.2

Fig. Hydraulic motor with two flow directions

Hydraulic pumps

Hydraulic pumps transform mechanical into hydraulic energy (triangles point to the outside g pump). An arrow < 45° ­indicates that the volume flow can be adjusted.

Fig. Hydraulic pump with a single flow direction and adjustable volume flow

5.3.3

Fig. Hydraulic pump with two flow directions

Hydraulic pump and hydraulic motor

Hydraulic pump and hydraulic motor can function as a unit and work both as a pump or motor (triangles point in the same ­direction g Hydraulic pump / -motor).

Fig. Hydraulic pump / -motor with one direction of flow

5.3.4

Fig. Hydraulic pump / -motor with two directions of flow

Direction of rotation

The direction of rotation of a hydraulic pump and motor is indicated with a curved arrow on the shaft.

������������

�����

(1 direction)

Fig. Hydraulic pump with one direction of flow constant displacement volume and one direction of rotation

58

(2 directions)

Fig. Hydraulic motor with one direction of flow, constant displacement volume / swept volume two directions of rotation

5.3.5

Hydraulic pumps and hydraulic motor classes

An additional motor is required to drive a pump, which is connected with shaft to the hydraulic pump or the hydraulic motor.

�

�

Fig. Hydraulic pump with one direction of flow, electro motor and one direction of rotation

5.4

Fig. Hydraulic pump with one direction of flow, combustion engine and one direction of rotation

Symbols for cylinders

Cylinders transform by means of a linear movement hydraulic into mechanical energy.

��������������������������� �������������

������ ���������

��������������

Fig. Simple acting hydraulic cylinder with return spring

5.5

Fig. Double acting differential cylinder

Symbols and naming (example: directional valve)

We take a directional valve to explain the basic symbols and names of hydraulic valves. Directional valves open and shut the hydraulic pipes and facilitate the exchange between pipe connectors. This way ­volume flows and consumers (cylinder, motors) can be controlled. Because of this function directional valves have at least two switching modes and at least two connectors. The symbols are always drawn in a non-operational state with connectors and designation.

5.5.1

Design of a directional valve

Switching Modes (Number of squares) 0 = neutral position a, b = functional position

�

�

�

2 switch positions

�

�

3 switch positions

Number of connectors and connections within a sitching mode, i. e.:



2 connectors

3 connectors

The arrows inside the squares show the possible direction of flow. These signs

4 connectors indicate that the flow is shut off.

Since there exist manifold demands on the valves there are numerous connections between the various connectors.

59

Example:

�

�

Flow shut off

Flow in two directions possible

�

Flow from P to A, T is shut off.

Which flow directions are opened or shut off depends on the tasks of the system to be designed.

5.5.2

Characteristics and naming of a directional valve

Naming of connectors: P = T = A,B = X,Y,Z =

Pump Tank, return flow Consumers Control connectors

The naming is not laid down in DIN Standard. So other symbols are possible g follow manufactor’s data. Symbols indicate the component always in the neutral position. Example: Connector P is connected with the pump, connector T with the tank and connectors A and B with the cylinder.

��������

�

�

����������������� �

��

�

� �

� �

�

��

������

�������� �����

������������������

� ����

Pronunciation: 4 stroke 3 directional valve

�������������� �������� �������� ���������� ����������������

60

Further examples of directional valves (no operational modes):



2/2 directional valve



3/2 directional valve



4/2 directional valve



4/3 directional valve



6/3 directional valve



Directional valve continually variable (any number of switch positions and intermediate positions)

The operational consequences of the different switch positions becomes obvious when you slide the whole sign against the fixed positions of the connectors.

5.5.3 ��

��

Different centre positions of directional valves � �

�

�

�

�

�

�

�

�

�

�

� �

� � �

� �

�

�

�

�

�

� �

�

A 4/3 directional valve with rotating central position is used to control double acting cylinders. If fixed displacement pumps are applied no heating of fluids occurs. If differential cylinders are used, the connection must not be exposed to pressure. Otherwise a creeping of the piston due to leakage oil transfer cannot be excluded. A 4/3 directional valve with blocked central position (all ports blocked) is used to control double acting cylinders or hydraulic motors. The central position ascertains a stop of the piston in any position (emergency stop, hard stop). Preferably these cylinders are used with cylinders of equal sized surfaces, since in differential cylinders leakage oil transfer from P to cylinder pipes might cause creeping of the piston. A 4/3 directional valve with floating mid-position (both cylinder lines are connected to tank by full flow diameter of the piston valve, P blocked) makes a soft halting possible. However the piston will move a little bit longer. Pressure relief via A, B and T prevents a creeping of piston in differential cylinders. This type of directional valve is also used in vertically built cylinders, which are safeguarded by pilot controlled check valves.

�

�

��

��

� �

��

��

�

� �





�

��

��

A 4/3 directional valve with throttled floating mid-position (both cylinder lines are connected to spool by small notches to tank, P blocked) are used to control double acting cylinders or hydraulic motors. A throttled floating mid-position results in a softer halt than could be obtained with valves with blocked midposition. A pressure relief due to the notches prevents a creeping of the piston in differential cylinders. A 4/3 directional valve with continuous flow in mid-position (all ports are connected to each other) results in a soft halting, but the piston needs more time to Stopp completely. The fluid flow in fixed displacement pumps occurs pressureless in order to avoid heating. This type of valve is usually only used in cylinders with equal surfaces, since pressure heads and flow resistance in the pipes can cause a creeping of the piston in differential cylinders.

61

5.5.4

Operational methods

So far we have named and discussed a 4/3 directional valve. In order to slide the symbols into the different switch ­ ositions (a, 0, b) the appropriate operational mode has to be added. p Operational modes are drawn on the left or right side of the symbol of a directional valve. The selection depends on the demands on the unit. You have to be careful here, since some operational modes require a return spring to reposition the valve into neutral. In circuit diagrams directional valves are drawn in the neutral position (0), in other words not when they are being ­operated but when they have been repositioned by a return spring.

� �

� �

� �

�

Fig. 4/3 directional valve with two-sided electro-magnetic switch and spring centering In order to keep the directional valve in the neutral position a return spring is attached left and right (spring centring). The directional valve can be moved into position a or b with the help of an electric element. The valve is repositioned with the return springs. Example:

� � �

� �

Fig. 3/2 directional valve with push button / pull-out knob No return spring is required fort this 3/2 directional valve since this valve is moved by mechanical force into the required position (push button / pull-out knob).

� �

� �

�

Fig. 4/2 directional valve with an electric element and return spring Here we need a return spring. The valve is switched with the help of an electric element into position b and returned to position a with a return spring.

62

5.6

Further symbols of hydraulic valves

5.6.1

Pressure valve

Pressure valves control the pressure within a hydraulic unit. They are symbolized by a single square with an arrow. The position of the arrow tells you, whether the connectors are connected with each other or not. The throttle cross section can be adjusted variably.

Fig. Throttle cross section open

Fig. Throttle cross section shut

Two main groups of pressure valves are called pressure limiting valves (to limit the pressure inside a unit) and pressure reducing valves (to reduce the operational pressure to a certain predetermined level). Pressure limiting valve: The control line sits before the valve. Therefore the valve can be controlled when the opening pressure increases by opening outlet (2) to the tank against an opposing force (adjustable spring force). ������������

���������� ������������

�

�

�����������������

Fig. Pressure limiting valve (Control line before valve, throttle cross section shut-off) Pressure reducing valve: Pressure reducing valves feature the control line after the valve. With an increasing opening pressure the valve can keep the pressure constant no matter how high the load. Outlet (2) needs to be shut off against an opposing force. Therefore pressure reducing valves are also called pressure control valves. ������������ �

���������� ������������

�

�����������������

Fig. Pressure reducing valve (Control line after valve, throttle cross section open)

5.6.2

Check valves

Check valves are valves which permit only one direction of flow. ���������

They where also called „now return valves“.

Flow is shut off by a locking element (i. e. ball, cone). Flow from A to B is prevented by a locking element. Volume flow is possible from B to A if the pressure before the locking element is higher than the spring pressure.

������ �

�

��������������� ������ Fig. Spring check valve

63

5.6.3

Flow control valve

Flow control valves influence the volume flow by changing the cross section of flow carrying device. In a flow (circuit) diagram it is drawn like this:

�������� �

�

���������� Fig. Adjustable throttle valve By reducing the cross section of a pipe you „throttle“ the volume flow. Since flow control valves and check valves are usually only used for one flow direction another check valve is added to the system. �

�

Fig. Adjustable throttle check valve

5.7

Further general hydraulic symbols

5.7.1

Symbols of storage and processing of pressure fluids

Symbols for storage components are usually ovals:

Examples:

Fig. Hydraulic accumulator (only for upright positions)

Fig. Closed accumulator (with three lines)

Symbols for processing the pressure medium are drawn like this:

Examples:

Fig. Separators

64

Fig. Cooling devices

Fig. Filters

Fig. Heating devices

5.7.2

Symbols for checking and measuring instruments

Additional components for hydraulic units, like control and measuring instruments are represented by circles (∅ ¾):

Examples:

Fig. Pressure control

Fig. Temperature control

Fig. Volume flow indicator

65

6

Basics of hydraulic circuit diagrams

6.1

The circuit diagram (general)

A hydraulic circuit diagram is a graphic representation of all components and their connections of a hydraulic unit for which standardized symbols are being used. Mandatory standard is DIN ISO 1219. A circuit diagram is always drawn – and also has to be read that way – in the direction of the volume flow starting, for example, with a hydraulic pump and ­finishing with the outlet port user, cylinder, motor. The hydraulic symbols should be drawn horizontaly, the lines ­preferably without crossings and directly. Components are always represented in neutral position, neither exposed to pressure nor volume flow. The basic symbols can be rotated and mirrored. They should display all necessary characters for listing relevant data like connectors, pressure, volume flow, electric connections and component settings You will find more information for drawing hydraulic circuit diagrams in DIN ISO 1219-1.

Fig. The circuit diagram

6.2

Assembly of a basic power unit

The power unit is necessary for the power supply of the whole system. It consists of a tank (3) for the hydraulic fluid. The pump (1) is connected over a suction hose with the tank an is ­activated with and electric motor (2). That is where the volume flow for the system is generated. The check valve (4) is needed to stop the refluent oil when the motor is stopped, so that the motor can’t run ­backwards and will be destroyed. To secure the system from over pressure, we have the pressure control valve (5). This valve leads the volume flow directly to the tank, if the load doesn’t need it. The manometer (7) shows the system pressure. In the return line of the system (T) is a return filter (6). It cleans the fluid from caused contamination.

�

�

�

�

� �

� � �

� Fig. Circuit diagram: assembly of basic power unit

67

6.3

Actuating users with directional valves

To actuate the cylinders, we use a 4/3 directional valve in this circuit. The volume flow from the power unit can be r­ edirected with this valve. In position 0 the directional valve, the cylinder stays in his position and the volume flow is lead over the pressure control valve to the tank. If we supply a voltage to the solenoid y2, the directional valve switches in position b. The volume flow runs from P to A and the cylinder extends. The fluid from the cylinder flows over the ­connectors B to T through the return filter into the tank. Switching the directional valve with solenoid y1, the fluid flows from P to B and in the return line of the cylinder from A to T. The cylinder retracts.

��������

�

�

����������������� �

��

�

� �

� �

�

��

������

�������� �����

������������������

� ���� Fig. Circuit diagram: 4/3 directional valve

68

6.4

Application of pilot controlled check valves in load holding circuits

One of the objectives of check valves is that they have to hold a load over a certain period of time in a given position, like with working platforms. If pulling loads act upon cylinders and hydraulic motors, for example with cranes, a tight sealing of the line exposed to the load pressure is required in order to prevent a slow but continuous sliding down due to leakage. Therefore check valves are being used, which can block volume flow in both directions effectively and which can be ­released easily under certain circumstances.

�

�

��

��

��

�� �

�

��

�� �

�

� Fig. Circuit diagram: twin-check valve (RPDR06)

69

6.5

Speed control of actuators with throttle valves

For speed control of actuators we use different throttle valves. • example 1 with a constant cross-section (orifice) • example 2 with a throttle valve (volume flow can be adjusted) • example 3 with a flow control valve, with a constant volume flow to the user (the necessary directional valves are missing due to better overview)

� �

� �

�

�

� �

�

�

Fig. Circuit diagram: speed control of actuators with throttle valves

70

6.5.1

Feed line control (primary control)

Applied if counterforces occur at the actuator Advantage: Pressure is only exerted directly at the actuator, this means the actuator is only exposed to this pressure. Disadvantage: A pressure relief valve, which diverts an excessive quantity of fluid of the pump back to the tank, must be set to the ­ ighest available actuator pressure in the system. Heat generation is fairly high and the pump has a higher power h ­consumption. The heat due to friction generated by the volume flow passing through the flow control valve is directly transferred to the actuator.

�

�

�

� � �

�

� �

�

Fig. Circuit diagram: feed line control (primary control)

71

6.5.2

Discharge control (secondary control)

Applied with pulling loads at the actuator, so that the actuator cannot run faster than the pump generates fluid flow. Advantage: No back pressure valve is necessary. The friction heat generated by the fluid passing through the flow control valve, is transferred to the tank. Disadvantage: The pressure relief valve has to be set to the highest possible level for the pressure of the actuator (generation of heat). The actuator is exposed to the pressure due to the hydraulic context.

�

�

�

� � �

�

� �

�

Fig. Circuit diagram: discharge control (secondary control)

�

72

Attention: pressure conversion possible!

6.5.3

Bypass control

Advantage: Since the flow control valve limits the flow to the actuator by diverting a certain part of the total volume flow back to the tank, only the pressure as required by the load is built up at the actuator. The heat generated by the passage of the fluid through the valve is transferred to the tank. Disadvantage: The actuator is not hydraulically involved g the actuator could move faster than it is supplied with fluid; a back ­pressure valve might have to be implemented.

�

�

�

� �

��

� �

�

�

� �

�

�

Fig. Circuit diagram: bypass control

73

6.6

Various rapid traverse-controls in differential circuit designs

Rapid traverse-controls (rapid moving of the cylinder) can be achived through adding another power unit (without figure) or by returning the fluid from the outlet cylinder chamber to the inlet of the cylinder.

�

Attention: pressure conversion possible!

Examples:

�

�

� �

�

�

�

�

extend

�

�

�

�

�

��

�

�

��

�

�

�

retract

�

�

�

�

�

�

��

�

�

�

retract

��

�

Fig. Rapid feed forward back with intermediate stop

�

�

��

extend

�

�

�

�

retract

Fig. Rapid feed forward back without intermediate stop

�

�

�

�� extend

� �

Fig. Rapid feed forward back with intermediate stop

� �

�

� �

�� �

�

�

�� working feed

�

�

rapid feed forward

�

back

��

retract

�

Fig. Rapid feed forward and working feed forward without intermediate stop

74

� �

� �

�

�

�

�� extend

Fig. Rapid feed forward and working feed forward with intermediate stop

II

Introduction to equipment technology

1

Hydraulic pumps and hydraulic motors

1.1

General remarks

Hydraulic pumps and motors are instruments in which mechanical energy is transferred into hydraulic energy and vice ­versa. In most cases hydraulic motors have the same principal technical design as hydraulic pumps. However leakage oil return in motors goes to the outside as opposed to pumps, where the leakage oil is returned to the suction chamber. Some pumps like constant axial piston pumps in bent axis design can be directly used as motors. Depending on the displacement principle we distinguish several types of pumps and motors, which differ in the designs.

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Fig. Overview hydraulic pumps and hydraulic motors

75

1.1.1

Hydraulic pumps

Hydraulic pumps are hydraulic components in which fluids are displaced and energy provided by electrical motors or c­ ombustion engines is being transformed into hydraulic energy. Pumps take fluids out of a tank or container and displace these fluids through pipes and control and distributing elements to the different drive units, which provide work by transformation of hydraulic energy into mechanical energy. Depending on the components, which transport the fluids the pumps are called rotating circulation pumps and oscillating piston pumps. Circulation pumps:

Gear pumps External gear pumps

Constant displacement volume

Up to 250 bar

Internal gear pumps

Constant displacement volume

Up to 315 bar

Gear ring pumps

Constant displacement volume

Up to 100 bar

Screw pumps

Constant displacement volume

Up to 175 bar



Vane pumps Fixed vane pumps

Constant displacement volume

Up to 175 bar

Rotary vane pumps

Constant displacement volume

Up to 175 bar

Vane pumps

Displacement volume (constant and variable), pressure adjustable in the NELL stroke pump

Up to 125 bar

Screw pumps

Constant displacement volume

Up to 175 bar

Thrust pumps: Piston pumps

76

Hand pumps

Constant displacement volume

Up to 700 bar For highest pressures

In-line piston pumps

Constant displacement volume

Radial piston pumps

Constant displacement vol. (also adjustable) Pressure control up to approx. 630 bar

Swash plate pumps

Constant displacement vol. (also adjustable)

Pressure / output control up to approx. 400 bar

Bent axis pumps

Constant displacement vol. (also adjustable)

Pressure / output control up to approx. 400 bar

1.1.2

Hydraulic motors

The job of hydraulic motors is simply to transform hydraulic energy as provided by the pump into mechanical energy. This transformation will produce a certain torque. Quite often the type classification of hydraulic pumps is also valid for hydraulic motors. The demands on the performance of hydraulic motors, like: • maximum operational pressure • torque • life-time circle • dirt resistance • maintenance • pulsation • noise level • spare parts • weight • size • build-in possibilities • cost are the same, which are placed on to hydraulic pumps. In this case one does not speak of volume flow per rotation but of capacity per rotation. The most important characteristics of hydraulic motors are the torque delivered to the shaft and the drive speed range. Classification of hydraulic motors: The drive speed range plays a major role in the discussion about the right hydraulic motor. Especially the lowest possible drive speed with which a hydraulic motor can deliver the torque to the shaft in a uniform steadiness is important. Therefore hydraulic motors have certain advantages over electrical motors or combustion engines: • advantageous weigh to performance relationship • compact design • reasonable cost • stepless drive speed adjustment Motors are distinguished as follows: 1

- 150 min-1

Slow

Piston motors

50

- 750 min-1

Medium

Vane motors

Fast

Gear and screw motors

300

-1

- 3.000 min

When designing a hydraulic unit with a hydraulic motor it is advisable to be able to use the motor as a pump by reversing the direction of flow on a short or long term basis.

77

1.2

External gear pump and motor

Description: Volume is created between the gears and housing.

V =m z b h p m

=

Module

z

=

Number of gears

b

=

Width of gears

h

=

Height of gears

p

=

Pressure

Function: By turning the upper cog wheel in the direction of the arrow the lower cog wheel is turned in the opposite direction. ­German engineers talk about the „combing“ of the two wheels. The fluid is taken out of the suction chamber and ­propelled through the gears and displaced on the pressure side. Since on the pressure side the cavities between the cogs of the one wheel are sealed by the cogs of the other before they are emptied, the pressure fluid enclosed has to betransported to the outside by means of some drilled holes in the housing. Depending on the manufacturer the fluid used for the ­lubrication of the bearings and let out at the upstream side. At the same time a pressure balance is achieved at the ­bearing brackets and the efficiency thus improved. The pumps are manufactured from different materials. Special designs according to customers' requirements are also available. Certain permissible operational parameters, like pressure upstream, time, peak pressure (downstream), range of viscosity, contamination and fluids are very different from manufacturer to manufacturer. There are different designs in relation to operational parameters, like time, reverse operation and interruptions. The pumps are either single pumps, multiple pums and combinations thereof. Until the last century a version with three wheels was built. They were used in lubrication units of water plants and combustion engines. Repairs in these pumps are economically not feasible. Therefore no spare parts are available. Application: The external gear pump is a simple, robust and inexpensive construction with a high degree of reliability. However they feature a high degree of irregularity and a high noise level. Technical Data: • constant displacement volume • up to 250 bar • can be used as a hydraulic motor

78

External gear motor: The external gear motor is not suitable for low speed drives, since it features a rather bad total torque for low speeds. A speed reduction gear unit can be built in and is in comparison to piston motors (slow running) rather inexpensive. In order to secure a smooth start of the engine the required load torque has to be limited. gears shaft

�

�

housing Fig. External gear pump

79

1.3

Internal gear pump and motor

Description: Volume is created between the gears, housing and spacing / sealing element.

V =m z b h π m

=

Module

b

=

Number of gears of the innermost cog wheel

z

=

Width of gears

h

=

Hight of gears

Function: By turning the innermost cog wheel in the direction of the arrow the outside cog wheel is turned in the same direction. The rotational movement causes the cog wheels to divert, so that the spaces between the gears are set free and can be filled with the fluid from the suction chamber. The pump is available in different designs and with different pressure ranges. Pressure compensation and increased efficiency is solved in different ways by the various manufacturers. Multiple pumps and pump combinations are available, also special designs for particular requirements and locations (mobile sector, ­offshore etc.). Repair kits are also available. Whether a repair is economically feasible or not, cannot be discussed here. Application: Noise reduced pump with low degree of irregularity Technical Data: • constant displacement volume • up to 315 bar • can be used as a hydraulic motor shaft

internal gear

outside gear

sickle housing Fig. Internal gear pump

80

1.4

Ring gear pump and motor

Description: The rotor has one gear less than the internally geared stator. Planetary movement of the rotor.

V = z  ( Amax - Amin )  b z

=

Number of gears of the rotor

b

=

Width of gears

A

=

Area

Description: The ring gear pump works like an internal gear pump. The out­side cog wheel has one more gear than the innermost cog wheel. The displacement of the fluid is created by the fact that the gears of the innermost cog wheel when turning always touch the ­outside wheel so that hermetically sealed chambers are formed, in which the fluid are transported from the suction side to the pressure side. For lubrication and other low pressure units, like filters, cooling circuits etc. a simple pump with fixed side panels is available. For higher pressure ranges there are models with one or two axial flexible side panels. A lot of effort was put into a pump design with limited fuel consumption. Pumps in standard versions cannot be repaired. Application: • pumps with lower disturbance whilst running and a compact, space saving design Technical Data: • constant displacement volume • up to 100 bar • different pressure stages • can be used as a hydraulic motor Ring gear motor: This type of motor has the highest performance in relation to its dimensions. The motor can be applied where low drive speeds are required, if a high degree of irregularity is not important.

internal gear

outside gear

����

����

shaft

housing Fig. Ring gear pump

81

1.5

Planetary srew pump and motor

Description: Ring gear pumps as discussed on the previous page can be used as a motor provided some changes in the design are made. It is then called planetary screw motor. Function: By using a commutator and a control plate with controls slots you can achieve 56 displacement events per stroke. Since a planetary screw motor features such a high capacity or swept volume it belongs to the slow running motor types. ­You will find very few designs and manufacturers of this pump on the market. A planetary screw pump is usually employed in polymer production as a dosage pump. In hydraulics it is mainly used as a motor unit, because it is a typical slow running aggregate with slow starting features when exposed to heavy load. Compared to their size these motors offer very high torques. rotor

control plate

�

outside rollers shaft

housing hollow wheel inside rollers

commutator rings Fig. Planetary screw motor

82

�

1.6

Screw spindle pump and motor

Description: Volume is created between the spindles and the housing. Function: Two or more spindles one is driving and the other been driven are placed in a housing. With the rotational movement chambers are created, which are limited by the housing and the spindle shaft. The fluid filled chambers continually move from the suction side to the pressure side when the pump is running. Application: • this reduced pump has a high degree of synchronism accuracy and continuous pulsation free volume flow. • the rotational parts are counter balanced to a high degree. Furthermore due to the design no extremely pressurized fluids can be found in the unit. Therefore high drive speeds can be obtained and large volumes can be transported despite the small dimensions of the pump. Screw spindle motor: Screw spindle motors are usually not used as a drive unit. However they are applied as sensors for the volume flow.

suction side pressure side

driven spindler

driving spindle

Fig. Driven spindles

83

1.7

Single chamber vane pump and motor (single stroke and pressure controlled)

Description: Volume is created between the circular stator, rotor and vanes.

V =2 π b e D b

=

Width of vanes

e

=

Eccentricity

d

=

Internal gear stator

Function: A single chamber vane pump features a variable rotor, which runs inside a circular stator and single or double vanes fixed into slots. The stroke movement of the vanes is limited by a ring with a circular internal form. The displacement ­chamber consists of the rotor, two vanes, the inside of the ring and the control plates. The volume inside the ­displacement chamber is proportional to the distance between stator and rotator. Caused by the rotation of the rotator the volume inside the displacement chambers changes continually. When the volume decreases the fluid inside the displacement chambers is compressed and the impact direction reversed. With this pump design the displacement volume can be adjusted by changing the eccentricity of the stator to the rotor. The maximum pressure can also be adjusted (zero-stroke). The increased pressure caused by the system presses on the inside of the stator, thus causing a force in the direction of the spring. Once the predetermined spring force is reached, which equals a certain pressure, the eccentricity of the stator is diminished. Just the required amount of fluid is produced. If the consumer does not require more fluid, the predetermined pressure point is reached and no more flow occurs. Application: • stationary pump • adjustable in pressure, volume and output • zero-stroke effect • bypass filter installation Technical data: • displacement volume constant and adjustable • pressure adjustable as zero-stroke pump • up to 125 bar • can be used as a hydraulic motor

stator rotor vane adjustment unit

� Fig. Single chamber vane pump

84

1.8

Double chamber vane pump and motor

Description: Single chamber vane pumps have only one displacement event per rotation. Since the internal curvature of the stator has a double eccentric cam design double chamber vane pumps obtain two displacement events per rotation. b

=

Width of vane

K

=

Vane strokes per rotation (= 2)

Function: The only difference between single and double chamber vane pumps is that the stator of the double chamber vane pumps has a double cam form internal surface. The effect is that each vane carries out two strokes per rotation.

�

� Fig. Double chamber vane pump

85

1.9

Radial piston pump and motor with eccentric shaft

Description: The rotating eccentric shaft causes radial oscillating piston movements to be produced.

V = dk 2 

π  2e  z 4

z

=

Number of pistons

e

=

Eccentricity

dk

=

Diameter piston

Function: This is how a valve controlled radial piston pump with eccentric shaft works. The drive shaft is eccentric to the pump elements. The pump elements consist of piston, cylinder sleeve, pivot, compression spring, suction valve and pressure control valve. The pivot is screwed into the housing. The piston is positioned with a slipper pad on the excenter. The compression spring causes the slipper pad to always lie on the excenter, when the eccentric shaft rotates and the cylinder sleeve is to be supported by the pivot. There are different designs and pressure stages depending on the manufacturer, but always with an odd number of ­pistons (3 / 5 / 7 / 9 etc.). The connection of the individual pistons on the pressure side is regulated by means of a check valve in different designs. For example pipes are fitted into the pump or holes are drilled into the housing. On the suction side, the pump has to be fitted always below the oil level. Breathing is mandatory before starting the unit. Thus the ­mounting position is predetermined by the manufacturer, because they determine the position of the breather valve. This pump is very prone to conatamination due to its valve control mechanism and its very narrow orifices. Some pumps of some manufacturers are not suitable for large load changes with high pressure alterations. Problems on the suction side often lead to an immediate destruction of the pump. Most manufacturers provide repair kits. Application: Radial piston pumps are used for high pressure units (operating pressure above 400 bar). In presses, machines for ­processing plastic, in clamping hydraulics for machine tools and in many other applications, operating pressures of up to 700 bar are required. Only radial piston pumps can satisfactorily operate at such high pressures even under permanent use. Radial piston motor: A radial piston pump is a typical slow running unit without swept volume control. Sequential motors generate extremely high torques.

housing dK

spring eccenter slipper pad piston

e

86

Fig. Radial piston pump with eccentric shaft

1.10

Radial piston pump and motor with eccentric cylinder block

Description: The pistons rotate within the rigid external ring. Eccentricity „e“ determines the stroke of the pistons.

π  V =  d k 2    2e  z 4  e

=

Eccentricity

z

=

Number of pistons

dk

=

Diameter piston

Function: A radial piston pump with eccentric cylinder block operates as follows: The pistons fixed to slipper pads rotate in a static outside ring or cylinder. Eccentricity „e“ determines the stroke of the pistons. The volume in the cylinders diminishes (gpressure built-up) or increases (gsuction) due to the stroke movements. There are different designs, but always with an odd number of pistons (3 / 5 / 7 / 9 etc.). The connection of the individual pistons on the pressure side is regulated by means of a check valve in different designs. For example pipes are fitted into the pump or holes are drilled into the housing. On the suction side, the pump has to be fitted always below the oil level. Breathing is mandatory before starting the unit. Thus the mounting position is predetermined by the manufacturer, ­because they determine the position of the breather valve. Swept volume control is possible. This pump is very prone to contamination due to its valve control unit and extremely small orifices between piston and bush (5 to 8 µm). Some pumps of some manufacturers are not suitable for large load changes with high pressure ­alterations. Problems on the suction side often lead to an immediate destruction of the pump. By repositioning the stator ring the performance of the pump can be adjusted to the requirements of the unit. Most manufacturers supply repair kits. Technical data: • displacement volume constant or adjustable • pressure adjustable up to approx. 630 bar • different manufacturers g different pressure stages • can be used as a hydraulic motor

housing dK

slipper pad piston shaft

e

Fig. Radial piston pump with eccentric cylinder block

87

1.11

Axial piston pump and motor in swash plate design

Description: The rotating displacement pistons are supported by a swash plate. The angle of the swash plate determines the piston stroke.

V = dk 2 

π ( 2 rh  tan π )  z 4

z

=

Number of pistons

dk

=

Diameter piston

Function: The cylinder, tightly connected to the shaft and the pistons, is in a parallel position to the drive shaft. The ends of the pistons are designed as ball- and socket joint, are positioned on slipper pads held in place by discs at an angle. When the shaft starts to rotate, the cylinder, pistons and slipper pads start to rotate as well. Since the pistons with the slipper pads are attached to the swash plate piston strokes occur inside the cylinder. The fluid is controlled by kidney shaped slots in the control plate. With the exception of the housing all components of the pump are manoeuvrable. Depending on the manufacturer pumps must be pre-charged with fluid via the leakage oil port before they are brought into service. The pumps must be fitted below tank fluid level. The position is determined anyway by the manufacturers, since they decide on the position of the leakage oil port. With revolutions larger than 1500 per minute most pumps must be fed from the entry side. Occasionally these pumps are either fitted with feed pumps or the tank must be pre-charged. Problems on the suction side can cause immediate destruction of the pump. These pumps are very prone to ­contamination due to the revolving movement of the drum on the control plate and the stroke of the piston with the pressure ­compensation nozzles. The pumps are supplied with a huge variety of control units for pressure and volume flow. A great advantage of these pumps is the fact that a compensation of suction and pressure side occurs during operation with a concurrent drive turn of the pump due to the swivelling of the tilted axle. There are other designs with connection possibilities for more pumps. Repairs are possible with knowlegeable mechanics. Application: • mobile technology • stationary hydraulics • injection moulding machines • presses Technical data: • displacement volume constant or adjustable • pressure and output adjustable • always an odd number of pistons • high rest pulsation • different manufacturers g different pressure stages • up to approx. 400 bar • can be used as hydraulic motor

88

Axial piston motor in swash plate design: There are three different design types: • bent axis design • swash plate design • displacement design With the exception of the displacement design all designs permit a swept volume regulation. A motor in swash plate design permits high revolutions per minute since it is perfectly counter balanced. The motor in swash plate design in cylindrical form increases the possibilities for applications in comparison to the bent axis design motors. With axial piston motors in swash plate design a complete mass balancing is not possible, which is the reason why high revolutions per minute cannot be obtained. swashplate displaement piston

cylinder control plate shaft rh

dK

Fig. Axial piston pump in swash plate design

89

1.12

Axial piston pump and motor in bent axis design

Description: Depending on the swivel angle, the pistons move within the cylinder bores when the shaft rotates.

V = dk 2 

π ( 2 rh  sin π )  z 4

z

=

Number of pistons

dk

=

Diameter piston

Function: The stroke plate in which the middle pivot and the axial pistons are positioned, stands vertical to the drive shaft. The ­cylinder with the piston is positioned at an angle of usually 25° to the drive shaft. When the drive shaft rotates the ­cylinder rotates as well thus causing the pistons to perform a stroke. A control plate, also called pilot plate, with kidney shaped slots deals with the input and output of pressure fluid. Over the years the kink angle has been changed repeatedly. It started with an angle of 25° to 32°. Today we find angles of 40°. It also depends on the manufacturer with which angle the pump is equipped. Therefore it is very important to check on the angle, when faulty pumps have to be replaced. Depending on the manufacturer pumps must be pre-charged with fluid via the leakage oil port before they are brought into service. The pumps must be fitted below tank fluid level. The position is determined anyway by the manufacturers, since they decide on the position of the leakage oil port. With revolutions larger than 1500 per minute most pumps must be fed from the entry side. Occasionally these pumps are either fitted with feed pumps or the tank must be pre-charged. Problems on the suction side can cause immediate destruction of the pump. These pumps are very prone to ­contamination due to the revolving movement of the drum on the control plate and the stroke of the piston with the pressure ­compensation nozzles. The pumps are supplied with a huge variety of control units for pressure and volume flow. Repairs are possible with knowlegeable mechanics. Application: • mobile hydraulics • stationary hydraulics • injection moulding machines • presses Technical data: • displacement volume constant and adjustable • pressure and output controllable • different manufacturers g different pressure stages • high rest pulsation • up to ca. 400 bar • two design types (constant and variable) • can be also used as hydraulic motor

90

Axial piston motor in bent axis design: Axial piston motors in bent axis design are variable displacement motors in bent axis design with hydraulic adjustment. The adjustment is done through a regulating piston and pivot attached to the backside of the control plate. The regulating piston is controlled by the pilot piston which is activated either by applied pressure or a solenoid. A separate pilot oil pump is not necessary since the respective highest operational pressure is taken from connectors A or B as adjusting oil. In order to guarantee a proper functioning of the adjustment the pressure has to be at least 15 bar.

drive shaft drive flange

piston cylinder

pilot plate

rh dK Fig. Axial piston pump in bent axis design

91

1.13

Rotary vane motor

With rotary vane motors the swept volume can be increased by multiple fillings per rotation. The result is a higher ­specific torque and a considerably smaller load bearing. This is achieved by a higher pressure fluid intake than with vane motors. A disadvantage is the reduced sealing and the constant swept volume.

1.14

Roll vane motor

This type of motor is mostly used in the tooling industry as a feed drive. It is a full-TLA-constant machine with low ­­mass-moment of inertia, which shows a good response sensibility, low reversing time and a high torque. In combination with servo valves this motor is being used as positioning drive for control circuits. Function: The rotor features two vanes on opposite sides. Therefore a radial load balancing is secured. The main drive cog wheel is attached to the rotor, which drives the roll vanes in a way that rotor movement and roll vane movement are synchronized timewise. The cog wheel gear ratio is chosen in such a way that the peripheral speed of the rotating elements is equal. Therefore there is no sliding against the sealing surfaces but a rolling ­movement. In order to obtain a high volumetric torque, highest degrees of tolerances (5 µm) are necessary. A very fine filtration of the system is necessary.

1.15

Selection criteria

When planning a hydraulic unit several criteria for choosing the appropriate pump or motor have to be considered. The criteria for the individual design principles are shown in the table at the bottom of this page.

Ax

1

2

2

3

3

2

2

2

2

Usable pressure range

2

2

3

3

3

3

1

1

1

1

Viscosity range

1

2

3

1

3

3

1

1

1

1

Maximum noise level

4

1

2

1

2

2

3

3

3

3

Life time circle

3

2

2

1

1

1

2

2

2

2

Cost

1

2

2

3

2

2

3

3

3

3

hp as

sw st on

pi ia l

Ax

2

la

is ax

nt be

to n is

lp

te

cy ric

n

R ad

ia

ia

lp

is

to

n to is lp

R ad

ia

ec

ce

nt ce ec

rv be

m

D ou

bl

e

ch a

am ch

le Si

ng

w Sc

ro

nt

e an

e an rv be

e dl in

in

G

ea

rr

ne

In

te r

sp

g

lg

ea

r

ar ge al rn te Ex

Usable rpm range

Fig. Selection criteria in planning a hydraulic unit

92

ric

sh

af

lin

t

de

rb

lo

ck

1 = very good / very large 2 = good / large 3 = medium 4 = low

3

Cylinders

3.1

General remarks

3.1.1

Drive units

Drive units like hydraulic cylinders, swivel drives and hydraulic motors with linear, swivel ­motion and rotary movements are components which transform hydraulic into mechanical energy. When planning a hydraulic unit you usually start with the drive unit, because the necessary forces, paths and times of the machines to be built are predetermined. Having said this it is clear that the most important parameters for pumps, motors, swivel drives and cylinders like volume flow, pressure, torque and payload are obvious, since the volume flow ­determines the speed and the pressure determines the torque and thus the payload of the drive units.

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Fig. Overview: hydraulic cylinders

97

3.1.2

Hydraulic cylinder

Hydraulic cylinder perform linear movements and thus transfer forces. The maximum force of the cylinder (traction and force generated by pressure) depends on the area acted upon (piston and ring area) and the maximum permissible ­operation pressure. Hence follows the equation:

F=pA

The force of the cylinder is constant throughout the entire stroke area. There are countless types of hydraulic cylinders, but the basic design is always the same. Piston rod, piston with washer, cylinder casing, two lids and ports. As of a certain stroke speed shock absorbers at stroke end are built in. Furthermore breathing valves at both ends of the cylinder are necessary. We distinguish between single and double acting cylinders. piston rod

piston with washer cylinder casing

lids

lids

ports Fig. Hydraulic cylinder

The core characteristics of the hydraulic cylinder are: • the force will be generated directly without a connecting link. • the force can be used on every point of the movement in any dimension to the nominal force. • the usable deviation can be modified in the constructional limits. • the speed of the movement can be controlled by the volume flow. • with the choice of the force you can adjust the dimensions of the cylinder. Cylinders are used in single and double acting design in hydraulics and pneumatics. In principle they differ only in the use of the medium and the force.

3.2

Design

3.2.1

Single acting cylinder

Single acting cylinder transfer force only in one direction. They either exert traction and force generated by pressure. To return the piston into the start position a return spring is used or the weight of the piston and the load does the job. ­Basically simple acting cylinders have only one effective piston area on which the forces can act. Depending on the ­design we distinguish between plunger piston cylinders with or without an internal stroke limiter.

�

�

�

�

� � 98

�

Fig. Plunger cylinder with internal stroke limiter (left) and plunger cylinder without internal stroke limiter (right)

� Fig. Single acting cylinder with return spring

3.2.2

Double acting cylinder

Double acting cylinder have two opposing effective areas which are of the same or different size. They are fitted with two pipe ports, which are isolated from each other. By feeding fluid via ports „A“ or „B“, the piston may transfer pushing or pulling forces in both stroke directions. We distinguish between single and double rod cylinders.

�

�

�

Fig. Single rod cylinder

�

Fig. Double rod cylinder

3.2.2.1 Single rod cylinder A single rod cylinder or „differentiate“ cylinder have a piston rod only on one side of the piston. The name „differentiate“ came into use, because with this design you have to „differentiate“ effective areas. The area ratio of piston area to ­annulus area is indicated by the factor φ . Since we have two different effective area sizes we have also two different speeds for extension and retraction. The stroke velocities are inversely proportional to the areas. (large area g low ­velocity, small area g large velocity). The larger the effective area the bigger the force which can be transferred. Hence a bigger force is available for the ­extension of the piston. The force acting on the piston is calculated as follows:

F = p  A h force [F] = N pressure [p] = Pa effective area [A] = m² torque [h] Some advice: The torque depends on the used seal kit and ranges from 0.8 to 0.98. A very common ratio φ = 0.5.

� piston area

� annulus area

�

�

�

�

Fig. Single rod cylinder

99

3.2.2.2 Double rod cylinder Double rod cylinder have a piston, which is rigidly connected to two piston rods, which have diameters smaller than that of the piston. As a rule of thumb you can say that the effective areas of both pistons are equal in size. Thus the force transferred as well as the velocities on extension and retraction is equal in size.

�

�

�

�

Fig. Double rod cylinder

3.2.2.3 Single acting rod cylinder with different piston rod diameter In some cases single acting rod cylinder with two rods are needed. With this design force transference and velocity r­elate to each other according to the area ratio j.

�

�

�

�

Fig. Single acting rod cylinder with different piston rod diameters

3.2.3

Tandem cylinder

In double acting cylinder operating in tandem the effective areas of both piston are added. By using this arrangement large forces may be transferred for relatively small external diameters without increasing the operating pressure. ­However the longer length of this type can be a disadvantage. Usually this model is used in large presses.

�

��

Fig. Tandem cylinder

100

�� ��

��

3.2.4

Rapid traverse cylinder

Rapid traverse cylinder are used primarily in presses. In this cylinder, as long as the complete working force is not ­required, only part of the effective piston area, the so-called rapid traverse piston is placed under pressure. The ­complete effective piston area is only later connected to the hydraulic pump via a control system by means of pressure control valves or limit switches. The high rapid traverse velocity due to small volume and the high pressing force due to large effective piston areas are very advantageous.

�

� ��

��

��

�� Fig. Single acting rapid traverse cylinder

�

Fig. Double acting rapid traverse cylinder

Legend: A1: rapid travers process A2: pressing force S: suction Return flow (A1) and (A2).

3.2.5

Telescopic cylinder Telescopic cylinder consist of several rods sliding into each other. In general telescopic cylinders are manufactured in a simple ­design form and used wherever little space is available but relatively large stroke forces are required. Within the ­individual stroke units ­different velocities and stroke forces may occur. Due to new developments in the material sciences nowadays more and more lighter materials are used in mobile technologies. ­Therefore double acting telescopic cylinders have to be used, since the own weight of the much lighter containers cannot move them back to the start position. Fig. Telescopic cylinder

­

�

�

�

�

�

Fig. Single acting and double acting telescopic cylinder

� 101

3.2.6

Cam system

The cam system works as a force diffuser, witch is used in the moulding technology. With this system, valves can be saved and the hydraulic system can be designed easier and more efficiently. Design: The system consists of an input cylinder (drive unit), which is fitted very accessibly in the unit. The receptive cylinder (working unit) is connected with the input cylinder by means of a pipe system. Function: Due to the extension and retraction of the input cylinder the displaced hydraulic fluid is squeezed into the receptive ­cylinder. When the displaced volume of cylinder Z1 equals the received volume in cylinder Z2 a synchronization control can be achieved. By changing the volume of the cylinders (length, diameter), also a reduction of forces or an elongation of paths can be achieved.

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�

� ��

�

��

�

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�

�

�

� �

�

� �

�� �������������

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Fig. Cam system

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Please note:

When several cylinders are used on the receiving end in order to obtain a synchronization control, they should be ­connected mechanically. Synchronization is difficult to achieve if you use a pipe system with different pipe lengths. The same is true for hoses, since they expand if pressurized.

102

3.3

Design principles

The design of a cylinder is determined by its purpose, demands placed upon it and its application.

3.3.1

Tie rod cylinder

In tie rod cylinder cylinder head, cylinder pipes and cylinder bottom are tightly attached to each other with tie rods. They have a very compact design and are mainly used in machine tool industry, manufacturing devices and automotive ­industry.

Fig. Tie rod cylinder

3.3.2

Mill type cylinder

In mill type cylinder, the top and the base of the cylinder and cylinder tube are connected together via threads or ­retaining rings. Due to the robust design, hydraulic cylinders with screwed or welded constructions are also suitable for use in applications with extreme operating conditions.

Fig. Mill type cylinder

103

3.4

Mounting

3.4.1

Joint mounting

The joint mounting gives the cylinder moving possibilities in one or two ways. This mounting can be fixed on the bottom or on the rod.

Fig. Joint mounting on the bottom of the cylinder

a.

b.

tilting angle α

Fig. Possible fixtures for joint mountings a. swivel bearing on the bottom and rod with swivel bearing misalignment only in one direction b. swivel bearing on the bottom and rod with joint bearing equalisation of inaccuracies in the parallelism of the axle bolts will be equalized c. joint bearing on the bottom and rod with joint bearing misalignment across the normal rotatable direction

104

c.

3.4.2

Trunnion mounting

The cylinder is fitted by means of a swivel mechanism attached to the cylinder. Any position on the cylinder is possible. The most favoured position, however, is the the centre of gravity of the cylinder.

Fig. Cylinder with swivel mechanism

3.4.3

Flange mounting

The cylinder is fitted by means of a flange either at the head or the bottom of a cylinder. The screws are put under stress during the pulling sequence of the cylinder.

Fig. Flange at cylinder head

Fig. Mounting hints for flange attachments

Please note for both designs: • mainly vertical positioning • the screws of the flange should be relieved during main functional stress

105

3.4.4

Foot mounting

Fitting or mounting of cylinder by means of brackets attached to the cylinder. The screws of the brackets are mainly put under stress by shearing strain. Depending on the position of the brackets, you have to watch out for an additional tilting momentum.

Fig. Cylinder with brackets

106

3.5

Buckling / bending of piston rod

Wherever cylinders are built in either horizontally or in a strongly tilted position, you have to take into consideration, that the cylinders might buckle or bent due to their dead weight. This is especially true for very large cylinders with a ­considerable dead weight and stroke length. A load calculated with this formula will actually make the piston rod buckle: Maximum operational load: K

=

F=

K =π2  E 

K S

I Sk 2

Buckling load N

Sk =

Length of buckling in mm, check the table on this page

E

=

Flexibility = 2.1 • 105 N/mm2 for steel

S

=

Safety (ca. 2.5 - 3.5)

I

=

Torque of inertia for circle diameter mm4

�

�

�

� �

�



one end free, one end fixed

two ends guided with joints



�

�

�

� �

�



one end guided with joint, one end fixd

two ends fixed



load has to be carefully guided since tensions may accur otherwise

not suitable, since tensions are to be expected

Fig. Buckling / bending of piston rod

107

3.6

Shock absorber at stroke end (end cushioning)

The stroke end cushioning system causes and secures a deceleration of the piston velocity at one or both stroke ends, in order to generate mass forces. If stroke end and the end position of the piston are identical, a stroke end cushioning system is used. That of course requires constructive measures at the cylinders. A deceleration is brought about by ­measures at the control unit by means of appropriate valves. The end cushioning serves as a protection for the cylinder and the entire unit. Pressurization at starting point is initiated by check valves and acts immediately in the piston surface (or ring surface) in such a way that there are no losses in performance or delays in the start-up process. throttle valve

piston chamber

piston rod

bore throttle

damping bush check valve Fig. Shock absorber at stroke end Function: Attached to the piston is a conical damping bush. When the piston moves with the damping bush into the drilling in the cylinder bottom, the cross-section through which the fluid can escape, is continuously decreased until it is completely closed. Now the fluid from the piston chamber has to flow through the drilling to the throttle and the adjustable throttle valve. The cushioning effect can be controlled by means of the throttle valve. A small cross-section of the throttle valve results in a high cushioning effect. As an extension support for the piston to move out of its end position a check valve is fitted as well. This has the effect that during the extension the throttle is bypassed. We distinguish between three different kinds of end cushioning: • constant shock absorber slot • progressive shock absorber slot • ring hole shock absorbe

108

3.6.1

Constant cushioning

A constant cushioning causes a sudden breaking and a slow sliding into the end position.

�

� Fig. Constant cushioning

3.6.2

Progressive cushioning slot

Progressive shock absorbing has the advantage that the velocity at end point is low and therefore a soft sliding into the end position is possible.

�

� Fig. Progressive cushioning slot

3.6.3

Ring hole shock absorber

Ring hole shock absorbing causes a deceleration of velocity into the final position.

�

� Fig. Ring hole shock absorber

109

4

Hydraulic valves

4.1

Check valves

Check valves block the volume flow of a hydraulic unit in a certain direction. The pressure fluid can flow unhindered in the opposite direction. It is possible to compare this valve with a diode in electronics. Since the valve is designed as a seat valve no leakage will occur. The following closing elements can be used in these valves:

Fig. Cone-seat

Fig. Ball-seat

Fig. Disc-seat

A ball is by far the cheapest closing element, but because of its mass it is only meaningful to use it in small valves. A cone is the most widely used closing element and turned hollow due to its weight. The production of a cone however is more tedious than that of a ball or disc.

4.1.1

Check valve

Description: They are valves which permit flow in one direction and block flow in the opposite direction. Design: The check valve basically consists of a housing with integrated valve seat, a cut and tempered cone and a pressure spring. The valve is also being produced with a ball as a closing element. Function: As and when fluids flow through the valve a force generated by the system pressure acts on the closing element, which works against the spring force. When this force exceeds the spring force, the closing element, in this case a cone, and releases the flow from B to A. If there is a volume flow from A to B, the cone is pushed into the sealing lip and seals the valve without loss of leakage oil.

housing

seat valve

� �

�

� seat valve

cone

Fig. Check valve (RV)

�

Caution!

In a design without spring it is important that the valve is fitted vertically. This way the deadweight of the cone helps to let it sit in the neutral position.

111

Design: HYDAC offers check valves in the following designs: • pipe fitting (rv) • sandwich design (rvp) • valves in a threaded plug (RV and RVE)

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Please note: The opening pressure of the valve is increased by the pressure at port A. Model code: The model code is composed of the following sequences: ��

� �� � �� � � � �

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112

4.1.2

Pilot operated check valves

Design: Hydraulic pilot operated check valves can be opened via an additional pilot oil connector for flow in the opposite ­direction. They consist of a housing, control spool, a valve seat, a ball and a closing spring. Function: The valve permits free flow from 2 to 1 in the direction of flow. In the opposite direction the ball is pushed on the valve seat by the closing spring and the pressure at port 1 and thus blocks the flow direction from 1 to 2 without loss of l­eakage oil. The pressure at port 1 acts on the control spool and counteracts the pilot pressure at port 3. Therefore during the hydraulic opening port 1 has to be without pressure. If the pilot pressure at port 3 is sufficiently high the piston is moved and the ball is pushed away from the valve seat. Now the valve is unblocked and fluid can flow from 1 to 2. The return spring fitted underneath the piston permits an undelayed back switch once the pressure is lowered.

housing

piston 3 return spring 2 ball

� closing

�

�

1 Fig. Hydraulically pilot operated check valve (ERVE)

�

Please note: The existing pressure countervails the opening pressure at port 3.

113

Application: With hydraulically pilot operated check valves, creeping movements in cylinders, which are operated by spool valves and which are loaded, can be prevented. The valve is located in the return line of the cylinder and prevents the cylinder from extending even more under loading. Only as and when pressure is built up in the feed line, the pilot control unit unblocks the check valve in the return line of the cylinder.

�

�

�

�

�

�

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�

�

��

��

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Fig. Circut diagram: hydraulically pilot operated check valve (ERVE)

114

4.1.3

Check valves with pre-decompression

Description: By opening a check valve a sudden opening of the entire cross section might happen. Decompression shocks might occur with high volume flow, which not only produce a lot of noise, but also might be harmful to the entire unit. Damage to screwed fittings and valves could be the result. Function: In order to prevent this, you could choose valves with preopening features. If pressure acts on the piston, it pushes down on the preopening ball. Only a small part of the cross section is opened. Only then the main cone is pushed off its seat. This way a smooth release of the pressurized fluid is possible. piston

housing

�

check valve

� ball

� closing spring

�

�

piston

� Fig. Pilot operated check valve with predecompression (ERVE-R1)

115

Model code: The model code is composed of the following sequences: ���� � ��� �� � ����������� ����������������������������������

���� ����� ��� ������� ��� �������������

������ ����������������������������

Pre-decompression: • for large volume flows • for very high pressures (otherwise the unit will become too large)

�

Please note: The existing pressure countervails the opening pressure at port 3.

116

4.1.4

Lowering speed controlled valves

Description: Lowering speed controlled valves belong to the group of check valves. These are valves whose block position is ­canceled by a hydraulic operation or if the pre-set pressure has been reached. They fulfil the following tasks: • actuator velocity control depending on the volume flow intake • overreaction of the actuator with pulling loads is avoided • closed flow passages in block positions (the actuators can maintain their respective position) • limitation of actuator pressure (maximum load) to the predetermined pressure • free volume flow intake by integrated check valve • burst control at pipes leading to the actuator or at control pipes adjustment unit

adjustment spring

control piston

A3

�

A1

�

�

RV-piston

�

�

�

closing spring

Fig. Lowering speed controlled valves (SBVER1) Application: Lowering speed controlled valves in combination with double acting actuators, like cylinders and hydraulic motors, are ­mainly used for safety measures and control functions. They have to fitted at the outlet side of each actuator, because of a ­possible reversal of the load and direction of movement. Lowering speed controlled valves help to control the lowering of loads and also work as a speed control mechanism. They also serve as a safety measure if loads have to be held in place in case of pipe and hose bursts. Function: Lowering speed controlled valves are directly controlled piston seat valves for oil-hydraulic tasks. They permit a smooth moving of actuators for pulling and pushing loads. These valves basically consist of a housing, check valve piston (RV-piston), closing spring, adjustment spring, ­adjustment unit for presetting of spring force and a cut and tempered control piston. For lifting loads fluids flow from port 2 to 1 of the in-built check valve. The RV-piston is moved against the closing spring and opens the appropriate cross section for the fluid flow. If the valve is closed the actuator is held in its present position. The RV-piston is pushed against the control piston and seals at the seat edge. You have to make sure that control port 3 is not under pressure when the valve is closed. The pressure at the actuator (load pressure) at port 1 acts upon the control piston area A1 and thus against the force of the adjustment spring. This way a limitation of the load pressure can be achieved. The maximum preset pressure at the actuator should always be at least 20% above the highest load pressure permitted for standard operations. When the load is lowered (volume flow from port 1 to port 2) the valve is being acted upon via port 3. the volume flow of the load is controlled at the spool land of the control piston. A sudden drop of the load is thus prevented.

117

Example:

� �

�

�

�

�

�

�

�

�

�

�

�

Fig. Lowering speed controlled valve The opening point of a lowering speed controlled valve can be calculated by means of the following formula:

FFed = p1  A1 + p3  A3 p3,erf =

p1 = p2 = p3,erf = pe =

pe - p1 + p2 j highest pressure to move the max. load (load pressure) in bar pressure at connection 2 in bar required control in bar at connection 3 to unlatch adjusting pressure in bar (pe ≥ p1 • 1,2)

Please note: Pressure at p2 is incorporated as disturbance variable! Pilot ratio:

j=

A3 A1

Pilot ratio φ: Pilot ratio is a very important parameter in selecting the right valve. The pilot ratio should be selected in such a way, that an excellent load control is possible with an energy loss as low as possible. It depends first and foremost on its application. In order to obtain good dynamic properties, valves with high pilot ratio are applied in hydraulic motor applications. It is the other way round with the application of cylinders, where small pilot ratios are preferred.

118

4.1.5

Twin check valves

Design: If you combine two pilot operated check valves you get a twin check valve. Built into a hydraulic unit it prevents ­cylinders from re-traction or extension due to external forces. The circuit diagram down below shows how it works. Function: The hydraulic check valve RPDR06 is a twin check valve in one housing, consisting of 2 check valves, which allow the volume flow only in one direction. The volume flow in the other direction is reversed through the other valven. check valve

��

��

��

�� piston

housing

Fig. Twin check valves (RPDR06) Function hydraulic diagram: Let us assume a free volume flow in flow direction A1 to A2 and B1 to B2 respectively. In the direction A2 to A1 and B2 to B1 the valves block the volume flow. Therefore the cylinder cannot be moved due to external forces. Now if you set the directional valve in a way that the cylinder can extend the volume fluid can flow freely from B1 to B2. Thus the pressure line to the second valve is activated and the valve opens. The line from A2 to A1 is set free. The ­cylinder can extend without any problems.

�

�

��

��

��

�� �

�

�

�

��

��

� Fig. Circut diagram: twin check valves (RPDR06)

119

4.1.6

Pipe bursting protection

Design: Pipe bursting protection devices are volume flow dependent safety components, which in the case of a bursting pipe prevent the actuator to carry out uncontrollable and inadmissible actions. These valves are known for their high safety standards, quick response time and a compact design. In cylinders pipe bursting protection devices are directly fitted into the ports. They consist of a closing element, which is kept open by a spring and in case of a pipe burst is pressed on the valve seat to seal the valve without any leakage. Function: In a normal operational mode the valves are open. The closing element is kept open by a spring as long as the spring force is greater than the force exerted on the closing element by the flow restriction caused by volume flow from 1 to 2. the valve remains open and flow in both directions is possible. If the volume flow exceeds the pre-set value at the valve, the increase of the flow restriction causes the spring to react and the closing element is pressed abruptly on the valve seat. The closing element already sits tightly on the valve seat. Leakage via the thread can be prevented by glueing the valve into the thread. If you feed pressure into the system via port 2 the valve opens automatically. Application: • hydraulic lifts • lift tables / platforms • loading bridges • forklifts • other safety devices

� closing element

valve seat

spring

�

�

�

Fig. Pipe bursting protection device (RBE)

120

�

Please note:

• pipe bursting protection devices may only be used as a safety measure for protecting the actuator in the case of pipe bursts. The application of these devices for repetitive closing actions is not permissible. • if these devices react during standard operational procedures the pre-setting of the pipe bursting protection does not meet the operational parameters of the unit. It must be replaced with a corrected pre-setting. • to avoid a reaction of the device caused by normal volume flow oscillations the response threshold should be at least 20% higher than the normal volume flow during standard operational procedures. • the functioning of the valve depends largely on the viscosity of the fluid. This you should bear in mind, when a hydraulic unit is designed. • after pipe bursts the protection devices have to be replaced. Model code: The model code is composed of the following sequences: ���

� ���� � � � ��

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121

4.1.7

Change-over valves (shuttle valves)

Discription: Change-over valves are produced in poppet valve design. The cross-over switching functions automatically. Function: Change-over valves are check valves with two inlet ports and one outlet port. The inlet port with the higher pressure is automatically connected to the outlet port. The other port is closed.

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�

�

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� Fig. Change-over valve (WVT)

Application: Change-over valves are particularly suited for fitting in hydraulic circuits of pilot controlled or remote controlled ­directional valves, in variable displacement pumps, control pumps and in logic circuits.

�

Please note:

The pipe fitting of change-over valves should be in accordance with the mounting regulations of the manufacturer.

122

4.2

Flow control valves

Flow control valves control the volume flow in a hydraulic circuit. By changing the cross-section you can change the fluid flow at the throttle unit. Fluid control is affected, depending on the requirements, at inlet, outlet or bypass section. In general you can distinguish 2 types of flow control valves: • pressure dependant throttle valves (due to pressure loss via throttle) • pressure independent flow control valves (loss of pressure is being compensated by means of a pressure compensator)

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Fig. Overview: flow control valves Function: With throttle valves the volume flow depends on the differential pressure at the location where the throttle sits and on the viscosity of the fluid: • a greater differential pressure causes a greater volume flow. • a higher viscosity causes a lesser volume flow. Application: • presetting of velocity of actuators • pressure dependent throttleing of volume flow in general

123

Design:

Fig. For pipe systems

Fig. Sandwich system

Fig. Valve in a threaded plug

Difference throttle to orifice: As of a length of

l=

1 D 2

it is called an orifice.

An orifice is more susceptible to viscosity, but also more prone to turbulences. In other words, for pressure drops ­turbulences are more decisive than the viscosity.

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�

��������

� Fig. Throttle

With the following equation you can calculate the volume flow Q at the orifice:

Q =a  A 

2  Dp r

a = flow coefficient (in most cases 0.6 to 0.7; depending on the shape of the orifice) r = density A = surface of orifice Dp = pressure difference before and after the orifice

124

Overall view of designs: Throttle �

The cross section of the throttle is excellent, but very much susceptible to the viscosity due to the rather long throttle length.

Orifice �

The cross section of the throttle is excellent. The length of the throttle is almost zero and therefore independent of the viscosity.

Needle throttle �

Throttle length is short, the extent rather small and the influence of viscosity negligible. There is the danger of clogging with low volume flows, since the throttle possesses a small circular slot. Bad power of resolution.

�

�

Notch (triangle) �

�

Throttle length relatively short, circumference small and influence of viscosity low.good power of resolution. Suitable for small volume flow.

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Notch (rectangle)

�

Throttle with slit

Throttle length short, but large circumference. Not suitable for small volume flow, because restrictor becomes a narrow slit with the danger of clogging. Very bad power of resolution.

Extent throttle (circumference) triangle

Throttle length long, thus viscosity plays a major role. Power of resolution not very good, because angle of rotation generally possible only between 90°C to 180°C.

�

� �

Power of resolution = setting range for volume flow cross section Fig. Throttle designs

125

4.2.1

Throttle valves

Description: Deceleration valves are used to smoothly decelerate or accelerate hydraulically moved loads. Throttle and shut-off ­function work in both directions. Design: Throttle valves basically comprise housing, a special throttle setting screw or spindle and a rotary knob. Function: Volume flow increases with the number of turns of the rotary knob. Otherwise with a completely closed spindle no volume flow is possible. The rotary knob with a colour scale and ring (1 - 5) permits the repetition of the preset values. The area size of the triangle indicates the size of the cross section of the volume flow. An increase of the coloured triangle also means an increase of the cross section. Safe-guarding the adjustment knob is done by a clamping screw. Deceleration is possible in both flow directions.

�

rotary knob

� �

throttle spindle housing

throttle cross section

� �

�

�

Fig. Throttle valve (DV)

126

4.2.2

Throttle check valves

Description: Throttle check valves like throttle valves permit the same precise adjustment of the volume flow. The throttling and ­shut-off function is only possible in one direction. The in-built check valve however permits uninhibited return flow in the opposite direction. Design: Throttle check valves comprise the following components: housing with integrated valve seat, a cut and tempered cone (poppet), a pressure spring, a throttle spindle and a rotary knob. Function: The poppet is pressed against the valve seat by the spring and thus blocks port A from port B. If the throttle spindle is completely closed no flow is possible. With an increasing number of turns of the rotary knob the volume flow increases from A to B. the throttling and shut-off function only works in one direction. The rotary knob with colour scale and ring permits the repetition of the pre-set values. The size of the coloured triangle signifies the size of the cross section of the volume flow. An increase of the size of the triangle also means an increase of the cross section. Safeguarding the adjustment knob is done by a clamping screw. The poppet opens, if the pressure at port B is higher than at port A, including the opening pressure induced by the spring.

� � � � �

rotary knob

throttle spindle

housing

spring

� �

�

�

throttle cross section

check valve

Fig. Throttle check valve (DRV)

127

�

Please note:

If throttle check valves are used the opening pressure of the poppet is increased by the pressure at port A (throttle ­spindle closed). Model code: The model code is composed of the following sequences: ���������������������� ����������� ��� �� ������������������ ���� �� ������������������� ���� �� �� �� �� �� �� �� ���� ����� ��������������������������������������������� ����� ������������������������������������������� ������������������������������ ����� ��������������������������������������� ������������������������������������������������� �������������������������������������������� ����� ��������������������������������������������������������� ���������������������������������������������� ����������������� ������ ���������������������������� ������������������� �������������������������������������

128

4.2.3

Pressure compensator

The task of a pressure compensator is to keep a preset volume flow constant, independent from pressure oscillations. The volume flow is controlled by a adjustable throttle (1) and one more mobile pressure compensator (2). The mobile throttle operates as a control orifice and as a reference point for a control circuit. Function: • the measuring throttle (1) is adjusted to a desired flow. • the pressure difference (p1 - p2) at entry point changes g change of volume flow. • the pressure compensator (2) controls the deceleration and thus the differential pressure of the throttle (1). Thus the volume flow remains constant. We distinguish between an increase and a decrease of the differential pressure: • increase of differential pressure (p1 - p2): the piston is pushed against the spring, thus s becomes smaller. g Volume flow at exit point remains constant. • decrease of differential pressure (p1 - p2): the piston is pushed away from spring, thus s becomes larger. g Volume flow at exit point remains constant.

p1  AK = ( p2  AK ) + FF Dp = p1 - p2 =

FF = constant AK

AK =

Area of piston

FF

Spring force

=

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Fig. Pressure compensator (DWY)

129

4.2.4

2-way flow control valve (cartridge valve)

Description: A cartridge valve is basically a fixed orifice valve with a downstream differential pressure control for fluid-hydraulic units. It's purpose is to keep the volume flow constant by means of a control unit. The volume flow itself is to a large extend independent of pressure and viscosity. The size of the volume flow is determined by means of a variable orifice and can be adjusted in a certain range. Design: The differential pressure control consists of a control piston, pressure spring, variable orifice and an adjustment screw. Function: The variable orifice determines the volume flow adjustment range. If there is a flow from 1 to 2, we have a pressure drop at the variable orifice. The pressure compensator moves into a control position, which corresponds exactly to the force equilibrium derived from the formula pressure drop at the variable orifice times piston area on the one side and the spring pressure force on the other side. With increasing volume flow the variable orifice is reduced up to the point until an force equilibrium is yet again achieved. Due to a continuous re-adjustment of the pressure compensator according to the ­existing pressure drop, a constant volume flow from 1 to 2 is generated. In the opposite direction from 2 to 1 an ­uncontrolled flow through the valve is possible. The result is a pressure drop according to the variable orifice implemented.

threaded slot lock screw orifice (control)

adjustment screw

�

pressure spring

hardened control piston �

�

variable orifice threaded slot

� Fig. 2-way flow control valve (cartridge valve)

130

4.2.5

2-way flow control valves (pipe fitting)

Design: 2-way flow control valves are controllers of pressure differences with variable orifice for hydraulic units. The controller of pressure difference (pressure compensator) consists of a control spool, pressure spring and variable orifice. Function: With the variable orifice, which consists mainly of a spindle and rotary knob, the volume flow cross section is determined. If there is a flow through the valves from A to B the pressure drops at the variable orifice. The pressure compensator moves into a control position, which is equal to the balance of force and pressure drop at the variable orifice on the one side and the force created by the pressure spring on the other. If the pressure drop increases the cross section of the variable orifice is readjusted according to the increased pressure drop until a balance of forces is established. Due to the continuity of the pressure compensator, which corresponds with the existing pressure drop, a constant volume flow from A to B is achieved. In the opposite direction from B to A the volume fluids can flow through the valves in a uncontrolled way and with little loss of pressure. With the rotary knob the flow control valve can be easily and precisely adjusted. The more often you turn the knob the more fluid flow is possible. It increases with a steady rate. The knob with the colour code permits the replication of the values. The size of the coloured triangle tells you how large the cross-section actually is. If you turn the knob so that the triangle becomes larger, it also increases the cross-section. The adjustment knob can be fixed by a clamping screw. colour display � � � � �

rotary knob

spindle check valve spring check piston

� �

�

�

control orifice

variable orifice

Fig. 2-way flow control valve (pipe fitting) (SRVR)

131

Model code: The model code is composed of the following sequences: ������������������������� ����������� �������� �������������������������������������� ����������������������� �������� ���������������������������������������� ����������������������� ������������ �� �� �� �� ���� ���� ��������������������������������������������� ���� ����������������������������������������������� ����������������������������������������������� �������������������� ������ ���������������������������� ��������������������������������������������� ����������������������������������������

132

4.2.6

Flow divider / combiner

Description: The flow divider is used to synchronize the volume flow for cylinders and engine controls. Design: The flow divider consists of a housing, 2 check pistons and a spring, with 4 ports, whereas port 1 is not used. Function: In dividing mode, the flow divider will divert input flow, according to the specified ratio, from port 3 to ports 2 and 4. In combining mode, input flow will be combined, according to the specified ratio, from ports 2 and 4 into port 3. Port 1 is not used and should be blanked. housing

check piston

�

�

spring

� �

�

� Fig. Flow divider / combiner (ST16-01)

133

Model code: The model code is composed of the following sequences: �������������������������������� ����������� �������������� � ��� ���� ��� ����

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134

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4.3

Pressure valves

Pressure control valves influence and limit the pressure in hydraulic systems. Through forces acting on a closing element certain pressures in segments of a hydraulic system are limited or regulated. The piston rods can take up innumerable positions from „completely closed“ to „completely open“, depending on the volume flow and differential pressure. This functions on a balance base between pressure and spring force. Valves are produced in the following designs: • pipe fitting • subplate mounting • sandwich plate mounting • valves in a threaded plug • insert valves Depending on the function they are distinguished as follows:

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Fig. Overview: pressure valves

135

4.3.1

Pressure relief valves

Description: Pressure relief valves are used in hydraulic systems to limit the system pressure to a specific set level. If this set level is reached, the pressure relief valve is activated and feeds the excess flow back to the tank. The diagram shows this function:

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136

Design: Basically this valve consists of a valve body with ­integrated seat valve, a cut and tempered steel cone, a spring and a check unit for adjusting the spring load. Function: The force of the pretensioned spring acts in the direction of the closure. As long as the spring force is larger than the pressure force, the seating element stays on its seat. If the pressure exceeds the spring force, the element pushes against the spring and opens the connection. The excess fluid returns to the tank. As the fluid flows away via the ­ressure control valve, hydraulic energy is converted into heat.

adjustment unit

housing

spring

cone

seat valve

� �

�

� Fig. Pressure relief valve (DB3E)

�

Please note:

If the connectors are mixed up or if the pressure preset has been improperly adjusted, the safety function of the valve is jeopardized. The opening pressure of the valve is increased by the pressure of tank connector 2.

Fig. Pilot operated pressure relief valve (DB3)

137

4.3.1.1 Pressure relief valve pilot operated Description: This pressure relief valve is a spring-mass system, which causes oscillations when it is moved. These oscillations affect the pressure and must be eliminated by damping. Design: Underneath the damping mechanism we find a hollow space, which accommodates the oil. When the piston moves down, the oil is slowly displaced and thus dampens the downward movement. When the piston moves upwards a vacuum is created inside the said hollow space, which equally dampens the oscillations. adjustment unit

housing

spring

spring partition

closing cone with damping piston

� �

�

�

Fig. Pressure relief valve with shock damping (DB4E)

�

Please note:

If the connectors are mixed up or if the pressure preset has been improperly adjusted, the safety function of the valve is jeopardized. The opening pressure of the valve is increased be the pressure of tank connector 2.

138

4.3.1.2 Pressure relief valve pilot operated Description: The larger the volume flow in a hydraulic unit, the larger has to bet he diameter of the inlet. Therefore the piston ­diameter is automatically greater as well. This results in a larger force, which in turn corresponds to a higher spring force. Area and spring force rise by the power of two with the piston diameter. With volume flows of this magnitude, you have to use spring sizes, which are not longer meaningful. Therefore pressure relief valves are pilot operated so that the space required is as small as possible, even with large volume flows. Design: In addition to a main control unit pilot operated pressure relief valves feature a pilot operated unit made of a cut and ­tempered cone and a second spring. Function: The pretension of the spring pushes the cone on the seat of the valve. The unit pressure acts on the underside of the cone via small holes in both locking elements. adjustment unit

housing

pilot operated unit

spring cone return spring

main control unit

piston

�

�

�

�

Fig. Pilot operated pressure relief valve (DB10)

�

Please note:

Due to the spool valve design this valve is not suitable for a leakage free supply to the consumer. If the connectors are mixed up or the preset pressure is not properly adjusted, the safety function of the valve is jeopardized. Application: • as a safety valve for pressure limiting up to the maximum allowable pressure • as a safety valve for pumps

Fig. Pilot operated pressure releif valve (DB10)

139

4.3.1.3 Pressure relief valve, pilot operated, electric relief function Description: This pressure relief valve is mechanically and electrically dischargeable (pressure less circulation). This combination of a pressure relief valve and a 2/2 directional valve is space and cost saving. Design: In addition to a main control unit pilot operated pressure relief valve features a pilot operated unit made of a cut and ­tempered cone and a second spring. For the electric discharge, the valve has a solenoid. Function: The DB12120 AMPZ is a pilot operated pressure relief valve in spool design with electric relief function. If the pressure exceeds the spring force at port 1, the pilot operated unit opens and the oil flows to the back side of the piston to port 2. Because of the pressure difference the main piston opens and the oil can flow to port 2. With the electric relief function it is possible to switch the valve in depressurised flow. adjustment unit

solenoid coil

housing spring cone return spring piston �

� �

�

Fig. Pressure relief valve, pilot operated, electric relief function (DB12120APMZ)

�

Please note:

Due to the spool valve design this valve is not suitable for a leakage free supply to the consumer. If the connectors are mixed up or the preset pressure is not properly adjusted, the safety function of the valve is jeopardized. Application: • ideal for basic modules in the power unit production, integration in pumps • used with constant pumps as an alternative for regulated pumps • electric accumulator diagrams

140

Model code: The model code is composed of the following sequences: ������������������������������������������� ���� �������������������������� ��� ��������������������������������� ������ �������������������������� ���������������������� ������������� ������������� ������������������ ������������������������ ����������������������������������� ������������������������������������� �������������������������������������������� ��������������������������������������������

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141

4.3.2

Sequence valves

Design: The design of a sequence valve is basically a pressure relief valve with an additional check valve as a bypass. It is used when the opening pressure should not be influenced by the tank pressure or if you need a pressure relief valve which permits flow in both directions. The valve consists of the following components: valve body, check valve, a cut and ­tempered piston, a spring and an adjustment unit for presetting the spring. Function: A pressure spring acts with a preset force on the piston and keeps it in the start position against the spring force of the check piston. The connection between port 1 to 2 is closed. If the pressure increases at port 1 via the preset pressure of the spring, piston and check valve move jointly up to the upper stroke end. If the pressure increases still the piston sets the connection between 1 and 2 free, thus consumers are connected which are fed through port 2. The pressure is ­independent from the pressure at port 2, since the spring casing is sealed against the hydraulic unit. For a free flow from 2 to 1 the piston is pushed back by the spring into the starting position. The check valve then opens against the pressure of the spring.

adjustment unit

housing

spring

spring partition

return piston spring

�

�

�

142

�

Fig. Sequence valve (DZ5E)

Sequence valves are applied: • for adding cylinders in sequence • as a pressure relief valve, when you do not want the tank pressure to influence the opening pressure • as a pressure relief valve, when a free flow is required in both directions The circuit diagram shows the application of the valve for adding cylinders in sequence:

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�

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�

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� ���� Fig. Circuit diagram: sequence valve (DZ5E)

143

Model code: The model code is composed of the following sequences: ��������������������������������������������� ���� ���������������������������� ���� ������������� ������ �������������������������� ���������������������� ������������� ������������� ������������������ �������������� ������������������������� �����������������������

144

4.3.3

2-way pressure control valve (pressure reduce valve)

Design: A pressure control valve maintains a reduced pressure in a defined area of a hydraulic system. It reacts when the ­ ressure increases and blocks the volume flow to the controlled area until the pressure falls again below the preset value. p The valve consists basically of the following components: valve housing, a spring and a spool (piston). Function: The figure shows the characteristic design of a 2-way pressure control valve, which controls the secondary pressure with a spring-loaded piston valve. If the value of the system pressure lies below the preset value of the valve, ­unrestricted flow is possible from 1 to 2. The outlet pressure is transferred via a internal channel to the piston side opposite of the spring. If the pressure at outlet 2 reaches the reacting point of the valve, the piston moves towards the spring and thus reduces the outlet cross section. By this means of throttling exactly as much fluid flows through the valve as to support the preset pressure. If the pressure at port 2 reaches the preset value, the valve shuts down completely and thus ­prevents a fluid flow from 1 to 2.

adjustment unit

spring

piston

� piston valve

�

�

� Fig. 2-way pressure control valve (pressure reduce valve)

145

4.3.4

3-way pressure control valve

Function: Pressure Control Function from 2 to 1: In the start position a free flow through the pressure control valve from the high pressure side (port 2) through the check valve to the low pressure side (port 1) is possible. The pressure increases at port 1 due to an actuator, i. e. a cylinder acts upon the piston surface and creates an opposing force to the spring (preset pressure). If this opposing force is smaller than the preset force the piston remains in its start position. If the pressure increases due to the ­demands of the actuator the piston is moved against the spring. The inlet drillings at port 2 are closed in so far as only that much flow is permitted which is consumed by the actuator without pressure increase. If the actuator does not take more fluid, i. e. if the piston has reached stroke end of the cylinder, the piston reverses the complete stroke and closes the access drillings. If the starting pressure drops below the preset pressure by a lesser demand of the actuator, the piston is repositioned by a spring in the respective position (flow from 2 to 1) and the process starts again. The ­maximum adjustable starting ­pressure is limited by a pressure spring. Due to the design of a piston of a valve there are minute losses of leakage oil during the check processes. External pressure control from 1 to 3: If the pressure increases at port 1 through external forces via the preset pressure the piston is further slid against the sring and takes pressure off the of actuator in relation to the tank (external pressure reduction limiting). The pressure at port 1 is limited. Flow control from 1 to 2: Pressure control valves can be basically flushed trough port 1 to 2 . On the other hand you have to bear in mind the other volume flows. adjustment unit

spring

housing

check piston

�

�

�

�

� �

Fig. 3-way pressure control valve (DMVE)

146

Model code: The model code is composed of the following sequences: ��������������������������������������������������� ���� ������������������������������

���� ����� ���� ������������� ������ ���������������������������� ���������������������� ������������� ������������� ������������������ ������������������������ �������������� �������������������������������������������� ����������������������������������� ������������������������������������������������������������������

147

4.4

Directional valves

4.4.1

General remarks

Directional valves „control“ the volume flow in a hydraulic unit. „Control“ in this respect means that you can „steer“ or „stop“ or „start“ the volume flow in different directions. The valves are distinguished according to the following main features: • mode of operation • number of passages • number of switching positions • design of control element (spool or poppet valves) • depending on circumstances there are different possibilities to operate a directional valve We distinguish between the following operational modes: • manual operation: a manually operated directional valve features a lever, which is connected by rods with the control element. • mechanical operation: mechanical adjustment elements exist either in roller shaft or plunger design. It is operated through a cylinder or cam. • pneumatic operation: the control element is being operated by compressed air. • hydraulic operation: pilot pressure is being used to switch the control elements. However the pilot fluid is controlled by a separate directional valve. Hydraulically controlled valves cannot act out of their own accord. • electric operation: magnets are used with electrically operated valves. They respond to a electric signal and thus move the control element by means of a magnetic field.

148

Operational mode

Manual

Example

Symbol DIN 1219

Lever



Pedal



Push button



Mechanical



Plunger

Roller shaft



Pneumatical

Compressed air



Hydraulical

Pilot fluid



Electrical

Magnetic spool

Fig. Operation symbols Description: The naming of a directional valve is a fairly easy process. The number of connectors and switching positions of the valve is part of the product name. Example: A directional valve with 3 connectors (pump, tank, consumer) and 2 switching positions is called a 3/2 directional valve. The switching positions are indicated in small letters, like a and b.

� �

� �

�

The centering position of a directional valve with 3 switching positions is also the neutral position. The individual ­connectors are indicated either with capital letters like P for pump, T for tank, B and A for consumer connections or numbers 1 to 4.

149

We distinguish two different kinds of control elements: • directional spool valves • directional poppet valves In a directional poppet valve the volume flow is controlled by a closing element. It has the advantage of sealing the connectors so that no leakage occurs. In a directional spool valve the volume flow is controlled by the movements of a piston. Due to their design these valves permit some leakage oil to escape. In the housing of directional poppet valves you always find one or more moving parts in different shapes like balls, cones or plates which tightly seal the valve. Therefore no leakage oil can escape.

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4.4.2

Poppet directional valve

Advantages: • no leakage • high operational endurance, no leakage oil and no slots, which might get clogged • very high pressure resistance • isolating function without additional check valves Disadvantages: • very low volume flow • pressure loss during switching modes due to negative overlap • rough dirt can damage the poppet and thus causing leakage

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151

4.4.2.1 2/2 poppet directional valve Design: The picture on this page shows a directional valve in poppet design with 2 connectors and 2 switching possibilities. Hence the name 2/2 poppet directional valve. The valve is operated by a magnetic spool. The switch-back to the ­starting ­position is done by an in-built spring. Function: If there is no electric current on the solenoid coil the valve poppet is kept by the opening spring in the neutral position. The valve is open and the fluid can flow from port 2 to port 1 and also in the opposite direction. If there is a current on the solenoid coil the armature of the solenoid lifts. At the same time the plunger moves the valve poppet onto the valve seat against the force of the opening spring. Now the valve is closed for the flow direction from port 2 to port 1. The force of the solenoid keeps the valve closed in the opposite direction up to a differential pressure of approx. 400 bar.

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152

4.4.2.2 3/2 poppet directional valve Design: The figure shown below shows a directional valve in poppet design with 3 ports and 2 switching possibilities. It is ­therefore a 3/2 poppet directional valve. This valve is activated via a solenoid coil. A spring returns the valve in the neutral ­position. Function: In the neutral position the ports of the pump are closed without any leakage. Port 2 is connected to the tank. If the ­solenoid is activated the sealing element is moved and tank port 3 sealed without any leakage possibility. Flow is ­possible from pump port 1 to port 2.

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153

4.4.3

Directional spool valve

Description: A directional spool valve consists of a housing with a drill hole and a piston with clearance adaptation, which permits it to move. Additionally there are annular passages inside the housing, which serve as a spool land and cooperate with the edges of the control piston. The separation and connection of the annulus areas is done by the movements of the control piston. Due to the clearance adaptation it is not possible to seal the valve completely against leakage. Therefore we need an additional check valve in the hydraulic unit. The quantity of leakage oil depends on the viscosity of the fluid, the ­dimensions of the slits and the cover. Advantages: • simple design • suitable for large volume flows Disadvantages: • leakage caused by clearance adaptation, differences of viscosity and pressure differences • high losses of leakage oil with high pressures • any direction of volume flow possible • contamination can lead to clogging solenoid coil

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piston Fig. 4/2 directional spool valve (WK10Y-01)

154

4.4.3.1 4/2 directional spool valve Design: The picture shows a spool valve in directly operated spool valve design with 4 ports and 2 switching possibilities. ­Therefore it is called 4/2 directional spool valve. The valve is operated via a solenoid coil. A spring returns the valve to its neutral position. Function: The tempered and cut piston is kept in the neutral position by a pressure spring, when there is no current in the ­solenoid coil. Port 3 is connected to 2 and port 1 with port 4. A flow is possible only in the direction of the arrow. If there is current flowing through the solenoid coil the armature of the solenoid pushes the piston via the plunger into the foremost position. Port 3 and port 2 as well as port 1 and port 4 are now connected.

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155

4.4.3.2 4/3 directional spool valve Design: A 4/3 directional spool valve in directly operated spool design features 4 ports and 3 switching possibilities. The valve is operated via two solenoid coils. A spring repositions the valve into neutral. Function: In the neutral position the pilot piston is kept in place by a built-in spring. The pilot piston is operated by solenoids ­immersed in oil. The solenoid pushes the pilot piston out of neutral into its final position, thus permitting fluid flow as ­specified by the symbols. When the solenoid is switched off a return spring pushes back the pilot piston into neutral.

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156

4.4.4

4.4.4.1 4/3 directional spool valve Design: The directional spool valve has 2 solenoids and one piston which is kept in mid-position by return springs. An emergency feature permits the activation of the valve without solenoids. Function: In the neutral position (0) the spool valve is in a floating mid-position. In positon (a) the valve has a flow path from P to B and in position (b) from P to A. piston

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Directional spool valve (sandwich plate design)

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157

4.4.4.2 4/3 directional spool valve, pilot operated The spool valves so far discussed have the disadvantage that they cannot be used for larger volume flows. The reason for that are the tremendous moving forces to push the pilot piston, which cannot be supplied by solenoids. If we have large volume flows pilot operated directional valves must be used. They consist of a main valve and a pilot operated valve. The pilot operated valve is operated via solenoids. This valve is operated via a hydraulic pilot signal, which moves the main pilot piston. Design: For lager volume flows pilot operated spool valves are used. They consist of a main valve (1) and a pilot valve (2). Function: In the neutral position (0) the pilot valve is in a floating mid-position. The main valve is relieved from pilot pressure and is located in a spring centered mid-position. In switching position (a) the main valve permits fluid flow from P (A) to B (T) and in switching position (b) from P (B) to A (T).

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4.5

Proportional valves

With proportional directional control valves fairly complex procedures and programs of an actuator can be controlled (accelerate, brake etc.). The output is proportional to an electric input signal. Thus the direction of motions and velocities can be controlled with just one device.

4.5.1

4/3 proportional valves, pilot operated

Design: A proportional valve consists of a housing (casing), a piston with two return springs and two proportional solenoids. Function: The piston can be moved continuously with the proportional solenoids. If the magnet y1 switches the piston, the volume flow from P to B and A to T is open. By switching on the magnet y2 the passages from P to A and B to T are open. piston proportional solenoid

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4.5.2

4/3 proportional valves, balanced

Design: A proportional directional control valve consists of a pilot valve (pressure control valve) and a main valve (directional control valve). A pilot valve consists of a housing, two pilot pistons and two proportional solenoids. The main valve consists of a housing, a main piston and a centering spring. Function: If solenoid b is activated, the pilot piston moves to the right. The pilot fluid flows into spring chamber. At the same time pilot pressure port X on the opposite main piston side is relieved from pressure at port Y. Inside the spring ­chamber ­pressure increases depending on the force of the solenoid. The resulting pilot pressure pushes the main piston to the left. This creates a certain flow whose force depends on the incoming flow. An advantage of ­proportional valves is the lack of steps in the cross-section of the openings. Proportional directional control valves can also be equipped with pressure compensators. They can be attached on a middle plate under the proportional valve. In doing so you ­ascertain a flow independent from any pressure fluctuations at the throttle. Pressure compensators can be used in feed lines, as well as in discharge lines. pressure control valve proportional solenoid

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4.6

2-way cartridge valve (logic function elements)

Description: A 2-way cartridge valves are check valves. The main ports A and B can be shut or opened. With this simple construction the valve is able to control a high volume flow. The assembly of complex hydraulic diagrams with valves or complete valve functions is possible. Design: A 2-way cartridge valve consists of a lid and a cartridge with a valve cone, which is kept in position on the valve seat by a spring. Function: • volume flow inside the valve is possible in both directions (A to B, B to A). • the 2-way cartridge valve works with pressure dependence. • to fullfil its function the valve cone is shaped in three steps (A1 bis A3).

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Example diagram for a 4/3 directional valve with logic function elements: With the 4/3 directional valve it is possible to switch the corresponding logic function elements to move in or out the c­ ylinder.

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Introduction to Accumulator Technology

V

Introduction to accumulator technology

1

Introduction�������������������������������������������������������������������������������������������������������������������������� 299

2 2.1 2.2 2.3 2.4 2.5 2.6 2.7 2.8 2.8.1 2.8.2 2.8.3 2.8.4 2.8.5 2.8.6

Product range hydraulic accumulators HYDAC�������������������������������������������������������������������������� 301 Bladder accumulator���������������������������������������������������������������������������������������������������������������� 301 Piston accumulator������������������������������������������������������������������������������������������������������������������ 301 Diaphragm accumulator������������������������������������������������������������������������������������������������������������ 301 Metal membrane accumulators��������������������������������������������������������������������������������������������������� 302 Hydraulic dampers ������������������������������������������������������������������������������������������������������������������ 302 Special design accumulators������������������������������������������������������������������������������������������������������ 302 Accumulator stations���������������������������������������������������������������������������������������������������������������� 302 Accessories for accumulators����������������������������������������������������������������������������������������������������� 303 Back-up of N2-bottles���������������������������������������������������������������������������������������������������������������� 303 Charging and testing unit����������������������������������������������������������������������������������������������������������� 303 Safety and shut-off block����������������������������������������������������������������������������������������������������������� 303 Safety devices for hydraulic accumulators (fluid side)���������������������������������������������������������������������� 303 Fitting elements for hydraulic accumulators����������������������������������������������������������������������������������� 303 ACCUSET SB330�������������������������������������������������������������������������������������������������������������������� 303

3 3.1 3.1.1 3.2 3.3 3.4 3.5 3.6 3.6.1 3.6.2 3.6.3 3.6.4.

Designs of hydropneumatic accumulators and accessories�������������������������������������������������������� 305 Bladder accumulators��������������������������������������������������������������������������������������������������������������� 307 Special design High-Flow-Accumulator���������������������������������������������������������������������������������������� 309 Piston accumulator������������������������������������������������������������������������������������������������������������������ 310 Diaphragm accumulators����������������������������������������������������������������������������������������������������������� 314 Metal bellows accumulators������������������������������������������������������������������������������������������������������� 316 Weight reduced hydraulic accumulators��������������������������������������������������������������������������������������� 317 Hydraulic dampers������������������������������������������������������������������������������������������������������������������� 318 Pulsation dampers������������������������������������������������������������������������������������������������������������������� 319 Suction stabilizers�������������������������������������������������������������������������������������������������������������������� 320 Silencers�������������������������������������������������������������������������������������������������������������������������������� 321 Shock absorption��������������������������������������������������������������������������������������������������������������������� 322

4 4.1 4.2 4.3 4.4

Accumulator station��������������������������������������������������������������������������������������������������������������� 325 Sequential fitting of nitrogen bottles in bladder accumulators������������������������������������������������������������ 325 Connecting nitrogen bottles to bladder accumulators����������������������������������������������������������������������� 326 Sequential fitting of nitrogen bottles to piston accumulators�������������������������������������������������������������� 327 Connecting nitrogen bottles to piston accumulators������������������������������������������������������������������������� 328

5 5.1 5.2 5.3 5.3.1 5.3.1.1 5.3.1.2 5.3.1.3 5.3.2 5.3.2.1 5.3.2.2 5.3.2.3 5.3.2.4

Accessories�������������������������������������������������������������������������������������������������������������������������� 331 Universal charging and testing unit FPU-1������������������������������������������������������������������������������������ 331 Nitrogen charging unit��������������������������������������������������������������������������������������������������������������� 332 Safety measures��������������������������������������������������������������������������������������������������������������������� 333 Safeguarding on fluid side��������������������������������������������������������������������������������������������������������� 334 Safety and shut-off block (SAF)�������������������������������������������������������������������������������������������������� 334 DSV 10���������������������������������������������������������������������������������������������������������������������������������� 336 Special designs with logic valves������������������������������������������������������������������������������������������������ 336 Safeguarding the gas side��������������������������������������������������������������������������������������������������������� 337 Fire protection������������������������������������������������������������������������������������������������������������������������� 337 Burst plates���������������������������������������������������������������������������������������������������������������������������� 338 Gas safety valves�������������������������������������������������������������������������������������������������������������������� 338 Gas safety block���������������������������������������������������������������������������������������������������������������������� 339

6 6.1 6.1.1 6.1.2 6.1.3 6.2 6.2.1 6.3 6.3.1 6.4 6.4.1 6.5 6.6 6.7

Application of hydraulic accumulators������������������������������������������������������������������������������������� 341 Energy storage������������������������������������������������������������������������������������������������������������������������ 341 Energy storage in injection moulding machines������������������������������������������������������������������������������ 341 Energy storage and reduction of stroke time���������������������������������������������������������������������������������� 342 Energy storage for emergency function of hydraulic cylinders ����������������������������������������������������������� 342 Volume compensation�������������������������������������������������������������������������������������������������������������� 343 Volume compensation for leakage oil compensation����������������������������������������������������������������������� 343 Pulsation damping������������������������������������������������������������������������������������������������������������������� 344 Pulsation damping in a displacement pump����������������������������������������������������������������������������������� 344 Pressure surge damping����������������������������������������������������������������������������������������������������������� 345 Pressure surge damping in mobile units��������������������������������������������������������������������������������������� 345 Fluid separation����������������������������������������������������������������������������������������������������������������������� 346 Silencers�������������������������������������������������������������������������������������������������������������������������������� 346 Offshore applications (oil- and gas industry)���������������������������������������������������������������������������������� 347

7 7.1 7.2 7.3 7.3.1 7.3.2 7.3.3 7.3.4 7.3.5

Selection of hydro-pneumatic accumulators����������������������������������������������������������������������������� 349 Selection criteria���������������������������������������������������������������������������������������������������������������������� 349 Definition of operational parameters�������������������������������������������������������������������������������������������� 350 Definitions������������������������������������������������������������������������������������������������������������������������������ 351 Isobaric��������������������������������������������������������������������������������������������������������������������������������� 351 Isochoric�������������������������������������������������������������������������������������������������������������������������������� 351 Isothermal������������������������������������������������������������������������������������������������������������������������������ 351 Adiabatic�������������������������������������������������������������������������������������������������������������������������������� 351 Polytropic������������������������������������������������������������������������������������������������������������������������������� 352

297

7.4 7.5 7.6 7.7

Calculation by means of basic physical equations��������������������������������������������������������������������������� 352 Computation in case of deviation from ideal gas behaviour��������������������������������������������������������������� 356 Computation of thermal time response����������������������������������������������������������������������������������������� 356 Design of hydro-pneumatic accumulators with ASP-software������������������������������������������������������������ 357

8

Safety directives and regulations��������������������������������������������������������������������������������������������� 359



List of figures: accumulator technology������������������������������������������������������������������������������������ 360



298

V

Introduction to accumulator technology

1

Introduction

Hydraulic accumulators are pressurized containers in which a certain useful volume is stored. If necessary, the stored fluid volume can be returned to the system without further supportive energy. In order to store energy in accumulators, the fluid in an accumulator is weight or spring loaded or pressurised by a gas (hydropneumatically). Due to their system specific advantages hydraulic accumulators are preferably used for miscellaneous tasks in hydraulic systems. Application: • energy storage • emergency and safety functions • leakage oil compensation • volume compensation • suction stabilization • damping and cushioning of vibrations, fluctuations, pulsations (pulsation damping) and shocks and surges (shock absorption) as well as noise levels (noise absorption) • separation of fluids • mass balancing • energy recovery

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Please note:

All accumulators have to comply to the European Pressure Equipment Directive PED / DGR (97/23/EG) (see chapter 8)!

Fig. Hydraulic accumulators

299

2

Product range hydraulic accumulators HYDAC

2.1

Bladder accumulator Type:

SB

Nominal volume: 0.5 - 450 liter Max. excess operational pressure: up to 1000 bar Standard design: Nominal volume: 0.5 - 130 liter Max. excess operational pressure: 330 - 550 bar Special edition: stainless steel, compounds and composites SB 800 / 1000 in high pressure bladder accumulator design Low pressure design: Nominal volume: 2.5 - 450 liter Max. excess operational pressure: up to 40 bar High pressure design: Nominal volume: 1 - 54 liters Max. excess operational pressure: 690 - 1000 bar

2.2

Piston accumulator Type:

SK

Nominal volume: 0.2 - 1200 liter Max. excess operational pressure: 1000 bar Hydraulic piston accumulator Nominal volume: 0.2 - 1200 liter Max. excess operational pressure: 210 - 350 bar Type series SK280 Nominal volume: 0.16 - 6 liter Max. excess operational pressure: 280 bar Position control: • limit switch • extended piston rod • supersonic position-measuring system • cable control measuring system • magnet flap indicating system • piston positioning switch

2.3

Diaphragm accumulator Type:

SBO

Nominal volume: 0.075 - 4 liter Max. excess operational pressure: up to 750 bar Welded design Nominal volume: 0.075 - 4 liter Max. excess operational pressure: 50 - 330 bar Threaded design: Nominal volume: 0.1 - 4 liter Max. excess operational pressure: 210 - 750 bar

301

2.4

Metal membrane accumulators Type:

SM

Nominal volume: 6 liter Max. excess operational pressure: up to 350 bar

2.5

Hydraulic dampers Type:

• pulsation damper • suction stabilizer • fluid based silencer

Nominal volume: 0.075 - 450 liter Max. excess operational pressure: 10 - 1000 bar

2.6

Special design accumulators Weight-reduced bladder, piston and diaphragm accumulators. Weight can be saved by special production methods and usage of alternative materials, like aluminium (aluminum). This is especially used in mobile versions of the aggregates. Spring loaded accumulators are equipped with a spring, hence the name. In this case the energy is derived from that spring and not from gas. If bladder accumulators are produced as a composite structure (plastic / steel) the weight is cut by half (in comparison to the steel version).

2.7

Accumulator stations Type:

SS

Nominal volume of accumulators: 250 liter Nominal volume of nitrogen accumulators: 320 liter Max. excess operational pressure: up to 350 bar

302

2.8

Accessories for accumulators

Fig. Accessories for accumulators

2.8.1

Back-up of N2-bottles

The nitrogen bottles are used as a back-up for bladder and piston accumulators. They increase the gas volume in the accumulator. Nominal volume: 20 - 50 liter Effective gas volume: 17.5 - 47.5 liter

2.8.2

Charging and testing unit

Type: FPU

Typ: F+P

Technical data: • FPU-1 universal charging and testing unit up to 400 bar p0 • FPS600 for bladder accumulators up to 600 bar p0 • FPK600 for piston and diaphragm accumulator up to 600 bar p0 • FPH800 for SB, SBO and SK up to 800 bar p0

Technical data: • max. operational excess pressure up to 400 bar • nominal diameter: din16 / dn32 • materials: c-steel, rust-resistant steel • optimized dimensions

2.8.3

Safety and shut-off block

Type: Nominal volume: Max. excess operational pressure: Pressure relief valve:

2.8.4

SAF / DSV 10 - 32 liter up to 400 bar (DSV to 350 bar) nominal value DN12

Safety devices for hydraulic accumulators (fluid side)

Technical Data: • design: direct acting safety valve, spring loaded • gas: Nitrogen (N2) min. class 4.0 • response pressure: up to 370 bar • design: DGRL 97/23/EG CE0034 By default you can choose the following HYDAC safety elements for the gas inlet side: • direct acting gas safety valve GSV6 with • CE-certification • safety fuse against melting and • rupture disk with CE-certification. • gas safety block GSB450

2.8.5

Fitting elements for hydraulic accumulators

Accumulator sets, clamps and brackets for optimal fixing / mounting of hydraulic accumulators.

2.8.6

ACCUSET SB330

Nominal volume: 1 - 50 liter Max. excess operational pressure: up to 330 bar

303

3

Designs of hydropneumatic accumulators and accessories

We distinguish hydraulic accumulators according to the design of their separators between gas and fluids, as well as the storage medium (weight, spring or gas). All designs can store pressure energy. In mechanical systems (weight and spring loaded) energy is stored by the transformation of potential energy.

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weight-loaded

spring-loaded

air dome / air chamber with seperator between gas and fluid side

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bladder diaphragm piston accumu- accumulator accumulator lator

metal ­bellows accumulator

Fig. Designs of hydropneumatic accumulators Accordingly in gas-loaded accumulators the inert energy in a gas is transformed. Hydropneumatic ­accumulators can be distinguished as follows: • accumulator with separator • accumulator without separator According to the separator between gas and fluid, hydraulic accumulators can be also distinguished as follows: • bladder accumulator • piston accumulator • diaphragm accumulator • metal membrane accumulator The mode of operation of these accumulators is based on the compressibility of an inert gas, which is used for storing fluids. Usually nitrogen is used as the energy carrier.

�

Please note:

Due to danger of explosion only inert gases may be used for charging hydraulic accumulators. Hydropneumatic accumulators essentially consist of a fluid and a gas section, divided by a gas tight seperator. The fluid section is connected to the hydraulic circuit. So when the pressure increases, the gas on the opposite side is compressed. If the pressure decreases, the compressed gas can expand again and the stored liquid is pushed back into the hydraulic circuit. Best suited as charging gas is nitrogen.

305

Technical comparison of standardized accumulators: Bladder accumulator SB

Pressure Volume p0 : p2 bar 1litres l/min 5 330 550 0.5...450 1:4 690 (1000)

Piston accumulator SK 1...350 350 1:00 (1000) (1200) Diaphragm- accumulator 0.075...3.5 SBO 210 1:8 350 (1000) 0.1...4 1:10 Special designs: Pressure Volume Advantage Application bar Liter Metal bellows Metal bellows accumulator accumulator also as pulsation damper SM 5...330 to 6 • Gas tight • Large scale diesel engines in • No maintenance required ships, power plants and. in • Media consistent in large support or temperature ranges (-65...+160°C) emergency power generators • Large scale or compact • Unsusceptible to contamination • Automotive industry • Agricultural machines / construction machines • Aviation Spring loaded • No gas charge g no loss of gas accumulator SF

5...330 to 6 • Compact design • Mode of operation independent of temperatures as opposed to accumulators charged with gas

306

Low volume intake, Dependent on spring size and strength • Automotive industry • Agricultural machines / construction machines

3.1

Bladder accumulators

Fig. Bladder accumulator Design: The bladder accumulator depicted in the illustration above consists of a solid and sturdy pressure container, which has to meet the system requirements in terms of pressure. Inside the container is fitted a sealed and closed bladder made of a rubber-like material (elastomer). In order to fill or charge the bladder a gas valve is attached at the upper section of the bladder. At the lower end of the pressure container or accumulator a fluid valve is fitted, which first and foremost ­prevents the bladder to be sucked out as well, when the fluid inside the accumulator is discharged. The cross-section of the valve has to be dimensioned in such a way, that a volumetric flow, which in turn depends on the capacity of the ­accumulator and viscosity of the fluid, may not exceed values of 4 to 30 l/s in the standard version.

gas valve body

bladder

container

oil valve air bleed plug

Fig. Bladder accumulator

307

Function: The bladder is pre-filled with pressurized nitrogen (p0 = gas prefill pressure, as defined by the manufacturer). The fluid valve is closed and prevents the bladder from discharging. As and when the system pressure exceeds the prefill ­pressure, the valve opens and hydraulic fluid can flow into the accumulator. If the pressure increases still, the gas volume is ­compressed until the maximum operational pressure has been reached. (p2). The change in volume between the levels at min. and max. operating pressures correspond to the useful fluid volume.

�� ��

�� ��

���

�� ��

p0 = gas pre-charge pressure V0 = effective gas volume V1 = gas volume at p1

p1 = minimum operational pressure

p2 = maximum operational pressure

V2 = gas volume at p2 DV = effective volume Fig. Function principle of bladder accumulators

�

Please note:

In some exceptional cases an assembly and dismounting is also possible from the gas side, if the removal of the ­accumulator for bladder exchanges is very complicated or if the bladder exchange has to be done very rapidly (top-repair version). The bladder accumulators should always be fitted in an upright (vertical) position. This means that the gas valve is always at the top and the fluid valve always at the bottom. The more a bladder accumulator is tilted, the worse the fluid exchange gets in respect to fluid quantity and flow velocity. Compatibility of elastomers in bladder accumulators: The bladder material has to be adjusted to the operational media and temperature. In choosing the appropriate elastomer one thing has to be considered. In some cases, if the fluid discharge situation is unfavourable (high pressure ratio p2 / p0, high discharge velocity), the gas could cool down below the permissible elastomer temperature, which might cause ruptures in the bladder. By means of HYDAC‘s accumulator simulation program ASP, the gas temperature can be calculated. Depending on the operational medium and application different kinds of elastomers are available. Please use our ­technical data sheets for further reference.

308

Advantages: • large volume flows are possible • low inertia • compact • low maintenance costs • good value for money • high frequency is possible Disadvantages: • limited pressure ratios • bladder wear-out • restricted mounting positions • sudden loss of gas when bladder rupture occurs

3.1.1

Special design High-Flow-Accumulator

High-flow accumulators are especially designed constructions, which permit volumetric flows up to 140 l/s. The ­distinctiveness of these high performance accumulators is due to the fact that the fluid connection is enlarged to allow higher flow rates. One option or version of a high performance accumulator is shown below this paragraph. It can be used for operational pressures up to 330 bar. The adapter contains a pre-charged check valve. A discharge of the bladder due to a sudden pressure drop in the system or a complete draining is thus prevented. Furthermore the valve shaft is fitted with a damping device, which ensures that the valve is not damaged by high volumetric flows during the opening and closing ­procedure. Consequently the borehole in the pressure vessel for the mounting of the fluid valve has a larger diameter than the ­ orehole for the attachment of the gas valve. Thus the fitting and removal of the bladder from the fluid side is b ­predetermined.

Fig. Low pressure bladder accumulator (High-Flow)

Fig. High pressure bladder accumulator (High-Flow)

Due to the application of different oil valve designs, the maximum possible volume flow can be adjusted to the actual requirements.

309

3.2

Piston accumulator

Fig. Piston accumulator Design: The typical design of a piston accumulator is shown in the following picture. The main components of piston accumulators are a cylinder with a finely finished internal surface and an end cap on the gas side.They are sealed with o-rings and have a lightweight metal piston. The cylinder fullfills two functions. Firstly it contains the internal pressure and secondly it ­guides the piston, which serves as a separation element between gas and fluid side. gas valve body sealing cover

gas valve body sealing cover cylinder pipe

cylinder pipe piston piston fluid port fluid port sealing cover sealing cover

Fig. Piston accumulator (screw-type)

Fig. Piston accumulator (SK280) (crimped)

Function: The charging with nitrogen up to the pre-charge pressure level causes the piston to move to the lid on the fluid side and thus covers the borehole of the fluid inlet. Once the precharge pressure level has been reached and the initial breakaway momentum of the piston has been transgressed, the piston moves into the gas chamber and compresses the gas. The effective volume V can be derived from the compressed gas volume V1 and V2. Initial breakaway momentum and losses due to friction during movements depend entirely on the piston design being used.

310

��

�� ��

�� ��

���

��

p1 = minimum operational pressure

p0 = gas pre-charge pressure V0 = effective gas volume V1 = gas volume at p1

p2 = maximum operational pressure

V2 = gas volume p2 DV = effective volume Fig. Function principle of piston accumulators

In order to create a preferably balanced pressure level, friction between piston sealing and interior cylinder wall must be as low as possible. Therefore the inside of the cylinder has to be finely finished. However a pressure difference between gas and fluid chamber cannot be avoided. The diagram below shows the course of oil and gas pressure in relation to time for an accumulator cycle with two different sealing systems. As you can see a low-friction sealing system generates smaller differences between both pressures and thus better operational behaviour. However, the friction resistance is not constant but increases with increasing operational pressure. As long as the operational pressures are low, friction resistance dominates in comparison to the movement of the piston. Thus it seems that operations with low pressures are not meaningful. Certain functions of the hydraulic system, like turning off the supply pump or monitoring the fluid level in the accumulator can be directly influenced by modification of the piston accumulator. The problem is solved by attaching the piston rod to the piston and subsequently led out of the accumulator. Measurment of oil and gas pressure in piston accumulators

pressure [bar]

360 340 320 300 280 260 240

0

40

80

120 time [s]

160

200

240

Fig. Diagram oil and gas pressure This presents the possibility to realize certain control functions by different means, like: • mechanically by means of a cam switch or • electrically by means of permanent magnets or • inductive proximity switches Another possibility to determine the piston position is to use an ultra-sonic measuring system. By means of a micro ­processor data, like piston position, together with a simultaneous measurement of gas pressure are being used for ­various control functions. Advantages: • pump control (no control switch required) • emergency function • safety function • machine control • measurement of gas pressure

311

piston monitoring: piston accumulator with electric limit switch code letter: A = stroke 35 mm B = stroke 200 mm C = stroke 500 mm piston diameter (optional) ranges from 100 to 355 mm

extended piston rod code letter: K piston diameter (optional) from 100 to 355 mm

ultra-sonic position measuring system code letter: U piston diameter (optional) from 180 to 355 mm measurement on fluid side

cable control code letter: S piston diameter (optional) from 180 to 490 mm measurement on gas side by means of pressure tight cable duct max. pressure: 350 bar

magnetic flap indicator code letter: M piston diameter (optional) from 150 to 490 mm for slow (< 0,5 m/s) and infrequent (< 5 / day) piston movements

312

piston positioning switch: code letter: UP / UPEX different designs, available for Øi = 150 - 490 mm to 500 mm²/s viscosity + EX-protection LS 06.14: standard design, viscosity of medium < 100 mm²/s LS 06.14 BF3: for highly viscous < 500 mm²/s SONOCONTROL 14 BF5: application: explosion prevention, viscosity of medium < 100 mm²/s

types of pistons design 1: Vmax = 0.5 m/s • for general usage of accumulators • optimized for application in high contamination situations design 2: Vmax = 3.5 m/s • low friction design for high piston speed (velocity) • low „stick-slip-effect“ during slow piston movements filter element pore size: NAS 1638, class 6; ISO 4406, class 17/15/12 design 3: Vmax = 0.8 m/s • low friction design, simple sealing structure • low „stick-slip-effect“ during slow piston movements filter element pore size: NAS 1638, class 6; ISO 4406, class 17/15/12 design 4: Vmax = 5 m/s • low friction design with emergency features • low „stick-slip-effect“ during slow piston movements • very low oil supply on gas side filter element pore size: ISO 4406 class 17/15/12 Advantages: • no limited pressure ratio • all fitting positions are possible • piston position can be made visible, where required • large efficient volumes are possible • especially suited for back-up aggregates • extreme volume flows are possible • no abrupt and sudden gas discharge due to faulty sealing Disadvantages: • extended reaction time • restricted suitability for very small volume changes in short time intervals • friction • gas discharge (leakage) via sealing • fluid discharge (leakage) via sealing • susceptibility to contamination

313

3.3

Diaphragm accumulators

Fig. Diaphragm accumulator Design: Diaphragm accumulators as shown in the picture below consist of a compression-proof steel container in the shape of a ball or cylinder. Inside the container is fitted a membrane or diaphragm, made of an elastic millable material (elastomer or a PFFE teflon membrane), which serves as a separator. Certain situations put a high demand on the durability of the elastomer ­material, i. e. when it is exposed to aggressive fluids. Therefore it is advisable to change the membrane in regular intervals. For that reason HYDAC has developed a hemispherical membrane with high resistance to aggressive fluids. It also can be used in a temperature range of -30°C and +160°C. HYDAC supplies two versions: welded and screwed diaphragm accumulators. In the welded version the membrane is pressed into the bottom part before the circumferential seam is welded. A ­suitable welding technique combined with careful fitting of the membrane ensures that the latter is not harmed during the welding process. In the screwed version the membrane is fixed by an o-ring shaped bead between upper and lower part. Compression strength is ascertained by a swivel or spigot nut. Both versions are fitted with a centrally located valve plate, which prevents the membrane to be pushed out through the fluid port, something that always can happen when the accumulator is emptied completely.

screw plug

screw plug

membrane container

valve plate

connection

Fig. Diaphragm accumulator, weld type

membrane vessel

valve plate connection

Fig. Diaphragm accumulator, screw type

Function: As to how a diaphragm accumulator works is depicted in the pictures above and on the next page. In the initial position the membrane is exposed to pressure p0 from the gas side. The membrane nestles up to the inner contour of the ­accumulator and blocks the fluid inlet. As in a bladder accumulator the valve plate is lifted as and when the appropriate pressure has been reached. Thus the fluid can flow into the accumulator. The effective fluid quantity can now be ­calculated as the difference between ­minimal and maximum operational pressure. Diaphragm accumulators can be fitted in any postion. However, an upright position is always more suitable.

314

�� ��

p0 = gas pre-charge pressure V0 = effective gas volume V1 = gas volume at p1

�� ��

���

�� ��

p1 = minimum operational pressure

p2 = maximum operational pressure

V2 = gas volume at p2 DV = effective volume Fig. Function principle of diaphragm accumulators

Compatibility of elastomers in diaphragm accumulators: The material for the membrane has to be attuned to the respective operational medium and temperature. Unfavourable discharging conditions (high pressure ratio p2/p0, high discharging velocity) can cause the temperature of the gas to drop below the permissible temperature of the elastomer, which may result in ruptured bladders. This fact has to be ­considered during the selection process of the elastomer material. HYDAC‘s accumulator simulation program ASP helps you to ­calculate the correct gas temperature. Advantages: • any fitting position is possible • higher pressure ratios than in bladder accumulators • small nominal volumes (as of 0.075 l) • almost maintenance free • in practical terms almost no inertia • high endurance, long life-span • good value for money Disadvantages: • notching during power surges • material fatigue in seperator (diaphragm / membrane) • destruction of membrane leads to sudden loss of gas

�

Please note:

Depending on the operational medium and individual situations different kinds of elastomers are available. Please refer to our technical data sheets.

315

3.4

Metal bellows accumulators

Fig. Metal bellows accumulators Design: Instead of a bladder / diaphragm, special types of construction, e.g. metal bellows accumulators, use a metal bellows to separate gas and oil side. Therefore they are available as virtually gas-tight dampers. A baffle plate / diffuser block is ­integrated to improve damping characteristics. A metal bellows serves as separator between gas and oil side. Function: Without friction and wear the metal bellows moves inside the accumulator. Adjusted only once it can operate over a long period of time (years). Monitoring and maintenance are minimalized. Advantages: • up to 160°C resistant to all conventional fuels • virtually gas-tight, no friction between parts (no wear), thus maintenance and monitoring is minimalized • maintenance free • no recharging necessary • can be used with all hydraulic fluids • low operational costs Application: • pulsation damping in ship diesel engines and power plant motors, as well as in proportioning pumps / metering pumps • volume compensation and energy storage in aircraft / aviation

316

3.5

Weight reduced hydraulic accumulators

Fig. Hydraulic accumulator Bladder, piston and diaphragm accumulators are also supplied in a weight-reduced hydraulic accumulator version. Weight reduction can be achieved by a reduced accumulator wall thickness or by using aluminum or compounds. Application of these materials lead to a weight reduction of up to 80% in comparison to standard accumulators made of C-steel. Weight reduced accumulators save energy in various ways: On the one hand the weight reduced accumulators contribute to saving energy, i. e. fuel, on the other hand these ­accumulators are being used to store energy. For instance during break application the energy is stored and used again when the engine or machine accelerates. Aided by weight reduced hydraulic accumulators conventional transport vehicles can save energy particularly in „stop and go“ situations. Some examples are buses (shuttles, public transport in inner cities), public service vehicles, delivery trucks and rail-based vehicles (driving units of locomotives). Application: • mobile technology • automotive industry • aviation and aerospace industry

317

3.6

Hydraulic dampers

Fig. Hydraulic damper In hydraulic systems pressure fluctuations can occur. This might be caused by various inherent processes which change the flow condition of the hydraulic fluid, like: • irregularities within the displacement pump • spring-mass-systems (pressure compensator in valves) • sudden connection of pressure chambers with different pressure levels • operation of check and control valves with short opening and closing times (Jaukowsky-pressure surges) • connection and disconnection of displacement pumps In addition to what we have said before fluctuations of volume flow and pressure have a negative effect on the lifespan of components. We distinguish between stochastic (pressure surges) and periodic (pulsation) pressure fluctuations. In order to ensure that these do not cause interferences in their function, pressure fluctuations have to be taken into account already during the planning stages of a hydraulic unit. Appropriate countermeasures have to be taken. Possibilities for damping are numerous. However, hydraulic dampers applied in hydraulic systems have turned out to be particularly suitable and efficient. Pulsation types: Pressure pulsations are fluctuations in fluid systems, which occur: • periodically and uniformly • irregularly or • cyclically Pressure pulsations occur at both ends of the pump (pressure side and suction side). The requirements made on such dampers can be categorized into physical, constructive and operational or managerial aspects. The physical parameters relate to a very distinctive damping process covering a large frequency range and – at the same time – a low drop in pressure. The constructive design essentially comprises a simple design combined with excellent fitting possibilities and sufficient temperature, fluid and pressure reliability. The managerial aspect deals with maintenance work (input) as low as possible in such a way, that the operational reliability is not compromised. General aspects: Depending on the mode of operation hydraulic dampers are based on the physical principle of hydropneumatic bladder or diaphragm accumulators or fluid based sound absorbers. In hydropneumatic dampers the compressability of a gas ­(nitrogen) is applied. For instance in bladder accumulators depending on the amount of pressure applied, the bladder is compressed or extended. Something similar can be said about membrane (diaphragm) accumulators. During the application of standardized ­bladder or diaphragm accumulators the dampening process can be jeopardized due to the unfavourable connection ­between fluid and gas volume. Therefore special hydropneumatic dampers (pulse-tone dampers) have been developed, which by means of an inline-connection-block couple or link the fluctuations of volume or pressure to the volume of gas perfectly well. Thus excellent damping characteristics can be achieved up to a frequency level of approximately 500 Hz. We distinguish between the following types of damping processes: • pulsation damping • pressure surge damping • noise absorption There are the following types of hydraulic dampers: • pulsation dampers • suction stabilizers • fluid based sound absorbers • pressure surge dampers (shock absorbers)

318

3.6.1

Pulsation dampers

Fig. Pulsation damper Design: Pulsation dampers consist of a welded or forged pressure container made of C-steel resistant to chemically aggressive fluids. A specially designed fluid valve with inline connection guides the volume flow into the container. Function: A pulsation damper has two fluid ports. Therefore it can be fitted directly into a pipe. Due to the diversion of the volume flow inside the valve, the flow is immediately directed on the bladder or diaphragm, in other words, there is an immediate contact between bladder, diaphragm and fluid, which compensates the fluctuations of the volume flow via the gas volume. This also includes the pressure fluctuations with higher frequencies. The charging pressure has to be attuned to the ­existing operational conditions. Application: Pulsation dampers are predominantly used in hydraulic units, displacement pumps, sensitive measuring and control units and widely ramified pipe systems, for instance in process circuits of the chemical industry. Pulsation dampers: • prevent pipe bursts caused by material fatigue, vibrations of pipes and irregular volume flows • protect controls and instruments and other components • improve sound absorption Designs: • pulsation dampers working as diaphragm or bladder accumulator • suction stabilizer • fluid based sound absorber • standardized bladder, diaphragm and piston accumulator • metal bellows accumulator Sources of pulsations: Pulsations are generated by the degree of irregularity of: • piston pumps • membrane pumps / diaphragm pumps • cog wheel pumps • vane pumps or when pressure relief valves are activated, especially when the operational pressure is close to the opening pressure.

319

3.6.2

Suction stabilizers

Fig. Suction stabilizer Another way to dampen pulsations is to apply suction stabilizers, especially when they have to compensate pressure fluctuations generated at the suction side. Design: These stabilizers essentially consist of a fairly large housing (compared to the size of the bladder), which serves as a reservoir, a gas valve fitting and a cage to accommodate the bladder. gas valve insert

accumulator bladder

bubble sieve

housing

Fig. Suction stabilizer Function: The gas volume is surrounded by a fluid volume several times larger than the gas volume. Due to this fact and its special design in the vicinity of the ports, the effects of acceleration of the volume flow are reduced. This leads to the following effects: • improvement of NPSH-values (unit holding pressure) • cavitation of pump can be avoided • prevention of pipe vibrations Application: • in displacement pumps of all designs • in sensitive measuring and control instruments • in process cycles of chemical industries • in the low pressure range on the suction side of displacement pumps In general auxiliary pumps (rotatory pump) are connected in line before said pumps to generate a pilot pressure.

320

3.6.3

Silencers

Fig. Membrane accumulator housing (body) SD330 Design: A silencer consists of a welded or forged outer housing, an internal pipe and two pipe ports at opposite ends. A silencer has no moving parts and no gas filling. Therefore it does not require any maintenance work whatsoever. It can be used for mineral oils, phosphoric acid-ester and glycol. Other fluids might require a stainless-steel version. fluid chamber

inlet

outlet Fig. Silencer

Function: A silencer is based on the principle of an extension tank with interference duct. The oscillations are reflected within the tank. A major part of the oscillations are damped for a wide range of frequencies. Application: • displacement pumps (all types) • vehicles, machine tools, injection moulding machines, aircraft • hydraulic drive units and other systems with large „active area“ They are used to: • reduce pressure fluctuations in dB • reduce noise level (dB) by means of a silencer • compensate pulsations with different pressure changes • no maintenance required (no initial pressure) Application: All displacement pumps, like axial and radial piston pumps, vane pumps, gear pumps or screw spindle pumps generate fluctuations of volume and pressure. This causes vibrations and noise. Considerable noise levels are not generated by the pump alone, but also by the fluids and their mechanical pulsations. If they are transferred to larger surfaces, they are even amplified. Insulation and the application of flexible hoses or sound insulation caps resolve only part of the problem, since they cannot prevent the effects of the pulsations being transferred to other areas. A silencer can improve the ­situation considerably.

321

3.6.4.

Shock absorption

Fig. Pressure shock damper For pressure shock damping purposes all hydraulic accumulator designs can be used: • bladder accumulator • diaphragm accumulator • piston accumulator gas valve insert

accumulator bladder

mounting flange

pressure vessel

Fig. Low pressure bladder accumulator (shock absorber) Design: A welded pressure container made of C-steel or stainless steel, the fluid port with a punched disk, which prevents the bladder to be pushed out, plus a gas valve are the main components in a pressure shock damper. Function: Sudden changes in the stationary status of pipes, normally exposed to fluid flow, as may occur during pump failure or when a valve is opened or closed, can lead to pressure levels several times higher than permitted. A pressure shock damper prevents this incident by translating potential into kinetic energy or respectively kinetic into potential energy. Thus ­pressure surges are avoided and pipes, control units and other components are protected against destruction.

322

Where are pressure surges generated? Pressure surges can be generated by control and adjustment procedures: • influx into pipe systems • opening and closing of components • rapid pressure unloading of containers (accumulators) • quick flow control procedures Surges can also be generated by operational failures: • pump or compressor failures • pipe bursts Application: • pipe systems with quickly closing valves or flaps • switching on and off of pumps • rapidly changing operational pressures General remarks: A pressure shock damper or shock absorber reduces pressure surges and prevents both pipe systems and components from being destructed. Pressure surges are defined as pressure waves in fluid systems, which occur discretely as an individual damped oscillation. These pressure surges cannot be repeated. Depending on the circumstances, they cannot be predicted either (cavitation at peak level).

323

4

Accumulator station

4.1

Sequential fitting of nitrogen bottles in bladder accumulators

If the pressure difference (Dp) between minimal fluid pressure (p1) and maximum fluid pressure (p2) is relatively small, we have the possibility to increase the gas volume by means of sequentially fitted additional nitrogen bottles. In this case the nitrogen inside the accumulator can be compressed only a little bit. Consequently the accumulator volume designated to be employed for storage is not being used completely. Depending on the existing operational conditions the gas volume can be doubled (max.) by means of sequentially fitting more nitrogen bottles. The picture shows such an application in connection with a bladder accumulator. In this case a special port for the nitrogen bottles is fitted on the gas side of the accumulator. This design requires an additional creep-in rod inside the bladder, which prevents damage to the bladder during loading procedure. gas valve and creep-in rod

pipe

bladder accumulators

nitrogen bottle

Fig. Sequential fitting of nitrogen bottles in bladder accumulators

�

Please note:

Only one accumulator and one nitrogen bottle may be connected to each other.

325

4.2

Connecting nitrogen bottles to bladder accumulators

RIGHT! Always connect a nitrogen bottle to a bladder accumulator on the gas side. The fluid side is symmetrically fitted.

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Fig. Correct connection of nitrogen bottles to bladder accumulators

WRONG! The fluid side is asymmetrically fitted.

Fig. Faulty connection of nitrogen bottles to bladder accumulators

326

4.3

Sequential fitting of nitrogen bottles to piston accumulators surplus gas volume

superior end position of piston

i. e. nitrogen bottles gas exposed to high pressure gas exposed to low pressure fluid exposed to high pressure fluid exposed to low pressure

inferior end position of piston

the volume of nitrogen bottles may never exceed the surplus gas volume.

Fig. Schematic display of downstream nitrogen bottles in piston accumulators

�

Please note:

The excess gas volume can be fitted sequentially by means of additional nitrogen bottles. However, during the design process of such a unit you have to observe that the maximum additional gas volume in bladder and piston accumulators has to be attuned by taking into consideration the operational and ambient temperature. HYDACs software ASP will help you to do that.

Fig. Sequential fitting of nitrogen bottles in piston accumulators

327

4.4

Connecting nitrogen bottles to piston accumulators

RIGHT! Always connect a nitrogen bottle to a piston accumulator on the gas side. The fluid side is symmetrically fitted.

Fig. Correct connection of nitrogen bottles to piston accumulators

WRONG! The fluid side is asymmetrically fitted.

Fig. Faulty connection of nitrogen bottles to piston accumulators

328

Application:

Bladder accumulator inside oil channel of a 1340 MW steam turbine for emergency supply of turbine control Type: SB210-32A1/114A...

Accumulator station for central hydraulic unit of a veneer press Type: 6xSK350-380 and 6x SN350-1500

Piston accumulator station in an injection moulding ­machine Type: 2xSK350-70 and 22x SN360-50

Piston accumulator for energy storage purposes Type: 1x SK350-460l 1x SK350-440l 3x SN350-1570l

329

5

Accessories

5.1

Universal charging and testing unit FPU-1

Fig. FPU-1 Design: Charging and testing unit for bladder, piston and diaphragm accumulators consist of: • valve housing • spindle with elastic sealing • check valve and pressure relief valve • manometer with indication range up to 400 bar • charging hose (according to European directive, DIN EN 982, DIN EN 853-857) • adapter A3 for bladder accumulators Function: With the aid of charging and testing unit FPU-1 hydraulic accumulators are filled with nitrogen or, respectively, the ­ recharge pressure is tested and – if need be – changed. To this end FPU-1 is screwed to the gas valve of the hydraulic p accumulator and then connected to a standard nitrogen bottle via a flexible charging hose. If the purpose is only controlling or reducing the pilot pressure, the filling hose does not have to be connected. This apparatus is basically a screw connected ­instrument with an in-built manometer, check valve and spindle, which opens the accumulator gas valve for pressure control. Piston and diaphragm accumulators are directly charged and tested. Bladder accumulators are charged and tested by means of adapter A3. pressure relief valve adapter G

charging hose G1

charging and testing unit FPU-1 M adapter A

D

N2 D

N2

nitrogen bottle

hydraulic accumulator

fluid inlet

Fig. Charging and testing unit Inspection intervals: Generally speaking nitrogen losses in hydraulic accumulators are minute. However, in order to prevent the piston from ­hitting the cylinder lid or a substantial deformation of bladder or membrane due to a possible decrease of pressure, it is strongly recommended to check the gas pressure regularly. Precharge pressure p0 – as indicated on the manufacturer label – has to be checked and adjusted after repair work or fitting an accumulator. After that it should be tested at least once the ­following week. If there are no nitrogen losses it should be tested again after 4 months. If still no pressure losses could be observed, testing and checking on a yearly basis suffices.

331

5.2

Nitrogen charging unit

Fig. Nitrogen charging units HYDAC nitrogen charging units guarantee a quick and efficient charging or replenishment to the required precharge pressures in bladder, diaphragm and piston accumulators. They equally guarantee an optimal utilization of standardized nitrogen bottles up to a residual pressure of 20 bar and a maximum precharge pressure of 350 bar. Description Depending on their design nitrogen charging units N2-server are suitable for charging small accumulators, charging and replenishment of precharge pressure of accumulator stations. Design: They consist of an oil supply unit, an electric and hydraulic control unit, a piston accumulator and connecting hoses. The piston accumulator is being used as nitrogen pump. They also feature a safety and shut-off block according to pressure equipment directives. N2-Server Typs:

332

1. N2S-M Compact mobile charging unit N2-server

3. N2S-V Mobile nitrogen charging unit N2-server with fluid (oil) supply.

2. N2-T Mobile nitrogen charging unit N2-server without fluid (oil) supply. This charging unit must be connected to an existing hydraulic system (max. ­flow rate 8 l/min).

4. N2S-F (mobile) or N2S-S (stationary) Nitrogen charging unit N2-Server. The said nitrogen charging unit is fitted with an entire drive aggregate plus a charging and testing unit.

�

Please note:

When charging hydraulic accumulators an appropriate pressure relief valve has to be fitted in, if the permissible excess operational pressure of the hydraulic unit is lower than the pressure of the nitrogen bottles being used for charging. The pressure relief valve must be attuned or adjusted to the required gas-charge pressure.

5.3

Safety measures

When working with pressurized containers (accumulators etc.) you have to comply to international safety regulations and directives. In Germany DGR and TRB directives have to be met. The safety components as required by TRB standards, leading to the accumulator at the fluid side are integrated in the SA-Block. Other safety relevant functions are optional. The safety measures required for the gas side are met by gas safety blocks.

Fig. AD2000-regulations

Fig. TRB

333

5.3.1

Safeguarding on fluid side

The fluid side has to be protected against transgression of permissible levels of pressure by means of suitable and ­ pproved safety valves. a 5.3.1.1 Safety and shut-off block (SAF)

Type: SAF Nominal value: 10 bis 50 liter Max. operating excess pressure: up to 400 bar Relief / unloading / discharge: manually and/or electromagnetically Pressure relief valve: Nominal value DN12

Fig. SAF This safety and shut-off block is an accessory in hydraulics to shut-off and relief hydraulic accumulators or actuators. Pertinet and relevant safety regulations are being met. Design: SAF safety and shut-off block consist of a valve block, the in-built pressure relief valve, main shut-off valve and a manually operated relief valve. Furthermore there is a tank port plus the required manometer ports. An optional electromagnetically operated directional valve permits an automatic unloading of the accumulator or actuator in case of emergency or if the unit has to be set to idle. accumulator

manometer port for testing manometer �

��

� ��

main shut-off valve

valve block

�

�

relief valve

safety valve DB

Fig. Safety and isolating block Function: During operation of safety and shut-off blocks the main shut-off cock is open and the relief valve is closed. Thus the ­accumulator is always charged with oil as determined or discharges fluid if the pressure in the system decreases.

334

Electronic unloading optional:

manometer

accumulator

port for testing manometer �

��

�

�

� ��

valve block

main shut-off valve

decompression (safety) valve

safety valve DB

electromagnetic decompression (safety) valve

Fig. Safety and isolating control block with electric unloading

335

5.3.1.2 DSV 10

Type: DSV-10-M-2.1/X/.... Nominal value: 10 liter Max. operating excess pressure: up to 350 bar Pressure relief valve: nominal value DN10

Fig. DSV A 3-way safety valve DSV 10 serves to secure and unload hydraulic pressure accumulators and actuators. The ­essential difference between SAF 10 and DSV 10 is their shut-off and unloading capabilities. Design: DSV-3-way safety valve consists of a valve block with in-built pressure relief valve and shut-off cock. Ports for pump, manometer, tank and accumulator exist. An optional electromagnetically operated 2-directional valve permits an automatic unloading of accumulators or ­actuators. Function: During the operation of the accumulator the ball valve connects the pump port with the accumulator. A pressure relief valve constantly checks on the status of the accumulator. By turning the ball valve the pump port is closed without leakage and at the same time unloaded to the tank. The fitting of an electro-magnetically operated 2/2 poppet directional valve we can achieve an automatic unloading in case of a power cut or if the machine has to be set to idle.

�

Please note:

Due to their design ball valves are not fit to function as a throttle. Therefore ball valves have to be turned as far as they will go in order to avoid destruction of the sealing cups. To ascertain proper operation recommendations of the manufacturer regarding pressure and temperature have to be adhered to.

5.3.1.3 Special designs with logic valves

Type: special edition Nominal value: up to DN 160

Fig. Special design with logic valve HYDAC-SYSTEM offer also other safety and shut-off blocks up to a nominal value of DN 160. They are custom-made for special accumulator stations.

336

5.3.2

Safeguarding the gas side

Fig. GSV Inadmissible transgressions of pressure on the gas side especially due to increased ambient temperature, like in the case of a fire, have to be secured by means of a complete unloading or controlled depressurization. HYDAC offers three different and standardized means of protection: • fire protection (prevents the machine from melting) • burst plates • gas safety valves

5.3.2.1 Fire protection Protection by means of complete unloading if pressure and limiting temperature are breached. Protection units against melting is possible up to an admissible operational pressure of 690 bar and a temperature range of -10° to +80°C. Their melting point is at approximately 160°C to 170°C and unloads the gas pressure by releasing the nitrogen completely, if the limiting temperature is breached, like in the case of a fire.

Fig. Component with safety function

Melting point safety device: Simple retrofitting by replacing the sealing cap with the melting point safety device.

Fig. Melting point safety device

337

5.3.2.2 Burst plates Protection by means of complete unloading if pressure limits are breached. Burst plates are designed for different response pressures and are delivered with a declaration of conformity. If the ­pressure limit is breached the burst plate is destroyed thus unloading the gas pressure by releasing the nitrogen ­completely. Stainless steel or steel / nickel alloys are materials used for burst plates. hexagonal SW 19

thread: 1/4'' NPT

rupture disk

Fig. Rupture disks / bursting disks

5.3.2.3 Gas safety valves Protection by means of controlled pressure reduction if pressure limits are breached. Gas safety valve GSV6 is a directly acting, spring-loaded safety valve with a setting range of 30 to 370 bar and a ­temperature range of -20°C to +80°C. All parts of the valve are made of stainless steel and therefore, it can be used for a multitude of applications. GSV6 is also delivered with a declaration of conformity. Due to the self-centering washer a mounting of this valve is very easy and safe. thread: G 1/2''

Fig. Gas safety valve

338

5.3.2.4 Gas safety block Design: Gas safety block GSB450 consists of a block made of brass (other materials upon request) with integrated ­breather and shut-off valve plus ports for: • gas safety valve (GSV6) • gas charging valve (i. e. minimess) • manometer • pressure transducer or pressure switch • burst plate • fire protection unit (against melting) The connector (port) for the gas safety valve is designed as a check valve. Therefore the valve can be changed, even if the system is pressurized.

Fig. Gas safety block Function: GSB450 is an adapter block, which is mounted to a hydraulic accumulator on the gas side and which can be fitted with v­ arious pressure components, charging inlets, safety valves and other safety components. Advantages: • compact design • flexible fitting possibilities • variable indicators are possible (bar. mpa, analogous) • pressure indicators according to customer needs • charging of accumulator with nitrogen without FPU-1, directly via Minimess-valve • checking of pre-charge pressure without FPU-1 Technical data: Medium: Nitrogen (N2) Admissible operational temperature: -20 to +80°C (other options upon request) Max. operational excess pressure: 400 bar Dimensions (basic design): max. hight: max. approx. 120 mm width: max. approx. 86 mm depth: max. approx. 125 mm

339

6

Application of hydraulic accumulators

6.1

Energy storage

The stored hydraulic energy is available for the following purposes: Reserve pump (emergency function ­– support pump) and leakage compensation.

Fig. Energy storage

6.1.1

Energy storage in injection moulding machines

Hydraulic energy, which was for example saved and stored during a break, can be used as support for the pump and/or increased output at peak production times. With a clever design you can half the required electrical energy.

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6.1.2

Energy storage and reduction of stroke time

Hydraulic energy saved and stored during a break can be used as support for the pump or reduction of stroke time.

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6.1.3

Energy storage for emergency function of hydraulic cylinders

In case of an emergency (i. e. power cuts) the hydraulic accumulator returns the cylinder automatically into the end ­position.

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6.2

Volume compensation

The hydraulic accumulator compensates surplus volume, for instance when the volume of the fluid increases due to ­increased temperature.

Fig. Volume compensation

6.2.1

Volume compensation for leakage oil compensation

The accumulator is charged by a pump up to a certain pressure level. Then the pump is switched off. Now the ­accumulator takes over and compensates fluid losses due to leakage until the minimum pressure level has been reached and the pump is activated once again.

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� 343

6.3

Pulsation damping

Pressure pulsations are smoothed by the compressed gas inside the accumulator (suction flow stabilization, reduction of noise level and vibrations).

Fig. Pulsation damping

6.3.1

Pulsation damping in a displacement pump

Due to the degree of uniformity of displacement pumps pulsations are being created in the fluid, which can be reduced with a pulsation damper.

� �

� �

344

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6.4

Pressure surge damping

Pressure surges caused for instance by quick release valves are damped by the compressible gas in the accumulator.

Fig. Systems engineering

6.4.1

Fig. Mobile technology

Pressure surge damping in mobile units

The accumulator acts like a spring due to the compressibility of the gas inside. The extension and retraction of the spring or „spring hardness“ can be adjusted using a throttling mechanism on the accumulator.

Fig. Pressure shock damping

345

6.5

Fluid separation

Fluid separators separate two different fluids exposed to pressure with an elastic divider, i. e. a membrane.

Fig. Separation of fluids

6.6

Silencers

A silencer is based on the principle of an extension tank with interference duct. The oscillations are reflected within the tank. A major part of the oscillations are damped for a wide range of frequencies. Effect: Reduction of pressure pulsations and noise.

Fig. Fluid based sound absorber

346

6.7

Offshore applications (oil- and gas industry)

HYDAC offers specially designed hydraulic accumulators for the offshore industry. They are adapted to the extreme ­requirements imposed by this field of application. The surface is specially treated and stainless steel gas valves are fitted to withstand the aggressive salt water. Their maximum excess pressure reaches up to 1000 bar. The following components are being employed: • bladder, piston and diaphragm accumulators, pressure surge dampers • accumulator stations with bladder or piston accumulators or sequentially fitted nitrogen bottles They are specially used: • for pulsation dampers in injection pumps for chemical fluids • for quick shut-off procedures and separation • for energy storage Application: • hydraulics in general • closing of valves in pipelines • blow-off prevention systems (BOP) • wellhead control systems • emergency application • dampers

Fig. Piston accumulator for a deep water pole ram (Firma Menk)

Fig. Valve operation by means of a bladder accumulator with special coating (Rotork fluid system) – topside systems

347

7

Selection of hydro-pneumatic accumulators

7.1

Selection criteria

Depending on the area of application requirements for hydro-pneumatic accumulators can be very different. With regard to the design of a hydraulic accumulator the first and foremost interest are the requirements for effective volume and ­pressure energy. Of course, other requirements have to be considered as well. Requirements, which are usually specific for just one hydraulic unit. For example mobile technology applications demand a maximum ratio of energy content to mass. When the size of the container has been decided upon, more specific details in terms of design have to be worked out, like the quality of elastomer for sealings and separators. Basic criteria: • pressure range pmax and pmin [bar] • temperature range Tmax and Tmin [°C] • content / effective volume = fluid displaced volume DV [l] • extraction- / charging velocity Q [l/min] • fluid • certification directives • mounting possibilities Accumulator type: • mode of operation • pressure ratio • internal friction, losses • losses due to throttle during inflow and outflow • mounting space, mounting position Stress due to fluids: • of bladder and sealing material - due to exposure to chemical fluids - stress due to temperature • abrasiveness of fluids • degree of contamination • corrosion of steel components External environmental influence: • ambient temperature • vibrations, dynamic stress (due to external forces) • atmosphere (chemically aggressive) Special applications: • extreme extraction velocity • carry-over pulsations in pulsation dampers • losses due to throttle during pass through of ports • sequential fitting of nitrogen bottles • suction stabilization • pressure surge damping

349

7.2

Definition of operational parameters �� ��

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Fig. Schematic illustration of current operational status for a piston accumulator (incl. related parameters) The necessary operational parameters required for the design of a hydro-pneumatic accumulator can be clarified by means of a schematic illustration of a piston accumulator. This is equally valid for the other hydro-pneumatic accumulator designs. The parameters for describing the actual status of the gas are pressure, temperature and volume. For the individual actual situations during operation of an accumulator the following parameters are defined: Pressures: p0 p 1 p2 p0/p2

Pre-charge pressure of gas chamber without pressure admission of fluid chamber Minimal working pressure of the unit. Keep in mind that the maximum gas pressure (p0max) may only go up to 90% of working pressure (p1) Maximum operational excess pressure of the hydraulic system Admissible pressure ratio = limit value of operation

Volume: V 0 V 1 V 2 V

Effective gas volume at pre-charge pressure level Gas volume at min. pressure Gas volume at max. operational pressure Effective volume

Temperatures: Ti

Gas temperatures according to individual condition (i = 0, 1, 2). The temperature of the hydraulic fluid has an impact on the heat exchange with the compressed gas. Therefore this parameter is only required indirectly during the design process of the accumulator.

Since we use gases as energy carrier, the processes of charging and unloading a hydraulic accumulator are subjected to the laws of thermodynamics. With gases there is a physical correlation between pressure and volume. This has been discovered by the Englishman Boyle in the 17th century. Later his findings were confirmed by the Frenchman Mariotte. The product of pressure and volume of an encased gas takes on the same value if the temperature remains constant.

p1  V1 = p2  V2

oder

p  V = constant

Experiments in which two of the three parameters pressure, volume and temperature were varied whilst the third was kept constant, led to the laws of thermodynamics.

350

7.3

Definitions

7.3.1

Isobaric

A change of state is called isobaric, when the pressure remains constant. Feeding heat means that the volume increases, if the pressure remains constant. Example: • transformer-equalizing tank (compensation accumulator)

7.3.2

Isochoric

A change of state is called isochoric, if the volume remains constant. Pressure changes when the temperature ­changes.

p p1 = = constant T T1 Example: • charging an accumulator with nitrogen • compressing a gas creates warmth (warming of a pressure accumulator) • gas pressure measuring at different gas temperature levels

7.3.3

Isothermal

A change of state is called isothermal, if the temperature remains constant. The product of pressure and volume also remains constant. If the pressure changes, so does the volume. This happens extremely slowly. Therefore a thermal compensation to the outside is possible.

p  V = p1  V1 = constant Application: • leakage oil compensation • volume compensation Example: • pressure increase by means of a piston in a cylinder = bicycle tyre inflator (air pump)

7.3.4

Adiabatic

A change of state is called adiabatic, when no compensation of warmth occurs during a change of the gas volume with the environment. Even with the best insulation, a little heat will always escape. However, if the densification occurs very quickly, there is no time left for heat exchange, which means that the heat created during the densification process ­remains in the gas. Adiabatic change of state is also called isentropic (quick change). Inversely during an unloading process cold is always being created. Pressure, temperature and volume always change simultaneously during an adiabatic process. This change of state is the main functional principle and purpose of hydraulic accumulators – energy storage. In emergency and safety functions energy storage occurs in connection with isothermal changes of state. During this process the charging is always ­isothermal and the unloading adiabatic. K

K

p  V = p1  V1 = constant T V T p

K −1

= T1  V1

( 1− K ) / K

K −1

= T 1  p1

and ( 1− K )/ K

k = adiabatic index (exponent) for 2-atomic gas, like nitrogen = 1.4 depending on pressure and temperature Application: • catapult (sling-shot)

351

7.3.5

Polytropic

Isothermal and adiabatic changes of state are ideal „border-line“ cases, which cannot be realized technically in their ­purest form. Neither is it possible to keep the working temperature of a gas constant, nor is it possible to prevent an inflow or outflow of warmth. A change of state, which is neither isothermal nor adiabatic, is called polytropic. Ideal behaviour of gases: • effective volume

VN  DV = V1  V2 p

2

1

useful volume / useable volume = DV

0 V

Real gases: Especially with low temperatures and high pressures the behaviour of real gases deviates from the laws as laid down by Boyle and Mariotte. If for example the pressure increases, the volume is reduced by a larger extent than predicted by the physical law p • v = constant. The reason behind that is the fact that the molecules gravitate towards one another ­(cohesion). Furthermore the molecules may not be looked at as minute dots – as it is possible in „ideal“ gases. The ­volume has to be degraded by a factor, which reflects the actual space required by the molecules.

7.4

Calculation by means of basic physical equations

A gas charge in a hydraulic accumulator in thermodynamic sense can be looked at as a homogeneous closed system with equivalent variables. Without restriction to universal validity we will look at a piston accumulator to explain the ­necessary basic physical equations (we will neglect however the friction between piston and internal cylinder surface). The charging and extraction of hydraulic fluids in a cylinder correlates directly to a change of the gas charge. On the one hand there is a work compensation due to the fluid with the gas and on the other hand we have a heat exchange between the ­environment and the gas, when the gas and ambient temperatures are different (environment = separator, accumulator and ­hydraulic fluid). To move the piston by path length ds, the work input causes a change of volume dv. Simultaneously with a change of

dWv = - p  A  d s = - p  dv ( 1 )

volume a change of the state of the gas occurs. Equation 1: Firstly, the following is determined: Prefix (+) = work input / prefix (-) = work output. Thus during densification (dv < 0), the work for change of volume is positive. Given that we can call the gas „ideal“, the correlation between pressure, temperature and volume can be described by means of a state equation. R is a constant, depending on what type of gas we use.

p  V = m  R  T (2) For nitrogen (N2) the constant is:

R = 297 J / kg K Knowledge of the individual processes inside the accumulator in regard to changes of status of the gas is vitally ­important. To this end the following states and changes thereof can be given: a) Pre-charge of gas chamber at low temperature with a subsequent change of pre-charge pressure due to heat exchange with the environment. b) The charging and discharging cycle of the accumulator via the fluid takes such a long time, that a complete heat exchange with the environment is possible.

352

c) The charging and discharging cycle of the accumulator via the fluid takes such a long time, that a complete heat exchange with the environment is not possible. During the change of status as described under a) no work for volume change is put in. This means no change in volume. This change of status is called isochoric and can be described by means of a simplified state equation.

p / T = p1 / T1 = constant ( 3 ) A change of state as described under b) is called isothermal and occurs under the assumption of a complete heat ­exchange with the environment without a change of temperature. A mathematical correlation between the state variables can be deduced from the thermal state equation, thus for isothermal change:

p  V = p1  V1 = constant ( 4 ) A change of state as described under c) is called adiabatic. There is only an exchange (compensation) of work between hydraulic fluid and gas. Thus we have the correlation: K

K

p  V = p1  V1 = constant ( 5 ) The dependency between temperature, volume and pressure is also derived from the thermal state equation.

T V

K −1

= T1  V1

K −1

(6 )

and

T p

( 1− K )/ K

= T1  p1

( 1− K )/ K

(7 )

k in these equations means the adiabatic index (exponent), which can be used for 2-atomic gases like nitrogen under normal circumstances (diagram) with a value of 1.4.

2

2

polytropic state

isochoric state adiabatic state

isothermal state

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pressure p

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1

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Change of state shown in p-V diagram

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gas volume V

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Since accumulators are never operated according to theory, we will finally get a change of status, which lies somewhere between isothermal and adiabatic. This status is called polytropic. The mathematical correlations are valid in analogy to the adiabatic changes of state, whereby the adiabatic exponent is replaced by the polytropic exponent. In the diagram (pVdiagram) the individual changes of state are represented. Here you can see, that both isothermal and adiabatic changes of state are „border-line“ situations of a polytropic change. What equations you need to design a hydraulic accumulator depends on the time required for charge and discharge. As a rule of thumb you might consider this: cycle time < 1 minute adiabatic change of state cycle time > 3 minutes isothermal change of state cycle time between 1 und 3 minutes polytropic change of state In order to get a more precise approach to the actual changes of state, we need more information about the thermal time constant. Therefore we strongly recommend to use exclusively HYDAC's software (ASP) and when in doubt – activate the ­adiabatic calculation process.

353

For design purposes it would be of great advantage to change the equations given to the effect that the required ­parameters can be computed. Primarily these refer to the effective gas volume against the corresponding pressure ratios as well as the pre-charge pressure P0. The following table shows the relevant equations for accumulator design. Furthermore certain empirical values gained from the experience in designing accumulators should be considered as well. They ascertain an optimal utilization of accumulator volume and a long life-time. The table also shows these ­empirical values for individual accumulator designs. If you want to develop a sequential design with nitrogen bottles, the effective volume of the accumulator has to be considered as well. To this end you assume an isothermal charging starting at ­pre-charge pressure level up to maximum operational pressure. The extended effective volume V can be computed as follows:

V = V0 G ( 1 - p0 / p2 )

(8)

V’ = 0.75 • V0G should not be breached in sequential designs of bladder accumulators. Otherwise there would be to much stress on the bladder due to churning. V0G signifies the complete effective volume (accumulator plus nitrogen bottles). In any case the extended effective ­volume V must be smaller than the effective gas volume of the accumulator. You probably have to change the gas volume until these conditions are met.

354

Cycle (change of state) Equation Notes p0(T1) = pre-charge pressure at min. � Temperature T1 (in Celvin) ��������� p0(T2) = pre-charge pressure at max. �� operational temperature T2 (in Celvin)

p0( T1 ) = p0( T2 ) 

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V0 =

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1

(p0 at T1)

 p2   1    p1  n

DV  �

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V0 =

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V0 = �

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(p0 at T1)

p  - 0 p2 

Application: Leakage oil compensation, Volume compensation

DV p0 p0 p1 p2

1- n  

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p2 p0

1

� ��  ��������� �� p V  ���������� 1 −  1 1 �� W= �� �

Application: Emergency function, safety function

 p2  n   -1  p1 

�������������� � p �������������� DV = V0  0 �  p1

�

Application: Energy storage

 p0   p0  n   −   p1   p2 

2

�

n = K = 1.4 for nitrogen (p0 at T1)

DV 1 n

� ������������������������  ��������������������������� p  DV2 = V0 0  � p

�

Application: Computation of pre-charge pressure at deviation of operational temperature in comparison to pre-charge temperature

1   p0  n  -   p2   

1  n   p DV = V0  0   p1  

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��

T1 T2

p1     p2 

 1- n     n 

   

During adiabatic compression work is performed, which can be computed by means of the following formula on the left.

 1-n      n    p   1 Dp = p2 1 −     p2    

355

7.5

Computation in case of deviation from ideal gas behaviour

The state equations discussed above are only valid under one condition, ideal gas behaviour. Different gases like ­nitrogen deviate from ideal gas behaviour if exposed to high pressure. This behaviour is called „real“ or „non-real“.

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����������������� Fig. Diagram of ideal gas behaviour The mathematical correlation between pressure, temperature and volume (p, T, V) for actual „real“ gas behaviour can be described by an equation, which is in fact only an approximation computation. To handle such an equation with sufficient accuracy is very tedious work and requires a lot of computation time and effort, which only can be dealt with by means of a powerful computer. Therefore it is quite useful to introduce a corrective parameter, which reflects the „real“ gas ­behaviour. From this it follows that as we can describe the volume in a isothermal change of state as and in a adiabatic change of state as

Vreal = Ci  Videal

(9)

Vreal = Ca  Videal

( 10 )

Corrective parameters Ci and Ca in equations (9) and (10) can be directly looked up in the diagram (correlation of ­ ressure ratio p2 / p1 and maximum operational excess pressure). p

7.6

Computation of thermal time response

So far we have only given approximate time limits in order to determine the type of changes of state. Since we want to achieve a more precise accumulator design, it is necessary to analyze the thermodynamic exchange processes. ­Particularly in intermittent operational processes with quick changes these processes are influenced by the intensity of heat transmission. To this end for purposes of describing and evaluation of thermal time response we refer back to a thermal time constant.

ι = cv  m / a  Α

( 11 )

To explain the parameters: cV = specific heat capacity at constant volume, m = mass of gas, α = heat transfer coefficient and A = the entire heat transfer area. The time constant can be determined with little experimental effort. Since there is a correlation between the time constant and pre-charge pressure, accumulator design and size, the time constant has to be experimentally computed for every single accumulator type. The test results for the individual designs are shown in the diagrams above according to (11). Here you can find the thermal time constants against pre-charge pressure for different nominal volumes of individual accumulator types. With the aid of these time constants it is possible to run a simulation program for a given charging cycle.

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List of figures: accumulator technology Fig. Hydraulic accumulators������������������������������������������������������������������������������������������������������������������� 299 Fig. Accessories for accumulators������������������������������������������������������������������������������������������������������������ 303 Fig. Designs of hydropneumatic accumulators�������������������������������������������������������������������������������������������� 305 Fig. Bladder accumulator����������������������������������������������������������������������������������������������������������������������� 307 Fig. Bladder accumulator����������������������������������������������������������������������������������������������������������������������� 307 Fig. Function principle of bladder accumulators������������������������������������������������������������������������������������������ 308 Fig. Low pressure bladder accumulator (High-Flow)������������������������������������������������������������������������������������ 309 Fig. High pressure bladder accumulator (High-Flow)������������������������������������������������������������������������������������ 309 Fig. Piston accumulator������������������������������������������������������������������������������������������������������������������������� 310 Fig. Piston accumulator (screw-type)�������������������������������������������������������������������������������������������������������� 310 Fig. Piston accumulator (SK280) (crimped)������������������������������������������������������������������������������������������������ 310 Fig. Function principle of piston accumulators�������������������������������������������������������������������������������������������� 311 Fig. Diagram oil and gas pressure����������������������������������������������������������������������������������������������������������� 311 Fig. Diaphragm accumulator������������������������������������������������������������������������������������������������������������������� 314 Fig. Diaphragm accumulator, weld type����������������������������������������������������������������������������������������������������� 314 Fig. Diaphragm accumulator, screw type��������������������������������������������������������������������������������������������������� 314 Fig. Function principle of diaphragm accumulators�������������������������������������������������������������������������������������� 315 Fig. Metal bellows accumulators�������������������������������������������������������������������������������������������������������������� 316 Fig. Hydraulic accumulator��������������������������������������������������������������������������������������������������������������������� 317 Fig. Hydraulic damper��������������������������������������������������������������������������������������������������������������������������� 318 Fig. Pulsation damper��������������������������������������������������������������������������������������������������������������������������� 319 Fig. Suction stabilizer���������������������������������������������������������������������������������������������������������������������������� 320 Fig. Suction stabilizer���������������������������������������������������������������������������������������������������������������������������� 320 Fig. Silencer���������������������������������������������������������������������������������������������������������������������������������������� 321 Fig. Membrane accumulator housing (body) SD330������������������������������������������������������������������������������������ 321 Fig. Pressure shock damper������������������������������������������������������������������������������������������������������������������� 322 Fig. Low pressure bladder accumulator (shock absorber)����������������������������������������������������������������������������� 322 Fig. Sequential fitting of nitrogen bottles in bladder accumulators������������������������������������������������������������������� 325 Fig. Correct connection of nitrogen bottles to bladder accumulators���������������������������������������������������������������� 326 Fig. Faulty connection of nitrogen bottles to bladder accumulators������������������������������������������������������������������ 326 Fig. Sequential fitting of nitrogen bottles in piston accumulators��������������������������������������������������������������������� 327 Fig. Schematic display of downstream nitrogen bottles in piston accumulators�������������������������������������������������� 327 Fig. Correct connection of nitrogen bottles to piston accumulators������������������������������������������������������������������ 328 Fig. Faulty connection of nitrogen bottles to piston accumulators�������������������������������������������������������������������� 328 Fig. FPU-1������������������������������������������������������������������������������������������������������������������������������������������ 331 Fig. Charging and testing unit����������������������������������������������������������������������������������������������������������������� 331 Fig. Nitrogen charging units�������������������������������������������������������������������������������������������������������������������� 332 Fig. AD2000-regulations������������������������������������������������������������������������������������������������������������������������ 333 Fig. TRB��������������������������������������������������������������������������������������������������������������������������������������������� 333 Fig. SAF��������������������������������������������������������������������������������������������������������������������������������������������� 334 Fig. Safety and isolating block����������������������������������������������������������������������������������������������������������������� 334 Fig. Safety and isolating control block with electric unloading������������������������������������������������������������������������ 335 Fig. DSV�������������������������������������������������������������������������������������������������������������������������������������������� 336 Fig. Special design with logic valve���������������������������������������������������������������������������������������������������������� 336 Fig. GSV�������������������������������������������������������������������������������������������������������������������������������������������� 337 Fig. Component with safety function��������������������������������������������������������������������������������������������������������� 337 Fig. Melting point safety device��������������������������������������������������������������������������������������������������������������� 337 Fig. Rupture disks / bursting disks����������������������������������������������������������������������������������������������������������� 338 Fig. Gas safety valve���������������������������������������������������������������������������������������������������������������������������� 338 Fig. Gas safety block���������������������������������������������������������������������������������������������������������������������������� 339 Fig. Energy storage������������������������������������������������������������������������������������������������������������������������������ 341 Fig. Volume compensation��������������������������������������������������������������������������������������������������������������������� 343 Fig. Pulsation damping�������������������������������������������������������������������������������������������������������������������������� 344 Fig. Systems engineering����������������������������������������������������������������������������������������������������������������������� 345 Fig. Pressure shock damping������������������������������������������������������������������������������������������������������������������ 345 Fig. Mobile technology�������������������������������������������������������������������������������������������������������������������������� 345 Fig. Fluid based sound absorber������������������������������������������������������������������������������������������������������������� 346 Fig. Separation of fluids������������������������������������������������������������������������������������������������������������������������� 346 Fig. Piston accumulator for a deep water pole ram (Firma Menk)������������������������������������������������������������������� 347 Fig. Valve operation by means of a bladder accumulator with special coating (Rotork fluid system) – topside systems����������������������������������������������������������������������������������������������������������������������������� 347 Fig. Diagram of ideal gas behaviour��������������������������������������������������������������������������������������������������������� 356 Fig. Software ASP�������������������������������������������������������������������������������������������������������������������������������� 357

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