2005/2006 I. Hydraulic and Pneumatic Systems1 Valves If the pump is the heart of a hydraulic system then the valve is the brain. Valves are used to perform.

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


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


Valves


xA k C

Fspring

m

N

psys

Qp

Symbol



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



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 :


Pressure relief valveTheoretical 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

QrvQpump

Qag1Qag2

psysn = 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.


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.


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


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


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


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


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


Pressure relief valveSafety valve and overflow valve:

RV

p

pl

Safety valve Overflow valve (pressure source)

Qp Qs

p

RV

p

pl


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.


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


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.


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


Big pictures

End of normal presentationBeginning of big pictures


ValvesForm a) Spherical

b) c) d) e) Conical

Poppet valve

f) Plate

g) Spool valve


ValvesPressure

relief

valve


ValvesPressure

relief

valve


ValvesPressure relief valve


ValvesPressure relief valve


Valves


xA k C

Fspring

m

N

psys

Qp

Symbol

2005/2006 I. Hydraulic and Pneumatic Systems1 Valves If the pump is the heart of a hydraulic system then the valve is the brain. Valves are used to perform.

Documents

higher pressure

outside pressure

selfoperated valve

hydraulic resistance

q rv q pump slide

pneumatic systems1 valves

x max slide

p p max