Threaded Pipe Union — Three-Piece Ductile Iron Removable Joint

By Nico Yin · Qingdao Zhaoyuqi Machinery Co., Ltd.

Overview

A threaded union is a three-piece fitting that joins two pipes while allowing the line to be opened by hand tools, quickly and repeatedly, without cutting pipe or rotating either run. The three pieces are a female-threaded end screwed onto each pipe and a central nut that draws the two ends together against a seated joint. That is the fundamental difference between a union and a coupling: a coupling becomes permanent once installed, while a union is the deliberate service joint of the system — specified at equipment connections, ahead of valves, meters, and pumps, and anywhere a strainer or coil will need removal for cleaning or replacement within the plant's life.

Our unions are sand cast in ASTM A536 ductile iron (typically 65-45-12; 80-55-06 and the EN-GJS series available to specification), a material choice that matters at the seating faces. The joint face between the two threaded ends is machined — commonly as a ground or lapped seat that mates metal-to-metal around a central gasket or seating ring ( configuration per drawing) — and ductile iron's toughness resists the brinelling and chipping that a brittle grey iron seat suffers from repeated make-ups. Threads follow ASME B1.20.1 NPT or ISO 7 BSP, machined and gauge-verified on both ends, so the union screws onto standard threaded pipe anywhere in the world.

Specifications

Customization & Sampling

Union design is seat-driven, so customization starts there: flat face with gasket, male-female ground joint, or O-ring groove — chosen for your medium and the number of expected open-close cycles. Other frequent modifications include special overall length for cramped equipment bays, one end NPT and the other BSP in the same set, larger hex flats for strap wrenches on big sizes, and customer logo casting on the nut. For first-time buyers we offer a sample set with seat-finish measurements and an assembly/leak trial report, typically within 3–4 weeks. Replacement seats, nuts, or gaskets can also be ordered as spares against the same part number.

Quality Inspection & Packaging

Because the union is three mating parts, inspection includes fit checks, not just piece checks: thread gauging on both ends, seat flatness and surface finish verification, nut thread engagement, and an assembled leak test on sampled sets at working pressure. Chemistry and mechanical testing per ASTM A536, ISO 8062 dimensional audits, and coating thickness measurement complete the lot file. Sets are packed assembled with gasket in place or in matched three-piece bags — never mixed sizes in one bag — and cartons are size-labeled on plywood pallets so the sets reach your shelf complete and correct.

FAQ

Q1: When should I use a union instead of a coupling?

Use a union wherever the joint will be opened during the equipment's life: at pumps, valves, meters, heat exchangers, and appliance connections. Use a coupling for permanent straight joints where cost matters more than serviceability. A common scheme is couplings for the runs and unions at every equipment tie-in, which minimizes total fitting cost while keeping the system maintainable.

Q2: Does the union seal metal-to-metal or with a gasket?

Both designs exist. The ground-joint style seals on machined mating seats, with optional gasket for imperfect field conditions; the flat-face style always uses a replaceable gasket (EPDM for water, NBR for oil). Gasket styles are more forgiving and cheaper to re-service; ground joints resist higher cycle counts. Tell us the medium and how often the joint opens, and we will recommend the seat type in the quotation.

Q3: Can I replace just one damaged piece?

Yes. Ends, nuts, and gaskets are separate components and we supply them individually against the same part number — useful for distributors who find one damaged end in a set, or plants that stock gaskets only for annual overhauls. Note that seat faces are machined as pairs for ground-joint styles, so matched sets from one lot are recommended for critical lines.

Related Products

• Coupling — permanent straight joint alternative

• Coupling with EG Gasket — gasket-sealed straight coupling

• 90° Elbow with Female Union — elbow with built-in union end

• Check Valve — typical valve installed between unions

• Flange Adapter — alternative demountable connection via flange

• Casting Advantages & Classification — process background article

Get a Quote

Send seat type, sizes, thread standard, and quantity to WhatsApp: +86 185 6156 7088 or email, and we will return a foundry-direct quotation with lead time within one business day. Mixed-size container programs and annual stocking agreements are welcome.

Need a formal quotation? Send your Threaded Pipe Union drawing, quantity and target price through our quotation request form — it goes straight to the foundry inbox, and we reply with a priced offer, material certificate draft and production lead time within one business day.