1. HYDRAULICS & PNEUMATICS
Flow Control Valves
Presented by: Dr. Abootorabi

2. Flow Control Devices
Flow control devices produce the desired rate of actuator operating speed by controlling the volume of fluid allowed to reach the actuator.
Flow control devices can be divided into two general types:
Restrictor
Bypass

3. Flow Control Devices
Restrictor-type flow control valves limit the volume of fluid through the valve.
Excess pump output is forced to return to the reservoir through the system relief valve.

4. Flow Control Devices
Circuit containing a restrictor-type flow control valve

5. Flow Control Devices
Bypass type flow control valves use an integral control port to return excess pump output to the reservoir.
The returned fluid is at a pressure less than system relief valve pressure.

6. Flow Control Devices
Circuit containing a bypass-type flow control valve

7. Flow Control Devices
Conceptual operation of a flow control valve may be traced to a basic orifice.

8. Flow Control Devices
The flow rate through a simple, sharp-edged orifice depends on:
Area of the orifice
Pressure difference between the inlet and outlet sides of the orifice
Viscosity of the fluid, which varies with fluid temperature
Simplified formula:

9. Flow Control Devices
Flow control valves may be noncompensated or compensated:
The flow rate through noncompensated valves varies as the load or fluid viscosity changes
Compensated valves automatically adjust for fluid pressure variations to produce a consistent flow rate under varying load and temperature conditions

10. Flow Control Devices
Noncompensated and compensated flow control valves may have:
Fixed flow rate
Adjustable flow rate

11. Flow Control Devices
The simplest restrictor-type flow control valve is a simple orifice:
Basically a calibrated hole
Serves as a noncompensated, fixed-rate flow control device

12. Flow Control Devices
A needle valve is the simplest restrictor-type, noncompensated adjustable flow control device:
Consists of an orifice fitted with a tapered needle machined on a threaded stem
Turning the threaded stem changes the effective area of the orifice, which adjusts the flow rate through the valve

13. Flow Control Devices Basic adjustable flow control valve:
A simple needle valve without check valve is called also metering valve.

14. Flow Control Devices
Adjustable restrictors:

15. Flow Control Devices
Adjustable restrictors:

16. Flow Control Devices
One-way flow control valve:

17. Flow Control Devices
When using a restrictor-type, noncompensated flow control valve, actuator speed varies when system loads change.
Caused by the change in pressure drop across the control valve, which varies the flow rate through the valve.

18. Flow Control Devices
A pressure compensator maintains a constant pressure difference across the metering orifice of a flow control valve:
Senses pressure on the inlet and outlet sides of the orifice
These pressures generate forces that act on the end surfaces of a sliding spool that is preloaded by a biasing spring

19. Flow Control Devices
Force generated by the biasing spring establishes the constant pressure difference across the orifice.
This constant pressure difference maintains constant fluid flow through the valve even when system loads change.

20. Flow Control Devices
A basic pressure-compensated flow control valve

21. Flow Control Devices
Pressure compensator operation:

22. Flow Control Devices
Pressure compensator operation:

23. Flow Control Devices
Temperature compensation is necessary in flow control devices if an accurate, consistent flow rate through a valve is needed.
This is due to the fluid viscosity changes that occur as fluid temperature changes.

24. Flow Control Devices
Temperature compensation is typically accomplished in flow control devices by:
Specially designed, sharp edged orifice
Heat-sensitive metal rod that operates a needlelike control device in the metering orifice of the valve

25. Flow Control Devices
Temperature compensation using sharp-edged orifice:

26. Flow Control Devices
Temperature compensation using a heat-sensitive metal rod:

27. Flow Control Devices
In a circuit using a restrictor-type, pressure-compensated flow control valve:
Pressure drop across the internal flow-control device in the valve remains constant, which produces a constant flow rate through the valve
Actuator speed will not vary when system loads change

28. Flow Control Devices
In a circuit using a restrictor-type, temperature-compensated flow control valve:
Internal flow-control device is adjusted for viscosity variations that occur during fluid temperature changes
Flow remains constant as system operating temperatures change

29. Flow Control Devices
Circuit containing restrictor-type, compensated flow control valve

30. Flow Control Devices Bypass-type flow control valves:
Provide partly accurate flow to actuators
Direct any excess flow from the pump directly to the reservoir through an integral port

31. Flow Control Devices
Typical bypass-type flow control valve:

32. Flow Control Devices
The operating pressure of a system using a bypass-type flow control valve is determined by the load on the actuator plus the pressure needed to overcome the force of the biasing spring.
The relief valve functions only when actuator loads are great enough to increase system pressure above the cracking pressure of the relief valve.

33. Flow Control Devices
Operation of a bypass flow control valve during increasing or decreasing load:

34. Flow Control Devices
Operation of a bypass flow control valve during steady load:

35. Flow Control Devices
Operation of a bypass flow control valve with stalled actuator:

36. Flow Control Devices
The bypass flow control design provides an efficient operating flow control circuit
Pressure in the system is only as high as needed to move the load and operate the valve compensator
This reduces system heat generation and energy consumption
Care must be taken to accurately determine actuator loads and the cracking pressure of the system relief valve

37. Flow Control Devices
Priority and proportional divider valves are designed to divide one fluid supply between two circuit subsystems.
Many of these valves can also be used to combine the flow from two different circuits.

38. Flow Control Devices
Priority divider valves provide flow to one port before providing flow to a second port.
Often used in mobile equipment where pump output is controlled by engine speed.

39. Flow Control Devices
Typical priority valve:

40. Flow Control Devices
Circuit containing a priority divider valve

41. Flow Control Devices
Proportional divider valve splits input port flow into two proportional output flows.
Ratio between the output flows may be fixed or variable.
Ratio of is most common.

42. Flow Control Devices
A typical use of proportional flow divider valve:

43. Design and Operation of Basic Flow-Related Circuits
Three basic flow control circuits are used in hydraulic systems:
Meter in
Meter out
Bleed off
These basic circuits meet the varying flow-control demands for systems with positive and negative loads.

44. Design and Operation of Basic Flow-Related Circuits
The meter-in flow control design places the flow control valve between the pump and the inlet of the actuator:
Should only be used for positive loads
Cannot provide accurate control under a negative load condition
The prime mover is always operating against the maximum pressure setting of the system relief valve

45. Design and Operation of Basic Flow-Related Circuits
Basic meter-in circuit

46. Design and Operation of Basic Flow-Related Circuits
The meter-out flow control design places the flow control valve between the actuator outlet and the reservoir:
Can provide accurate control for positive and negative loads
The prime mover is always operating against the maximum pressure setting of the system relief valve

47. Design and Operation of Basic Flow-Related Circuits
Basic meter-out circuit

48. Design and Operation of Basic Flow-Related Circuits
The bleed-off flow control design places the flow control valve in a tee in the working line between the directional control valve and the actuator inlet:
Outlet of the flow control is directly connected to the reservoir
Measured flow is diverted to the reservoir while remaining flow operates the actuator
The prime mover operates against a pressure only high enough to move the load

49. Design and Operation of Basic Flow-Related Circuits
Basic bleed-off circuit

50. Design and Operation of Basic Flow-Related Circuits
Meter-in and meter-out circuits provide the most accurate actuator speeds.
Both meter fluid flow delivered directly to or from the actuator.
The meter-out circuit is the best method for negative loads that may pull the actuator.

51. Design and Operation of Basic Flow-Related Circuits
The bleed-off flow control circuit is less accurate than either the meter-in or meter-out system:
Flow is metered back to the reservoir while the remaining pump output establishes actuator speed
The remaining flow can vary because of pump efficiency and system leakage
Flow control valve metering accuracy under varying load conditions is also a factor

52. Design and Operation of Basic Flow-Related Circuits
The bleed-off circuit is the most energy-efficient design.
This is due to the fact the prime mover operates at a pressure only high enough to move the load.

53. The end.