Shuttle Valve?

                A shuttle valve allows two alternate flow sources to be connected in a one-branch circuit. The valve has two inlets P1 and P2 and one outlet A. Outlet A receives flow from an inlet that is at a higher pressure. Figure 1.5 shows the operation of a shuttle valve. If the pressure at P1 is greater than that at P2, the ball slides to the right and allows P1 to send flow to outlet A. If the pressure at P2 is greater than that at P1, the ball slides to the left and P2 supplies flow to outlet A .

               One application for a shuttle valve is to have a primary pump inlet P1 and a secondary pump inlet P2 connected to the system outlet A The secondary pump acts as a backup, supplying flow to the system if the primary pump loses pressure. A shuttle valve is called an “OR” valve because receiving a pressure input signal from either P1 or P2 causes a pressure output signal to be sent to A

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Pilot-Operated check Valve?

               A pilot-operated valve along with its symbol is shown in Fig. This type of check valve always permits free flow in one direction but permits flow in the normally blocked opposite direction only if the pilot pressure is applied at the pilot pressure point of the valve. The check valve poppet has the pilot piston attached to the threaded poppet stem by a nut. The light spring holds the poppet seated in a no-flow condition by pushing against the pilot piston. The purpose of the separate drain port is to prevent oil from creating a pressure build-up at the bottom of the piston. The dashed line in the graphical symbol represents the pilot pressure line connected to the pilot pressure port of the valve. Pilot check valves are used for locking hydraulic cylinders in position

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Advantages and Disadvantages of a Poppet Valve?

Advantages of a poppet valve 

 Virtually zero leakage in closed position.

 Poppet elements do not stick even when left under pressure for long periods
.
 Fast, consistent response time: typically 15 ms.

Disadvantages of a Poppet Valve A poppet valve has the following disadvantages:

 Axial pressure balance is impossible and considerable force may be needed to open the poppet against the flow at a high pressure. This limits valves that have direct mechanical actuation to low flow duties.

 Generally individual poppets are required for each flow path that significantly increases the complexity of multi-port valves
.
 Lapping and super finishing of valves add cost.

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Check Valve?

               The simplest DCV is a check valve. A check valve allows flow in one direction, but blocks the flow in the opposite direction. It is a two-way valve because it contains two ports

               provides two schematic drawings showing the operation of a poppet check valve. A poppet is a specially shaped plug element held on a valve seat by a light spring. Fluid flows through the valve in the space between the seat and poppet. In the free flow direction, the fluid pressure overcomes the spring force. If the flow is attempted in the opposite direction, the fluid pressure pushes the poppet in the closed position. Therefore, no flow is permitted.

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Graphical symbols of Actuating Devices?

What is meant by Actuating Devices?

                      Direction control valves may be actuated by a variety of methods. Actuation is the method of moving the valve element from one position to another. There are four basic methods of actuation: Manual, mechanical, solenoid-operated and pilot-operated. Several combinations of actuation are possible using these four basic methods. 

 Manually operated: In manually operated DCVs, the spool is shifted manually by moving a handle pushing a button or stepping on a foot pedal. When the handle is not operated, the spool returns to its original position by means of a spring.

 Mechanically operated: The spool is shifted by mechanical linkages such as cam and rollers.

 Solenoid operated: When an electric coil or a solenoid is energized, it creates a magnetic force that pulls the armature into the coil. This causes the armature to push the spool of the valve.

 Pilot operated: A DCV can also be shifted by applying a pilot signal (either hydraulic or pneumatic) against a piston at either end of the valve spool. When pilot pressure is introduced, it pushes the piston to shift the spool.

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4/3 Valves ?

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4/2 and 5/2 Valve ?

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2/2 and 3/2 Valves ?

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Classification of DCVs based on the Construction of Internal Moving Parts?

Rotary spool type: 

                    In this type, the spool is rotated to change the direction of fluid. It has longitudinal grooves. The rotary spools are usually manually operated. 

Sliding spool type: 

                       This consists of a specially shaped spool and a means of positioning the spool. The spool is fitted with precision into the body bore through the longitudinal axis of the valve body. The lands of the spool divide this bore into a series of separate chambers. The ports of the valve body lead into these chambers and the position of the spool determines the nature of inter-connection between the ports.

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Classification of DCVs based on the Control Method?

Direct controlled DCV:

                              A valve is actuated directly on the valve spool. This is suitable for small sized valves.

 

Indirect controlled DCV:

                              A valve is actuated by a pilot line or using a solenoid or by the combination of electro hydraulic and electro-pneumatic means. The use of solenoid reduces the size of the valve. This is suitable for large-sized valves. 

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Classification of DCVs based on Design Characteristics?

Based on design characteristics, DCVs can be classified as follows:

+) An internal valve mechanism that directs the flow of fluid. Such a mechanism can either be a poppet, a ball, a sliding spool, a rotary plug or a rotary disk.

+) Number of switching positions (usually 2 or 3).

+) Number of connecting ports or ways.

+) Method of valve actuation that causes the valve mechanism to move into an alternate position.

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Classification of DCVs based Fluid Path?

1) Check valves.

2) Shuttle valves.

3) Two-way valves. 

4) Three-way valves. 

   
5) Four-way valves.

     

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Purpose of DCV's?

 To start, stop, accelerate, decelerate and change the direction of motion of a hydraulic actuator. 

 To permit the free flow from the pump to the reservoir at low pressure when the pump’s delivery is not needed into the system. 

 To vent the relief valve by either electrical or mechanical control. 

 To isolate certain branch of a circuit.

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types of valves?

1. Directional control valves (DCVs): 

                      They determine the path through which a fluid transverses a given circuit. 

Pressure control valves: 

                     They protect the system against overpressure, which may occur due to a sudden surge as valves open or close or due to an increase in fluid demand. 

2. Flow control valves: 

               Shock absorbers are hydraulic devices designed to smooth out pressure surges and to dampen hydraulic shock. In addition, the fluid flow rate must be controlled in various lines of a hydraulic circuit. For example, the control of actuator speeds can be accomplished through use of flow control valves. Non-compensated flow control valves are used where precise speed control is not required because the flow rate varies with pressure drop across a flow control valve. It is important to know the primary function and operation of various types of control components not only for good functioning of a system, but also for discovering innovative methods to improve the fluid power system for a given application.

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WHAT IS THE FUNCTION OF PRESSURE RELIEF VALVE?

A pressure Relief Valve is a safety device designed to protect a pressurized vessel or system during an over pressure event.
An over pressure event refers to any condition which would cause pressure in a vessel or system to increase beyond the specified design pressure or maximum allowable working pressure (MAWP).
The primary purpose of a pressure Relief Valve is protection of life and property by venting fluid from an over pressurized vessel.

Many electronic, pneumatic and hydraulic systems exist today to control fluid system variables, such as pressure, temperature and flow. Each of these systems requires a power source of some type, such as electricity or compressed air in order to operate. A pressure Relief Valve must be capable of operating at all times, especially during a period of power failure when system controls are non functional. The sole source of power for the pressure Relief Valve, therefore, is the process fluid.

Once a condition occurs that causes the pressure in a system or vessel to increase to a dangerous level, the pressure Relief Valve may be the only device remaining to prevent a catastrophic failure. Since reliability is directly related to the complexity of the device, it is important that the design of the pressure Relief Valve be as simple as possible.

The pressure Relief Valve must open at a predetermined set pressure, flow a rated capacity at a specified over pressure, and close when the system pressure has returned to a safe level. Pressure Relief Valves must be designed with materials compatible with many process fluids from simple air and water to the most corrosive media. They must also be designed to operate in a consistently smooth and stable manner on a variety of fluids and fluid phases.

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