Hydraulic Systems Design Guidelines

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Hydraulic Systems Design Guidelines 1. 2. 3. 4. 5. 6. 7. Hydraulic Diagram Symbols Types of Hydraulic Cylinder Mountings Positions Power Pack and Accessories Hydraulic valves and Functions Hydraulic Circuit Design Calculations Rajesh Kumdale Engineer Design inpac@rediffmail.com Hydraulic Circuits Design Symbols Hydraulic symbols can provide a clear representation of the function of each hydraulic component and therefore laying each symbol out on the page in the same way the components are placed in the circuit allows for the complete function of the hydraulic equipment to be diagnosed and understood. Lines -continuous line - flow line -dashed line - pilot, drain -envelope - long and short dashes around two or more component symbols. Circular -large circle - pump, motor -small circle - Measuring devices -semi-circle - rotary actuator Square -one square - pressure control function -two or three adjacent squares - directional control Diamond -diamond - Fluid conditioner (filter, separator, lubricator, heat exchanger) Miscellaneous Symbols -Spring -Flow Restriction Triangle -solid - Direction of Hydraulic Fluid Flow -open - Direction of Pnematic flow Pumps and Compressors Fixed Displacement hydraulic pump -unidirectional -bidirectional Variable displacement hydraulic pump -unidirectional -bidirectional Compressor Motors Fixed displacement hydraulic motor -unidirectional -bidirectional Variable displacement hydraulic motor -unidirectional -bidirectional Pneumatic motor -unidirectional -bidirectional Rotary Actuator - hydraulic - pneumatic Cylinders Single acting cylinder -returned by external force -returned by spring or extended by spring force Double acting cylinders -single piston rod (fluid required to extend and retract) -double ended piston rod Cylinders with cushions - single fixed cushon - double fixed cushion - single adjustable cushion - double adjustable cushion Directional Control Valves Directional control valve (2 ports / 2 positions) -Normally closed directional control valve with 2 ports and 2 finite positions. -Normally open directional control valve with 2 ports and 2 finite positions. Directional control valve (3 ports / 2 positions) -Normally closed directional control valve with 3 ports and 2 finite positions. -Normally open directional control valve with 3 ports and 2 finite positions. Directional control valve (4 ports / 2 positions) -directional control valve with 4 ports and 2 finite postions Directional control valve (4 ports / 3 positions) -directional control valve with 4 ports and 3 finite postions *-(center position can have various flow paths) Directional control valve (5 ports / 2 positions) Normally a pneumatic valve -directional control valve with 5 ports and 2 finite postions Directional control valve (5 ports / 3 positions) Normally a pneumatic valve -directional control valve with 5 ports and 3 finite postions Proportional directional control valve Electro-hydraulic servo valve -The spool positions on these valves is variable allowing for variable flow conditions. -single-stage direct operation unit which accepts an analog signal and provides a similar analog fluid power output -two-stage with mechnical feedback indirect pilot operation unit which accepts an analog signal and provides a similar analog fluid power output Control Methods Manual Control -general symbol (without showing the control type) -pushbutton -lever -foot pedal Mechanical Control -plunger or tracer -spring -roller -roller(one direction only) Electrical Control -Solenoid (the one winding) Pilot Operation -pneumatic -hydraulic Pilot operated two-stage valve -Pneumatic: Sol first stage -Pneumatic: Air pilot second stage -Hydraulic: Sol first stage -Hydraulic: Hyd pilot second stage Check valves, Shuttle valves, Rapid Exhaust valves -check valve -free flow one direction, blocked flow in other direction -pilot operated check valve, pilot to close -pilot operated check valve, pilot to open Shuttle valve -to isolate one part of a system from an alternate part of circuit. Rapid exhaust valve/Pneumatic -installed close to an actuator for rapid movement of the actuator. Pressure Control Valves Pressure Relief Valve(safety valve) normally closed - line pressure is limited to the setting of the valve, secondary part is directed to tank. Proportional Pressure Relief - line pressure is limited to and proportional to an electronic signal Sequence Valve - when the line pressure reaches the setting of the valve, valve opens permitting flow to the secondary port. The pilot must be externally drained to tank. Pressure - pressure downstream of valve is limited to the setting of the valve Reducing valve Flow Control Valves Throttle valve -adjustable output flow Flow Control valve -with fixed output (variations in inlet pressure do not affect rate of flow) -with fixed output and relief port to reservoir with relief for excess flow (variations in inlet pressure do not affect rate of flow) -with variable output -fixed orifice -metered flow toward right free flow to left -pressure compensated flow control fixed output flow regardless of load -pressure and temperature compensated -with variable output and relief port to reservoir Flow dividing valve -flow is divided equally to two outputs. Shut-Off Valve -Simplified symbol Accumulators Filters, Water Traps, Lubricators and Miscellaneous Apparatus Filter or Strainer Water Trap -with manual drain -with automatic drained Filter with water trap -with manual drain -automatic drain Air Dryer refrigerant, or chemical removal of water from compressed air line Lubricator -oil vapor is indected into air line Conditioning unit -compound symbol of filter, regulator, lubricator unit -Simplified Symbol Heat Exchangers -air or water cooled unit designed to remove heat from oil returning to reservoir Hydraulic Cylinder Types Cylinder Description design This single-action cylinder is pressurized only in one direction. Plunger cylinders are also available with an upper limit stop, which returns through its own weight. Application: Presses, construction machines, heavy vehicle construction, and so on. According to pressurization of the piston area, this cylinder may be either a pressure or a traction cylinder. The unpressurized piston side has an air filter to prevent dust from penetrating into the cylinder and thus the wear connected with this problem. If a spring has been provided as reset force, the spring pretension must be determined. Telescopic cylinders adapt to limited installation possibilities and, at the same time, fulfill the requirement for the longest stroke possible. All stages of the cylinder are pressurized on one side and the cylinder releases its power during extension. Its own weight on the consumer side makes the cylinder retract. Applications: Vehicle, excavator, and machine construction, and so on. These cylinders are the most commonly used hydraulic cylinders. They are also manufactured with damping on the piston or rod side or on both sides.In the case of this model, a force can be transmitted in both directions of movement. This telescopic cylinder does not return through its own weight and is thus effective in every position. Application: Feed mechanism for large furnaces and brick kilns. Symbol Plunger cylinder Singleaction cylinder Singleaction telescopic cylinder Doubleaction cylinder (Differential cylinder) Doubleaction telescopic cylinder Rod End Connection Fix and Rigidly Guide Mountings Pivoted and Rigidly Guide Fix and Rigidly Guide Pivoted and Rigidly Guide Pivoted and Rigidly Guide Supported but not Rigidly Guide Pivoted and Rigidly Guide Supported but not Rigidly Guide Supported but not Rigidly Guide Power Pack. 1) Hydraulic Driven Power Unit and Sizing 2) Hydraulic Motor 3) Valve Panel Assembly l   Reservoir Size 4 to 5 times pump capacity Suction Strainer 4 to 5 times pump Flow capacity 1. 2 3 4 5 6 7 8 9 10 Oil reservoir Filler/breather Float switch Thermostat with display Return filter Electric motor Axial piston pump Pressure safety block Pressure relief valve Directional valve 11 Pressure gauge 12 Pressure hose 13 Check valve 14 Suction hose 15 Check flap with monitoring of the position 16 Motor pump assembly 17 Filter 18 Oil/water cooler 19 Water control valve, electric Required for Design the Hydraulic Circuit, Pump capacity / Hose Power required by Driven the Pump / Working pressure and size of Reservoir Service to Store for Hydraulic Oil. 2) Breather Filter. Function this filter the air which breathes into and out of the reservoir, helping to maintain hydraulic fluid purity. As well as help to fill tank.  Choose a Breather Filter of 5 CFM Air flow capacity when the reservoir less than 50 Ltr, Flow Over 50 Ltr 25 CFM air flow breather can select. 3) Flot Switch . Float switch is a highly reliable product used for automated liquid control the regulator can be used for filling or empying function, according with the demands 4) Electronic Contact thermometer. Serve the temperature control of hydraulic system  The contact thermometer have two or four programmable temperature switching output alternatively one program mobile switch out put one analog output 4-20 mA with display and control unit. Hydraulic Pump Power Calculations. Power Pinkw  PxQ 600 xn P = Pressure in Bar Q = Flow rater of Pump LPM N = Efficiency 80 % (0.8) Maximum Pressure Type of Pump Centrifugal Gear Vane Axial Piston - portplate Axial Piston - valved In-line Piston (bar) 20 175 175 300 700 1000 Maximum Flow (l/min) 3000 300 500 500 650 100 Comment Positive displacement Positive and variable displacement Positive and variable displacement Positive and variable displacement Positive and variable displacement Hydraulic Valves. Directional Control Valve, Hydraulic - Description Hydraulic valves function to control pressure, control flow or direct flow in response to external commands. Directional valves are valves that direct flow in response to external commands. Directional valves are referenced by the number of positions the spool will take (2, 3 or 4 positions are typical) and the number of hydraulic ports in the valve (2 way, 3 way and 4 way are typical). Examples are shown below. Two way, Two Position (2/2) Valve In a two way, two position valve, the servo can be in one of two positions and the two ways because there are 2 fluid ports in the valve (or, if you prefer, the valve housing). Although a spool arrangement is shown, any type of check valve could be considered a two way, two position valve. (Two Positions Shown) Three way, Two Position (3/2) Valve In a three way, two position valve, there are three inlet/outlet ports in the valve and the spool can be in one of two positions. A 3/2 valve would be used to allow fluid flow into or out of actuator or motor. Four Way, Two Position (4/2) Valve In a four way, two position valves there are four inlet/outlet ports in the valve and the spool can be located in one of two positions. For 4/2 valve fluid is always flowing through the valve with system pressure supplied to one of the two outlet ports at all times. The other port would then be ported to return. 4/2 valves would normally be used in hydraulic systems in conjunction with an upstream shut valve (or 2/2 valve). In this case a 4/3 valve usually makes more sense. However, 4/3 valves can be found in power control units (PCUs), where a shutoff valve is installed in the PCU where a shut valve is not packaged with the 4/2 valve due to other design considerations in the PCU. Four way, Three Position (4/3) Valve In a four way, three position valve, the spool is in one of three positions and there are 4 inlet/outlet ports in the valve. In the midstroke position there is no flow through the valve. A good application of a 4/3 valve is actuator control, where the actuator control goal is to extend, retract or hold a position. 4/3 valves are used in servovalves, where the spool is controlled by a flapper valve or a jet pipe valve. When specifying a directional control valve, the following parameters should be evaluated: Pressure Rating – make sure valve is rated for your system pressure Pressure Drop – this is the manufacturer’s pressure drop at a rated flow through the valve. There may be a tolerance on the pressure drop which may need to be evaluated. Pressure and Flow Characteristics for a 4/3 Valve Reference the 4/3 directional schematic above, note the flow areas from Ps to PA and PB to Pr are equal (matched and symmetrical valve). Ignoring leakage through the servo piston, the flow rates are characterized by the orifice flow equations, Flow Control Valve, Hydraulic - Description The two methods of controlling flow rate in a hydraulic circuit are (i) using a fixed orifice and (ii) using a flow control valve. For accurate flow control, a device that regulates to a Δp across an orifice is required – referred to as pressure compensated flow control valve. Figure 1 shows a simplified pressure compensated flow control valve. Schematic of a Inlet Pressure Regulated Flow Control Valve As the Δp (force balance) across the servo varies the flow opening by the servo, the metering orifice inlet pressure is regulated. Hence as P1 increases (or P2 decreases), the servo moves to the left and reduces the servo flow area. And as P1 reduces (or P2 increases), the servo moves to the right thereby increasing the flow area. The flow control valve shown in Figure 1 modulates P1 to control flow. Calibration of the flow control valve is obtained by adjusting the metering orifice. The spring preload may also be adjustable. A second example of the flow control valve is shown in Figure 2. In this valve, the servo modulates P2. However, the overall function of the valve is similar to the valve in Figure 1. Regulating P2 may be an advantage over regulating P1 if the servo port in Figure 1 could become the controlling orifice (flow area becomes smaller than the metering orifice). In this case the servo port opening would be controlling flow. Pilot operated directional valve This symbol shows a complete pilot operated directional valve. This would probably be a large, high flow, hydraulically operated bottom valve with a smaller electrically operated pilot valve. The solenoids show the hydraulic pilot as well as an external pilot pressure supply X and external pilot pressure drain Y. The symbol also shows and open centre P to T spool. Proportional directional valve The proportional valve symbol includes several key differences from the ON/OFF type directional valve. Firstly the outside of the spool section contains lines either side. The solenoids also have arrows running through them to show they operate gradually rather than just on and off. The square box with the triangle inside represents internal electronics although this will not be present on all proportional valves. Schematic of a Outlet Pressure Regulated Flow Control Valve Check Valve, Hydraulic, Description The function of a check valve is to prevent flow in one direction and allow flow in the other direction. Check valves commonly use a poppet and light spring to control flow as shown in the figure below. If P1A1 > P2A2 + spring force + friction, then flow occurs in the direction of the arrows. If P1A1 < P2A2 + spring force + friction, then the poppet would be pushed to the left, against the stop, prohibiting flow in the reverse direction. Check Valve Schematic The most common method for designing a check valve is illustrated in the Figure 1. Different manufacturers may utilize other design approaches. For example, another type of check valve is a ball that pushes against a spring. Operation is similar to the check valve shown in Figure 1 except a ball replaces the piston.Check valves are used in hydraulic systems anytime flow in a selected direction is not desirable or may create a problem. Check valves are not used in bidirectional flow lines, such as to and from actuators. Some examples where check valves are used are Accumulator Operation and Applications An accumulator: - is an energy storage device. It stores potential energy through the compression of a dry inert gas (typically nitrogen) in a container open to a relatively incompressible fluid (typically hydraulic oil). There are two types of accumulators commonly used today. The first is the bladder type (including diaphragm designs) and the second is the piston type. While other types of accumulator designs exist, compressed gas accumulators’ ar far and away the most common. The bladder style uses a compressible gas contained in an elastic bladder mounted inside a shell. Working: - 1.) When the hydraulic pump in the system is turned on it causes fluid to enter the accumulator. When fluid fills the shell, accumulator charging begins as the nitrogen in the bladder is compressed by a fluid pressure greater than its pre-charge pressure. This is the source of stored energy. (2.) As the bladder compresses due to the fluid filling the shell, it "deforms" in shape, taking up less space in the shell while at the same time, pressure in the bladder increases. This bladder "deformation" ceases when the pressure of the system fluid and the now compressed nitrogen become balanced. (3.) Upon downstream system demand, fluid system pressure falls and the stored fluid is pushed out of the accumulator shell and returned to the system under pressure exerted by the compressed nitrogen, whose pressure is now greater than the fluid pressure. Upon completion of whatever hydraulic system function the accumulator was designed to do, the cycle starts all over again with step one. One of the most important considerations in applying accumulators is calculating the correct pre-charge pressure for the type of accumulator being used, the work to be done and system operating parameters. Pre-charge pressure is generally 80 - 90% of the minimum system working pressure to allow a small amount of fluid to remain in the accumulator. cause accumulator damage or failure. Conversely, a properly designed and maintained accumulator should operate trouble-free for years. Hydraulic Hose Sizing Hydraulic Formula (v)VELOCITY OF FLUID IN PIPE (m/s) d = bore of pipe (mm) f = flow rate (l/min) f  21.22 d2 V= Note: Normally accepted flow range for intake lines 0.6 – 1.2 (lit/min) for pressure lines 2 – 5 (lit/min). Stated flow rates for continuous duty application, for intermittent duty cycles, with due considerations to ∆t pressure lines could be exceeded. Example Hitachi EX450 dipper cylinder annulus (rod side) delivery flow v = 10.2 m/s head side exhaust flow v = 20 m/s. ∆t = TEMPERATURE RISE DUE p p = pressure drop (bar) TO PRESSURE DROP IN A ∆t = 600 = Kw SYSTEM In a pipeline system the total heat produced will be dissipated over the whole length of the run via radiation & convection. That may be sufficient in effecting sufficient heat dissipation to maintain acceptable oil temperature 650 - 700C. (p) PRESSURE REQUIRED IN A F F = Newtons (N) CYLINDER TO LIFT A LOAD A = Area (cm2) P= A (bar) (1bar = 10N/cm2 ) p = Pressure ( F ) FORCE GENERATED BY A (bar)=(10N/cm2) pxA CYLINDER (Newtons) (N) A =Area (cm2) p = Pressure ( A ) AREA (cm2) OF A F (bar)=(10N/cm2) CYLINDER REQUIRED FOR A A = p F = Newtons (N) GIVEN FORCE Example F p A Force (F) = 5000N Pressure(p) 50 bar (50 x 10 = 500Ncm2) Area (A) = 10 cm2 5000 P = 10 = 500 N/cm2 = 50 bar POWER REQUIRED TO DRIVE A PUMP TORQUE REQUIRED TO DRIVE A PUMP TORQUE GENERATED BY A HYDRAULIC MOTOR bar  cc  (rev / min) 20 Nm = l / min bar 600 = kW kW  9550 rpm = Nm F = 500  10 = 5000 N CONVERSION FACTORS 100,000 pascal (Pa) / 100 kilo pascal (kPa) / 10Ncm2 / 100,000 (105) Nm / 14.504 psi / 1.02kgf/cm2 / (10 bar = 1 mega pascal (1 MPa) 1 bar = The SI standard unit for pressure is the “pascal” (Pa) though used mainly in academic applications 1 newton (N) = 1 kiogram force (kgf) = 1 Pound force (lbf) = 1 Kilo watt (kW) = 1 Horse power (hp) = 1 Litre (ltr) = 1 Imperial gallon (gall) = 0.1019367 kgf 9.81N / 2.204 lbf 4.45N 1.34 hp 0.745 kW / 2545 British thermal units / hour (BTU/h) 0.22 Imperial gallons 4.546 l F acts on a piston, this force creates a pressure on the piston area Ak. Conversely, this force is also a resistance that the pump delivery Q meets. In this way, a pressure builds that acts evenly on all sides, including the piston area Ak. The piston can thus now exert a force itself according to the following formula: F  pxAk F Ak = = Piston force F = p * Ar (Analogously for the return flow) in [N] (D² x 3,14) in [mm²] 4 (D²-d²) x 3,14 Piston area Ar D d h Q t = = = = = = Piston ring area 4 Piston diameter Piston rod diameter Stroke length Pump delivery Stroke time in [mm²] in [mm] in [mm] in [mm] in [l/min] in [s] v = Speed in [m/s] Since friction and pressure losses reduce the piston force, a safety margin of 15-25% should be allowed for compensation when you select a cylinder. The theoretical compressive or attractive forces of the hydraulic cylinders can be found in the documents for every series. Piston Speed The feed flow or return flow speeds are calculated using the flows of liquid coming from the pump, which fill the stroke volume of the hydraulic cylinder within a certain unit of time. For the speeds specified above, the following equations result under consideration of the usual units: V Q = Feed Flow Ak Q = Return Flow Ar V Please observe that the volume of liquid flowing away during the return flow is always greater than the in-flowing liquid due to the difference in area. As a result, the return flow is faster than the feed flow. If the length of the cylinder stroke is known, the stroke time t can be calculated as follows: t  t  A kxh Q A rxh Q = Feed Flow = Return Flow Buckling strength of the Piston Rod The danger of the piston rods buckling during long strokes and an additional unfavorable cylinder mounting determine the cylinder sizes and their limits. The calculations are usually carried out according to Euler since the piston rods are to be seen as slender rods. The following applies: Fk  Fk E I Lk  2 xE xI Lk 2 = Force at which the piston rod buckles = Modulus of elasticity (Steel 210000 N/mm2) = = Moment of inertia (for circle cross-section) Free buckling length N N/mm2 mm4 mm The following diagram shows the four so-called Eulerian load cases. The permissible stroke lengths of every hydraulic cylinder series are listed in the appendix. the case of end position damping, the kinetic energy is converted to heat energy. At a constant delay, the following applies: m v² 2 m v A p s = = = = = = A*p*s (Energy conservation law) [kg] [m/s] [m²] N Average damping pressure in m² Damping distance [m] Moved mass Speed Effective damping area This results in the average damping pressure: P  m xv 2 2 xA xs Permissible Deviation in mm Stroke and Total Length Tolerances According to DIN 7169G L + Stroke = Installation Length in mm 0...499 500...1249 1250...3149 3150...8000 +/-1,5 +/- 2 +/- 3 +/- 5 Formula Recommended Maximum Oil Velocity in Hydraulic Lines mps = meters per second Pump suction lines -0 .6 to 1.2 mps Pres. lines to 35 bar - 3 to 4½ mps Pres. lines to 206.8 bar - 4½ to 6 mps Pres. lines over 200 bar - 7½ mps Oil lines in air/oil system - 1¼ mps Note:- Avoid Flow rate > 8 m/sec.resulting force is high can be destroy tube lines. Line Sizing. The velocity of hydraulic fluid through a conductor (pipe, tube or hose) is dependent on flow rate and Cross sectional area. Recommended fluid velocities through pipes and hoses in hydraulic systems are as Follows Service Velocity ( Ft/Sec) Velocity ( m/sec) Suction / intake 2-4 0.6 to 1.2 Return Pressure / discharge 4- 13 7- 18 1.5 to 4 2, to 5.5 Use values at the lower end of the range for lower pressures or where operation is continuous. Refer to the flow/velocity nomograms on pages four and five for more information, alternatively, fluid velocity In metric units Flow calculated formula Tube ID = 4.61 FlowLPM Vm/sec Where V = velocity in meters per second (m / Sec) Q = Flow rate in liters per minute (L / min) D = inside diameter of pipe or hose in millimetres (mm) Pressure drop Friction between the fluid flowing through a conductor and its inside wall causes losses, which are Quantified as pressure drop. Pressure drop in conductors is an important consideration for the designer Q = Flow rate in litres per minute (L/min) D = Inside diameter of pipe or hose in millimetres (mm) (cSt) = Kinematic viscosity of fluid (at operating temperature) in centistokes P = Density of the fluid in kilograms per cubic metre (kg/m³) L = Length of the pipe, tube or hose in metres (m) Calculate the Reynolds Number (Re) Calculate Friction factor: - F = 64 / Re f = Friction Factor. Re = Reynolds Number < 2300 = 1000 xV xD V If the Reynold Number is between 2300 and 4000 flow is transition and greater than 4000 Flow is turbulent for reynold Numbers greater than 2300 and less than 1000,000 the following formula can be used to calculate the friction factor. Calculate Pressure drop P  V 2 xF xL xP 2D p = Pressure drop in Pascal (Pa) V = velocity in meter per second (m /sec) F = Friction factor L = Length of Pipe in meter (m) P = density of fluid in kilogram cubic meter (870 - 890 kg/m3 for hydraulic oil) D = inside diameter of pipe in (m) Flow / Velocity (Metric)
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