Lecture 2. Review momentum, continuity, least squares Bernoulli Equation: => orifice equation Flow control valves:

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1 Fluid Power Controls Laboratory (Copyright Perry Li, ) Lecture 61 Your own goal in this course Office hour: Monday 11:00-1:00? Syllabus Feedback on labs so far Review momentum, continuity, least squares Bernoulli Equation: => orifice equation Flow control valves: Needle valve (lab 3a) Pressure compensated flow control valve (lab 3b) Pressure control valves (lab 4): Direct acting relief valve Pilot operated relief valve Simulation of hydraulic components and circuits - Example M..E., University of Minnesota (updated 1.010) 1

2 Fluid Power Controls Laboratory (Copyright Perry Li, ) Bernoulli Equation 6 When fluid is not at rest, Pascal s law is not strictly applicable If energy is conserved, as fluid speeds up, pressure decreases 1 P1 + ρ gh1 + ρv1 = P + ρgh + 1 ρv P 1, v1 P, v M..E., University of Minnesota (updated 1.010)

3 Fluid Power Controls Laboratory (Copyright Perry Li, ) How small does a restriction have to be for Bernoulli to be important? 63 Q = 30lpm (liters/min) = 5x10-4 m 3 /sec D = cm Pressure variation = 3.6 kpa = 0.5 psi D = 0.cm Pressure variation = 36 MPa = 50 psi!! ρv 8ρ Q = π D 4 Very sensitive to changes in diameter!!! M..E., University of Minnesota (updated 1.010) 3

4 Fluid Power Controls Laboratory (Copyright Perry Li, ) Orifice Equation 64 Upstream area assumed large (so that v 1 = 0) Bernoulli equation applies to jet ρ v 1 + P 1 = ρ v + P ( P ρ 1 P ) = v = Q A Q = C A d 0 P1 Ao A P 0.6 C d = A / A0 1 Discharge coefficient Q = Cd A0 ( P1 P ρ ) M..E., University of Minnesota (updated 1.010) 4

5 Fluid Power Controls Laboratory (Copyright Perry Li, ) Needle Valve as Flow Control Flow equation is generally given by the orifice equation: 65 Q = Cd A0 ( P1 P ρ A needle controls the opening of the flow channel (effective orifice area) ) Needle valve controls the resistance to flow, not flow directly Can be characterized by P-Q relationship either graphically or as an equation. M..E., University of Minnesota (updated 1.010) 5

6 Fluid Power Controls Laboratory (Copyright Perry Li, ) Flow Control Valves: Needle Valve 66 The goal of flow control valves is to maintain the flow rate at the specified setting Needle valve is essentially an orifice If pressure difference is fixed, flow will be constant Sensitivity depends on dq/dp You can determine from your experiment Needle valve can also be used as pressure control valve then flow must be constant Q = Cd A0 ( P1 P ρ Orifice area somestimes adjustable ) Is a needle valve a better flow control valve at lower pressures or at higher pressure? How about as a pressure control valve? M..E., University of Minnesota (updated 1.010) 6

7 Fluid Power Controls Laboratory (Copyright Perry Li, ) Pressure Compensated Flow Control Valve (PCFC) 67 Needle valve cannot maintain flow if pressure varies PCFC uses feedback to ensure that pressure across an orifice is constant As pressure across fixed orifice varies, the adjustable orifice tries to compensate - how? How does spring rate affect accuracy in flow control? Why must there be a minimum working pressure? Poor energy efficiency - where is the energy lost?? M..E., University of Minnesota (updated 1.010) 7

8 Fluid Power Controls Laboratory (Copyright Perry Li, ) PCFC Continued 68 Restricter type Q P What happens here? How does spring constant affect flow control accuracy? M..E., University of Minnesota (updated 1.010) 8

9 Fluid Power Controls Laboratory (Copyright Perry Li, ) Q PCFC - Operation Identify all components What is the flow path? What is the P-Q relationship when the control spool is stuck open? What are the forces and pressures that cause the control to move? How does spring rate affect the shape of the P-Q curve? Comment on efficiency What happens when pressure is too low? 69 P M..E., University of Minnesota (updated 1.010) 9

10 Fluid Power Controls Laboratory (Copyright Perry Li, ) Pressure control valve: Direct Acting Relief Valve 70 Poppet is seated against a spring Used to limit or to maintain pressure: Why is this important?? As pressure in main line increases above the preset spring force, poppet is moved up Orifice to drain is opened Relief flow occurs Cracking pressure versus full flow pressure? Questions: Formula for cracking pressure??? Relieve valves are Non-passing valve M..E., University of Minnesota (updated 1.010) 10

11 Fluid Power Controls Laboratory (Copyright Perry Li, ) Direct Acting Relief Valve 71 A relief valve is a non-passing pressure control valve Force balance on the spool? Cracking pressure versus spring rate, spring adjustment screw? Over-ride versus spring rate? M..E., University of Minnesota (updated 1.010) 11

12 Fluid Power Controls Laboratory (Copyright Perry Li, ) 7 Direct Acting Relief Valve Inlet pressure (P 1 ) versus spool displacement (x) is related by spring constant and spool area. How to draw the pressure (P 1 ) versus flow (Q) curve? How does the spring constant (K) affect the shape of the (P 1 versus Q) curve? What is an ideal relief valve? For sensitivity: Stiffer or softer spring? If spring is too soft, there may be stability problems. Cracking pressure is set by: Preload in spring = K x 0 (long springs) Conflict between high cracking pressure and sensitivity Open question: how does outlet pressure P affect P versus Q curve? M..E., University of Minnesota (updated 1.010) 1

13 Fluid Power Controls Laboratory (Copyright Perry Li, ) Pilot Operated Relief Valve 73 Soft spring Pilot operated relief valves are supposed to be more sensitive than direct acting relief valves M..E., University of Minnesota (updated 1.010) 13

14 Fluid Power Controls Laboratory (Copyright Perry Li, ) Pilot Operated Relief Valve - Schematic 74 Consists of a main stage and a pilot stage Main stage is lightly spring loaded Pilot stage dart is seated by a preloaded spring M..E., University of Minnesota (updated 1.010) 14

15 Fluid Power Controls Laboratory (Copyright Perry Li, ) Main spool and Pilot 75 Pilot - high pressure, low flow Main Spool - high flow, low pressure M..E., University of Minnesota (updated 1.010) 15

16 Fluid Power Controls Laboratory (Copyright Perry Li, ) Operation of Pilot Operated Relief 76 Pilot stage limits pressure above the spool (set by pilot spring) When pilot is closed, main spool is seated by the light spring When pilot is open, main spool sees more force from lower chamber than from upper chamber and spring - hence spool opens It is an avalanche effect! M..E., University of Minnesota (updated 1.010) 16

17 Fluid Power Controls Laboratory (Copyright Perry Li, ) 77 Pilot Operated Relief Valve This solves the dilemma of directing acting relief valve by separating the roles of sensitivity and cracking pressure setting. Cracking pressure determined by pilot dart (poppet) and spring stiff spring and small area can be used for high cracking pressure As pilot cracks, pressure above main spool is rapidly released. An avalanche results, as the main spool is held by a light spring. Sensitivity is controlled by the soft spring in the main spool M..E., University of Minnesota (updated 1.010) 17

18 Fluid Power Controls Laboratory (Copyright Perry Li, ) Remote Operated Pilot 78 M..E., University of Minnesota (updated 1.010) 18

19 Fluid Power Controls Laboratory (Copyright Perry Li, ) Ideal P-Q curve for relief valves 79 Pilot operated Q Which one is better?? Direct acting How do spring rates affect this curve For direct acting, or Pilot operated relief valve? Cracking pressure P M..E., University of Minnesota (updated 1.010) 19