Flow Control Valve

 

FLOW CONTROL VALVE (FCV)

In a Hydraulic system, hydraulic power, P_H (in kW), is a product of system pressure p (N/m²) and fluid flow rate, Q (m³/sec). Mathematically,

From above basic equation, it is clear that modulation of hydraulic power is possible by modulating the system pressure and/or fluid flow rate.

In hydraulic systems, pressure control valves are used to modulate the pressure conditions at various parts. Similarly flow control valves helps in varying/modulating the fluid flow rate.

This post focuses on the function, working principle, types and application of flow control valves used in industrial hydraulic systems.

 

Introduction:

1.   What is the purpose of flow control in a hydraulic system?

Flow control in fluid power (hydraulic) system regulates the flow rate or Liters/minute which may be directed to an actuator such as a hydraulic cylinder or a hydraulic motor. Since the speed of the cylinder piston rod or rotary speed of hydraulic motor shaft is determined by the flow rate through the actuator, Flow (volume) controls may vary or maintain the selected actuator speeds.

 

2.   When is the flow control necessary?

Flow control becomes necessary when the pump delivery rate is more than the actuator requirement. This situation arises when the pump supplies flow for more than one actuator. Typical applications are: regulating cutting tool speeds, spindle speeds, travel rate of vertically moving loads in forklift, hydraulic winch speed to control the rate of hauling of the load, etc.,

 

 3.   How is the flow rate changed?

A flow control valve has a restricted passage whose size is either fixed are adjustable. The restricted passage increases friction to reduce the flow and decreases friction to increase the flow. Thus, flow rate can be changed by changing the size of the restricted passage.

 

Main functions of a Flow Control Valve:

Flow control valves are used to influence the speed of a hydraulic actuator (Cylinder, motor) by changing the opening to flow (decreasing or increasing) at the throttle point.

Flow control valves in hydraulic systems are necessary to control the rate of flow from one part of the system to another.

Flow control valves accomplish one or more of the following control functions;

 

Ø Limit the speed (v) of the linear actuator and hydraulic motors.

                     

Ø Limit the maximum power available to sub circuits by controlling the  flow to them, because

Ø Proportionately divide or regulate pump flow to various branches of the circuit.

  

A flow control valve is designed such that it becomes capable of reducing fluid volume down stream of itself relative to upstream.

Volumetric flow rate, Q, expressed in units of cc/sec or cc/min in SI metric measure is used to calculate the linear speeds of piston rods or rotational speeds of motor shafts.

 

Reference:

 

https://www.fluidpowerworld.com/hydraulic-symbology-204-flow-control-valves/

 

Flow control valve and the method of varying flow depends on the type of design and its location in a hydraulic circuit.

A simplest method of fixing a particular flow rate is with an orifice. An orifice is a small opening incorporated into a cavity.

                  Figure 1 (a)                                              Figure 1 (b)

Fig. 1 (a) and (b) shows the elemental types of fixed orifice. A simple disk with an orifice hole in the center will provide a method of flow control within a ferrule-type fitting as in fig. 1(a) or drilling out a hole in fitting forms another type orifice as shown in Fig. 1 (b). An orifice shall be as short as possible in depth while it is designed to be strong enough to withstand the effects of pressure.

 

Fig. 2 (a) and (b) shows the hydraulic symbols for a fixed flow control valve. Fig. 2 (a) shows the flow path which depicts smooth compression of the fluid. In this design the orifice has certain length in order to provide the gentle passage for fluid flow. Fig. 2 (b) depicts that the orifice is with sharp edge indicating the length of the orifice is minimum.

 

Fixed orifices are normally applicable for factory settings in pumps, manifolds and valves, but they rule out user adjustability.

Factors that determine the flow rate, Q, across an orifice are:

·        Cross sectional are of the orifice (mm²)

·        Shape of the orifice ( round, square, triangle)

·        Length of the restrictor/orifice as shown below

·        Pressure differential (∆p) across the orifice

·        Viscosity of the fluid (cSt) depending on temperature.

A variable orifice flow control valve provides a method to control the size of the gap between the needle and its seat, thus changing the flow rate through itself.

Fig.3 shows a needle valve where in the flow rate is adjusted by turning adjustment stem with a screw driver or a wrench. This design allows the flow to pass through the opening or orifice around the needle.  When the adjusting stem is raised, the orifice is made larger and the restriction to flow is less. When the adjusting screw is lowered the orifice is made smaller and the restriction for flow is greater. Once the adjustment stem is positioned to get the desired flow rate the setting is held by tightening the locknut.

 

The hydraulic symbol shows a diagonal arrow across the restrictor depicting adjustability.

In practice, the variable orifice flow control valves are fitted with an integral check valve such that the flow rate is controlled in only one direction and allows free flow in the reverse direction. In this design, Fig. 4 and Fig. 5 shows a typical designs where in the spool functions as needle during restricted flow and as poppet during reverse flow. In Fig. 3(c) the check valve blocks the flow from A to B there by forcing the fluid to pass through the restrictor. In the reverse flow, from B to A, the fluid unseats the ball poppet and bypasses the restrictor there by ensuring the free flow.

Typical hydraulic circuits incorporating Flow Control valves for speed control of actuators are discussed below:

Meter-in flow control:

Fig. 6 (a) (Reference taken from Yuken Kogyo text book), shows control of actuator speed, in forward direction, using flow control valve in the pressure line (feed line) of the actuator. FCV is located between the Direction control valve and the working port of Piston end. This position ensures that only forward stroke of the actuator is speed controlled. In the reverse direction of the actuator the check valve allows free flow of fluid, bypassing the FCV, thereby ensuring fast retraction.

Flow control valves with integral check valves are used when the reverse flow is not to be restricted.

The type of speed control illustrated in Fig. 6 (a) is known as Meter-in type of control which implies that the fluid flow is metered before it enters the actuator. Fig. 6 (a) depicts that the fluid is metered into the cap end of the cylinder to extend the piston rod at a fixed speed (as set on the variable flow control valve). The displaced fluid from the rod end of the cylinder flows unrestricted to the reservoir.

Take note of the pressure gauge (PG) readings.

PG1 shows the relief valve setting, 10 MPa, the system pressure.

PG2 shows 10 MPa equal to relief valve setting.

PG3 shows 4 MPa, which is equal to the load pressure. Note that the pressure in the system is dependent on the resistance to fluid flow by the load.

PG4 and PG5 indicates zero reading because the fluid is returning to the reservoir without any restriction.

Meter-in type of speed control is highly accurate, and is used where the load on the actuator resists the stroke at all times (no “runaway” situation)

 

Meter-out flow control:

Fig. 6 (b) (Reference: Yuken Kogyo text book), shows control of actuator speed, in forward direction, using flow control valve in the return line of the actuator. FCV is located between the rod end of the actuator and the Direction control valve. This position ensures that only forward stroke of the actuator is speed controlled by metering the exhaust fluid from the actuator. In the reverse direction of the actuator the check valve allows free flow of fluid, bypassing the FCV, thereby ensuring fast retraction.

The type of speed control illustrated in Fig. 6 (b) is known as Meter-out type of control which implies that the fluid flow is metered before it leaves the actuator. Fig. 6 (b) depicts that the fluid is metered out of the rod end of the cylinder to extend the piston rod at a fixed speed (as set on the variable flow control valve). The displaced fluid from the rod end of the cylinder flows restricted by the flow control valve and drains into the reservoir.

Take note of the pressure gauge (PG) readings.

PG1 shows the relief valve setting, 10 MPa, the system pressure.

PG2 shows 10 MPa equal to relief valve setting.

PG3 shows 10 MPa, which is equal to the system pressure.

PG4 shows 12 MPa (observe pressure intensification due to area difference between the piston and rod end). 10 MPa pressure on 100 cm² piston area pushes the piston with 100 kN force. Load pressure 4 MPa on the area 100 cm² offers a resistive load of 40 kN. The net force from the piston is (100 – 40 = 60 kN). Equivalent load from piston to the rod end oil is 60 kN on 50 cm² annular area. Hence the PG4 shows 12 MPa (=60 kN / 50x10^-4 m²), which is more than the system pressure of 10 MPa. Designers shall select the fluid components considering the pressure intensification.

PG5 indicates zero reading because the fluid is returning to the reservoir without any restriction after exiting the flow control valve.

Meter-out type of speed control is highly accurate, and is used where a free falling load or overhauling load tends to get out of control of the actuator. (“Runaway” situation)

  Bleed of flow control:

Fig. 6 (c) shows speed control of the actuator by Bleed-off method. In this method the flow control valve is installed on a bypass line to regulate flow to the tank and control the actuator speed. Compared to meter-in and Meter-out circuits, this method works with small power consumption because the pump’s discharge pressure is fully delivered to the load resistance. Given that the bleed flow is constant, the fluctuation of pump flow determines the actuator speed. i.e., the pump flow directly influences the load and pump’s volumetric efficiency. This circuit does not allow for control of multiple cylinders.

 

Bleed-off speed control method has a power saving advantage, as the pump operates always at the pressure required by the work load, and the excess pump flow returns to the tank via the flow control valve without being pushed over the relief valve.

Bleed – off method is not as accurate as Meter-in, since the measured flow goes to tank and the remaining flow into the actuator.

Observe pressure gauge readings and reason out the validity of the indicated readings.

 

Practice problems:

Identify the flow control valve in the circuit. Discuss the functioning of the circuits shown in Fig. 7, Fig.8 and Fig. 9 identifying the speed control methods adopted in it. Explain how both the forward and return strokes of the cylinders are speed controlled.