To work, valves generally have at least two settings: opened and closed. Basically, if the valve is open, fluid may flow freely through it and if the valve is closed, flow is restricted. Many hydraulic valves also allow for partial flow obstruction. This straightforward operation makes 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.
Most hydraulic valves consist of, at least, a main casing, a bonnet, a seat, and a disc.
The main casing, also called the body, is the valve’s outer enclosure; it contains all the internal components, which are collectively called the trim. Usually, 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. To access interior parts, one may remove the bonnet. Note that some valves, such as plug valves, do not have a bonnet, because of the way they are constructed.
Next, the term “seat” refers to the interior surface of the casing, which connects to the disc in order to create a leak-proof seal.
Finally, the disc, also called a valve member, is that part of the valve that restricts flow and prevents leaking at the seat.
Other components associated with hydraulic valves include springs, gaskets, stems, and valve balls.
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.
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.
Valve balls are a common feature in hydraulic valves that are made for high pressure, high tolerance, and/or severe duty applications. They are perforated and pivoting, and allow for fluid flow when their hole is in line with said flow. Most often, they are made of nickel, brass, stainless steel, titanium, Stellite, or Hastelloy. Sometimes, they’re composed of a plastic, like PVC, PP, ABS or PVDF.
While they all operate using the same basic principles, the way that they go about using those principles may look very different from one valve to another. For example, some valves are manually operated with a handle. One example of these is the hydraulic ball valve, which features a handle that can be quickly rotated 90° between opened and closed positions.
Many others are electrically operated, and/or guided remotely with computer controls. Hydraulic solenoid valves are an excellent example of this; 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.
Beyond this, there are many other unique valve types with individual functions. Hydraulic needle valves, for instance, are composed of small ports and threaded plungers; this shape allows them to regulate flow in tight spaces.
Other valve types include proportional valves, which carry an output flow that is unequal to the input flow, hydraulic manifolds, which are made up different valves that are connected to one another and check valves, cartridge valves, control valves, directional control valves, and relief valves.
Hydraulic valves also vary in shape and size, which affects their uses. In all, hydraulic valves range in size from less than an inch to a foot long. On average, they can fit in the palm of a hand.