Chapter 6 Pressure Control Valves

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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.
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Measure methods of some usually used physical
quantities
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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
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Chapter 6 Pressure Control Valves
1 Structure
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.
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Chapter 6 Pressure Control Valves
3 Equilibrium equation Neglecting the
poppet weight, friction force, and flow force,
the static force equilibrium equation of the
poppet is
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Chapter 6 Pressure Control Valves
ps----starting pressure pT----setting or
predetermined level ?p pT-ps steady
overshoot
4 Characteristic curve
attention
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.
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Chapter 6 Pressure Control Valves
6.2.2 Pilot-operated pressure relief valves
1 View and constitution
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Chapter 6 Pressure Control Valves
1 Schematic 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.
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Chapter 6 Pressure Control Valves
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Chapter 6 Pressure Control Valves
As soon as the systematic pressure 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)
2 Operating process
When the systematic pressure 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
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Chapter 6 Pressure Control Valves
3 Equilibrium equation Neglecting the
poppet weight, friction force, and flow force,
the static force balance equation of the poppet
is
if the Kx is very small and x0gtgtx, thus
p2Kxx0/A3(constant)

main valve SFy Ky(y0y) -p1A1p2A20
That is p1Ky
(y0y)/A1 p2A2 /A1 if the Ky is very
small and y0gtgty, thus
p1Kyy0/A1 (Kxx0/As) A2 /A1 (constant)

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4 Remotely adjusting pressure
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.
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Chapter 6 Pressure Control Valves
4 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)
5 advantage The pilot-operated relief valve
usually is smaller than a direct-acting relief
valve for the same flow and pressure ratings.
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6.2.3 Solenoid pressure relief valves
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6.3 Pressure reducing valves
The pressure reducing valve maintains a
reduced pressure level in a branch circuit of a
hydraulic system.
6.3.1 Pressure reducing valve with pilot oil from
outlet
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Pilot valve SFxFs-pA-Ks(x0x)-
p3As0 That is
p3Ks (x0x)/As if the Kx is very small and
x0gtgtx, p3Ksx0/As(constant)
main valve SFy K(y0y)
p2Ap3A0 That is
p2K (y0y)/Ap3 if the K is very small and
y0gtgty,p2Ky0/A Ksx0/A3(constant)

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Measure quantity and control quantity outlet
pressure Drain manner external drain Outlet
port to an actuator
Problem how does a pressure relief valve?
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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 typesdirect-acting
and pilot-operated. They can also be
classified as internal control and external
control types according to where the control
pressure is from.
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Chapter 6 Pressure Control Valves
6.4.1 Direct-acting sequence valves
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Chapter 6 Pressure Control Valves
One-way sequence valve and counterbalance valve

• One-way internal control sequence valve
• One-way external control sequence valve
• One-way internal control counterbalance valve
• (d) One-way external control counterbalance valve

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Summary
No. Pressure valve Measure and control quantity Outlet port to leakage symbol
1 Pressure relief Inlet pressure tank to have no drain port
2 Unloading valve Pilot pressure tank to have no drain port
3 Pressure reducing valve Outlet pressure actuator to have a drain port
4 Sequence valve Inlet/pilot pressure actuator to have a drain port
5 Counterbalance Inlet/pilot pressure tank to have no drain port
6 Pressure switch Inlet port pressure
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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 multi-pressure 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.
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Chapter 6 Pressure Control Valves
6.6.1.2 Bi-directional pressure regulation
circuit
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Chapter 6 Pressure Control Valves
6.6.1.3 Multi-pressure regulation circuit
Bi-stage pressure regulation circuit and
multi-stage pressure regulation circuit can
reduce overflow power loss.
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Chapter 6 Pressure Control Valves
6.6.2 Pressure reducing circuit