Evolution of reducing valves

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EVOLUTION OF REDUCING VALVESDR.P.N.REDDY

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INTRODUCTION

Anesthetic gases are compressed. Must be reduced to working pressure. Problems with high pressure:

Periodic adjustment of bobbin. Fine adjustment difficult. Delicate parts can be damaged. Can damage the pt- barotrauma.

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SIMPLE VALVE

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SIMPLE VALVE

FORCE = PRESSURE × AREA Force acting on area “ a “ due to pressure in

the cylinder = Pa. The upward forces acting on j rod is RA. R=reduced pressure acting on diaphragm Y

of area A of low pressure chamber L. The outlet T on the side.

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SIMPLE VALVE CONTD…,

If Pa > RA gas flow from HL Pr in L raises Y & pr in L chamber; RA = Pa Then gas stops flowing. Pa =RA R=Pa /A R/P =a/A

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SIMPLE VALVE CONTD…

When T is opened gas flows out reducing pr in chamber L & Pa increases & gas flows to chamber L

During administration of gases to pt pressure in chamber H decreases progressively.

if pr in H chamber reduced to p-p¹ & reduced pr R falls to R¹

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P¹a = R¹A R¹ = P¹ a/A R¹P¹ = a/A This shows ratio of decreased pressure to

high pr is proportional to the ratio of area of high pr seating to area of low pr seating.

But a/A is fixed Hence in simple valve as pr P falls the R

reduced pr also falls proportionately.

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So , the bobbin falls slowly with reduced flow rate.

This simple valve satisfies two points:1. pressure is reduced2. fine adjustment possible

But the bobbin needs frequent readjustments.

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RATE OF FALL OF REDUCED PRESSURE

The fall of reduced pr is proportional to cylinder pr.

With full cylinder if 3 lt of flow is kept after completion of half cylinder the output is only 1.5 lt /min.

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IMPROVED VALVE SCHEME 1

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Z is high tension spring exerting downward pr S on diaphragm Y.

The force S can be changed by the screw Q. Upward pr is RA. Downward force is (Pa+S). At equilibrium., RA = Pa + S = R(Pa+S) / A. R=Pa/A + S/A .

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CONTD,…

The reduced pressure R now depends on downward force S.

S/A is the main factor controlling the reduced pr.

R can be adjusted to any level by varying the tension of the spring.

The rate of decrease of reduced pr depends on 3 factors :

1). P-P¹ 2). R 3). a/A

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CONTD…,

No control on P-P¹ Ratio of a/A can be controlled R can be controlled by spring Z The reduced pr R also depends on ratio of

a/A If a/A is reduced the rate of fall of R also

decreases. It is also observed that if R is set initially at

high level the valve works better.

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CONTD,…

Therefore if the tension S is made very large compared to Pa the valve works better and rate of reduction of reduced pr R further decreases.

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In the scheme 1 tap T is closed or partially opened to deliver small flows & so R falls slowly.

If large gas flows out of chamber L the R decreases faster. Such high flows are needed to run the ventilator,

A small spring s is placed below J rod and it can be adjusted to vary its tension with the help of thumb screw.

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If cylinder is 132 atm pr the spring s fully closes the seating a and no gas flows to chamber L.

If z spring is unscrewed & T is opened the small spring s prevents air leak & pr in L is 0.

If z spring is screwed down it pushes small s down and gas flows into chamber L.

Here the nozzle size and seating area a is increased to give high flows.

But if the ratio of a/A decreases the valve cannot function efficiently.

Hence a is increased to a small extent & A is increased to large extent so that the ratio is maintained.

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Here soft rubber metal diaphragm is replaced by a rigid hard diaphragm so that it can withstand greater changes in pressure.

The J rod is broken into two parts , the seating a and vertical rod x with a screw d .

The seating is adjusted & fixed at a level where there is no flow when z is not exerting any pr.

Here no small spring & R is zero.

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The action of small spring s is taken over by diaphragm y.

When z spring is screwed down it produces tension S on the diaphragm and J rod and seating come down & gas flows into L.

Here downward forces are (Pa+S) & upward forces are RA

R= (Pa +S) /A Advantages:

Metal diaphragm R can be set at high level (4 kg/sq cm²)

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IN 1890, CLARKSON designed a valve with a seating a in high pr side of the valve.

All levers rubber parts are eliminated. Servicing became very easy. The features are:

Seating a in high pr chamber Presence of safety blow valve High pr guage Low pr guage Diaphragm made of tough rubber.

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The spring s is controlled by adjustable thumb screw.

This arrangement is suitable when R is kept high and it can be varied by the operator.

Here as the cylinder pr comes down the R slightly goes up and bobbin raises slightly higher and comes down to 0 as cylinder becomes empty.

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ADVANTAGES OF MODERN VALVES

1. To run ventilators.2. To use with high resistant devices like

vaporizers.3. Bobbin will not fall n rise with IPPV4. Periodic adjustment of bobbin not

necessary5. Slave valve mechanism with N2O 6. Pressure settings in manifold

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Fire hazards

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THANK USorry for the boring lecture!!!

Evolution of reducing valves

Health & Medicine

Transcript of Evolution of reducing valves