Chapter 8 1 Flow Control Valves 15

8/13/2019 Chapter 8 1 Flow Control Valves 15

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In order to control

the velocity of ahydraulic motor or

control the flow.

In this exam le theflow to the cylinderis controlled by a

.

the load on the cylinder and is in this case 80 bar.

.hydraulic pump delivers 12 l/min so a part of the pumpflow, 4 l/min flows throu h the ressure relief valve backto the reservoir.

The ressure before the flow control is determined b thepressure relief valve, in this case 120 bar.

The pressure drop in the flow control (40 bar) and in thepressure relief valve (120 bar) is converted into heat.

This kind of flow control is relatively cheap but has a lowenergy efficiency.


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The simplest and most common of the flow restricting valvesis a simple needle valve.

This type of valve consists of a valve body that contains an,

in and out to vary the orifice opening.

type valves, regardless of the actual restrictor mechanism.


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the valve, There is, however, a preferred direction of flow,normally indicated by an arrow on the valve body. When the flow

is in this direction, the pressure loss across the valve is slightlyless and the valve is self-cleaning and not likely to accumulate

.

The addition of an internal check valve allows for restrictedflow in one direction and free flow in the other direction Fi ure9.2). Flow entering the P port of the valve is obstructed by thecheck valve and directed through the metering orifice. When

, .

direction opens the check valve and flows freely through thevalve. No control is exerted in the reverse direction, so thevalve must be sized to accommodate the maximum flow that canbe expected in that direction.

nee e va ve w an n erna c ec va veallows for restricted flow in one direction and

free flow in the other direction.


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In a simplified equation (often referred to as the orifice

through the orifice aspg2

where C is a constant that is a characteristic of the particularorifice design, A is the area of the orifice, g is the acceleration of

, .

For any given orifice, C and A are fixed, g remains constant, and the.

leaves only the differential pressure as a variable parameter.

as a control device in common fluid power systems. Any time thepressure differential across the orifice changes, the flow ratethrough the orifice changes. As we have seen, the speed of anyactuator (motor or cylinder) depends on fluid flow rate. Thus, if the

pressure varies (as will happen if the load changes), then the speedof the actuator changes.

o e p overcome s pro em, ev ces suc as nee e va ves areused. Needle valves are nothing more than variable orifices thatuse the stem to chan e the size of the orifice. This ives us theability to compensate for changes in the differential pressure inorder to maintain a constant flow rate through the valve and,consequen y, a cons an ac ua or spee . a so a ow us oestablish a desired flow rate without removing a fixed orifice andre lacin it with one of a different size.The ability to vary the orifice size in a needle valve also causesthe orifice constant, C, to change.


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pressure differential. Fig. 9.4 shows the pressure-flowcharacteristics of a typical needle valve. It is obvious from the plotthat this valve provides no real measure of control unless thepressure differential remains constant. While a constant pressure

operations, it is not typical of most working fluid power systems. Thenormal working system may require more sophisticated flow controlthan can be provided by a simple needle valve. A pressure-compensated flow control valve is often necessary.

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pressure-compensated valve. As the upstream pressure increases, aforce is exerted on the face of the compensating spool that causes it

orifice. An increase in the downstream pressure pushes the spool the

other way. The purpose of the compensator is to create a pressurerop t at eeps t e pressure i erentia across t e a justa e nee e

valve constant- usually about (689.5 kPa). This allows the valve tomaintain a constant downstream flow rate even though the pressuredrop across the entire valve may be changing frequently and over a widerange.

. . ,valve setting. Notice that there is an apparent increase in the flow rateat the lower pressure settings. This occurs because of the positiveorce exer e n e com ensa or s oo y e s r ng. n e n e

pressure is high enough to generate sufficient force to begin tocompress the spring, the valve acts as a simple needle valve. The valvealso acts as a simple needle valve if the flow is backward through it.


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Pressure-Compensated FlowControl Valves

-control than a simple needle valve.

It is obvious that apressure-compensatedvalve would be much moresu a e an a non-compensated valve in anys stem where a steadactuator speed isrequired.

ese va ves w yp ca ymaintain output flowwithin + 5% of theselected setting over awide pressure range. Thiss su c en y accura efor the majority ofindustrial and mobileequipment applications.

yp ca pressure-compensa eflow control valve performance


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e con ro e ow rec on w e c ear y n ca e on evalve by either an arrow on the valve body or by marking the inletand outlet orts.

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pressure-

com ensated

flow-controlvalve

The pressure compensated flowcontrol


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The pressure compensated flow control To control the velocity of a hydraulic motor or cylinder one has to

control the flow to these components. This can be done with a simpleow con ro

The flow through a flow control is determined by:a) The area of the flow control: a larger area means a higher amount of

flow andb) the pressure drop across the flow control: an increase of theressure dro means an increase of flow

Example: in a system with a flow control the pressure at the pump sideis determined by the pressure relief valve (see also flow control). When

increase of the load on the cylinder the flow and velocity of the cylinderwill decrease. If the velocity has to remain constant and independent oft e oa one as to use a pressure compensate ow contro

How does it work?The ressure at the outlet o the ressure com ensated low control isdetermined by the load on the cylinder. The load is 50 bar andincreases to 90 bar when the mouse cursor is put on the picture. The

, .

The ressure com ensated flow control The pressure compensated flow control is adjusted on 10 l/min. The

um delivers 12 l/min: this means that the a flow of 2 l/min flowst roug t e pressure contro va ve ack to t e reservoir.

The pressure compensated flow control has two parts: a flow controlvalve the needle valve and a ressure reducin valve or ressurecompensator. The desired flow is adjusted with the needle valve.

The pressure compensator with spring loaded plunger at the leftside measures the ressure at the inlet of the needle valve ( 2). Att e rig t si e o t e p unger t e pressure o t e oa p3 an othe spring are pushing the plunger to the left. The pressure of thespring is 8 bar.

e p unger n s s a ance w en:p2 = p3 + pspring ==> p2 - p3 = pspringand because of the fact that pspring= constant (8 bar) the pressurecompensator keeps the pressure drop across the needle valve on acons an va ue o ar.

This means that the flow through the needle valve remains constant!

When the load increases the pressure p3 increases and the plungers ou o a ance an pus e o e e . en e pressure p2 wincrease as well and the plunger finds it's balance again.

The pressure drop across the needle valve is still 8 bar so the flowremains 10 min an t ere ore t e ve ocity o t e cy in er remainsconstant and independent of the load!!


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van ages o s va ve over e res r c or- y e va ves arereduced heating of the fluid and a significant reduction in thehorse ower in ut re uired durin ortions of the o eratin c cle.

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A motor using a Bypass

Flo Control Valve A licationsFlow control valves regulate the speed of a hydraulic actuator by

controllin the volume flow rate to the actuator. This can beaccomplished in three ways:

1. Controlling the flow rate into the actuator (termed meter-in).. on ro ng e ow ra e ou o e ac ua or erme me er-ou .

3. Diverting a portion of the fluid before it reaches the actuatortermed bleed-off .

In some instances, a combined meter-in and meter-out circuit may beused.

ues on: e ow con ro va ve s se o pass gpm , u efixed displacement pump has a 10 gpm flow rate. Where is theother 5 m oin ?

The only place it can go is through the relief valve and back to thereservoir. For it to do that, there must be sufficient pressure toopen e re e va ve. e re e va ve s se a ps , so wecan assume that the pressure in the circuit is 2000 psi.


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Flow Control Circuits

In a meter-in circuit, a flow controlvalve is placed in the line to the inletport of the actuator. The actuator may

, ,or a hydraulic motor. A meter-in circuitof a cylinder is shown in.Adjusting the valve setting varies the

flow rate into the cylinder. Meter-in

arrangements where the load is alwaysresisting the force of the actuator.Where the force is not resistive, as inthe case of a suspended load, it is

uncontrolled, even if the meter-in valveis fully closed.

Meter-in speed control(extending) works by means

placed in the inlet line ofthe actuator.

A meter-out circuitis shown.The valve placed in the outlet line controlsthe rate at which the fluid leaves the

.inlet and outlet depend on the direction oftravel of the cylinder.)The meter-out technique is usually

preferred over the meter-in technique

eliminates the possibility of a suspendedload running away.The meter-out circuit presents thelikelihood of pressure intensification when

.the flow control valve restricts the outlet

flow, a back-pressure results on the rodend of the cylinder. The degree ofintensification depends on the load, the

Meter-out speed control(extending) works by

,relative to the piston.

valve placed in the outlet

line of the actuator


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You must be very careful in how you use meter-out flow controls

- very high pressures may result due to pressure intensification in

the cylinder and the accompanying pressure drop across the

valve may cause a high rate of heat generation. Figure 9.12 shows a circuit in which a three-port flow control

- . This type of valve can be used only on the inlet side of the

actuator. A major advantage of the three-port valve is that thesys em pressure s e erm ne en re y y e oa an neverexceeds that required to move the load. In circuits utilizing thetwo- ort flow control valves, the ressure u stream of the valvewill continue to rise until it reaches the setting of the systemrelief valve. A major disadvantage is that three-port valves do

control is unsuitable for parallel circuits where there is morethan one actuator.

s ree por ow con ro va vecan be used only on the inlet side

o e ac ua or.

A variation on the use of a port flow

. . .circuit, termed a bleed-off circuit, arestrictor type flow control valve is used,but it is connected in parallel with the lineleading to the cylinder.


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The outlet port of the valve dumps directly to the reservoir.s prov es a ow pa y w c e excess ow no nee e

to operate the cylinder) can return to the reservoir withoutgoing through the relief valve. The advantage of this bleed-offcircuit is that, as with the three-port valve, the load rather thanthe relief valve determines the system operating pressure.

There is a pressure drop through any flow control valve thatresults in horse ower loss and heat eneration. In a meter-incircuit, for example, the pressure drop across the flow controlvalve depends on the force required by the actuator. If there is

. . ,valve outlet is essentially zero. The result is a high pressuredrop that results in a high horsepower loss. As the load

, ,

pressure in the portion of the circuit between the valve and thecylinder increases. The pressure drop decrease means thatere s ess orsepower os an ess ea genera e . a

happened to the horse power difference between the twocases? It was used to do useful work in lifting the load.

When a three-port flow control valve is used, the system pressure isgoverned by the force required to move the load. Consequently, thepressure drop across the valve will be much less than that across a

- -. ,off type flow control can be tolerated, significant energy savings canbe realized. As the previous example shows, this decreased pressuredrop will also reduce the heat buildup in the system.


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Drops

In this chapter, we have looked at the pressure drops and heateneration associated with flow control valves. And we looked at

those same things for pressure control valves. In the real world,though, these are not separate, unrelated events. Instead, thepressure rops across o ypes o va ves, an , n ee , everyother device in the circuit combine to give the total heateneration rate for the s stem.

In a normal system, much of this heat will be dissipated throughthe components, reservoir, and system piping, but it may benecessary to add an oil cooler (heat exchanger) if there is notsufficient natural dissipation.

- ,adjustable flow regulator

Free flow in one direction

Adjustable restricted

flow in the otherrect on

Free Flow


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adjustable flow regulator

Free flow in one direction

Adjustable restrictedflow in the other

rect on

Controlled Flow

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Chapter 8 1 Flow Control Valves 15