Flow Control Valve vs Directional Control Valve

A flow control valve regulates the rate of the fluid that moves through a circuit, regulating the speed of an actuator by limiting or measuring flow. The directional valve controls the speed at which fluid moves through a circuit. Path fluid, and controlling the way the actuator moves (extend the actuator, retract it, rotate it, or stop it) through the opening and closing of various flow passages. In short, flow control valves control the speed of the flow, and directional control valves determine the direction. The majority of hydraulic circuits utilize both, as neither type of valve can fulfill the same function as the other.

Why is this important?

Maintenance and design engineers often face confusion between these two types of valves in particular, as they both are integral to nearly every circuit powered by fluids and are usually placed close to each other in a manifold or a valve stack. Making the wrong choice of valve or not understanding how they interact could cause circuits to move in the correct direction but at a different speed, or to systems that control speed properly but are unable to stop or reverse at the time necessary. Understanding the distinct functions for each type of valve can be essential for correct circuit design, troubleshooting, diagnosis, and replacement.

How do you define a flow valve?

A flow valve controls the amount of hydraulic fluid that flows through a certain point of the circuit over time. It is expressed in terms of liters per minute (LPM) or gallons per minute (GPM). Because actuator speed is directly proportional to flow speed, they are the most important device to regulate the speed at which an actuator extends or the speed at which an engine rotates.

What is the function of flow control valves?

Most flow control valves employ an orifice that is variable, an adjustable restriction that allows fluid to traverse. The narrowing of the orifice slows down the flow of fluid and slows down the actuator. However, widening it boosts flows and accelerates the speed of the actuator. The restriction can be manual (a needle valve adjusted by hand), pressure-compensated (maintaining constant flow despite load pressure changes), or temperature-compensated (adjusting for viscosity shifts as fluid heats up).

Common kinds

• Needle valves - easy manually adjustable restrictors to regulate basic speed
• Pressure-compensated flow valves - keep the flow rate constant regardless of the pressure changes caused by loads, essential for applications that require steady flow rate under varying loads
• Pressure-compensated and temperature-controlled valves provide viscosity correction to ensure precision applications over a wide range of operating temperatures
• Dividers of flow—divide an individual flowing source of flow into two or three outputs that are proportioned, ideal for synchronizing multiple actuators

When flow control valves are utilized

The flow control valves can be crucial where speed control is required, such as feed rate control for hydraulic presses or lower speed of the boom for excavators. They also have synchronized cylinder movements on lifts and metering circuits for injector molding equipment. They are typically installed in meters-in (restricting flow to the actuator) and meters-out (restricting flow out of the actuator) or bleeding off (diverting the excess flow into tanks) models, each with different efficiency and stability tradeoffs.

What is a directionally controlled valve?

A directional control valve (DCV) controls the direction that the fluid follows through the circuit and routes it to various ports to regulate actuator movement. Instead of controlling the amount of fluid that flows through the circuit, it controls the flow of fluid. DCV regulates the direction it is flowing, allowing forward and reverse motion, retaining its place, or even stopping completely.

How do directional valves function?

DCVs make use of internal spools, poppets, or rotary elements that change their position to allow or block certain flow pathways. A typical spool valve is sliding spools with grooves and lands; when the spool is moved, it will let different combinations of the outlets, inlet, and tank ports be connected to or disconnected. Valves are classified according to their ports (2-way, 3-way, 4-way) and also the number of positions for spools (2-position, 3-position), typically described in shorthand as "4/3" (four ports, three positions).

Common kinds

• Manually operated DCVs are shifted using the pedal or lever used in mobile devices
• Solenoid-powered DCVs that are electrically actuated to control and integrate PLC
• Pilot-operated DCVs utilize the hydraulic pressure of a pilot to move larger spools. They are used in high-flow industrial equipment
• Check valves - a streamlined control element for directional flow that permits flow only in one direction
• Directional valves—proportional and servo—can provide an electronically controlled, variable spool position for a precise motion profile that bridges some directions and metering capabilities.

When directional control valves are employed, directional control valves

DCVs form the basis of almost every hydraulic circuit. They are responsible for reversing strokes of cylinders on presses, changing the direction of motor rotation on winches, and controlling multi-actuator systems in mobile equipment as well as providing fail-safe or emergency stop positions for center spool designs (open centers, closed centers, tandem centers, and floating centers).

Direct comparison

What are the reasons why they both are essential

A circuit for hydraulics that has only a directional valve can move an actuator in a direction that is backwards and forwards but provides no precise control over the speed at which this motion occurs, and the speed will depend on the pump's output and resistance to load, something that is not appropriate for precise applications. A circuit constructed with just a flow control valve will regulate speed but does not have any way of reverse movement or returning fluid to the tank in a controlled manner. This is the reason why the majority of hydraulic systems, ranging from mobile and industrial presses, combine both types of valves into one circuit, usually within the same manifold or valve bank, with each valve performing its own specific job.

Considerations for selection

When determining the type of valve, engineers must take into consideration the following:

• Capacity of flow - matching valve size to flow rate to prevent an excessive drop in pressure or cavitation
• Pressure rating: ensuring that the valve's nominal working pressure is greater than the the pressures of the system operating and spike pressures
• Method of operation - selecting either a solenoid or a manual pilot action based on automation needs and access restrictions
• Needs for compensation for temperature and pressure compensation for flow control valves operating in variable load or variable temperature conditions
• Spool configuration center condition (open closed, closed or tandem, float) for directionally valves is determined by how the circuit will behave when it is centered
• Response time is critical in applications that require rapid cycle times or precision motion profiles

Selecting the appropriate mix and layout directly impacts the efficiency of the system, cycle speed reliability, component wear, and overall reliability of the circuit.

1. Can a directional valve also control the flow rate?

The standard directional control valves are intended to channel the flow of fluid, not measure it precisely. However, proportional and servo-directional valves are able to alter the spool's opening in order to affect flow rate in a small degree, mixing metering and directional functions, but they can't replace flow control valves for the majority of precise applications.

2. What happens when the flow control valve malfunctions in the hydraulic circuit?

A malfunctioning flow control valve is usually the cause of those actuators running too quickly or slowly or in an erratic manner because the flow is not getting a proper amount of flow. This could cause instability in the control system, excess heat production, or damage to components due to sudden or uncontrolled movements.

3. Are controlled directional valves rated according to the flow of air or by pressure?

Control valves for direction are rated with regard to the maximum flow capacity (to ensure that there isn't a huge drop in pressure through the valve) and the maximum working pressure (to make sure the bodies of the valve and their spools can withstand the pressures of the system without failing).

4. Are flow control valves employed to shut down an actuator totally?

A flow control valve may be completely shut off in order to shut off flow. This will prevent actuator movement, but this isn't the intended use, and it doesn't offer the secure positive shutoff or directional switch that a direction valve can provide. When used this way, it can result in pressure spikes and shouldn't be recommended for regular stop-start cycles.

5. What valve should be considered first in the design of a hydraulic circuit?

The selection of the directional control valve usually comes first, as it determines the fundamental circuit's architecture (how many actuators there are and what the possible directions for motion as well as the centering behavior are). Flow control valves are determined and placed in order to fulfill the speed specifications that are specific to the specific directional framework.