Presented By :

SRICHANDAN SUBUDHI GET, CSP MILL – E & A Bhushan Power And Steel Ltd.

After this presentation you will be knowing :

• What are DCVs, its type and their uses

• About Check Valves and pilot controlled check valves

• What are solenoid actuated valves and their operation

• What are proportional solenoid valves and their operation

• Servo Valve Operation

• Servo Valve Connector

DIRECTIONAL CONTROL VALVE

• The primary function of a DC valve is to direct or prevent fluid flow to specific piping or system actuator.

• These valves usually consists of spool inside it which is electrically/hydraulically controlled.

• There are two fundamental positions of these valves :

1. Working Position2. Normal Position

• According to no. of valve ports and spool position DC valve can be categorized into following :

1. 2-way, 2-position2. 3-way, 2-position3. 4-way, 2-position4. 4-way, 3-position and many more…

Common Abbreviation used in Hydraulics

P = Pressure line

T = Tank Line

A,B = System Actuator line

X = Pilot Line

Y = Drain Line

Spool Position

2 - Way, 2 – Position DC Valve

3 – Way, 2 - Position DC Valve

4 – Way, 2 – Position DC Valve

4 – Way, 3 – Position DC Valve

• Electrically actuated DC Valves are actuated by means of solenoid. They can be single/double solenoid actuated. When these solenoids are energized the DC valves are shifted to Working position.

• Along with solenoid, there are spring actuators which are used to shift the spool of a DC valve to normal position when the solenoid is de-energized.

Single solenoid, 2 – position, Spring Offset

Double Solenoid, 2 - Position

Double Solenoid, 3 – Position, Spring Centre

1. Continuous line – Flow line

2. Dashed line – Pilot , drain

3. Spring 4. Flow Restriction

5. Single Acting Cylinder

6. Double Acting Cylinder

7. 2-way, 2-position DC valve (NC)

8. 2-way, 2-position DC valve (NO)

9. 3-way, 2-Position DC valve (NO)

10. 3-way, 2-Position DC valve (NC)

Common Hydraulic Symbols

11. 4-Way, 2-position DC valve

12. 4-way, 3-position DC valve

13. Solenoid actuated

14. Hydraulic Actuated

15. Check Valve

16. Pilot operated Check Valve

17. Pressure Relief valve

It is an electro-mechanical device that take electrical energy and produces a linear force by the use of magnetism. It is basically a winding of wire around a metal core.

SOLENOID ACTUATED DC VALVES

When the solenoid is energized, the air gap is closed quickly and a force is developed in the direction of the valve spool.

SOLENOID CONTROLLED, PILOT-OPERATED DCV

Although a valve could be shifted directly by the force of a solenoid, large flow DCVs are most often shifted using fluid at system pressure. Larger flow valves demand larger shifting forces and it is no longer practical to use a solenoid there.

CHECK VALVE

• A check valve, non-return valve or one-way valve is a valve that normally allows fluid to flow through it in only one direction.

• Check valves are two-port valves, meaning they have two openings in the body, one for fluid to enter and the other for fluid to leave.

• An important concept in check valves is the cracking pressure which is the minimum upstream pressure at which the valve will operate.

PILOT OPERATED CHECK VALVE

• This type of check valves are remotely operated through any one of the directional valves.

• This is controlled by a pilot pressure line which is indeed controlled by a directional control valve.

X : Pilot lineA : I/P line to the check valveB : O/P line from the check valve

• The main application of check valves is to hold the pressure of a load and for safety purposes.

PROPORTIONAL VALVE

• The proportional DCV is actuated by means of an electrical control signal. The control signal influences the flow rate and flow direction.

• It is exactly the same as the solenoid valve except that solenoid valve acts as ON/OFF switch whereas this proportional solenoid valve can achieve each and every point in between thus varying the flow rate.

• Different flow rate is achieved by varying the i/p current signal.

DIFFERENCE BETWEEN SOLENOID AND PROPORTIONAL VALVE

Normally close position

1. Solenoid valve

Fully Open position

2. Proportional Valve

Normally close position

Intermediate Position ( 30% Open )

Intermediate Position ( 60% Open )

Fully Open Position

• The difference between a solenoid and a proportional solenoid valve is in the construction of their spool.

How Signal Flows in It ?

i. Electrical Voltage ( -10V to +10V ) acts upon an amplifier.

ii. The amplifier converts voltage into current signal.

iii. The current acts upon the proportional solenoid, which actuates the valve.

So, In general a proportional valve is a combination of both flow control valve and directional control valve.

SERVO VALVE OPERATION

• A hydraulic servo valve is a servo (spool) with a flapper nozzle system used to position the servo. The term electro-hydraulic servo valve is often used because servo valves are controlled through an electrical signal.

• The primary components in a servo valve are a torque motor, flapper nozzle , and spool.

• The Servo valve has a hydraulic pressure inlet and an electrical input for the torque motor. The input current controls the flapper position. The flapper position controls the pressure in rod side or piston side of the cylinder. So, a current (+ or -) will position the flapper, leading to a delta pressure on the servo, which cause the servo to move in one direction or the other. Movement of the servo ports hydraulic pressure to one side of the actuator or the other, while porting the opposite side of the actuator to return.

• Flapper position is controlled by the electromagnetic torque motor. A torque motor consists of two permanent magnets with a coil winding attached to a magnetically permeable armature. The armature is part of the flapper piece. When a current is applied to the coils, magnetic flux acting on the ends of the armature is developed. The direction of the magnetic flux (force) depends on the sign (direction) of the current.

• The magnetic flux will cause the armature tips to be attracted to the ends of the permanent magnets (current direction determines which magnetic pole is attracting and which one is repelling). This magnetic force creates an applied torque on the flapper assembly, which is proportional to applied current.

• As the applied current is increased, the armature and flapper will rotate. As the flapper moves closer to one nozzle, the flow area through this nozzle is decreased while the flow area through the other nozzle increases.

• In the above the figure the flapper nozzle consists of the flapper, two inlet orifices (O1 and O2), two outlet nozzles (n1 and n2), nozzle backpressure nozzle (n3) and usually a feedback spring. As described above, the torque motor positions the flapper, which in turns controls the flow through the nozzles. When the flapper is in the neutral position, the nozzle flow areas are equal and the pressures Pn1 and Pn2 are equal. When the flow areas and inlet nozzle pressures are equal, the flow forces through each nozzle keep the flapper centered in the neutral position.

• As the flapper moves towards one of the nozzles, the outlet flow area is reduced for this nozzle. Outlet flow area increases for the other nozzle. For example, looking at Figure let the flapper move towards the n1 nozzle. This will reduce the outlet flow area and the pressure Pn1 will increase. At the same time, the outlet flow area at the n2 nozzle will increase and the pressure Pn2 will decrease. A delta pressure ΔP = Pn1 – Pn2 will occur across the pilot spool piston and the pilot spool will displace to the right. High pressure fluid will then flow to the PA actuator chamber while the PB actuator chamber is ported to return.

• Servo valves are normally used when accurate position and force control is required.

• The main advantage of a servo valve is that a low power electrical signal can be used to accurately position an actuator or motor.

• The disadvantage is complexity and cost which results from a component consisting of many detail parts manufactured to very tight tolerances. Therefore, servo valves should only be used when accurate position (or rate) control is required.

SERVO VALVE CONNECTOR

PIN FUNCTION

VOLTAGE / CURRENT RATING

A

SUPPLY VOLTAGE 24 V DC

B GROUND

C ENABLE INPUT < 6.5 V DC DISABLED

D COMMAND SIGNAL

I/P-10 to +10 mA OR

E 4 – 20 mA

F COMMAND SIGNAL O/P 4 – 20 mA

G P.E.

SERVO CONNECTOR

24 v dc

A B C D EG CONNECTOR

(4 – 20 mA)

X Y

POTENTIOMETER

In No Load Condition

When the wiper is at ‘X’ end : Current across DE will be, 24/970 = 24 mA (Apprx. )

When the wiper is at ‘Y’ end : Current across DE will be 0 mA.