Valves

1
Valves
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.
Aspects of classification
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
2
Valves

• Pressure control valves

• pressure relief valves
• pressure reducing valves

• pressure difference valve
• pressure ratio valve

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

• 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

3
Valves

• Pressure relief valve

Symbol
A
C
k
Fspring
psys
Qp
4
Pressure relief valve

• Theoretical basics

Curtain surface Circumference height k x
Ac
Force equilibrium
(1)
Bernoulli equation
(2)
(3)
Qrv depends nonlinearly on x or if we express x
from (1) then
(4)
5
Pressure relief valve

• Theoretical basics

Let be the pressure when the valve just
opens (x 0)
? subst. to (4)
(5)
(5) is valid from x 0 to x xmax, xmax being
the displacement when the spring is completely
compressed
If p gt pmax
6
Pressure relief valve

• Theoretical basics

7
Pressure relief valve

• Hydraulic aggregate

The simplest hydraulic system Pump pressure
relief valve
Qag user
Qp
psys
Qrv
M
reservoir
8
Hydraulic aggregate
Let us derive the characteristics of the complete
aggregate. Parallel circuit so

n n1
lt n n2
psys
Qrv
Qpump




Qag2
Qag1
Qpump
Qrv
9
Pressure relief valve
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.
10
Pressure relief valve

• 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.
11
Pilot operated pressure relief valve
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.
Valve constant has to be increased
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)

12
Pressure relief valve

• Pilot operated

Q
Thr
p1
p2
spring 1
spring 1
A
main valve
pilot valve
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.
13
Pressure relief valve

• 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 gt p2. If A(p1-p2) gt 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
14
Pressure relief valves

• Pilot-operated pressure relief valve

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
1 - foszelep, 2 - elovezérlo szelep, 3 -
fotolattyú, 4 - 5 - 11 - fojtás, 6 - 7 - 13 -
vezérlo vezeték, 8 - szeleptest, 9 - rugó, 15 -
tehermentesítés
15
Pressure relief valve
Simplified symbol of a pilot operated pressure
relief valve
Characteristic curves of pressure relief valves

• 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)

16
Pressure relief valve

• Safety valve and overflow valve

17
Pressure relief valve

• 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.
18
Pressure relief valve

• Sequence valves

Sequence valve (turn off)
19
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.
20
Pressure relief 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.
21
Big pictures

• End of normal presentation
• Beginning of big pictures

22
Valves

• Form

a) Spherical b) c) d) e) Conical
f) Plate g) Spool valve
Poppet valve
23
Valves

• Pressure
• relief
• valve

24
Valves

• Pressure
• relief
• valve

25
Valves

• Pressure relief valve

26
Valves

• Pressure relief valve

27
Valves

• Pressure relief valve

Symbol
A
C
k
Fspring
psys
Qp