Valves are used to handle a wide variety of functions in controlling the flow of liquids and gases. With so many options available, it’s important to look at all of the factors to ensure you are selecting the correct valve for your application.
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1. Media Type
The characteristics of the liquid or gas type being controlled will determine the valve material that should be used. When monitoring harsh or corrosive materials, polytetrafluoroethylene (PTFE) and perfluoroalkoxy alkanes (PFA) are great choices, as they are able to withstand these conditions. When monitoring pressurized gases, it’s important to use metal valves for the highest level of safety.
2. Application Conditions
The selection of the correct valve sizing is the most important step in selecting the right valve for your application. An improperly sized valve can cause operation issues within the valve or elsewhere in the system. In addition to this, determine the temperature, pressure, and flow rate that your valve will be operating under. In most cases, metal valves can withstand higher temperatures and pressures than plastic ones.
3. Valve Function
Determine the need that your valve needs to fill. 2-way valves perform well for on/off control in a system, while 3-way valves can be used for on/off control, as well as for diverting or mixing media. Also, take note of whether your valve will be open or closed the most often. In order to extend the life expectancy, order a valve that is normally in that orientation and changes to the other direction when energized.
4. Actuation Method
Another consideration to be made is how the valve is to be operated. Identify if the valve will be manually hand-operated or if the valve will need to be actuated to turn off and on automatically. If the latter is chosen, decide whether the unit is going to be used with an electric, pneumatic, or hydraulic actuator.
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5. Maintenance Requirements
If maintenance needs to be done regularly, ball valves are a great option as they resist clogging and are one of the simplest valve types to service. Ball valves are also available in 3-piece configurations, which consist of two end caps and a body. This allows the main body section to be easily removed for cleaning without disconnecting the end caps from the pipe, preventing the line from being shut down during maintenance.
The W.E. Anderson™ division of Dwyer Instruments, Inc. produces a broad line of competitively priced valves and valve control products, including: globe, butterfly, ball, manifold, actuators, and solenoid valves. Each valve type has a variety of options to meet specific application needs. Valve position indicators, valve positioners, and electric or manual pneumatic actuators are also available for a complete valve package. To learn more about Dwyer’s valve product line, please visit our website.
Directional control valves perform only three functions:
These three functions usually operate in combination.
The simplest directional control valve is the 2-way valve. A 2-way valve stops flow or allows flow. A water faucet is a good example of a 2-way valve. A water faucet allows flow or stops flow by manual control.
A single-acting cylinder needs supply to and exhaust from its port to operate. This requires a 3-way valve. A 3-way valve allows fluid flow to an actuator in one position and exhausts the fluid from it in the other position. Some 3-way valves have a third position that blocks flow at all ports.
A double-acting actuator requires a 4-way valve. A 4-way valve pressurizes and exhausts two ports interdependently. A 3-position, 4-way valve stops an actuator or allows it to float. The 4-way function is a common type of directional control valve for both air and hydraulic circuits. A 3-position, 4-way valve is more common in hydraulic circuits.
The 5-way valve is found most frequently in air circuits. A 5-way valve performs the same function as a 4-way valve. The only difference is an extra tank or exhaust port. (Some suppliers call their 5-way valves, “5-ported 4-ways.") All spool valves are five ported, but hydraulic valves have internally connected exhaust ports going to a common outlet. Because oil must return to tank, it is convenient to connect the dual tank ports to a single return port. For air valves, atmosphere is the tank, so exhaust piping is usually unimportant. Using two exhaust ports makes the valve smaller and less expensive. As will be explained later, dual exhausts used for speed-control mufflers or as dual-pressure inlets make this configuration versatile.
Following are schematic symbols for commonly used directional control valves.
2-way directional control valves
A 2-way directional valve has two ports normally called inlet and outlet. When the inlet is blocked in the at-rest condition, as shown in Figure 8-1, it is referred to as "normally closed" (NC). The at-rest box or the normal condition is the one with the flow lines going to and from it.
The boxes or enclosures represent the valve’s positions. In Figure 8-1, the active box shows blocked ports, or a closed condition, while the upper box shows a flow path. When an operator shifts the valve, it is the same as sliding the upper box down to take the place of the lower box. In the shifted condition there is flow from inlet to outlet. Releasing the palm button in Figure 8-1 allows the valve spring to return to the normal stop flow condition. A 2-way valve makes a blow-off device or runs a fluid motor in one direction. By itself, a 2-way valve cannot cycle even a single acting cylinder.
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