HART Female Threaded (FNPT) O-Ring Unions made out of Brass / Copper.

HART Female Threaded (FNPT) O-Ring Unions made of Brass / Copper.

HART Industrial O-Ring Unions are available in female threaded end connections to suit various applications and configurations. Female Threaded end connections are available in both Class 3000 and Class 6000 union configurations and offer the same reliable, leak-free seal.

Female Threaded Unions come standard with Female NPT (National Pipe Taper) connections, but BSPT (British Standard Pipe Taper) threads are also available as a custom option. Female threads are internal threads that you would typically see on the end of a pipe or system—these mate into a corresponding male or external thread. The tapered design of the threads allows for a level of mechanical sealing without the need for a gasket, though typically pipe sealant compound (“pipe dope”) or tape is used at higher pressures.

HART’s female threaded end connections are ideal for applications where a compact seamless connection to a male threaded connection on a piece of equipment, manifold, or other system is required.

HART O-Ring Union Features:

  • Turbulence Free: All unions provide a turbulence-free fit across the seats
  • O-Ring Isolation: The O-Ring seal is strategically located in the threadpiece face, eliminating O-Ring media contact and added protection against abrasives and erosion. A variety of O-Rings for any application are available.
  • Precision Machined Components: All unions are precision machined to provide high quality and fail-safe leakproof reliability.
  • Interchangeable End Connections: HART’s male threaded connections feature interchangeable configurations with other threaded or welded connections, reducing the need for nipples or additional fittings.
  • Material Versatility: Unions can be provided in all standard metals or combinations of metals. This includes 304 Stainless, 316 Stainless, A105 Carbon Steel, Brass, Monel 400, Hastelloy, and Titanium.
  • Excellent Vibration Resistance: Seals will not loosen, even under extreme pressure and surges.
  • No Maintenance: Once the initial seal is made, no further tightening is required.

Class 3000 Female Threaded Unions

HART offers Female Threaded end connections in our Class 3000 series of unions in a wide variety of materials. HART’s Class 3000 are ideal for piping systems that are pressure rated up to 3000 psig W.O.G. Our female threaded unions are interchangeable with all of HART’s threaded and welded connections.

  • Specifications


  • 3D Animations
  • Datasheets
    HART 3131 Series - Technical Data Sheet Thumbnail

    HART 3131 Series – Technical Data Sheet.pdf

    HART 3131 Series - Assembly Reference

    HART 3131 Series – Assembly Reference.pdf

Class 6000 Female Threaded Unions

HART offers Female Threaded end connections in our Class 6000 series of unions in a wide variety of materials. Class 6000 female threaded unions are ideal for higher pressure applications that require connecting a female threaded union to a male threaded piping system that is pressure rated up to 6000 psig W.O.G.

  • Specifications


  • 3D Animations
  • Datasheets
    6161 Series Union Tech Sheet

    HART 6161 Series – Technical Data Sheet.pdf


Female threaded unions are ideal for a wide range of applications and industries, often including, but not limited to:

  • High-pressure fluid and gas systems
  • Instrumentation connections
  • Commercial HVAC and steam systems
  • Fire protection
  • High-pressure fluid loads
  • Industrial water cooling systems
  • Pipe applications for municipal water supply and wastewater, among others
  • Piping systems that require a flat face seal
  • Use cases under static, low-pressure, and no-pressure loads

HART Industrial Unions’ Capabilities

At HART, we specialize in creating high-quality unions for commercial and industrial applications. Our male and female NPT unions provide 3000 and 6000 Class services, and we also offer custom-made male and female BSPT unions. HART’s trusted Sure Seal O-ring unions ensure a tight seal even in environments with little to no pressure.

Along with our male and female threaded unions, we manufacture buttweld unions, socketweld unions, dielectric insulated unions, hammer lug unions, and more. Choose from interchangeable ends and fitting types, metal and seal materials, sizes, and pressure ratings. HART utilizes stainless and carbon steel, brass, aluminum, hastelloy, and Monel 400 to meet your high-pressure pipe union needs.

To learn more about our Engineering and Manufacturing Capabilities, visit our CAPABILITIES

Our team is here to answer any questions about our available inventory, production processes, and adherence to regulatory standards. Contact us today to start your order.