Hydraulic systems have existed in some way or another since the sixth century BC, when the Mesopotamians and Egyptians used water power for irrigation. Use of hydraulics was also seen in the Hellenistic age and in ancient Persia, China, Sri Lanka, and Rome. The modern age of hydraulics began in the early 1600s, with the innovations of scientists like Benedetto Castelli and Blaise Pascal. Pascal, in particular, played a pioneering role in the field of modern hydraulics.
Pascal’s law summarizes the basis upon which the principles of hydraulics are founded. In essence, this law states that when pressure is placed on any point of a confined liquid, such pressure will transmit equally to all other parts of the confined liquid. Correspondingly, if pressure increases at any point in a confined liquid, equal and proportional increases will appear at all other points in the confined liquid. It is important to note that Pascal’s Law is made possible by the fact that liquid is incompressible. It is equally important to note that is does not apply to liquids which are not confined in some type of enclosed area. Using this principle, engineers and scientists have successfully designed systems that generate, control, and transfer power via pressurized fluids, eliminating much need for manual human effort. (Fuller explanations of Pascal’s Law can be found at treatments of other hydraulic parts, including our sites on hydraulic pumps and hydraulic cylinders).
Most hydraulic valves minimally consist of a main casing, a bonnet, a seat, and a disc. The main casing is the valve’s outer enclosure; it contains all the internal components, which are collectively called the trim. Most often, the casing is made from a metallic or plastic material. Common metallic materials include steel, stainless steel, alloyed steel, cast iron, bronze, brass, and gunmetal (red brass), while among the most common plastic options are PVC, PVDF, PP, and glass-reinforced nylon.
The bonnet is a semi-permanent, removable part of the valve that acts as a cover. For access to interior parts of the valve, the bonnet needs to be removed. Some valves do not have a bonnet because of the way they are constructed. (One example of such a valve is a plug valve.)
The term seat refers to the interior surface of the casing, which connects to the disc in order to create a leak-proof seal. The seat typically possesses sealing made of either rubber or plastic.
Finally, the disc (also called a valve member) is the part of the valve that slides into the seat to restrict flow and prevent leaking.
How They Work
Hydraulic valves can only be properly understood within the context of entire hydraulic systems. An entire unit that generates power hydraulically is known as a hydraulic power pack or a hydraulic power unit. Such packs or units typically consist of a reservoir, a pump, hydraulic valves, and hydraulic actuators such as motors or cylinders.
The purpose of hydraulic valves within a hydraulic power pack is to connect the power source (i.e., the pump) to the actuators which translate hydraulic power into mechanical motion (i.e. hydraulic cylinders, hydraulic motors). Through its valves, a hydraulic power system can supply its actuators with hydraulic fluid and modify the flow of such fluid as needed.
While functional, valves generally have at least two settings: open and closed. Generally speaking, fluid may flow freely through the valve if it is open. Conversely, fluid flow is restricted if the valve is closed. Valves with a default status of open are also known as open center valves, while valves with a default status of closed are known as closed center valves.
Valves are either open or closed based on the positioning of their interior pieces; more specifically, a valve’s status depends on whether or not the disc is inside the seat. Hydraulic valves (and especially ones used for directional control) are often referred to as spool valves since they visually resemble spools of thread (by containing interior trim within exterior housing). The flow of hydraulic fluid (or lack thereof) is dependent on the position of the interior “spool” portion of the valve within the exterior housing. The default or “neutral” position of many valves has the spool in a central position which blocks the flow of hydraulic fluid. In order to open the valve and let fluid through, the spool simply slides to one side of the housing and away from the neutral position. Nowadays, many hydraulic valves also allow for partial flow obstruction.
As alluded to in the introduction, hydraulic valves can be categorized in several different ways. Some methods of categorization emphasize a valve’s physical characteristics or construction. Other methods emphasize a valve’s method of actuation or control. Still other categorization methods classify hydraulic valves according to their specific application or function.
Classifications by Construction
A common way to label hydraulic valves is by its number of ports. The term port simply refers to an avenue that hydraulic fluid can use to flow into or out of a valve. Standard hydraulic valves are double port, since they possess both an inlet port (to draw in fluid from the pump) and an outlet port (to pass fluid on to the actuators). However, hydraulic valves can also be three-port, four-port, or multi-port. Hydraulic manifolds are another type of valve which is classified primarily on the basis of physical characteristics. Such mechanisms are actually separate hydraulic valves that are connected to one another within hydraulic systems.
Hydraulic cartridge valves (also known as slip in valves, logic valves, or 2/2-way valves) are some of the more popular valves which derive their classification from their configuration. These valves are screwed into a threaded cavity and are typically composed of only a sleeve, a cone or poppet, and a spring. They open when incoming fluid pushes the holding cone or poppet (held in place by the spring) aside. The ease of installing cartridge valves makes them very popular in the hydraulic world.
Overall, hydraulic valves vary widely in physical shape and size. They can range in size from less than an inch to a foot long. On average, they can fit in the palm of a hand. The broad physical variety that characterizes hydraulic valves directly affects their differing uses.
Classifications by Actuation/Control
It is important to note that valves can only function properly with some type of valve actuator. While not strictly a part of the valve itself, valve actuators are important since they are responsible for actually moving the machinery within a valve to change its status. Valve actuators can be either manual or automatic.
An example of a manually operated valve is the hydraulic ball valve. This valve derives its name from a spherical, internal disk containing a hole and is activated with a handle that can be quickly rotated 90° between opened and closed positions. When the valve is open, the hole in the ball disk lines up with the direction of fluid flow and allows fluid to pass through. When the valve is closed, the hole is not lined up with the fluid flow, thus blocking the flow of fluid.
Valve balls are perforated and most often made of nickel, brass, stainless steel, or titanium. (Sometimes, they are composed of a plastic, like PVC, PP, ABS, or PVDF.)
Many manually operated hydraulic valves typically require high amounts of force in order to successfully stop high-pressure flows of hydraulic fluid. Thus, many manual hydraulic valves other than ball valves are operated by oversized wheels, levers, and even hydraulic rams.
Other hydraulic valves are electrically operated, and/or guided remotely with computer controls. Hydraulic solenoid valves are an excellent example of such valves. They open and close based on the charge of a magnetic field that pushes on a plunger. The magnetic field is signaled by a current, which is received by a wire coil when the solenoid converts electrical energy into mechanical energy. Other types of electronically or remotely controlled hydraulic valves can be found in places such as construction sites, where they are critical to the operation of many hydraulically-powered construction machines.
Classifications by Application or Function
Overall, mechanical valves in general are often classified by the exact function they are designed to exact on a fluid (e.g. completely cutting off a flow, preventing backflow, etc.). Since hydraulic valves are essentially general types of valves expressly applied in hydraulic scenarios, hydraulic valves are often also classified according to their exact regulatory function.
Control valves are valves specifically designed to control or modify the amount and speed of a fluid flow. These types of valves are particularly capable of occupying a spectrum of positions between fully open and fully closed. They are sometimes further classified as pressure control valves and flow control valves. (Control valves contrast with simple on/off valves or shutoff valves, which are designed to completely stop or start a fluid flow rather than simply modifying it.)
Directional control valves (or simply directional valves) may arguably be the “basic” type of mechanical valve, since their purpose is to control or modify the direction (rather than the amount) of fluid flow. On average, many standard hydraulic “spool” valves are used expressly for directional control and occupy a few discrete positions. Check valves (or non-return valves) are specific types of directional control valves that are used to force fluid flow in one direction only; if fluid within a hydraulic system somehow begins flowing in an undesirable direction, the check valve will close and block the flow. Check valves are critical to hydraulic systems in environments where substances of varying compositions and pressures must be kept separated (such as wastewater management plants).
Proportional valves can be considered as “extensions” of directional control valves. In addition to modifying flow direction, these type of valves can occupy intermediate positions and carry an output flow that is unequal to the input flow. In other words, proportional valves are designed to control the speed as well as the direction of fluid flow. (From this perspective, they can also be considered as extensions of control valves, which are designed to control the speed and amount of fluid flow.)
Pressure relief valves (or simply relief valves) are primarily designed to keep hydraulic systems from over-pressurizing. Instead of closing when undesirable conditions are met (such as check valves), these types of valves open in order to draw hydraulic fluid back into the reservoir when internal pressure has exceeded a certain point (e.g. due to a blocked pipe in the system).
It should be noted that differing functions accomplished by the aforementioned valves can also be performed by other, more specific types of valves. For example, hydraulic cartridge valves are often used for directional or check control as well as pressure or flow control. Beyond these few examples, there are many other unique valve types with individual functions. Hydraulic needle valves, for instance, are composed of small ports and threaded plungers. Their unique shape allows them to regulate flow in extremely tight spaces.
Other components associated with hydraulic valves include springs, gaskets, and stems. Those valves that include springs do so in order to shift the disc and control repositioning. Common spring materials include stainless steel, zinc-plated steel and, for work with exceptionally high temperatures, Inconel X750. Gaskets are mechanical seals, usually made from an elastomer. Their purpose is to prevent leakage of fluids from the valve or in between separate areas of the valve. The term metal face seal refers to a gasket that is located between two fittings in a sandwich-like arrangement. Stems are not always present, because they are often combined with the disc or handle. However, when present, they transmit motion from the controlling device, like the handle, through the bonnet and to the disc.
Hydraulic valves can be connected to hydraulic systems with a variety of different mechanisms. Some of these mechanisms include flanges (bolt or clamp), welds (butt or socket), union connections, and fittings (tube or compressions).
The value of hydraulic valves to the industrial world is inextricably bound up in the value of hydraulic systems as a whole. Overall, hydraulic power systems offer energy sources that are simpler and safer than other types (such as electrical power systems) while still being incredibly effective. Hydraulic valves are thus valued and widespread because they enable effective movement of hydraulic fluid, which forms the “lifeblood” of hydraulic power systems.
Hydraulic valves make flow control possible for many, many applications, including those in the 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 industries.
Since hydraulic valves vary so widely, it can be difficult determining the correct valve for a specific application. The below points offer a brief sketch of various factors to keep in mind during the determination process.
• What type of flow coefficient is best for this application? A hydraulic valve’s flow coefficient is a combined measure that indicates the amount of energy that is lost by fluid as it flows through or across the valve. Different valves possess differing coefficients, and similar valves can diverge in flow coefficients if their diameter (often measured in inches) is different. Generally speaking, higher flow coefficients indicate lower drop pressures that occur across the valve (if the flow rate remains the same). Determining the proper flow coefficient is one of the best methods to determine the proper valve to use in a given scenario. For example, a valve with a low head loss (one of the combined measures that makes up a flow coefficient) to conserve energy is best in a scenario where the valve will be normally open rather than closed. Hydraulic cartridge valves are popular in scenarios where energy needs to be conserved, since they cause far less energy and/or pressure loss than other types of valves.
• What is the maximum temperature you will reach in a given hydraulic scenario? Different types of hydraulic valves are designed to handle different maximum temperatures. You will want to investigate the maximum temperatures reached during your hydraulic operations and select hydraulic valves accordingly.
• How rigorous will my hydraulic application be? Some valves are better than others for high-intensity hydraulic situations. For example, valve balls are a common feature in hydraulic valves that are made for high pressure, high tolerance, and/or severe duty applications.
• How many ports or directional stages does my hydraulic scenario require? Although many times a standard double-port hydraulic valve will work, there may be times when a multi-port hydraulic valve is preferable.
Not all of the decisions that go into hydraulic valve selection need to be made alone. Investing in a quality hydraulic parts manufacturer or supplier is well worth the cost the majority of the time. Some characteristics of hydraulic parts suppliers to look for include:
• Accreditation. A company that possesses an accreditation such as the ISO 9001 is a good bet.
• Adherence to hydraulic industry standards. This characteristic will be closely tied to a company’s level of accreditation. Some specific ISO standards to look for include ISO 6403 (flow and pressure valves), ISO 6263 (hydraulic fluid power and mounting surfaces), SAE J748 (hydraulic directional control valves), and SAE J1235 (standards for reporting the hydraulic valve leakage)
• Depth of experience/expertise. Sometimes, a supplier may regularly offer only a small selection of hydraulic valves. However, a supplier’s level of industry expertise may be able to offset this reality and provide you with customized hydraulic valves as needed.
• Turnaround time. All types of industrial breakdowns are undesirable, but the failure of power systems (hydraulic and otherwise) is particularly undesirable. If your hydraulic power system fails due to valve trouble, you want to be sure you are working with a supplier that can advise on and provide needed replacements in record time.