Superior Fluid Solutions · Cross Reference
Tube Fitting Union Cross Reference: Straight, Reducing, Bulkhead and Union Tee
The union is the part a maintenance crew replaces more often than any other, and the part most likely to be ordered wrong.
A union does one job: it joins two tube ends and lets you take them apart again. That second half is why unions wear out first. Every time a transmitter is swapped, a filter element is changed or a section of tube is cut back, somebody breaks a union. According to Swagelok®, whose published technical material describes its own two-ferrule fittings as producing “a repeatable, leak-tight seal, even after being reassembled multiple times,” reassembly is a designed in part of how these connections are used. That is a manufacturer statement about a manufacturer’s own product, and it explains why the union bin empties faster than the elbow bin.
Why a union is the part maintenance touches most
Count the joints on any instrument hook up that exist purely so the line can be opened. A union upstream of a gauge. A union either side of a filter body. A union where a skid meets plant tubing, so the skid can be lifted out without cutting. None of them changes direction or size. They exist to be undone.
Because they get undone, ferrules get reused and nuts get crossed. A stores list for a shutdown is usually half unions, so the cross-reference you do at ordering time is repeated more often for this one shape than for anything else in the tube fittings range. Getting the process right once pays back every outage.
Four unions, four jobs
The straight union is the default. Same tube size both ends, in line, nothing else happening. If a drawing just says “union” with one size against it, this is almost always what it means.
The reducing union carries two different tube sizes on one body. It appears where a main run steps down to an instrument tail, for example a half inch header feeding a quarter inch impulse line. Note that a quarter inch tube is 6.35 mm and a half inch tube is 12.7 mm, so a drawing mixing inch and metric callouts on the same reducing union is worth a phone call before you order. When the step is large or the line also needs a different end style, a separate reducer may be the cleaner part.
The bulkhead union passes through a panel or enclosure wall and is held by a locknut, so the tube outside can be disturbed without loading the tube inside. It is the one union where the panel itself is part of the order, because the threaded barrel has to suit the plate you are mounting through.
The union tee is a union with a third leg. It is a branch, not a size change, and it is where people most often mis-order, because a drawing may show a tee symbol without saying whether the run and branch are the same size. Our union fittings and tee fittings pages separate these so you are not guessing from a symbol.
| Union shape | Where it belongs | Detail most often missed |
|---|---|---|
| Straight union | Any point a run needs to be openable without changing size or direction | Tube wall thickness, which the drawing may only give once for the whole line |
| Reducing union | Header to instrument tail, or where a line steps down to a valve tail | Which end is which size, and whether both callouts are in the same unit |
| Bulkhead union | Through a panel, cabinet or enclosure wall | Panel thickness and hole size, neither of which is a fitting dimension |
| Union tee | A branch to a gauge, a bleed point or a second instrument | Whether run and branch are the same size, and which leg is which |
Shapes and applications only. Pressure rating, material grade and certification are confirmed per order against the specific part supplied.
Reading a union off a drawing
Older isometrics often name a brand instead of a description, which is the moment a cross-reference starts. The method does not change: read the physical facts off the drawing, then identify a part that matches those facts. Tube outside diameter at each end. Tube wall. Body shape. Material. Rated working pressure, confirmed for the part you are actually buying. That is a cross-reference by form, fit, and function, and it is the only claim we make. We do not say a Superlok® union is equivalent to, compatible with or a replacement for a Swagelok®, Parker®, Hy-Lok® or FITOK® union, because none of those manufacturers publishes anything that would support the statement.
Where the drawing is silent, go and measure. Calipers on the tube settle more arguments than an hour with a legacy document. In a corrosive service, check whether the specification asked for a corrosion-resistant grade and whether that is what is installed. Superlok® unions are listed in 316 stainless steel, and Superlok® publishes that its fittings offer the finest interior surface finish of any tube and pipe fitting on the market, which matters most on the small bore lines where unions cluster.
What to confirm before the order goes out
Five things. Both tube sizes, written as separate numbers even when they match. The tube wall, because a thin wall and a heavy wall in one nominal size are not the same job for the ferrules. The body shape, named in words rather than read off a symbol. The material grade and the certificate type. And the quantity of complete assemblies, including spare ferrule sets, since a union that is opened repeatedly is the one most likely to need fresh ferrules mid outage.
Write the result down against the line or tag number. A short record of what replaced what is the difference between a tidy turnaround and a scramble the next time the line is opened. The chemical turnaround post covers the stores side of that, and the certifications page lists what comes with an order.
Buying questions
My drawing says “union” with one size. Is that a straight union?
Almost always, yes, but confirm at the pipe. If the two tube stubs measure differently you need a reducing union, and a single size callout on the drawing is simply out of date.
Can I use a reducing union instead of a reducer?
They are different parts. A reducing union takes tube on both ends. If one side has to land on a thread or another connection style, look at reducers and adapters instead.
What do I need to know to order a bulkhead union?
The tube size, and the thickness of the plate you are passing through plus the hole diameter drilled in it. Those last two are properties of your panel, not of the fitting, so they have to come from you.
Are ferrules from an old union reusable?
Treat ferrules as belonging to the joint they were pulled up on, from one supplier, and order fresh sets for planned work. Mixing components from different sources is not supported by any manufacturer’s published material.
The old fittings carry another brand’s name. What can you tell me?
We can identify a part that matches on form, fit, and function: tube sizes, body shape, material and rated working pressure confirmed per order. We do not rank brands and we make no interchangeability claim in either direction.
Building a union list for the next outage?
Work from the shape and the tube sizes, and confirm grade and rating on the order.
Superior Fluid Solutions
Unions identified by shape, size and material, never by assumption.
Sources
- Swagelok, “The Swagelok Tube Fitting Advantage,” manufacturer technical page, accessed September 2026, quoted for the manufacturer’s own statement about reassembly of its two-ferrule fittings. swagelok.com
- ASTM International, “A269 Standard Specification for Seamless and Welded Austenitic Stainless Steel Tubing for General Service,” standard abstract, supports the reference to nominal wall stainless tubing for corrosion resisting service. astm.org
- ASME, “B31.3 Process Piping,” code description page, supports the statement that the governing code covers materials, examination and testing of process piping. asme.org
- McMaster-Carr, “Bulkhead Unions,” distributor catalog category, accessed September 2026, supports that bulkhead unions are offered as a distinct catalog shape across materials and sizes. mcmaster.com
Technical note: the sources above support what one manufacturer publishes about reassembly of its own fittings, the existence of bulkhead unions as a catalog shape, and the scope of the ASTM tubing specification and the ASME piping code. They do not show that unions from different manufacturers will assemble together, and no such claim is made here. The inch to millimetre figures are direct conversions. Material grade, pressure rating and certification are confirmed per order.
Trademark disclaimer: Swagelok® is a registered trademark of Swagelok Company. Parker® is a registered trademark of Parker Hannifin Corporation. Hy-Lok® is a registered trademark of Hy-Lok Corporation. FITOK® is a registered trademark of FITOK Group. Superlok® is a registered trademark of BMT Co., Ltd. Superior Fluid Solutions is not affiliated with, endorsed by, or sponsored by any of these companies. All brand names are used for identification only. Cross-references are made by form, fit, and function only.
