Union Tube Fittings: Stainless Steel & High-Nickel Alloy Instrumentation Unions

A union tube fitting is a straight instrumentation compression fitting. It joins two tubes of matching outside diameter, end to end, gripping and sealing each tube on its OD with a ferrule set driven forward by nut torque. Crestflo makes these unions in 316/316L, duplex, and nickel alloys, up to 2″ (50 mm) OD, in single- or double-ferrule configurations. Each one is tested to ASTM F1387 and built to the exact dimensions needed to serve as a drop-in replacement for the major brands.

Reviewed by the Crestflo Engineering Team

A union, in this context, means a stainless-steel or alloy instrumentation compression tube fitting that joins two tubes of equal outside diameter in a straight line. It isn't a threaded plumbing or pipe union. It's not a copper or brass compression coupling either, and it isn't a dielectric water union.

Union Tube Fitting: Also Known As

Across drawings, catalogs, and RFQs the same part is specified under many names. Buyers call it a tube fitting union, a tube union or tube-to-tube union, a straight union or straight tube union, and, by the connection it makes, a union connector, union coupling, or tube coupling union. In fluid-system and analyzer work it is written as an instrument union, instrumentation union, or instrument tube union, and by its sealing mechanism as a compression union or compression tube union, further split into the double-ferrule (two-ferrule) union and the single-ferrule union. All of these describe the straight equal-ended union covered on this page; the variants below split it by shape, port, or non-tube end.

What Is a Union Tube Fitting? (Compression Tube Union Working Principle)

A union closes the gap between two tube runs of the same size. Each end carries a ferrule, or a ferrule pair, that swages onto the tube OD as the nut is pulled up. The result is a leak-tight, gasket-free metal seal that also grips the tube against pull-out. Same compression mechanism on both ends, mirrored around the body center. That's what sets a union apart from its neighbors. A union joins tube to tube, with a ferrule set on both ends. A connector, by contrast, joins tube to thread or tube to weld. And a coupling, in plumbing terms, threads or solders pipe rather than compressing tube. A union, then, is the part you reach for to splice a tubing run, extend a line, or drop a replaceable section into an instrument, hydraulic, or sampling loop, all without cutting a thread.

Instrumentation Union vs Plumbing Union & Copper Compression Coupling

Crestflo union is a stainless-steel or corrosion-resistant-alloy compression union built for fluid and gas systems. It's an all-metal, ferrule-sealed tube fitting, rated by the tube it grips. Since the term "compression union fitting" gets so much search traffic dominated by copper and plumbing use cases, it's worth stating the scope plainly here. This isn't a threaded pipe union with a seat and ring. It's not a copper or brass compression coupling for water lines, either, and it's not a dielectric union for joining dissimilar plumbing metals. Throughout this page, "compression union" means the instrumentation double-ferrule kind: two ferrules gripping the tube OD. No gasket. No thread on the tube.

Union Design & Ferrule Technology: Single vs Double Ferrule

Crestflo unions come in both double- (two-) ferrule and single-ferrule designs. The two-ferrule geometry splits the sealing and gripping jobs apart, so the seal holds up under vibration, pressure cycling, and thermal movement. The single-ferrule design, by contrast, gives you a simpler, lower-part-count option for suited services. Ferrules ship hardened or non-hardened depending on what's required, and every part carries a heat/lot number stamped on it for full traceability. Take a look at the single-ferrule design, the ferrule-less option, and matching nut-and-ferrule sets or replacement ferrules. The trade-offs between them are laid out in our single vs double ferrule and hardened vs non-hardened ferrules guides.

Union Materials: 316/316L Stainless, Duplex, Inconel & Hastelloy

316/316L stainless remains the standard choice for general instrumentation service. For corrosive, sour, high-temperature, or high-chloride media, the full alloy ladder is available. Grades are supplied to ASTM A269 / A276 / A479, with values set per the applicable standard and customizable to requirement. A union is an all-metal fitting, so its service temperature depends on the chosen alloy rather than any soft seal.

Not sure which grade the job needs? Start with our material selection by service guide, or try the alloy selector.

Union Sizes & Tube OD: 1/8″ to 2″, Imperial and Metric

Unions come in tube OD sizes up to 2″ (50 mm), covering both fractional/imperial sizes and their metric counterparts, every fitting lists both. Popular sizes get their own landing pages, so you can head straight to the one you need:

Both metric and imperial/fractional tube unions come by tube OD. Check the full tube fitting size chart for details.

Union Types: Bulkhead, Reducing, Elbow, Tee & Cross Unions

The straight, equal-ended union is the base case. That same union principle carries through the shaped, reducing, and through-wall variants as well, each with its own page. Pick based on whatever geometry the run calls for:

For the pure tube-to-thread or tube-to-weld case, where only one end grips tube, see the connector tube fitting instead.

Union End Options: Weld & Threaded Ends via the Fitting Family

By definition, a straight union has a compression tube-fitting on both ends. But when a run needs to end in a weld prep or a thread rather than a second tube end, the right term for that transition is a connector, not a union. Those weld and threaded configurations get their own pages: socket-weld, butt-weld, and weld connector ends, plus NPT, NPTF, BSPT, and BSPP threads, built to their published standards.

Union Specifications, Pressure Rating & Standards Conformance

Working pressure tracks the tube. Tube OD, wall thickness, and alloy set the number, calculated on the allowable-stress basis of ASME B31.3, so a correctly matched union sustains the rated pressure of whatever tubing it grips. A thick-wall, small-OD assembly gives you a high-pressure union, and the fitting body doesn't need to change at all. Values follow the applicable standard and can be customized to the requirement. The seal is all-metal, with no elastomer in the mix, so service temperature comes down to the alloy you pick rather than a soft seal's tolerance. Sour service works the same way: choose the right alloy, and NACE MR0175 / ISO 15156 compliance is available on request. Size the assembly with our tube-pressure calculator, then check the limits against the pressure-temperature derating guide.

Crestflo carries every standard approval that applies to compression tube fittings. It's also certified to ISO 9001 / 14001 / 45001 & PED. Certificates are published in quality; the underlying standards sit at ASTM F1387, ASTM A269, NACE MR0175, and EN 10204 MTC.

Swagelok & Parker Union Interchange, Backed by ASTM F1387

Crestflo unions are built as dimensional drop-in equivalents for Swagelok, Parker, Hoke, Hy-Lok, Gyrolok, and Fitok unions. That interchange rests on ASTM F1387 performance qualification, not a resemblance claim. We describe it as engineered equivalence, never as a compatibility or intermix statement. Rather than post a static part-number table, we build the mapping for your bill of materials directly: request your Swagelok® or Parker® cross-reference, and our engineering team sends back the matched Crestflo equivalents. Specifying a Swagelok alternative? Our Swagelok alternative page lays out the F1387-backed equivalence before you commit an RFQ. Start at the Swagelok cross-reference, the Parker A-Lok cross-reference, the Hy-Lok cross-reference, the Gyrolok cross-reference, or the interchange lookup. Swagelok® and Parker® belong to their respective owners; Crestflo isn't affiliated with either.

Installing a Tube Union

Push each tube in until it bottoms out against the union shoulder. Finger-tighten the nut, then pull it up to the marked position so the ferrule (or ferrules) bite into the tube OD and form the seal. Do this the same way on both ends. Gauge each make-up consistently and the joint comes out repeatable and leak-tight every time. Full method and gap-gauge checks are covered in our ferrule installation and gauging guide, and dimensional data is on the tube fitting size chart.

Why Buy Union Tube Fittings From Crestflo

Crestflo makes and supplies union tube fittings for the US market. It's the instrumentation division of an export house that's been around for four decades, one recognized by the Government of India, with a track record of delivering into the Middle East. That history built the kind of supply discipline you'd expect from a focused instrumentation brand. For unions, here's what that means:

• ASTM F1387-qualified drop-in interchange for Swagelok, Parker, Hoke, Hy-Lok, Gyrolok, and Fitok, something most competitors won't put in writing.
• Standard items ship from ready stock, specials are made to order, and delivery runs 6–8 weeks, well short of the 20-plus weeks typical of the majors.
• Both imperial and metric on every union, single or double ferrule.
• Small MOQ and private-label/OEM programs for distributors and equipment builders.
• We're certified to ISO 9001, ISO 14001, and ISO 45001, and we hold PED certification as well. Every part ships with an EN 10204 MTC 3.1 (3.2 available on request), and each one carries its own heat and lot number.
• NACE MR0175 / ISO 15156 compliance available for sour service.

Indian origin steel, with US or European melt available on request and full material traceability throughout. TAA and domestic-melt options are available where your specification calls for them.

Datasheets and CAD models are available to customers on request. For more, see the Crestflo Engineering Team page, our position on US-origin & compliance, and our approach to material traceability.

Request a Union Quote or Sample

Give us the size, the ferrule configuration, and the alloy, and we'll match the union and confirm delivery. To move forward, request a quote or request a free sample. For distributors and private-label partners, review the private-label program or become a distributor. Reach Crestflo's US team at sales@crestflousa.com · +1 346 594 9005 · Houston, TX.

These unions find work across oil & gas, petrochemical, offshore, hydrogen, semiconductor, and analytical sampling systems. For more detail, see oil and gas, analytical sampling systems, and instrument air.

Union Tube Fitting FAQ

Related Union Resources

• Also available in union form: bulkhead union · reducer · elbow · tee · cross · port connector
• Related fittings worth a look: connector · adapter · single ferrule
• Governing standards: ASTM F1387 · ASTM A269 · NACE MR0175 · EN 10204 MTC
• Pillar: tube fittings