Evolution of reducing valves


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Evolution of reducing valves

  1. 1. EVOLUTION OF REDUCING VALVES<br />DR.P.N.REDDY<br />2/24/2009<br />1<br />
  2. 2. INTRODUCTION<br />Anesthetic gases are compressed.<br />Must be reduced to working pressure.<br />Problems with high pressure:<br />Periodic adjustment of bobbin.<br />Fine adjustment difficult.<br />Delicate parts can be damaged.<br />Can damage the pt- barotrauma.<br />2/24/2009<br />2<br />
  3. 3. SIMPLE VALVE<br />2/24/2009<br />3<br />
  4. 4. SIMPLE VALVE<br />FORCE = PRESSURE × AREA<br />Force acting on area “ a “ due to pressure in the cylinder = Pa.<br />The upward forces acting on j rod is RA.<br />R=reduced pressure acting on diaphragm Y of area A of low pressure chamber L.<br />The outlet T on the side.<br />2/24/2009<br />4<br />
  5. 5. SIMPLE VALVE contd…,<br />If Pa > RA gas flow from HL<br />Pr in L raises Y & pr in L chamber; RA = Pa<br />Then gas stops flowing.<br />Pa =RA<br />R=Pa /A<br />R/P =a/A<br />2/24/2009<br />5<br />
  6. 6. SIMPLE VALVE contd…<br />When T is opened gas flows out reducing pr in chamber L & Pa increases & gas flows to chamber L<br />During administration of gases to pt pressure in chamber H decreases progressively.<br />if pr in H chamber reduced to p-p¹ & reduced pr R falls to R¹<br />2/24/2009<br />6<br />
  7. 7. P¹a = R¹A<br />R¹ = P¹ a/A<br />R¹P¹ = a/A<br />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.<br />But a/A is fixed<br />Hence in simple valve as pr P falls the R reduced pr also falls proportionately. <br />2/24/2009<br />7<br />
  8. 8. So , the bobbin falls slowly with reduced flow rate.<br />This simple valve satisfies two points:<br />1. pressure is reduced<br />2. fine adjustment possible<br />But the bobbin needs frequent readjustments.<br />2/24/2009<br />8<br />
  9. 9. RATE OF FALL OF REDUCED PRESSURE<br />The fall of reduced pr is proportional to cylinder pr.<br />With full cylinder if 3 lt of flow is kept after completion of half cylinder the output is only 1.5 lt /min.<br />2/24/2009<br />9<br />
  10. 10. IMPROVED VALVE SCHEME 1<br />2/24/2009<br />10<br />
  11. 11. IMPROVED VALVE SCHEME 1<br />Z is high tension spring exerting downward pr S on diaphragm Y.<br />The force S can be changed by the screw Q.<br />Upward pr is RA.<br />Downward force is (Pa+S).<br />At equilibrium., RA = Pa + S = R(Pa+S) / A.<br />R=Pa/A + S/A .<br />2/24/2009<br />11<br />
  12. 12. Contd,…<br />The reduced pressure R now depends on downward force S.<br />S/A is the main factor controlling the reduced pr.<br />R can be adjusted to any level by varying the tension of the spring.<br />The rate of decrease of reduced pr depends on 3 factors :<br /> 1). P-P¹ <br /> 2). R <br /> 3). a/A<br />2/24/2009<br />12<br />
  13. 13. Contd…,<br />No control on P-P¹<br />Ratio of a/A can be controlled<br />R can be controlled by spring Z<br />The reduced pr R also depends on ratio of a/A<br />If a/A is reduced the rate of fall of R also decreases.<br />It is also observed that if R is set initially at high level the valve works better.<br />2/24/2009<br />13<br />
  14. 14. Contd,…<br />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.<br />2/24/2009<br />14<br />
  15. 15. IMPROVED VALVE SCHEME 2<br />2/24/2009<br />15<br />
  16. 16. IMPROVED VALVE SCHEME 2<br />In the scheme 1 tap T is closed or partially opened to deliver small flows & so R falls slowly.<br />If large gas flows out of chamber L the R decreases faster. Such high flows are needed to run the ventilator,<br />A small spring s is placed below J rod and it can be adjusted to vary its tension with the help of thumb screw.<br />2/24/2009<br />16<br />
  17. 17. IMPROVED VALVE SCHEME 2<br />If cylinder is 132 atm pr the spring s fully closes the seating a and no gas flows to chamber L.<br />If z spring is unscrewed & T is opened the small spring s prevents air leak & pr in L is 0.<br />If z spring is screwed down it pushes small s down and gas flows into chamber L.<br />Here the nozzle size and seating area a is increased to give high flows.<br />But if the ratio of a/A decreases the valve cannot function efficiently.<br />Hence a is increased to a small extent & A is increased to large extent so that the ratio is maintained.<br />2/24/2009<br />17<br />
  18. 18. IMPROVED VALVE SCHEME 3<br />2/24/2009<br />18<br />
  19. 19. IMPROVED VALVE SCHEME 3<br />Here soft rubber metal diaphragm is replaced by a rigid hard diaphragm so that it can withstand greater changes in pressure.<br />The J rod is broken into two parts , the seating a and vertical rod x with a screw d .<br />The seating is adjusted & fixed at a level where there is no flow when z is not exerting any pr.<br />Here no small spring & R is zero.<br />2/24/2009<br />19<br />
  20. 20. IMPROVED VALVE SCHEME 3<br />The action of small spring s is taken over by diaphragm y.<br />When z spring is screwed down it produces tension S on the diaphragm and J rod and seating come down & gas flows into L.<br />Here downward forces are (Pa+S) & upward forces are RA<br />R= (Pa +S) /A<br />Advantages:<br />Metal diaphragm<br />R can be set at high level (4 kg/sq cm²)<br />2/24/2009<br />20<br />
  21. 21. IMPROVED VALVE SCHEME 4<br />2/24/2009<br />21<br />
  22. 22. IMPROVED VALVE SCHEME 4<br />2/24/2009<br />22<br />
  23. 23. IMPROVED VALVE SCHEME 4<br />IN 1890, CLARKSON designed a valve with a seating a in high pr side of the valve.<br />All levers rubber parts are eliminated.<br />Servicing became very easy.<br />The features are:<br />Seating a in high pr chamber<br />Presence of safety blow valve<br />High pr guage<br />Low pr guage<br />Diaphragm made of tough rubber.<br />2/24/2009<br />23<br />
  24. 24. IMPROVED VALVE SCHEME 4<br />The spring s is controlled by adjustable thumb screw.<br />This arrangement is suitable when R is kept high and it can be varied by the operator.<br />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.<br />2/24/2009<br />24<br />
  25. 25. IMPROVED VALVE SCHEME 4<br />2/24/2009<br />25<br />
  26. 26. Advantages of modern valves <br />To run ventilators.<br />To use with high resistant devices like vaporizers.<br />Bobbin will not fall n rise with IPPV<br />Periodic adjustment of bobbin not necessary<br />Slave valve mechanism with N2O <br />Pressure settings in manifold<br />2/24/2009<br />26<br />
  27. 27. Fire hazards<br />2/24/2009<br />27<br />
  28. 28. Thank u<br />Sorry for the boring lecture!!!<br />28<br />2/24/2009<br />