Table of Contents
What are Pneumatic Control valves?
Valve are defined as devices to control or regulate the commencement, termination, and direction and also the pressure or rate of flow of a fluid under pressure which is delivered by a compressor or vacuum pump or is stored in a vessel.
The function of valves is to control the pressure or flow rate of pressure media, to provide a signal which governs the sequence of operation and act as interlock and safety devices.
Valves used in pneumatics mainly have a control function that is when they act on some process, operation, or quantity to be stopped. A control function requires control energy, it is desirable to achieve the greatest possible effect with the least effort.
Types of Pneumatic Control Valves
Depending on the design, these can be divided into the following 4 categories:
- Direction control valve
- Non-return valves
- Flow control valves
- Pressure control valves
Direction Control Valve
Pneumatic systems like hydraulic systems also require control valves to direct and regulate the flow of fluid from the compressor to the various devices like air actuators and air motors. In order to control the movement of air actuators, compressed air has to be regulated, controlled, and reversed with a predetermined sequence. Pressure and flow rates of the compressed air to be controlled to obtain the desired level of force and speed of air actuators.
The function of the directional control valve is to control the direction of flow in the pneumatic circuit. DCVs are used to start, stop and regulate the direction of airflow and to help in the distribution of air in the required line.
As a signaling element, the directional control valve is operated for example, by a roller lever to detect the piston rod position of the cylinder.
As a processing element, the directional control valve redirects or cancels signals depending on the signal input received.
As a control element, the directional control valve must deliver the required quantity of air to match the power component requirements.
ISO Designation and Symbols of Direction Control Valves
The valve is described by the following methods:
The number of ports: 2-way, 3-way, 4-way, 5-way, etc.
The number of positions: 2 positions, 3 positions.
Methods of actuation of the valve: Manually actuated, mechanically actuated, pneumatically actuated, or electrically actuated.
Methods of return action: Spring return, air return, etc.
Valves are represented by symbols because actual construction is quite complex. A symbol specifies the function of the valve, method of actuation, no of ports, and ways. Pneumatic symbols have been standardized in ISO 1219-1:2006. Another standard ISO 1219-2:1995 establishes the rules for drawing diagrams of fluid power systems using symbols from ISO 1219-1. Port designations are described in ISO 5599.
Port markings: As per the ISO 5599, ports are designated using a number system. Earlier, a letter system was used to designate a port.
|
Port |
Old (Letter) system |
ISO (Number) system |
Remarks |
|
Pressure port |
P |
1 |
Supply port |
|
Working port |
A |
2 |
3/2 DCV |
|
Working ports |
A,B |
4,2 |
4/2 or 5/2 DCV |
|
Exhaust port |
R |
3 |
3/2 DCV |
|
Exhaust ports |
R,S |
5,3 |
5/3 DCV |
|
Pilot ports |
Z or Y |
12 |
Pilot line (flow 1-2) |
|
Pilot ports |
Z |
14 |
Pilot line (flow 1-4) |
|
Pilot ports |
Z or Y |
10 |
Pilot line (no flow) |
|
Internal pilot ports |
Pz, Py |
81,91 |
Auxiliary pilot line |
Ports and position: DCVs are described by the number of port connections or ways they control. For example Two-way, three-way, four-way valves.
Two-way Valve
Two-way valves are also simple two positions ON/OFF valves but actuated by actuators such as solenoid actuators, hydraulic actuators, etc.
|
|
| Two-way valve |
There is normally closed and normally opened two-way valves, external actuation is needed to do the position change of the valve actuator.
A flow path is established or closed when the actuator is moved. These valves are also called on-off valves
Three-way valve
These valves have three nodes that’s why named three-way valves and three-way valves allow two positions of actuation in ON condition.
|
|
| Three-way valve |
A three-way valve has two plug actuators each plug controls the flow in two different ways. The two plug positions are shown in the above figure. When one of the plugs closes a node other one opens the other node.
This normally closed valve neutralize the position of the actuator closing both the node to close the valve.
Three-way directional valves are available for manual, mechanical, pilot, solenoid actuation.
Four-way Valve
Four-way valve or three-position valves which got three working positions for the actuators to control the flow direction.
|
|
| Four-way valve |
These valves are generally used to operate cylinders and fluid motors in both directions hydraulically.
Also read,
- Different Types of Hydraulic Cylinders
- Different types of Gears and their Applications
- Bearing Materials and their properties
Methods of actuation
The methods of actuation of pneumatic directional control valves depend upon the requirements of the task. The types of actuation vary;
- Manually actuated
- Mechanically actuated
- Pneumatically/ air actuated
- Electrically actuated
- Combined actuation
The symbols of the methods of actuation are detailed in DIN ISO 1219. When applied to a directional control valve, consideration must be given to the method of initial actuation of the valve and also the method of return actuation. Normally these are two separate methods. They are both shown on the symbol on either side of the position boxes. There may also be additional methods of actuation such as manual overrides, which are separately indicated.
Non-return valves
Non-return valves permit the flow of air in one direction only, the other direction through the valve being at all times blocked to the airflow. Mostly the valves are designed so that the check is additionally loaded by the downstream air pressure, thus supporting the non-return action.
Among the various types of non-return valves available, those preferentially employed in pneumatic controls are as follows:
- Check valve
- Shuttle valve
- Quick exhaust valve
Check valve
The simplest type of non-return valve is the check valve, which completely blocks airflow in one direction while permitting flow in the opposite direction with minimum pressure loss across the valve. As soon as the inlet pressure in the direction of free flow develops a force greater than that of the internal spring, the check is lifted clear of the valve seat. The check-in valve may be a plug, ball, plate, or diaphragm.
|
|
| Fig. Check valve |
Shuttle valve
It is also known as a double control valve or double-check valve. A shuttle valve has two inlets and one outlet. At any one time, flow is shut off in the direction of whichever inlet is unloaded and is open from the loaded inlet to the outlet. A shuttle valve may be installed, for example, when a power unit (cylinder) or control unit (valve) is to be actuated from two points, which may be remote from one other.
|
|
| Fig. Shuttle valve |
Quick Exhaust Valves
A quick exhaust valve is a typical shuttle valve.
|
|
| Fig. Quick exhaust valve |
The quick exhaust valve is used to exhaust the cylinder air quickly into the atmosphere. In many applications especially with single-acting cylinders, it is a common practice to increase the piston speed during retraction of the cylinder to save the cycle time. The higher speed of the piston is possible by reducing the resistance to the flow of the exhausting air during the motion of the cylinder. The resistance can be reduced by expelling the exhausting air to the atmosphere quickly by using a Quick exhaust valve.
 |
| Fig. Quick exhaust valve symbol |
Flow Control Valves
The function of a flow control valve is self-evident from its name. A flow control valve regulates the rate of airflow. The control action is limited to the airflow passing through the valve when it is open, maintaining a set volume per unit of time.
|
|
| Fig. Flow control valve |
|
|
| Fig. Variable flow control valve symbol |
Pressure Control Valve
Compared with hydraulic systems, few pressure control valves are brought into use in pneumatics. Pressure control valves control the pressure of the air flowing through the valve or confined in the system controlled by the valve.
There are three types of pressure control valves:
- Pressure limiting valve
- Pressure sequence valve
- Pressure regulator or pressure reducing valve
Pressure limiting valve
Prevents the pressure in a system from rising above a permissible maximum. It is a standard feature of a compressed air production plant but is hardly ever used in pneumatic controls.
|
|
| Fig. Pressure limiting valve |
These valves perform a safety relief function by opening to the atmosphere if a predetermined pressure is exceeded in the system, thus releasing the excess pressure. As soon as the pressure is thus relieved to the desired figure, the valve is closed again by spring force.
Pressure sequence valve
The function of the sequence valve is very similar to that of a pressure limiting valve. It is however used for a different purpose.
The outlet of the pressure sequence valve remains closed until pressure upstream of it builds up to a predetermined value. Only then does the valve open to permit the air from the inlet to the outlet.
Sequence valve must be incorporated into a pneumatic control where a certain minimum pressure must be available for a given function and operation is not be initiated at any pressure lower than that.
There are also used in systems containing priority air consumers when other consumers are not to be supplied with air until ample pressure is assured.
Pressure reducing valve or regulator
Pressure regulators commonly called pressure-reducing valves, maintain constant output pressure in compressed-air systems regardless of variations in input pressure or output flow. Regulators are a special class of valves containing integral loading, sensing, actuating, and control components.
|
|
| Fig. Regulator |
Regulators can use either a piston or diaphragm to sense downstream pressure. Diaphragms are generally more sensitive to pressure changes and react more quickly. They should be used where sensitive pressure settings are required (less than 0.0025 bar). Pistons, on the other hand, are generally more rugged and provide a larger effective sensing area in a given size regulator. The functional difference between precision and general-purpose regulators are the degree of control accuracy of the output pressure. Output pressure accuracy is determined by the droop due to flow changes (regulator characteristics).
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