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Evolution of reducing valves

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reducing valves in anaesthesia machine

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Evolution of reducing valves

1. 1. 2/13/2014 EVOLUTION OF REDUCING VALVES 1 DR.P.N.REDDY
2. 2. INTRODUCTION 2/13/2014 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.  2
3. 3. SIMPLE VALVE 2/13/2014 3
4. 4. SIMPLE VALVE 2/13/2014 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.  4
5. 5. SIMPLE VALVE CONTD…, 2/13/2014 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  5
6. 6. SIMPLE VALVE CONTD… 2/13/2014 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  6
7. 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.  7
8. 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. 8
9. 9. RATE OF FALL OF REDUCED PRESSURE 2/13/2014 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.  9
10. 10. IMPROVED VALVE SCHEME 1 2/13/2014 10
11. 11. IMPROVED VALVE SCHEME 1 2/13/2014 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 .  11
12. 12. CONTD,… 2/13/2014 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  12
13. 13. CONTD…, 2/13/2014 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.  13
14. 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. 2/13/2014  14
15. 15. IMPROVED VALVE SCHEME 2 2/13/2014 15
16. 16. IMPROVED VALVE SCHEME 2 2/13/2014 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.  16
17. 17. IMPROVED VALVE SCHEME 2 2/13/2014 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.  17
18. 18. IMPROVED VALVE SCHEME 3 2/13/2014 18
19. 19. IMPROVED VALVE SCHEME 3 2/13/2014 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.  19
20. 20. IMPROVED VALVE SCHEME 3   Metal diaphragm R can be set at high level (4 kg/sq cm²) 2/13/2014 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:  20
21. 21. IMPROVED VALVE SCHEME 4 2/13/2014 21
22. 22. IMPROVED VALVE SCHEME 4 2/13/2014 22
23. 23. IMPROVED VALVE SCHEME 4 2/13/2014 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.  23
24. 24. IMPROVED VALVE SCHEME 4 2/13/2014 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.  24
25. 25. IMPROVED VALVE SCHEME 4 2/13/2014 25
26. 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 2/13/2014 1. 26
27. 27. Fire hazards 2/13/2014  27
28. 28. 2/13/2014 THANK U 28 Sorry for the boring lecture!!!