Process Plants Instrumentation

Directional Control Valves: Function & Principle of Operation

Directional control valves are used to regulate the flow rate of hydraulic fluid; they are also utilized in pneumatic systems for flow control. Directional control valve works by regulating the movement of the piston in the cylinder. It admits pressurized fluid to either end of the cylinder while providing a return path for fluid being squeezed out of the other end of the cylinder. An illustration of this kind of valve is shown in Figure 1.0 below.

Components of Directional Control Valve

The directional control valve consists of two main parts:

  • Valve body
  • Spool

These two main parts are shown in the figure below:

With a reference to the figure above, the valve body has four-fluid connection ports:

  1. High-pressure fluid supply (from the pump)
  2. Low-pressure return to the tank.
  3. Connection to right end of the cylinder
  4. Connection to left end of the cylinder

The spool is a solid-metal machined shaft that can slide back-and-forth in the valve body. The spool shown in Figure 1.0 above has two deep grooves that allow fluid to pass from one port in the valve body to another. The position of the spool within the valve body determines how fast, and in what direction, the piston will move. The spool is moved by a hand lever, an electrical actuator such as solenoid, or a small hydraulic or pneumatic actuator.

The Operation of Directional Control Valve

The Figure 1.0 above shows the valve with the spool centered. In this position, the fluid from the pump is completely blocked by the spool. The lines connected to the cylinder are also blocked; with the cylinder fluid thus trapped, the piston is locked in place.

Figure 1.1 below shows the same valve with the spool moved to the right of center.

Observe from Figure 1.1 above, that the right-hand groove is allowing the fluid to pass from the pump to the right end of the cylinder, causing the piston to move to left. The left-hand spool groove has moved into position to allow an escape route for the fluid being pushed out of the left end of the cylinder. This low-pressure fluid is allowed return to the tank.

Figure 1.2 shows the same valve with spool moved to the left of center.

With reference to Figure 1.2 above, the high-pressure fluid from pump is allowed to pass, via the left-hand groove, to the left end of the cylinder (causing the piston to move to the right). Now, the return fluid from the cylinder passes through the right-hand spool groove and then back to the tank.

John Mulindi

John Mulindi is an Industrial Instrumentation and Control Professional with a wide range of experience in electrical and electronics, process measurement, control systems and automation. In free time he spends time reading, taking adventure walks and watching football.

View Comments

Recent Posts

Standard Process Signals for Industrial Instrumentation

Industrial measurement and control processes employ standard process signals that are used throughout all the…

1 week ago

Top 5 Benefits of Combining CCTV Cameras with Biometric Systems

The integration of advanced technologies in security systems has become imperative for ensuring safety and…

1 week ago

Sources of Power Quality Problems

Power quality may be affected by a number of issues. Our discussion in this article…

2 weeks ago

Common Terms Used When Describing Power Quality Problems

Power quality has become an important issue to electricity consumers at all levels of consumption.…

2 weeks ago

What to Expect from PCB Assembly Services in China

The importance of printed circuit board (PCB) technology has escalated throughout the years with the…

2 weeks ago

Magneto-Optic Current Sensors for High Voltage, High Power Transmission Lines

One of the key challenges in measuring the electrical current in high voltage, high power…

3 weeks ago