Throttle Relief Valve

8/10/2019 Throttle Relief Valve

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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

Poppetvalve

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

F spring

m

N

p sy

s

Q p

Symbol





Theoretical basics

xc F F A p S S sys 0

f

sys

f

crv

p xk

p xk v AQ

22

A

cx F xk Q S

f

rv

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

F A p x

S sys 0

sys

f

S sys

rv pc

F A pk Q

20





Theoretical basics:

0S sys F A p

sys sys sys

const

f

sys

f

sys sysrv p p pc

Ak pc

A p pk Q

22

(5)

Let be the pressure when the valve just opens ( x = 0)

subst. to (4)

*

sys p

(5) is valid from x = 0 to x = xmax, xmax being the displacement when the spring is

completely compressed

max0 x x

sys sys sysrv pconst p p pconst Q

maxIf p > pmax :




Pressure relief valveTheoretical basics:

Qrv

negative part:

physically impossible

pmax

p sys

belongs to xmax

*

sys p


http://slidepdf.com/reader/full/throttle-relief-valve 7/272005/2006 I. Hydraulic and Pneumatic Systems 7


Hydraulic aggregate:

The simplest hydraulic system

Pump + pressure relief valve

M

Qrv

Qag userQ p p sys

reservoir



Hydraulic aggregate

Let us derive the characteristics of the complete aggregate. Parallel circuit so:

rv pag ag rv p QQQQQQ

Qrv

Q pump

Qag1Qag2

p sysn = n1 < n = n2

*

sys p

= =

= =

Qrv

Q pump




The pressure relief valve always has to be matched with the pump.

If for example the rotational speed is increased (orange curve) then therewill 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)

f c

Ak C

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 1 A

p1

Q

p2

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- p

2) > F

S 01 (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 characteristicsBetter dynamic characteristics



Pressure relief valvesPilot-operated pressure relief valve

1 - f őszelep, 2 - elő vezérlő szelep,

3 - f őtolattyú, 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





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 isswitched 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

pl 1

pl 2

Q p > QS

Here the RV1 is an

externally operated

valve.

Check valve

RV1RV2

pl 1

pl 2

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 pressureon 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 pumpand valve → capacity and inductivity

changes.

DV

p1

Qrv

Test rig

Q p

RV

p

t

p1 V 1 V 2 V 3

V 1 < V 2 < V 3




Big pictures

End of normal presentation

Beginning of big pictures

V l




Valves

Form a) Spherical

b) c) d) e) Conical

Poppet

valve

f) Plate

g) Spool valve

V l




Valves

Pressure

relief

valve

V l




Valves

Pressure

relief

valve

V l




Valves


V l




Valves


V l



d li d i

Valves


x A k C

F spring

m

N

p sy

s

Q p

Symbol

Date post:	02-Jun-2018
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Throttle Relief Valve

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