What is Flow Control Valve | Principle, Need, Type, Symbol

Flow control valve

A flow control valve regulates the flow or pressure of a fluid. Control valves normally respond to signals generated by independent devices such as flow meters.

Need

To change speed of hydraulic actuators

In hydraulic system, sometimes it is required to increase or decrease the speed of the hydraulic actuator as per application.
1) In hydraulic shaper, the return stroke is faster than the cutting stroke. Hence actuator speed is to be increased during return stroke.
2) Similarly, hydraulic motor is to be operated at various speed toperrform drilling, tapping, operations where speed changing is necessary.

Example – Fig. shows that,when flow port area is increased, it allows more quantity of oil to enter in the actuator and it moves at faster rate. Vice versa, when the flow passage is reduced by flow needle, it allows small quantity of pressurised oil to enter in the actuator and its movement will be at slower rate.

Thus by varing the flow rate (Q) of the oil, we can change the speed of the actuators. This function is performed by flow control valve.

Principle of flow control valve/ Function

The flow rate oil isthe quantity of oil flowing per unit time. It is expressed in lit/min by letter Q.
The flow rate Q and linear cylinder velocity is related by the formula –

                                             V =  Q/A

Where V = Cylinder velocity (cm/min)
Q = Flow rate (cm3/min)
A = Area of cylinder piston (cm2)

Similarly rpm of a hydromotor is related by the formula

                                             n = Q/V

where V = Volume capacity of motor ( cm3/revolution)

The cylinder velocity (V) and rpm of hydromotor (n) are directly proportional to flow rate (Q). Hence by changing flow rate (Q) we can vary speed ofcylinder and hydromot.
The flow rate can be changed by varying the flow port area.

When a obstruction is inserted in the flow passage like a needle or gate orific, its position in the flow area may increase or decrease the flow quantity of oil passing through it.

Type of FCV

1) Throttle valve – They are two types
a) Fixed throttle
b) Variable throttle (Two way)

2) Pressure dependent flow control valve ( Non compensated)
3) Pressure compensted flow control valve
4) Flow dividers

Location of flow control valve

When the fixed delivery pump is used in hydraulic system.The flow control valve can be located at various points in the system.

a) In between D.C. valve and the actuator

Flow control valve is placed to control flow of oil entering or leaving from the actuator. These are two methods as shown in fig. a and b.

Fig. a shows that flow of oil is controlled when oil enters in the cylinder. This method is called meter-in flow control.
Fig. b shows that flow of oil is controlled while oil leaves the cylinder and drain to the tank. This method is called as meter-out flow control.

b) Between D.C. valve and pump ( By pass arrangement)

In this method, the flow control valve is placed after the pump and some of flow of oil is by pass ( diverted ) to the reservoir. This method is known as bleed-off flow control.

1. Throttle Valve –
   When a small opening is provided to reduce the flow, it causes pressure drop acress the opening called as orifice. It is called as throttle valve.

a) Fixed throttle –

In this fixed throttle valve, there is a permanent small aperture (orifice) is providedin the flow direction.
At the throttle aperture,the velocity of oil increases and there is drop in the pressure.
The flow rate at the throttle valve is calculated using the relation.

This throttle valve has constant opening which can not be adjusted. They are generally inserted directly in the pipe lines.

Symbol –

b) Diaphragm orifice (throttle) –

Two way valve

Symbol –

In this type of throttle valve, the needle with pointed end is used for varying the flow opening.

2. NonPressure Compensated Flow Control Valve –

When there is no presssure compensation for drop in pressure in the flow control valve, It iscalledas noncompensated flow control valve.
It work on the principle of throttling.

Construction and working –

It consists of needle valve fitted with adjustable screw. The needle is the flow control element.
Non return valve is provided along with throttle valve to provide regulated flow in one direction.
This combination is commonly known as flow control valve with inbuilt check valve.
The inlet and outlet port is provided for input of oil supply and regulated flow at the outlet respectively.
The needle position is adjusted to set the throttle opening. The oil flow from the inlet and pass through throttle opening and delivers the required quantity of oil.
At the throttle opening, the reduced passage may cause return flow of oil which is restricted by the check valve fitted near the valve opening.

Application –
It is used where pressure is nearlyconstant and monitoring of speed is not critical. It is used in speed control circuits like meter-in, meter-out.

Symbol –

3. Pressure Compensated Flow Control Valve –

The flow control valve which allows flow control of oil independent of pressure drop and temperature across the valve it is known as pressure compensated flow control valve.
It work on the principle of maintaining a constant pressure drop on the adjustable throttle path with the help of compensating spring loaded spool.

Construction and working –

It consists of throttle in the form of needle located in front of inlet port.
The compensator spool is provided with the spring to compensate the drop in pressure across the throttle opening.
The inlet, outlet and other internal passages are provided for all flow in the valve.

Due to change in the load of actuator, the work resistance also changes and there is a variation in the pressure drop at the entrance and the throttle.
When there is a flow of oil from the inlet, it will flow through two passages,from left hand pilot line it will exert oil pressure on the compensator spool.
Similarly the oil flow from throttle opening and flow from right side pilot line and exerts pressure in addition with spring force.

              Pressure on left side =  Pressure on right side
                          P1. A     =   P2. A + F
                      ( P1 - P2 )A  =   F
                                P   =  F/A = Costant

Where P1 = Pressure before throttling
A = Cross sectional area of valve spool
F = Spring Force

As F and A are both constant P is also constant.

The spring force determines the position of the spool and the amount of oil allowed through the valve.

Application –
Precision machine tools and other equipments where constant speed is reqiured independent of external resistanceand temperature.

Symbol –

4. Flow Dividers –

When the flow of oil is to be divided equally so that it can control the two actuators simultaneously, flow dividers are required.
In this flow control valve,the oil flow is divided equally as shown in fig.

It is used in motion synchronisation of hydraulic actuators like lifting of load at same speed by two cylinders.

Symbol –

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