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# Mba Ebooks ! Edhole

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### Mba Ebooks ! Edhole

2. 2. Valves 2005/2006 I. Hydraulic and Pneumatic Systems 2 If the pump is the heart of a hydraulic system then the valve is the brain. Valves are used to perform a large variety of governing and controlling functions. Form: Function: • pressure control valves • flow control valves • check valves (non-return valves) • directional control valves a) Spherical b) c) d) e) Conical f) Plate g) Spool valve Poppet valve Aspects of classification http://ebooks.edhole.com
3. 3. Valves Pressure control valves: 2005/2006 I. Hydraulic and Pneumatic Systems 3 • pressure relief valves • pressure reducing valves Pressure relief valve Has the task to limit the pressure in a hydraulic system or in a part of the system. The pressure can rise in a hydraulic system if: • pressure difference valve • pressure ratio valve - the flow rate from the pump is larger than the flow rate through the actuator - the volume of a closed system is reduced - the load of the actuator rises - heat is introduced into a closed system - the hydraulic resistance of the system rises http://ebooks.edhole.com
4. 4. Valves Pressure relief valve 2005/2006 I. Hydraulic and Pneumatic Systems 4 x A k C Fspring       m N psys Qp Symbol http://ebooks.edhole.com
5. 5. Pressure relief valve Theoretical basics 2005/2006 I. Hydraulic and Pneumatic Systems 5 xcFFAp SSsys ⋅+==⋅ 0 f sys f crv p xk p xkvAQ ρ α ρ α 22 ⋅⋅⋅= ∆ ⋅⋅⋅=⋅= A cxF xkQ S f rv + ⋅⋅⋅⋅= 02 ρ α Force equilibrium: (1) Curtain surface: Circumference ∙ height = k ∙ x = Ac Bernoulli equation: (2) (3) (4) 7,0...6,0=α Qrv depends nonlinearly on x or if we express x from (1) then: c FAp x Ssys 0−⋅ = sys f Ssys rv p c FAp kQ ⋅⋅ −⋅ ⋅⋅= ρ α 20 http://ebooks.edhole.com
6. 6. Pressure relief valve Theoretical basics: 2005/2006 I. Hydraulic and Pneumatic Systems 6 0Ssys FAp =⋅∗ ( ) ( ) syssyssys const f sys f syssys rv ppp c Ak p c App kQ ∗ = ∗ −⋅⋅ ⋅⋅ =⋅⋅ ⋅− ⋅⋅=    ρ α ρ α 22 (5) Let be the pressure when the valve just opens (x = 0) ⇒ subst. to (4) * sysp (5) is valid from x = 0 to x = xmax, xmax being the displacement when the spring is completely compressed max0 xx ≤≤ ( ) syssyssysrv pconstpppconstQ ⋅=⋅−⋅= ∗ maxIf p > pmax : http://ebooks.edhole.com
7. 7. Pressure relief valve Theoretical basics: 2005/2006 I. Hydraulic and Pneumatic Systems 7 Qrv negative part: physically impossible pmax psys belongs to xmax * sysp http://ebooks.edhole.com
8. 8. Pressure relief valve Hydraulic aggregate: 2005/2006 I. Hydraulic and Pneumatic Systems 8 The simplest hydraulic system Pump + pressure relief valve M Qrv Qag userQp psys reservoir http://ebooks.edhole.com
9. 9. Hydraulic aggregate 2005/2006 I. Hydraulic and Pneumatic Systems 9 Let us derive the characteristics of the complete aggregate. Parallel circuit so: rvpagagrvp QQQQQQ −=⇒+= Qrv Qpump Qag1 Qag2 psys n = n1 < n = n2 * sysp = = = = Qrv Qpump http://ebooks.edhole.com
10. 10. Pressure relief valve 2005/2006 I. Hydraulic and Pneumatic Systems 10 The pressure relief valve always has to be matched with the pump. If for example the rotational speed is increased (orange curve) ⇒ then there will be flow through the aggregate even with higher pressure. Wrong! The last section of the curve has to be at the negative Q plane. http://ebooks.edhole.com
11. 11. Pressure relief valve Versions: 2005/2006 I. Hydraulic and Pneumatic Systems 11 There are various versions of pressure relief valves: 1. Directly operated → ← pilot operated 2. Self-operated → ← externally operated The self-operated valve is controlled by its own pressure, whereas the externally operated valve is controlled by an outside pressure. Directly operated – already explained. http://ebooks.edhole.com
12. 12. Pilot operated pressure relief valve 2005/2006 I. Hydraulic and Pneumatic Systems 12 Valve constant has to be increased: Pilot operated is applied when large flow rates have to be controlled. Consists of two valves: At higher flow rates the losses at a directly operated valve would be very large. Another problem is that with high flow rate the dynamics of the valve gets worse. The area can be increased but this increases size and costs and worsens the sensitivity of the control. The spring constant can be decreased but this increase against the size. (Large preloading is necessary.) Again bad for dynamics. Solution: two valves: pilot operated valve - Main valve (low c) - Pilot valve (high c) fc Ak C ρ α 2 ⋅ ⋅⋅ = http://ebooks.edhole.com
13. 13. Pressure relief valve Pilot operated: 2005/2006 I. Hydraulic and Pneumatic Systems 13 p1 is the system pressure that has to be limited. The valve is closed as long as the limit pressure is not reached at the pilot valve. x main valve spring 1A p1 Q p2 spring 1 pilot valve Thr http://ebooks.edhole.com
14. 14. Pressure relief valve Pilot operated: 2005/2006 I. Hydraulic and Pneumatic Systems 14 In this case the pressure p1 acts on both sides of the valve and the valve is in equilibrium: p1 = p2. In the pressure rises above the limiting pressure of the pilot valve then the pilot valve opens. A flow starts through the throttling valve so that p1 > p2. If A(p1-p2) > FS01 (preload force of spring 1) then the main valve also starts to open. This is more favourable because it allows a softer spring in the main valve. The pilot valve can be spatially separated from the main valve – the control can be exercised from a distance. More favourable static characteristics Better dynamic characteristics http://ebooks.edhole.com
15. 15. Pressure relief valves Pilot-operated pressure relief valve 2005/2006 I. Hydraulic and Pneumatic Systems 15 1 - főszelep, 2 - elővezérlő szelep, 3 - főtolattyú, 4 - 5 - 11 - fojtás, 6 - 7 - 13 - vezérlő vezeték, 8 - szeleptest, 9 - rugó, 15 - tehermentesítés 1 - Main valve 2 - Pilot valve 3 – Main spool 4 - 5 - 11 - Throttle 6 - 7 - 13 - Operation line 8 - Valve body 9 - Spring 15 - Discharging http://ebooks.edhole.com
16. 16. Pressure relief valve 2005/2006 I. Hydraulic and Pneumatic Systems 16 Simplified symbol of a pilot operated pressure relief valve Applications of pressure relief valves: - Safety valve (most common) - Overflow valve (pressure source always open produces large losses, should be used only for small power) - Sequence valves (turn-on or turn-off) p1 Qrv directly operated pilot operated ideal Characteristic curves of pressure relief valves http://ebooks.edhole.com
17. 17. Pressure relief valve Safety valve and overflow valve: 2005/2006 I. Hydraulic and Pneumatic Systems 17 RV p pl Safety valve Overflow valve (pressure source) Qp Qs p RV p pl http://ebooks.edhole.com
18. 18. Pressure relief valve Sequence valves: 2005/2006 I. Hydraulic and Pneumatic Systems 18 “Turn-on” valve is used when two or more users are fed by the same pump. When one working step is finished , the pressure rises and a second user is switched through the valve. “Turn-off” valve is applied when two pumps (one high pressure, one low pressure) work in parallel. If the pressure is high enough then only the high pressure pump works. If the pressure drops below the limiting value of RV1 then the valve closes and the pump delivers into the system again. http://ebooks.edhole.com
19. 19. Pressure relief valve Sequence valves: 2005/2006 I. Hydraulic and Pneumatic Systems 19 Sequence valve (turn off) RV1 Sequence valve (turn on) RV2 pl1 pl2 Qp > QS Here the RV1 is an externally operated valve. Check valve RV1RV2 pl1 pl2 HPP LPP Q http://ebooks.edhole.com
20. 20. Pressure relief valves Dynamic behaviour of pressure relief valves: 2005/2006 I. Hydraulic and Pneumatic Systems 20 Sudden opening or closing leads to oscillations of the valve. Stationary characteristics depend only on the valve, dynamics characteristics depend on the whole system. The dynamic behaviour can be tested by a sudden application of the pressure on the valve. http://ebooks.edhole.com
21. 21. Pressure relief valve Dynamic behaviour of pressure relief valves: 2005/2006 I. Hydraulic and Pneumatic Systems 21 First linear rise, then damped oscillation. There is a differential equation system which can be solved to simulate the process. V is the volume of fluid between pump and valve → capacity and inductivity changes. DV p1 Qrv Test rig Qp RV p t p1 V1 V2 V3 V1 < V2 < V3 http://ebooks.edhole.com
22. 22. Big pictures End of normal presentation Beginning of big pictures 2005/2006 I. Hydraulic and Pneumatic Systems 22http://ebooks.edhole.com
23. 23. Valves Form 2005/2006 I. Hydraulic and Pneumatic Systems 23 a) Spherical b) c) d) e) Conical Poppet valve f) Plate g) Spool valve http://ebooks.edhole.com
24. 24. Valves Pressure relief valve 2005/2006 I. Hydraulic and Pneumatic Systems 24http://ebooks.edhole.com
25. 25. Valves Pressure relief valve 2005/2006 I. Hydraulic and Pneumatic Systems 25http://ebooks.edhole.com
26. 26. Valves Pressure relief valve 2005/2006 I. Hydraulic and Pneumatic Systems 26http://ebooks.edhole.com
27. 27. Valves Pressure relief valve 2005/2006 I. Hydraulic and Pneumatic Systems 27http://ebooks.edhole.com
28. 28. Valves Pressure relief valve 2005/2006 I. Hydraulic and Pneumatic Systems 28 x A k C Fspring       m N psys Qp Symbol http://ebooks.edhole.com