What are the most important considerations when selecting a directional control valve?

Basics of Directional-Control Valves | Power & Motion

Above are common center-spool arrangements for matching neutral-position fluid routes to the application.

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These and other common center-position configurations can be quite specialized, depending on the application of the valve. Most manufacturers offer a variety of center-position configurations as standard, off-the-shelf items. Although the vast majority of directional-control valves for industrial applications are 2- and 3-position, many valves used in mobile equipment come in 4-position configurations to accommodate special needs.

When specifying the specific type of valve needed for an application, it has become common practice in North America to refer to the number of ports on a valve as the way, such as 2-way, 3-way, or 4-way. However, international standards use the word ports. Thus, what is known as a 2-way, 2-position directional valve in the U.S. is called a 2-port, 2-position valve internationally and can be abbreviated 2/2. The number before the slash identifies the number of ports, and the second number refers to the number of positions.

Spool Valves

The most common sliding-action valve is the spool-type valve (Fig. 5). Fluid is routed to or from the work ports as the spool slides between passages to open and close flow paths, depending on spool position. Spool valves readily adapt to many different spool-shifting schemes, which broadens their use over a wide variety of applications.

Many mobile applications require metering or throttling to enable the operator to slowly or gently accelerate or decelerate a load. In these instances, the spool may be modified with V notches, for example, so that a small displacement of the spool gradually permits increasing or decreasing fluid flow to gradually speed or slow actuator and load movement. This technique is also used in valves for industrial equipment. A beveled or notched edge on the spool is commonly referred to as a soft-shifting feature.

A variation of the single- or multiple-spool valve is the stack valve, in which multiple spool and envelope sections are bolted together between an inlet and outlet section to provide control of multiple flow paths. In addition to providing a central valve location for the machine operator, the valve grouping reduces the number of fluid connections involved and increases ease of sealing. The number of valves that can be stacked in this manner varies from one manufacturer to another.

Valve Operators

Valve operators are the parts that apply force to shift a valve’s flow-directing elements, such as spools, poppets, and plungers. The sequence, timing, and frequency of valve shifting are key factors in fluid power system performance. As long as the operator produces enough force to shift the valve, the system designer can select any appropriate operator for the conditions and type of control under which the system will operate.

Operators for directional-control valves are either mechanical, pilot, electrical, and electronic, or a combination of these. Different types of actuators can all be installed on the same basic valve design. A common directional valve often is used that makes provision for mounting a variety of different operators on its body.

With a mechanical operator, a machine element or person applies force on the valve’s flow-directing element to move or shift it to another position. Manual operators include levers, palm buttons, push buttons, and pedals. Purely mechanical operators include cams, rollers, levers, springs, stems, and screws. Springs are used in most directional valves to hold the flow-directing element in a neutral position. In 2-position valves, for example, springs hold the non-actuated valve in one position until an actuating force great enough to compress the spring shifts the valve. When the actuating force is removed, the spring returns the valve to its original position. In 3-position valves, two springs hold the non-actuated valve in its center position until an actuating force shifts it. When the actuating force is removed, the springs re-center the valve, leading to the common identification, spring-centered valve. Detents are locks that hold a valve in its last position after the actuating force is removed until a stronger force is applied to shift the valve to another position. The detents may then hold this new position after the actuating force again is removed.

Mechanical operation is probably the most positive way to control industrial fluid power equipment. If a valve must shift only when a machine element is in a certain position, the equipment can be designed so that the machine element physically shifts the valve through a mechanical operator when the element reaches the correct position. This arrangement virtually eliminates any possibility of false or phantom signals from shifting the valve at the wrong time.

However, mounting mechanically operated valves on a machine requires some special cautions. The valve and actuator may be exposed to a wet or dirty environment that requires special sealing. The actuator will probably be subjected to impact loads, which must be limited to avoid physical damage. Valve alignment with the operating element is also important, so the valve must be mounted accurately and securely for long service life.

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Pilot-actuated valves are shifted by pressurized fluid (often about 50 psig) that applies force to a piston that shifts the valve’s flow-directing elements. An important advantage of pilot operation is that large shifting forces can be developed without the impact and wear that affects mechanically actuated valves. Pilot-operated valves can be mounted in any convenient or remote location to which pressure fluid can be piped. The absence of sparks and heat buildup makes pilot-actuated valves attractive for applications in flammable or explosive environments.

Electric or electronic valve operation involves energizing a solenoid. The force generated at the solenoid plunger then shifts the valve’s flow-directing element. Solenoid-actuated valves are particularly popular for industrial machines because of the ready availability of electric power in industrial plants. However, mobile equipment makes extensive use of solenoid-operated valves as well. The selection of ac or dc solenoids depends on the form of electrical power available. At one time, dc solenoids offered longer service life, but improvements in ac solenoid designs have eliminated that advantage.

There is a practical limit to the force that solenoids can generate. This means they cannot directly shift valves requiring high shifting forces. Furthermore, valves using large solenoids also consume substantial electrical power when valves must remain actuated for long intervals. Heat buildup can also pose problems in these situations. The solution is to use small, low-power solenoids in combination with pilot pressure. The solenoid starts and stops pilot flow, and pilot pressure provides the high force to shift the valve’s flow-directing mechanism (Fig 5).

8 Considerations for Selecting a Directional Control Valve

Directional control valve reliability is critical in any heavy industrial application. Selecting the right valve may be the most critical decision you’ll make regarding whether or not you will experience failure, downtime, or breakdowns. Proper valve selection can help avoid possible business interruptions, environmental catastrophes, or even loss of life.

Every heavy industry application and its operating environment are unique. To select the most reliable directional control valve, you must consider all the variables of your specific application. Based on more than 70 years of experience creating the industry’s most reliable valves, our engineers at Versa Valves have compiled the following considerations as part of our recommended valve selection process:

1. Application Environment – Consider all applicable environmental concerns, which among others include the presence of dust, dirt, and insects; weather and temperature; and the mobility and accessibility of the location the valve will be used in.

2. Media – Determine what media the valve will control, which could include compressed air, hydraulic oil, water, natural gas, or inert gases.

3. Operating and Available Pressure – What are the minimum and maximum pressure requirements? Will the valve be pilot-operated, manual, or solenoid-pilot? If the latter, will it be internally piloted (InPilot) or externally piloted (ExPilot)?

4. Flow/Port Size and Location – It is important to base the valve size on the flow factor (Cv) instead of the application. Different types of valves can have widely varying flow factors at the same port size. Port size is not the most accurate way to properly size a valve and often leads to oversized valves, additional material, unnecessary expense, and higher installation costs. Fully ported internal flow areas ensure maximum flow for the corresponding port (pipe) connection.

5. Function – Do you require a 2- or 3-position valve? For example, a 2-position valve is most commonly used to pressurize or depressurize a fluid power circuit. A 3-position valve performs the same function as a 2-position valve but adds a third function to an unactuated valve. The third function occurs when the directional control valve is in the center position. The most common center positions are all ports blocked (APB) for fail-last applications, cylinder ports open to exhaust, inlet open to both cylinder ports, and all ports open (APO).

6. Type of Operators – Know the required voltage, current, area classification, and ingress protection. Specify the type of required electrical connection. Flying leads, DIN (Hirschman style) connectors, and junction boxes are most common. Class F coil insulation is standard, while Class H is available for high-temperature applications. Power consumption is also significant. Low-power solenoid coils are available in 1.8W, 0.85W, and 0.5W.

7. Regulation or Speed Control – Consider if pressure regulators, speed controls, flow control valves, and/or check valves are necessary for your application.

8. Enhanced Functionality for Safety and Redundancy – Versa offers many options for enhancing reliability and safety. For added safety, consider the Latching/Manual reset and the Shut Off Valve (SOV). Latching manual reset valves are particularly suited for applications where it is desirable or mandatory to manually reset or restart a system. Shut Off Valves provide a series pilot control circuit in a single valve for processes with various control signals. The loss of either signal will cause the valve to shift to the unactuated position. To add redundancy, consider the Redundant Valve (RS). Redundant Valves provide parallel control circuits in a single valve to avoid process interruptions for maintenance or failure of a single control circuit. Both coils must be de-energized to shift to the unactuated position.

While it’s likely that not all of these considerations will be relevant to your particular application, working through the entire process makes sure no applicable ones are missed. Additionally, by following this process, you can ensure you are considering all the significant environmental factors necessary to select a durable, proven, reliable valve for your exact application.

Learn more about valve selection for heavy industrial applications by reading our full white paper, How to Ensure Directional Control Valve Reliability in Extreme Environments.

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