reducing valves in anaesthesia machine

1. 2/13/2014
EVOLUTION OF REDUCING
VALVES
1
DR.P.N.REDDY

2. INTRODUCTION
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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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3. SIMPLE VALVE
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4. SIMPLE VALVE
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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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5. SIMPLE VALVE CONTD…,
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If Pa > RA gas flow from HL
 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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6. SIMPLE VALVE CONTD…
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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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7. 2/13/2014
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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8. 2/13/2014
So , the bobbin falls slowly with reduced flow rate.
 This simple valve satisfies two points:

1. pressure is reduced
2. fine adjustment possible
But the bobbin needs frequent readjustments.
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9. RATE OF FALL OF REDUCED
PRESSURE
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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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10. IMPROVED VALVE SCHEME 1
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11. 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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12. CONTD,…
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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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13. CONTD…,
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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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14. 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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
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15. IMPROVED VALVE SCHEME 2
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16. IMPROVED VALVE SCHEME 2
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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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17. IMPROVED VALVE SCHEME 2
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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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18. IMPROVED VALVE SCHEME 3
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19. IMPROVED VALVE SCHEME 3
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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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20. IMPROVED VALVE SCHEME 3


Metal diaphragm
R can be set at high level (4 kg/sq cm²)
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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:

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21. IMPROVED VALVE SCHEME 4
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22. IMPROVED VALVE SCHEME 4
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23. IMPROVED VALVE SCHEME 4
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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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24. IMPROVED VALVE SCHEME 4
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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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25. IMPROVED VALVE SCHEME 4
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26. ADVANTAGES OF MODERN VALVES
2.
3.
4.
5.
6.
To run ventilators.
To use with high resistant devices like vaporizers.
Bobbin will not fall n rise with IPPV
Periodic adjustment of bobbin not necessary
Slave valve mechanism with N2O
Pressure settings in manifold
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1.
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27. Fire hazards
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
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28. 2/13/2014
THANK U
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Sorry for the boring lecture!!!