Hydraulic Control Valve

Hydraulic control valves are hydraulic-operated devices that are used to modify fluid flow or pressure rate in a process system. Hydraulic control valves are used to control and direct the oil or fluid from the time it leaves the pump until it starts on its return to the reservoir in a hydraulic circuit. The control valve is almost always the first one encountered by the oil or fluid after it leaves the pump. From that point on throughout the circuit, the type of valve employed depends upon the design of the circuit. In a very simple circuit, the only other valve used will probably be the master control valve. In a complex circuit the oil may flow through a sequence valve, master control valve, reducing valve, flow control valve, check valve and several others before it finally returns to the reservoir.

Hydraulic valves as used in industrial applications are built in several pressure ranges. The majority of present applications are in the range of 0 to 1500 lb per sq in., but the trend is toward higher pressures. Various types of valves are used in hydraulic systems to control or regulate the flow medium. Basically valves are expected to control:
» Blocking or stopping of flow
» Direction of flow
» Pressure of flow media
» Flow quantity

Types of Hydraulic Control Valves

Closed Control Valve:

This valve has a blocked "open-center" flow passage. When the contol valve is in neutral position, it stops the pump flow from going to the tank.

Open Center Valve:

This has an open center flow passage provided by the spool land openings. Also known as tandem-center, this type of valve connects the pump flow to the tank when the control valve spool is in its neutral position.

Parallel Valve:

This is a multiple spool valve whereby all spools control different functions simultaneously . The oil flow will be divided equally considering that the operating pressure requirements are same for all functions. The function requiring the lowest pressure will operate first. When the first function is fully operated, the next function will operate.

Four Way Valve:

This is also known as double-acting valve. This type of valve has four functional port connections. Here either a reversible hydraulic motor or double-acting cylinder can be operated. Ports usually consist of two work ports, a "inlet" or "pump" port and a "return" or "tank" port.

Three Way Valve:

This is also known as single-acting valve. This type of valve has three functional flow port connections. Here either a undirectional motor or a single-acting cylinder can be operated with such a valve. Ports usually consist of one "work" port, a "pump" port and a "tank" port.

Motor Spool or Free Flow Valve:

This type pf valve is designed to provide flow from "work" ports to "tank" when it is in neutral position.

Controlled Parameters

The various parameters that need to be controlled in the case of oil energy are as follows:

1. In a hydraulic machine the actuators make to and fro motion. Hence the direction of oil feeding the cylinder or motor needs to be reversed. This means that the direction of oil flow is an essential aspect of hydraulic operation. This is achieved by direction control valves.
   
   
2. Oil energy does the work due to pressure in the oil system. In a mechanical or hydraulic system there may be a need to increase or decrease the oil pressure depending on the specific requirement. Hence a group of valves called pressure control valves have been designed.
   
   
3. The speed of the actuators needs to be altered as per operational requirement. For this purpose hydraulic systems use flow control valves.

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