2005/2006 i. hydraulic and pneumatic systems1 valves if the pump is the heart of a hydraulic system...
TRANSCRIPT
2005/2006 I. Hydraulic and Pneumatic Systems 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.
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
2005/2006 I. Hydraulic and Pneumatic Systems 2
ValvesPressure 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
2005/2006 I. Hydraulic and Pneumatic Systems 3
Valves
Pressure relief valve
xA k C
Fspring
m
N
psys
Qp
Symbol
2005/2006 I. Hydraulic and Pneumatic Systems 4
Pressure relief valve
Theoretical basics
xcFFAp SSsys 0
f
sys
fcrv
pxk
pxkvAQ
22
AcxF
xkQ S
frv
02
Force equilibrium:
(1)
Curtain surface:
Circumference ∙ height = k ∙ x = Ac
Bernoulli equation:
(2)
(3)
(4)
7,0...6,0
Qrv depends nonlinearly on x or if we express x from (1)
then:c
FApx Ssys 0
sysf
Ssysrv p
c
FApkQ
20
2005/2006 I. Hydraulic and Pneumatic Systems 5
Pressure relief valve
Theoretical basics:
0Ssys FAp
syssyssys
const
fsys
f
syssysrv ppp
cAk
pc
AppkQ
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 :
2005/2006 I. Hydraulic and Pneumatic Systems 6
Pressure relief valveTheoretical basics:
Qrv
negative part: physically impossible
pmax
psys
belongs to xmax
*sysp
2005/2006 I. Hydraulic and Pneumatic Systems 7
Pressure relief valve
Hydraulic aggregate:The simplest hydraulic system
Pump + pressure relief valve
M
Qrv
Qag userQp psys
reservoir
2005/2006 I. Hydraulic and Pneumatic Systems 8
Hydraulic aggregate
Let us derive the characteristics of the complete aggregate. Parallel circuit so:
rvpagagrvp QQQQQQ
QrvQpump
Qag1Qag2
psysn = n1 < n = n2
*sysp
= =
= =
Qrv
Qpump
2005/2006 I. Hydraulic and Pneumatic Systems 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.
2005/2006 I. Hydraulic and Pneumatic Systems 10
Pressure relief valveVersions:
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.
2005/2006 I. Hydraulic and Pneumatic Systems 11
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
AkC
2
2005/2006 I. Hydraulic and Pneumatic Systems 12
Pressure relief valvePilot 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
Qp2
spring 1
pilot valve
Thr
2005/2006 I. Hydraulic and Pneumatic Systems 13
Pressure relief valvePilot 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
2005/2006 I. Hydraulic and Pneumatic Systems 14
Pressure relief valvesPilot-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
2005/2006 I. Hydraulic and Pneumatic Systems 15
Pressure relief valveSimplified 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 operatedpilot operatedideal
Characteristic curves of pressure relief valves
2005/2006 I. Hydraulic and Pneumatic Systems 16
Pressure relief valveSafety valve and overflow valve:
RV
p
pl
Safety valve Overflow valve (pressure source)
Qp Qs
p
RV
p
pl
2005/2006 I. Hydraulic and Pneumatic Systems 17
Pressure relief valveSequence 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.
2005/2006 I. Hydraulic and Pneumatic Systems 18
Pressure relief valveSequence 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
2005/2006 I. Hydraulic and Pneumatic Systems 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.
2005/2006 I. Hydraulic and Pneumatic Systems 20
Pressure relief valveDynamic 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
2005/2006 I. Hydraulic and Pneumatic Systems 21
Big pictures
End of normal presentationBeginning of big pictures
2005/2006 I. Hydraulic and Pneumatic Systems 22
ValvesForm a) Spherical
b) c) d) e) Conical
Poppet valve
f) Plate
g) Spool valve