chapter four_ fluid power system lect.pptx

1. CHAPTER 4
VALVES

2. 2
4.1. Introduction
Contents
4.2. Types of Valves
4.2.1 Directional Control Valves
4.2.2 Pressure Control Valves
4.2.3 Flow Control Valves

3. 3
4.1 Introduction
• Control is the most important consideration in any
system.
• If control system components are not properly
selected, the entire system will not function as
required.
• Valves are used in fluid power system to control
direction, pressure and flow. Also the selection of
valves involves the type, size, actuating techniques
etc of the system.

4. 4
4.2 Types of Valves
There are three basic types of valves:
a) Directional control
b) Pressure control
c) Flow control
4.2.1 Directional Control Valves (DCV)
• DCVs are used to control the direction of flow in a
hydraulic circuit. By determining the path, the
direction change in cylinder or motor may be
accomplished.
• This control of the fluid path is accomplished by
check valves, shuttle valves, two-way, three-way,
four-way and five-way valves.

5. 5
• The number of ports on the DCVs is identified
using the term “way”: flow path through valve or
“port” : a connection to a valve passage fig-
chp4fig4.1.pptx.
• Types of DCVs
 One-way valve: simple check valve; flow in one
direction only, has two ports. Pilot-operated check valve
(Valves may be direct acting or pilot operated from an
adjacent or remote location).
 Two-way valve: flow in both direction; simple shutoff
valves
 Three-way valve: three flow path
 Four-way valve : four flow path
 Five- way valve : five flow path

6. 6
a) Check valves
• The simplest type of directional control valve.
• The purpose of check valve is to permit free flow in
one direction and prevent any flow in the opposite
direction.
• fig-chp4fig4.2.pptx provides two schematic
drawings ( one for the no-flow condition and one for
the free-flow condition) showing the internal
operation of poppet check valve. The figure also
shows the graphic symbol.

7. 7
b) Pilot-operated check valve
• It is a second type of check valve. fig-
chp4fig4.3.pptx
• This type of check valve always permits flow in the
normally blocked opposite direction only if pilot
pressure is applied at the pilot pressure port of the
valve.
• In Fig. 4-3, the check valve poppet has the pilot
piston attached to the threaded poppet stem by a nut.
The light spring holds the poppet seated in a no-flow
condition by pushing against the pilot piston.
• Frequently used for locking hydraulic cylinder in
position.

8. 8
c) Three-way valves
• A directional control valve whose primary function
is alternately to pressurize and exhaust on working
port
• Contains three ports, are typically of the spool
design rather than poppet design. fig-
chp4fig4.4.pptx
• A spool is a circular shaft containing lands (larger
diameter sections) that are machined to slide in a
very close fitting bore of the valve body. The
grooves between the lands provide the flow paths
between ports. Clearance between land and bore <
0.03mm.

9. 9
• Used to operate cylinder or motor in one direction.
Single acting cylinders retracted by spring or other
means.
• Can be two position or three position
• Designation by No. of Ports/ No. of Positions ( for
example: a three-way valve that uses two positions
of the spool is called a three-way, two position
directional control valve.
• The spool can be positioned manually, mechanically,
by using pilot pressure or by using electrical
solenoid.

10. 10
d)Four-way valves
• fig-chp4fig4.5.pptx shows a four-way valve design
and operation
• Has four ports (from functional point of view) and
four internal passages
• It can actuate a double acting cylinder
• Rotary or sliding
• 4/2 and 4/3 valves are common.
e) Five-way valve
• Used usually in pneumatic systems. fig-
chp4fig4.6.pptx

11. 11
f) Shuttle valve
• It is another type of directional control valve. fig-
chp4fig4.7.pptx
• Shuttle valve provides a means for connecting two
lines to one output (it permits a system to operate
from either of the fluid power source).
• One application is for safety in the event that the
main pump can no longer provide a hydraulic power
to operate emergency devices.

12. 12
Methods of actuating valves
• Manually actuated
• Mechanical actuated
• Pilot-actuated: applying air pressure against the
piston on either end of the spool
• Solenoid actuated
fig-chp4fig4.8.pptx

13. 13
4.2.2 Pressure Control Valves
a) Simple pressure relief valve
• Most widely used type of pressure control valve.
• Relief valve limits system pressure and establishes
working pressure.
• Cracking pressure-Pressure which initially lifts the
valve.
fig-chp4fig4.9.pptx
• Directly operated and pilot operated.

14. 14
b) Compound type relief valve
• Increase the pressure sensitivity by reducing the
pressure override.
• The inlet fluid goes through the back of the piston and
after moving the pilot poppet valve, it gets drained. This
effect reduces the pressure on the back of the piston
thus lifting the valve.
fig-chp4fig4.10.pptx
c) Pressure reducing valve:
fig-chp4fig4.11.pptx
• Limits the outlet pressure(normally open).
• Various operating ranges and also adjustable. When
pressure goes beyond the set value, the valve closes to
throttle the flow.

15. 15
• Useful for operation of branch circuits (sequence
valve)
Sequence valves: Control the order of operation
of two parallel branch circuits.
• Common examples of application include the
operation of two work cylinders in sequence such
that the second one activates after the complete
extension of the first (stalling increases the pressure
which raises the valve to allow flow to the other
cylinder).

16. 16
c) Counterbalance valves:
• Used to maintain back pressure to prevent a load
from falling. Common applications include vertical
presses, loaders, lift trucks, etc. fig-
chp4fig4.12.pptx
• The valve is to open at a pressure above the pressure
required to prevent the vertical cylinder from
descending due to the weight of its load. The
pressure is applied at the blank end.
• If operated directly, pressure on the rod end of the
ram must exceed the pressure setting of the valve.
Usually 10% greater than the pressure required to
sustain the vertical load.

17. 17
4.1.3 Flow Control (FCV)
• Primary function: controlling rate of flow,
velocity control of cylinders, speed control of
hydraulic motors. fig-chp4fig4.13.pptx
• Accurate speed control of pneumatic cylinders is
difficult to achieve because of compressibility effect
Orifice as a Flow Meter or Flow Control Device
An orifice ( a disk with a hole through which
fluid flows) installed in a pipe. Such a device can be
used as a flow meter by measuring a pressure drop
across the orifice. It is also used to control the flow
rate. The area of the orifice is fixed.

18. 18
4.1.3 Flow Control (FCV)
• Primary function: controlling rate of flow,
velocity control of cylinders, speed control of
hydraulic motors.
• Accurate speed control of pneumatic cylinders is
difficult to achieve because of compressibility effect
a) Orifice as a Flow Meter or Flow Control Device
Orifice is used to determine flow rate by
measurement of pressure drop between inlet and
exit. It is also used to control the flow rate. The
area of the orifice is fixed.

19. For a given orifice, there is a unique relationship
between pressure drop ( ) and flow rate ( ).
flow-rate (lpm)
C = flow coefficient (C=0.80 for sharp-edged orifice
and 0.60 for square-edged orifice)
A = area of orifice opening (mm2)
Δp = pressure drop across the orifice (kPa)
SG = specific gravity of flowing fluid

Q

SG
p
CA
Q 
 0851
0.

p
 Q


20. b) Needle Valve
fig-chp4fig4.13.pptx
Needle valves are adjustable valves that will allow
the change of flow rate. Usually a tapered needle is
used to control the flow rate by adjusting a screw.
• Flow control only in one direction
• By including a check valve, the flow can be
restricted in one direction and can freely flow in the
other direction.
For a given opening position, a needle valve
behaves as an orifice. However, unlike un orifice,
the flow area (A) in a needle valve can be varied.

21. Control is achieved by varying the flow area.
Q = volume flow rate (l/m)
Cv = capacity coefficient (lpm/√kPa)
Δp = pressure drop across the valve (kPa)
SG = specific gravity
SG
p
C
Q v




22. c) Types of Flow Control Valve
Basically two types of flow control valves:
1. None-pressure compensated and
2. Pressure compensated.
1. None-pressure compensated
None-pressure compensated valves are used where
system pressures are relatively constant. Pressure
drops are essentially constant.
2. Pressure compensated.
fig-chp4fig4.14.pptx

23. 23
• Temperature compensation: increase in
temperature reduces viscosity (hydraulics) which in
turn increases flow rate.
Compensation needed by a temperature sensitive
element to increase flow. Or use a knife sharp edge
orifice to eliminate temperature rise. Flow variations
<5% experienced within temperature range of 68o-
140oF using sharp-edged orifices.

24. Example 4-1

25. Example 4-2

26. Example 4-3

27. ASSIGNMENT II
1. The system shown in the figure below has a hydraulic cylinder with a
suspended load W. The cylinder piston and rod diameters are 50 mm and
25 mm respectively. The pressure relief valve setting is 5 bar. For a load
of (a) W=0, and (b) W= 10000 N, determine the cylinder speeds if the
flow control valve has a capacity coefficient of 0.72lpm/(kPa)1/2 . The
fluid is hydraulic oil with a specific gravity of 0.9. Assume constant
cylinder speeds.

28. 2. A 55-mm diameter sharp-edged orifice is placed in a
pipeline to measure flow rate. If the measured pressure
drop is 300kPa and the fluid specific gravity is 0.9, find the
flow rate in m3/s.
3. A pressure relief valve contains a poppet with a 4.20 cm2
area on which system pressure acts. During assembly a
spring with spring constant of 3200 N/cm is installed in the
valve to hold the poppet against its seat. In order to pass
full pump flow through the valve at the PRV pressure
setting, the poppet must move 0.3 cm from its fully closed
position. What should be the initial compression of the
spring in the PRV if the full pump flow pressure is to be
40% greater than the cracking pressure?

29. Fig.4.1 DCVs position
29

30. Fig.4.2 Check valve
30

31. Fig.4.3 Pilot-operated check valve
31

32. Fig.4.4 Three-way valve
32

33. Fig.4.5 Four-way valve
33

34. Fig.4.6 Five-way valve
34

35. Fig.4.7 Shuttle valve
35

36. Fig.4.8 Methods of actuating DCVs
36

37. Fig.4.9 Pressure relief valve
37

38. Fig.4.10 Compound type relief valve
38

39. Fig.4.11 Pressure reducing valve
39

40. Fig.4.12 Counter balance valve
40

41. Fig.4.13 Flow control valve (FCV)
41

42. Fig.4.14 Operation of pressure-compensated flow control
valve
42