Everything You Need To Know To Find The Best hydraulic flow control valve
A Guide to Flow Control Valves Types and Use Cases
Flow control is an important aspect of hydraulic system designs as it is used to regulate speed. Various types of flow control valves and other devices can be used which enables the speed of an actuator to be controlled by regulating the hydraulic system's flow rate.
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Flow rate also determines rate of energy transfer at any given pressure. The two are related in that the actuator force multiplied by the distance through which it moves (stroke) equals the work done on the load. The energy transferred must also equal the work done. Actuator speed determines the rate of energy transfer (i.e., horsepower), and speed is thus a function of flow rate.
Directional control, on the other hand, does not deal primarily with energy control, but rather with directing the energy transfer system to the proper place in the system at the proper time. Directional control valves can be thought of as fluid switches that make the desired "contacts." That is, they direct the high-energy input stream to the actuator inlet and provide a return path for the lower-energy oil.
It is of little consequence to control the energy transfer of the system through pressure and flow controls if the flow stream does not arrive at the right place at the right time. Thus, a secondary function of directional control devices might be defined as the timing of cycle events. Because fluid flow often can be throttled in directional-control valves, some measure of flow rate or pressure control can also be achieved with them.
Different Types of Flow Measurement
Controlling flow of a hydraulic system does not necessarily mean regulating volume per unit of time from a valve. Flow rate can be specified three different ways, so it is important to be aware of how flow is to be specified or measured:
Because they control the quantity of fluid that flows through the valve per unit of time, the same control valves are used for all three types of flow rates.
Control of Flow Rate with Valves
There are eight types of flow control valves which are used most often in hydraulic circuits.
Orifices
A simple orifice in the line, shown in Figure 1 (a) below, is the most elementary method for controlling flow. (Note that this is also a basic pressure control device.) When used to control flow, the orifice is placed in series with the pump. An orifice can be a drilled hole in a fitting, in which case it is fixed; or it may be a calibrated needle valve, in which case it functions as a variable orifice, Figure 1 (b). Both types are non-compensated flow control devices.
Rotary Flow Dividers
Another technique for dividing one input flow into proportional, multiple-branch output flows is using a rotary flow divider. It consists of several hydraulic motors connected together mechanically by a common shaft. One input fluid stream is split into as many output streams as there are motor sections in the flow divider. Because all motor sections turn at the same speed, output stream flow rates are proportional and equal to the sum of displacements of all the motor sections. Rotary flow dividers can usually handle larger flows than flow divider valves.
The pressure drop across each motor section is relatively small because no energy is delivered to an external load, as is the usual case with a hydraulic motor. However, designers should be aware of pressure intensification generated by a rotary flow divider. If, for any reason, load pressure in one or more branches drops to some lower level or to zero, full differential pressure will be applied across the motor section in each particular branch. The sections thus pressurized will act as hydraulic motors and drive the remaining section(s) as a pump(s). This results in higher (intensified) pressure in these circuit branches.
When specifying rotary flow dividers, system designers must be careful to minimize the potential for pressure intensification. A pressure relief valve should be placed in any actuator fluid line where this condition may occur. Rotary flow dividers can also integrate multiple branch return flows into a single return flow.
Read more about flow dividers.
Proportional Flow-Control Valves
Proportional flow control valves combine state-of-the-art hydraulic valve actuation with modern, sophisticated electronic control. These valves help simplify hydraulic circuitry by reducing the number of components a system may require while, at the same time, substantially increasing system accuracy and efficiency.
An electronically controlled, proportional flow control valve modulates fluid flow in proportion to the input current it receives. The valves can easily control cylinders or smaller hydraulic motors in applications that require precise speed control or controlled acceleration or deceleration. Most proportional flow-control valves are pressure compensated to minimize flow variations caused by changes in inlet or outlet pressure.
An electrohydraulic proportional valve consists of three main elements:
- a pilot or proportional solenoid
- a metering area (where the valve spool is located), and
- an electronic position-feedback device, often an LVDT (linear variable differential transformer).
Valve operation begins when it receives a signal from an outside controlling device such as a computer, programmable logic controller (PLC), traditional logic relay, or potentiometer. The control device delivers analog electrical signals to the valve driver card, which, in turn, sends a current signal to the solenoid on the valve.
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The electromechanical force on the spool causes it to shift, gradually opening a flow path from the pump to the actuator port. The greater the command input signal, the greater the current to the valve solenoid, and, thus, the higher the flow from the valve. The important feature of this proportional valve is that all elements are proportional; thus, any change in input current changes force signals proportionately as well as the distance the valve spool will shift, the size of the flow path, the amount of fluid flowing through the valve, and finally the speed at which the actuator moves.
As the spool shifts, its motion is detected and monitored very accurately by an LVDT or other type of position-feedback transducer. This signal is fed back to the driver card where it is continuously compared with the input signals from the controller. If the two differ, the driver adjusts spool position until the two signals match.
Pressure-compensated proportional flow control valves are 2-port valves in which the main control orifice is adjusted electronically. Similar to conventional pressure-compensated flow control valves, a proportional version maintains constant flow output by keeping the pressure drop constant across the main control orifice. The proportional valve, however, is different in that the control orifice is modified to work in conjunction with a stroke controlled solenoid.
In a 2-port, pressure-compensated proportional flow control valve, an electrically adjustable control orifice is connected in series with a pressure reducing valve spool, known as a compensator, shown below in Figure 11. The compensator is located upstream of the main control orifice and is held open by a light spring. When there is no input signal to the solenoid, the light spring force holds the main control orifice closed. When the solenoid is energized, the solenoid pin acts directly on the control orifice, moving it downward against the spring to open the valve and allow oil to flow from port A to port B.
When an electrical signal is fed into an electronic amplifier, the solenoid and controller adjust the pilot pressure supplied from port A to change spool position. An LVDT then feeds back the position to the amplifier to maintain the desired orifice condition for flow from port A to port B. The proportional logic valve is available with either linear or progressive flow characteristics, and the valve drivers respond to voltage (0-10V DC) or current (0-20 mA) command signals. The typical valve amplifier card requires a 24V DC power supply.
Because the valve remains relatively unaffected by changes in system pressure, it can open and close the orifice in the same length of time. This maximum time can be changed on the amplifier card by adjusting a built-in ramp generator.
The amplifier can be used in several ways. An external electronic control can make the orifice remotely adjustable while maximum spool acceleration is still limited by this internal ramp; or a switch can be added to turn the ramp on and off. In case of power failure, the element will return to its normally closed position.
Determining the best type of flow control valve to use depends on the design parameters of the application. The table below provides general guidelines based on common application characteristics.
Which Flow Control Valve to use for an Application ApplicationType of Flow Control Valve Load on the actuator and supply pressure both are constant: ±5% accuracy Non-compensated, fixed or variable flow control, depending on application Load on the actuator, supply pressure, or both undergo changes: ±3-5% accuracy Pressure-compensated, fixed or variable flow control, depending on application Load on the actuator, supply pressure, or both change, and fluid temperature varies ±30° F (±17° C): ±3-5% accuracy Pressure- and temperature-compensated, fixed or variable flow controlFlow Control Valves: Which are The Best Types to Use? - MPC
There are countless types of valves for use across a variety of industries and applications. When it comes to flow control valves, valve types range from simple to sophisticated; some valves are complex enough to adjust automatically to pressure and temperature variations.
No matter their construction, flow control valves are designed to regulate the flow or pressure of fluids, and they typically react to signals generated by flow meters or temperature gauges.
Function of a Flow Control Valve
Flow control valves can serve a number of different functions within a hydraulic flow system depending on the specific type that is used. One of the most common uses of a flow control valve is to regulate the speed of motors or cylinders within the system.
This function is possible due to the capability of a flow control valve to affect the rate of energy transfer at any given point in a system by impacting the flow rate.
The ability to reduce or increase pressure in a system has a number of benefits. System operators can use a flow control valve to rapidly depressurize a serviceable hose and change fittings quickly.
They are also used in many consumer applications such as showers, faucets, and lawn watering systems to easily reduce the amount of water consumed without impacting the overall system performance. Flow control valves are also known for their reliability and typically have a long operating lifetime as they are not prone to clogging due to their design.
Due to these flexible performance parameters, flow control valves have found wide use in applications across materials handling, food processing, and automated factory and warehouse equipment.
Let’s take a look at the different types of flow control valves and their functions.
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1. Gate Valves
Gate valves are general service valves primarily used for on/off, non-throttling service. Specifically, gate valves are used in applications requiring a straight-line flow of fluid with minimum restriction is desired.
Gate valves operate when the user rotates the stem in a clockwise to close (CTC) motion or a clockwise to open (CTO) motion. The gate moves up or down on the threaded step when an operator moves the stem, which is why it is a multi-turn valve; the valve must turn several times for it to go from open to closed, and it is the slow operation that prevents water hammer effects.
Engineers also utilize gate valves when minimum pressure loss and a free bore are required. Typical gate valves have no obstruction in the flow path, which results in a minimal loss of pressure.
Gate valves may be used for several fluids. Generally, gate valves are applicable for potable water, wastewater, and neutral liquids; in temperatures between -20 and 70 degrees Celsius; maximum 5 meter/second flow velocity; and up to 16 bar differential pressure.
Gate valves also are applicable for gases with temperatures between -20 and 60 degrees Celsius; maximum 20 meter/second flow velocity; and up to 16 bar differential pressure.
There are two types of gate valves: parallel and wedge-shaped. Parallel gate valves feature a flat gate between two parallel seats. Wedge-shaped gate valves are comprised of two inclined seats and an inclined gate that is just a bit mismatched.
2. Globe Valves
A linear motion valve, globe valves stop, start, and regulate flow. Globe valves initiate closure via a plug featuring a flat or convex bottom that is lowered onto a horizontal seat situated in the center of the valve.
When a user opens the valve, the plug raises to allow fluid to flow. Globe valves are used for on/off and throttling applications because the disk of the valve can be removed from the flow path completely or it can completely close the flow path. While this type of flow control valve does produce slightly higher pressure drops than straight-through valves like gate, plug, and ball valves, they are applicable in situations where the pressure drop through the valve is not a controlling factor.
The practical size limit for globe valves is NPS 12 (DN 300) because the entire system pressure exerted on the disc transfers to the valve stem. It is possible, however, to have globe valves larger than NPS 12 (DN 300), and manufacturers and engineers have created and used globe valves up to NPS 48 (DN ).
3. Pinch Valves
A cost-effective flow control valve, pinch valves are ideal for applications of slurries or liquids containing significant amounts of suspended solids. Pinch valves seal using one or more flexible elements like rubber tubes that become pinched to turn off the flow. These rubber sleeves are the valve’s only wetted part, and their flexibility allows pinch valves to close tightly around entrapped solids.
Air or hydraulic pressure is placed directly on the elastomer sleeve to actuate pinch valves. A pinch valve’s body acts as a built-in actuator, which eliminates expensive hydraulic, pneumatic, or electric operators and results in the cost-effectiveness of this type of flow control valve.
4. Diaphragm Valves
Diaphragm valves are characterized by a flexible disc that contacts a seat at the top of the valve body and forms a seal. The diaphragm is flexible and pressure-responsive; it transmits force to open, close, or control a valve.
While diaphragm valves are related to pinch valves, they use an elastomeric diaphragm rather than an elastomeric liner in the valve body. The elastomeric diaphragm is attached to a compressor and separates the flow stream from the closure element.
Diaphragm valves are ideal for handling corrosive, erosive, and dirty services.
There are many advantages to using diaphragm valves: they are extremely clean, feature a leak-proof seal, have a tight shut-off, are easy to maintain, and reduce leakage to the environment. Diaphragm valves also may be repaired without interrupting a pipeline.
On the other hand, the disadvantages of using diaphragm valves include only being able to use them in moderate temperatures of -60 to 450 degrees Fahrenheit and in moderate pressures of approximately 300psi.
Diaphragm valves cannot be used in multi-turn operations and do not have industry standard face-to-face dimensions. Also, the body of a diaphragm valve must be made of corrosive-resistant materials.
5. Needle Valves
Needle valves are volume control valves that restrict flow in small lines. Fluid moving through the valve turns 90 degrees and flows through an orifice that serves as the seat for a cone-shape-tipped rod.
The orifice size changes when the user positions the cone in relation to the seat. Needle valves are similar to globe valves in that they share a few design features and have similar benefits; for example, both needle valves and globe valves empower operators to change flow rate using a threaded rotating stem.
The difference between needle valves and globe valves is the precision that needle valves can achieve. In fact, needle valves are an ideal choice for calibration applications because they are capable of being fine-tuned.
Needle valves can provide positive shutoff in order to allow gauges and other measurement instruments to be installed or removed safely. That’s also why needle valves may be used in a range of industries, from petrochemicals to biofuels.
It is the needle valve’s finely-threaded valve stem that gives it a significant mechanical advantage by allowing operators to seal it using only minimal force. One disadvantage of needle valves, however, is that the visual inspection alone is not enough to determine whether a needle valve is open or closed.
Flow control valves are necessary components in a broad range of industries. Determining which flow control valve type is best for your particular situation depends on a host of criteria, but the most commonly used types include gate valves, globe valves, pinch valves, diaphragm valves, and needle valves.
Other Types of Flow Control Valves
While the five types of flow control valves described above are some of the most commonly used valve types, there are other types of flow control valves with features that make them suitable for different applications. Here’s a look at a few other types of flow control valves.
Butterfly Valves
A butterfly valve is operated by rotating a disk within the flow area and, due to this design, it does not have linear flow characteristics. This makes these valves less precise than the more common flow control valve types above. For this reason, it can often be dismissed as a flow control valve choice even though it is useful in some applications that do not require a very high degree of accuracy.
They are also a very affordable valve option, which makes it worthwhile to consider them in the right applications.
Plug Valves
Plug valves come in a variety of configurations and are operated by rotating a cylindrical or cone-shaped plug within the valve body to regulate the flow through a hollow area of the plug. For flow control applications the most common design is an eccentric plug valve, which uses a half plug to create a higher seating force with minimal friction as it is opened and closed.
This has the advantage of greater shut off capability which is ideal for flow control situations.
Ball Valves
Ball valves are commonly used in flow systems across numerous industries due to their low cost, durability, and excellent shutoff capability. Similar to butterfly valves, they are not as effective for flow control applications that require a high degree of accuracy and control.
One of the reasons for this is that a ball valve requires a high degree of torque to open and close that prevents an operator from making fine adjustments. There is also a certain amount of “play” between the stem and the ball which can make finding specific flow rates difficult.
For flow control applications where a ball valve is possible, such as filling a tank to a reasonable degree of accuracy, a trunnion or v-port ball valve design is usually the best choice.
Final Thoughts
Flow control valves are used in a variety of applications, such as plumbing, mechanical, and gas dispensing applications. There are many factors to consider when choosing the appropriate flow control valve for an application, such as the characteristics of the fluid, service conditions, how frequently the valve is operated, and maintenance and environmental considerations.
With a variety of valve types available, comparing the function and performance of various valves alongside your application specifications will help you identify the most suitable flow control valve for your application.
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