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Valves
2005/2006 I. Hydraulic and Pneumatic Systems 2
If the pump is the heart of a hydraulic system then the valve is the brain.
Valves are used to perform a large variety of governing and controlling functions.
Form: Function:
• pressure control valves
• flow control valves
• check valves (non-return valves)
• directional control valves
a) Spherical
b) c) d) e) Conical
f) Plate
g) Spool valve
Poppet
valve
Aspects of classification

Valves
Pressure control valves:
• pressure relief valves
• pressure reducing valves
Pressure relief valve
Has the task to limit the pressure in a hydraulic system or in a part of the system.
The pressure can rise in a hydraulic system if:
• pressure difference valve
• pressure ratio valve
- the flow rate from the pump is larger than the flow rate through the actuator
- the volume of a closed system is reduced
- the load of the actuator rises
- heat is introduced into a closed system
- the hydraulic resistance of the system rises

Valves
x
A k C
Fspring






m
N
psys
Qp
Symbol

Theoretical basics
xcFFAp SSsys ⋅+==⋅ 0
f
sys
f
crv
p
xk
p
xkvAQ
ρ
α
ρ
α
22
⋅⋅⋅=
∆
⋅⋅⋅=⋅=
A
cxF
xkQ S
f
rv
+
⋅⋅⋅⋅= 02
ρ
α
Force equilibrium:
(1)
Curtain surface:
Circumference ∙ height = k ∙ x = Ac
Bernoulli equation:
(2)
(3)
(4)
7,0...6,0=α
Qrv depends nonlinearly on x or if we express x from (1)
then:
c
FAp
x Ssys 0−⋅
=
sys
f
Ssys
rv p
c
FAp
kQ ⋅⋅
−⋅
⋅⋅=
ρ
α
20

Theoretical basics:
0Ssys FAp =⋅∗
( ) ( ) syssyssys
const
f
sys
f
syssys
rv ppp
c
Ak
p
c
App
kQ ∗
=
∗
−⋅⋅
⋅⋅
=⋅⋅
⋅−
⋅⋅=
  
ρ
α
ρ
α
22
(5)
Let be the pressure when the valve just opens (x = 0)
⇒ subst. to (4)
*
sysp
(5) is valid from x = 0 to x = xmax, xmax being the displacement when the spring is
completely compressed
max0 xx ≤≤
( ) syssyssysrv pconstpppconstQ ⋅=⋅−⋅= ∗
maxIf p > pmax :

Theoretical basics:
Qrv
negative part:
physically impossible
pmax
psys
belongs to xmax
*
sysp

Hydraulic aggregate:
The simplest hydraulic system
Pump + pressure relief valve
M
Qrv
Qag userQp psys
reservoir

Hydraulic aggregate
Let us derive the characteristics of the complete aggregate. Parallel circuit so:
rvpagagrvp QQQQQQ −=⇒+=
Qrv
Qpump
Qag1
Qag2
psys
n = n1 < n = n2
*
sysp
= =
= =
Qrv
Qpump

The pressure relief valve always has to be matched with the pump.
If for example the rotational speed is increased (orange curve) ⇒ then there
will be flow through the aggregate even with higher pressure. Wrong!
The last section of the curve has to be at the negative Q plane.

Versions:
There are various versions of pressure relief valves:
1. Directly operated → ← pilot operated
2. Self-operated → ← externally operated
The self-operated valve is controlled by its own pressure, whereas the externally
operated valve is controlled by an outside pressure.
Directly operated – already explained.

Pilot operated pressure relief valve
Valve constant has to be increased:
Pilot operated is applied when large flow rates have to be controlled. Consists of
two valves:
At higher flow rates the losses at a directly operated valve would be very large.
Another problem is that with high flow rate the dynamics of the valve gets worse.
The area can be increased but this increases size and costs and worsens the
sensitivity of the control.
The spring constant can be decreased but this increase against the size. (Large
preloading is necessary.) Again bad for dynamics.
Solution: two valves: pilot operated valve
- Main valve (low c)
- Pilot valve (high c)
fc
Ak
C
ρ
α 2
⋅
⋅⋅
=

Pilot operated:
p1 is the system pressure that has to be limited.
The valve is closed as long as the limit pressure is not reached at the pilot valve.
x
main
valve
spring 1A
p1
Q
p2
spring 1
pilot
valve
Thr

Pilot operated:
In this case the pressure p1 acts on both sides of the valve and the valve is in
equilibrium: p1 = p2.
In the pressure rises above the limiting pressure of the pilot valve then the pilot valve
opens.
A flow starts through the throttling valve so that p1 > p2.
If A(p1-p2) > FS01 (preload force of spring 1) then the main valve also starts to open.
This is more favourable because it allows a softer spring in the main valve.
The pilot valve can be spatially separated from the main valve – the control can
be exercised from a distance.
More favourable static characteristics
Better dynamic characteristics

Pressure relief valves
Pilot-operated pressure relief valve
1 - főszelep, 2 - elővezérlő szelep,
3 - főtolattyú, 4 - 5 - 11 - fojtás,
6 - 7 - 13 - vezérlő vezeték,
8 - szeleptest, 9 - rugó,
15 - tehermentesítés
1 - Main valve
2 - Pilot valve
3 – Main spool
4 - 5 - 11 - Throttle
6 - 7 - 13 - Operation
line
8 - Valve body
9 - Spring
15 - Discharging

Simplified symbol of
a pilot operated
pressure relief valve
Applications of pressure relief valves:
- Safety valve (most common)
- Overflow valve (pressure source always open produces large losses,
should be used only for small power)
- Sequence valves (turn-on or turn-off)
p1
Qrv
directly operated
pilot operated
ideal
Characteristic curves of pressure relief valves

Safety valve and overflow valve:
RV
p
pl
Safety valve Overflow valve (pressure source)
Qp Qs
p
RV
p
pl

Sequence valves:
“Turn-on” valve is used when two or more users are fed by the same pump.
When one working step is finished , the pressure rises and a second user is
switched through the valve.
“Turn-off” valve is applied when two pumps (one high pressure, one low
pressure) work in parallel. If the pressure is high enough then only the high
pressure pump works.
If the pressure drops below the limiting value of RV1 then the valve closes and
the pump delivers into the system again.

Sequence valves:
Sequence valve (turn off)
RV1
Sequence valve (turn on)
RV2
pl1
pl2
Qp > QS
Here the RV1 is an
externally operated
valve.
Check valve
RV1RV2
pl1
pl2
HPP
LPP
Q

Pressure relief valves
Dynamic behaviour of pressure relief valves:
Sudden opening or closing leads to oscillations of the valve.
Stationary characteristics depend only on the valve, dynamics characteristics
depend on the whole system.
The dynamic behaviour can be tested by a sudden application of the pressure
on the valve.

Dynamic behaviour of pressure relief valves:
First linear rise, then damped
oscillation.
There is a differential equation
system which can be solved to
simulate the process.
V is the volume of fluid between pump
and valve → capacity and inductivity
changes.
DV
p1
Qrv
Test rig
Qp
RV
p
t
p1 V1 V2 V3
V1 < V2 < V3

Big pictures
End of normal presentation
Beginning of big pictures
2005/2006 I. Hydraulic and Pneumatic Systems 22http://ebooks.edhole.com

Valves
Form
a) Spherical
b) c) d) e) Conical
Poppet
valve
f) Plate
g) Spool valve

Valves
Pressure
relief
valve

Valves

Valves
x
A k C
Fspring






m
N
psys
Qp
Symbol

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