Needle Valves

Michael
Nasser

Needle valves, sometimes called plunger valves, enable engineers to control and regulate subtle flow and pressure changes. The needle valve’s stem provides precise control by moving a piston tube open and closed. In this article, you will learn about needle valve operating principles, applications, and maintenance and repair considerations.

Needle Valve Operating Principles

Needle valves got their name from their narrow tapered plug and port arrangement. They provide fine process control. Needle valve applications target small process lines (1″ and under). The lines often use threaded connections, and as such, many needle valves constrict flow via the same threaded mechanism. Needle valve’s material of construction may be stainless steel, brass, bronze, or other metal alloys. To optimize valve life, valve material should be chosen with process fluid characteristics in mind.

cross-section of needle valve
Courtesy: Hydradyne

Needle valves comprise of a valve body with a hole and a stem that screws in and out of the hole opening. A tapered pin opens just enough to let a fine stream of fluid pass through. Spindle control actuation occurs via electric, hydraulic, pneumatic, or manual means. Needle valve may also feature an internal check valve to restrict flow in one direction and to allow free flow in the other direction. 

The long needle-like point at the end of the valve stem acts as the disc. That needle-point fits perfectly into the seats and allows for steady changes in fluid flow. The liquid turns sharply as it moves through a hole, resulting in a relatively high-pressure drop. The high-pressure drop does not cause operational issues as needle valve applications don’t typically require the minimization of pressure drop.

needle valve gif

Needle valves are sometimes characterized as a globe valve due to their similarity in operation. However, needle valves provide a much smaller seating surface area than typical globe valves. This ensures pressure build-up on the wetted part attached to the stem is minimal.

needle valve brass body
Courtesy: Grainger

Applications

Needle valves find their application in:

  • Gas bleeder lines
  • Sampling lines
  • Abrasive services and fluid power systems.
  • Flow metering applications
  • Calibration
  • Automatic combustion control systems
  • Engine carburetors

Turbine by-pass, pump starts, flow discharges, reservoir inlets, and flow regulation are also common process areas where needle valves are specified. In constant pressure pump applications, needle valves function as governors to reduce fluctuations in pump discharge pressure. Needle valves provide accurate flow in small line size applications as well as the ability to throttle and quickly adjust flow.

Needle valves typically cause high-pressure loss due to their highly restricted flow and small passage size. They work well with higher differential pressures. They do not typically fit rugged applications as excessive shut-off pressure may cause valve damage.

Maintenance and Repair

repair of a needle valve
Courtesy: Swagelok

Needles valves are easy to maintain and repair. For smooth-running operation, the following considerations must be kept in mind:

  • Hygiene and cleanliness – Due to the small size and precise control, fluids typically require a high degree of cleanliness. Furthermore, technicians must carefully ensure the orfice does not become plugged with debris during handling.
  • The valve’s outer body and connection to the pipe requires regular tightness checks.
  • Pressure drop check must occur at regular intervals.
  • Cleaning of the outer coating should occur once a month or at a manufacturer’s required maintenance schedule.
  • Diaphragm type needles valves require immediate change should leakage begin.