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Valves serve various functions such as starting or stopping flows, adjusting flow rates, changing the direction of flow, and controlling or relieving pressure. There are numerous types of valves, leading to various classification methods. Importantly, the term “hydraulic valve” denotes a specific category of valve designed for hydraulic fluid applications. In essence, a hydraulic valve is any valve that operates with hydraulic fluid.

History

Hydraulic systems have been in use in various forms since the 6th century BC, when the Mesopotamians and Egyptians utilized water power for irrigation. The application of hydraulics also appeared in the Hellenistic period, and in ancient Persia, China, Sri Lanka, and Rome. The modern era of hydraulics began in the early 1600s, thanks to the groundbreaking work of scientists like Benedetto Castelli and Blaise Pascal, with Pascal making significant contributions to the field.

Pascal’s law forms the foundation of hydraulic principles. It states that when pressure is applied to a confined liquid at any point, that pressure is transmitted equally throughout the liquid. Therefore, if the pressure increases at one point in the liquid, it will proportionally increase at all other points. This principle relies on the fact that liquids are incompressible and does not apply to liquids that are not confined within an enclosed space. By applying Pascal’s Law, engineers and scientists have developed systems that generate, control, and transfer power using pressurized fluids, significantly reducing the need for manual labor. For more detailed explanations of Pascal’s Law, refer to our resources on hydraulic pumps and hydraulic cylinders.

Hydraulic Valves – Fulflo Specialties, Inc.

Parts

Hydraulic valves typically consist of a main casing, a bonnet, a seat, and a disc. The main casing serves as the outer enclosure of the valve, housing all internal components collectively known as the trim. This casing is usually made from metals like steel, stainless steel, alloyed steel, cast iron, bronze, brass, or gunmetal (red brass), or plastics such as PVC, PVDF, PP, and glass-reinforced nylon.

The bonnet is a semi-permanent, removable component that covers the valve. To access the internal parts, the bonnet must be removed, although some valves, such as plug valves, are designed without a bonnet.

The seat is the internal surface of the casing that interfaces with the disc to form a leak-proof seal. This seat is generally equipped with a sealing material made of rubber or plastic.

Lastly, the disc, or valve member, is the component that moves into the seat to control flow and prevent leaks.

How They Work

Hydraulic valves are best understood in the context of entire hydraulic systems. A hydraulic power unit or power pack generates power hydraulically and typically includes a reservoir, a pump, hydraulic valves, and hydraulic actuators like motors or cylinders.

The role of hydraulic valves in a power pack is to manage the connection between the pump and the actuators, which convert hydraulic power into mechanical motion. Valves regulate the flow of hydraulic fluid to the actuators and can adjust this flow as needed.

Hydraulic valves usually have two main positions: open and closed. When a valve is open, fluid flows freely through it. When closed, the flow is restricted. Valves with a default open position are called open center valves, while those with a default closed position are known as closed center valves.

The operation of a valve depends on its internal components. Specifically, whether a valve is open or closed depends on the position of the internal disc relative to the seat. Many hydraulic valves, especially those for directional control, are referred to as spool valves because they resemble spools of thread. The flow of hydraulic fluid is controlled by the position of the “spool” within the valve housing. In many valves, the default or “neutral” position blocks fluid flow by having the spool in the center. To allow fluid flow, the spool moves to one side of the housing. Modern hydraulic valves also offer options for partial flow restriction.

Types

As mentioned in the introduction, hydraulic valves can be classified in various ways. Some methods focus on the physical characteristics or construction of the valve, while others consider how the valve is actuated or controlled. Additionally, valves can be categorized based on their specific application or function.

Classifications by Construction

One common way to classify hydraulic valves is by the number of ports. A port is an entry or exit point for hydraulic fluid. Most standard hydraulic valves have two ports: an inlet port for fluid coming from the pump and an outlet port for directing fluid to the actuators. However, valves can also have three, four, or multiple ports. Hydraulic manifolds, another classification, consist of several interconnected valves within a hydraulic system, categorized primarily by their physical structure.

Cartridge valves, also known as slip-in valves, logic valves, or 2/2-way valves, are classified based on their configuration. These valves screw into a threaded cavity and typically include a sleeve, a cone or poppet, and a spring. They operate by moving the cone or poppet aside when fluid pressure overcomes the spring, making cartridge valves popular due to their ease of installation.

Overall, hydraulic valves vary greatly in size and shape, ranging from less than an inch to over a foot long, with most fitting comfortably in the palm of a hand. This wide range of physical characteristics influences their diverse applications.

Classifications by Actuation/Control

Valves require a valve actuator to function properly. Although not a direct component of the valve, actuators are crucial as they move the internal mechanism of the valve to alter its state. Valve actuators can be either manual or automatic.

For instance, the hydraulic ball valve is an example of a manually operated valve. It features a spherical internal disk with a hole that can be quickly rotated 90° between open and closed positions using a handle. When open, the hole aligns with the fluid flow, allowing it to pass through. When closed, the hole is misaligned with the flow, blocking it.

Valve balls are typically made from materials like nickel, brass, stainless steel, or titanium, though some are made from plastics such as PVC, PP, ABS, or PVDF.

Manually operated hydraulic valves often require significant force to control high-pressure hydraulic fluids. To handle this, they may use oversized wheels, levers, or hydraulic rams.

In contrast, some hydraulic valves are electrically operated or controlled remotely via computer systems. Hydraulic solenoid valves are a prime example; they open and close in response to a magnetic field generated by an electrical current in a wire coil, converting electrical energy into mechanical motion. Electronically or remotely controlled hydraulic valves are commonly used in applications like construction sites, where they play a vital role in the operation of hydraulically-powered machinery.

Classifications by Application or Function

Mechanical valves are typically categorized based on their specific function with fluids, such as completely stopping flow or preventing backflow. Hydraulic valves, which are used in hydraulic systems, are similarly classified by their regulatory roles.

Control valves are designed to adjust or modify the volume and speed of fluid flow, capable of positioning between fully open and fully closed. These valves are further divided into pressure control valves and flow control valves, contrasting with simple on/off or shutoff valves that only start or stop fluid flow.

Directional control valves, often considered fundamental, control the direction of fluid flow. Standard hydraulic “spool” valves, which are used for directional control, have a few discrete positions. Check valves, or non-return valves, are a type of directional valve that allows fluid to flow in one direction only and blocks reverse flow, crucial in applications like wastewater management where separation of fluids is needed.

Proportional valves extend the functionality of directional control valves by not only controlling the flow direction but also adjusting the flow rate and speed, making them useful for more precise control.

Pressure hydraulic valves prevent over-pressurization in hydraulic systems by opening to release excess pressure and return fluid to the reservoir when a certain threshold is exceeded.

Other specialized valves can perform similar functions. For instance, hydraulic cartridge valves are used for directional, check, pressure, and flow control, while hydraulic needle valves, with their small ports and threaded plungers, regulate flow in confined spaces.

Accessories

Hydraulic valves often include several additional components such as springs, gaskets, and stems. Springs are used to shift the disc and control its repositioning. Common materials for springs are stainless steel, zinc-plated steel, and, for extremely high temperatures, Inconel X750. Gaskets, typically made from elastomers, are mechanical seals designed to prevent fluid leakage either from the valve or between its separate areas. A metal face seal is a specific type of gasket placed between two fittings in a sandwich-like configuration. Stems, while not always present as they may be integrated with the disc or handle, serve to transmit motion from the controlling device, such as the handle, through the bonnet to the disc.

Hydraulic valves can be connected to hydraulic systems using various mechanisms, including flanges (bolt or clamp), welds (butt or socket), union connections, and fittings (tube or compression).

Applications

Hydraulic valves are essential to the industrial world due to their crucial role in hydraulic systems. These systems offer energy solutions that are both simpler and safer than alternatives like electrical power, while maintaining high effectiveness. Hydraulic valves are vital because they facilitate the efficient movement of hydraulic fluid, which is the “lifeblood” of these power systems.

Hydraulic valves enable precise flow control across a wide range of applications. They are used in industries such as aerospace, automotive, chemical and laboratory, construction, cryogenic, fire and heating services, food processing, fuel and oil, gas and air, irrigation, medical, military, process control, refrigeration, and wastewater management.

Considerations

Choosing the right hydraulic valves for a specific application is essential and involves considering several important factors. One of the primary factors is selecting the appropriate flow coefficient (Cv). The Cv value indicates the valve’s capability to control fluid flow, ensuring that the correct amount of fluid passes through the system. Selecting a valve with the correct Cv value is crucial for optimal performance and helps prevent problems like pressure drops or flow restrictions.

Another key factor is the maximum temperature the hydraulic system will encounter. Hydraulic systems can generate significant heat during operation, so it is vital to choose valves that can withstand the maximum expected temperature to prevent performance issues or valve failure. High-temperature applications may require valves made of specialized materials or with advanced thermal resistance properties.

The intensity of the hydraulic application also affects valve selection. Applications like industrial or heavy-duty machinery demand robust and reliable valves that can withstand harsh conditions and heavy loads. Conversely, less demanding applications might allow for more cost-effective and lighter valves without compromising safety and efficiency.

Additionally, it’s important to consider the number and type of ports or directional stages needed for a hydraulic scenario. Different systems may require various valve configurations to effectively control fluid flow, pressure, and direction. Understanding the system’s needs and the specific roles each valve must perform is crucial for ensuring the system’s overall efficiency and functionality.

In summary, selecting hydraulic valves requires a detailed analysis of factors such as the appropriate flow coefficient, maximum temperature tolerance, application rigor, and the required ports or directional stages. A thorough understanding of the application’s demands and the properties and capabilities of different valve types is essential for making informed decisions that lead to a well-functioning and reliable hydraulic system. By considering these factors together, engineers and system designers can ensure optimal performance and longevity for their hydraulic applications.

Choosing the Proper Hydraulic Valve Supplier

To achieve the best results when purchasing hydraulic valves, it’s crucial to compare several suppliers using our comprehensive directory. Each supplier has a detailed business profile page that outlines their expertise and capabilities. You can also find a contact form to communicate directly with suppliers for more information or to request a quote. Utilize our patented website previewer to quickly assess each company’s specialties. Then, use our straightforward RFQ form to reach out to multiple hydraulic valve suppliers with a single request.