Chapter 6 Pressure Control Valves
Objectives:
The purpose of this chapter is to describe:
1. Operation of pressure control valves.
2. Pressure relief valves, pressure reducing valves, sequence
valves, and pressure switches.
3. Measure and control quantities of pressure control valve.
4. Applications of pressure control valves.
Upon completing this chapter, you should be able to:
Explain the operation of the various types of pressure control
valves.
Identify the graphic symbols used for pressure control valves.
Distinguish the different types of pressure control valves.
Measure methods of some usually used physical quantities
A
A
A
A
p
p1
A
p1
p2
p2
(a) pressure
pA  k (measure
x  x)
0
(b)
(p pressure
 p ) A sum
 k (measure
x  x)
1
2
0
(c)
(ppressure
 p ) Adifference
 k ( x measure
x)
1
2
0
(a) pressure measure (b) pressure sum measure (c) pressure difference measure
Chapter 6 Pressure Control Valves
6.1 Introduction
Pressure control valves are classified as pressure relief valves,
pressure reducing valves, sequence valves, and pressure
switches.
6.2 Pressure relief valves
Pressure relief valves limit/maintain the maximum pressure
in a hydraulic circuit by diverting pump flow back to the tank.
6.2.1 Direct-acting pressure relief valve
Chapter 6 Pressure Control Valves
1 Structure
T
P
hand wheel
adjustment spring poppet seat valve body
2 Operating process
When the hydraulic force is less than the spring force, the
poppet remains on its seat and no flow pass through the valve.
When the hydraulic force is greater than the spring force, the
poppet will be forced off its seat, and fluid will flow back to the
tank through port T.
Chapter 6 Pressure Control ValvesR
T
P
hand wheel
adjustment spring poppet seat valve body
3 Equilibrium equation
Neglecting the poppet weight, friction force, and flow force,
the static force equilibrium equation of the poppet is
F  FS  pA  k ( x0  x)  pA  0
that is
p  k ( x0  x) / A
If the k is very small and x0  x
p  kx0 / A  costant
Chapter 6 Pressure Control Valves
ps----starting pressure;
pT----setting or predetermined
level;
Δp= pT-ps steady overshoot;
q
4 Characteristic curve
qe
attentio
n
p
ps
pT
The relief valve should be able to pass through the overall flow
rate of the pump.
5 advantage and disadvantage
The direct-acting pressure relief valve has a simple construction
and a high sensitivity; however, it is not suitable for the application
of high pressure and high flow rate.
Chapter 6 Pressure Control Valves
6.2.2 Pilot-operated pressure relief valves
1) View and constitution
Chapter 6 Pressure Control Valves
1 Schematic
2) components
vent port K
a
Symbol
p
p2
9
8
K
T
p1
outlet port T
1
2
3
4
5
6
7
1-pilot valve;
2-poppet seat;
3-valve cap;
4-valve body;
5-orifice;
6-main spool;
7-main valve seat;
8-main spring;
9-adjustment spring
Port P1 is connected
to the pump line.
Port T is connected
to the tank.
Chapter 6 Pressure Control Valves
3) cuicuit
vent port K
a
p2
p1

outlet port T
5
a
hole
p1 

p2 

K central

T , p1  T
Chapter 6 Pressure Control Valves
4) Operating process
If p1 is very small, both close.
As soon as p1 reaches the setting,
it will force the pilot poppet off its
seat. A small amount of flow begins
to go through the pilot line and the
piston orifice back to tank.
(pilot valve opening)
When p1 further rises, the
opening of the pilot valve further
increases. As a result, the pressure
drop of the camping orifice causes
the piston to lift off its seat and the
flow goes directly from the pressure
port to the tank. (both valve
opening)
Measure and control quantity:
inlet pressure
Chapter 6 Pressure Control Valves
5) Equilibrium equation
Neglecting the poppet weight, friction force, and flow force,
the static force balance equation of the poppet is
Pilot valve:  F  F  pA  K ( x  x )  p A  0
x
s
x
0
2 s
p2  K x ( x0  x ) / As
if the Kx is very small and x0>>x,
thus
p2≈Kxx0/A3（constant）
main valve:
ΣFy= Ky(y0+y) -p1A1+p2A2=0
That is
p1=Ky (y0+y)/A1 +p2A2 /A1
if the Ky is very small and y0>>y,
thus
p1≈Kyy0/A1 + (Kxx0/As) A2 /A1 （constant）
6) Remotely adjusting pressure
p
s
T
a
p2
p1
The pilot relief valve
is set for the maximum
pressure
that
the
circuit is designed.
The remote relief
valve is set to a lower
pressure dictated by
the current operating
parameters.
When s is energized, the
systematic pressure is set
by the remote pilot relief
valve
When s is de-energized,
the systematic pressure is
set by the pilot relief valve.
Chapter 6 Pressure Control Valves
7) Unloading the pump
When the vent port is
connected to tank via a
solenoid
directional
control
valve,
the
system will be able to
be unloaded.
(only the main valve
opening)
a
s
p2
p1
8) advantage
The pilot-operated relief valve usually is smaller than a directacting relief valve for the same flow and pressure ratings.
6.2.3 Solenoid pressure relief valves
K
o u t le t ch a m b e r
P
ve n t p o r t K
T
( b)
K
in le t p o r t P
P
ta nk po rt T
T
( c)
( a)
6.3 Pressure reducing valves
1 Function: The pressure reducing valve maintains a reduced
pressure level in a branch circuit of a hydraulic system.
2 Components
6.3 Pressure reducing valves
3 Orifaces
P1
drain port L
L
x
inlet port P1
orifice f
outlet port P2
orifice e
e: camping oriface(fixed)
f: throattle oriface(variable)
X: openning
P2
6.3 Pressure reducing valves
4 oil circuits
P1
drain port L
L
x
inlet port P1
P2
orifice f
outlet port P2
orifice e
p f
pe
p1  p2 (pressure reducing), p2  p3
6.3 Pressure reducing valves
5 Euilibrun equation
pilot valve:
F  p3 A3  K3 ( x0  x)  0
p3  K3 ( x0  x) / A3
if K3 is very small and x0  x
p3  K3 x0 / A3  constant
main valve:
F  p2 A  p3 A  K ( y0  y )  0
p2  p3  K ( y0  y ) / A3
if K is very small and y0  y
p2  p3  Ky0 / A  constant
6 dynamic regulating
p1  p2  spool  x  px  p2  p2  C
p1  p2  spool  x  px  p2  p2  C
6 Measure quantity and control quantity: outlet pressure
Drain manner: external drain
Outlet port: to an actuator
Problem: how does a pressure relief valve?
Chapter 6 Pressure Control Valves
6.4 Sequence valves
Sequence valves cause a hydraulic system to operate in a
pressure sequence.
They are used to control the order of various actuators of a
hydraulic system.
As soon as the inlet pressure reaches a preset pressure value,
the sequence valve will open and let oil pass to a secondary
circuit.
Sequence valves have two types—direct-acting and pilotoperated.
They can also be classified as internal control and external
control types according to where the control pressure is from.
Chapter 6 Pressure Control Valves
6.4.1 Direct-acting sequence valves
adjustment screw
P1
P2
spring
L
drain port L
(b)
valve cap
P1
outlet port P 2
P2
valve body
L
(e)
inlet port P 1
valve spool
II
a
vent port
K
control piston
p2
I
L
p1
A
end cap
b
K
Chapter 6 Pressure Control Valves
One-way sequence valve and counterbalance valve
(a)
(b)
(c)
(a) One-way internal control sequence valve
(b) One-way external control sequence valve
(c) One-way internal control counterbalance valve
(d) One-way external control counterbalance valve
(d)
Summary
No. Pressure
valve
1 Pressure
relief
2 Unloading
valve
3 Pressure
reducing
valve
4 Sequence
valve
5 Counterba
lance
6 Pressure
switch
Measure and
control quantity
Inlet pressure
Pilot pressure
Outlet pressure
Outlet
port to
tank
leakage
to have no
drain port
tank
to have no
drain port
actuator to have a
drain port
symbol
p
K
T
T
p
K
T
P1
L
P2
Inlet/pilot pressure actuator to have a
drain port
Inlet/pilot pressure tank
to have no
drain port
Inlet port pressure
p
P1
P1
L
K
P2
P2
P
P
K
T
T
L
Chapter 6 Pressure Control Valves
6.6.1 Pressure regulation circuit
In a fixed displacement pump system, the supply pressure of
a hydraulic pump is regulated by a pressure relief valve.
In a variable displacement pump system, a pressure relief
valve is used as a safety valve to limit the systematic maximum
pressure and prevent a system from overloading.
When a system needs more than two pressure levels, a multipressure regulation circuit is used.
6.6.1.1 Single stage pressure regulation circuit
The supply pressure of pump can be
regulated by adjusting the pressure relief
valve.
The setting pressure of pressure relief
valve must be more than the sum of
hydraulic cylinder maximum pressure
and all pressure losses in the circuit.
Chapter 6 Pressure Control Valves
6.6.1.2 Bi-directional pressure regulation circuit
3
3
2
2
1
(a)
1
(b)
Chapter 6 Pressure Control Valves
6.6.1.3 Multi-pressure regulation circuit
3
to system
2
4
to system
to system
2
1
3
1
(a)
(b)
(c )
Bi-stage pressure regulation circuit and multi-stage pressure
regulation circuit can reduce overflow power loss.
Chapter 6 Pressure Control Valves
6.6.2 Pressure reducing circuit
to s y s t em
2
1
(a)
(b)
clamping cylinder