Flow Control Valve vs Directional Control Valve

A flow control valve controls the speed that fluids are moving through a circuit, controlling the speed of an actuator, whereas a directional control valve decides what path the fluid takes to determine the direction an actuator is moving (or whether it is moving in any way). Flow control valves determine "how fast"; directional control valves determine "where to." The majority of hydraulic circuits utilize both directional valves to route flows to the appropriate port of the actuator as well as flow control valves that meter the flow for smooth and controlled movement.

The confusion between the two valve types is a common error made by engineers and technicians less experienced in the design of a fluid power system. Both are crucial; however, they tackle completely different problems. This guide will explain the functions of each valve in the context of where they're employed and how to choose the one your circuit requires—or if you'll need both.

What is a directionally controlled valve?

The directional valve (DCV) regulates the path of hydraulic fluid in the circuit. It decides if the fluid flows towards the rod end or cap end of a cylinder. It controls a motor clockwise, counterclockwise, or is shut off completely so that an actuator is able to hold the position.

What is the function of directional control valves?

DCVs are usually classified according to:

• A variety of positions: 2-position, 3-position (most popular extend retract, extend and neutral)
• Ports available—3-way, 2-way, 4-way, or 5-way configurations
• Method of operation—a lever that is manual or solenoid, pilot-operated, or hydraulic pilot
• Condition of center (for 3-position valves)—the open or closed center, tandem center, or floating center. Each of these alters how the actuator and pump behave in the event that the valve becomes in a centered position.

A standard 3-position, 4-way solenoid DCV is the heart of all industrial hydraulic systems. It is that which is used to reverse the cylinder's or motor's direction at request.

Common applications

• Cylinder extend/retract controls on presses and lifts, as well as equipment for handling materials
• Reversal of motor direction on augers, winches and conveyors
• Safety blocking and circuit isolation in stationary and mobile equipment

Which flow control valve is it?

A flow control valve controls the flow rate of volumetric fluid that directly controls the speed of actuators. Because the speed of cylinder extension and motor RPM are both aspects that are governed by flow rates (not pressure), these valves serve as a primary instrument for controlling speed in the hydraulic system.

What is the function of flow control valves?

Control valves for flow typically are classified into three types:

• The non-compounded (needle) valves can be a simple adjustment orifice. The flow rate is dependent on the an increase in pressure across the valve and so the rate of flow changes with loads
• Pressure-compensated valves keep a constant flow regardless of pressure fluctuations caused by load with an internal compensator.
• Pressure-compensated valves and temperature sensors Add correction for viscosity fluctuations as the fluid temperature fluctuates, which is helpful in systems with large temperature swings. temperature fluctuations.

Flow control valves can also be set up in the form of meter-in (restricting flow to the actuator), meter-out (restricting flow out of the actuator), or bleeding off (diverting the excess flow of the pump to a tank) dependent on the characteristics of the load and the accuracy of control required.

Common applications

• Control of the speed of the cylinder on presses, feed systems and equipment for positioning
• Motor RPM control on the rotary actuators as well as hydraulic fans
• Regenerative circuits that control deceleration and stops load overruns

Control valve for flow and directionally controlled valve: side-by-side comparison

Why do circuits require two valve types?

In the real world the flow control and directional valves are not used in isolation. The typical circuit for cylinders may employ a 4-way DCV to control the length of extend or retract, coupled with a meter-out flow valve located on the rod's end line to stop the cylinder from running over under the pressure of a negative load, for example, an inclining platform or a hydraulic press ram with a gravity assist.

When you remove either valve type, it produces a specific failure condition:

• Without direction control, the actuator is able to be moved in a pre-planned direction, which makes the circuit ineffective for anything other than the single-purpose activation.
• If flow is not controlled, the speed of actuators is entirely dependent on the output of the pump along with load and operating conditions, rendering exact, repeatable, or even safe motion control difficult and especially dangerous in circuits that have overrunning loads.

The right valve to match your circuit

When determining the valves to be used in an upgrade or new circuit, take into account the following:

1. Does the actuator have to stop or reverse direction? If so, you'll require a directional control valve; the only thing to consider is the position number, port configuration, and type of actuation.
2. Do you think that the speed of actuators should be controlled without regard to load? If so, a pressurized flow valve can be worth the additional cost over a standard needle valve.
3. Is the load running out of time or resistive? Overrunning loads (lowering the weight or retracting under gravity) generally require meter-out flow controls to avoid losses of pressure and cavitation. Resistive loads can be more accommodating of the meter-in settings.
4. What's the duty cycle in terms of temperature range? Wide ambient swings justify temperature-compensated flow valves; steady indoor operation usually doesn't.
5. What is the appropriate pressure drop? Flow control valves naturally cause a drop in pressure proportional to flow restriction. This is manifested as heat generation, a crucial aspect in thermal management and selection of fluids.

Flow control valves as well as directional valves resolve two distinct engineering issues they solve: one controls speed, and the other handles path. They are not able to replace the other, and the majority of functional hydraulic circuits rely on both of them working together. Understanding this distinction is crucial not only for proper valve selection but also for diagnosing issues with the circuit when a machine operating at a wrong speed points at the directional valve or its actuation signal. On the other hand, an engine operating at an incorrect speed indicates a problem with the inflow control valve or the compensation settings.

Can directional control valves regulate the flow rate?

Not precisely. Certain DCVs have proportional or servo versions that regulate flow via partial spool shifts, providing some speed control; however, they aren't as precise or load-independent as a flow control valve that is pressure compensated.

What valve type regulates the speed of a cylinder, direction, or flow?

Control valves for flow regulate the speed of the cylinder by measuring the volume of fluid flowing into or leaving the cylinder. Control valves that direct flow only determine which ports receive flow, but not the quantity.

Why do flow valves have a higher price than needle valves?

They are equipped with an internal compensator system that automatically adjusts the metering aperture to ensure that the flow remains steady despite changes in pressure, making the process more complex but offering consistent speed regardless of different conditions of load.

What happens when the spool of a directional control valve is stuck in the hydraulic circuit?

A stuck spool could leave the actuator operating continuously in a single direction or stop it from ever moving and, in the worst case, result in a rise in pressure when the valve is unable to switch back to neutral—all of these are potentially dangerous malfunctions that need to be identified immediately.

Do you prefer metering-in or metering-out? flow control more appropriate for hydraulic presses?

It is dependent on the direction of the load The most common use of a meter-out is when the load could exceed the flow of the pump (such as an inclining press ram) because it keeps back-pressure on the pump and stops uncontrolled acceleration. Meter-in is suitable for resistive load-opposing applications.