Hydraulic and Pneumatic

Circuits

Dr. Omar Mehrez

(PhD) Assistant Professor,

Mechanical Power Engineering Department,
Faculty of Engineering, Tanta University

1
 - About the Course -

❑ Introduction to Fluid Power

❑ Hydraulic Pumps
❑ Hydraulic Cylinders and Hydraulic Motors
❑ Hydraulic Valves
❑ Hydraulic Circuit Design and Analysis
❑ Basic Electrical Control of Fluid Power Systems

Reference:
Esposito, A. “Fluid Power with Applications”, England: PEARSON, 2014

2
 Hydraulic Valves

❑ Introduction
❑ Directional Control Valves
❑ Pressure Control Valves
❑ Flow Control Valves

3
 Introduction
Introduction
Fluid power is controlled primarily through the use of control devices
called valves
Valves

Directional Pressure Flow

Control Valves Control Valves Control Valves

Determine the path Protect the system Control the fluid flow
through which a fluid against overpressure due rate in hydraulic circuit
traverses a given circuit to excessive actuator load lines
or closing a valve

4
 Hydraulic Valves

❑ Introduction
❑ Directional Control Valves
❑ Pressure Control Valves
❑ Flow Control Valves

5
 Directional Control Valves
Introduction

Two-way Three-way Four-way

Valve way
DCV DCV DCV

Valve port Directional Control Valves (DCV)

The valve port contain Manual

external opening Actuation Mechanical
through which fluid -based
Pilot
can enter and leave via
Solenoid
connecting pipelines
6
 Directional Control Valves
Introduction

What is valve positions (envelopes)?

7
 Directional Control Valves
Two-way DCV
Check valve

9
 Directional Control Valves
Two-way DCV
Pilot-operated check valve

10
 Directional Control Valves
Two-way DCV
Pilot-operated check valve

11
 Directional Control Valves
Three-way DCV
Shuttle Valve

12
 Directional Control Valves
Three-way DCV

Three-way, two-position
DCV
13
 Directional Control Valves
Three-way DCV

Three-way DCV application

14
 Directional Control Valves
Four-way DCV

Four-way, two-position
DCV
15
 Directional Control Valves
Four-way DCV Center-Flow Path Configurations

1) Open center

▪ All ports are open to each other in the center position

▪ Pump flow is directed to the tank at atmospheric pressure (Pump unloading)
▪ Actuator can be moved freely by applying an external force (Floating actuator)
▪ No heat buildup in the system
▪ Other auxiliary devices in the system using pressure energy cannot function 18
 Directional Control Valves
Four-way DCV
2) Closed center

▪ All ports are closed to each other

▪ pump produces flow at the high pressure setting of the pressure relief valve
(Pump loading) (when flow is not being used for any other parts of the circuit)
▪ Actuator can not be moved by applying an external force (Blocked actuator)
▪ waste pump design power, promotes wear and shortens pump life, heat buildup
which promotes oil oxidation , viscosity drop, which further raises the wear of
parts and increased leakage.
19
 Directional Control Valves
Four-way DCV
3) Tandem center

▪ Pump unloading
▪ Blocked actuator

20
 Directional Control Valves
Four-way DCV
4) Regenerative center

▪ Regenerative means the flow generated from the system itself

▪ is used whenever the actuator movement in one direction requires two different
speeds

21
 Directional Control Valves
Four-way DCV
5) Floating center

▪ Pump flow must go over the relief valve when flow is not being used for any
other parts of the circuit (Pump loading)
▪ Freely moving the actuator (Floating actuator)

22
 Directional Control Valves
DCV Actuation

23
 Directional Control Valves
DCV Actuation
Manually Actuated

Manually-actuated, spring-centered, 3-
position, 4-way valve
24
 Directional Control Valves
DCV Actuation
Manually Actuated

Manually-actuated, spring-offset, 2-
position, 4-way valve

25
 Directional Control Valves
DCV Actuation
Mechanically Actuated

Mechanically-actuated, spring-offset, 2-
position, 4-way valve
26
 Directional Control Valves
DCV Actuation
Pilot Actuated

Air pilot-actuated, spring-centered, 3-

position, 4-way valve
27
 Directional Control Valves
DCV Actuation
Solenoid Actuated

Solenoid-actuated, spring-centered, 3-
position, 4-way valve
28
 Directional Control Valves
DCV Actuation
Solenoid Actuated

Solenoid-actuated, spring-offset, 2-
position, 4-way valve
29
 Hydraulic Valves

❑ Introduction
❑ Directional Control Valves
❑ Pressure Control Valves
❑ Flow Control Valves

30
 Pressure Control Valves
Introduction

Pressure Control Valves (PCV)

Pressure Pressure Unloading Sequence Counterbalance

Relief Reducing Valves Valves Valves
Valves Valves

31
 Pressure Control Valves
Pressure Relief Valves
1) Simple Pressure Relief Valves

32
 Pressure Control Valves
Pressure Relief Valves
1) Simple Pressure Relief Valves

Increasing the
system pressure
over the setting
pressure

33
 Pressure Control Valves
Pressure Relief Valves
1) Simple Pressure Relief Valves

Pressure vs. flow curve of

simple relief valve

34
 Pressure Control Valves
Pressure Relief Valves
2) Compound Pressure Relief Valves

Compound relief valve 35

 Pressure Control Valves
Pressure Relief Valves 2) Compound Pressure Relief Valves

36
 Pressure Control Valves
Pressure Relief Valves 2) Compound Pressure Relief Valves

37
 Pressure Control Valves
Pressure Relief Valves
2) Compound Pressure Relief Valves

Increasing the system pressure

over the setting pressure 38
 Pressure Control Valves
Pressure Relief Valves
2) Compound Pressure Relief Valves
can be used to unload the pump at low pressure during periods between work cycles by:
• Venting the chamber above the piston to the tank via a solenoid valve.
• Low pressure at the bottom chamber (e.g. 20psi pressure difference) overcomes the bias
spring

Compound relieve valve with integral

solenoid actuated vent valve 39
 Pressure Control Valves
Pressure Relief Valves
2) Compound Pressure Relief Valves

Example: using compound relieve valve to unload pump at low pressures

40
 Pressure Control Valves
Introduction

Pressure Control Valves (PCV)

Pressure Pressure Unloading Sequence Counterbalance

Relief Reducing Valves Valves Valves
Valves Valves

41
 Pressure Control Valves
Pressure Reducing Valves
Example: Clamp and drill circuit of thin-walled tube

42
 Pressure Control Valves
Pressure Reducing Valves
Used to maintain reduced pressures in specified locations of hydraulic systems

Pressure reducing valve 43

 Pressure Control Valves
Pressure Reducing Valves

Increasing the outlet pressure

over the setting pressure 44
 Pressure Control Valves
Pressure Reducing Valves

45
 Pressure Control Valves
Introduction

Pressure Control Valves (PCV)

Pressure Pressure Unloading Sequence Counterbalance

Relief Reducing Valves Valves Valves
Valves Valves

46
 Pressure Control Valves
Unloading Valves
Used to permit the pump to build pressure to an adjustable setting and then
discharge oil to the tank at essentially zero pressure

are often used in the applications where:

❑ The first portion of the cylinder stroke requires:
▪ High flow
▪ Low pressure
❑ For the reminder of the cylinder stroke, requires:
▪ Low flow
▪ High pressure

Example: Metal stamping machine

47
 Pressure Control Valves
Unloading Valves
Unloading
valve

After reaching
unloading valve
setting 48
 Pressure Control Valves
Unloading Valves
Double pump hydraulic system
Using two pumps:
low flow rate / high
pressure pump +
high flow rate / low
pressure pump
instead of a very
expensive high flow
rate / high pressure
pump
49
 Pressure Control Valves
Unloading Valves

Unloading valve 50
 Pressure Control Valves
Introduction

Pressure Control Valves (PCV)

Pressure Pressure Unloading Sequence Counterbalance

Relief Reducing Valves Valves Valves
Valves Valves

51
 Pressure Control Valves
Sequence Valves
are used when two operations are performed in sequence, that
is one operation ends before the other begins

Example:
Operation of a clamp and bend machine
The clamp cylinder holds a workpiece in place, and then a
second cylinder bends it to a desired angle

52
 Pressure Control Valves
Sequence Valves
1

3
53
 Pressure Control Valves
Sequence Valves
Bend second

Clamp
First

54
 Pressure Control Valves
Sequence Valves

Extend the clamp

cylinder

55
 Pressure Control Valves
Sequence Valves

Extend the bend

cylinder

56
 Pressure Control Valves
Sequence Valves
Retracting both cylinders at the same time

57
 Pressure Control Valves
Sequence Valves
Example: sequence operation of two double-acting cylinders

58
 Pressure Control Valves
Sequence Valves

Sequence valve 59
 Pressure Control Valves
Introduction

Pressure Control Valves (PCV)

Pressure Pressure Unloading Sequence Counterbalance

Relief Reducing Valves Valves Valves
Valves Valves

60
 Pressure Control Valves
Counterbalance Valves
The purpose of the counterbalance valve is to maintain control of a
vertical hydraulic cylinder to prevent it from descending due to the
weight of the external load.

61
 Pressure Control Valves
Counterbalance Valves

Adding
counterbalance
valve

62
 Pressure Control Valves
Counterbalance Valves

63
 Pressure Control Valves
Counterbalance Valves
Extend the
cylinder

64
 Pressure Control Valves
Counterbalance Valves
Retract the
cylinder

65
 Pressure Control Valves
Counterbalance Valves

Counterbalance valve application 66

 Flow Control Valves
Introduction

Flow Control Valves (FCV)

Needle Non-pressure Pressure

Valves Compensated Compensated
Valves Valves

Same category of non-pressure

compensated valves

67
 Flow Control Valves
Orifice
• is a disk with a hole through which fluid flows, installed in pipe
• can be used as flowmeter or flow control device

Orifice relation:
𝑄 Lpm
∆𝑝 ∆𝑝 kpa
𝑄 = 0.0851𝐶𝐴 𝐴 (mm2 )
𝑆𝐺
𝑆𝐺: specific gravity 68
 Flow Control Valves
Needle Valves

are used to give fine control of flow in small-

diameter piping; they are adjustable orifice

Fully opened Partially opened Fully closed

69
 Flow Control Valves
Needle Valves 1psi
∆𝑝
𝑄 = 𝐶𝑣
Valve relation: 𝑆𝐺

Capacity coefficient 𝐶𝑣 : flow rate of water (GPM or Lpm) that will flow through the
valve at a pressure drop of 1(psi or kPa)

𝐶𝑣 : determined experimentally for

𝐶𝑣
valve type in fully open position;
listed by manufacturer (rated- 𝐶𝑣 )
1psi
 Flow Control Valves
Example (1)

A flow control valve experiences a pressure drop of 687 kpa for a flow rate
of 94.8 Lpm. The fluid is hydraulic oil with a specific gravity of 0.9.
Determine the capacity coefficient.

71
 Flow Control Valves
Example (2)

A needle valve is used to control the extending speed of a hydraulic cylinder.

The needle valve is placed in the outlet line of the hydraulic cylinder, as
shown in the figure. The following data are given:
• Desired cylinder speed = 10 in/s
• Cylinder piston diameter = 2 in
• Cylinder rod diameter = 1 in
• Cylinder load = 1000 lb
• Specific gravity of oil = 0.9
• Pressure relief valve setting (PRV setting) = 500 psi
Determine the required capacity coefficient of the needle valve.

72
 Flow Control Valves
Example (2)

73
 Flow Control Valves
Example (2)

74
 Flow Control Valves
Example (2)

75
 Flow Control Valves
Introduction

Flow Control Valves (FCV)

Needle Non-pressure Pressure

Valves Compensated Compensated
Valves Valves

76
 Flow Control Valves
Non-pressure Compensated Valves

are used when system pressures are relatively constant and

motoring speeds are not too critical.

∆𝑝
𝑄 = 0.0851𝐶𝐴
𝑆𝐺

Q is const. for const. Δp

77
 Flow Control Valves
Non-pressure Compensated Valves

78
 Flow Control Valves
Non-pressure Compensated Valves

Flow Control

No Flow Control
79
 Flow Control Valves
Non-pressure Compensated Valves
Example: Meter-in speed control of a hydraulic cylinder

80
 Flow Control Valves
Non-pressure Compensated Valves
Example: Meter-in speed control of a hydraulic cylinder

DCV position
upon cylinder
extension

81
 Flow Control Valves
Non-pressure Compensated Valves
Example: Meter-out speed control of a hydraulic cylinder

82
 Flow Control Valves
Non-pressure Compensated Valves
Example: Meter-out speed control of a hydraulic cylinder

DCV position
upon cylinder
extension

83
 Flow Control Valves
Introduction

Flow Control Valves (FCV)

Needle Non-pressure Pressure

Valves Compensated Compensated
Valves Valves

84
 Flow Control Valves
Pressure Compensated Valves

overcome the difficulty of non-pressure compensated valves by

changing the size of the orifice in relation to the changes in the
system pressure.

∆𝑝
𝑄 = 0.0851𝐶𝐴
𝑆𝐺

Q is const. even with

varying system pressure

85
 Flow Control Valves
Pressure Compensated Valves

87
 Flow Control Valves
Pressure Compensated Valves

Adjustable orifice

Inlet press.

load press.
Intermediate press.

88
 Flow Control Valves
Pressure Compensated Inlet
Valves press.

load
Intermediate press. press.

Inlet Load
pressure pressure

Variable orifice
pressure drop

Adjustable orifice
pressure drop

89
 Flow Control Valves
Pressure Compensated
Valves

Adjustable orifice
(hand wheel)
Inlet
press.

Inlet Load
pressure pressure load
Intermediate press. press.
Variable orifice
pressure drop

Adjustable orifice
pressure drop

90
 Flow Control Valves
Pressure Compensated Valves

Case-1: PL = 0 psi 100

Inlet psi
press.
Pintermediate = 100 psi 0 psi

∆Phw = 100 psi

load
∆Po = 400 psi press.
Intermediate press.

Inlet pressure:

Pfc = 400 + 100 + 0 = 500 psi

91
 Flow Control Valves
Pressure Compensated Valves

Case-2: PL = 200 psi Inlet

press.
Pintermediate = 300 psi

∆Phw = 100 psi

load
∆Po = 200 psi press.
Intermediate press.

Inlet pressure:

Pfc = 200 + 100 + 200 = 500 psi

92
 Flow Control Valves
Pressure Compensated Valves
Case-3: PL = 400 psi 500
Inlet psi
press.
Pintermediate = 500 psi 400
psi

∆Phw = 100 psi

load
∆Po = 0 psi press.
Intermediate press.

Inlet pressure:

Pfc = 0 + 100 + 400 = 500 psi

93
 Flow Control Valves
Pressure Compensated Valves
Example: Speed control of a hydraulic motor

94
 Servo Valves
Introduction

A servo valve is a directional control valve that has infinitely

variable positioning capability. It controls both the flow direction
and amount.

Mechanical- Electrohydraulic
Type Servo Servo Valve
Valve

95
 Servo Valves
Mechanical-Type Servo Valve

The output is fed to modify the input (a closed-loop system)

96
 Servo Valves
Electrohydraulic Servo Valve
Uses an electrical torque motor, a double nozzle pilot stage,
and a sliding spool second stage.

98
99
 Servo Valves
Electrohydraulic Servo Valve

Block diagram of an electrohydraulic servo system

(closed-loop)
100
 Proportional Control Valves
Proportional Control Valve
is similar to electrohydraulic servo valve for
controlling the flow amount and direction but
different in ways:

Electrohydraulic Servo Valves Proportional Control Valves

Used in closed-loop systems Used in open-loop systems

Spool valve movement is due to

Uses a solenoid that produces a
electrical current in a torque
force proportional to the current
motor coil causes a torque on an
in its coil
armature
101
 Summary

▪ Describe the purpose, construction, and operation of directional,

pressure and flow control valves
▪ Identify the graphical symbols of directional, pressure and flow
valves
▪ Explain different valves actuation methods

102
 References
▪ Directional Control Valves:
https://www.wisc-online.com/learn/career-
clusters/stem/dce9404/hydraulic-directional-control-valves
▪ Pressure Relief Valve:
✓ https://www.wisc-online.com/learn/career-
clusters/stem/hyp1206/pressure-relief-valves
✓ http://www.valvehydraulic.info/hydraulic-pressure-

control/pilot-operated-relief-valves-hydraulic-circuits.html
▪ Pressure Reducing Valve:
https://www.wisc-online.com/learn/career-
clusters/stem/hyp5509/direct-operated-pressure-reducing-valves 103
 References

▪ Unloading Valve:
https://www.wisc-online.com/learn/career-
clusters/stem/hyp3306/unloading-valves
▪ Sequence Valve:
https://www.wisc-online.com/learn/career-
clusters/stem/hyp3206/sequence-valves
▪ Counterbalance Valve:
https://www.wisc-online.com/learn/career-
clusters/stem/hyp3106/counterbalance-valves

104
THANK YOU

105