1. EVOLUTION OF REDUCING VALVES DR.P.N.REDDY 2/24/2009 1

2. 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. 2/24/2009 2

3. SIMPLE VALVE 2/24/2009 3

4. 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. 2/24/2009 4

5. SIMPLE VALVE contd…, 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 2/24/2009 5

6. 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 2/24/2009 6

7. 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. 2/24/2009 7

8. 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. 2/24/2009 8

9. 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. 2/24/2009 9

10. IMPROVED VALVE SCHEME 1 2/24/2009 10

11. IMPROVED VALVE SCHEME 1 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 . 2/24/2009 11

12. 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 2/24/2009 12

13. 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. 2/24/2009 13

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. 2/24/2009 14

15. IMPROVED VALVE SCHEME 2 2/24/2009 15

16. IMPROVED VALVE SCHEME 2 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. 2/24/2009 16

17. IMPROVED VALVE SCHEME 2 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. 2/24/2009 17

18. IMPROVED VALVE SCHEME 3 2/24/2009 18

19. IMPROVED VALVE SCHEME 3 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. 2/24/2009 19

20. IMPROVED VALVE SCHEME 3 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) 2/24/2009 20

21. IMPROVED VALVE SCHEME 4 2/24/2009 21

22. IMPROVED VALVE SCHEME 4 2/24/2009 22

23. IMPROVED VALVE SCHEME 4 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. 2/24/2009 23

24. IMPROVED VALVE SCHEME 4 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. 2/24/2009 24

25. IMPROVED VALVE SCHEME 4 2/24/2009 25

26. Advantages of modern valves 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 2/24/2009 26

27. Fire hazards 2/24/2009 27

28. Thank u Sorry for the boring lecture!!! 28 2/24/2009