The efficiency of a hydraulic circuit depends upon the functioning of various control devices used in the circuit. The control devices refer to the various types of hydraulic valves. As the valves do the task of controlling the hydraulic energy, a hydraulic circuit without valves is useless.
The following are the different types of valves used in hydraulic circuits.
Valves used to control the amount of flow in hydraulic circuits
- Gate valve
- Globe valve
- Needle valve
- Bye pass valve
- Isolation valve
Direction control valves
- Check valve
- Swing valve
- Poppet valve
- Pilot valve
- Two way plunger valve
- Three way plunger valve
- Four way plunger valve
Valve used to control the amount of flow in hydraulic equipment
- Flow control valve (or) Flow regulator
Pressure reducing valve
- Sequence valve
- Unloading vale
- Counterbalance valve
- Deceleration valve
Gate Valve – Hydraulic liquid passes through various pipelines in a circuit. Gate valve is a device useful in opening and closing of the passage for the liquid. A wedge shaped structure is operated by a screw to open or close to control the flow of liquid.
When the passage is fully opened, very little resistance is offered to the flow of the liquid. If the pressure of the liquid in the circuit is high, it is very difficult to operate the valve.
If the passage is opened partially, this valve functions normally. Gate valves are used to cut off the flow of the liquid when maintenance works are carried out in the circuit.
Globe Valve – Globe valve is used to open or close the flow of hydraulic liquid in the circuit like a gate valve. A taper hole is present at the centre of the valve. A globe like part fits into the hole. This part is operated to control the passage of the liquid.
This valve is operated even at high pressures. It is highly reliable when it is partially opened. It is not generally used in hydraulic installations due to the fact that it is intended for handling very high quantity of liquid.
Needle Valve – The needle valve is similar to a globe valve. The sliding part of the valve looks like a needle. It is suitable to be used in a hydraulic circuit since it is intended for handling smaller quantities of liquid. It is also used where it is not necessary to change the flow very frequently.
Byepass Valve – Hydraulic liquid is pumped into the circuit by a pump. At some point of time, a part of the pumped liquid or the whole of it may not be necessary to be passed into the circuit. A byepass valve attached in the circuit to divert such quantity of the liquid back to the reservoir.
Isolation Valve – In large hydraulic systems, the circuit may have many sub-circuits. When a repair or maintenance work is to be done at any part of the circuit, the liquid supply to the particular circuit should be stopped. An isolating valve is attached to each of the sub-circuit to cut it off from the main circuit.
Direction Control Valves (Plunger valves)
Direction control valves are used to allow the hydraulic liquid into various parts of the equipment and to reverse the direction of flow of the liquid. They are also known as Plunger valve, Selector valve or Main valve. Different types of direction control valves are
- According to the number of ports
- Uni-directional plunger valve or Check valve
- Two way plunger valve
- Three way plunger valve
- Four way plunger valve
- According to the state of the valve in neutral position
- Closed centre main valve
- Open centre main valve
Uni-directional Plunger Valve
This valve is useful in allowing the flow of liquid in only one direction. It does not allow the flow in the opposite direction. It is also known as plunger valve, check valve or selector valve. The valve now is in closed condition. It is connected in line with the circuit.
Two Way Plunger Valve
In this valve, there are two ports – a. Inlet and b. Outlet. The moving part inside the valve is known as spool or plunger. When the spool is positioned at the right side end, the inlet and outlet are disconnected. If the spool is moved to the opposite side, both the inlet and the outlet are connected and the flow of the liquid is established. An hydraulic device like a hydraulic motor can be switched on or off by this valve. As there is no drain in this valve to send back the liquid to the reservoir, it cannot be used to operate pistons in cylinders.
Three Way Plunger Valve
In this valve, there are three ports namely inlet, outlet and drain. If the plunger is pushed to the right hand end, the inlet A and the outlet B are conneced and the liquid is allowed into the circuit to operate the device. When the plunger is pushed to the opposite end, the inlet A and the drain D are conneced and the liquid is sent back to the reservoir. This type of valve is used to operate hydraulic motors like single acting hydraulic cylinders.
Four Way Plunger Valve
There are four ports in this valve – i) Inlet A ii) Outlets B & C and iii) Drain D. When the plunger is pushed to a side, the inlet A and a outlet B are connected to allow the liquid in a particular direction. In this position, the drain D is con- nected to the other outlet C. When the position of the plunger is altered, the inlet and the outlet C are connected to allow the liquid in a different direction. Outlet B and drain D are connected now. A four way plunger valve is used to operate a double acting hydraulic cylinder.
It is very important to ensure a correct clearance between the plunger and the cylinder in plunger valves. It is difficult to keep a very little clearance between them. But at the same time, more clearance will allow the liquid to pass through the clearance to the other side. When the valves are handling pressures in the range of 140Kg/cm2, no problems arise. But when pressure range reaches 210Kg/cm2, the system faces two difficulties.
- Leakage 2. Hydraulic lock
The plunger should have correct alignment with the cylinder moving concentrically within it. But while handling higher pressures, the plunger loses the alignment and dash against the walls of the cylinder to get stuck within it. Further movement of the plunger is prevented. This effect is known as hydraulic lock. The lock can be released by reducing the pressure of the system. The clearance between the plunger and the cylinder should be kept minimum preferrably about 0.0025mm by finishing these surfaces by honing or by lapping.
Flow Control Valve (Flow Regulator)
The speed of any hydraulic device (linear or rotary) depends upon the quantity of flow of liquid. If the quantity of liquid pumped by the pump is increased, the speed of the equipment also increases. If the speed of a particular equipment of a circuit is to be changed, the quantity of the liquid is controlled by attaching a flow control valve. It is also known as flow regulator.
Pressure Reducing Valve (PRV)
A pressure relief valve is attached to a hydraulic circuit to control the pressure of the liquid within the required limits. But a pressure reducing valve is used in specific sub-circuits to further reduce the pressure in them.
- Sequence valve – When a series of operations are to be performed in a hydraulic equipment in a specific order, a sequence valve is attached to the circuit. This will operate the devices in the specific order. It is used in machines where production work is carried out.
For example, a work has to be held firmly before any machining is carried out on it. The second operation cannot be done before the first is performed. So, a sequence valve is very important in executing the operations in a specific sequence.
- Unloading valve – This valve is used where it is necessary to provide the equipment with different speeds one by one. The different speeds shall be used alternately as in the case of a hydraulic shaper.
- Deceleration valve – The speed of a hydraulic actuator is made to get slower on reaching a specific point. The first phase moves at relatively faster speed. The valve used to reduce the speed at a particular point is known as deceleration valve.