Hydraulic Needle Valves
Hydraulic valves generally consist of the main casing, the outer enclosure that holds all the other components, the disc, which is the actual seal and the seat, an interior part that contacts the disc to form a leak-tight seal. Needle valves are usually manually operated and have a handwheel or other handle for ease of use. They range in size and are identified by the size of the opening that allows fluid to pass through, which is typically a fraction of an inch. Standard sizes are measured by eighths of inches. Common materials include metals such as brass, bronze,
copper, cast iron and stainless steel. They are available in plastic or composite materials but these are much less widespread then the metal versions. Unlike many
hydraulics valves such as
hydraulic solenoid valves, needle valves are not used for simple shutoff applications nor are they able to provide directional control. They are utilized as bleed valves for instrumentation devices to vent the pressure of a line before the instrument is removed in addition to calibrating control devices.
Hydraulic needle valves are a kind of
control valve. They usually have small holes or openings with a long tapered seat. The plunger is shaped like a needle and is attached to the end of a screw that fits the seat exactly. To permit flow between the seat and plunger, the screw must be turned as the plunger is retracted. Until the plunger is totally pulled back, the flow will be very restricted. The screw is finely threaded; to retract the plunger, the screw must be turned many times. This allows for exact and careful regulation of the flow rate because the tighter the screw is turned, the less space there is around the needle plunger and therefore the less fluid that can fit through the hole. This precise control is a result of the ratio between the plunger's length and diameter or the axial versus radial relationship. Due to the small size of the hole and the fine threading of the screw, hydraulic needle valves can be closed off completely by hand. It is not easy to determine whether a needle valve is open or closed because there is little indication, unlike a ball valve where the handle position would identify it.
