Bulkhead Union Tube Fittings: Stainless & Alloy, Panel-Mount, ASTM F1387
A bulkhead union tube fitting is a gaugeable, two-ended compression fitting. Its central body carries an external thread and a panel locknut, which lets it mount through a hole in a panel, enclosure, or bulkhead and join two tubes of equal outside diameter on opposite sides of that wall. Crestflo builds panel-mount bulkhead unions in stainless steel and exotic alloys alike, with single or double ferrule options, qualified per ASTM F1387 for drop-in interchange with the major brands. Here, "bulkhead union" refers specifically to an instrumentation-grade compression tube fitting meant for panel-mount tube runs. It doesn't mean a tank, RV, or aquarium plumbing bulkhead fitting, and it isn't an electrical bulkhead connector. Request a quote or a sample to move a spec forward.
What Is a Bulkhead Union Tube Fitting? Panel-Mount Tube Union Explained
A bulkhead union differs from a plain tube-to-tube union in just one way: the body is threaded and carries a locknut, so the fitting can be anchored right to a panel. Both ends function as gaugeable compression connections, sealing on the tube itself, never on the panel, through single or double ferrules. That's what lets a fabricator run an instrument tube cleanly through an enclosure wall and still service the joint from either side.
This fitting also goes by several other names: bulkhead union fitting, bulkhead tube union, bulkhead tube fitting, panel-mount tube union, panel-mount bulkhead union, through-wall tube union, union bulkhead, straight bulkhead union, or double-ferrule (twin-ferrule) bulkhead union. Within instrumentation, the family breaks down into three types: the straight bulkhead union (equal tube ODs), the reducing bulkhead union (two different tube ODs), and male-thread bulkhead connectors, which Crestflo covers on its thread-variant pages. Each comes with single or double ferrules, and spans the full alloy range.
How a Bulkhead Union Works & Installs Through a Panel
Installers use the same gaugeable make-up process common to two-ferrule instrumentation fittings. First, the threaded body goes through a prepared panel hole and gets secured against the wall with the panel locknut. Then each tube is bottomed out in its fitting body, and the compression nut is pulled up to the manufacturer's gauge position. This advances the ferrule, or ferrules, onto the tube, forming a metal seal that grips tight and won't leak. Since the seal forms on the tube itself rather than on the panel, the fitting works across a range of panel thicknesses and can be re-made after inspection.
Mounting-hole diameter and locknut thread follow the fitting series. Those dimensions, plus the recommended pull-up and panel-thickness limits, appear in the ordering guide, which you'll need to request from Crestflo's engineering team, specifying your size and alloy. For make-up detail, see our ferrule installation and gauging guidance.
Bulkhead Union Design: Single vs Double (Twin) Ferrule, Gaugeable Make-Up
Crestflo makes gaugeable bulkhead unions in two configurations: single-ferrule and double-ferrule (also called twin-ferrule), with hardened or non-hardened ferrules available to spec. In the double-ferrule setup, sealing and gripping are handled separately by a front and back ferrule, which gives you consistent performance you can re-make reliably. The single-ferrule design cuts down on part count for services where that works. Replacement nuts and ferrules are sold as spares, too.
Trying to decide which design fits your application? Check the single vs double ferrule guide and the piece on hardened vs non-hardened ferrules.
Bulkhead Union Sizes: Imperial (1/8"–2") & Metric (3–50 mm)
Crestflo offers bulkhead unions in both imperial-fractional and metric tube ODs. End connections go up to 2" (50 mm) OD, and reducing configurations are available too. Tube OD values follow standard published dimensions. Per-fitting envelope and mounting dimensions, though, depend on the applicable series, and they can be customized to what the application requires.
| Tube OD (imperial) | Metric equivalent | Ferrule options | Reducing option |
|---|---|---|---|
| 1/8" | 3 mm nominal | single or double | Yes |
| 1/4" | 6 mm | single or double | Yes |
| 3/8" | 10 mm | single or double | Yes |
| 1/2" | 12 mm | single or double | Yes |
| 5/8" | 16 mm | single or double | Yes |
| 3/4" | 18–20 mm | single or double | Yes |
| 1" | 25 mm | single or double | Yes |
| 1-1/2" | 38 mm | single or double | Yes |
| 2" | 50 mm | single or double | Yes |
Standard metric sizes of 3, 6, 8, 10, 12, 25 mm and larger are stocked as standard within the range. Need a full rundown? Check the tube fitting size chart.
Bulkhead Union Materials: 316/316L, 904L, Super Duplex 2507, Monel & Hastelloy
Bulkhead unions come in stainless, super-austenitic, duplex, high-nickel, and titanium grades. No brass, not in the US range. The UNS designations below are standard published values. Pick by service, then confirm your requirement on the RFQ.
| Alloy | UNS | Family | When to choose | Grade page |
|---|---|---|---|---|
| 316 / 316L | S31600 / S31603 | Austenitic SS | Default for most instrument service | 316/316L |
| 904L | N08904 | Super-austenitic | Higher chloride and acid resistance | 904L |
| Duplex 2205 | S32205 | Duplex | Strength plus chloride SCC resistance | Duplex 2205 |
| Super Duplex 2507 | S32750 | Super duplex | Seawater and high-chloride service | Super Duplex 2507 |
| 254 SMO (6Mo) | S31254 | Super-austenitic | Chloride-rich and marine media | 254 SMO |
| Monel 400 | N04400 | Nickel-copper | Hydrofluoric and reducing acids | Monel 400 |
| Inconel 625 | N06625 | Nickel-chromium | High-temp and aggressive corrosion | Inconel 625 |
| Hastelloy C-276 | N10276 | Nickel-moly-chrome | Severe wet-chlorine and sour media | Hastelloy C-276 |
| Titanium Gr2 | R50400 | Titanium | Seawater and oxidizing chlorides | Titanium Gr2 |
Plenty of other grades are on offer too: 304/304L, AL-6XN, Alloy 20, Inconel 600/601/718/825, Incoloy 800/800H/825, Monel K500, Hastelloy C-22/B-2/B-3/X, and Nickel 200/201. Full details live in the materials library. Think of these grade listings as educational. They aren't a spec sheet. Crestflo supplies these alloys as instrumentation fittings only, not as bar, sheet, or mill forms.
High-Pressure Bulkhead Union Ratings & Temperature Limits
Working pressure on a compression bulkhead union depends on size, wall thickness, and alloy. It comes from standard tube working-pressure practice for the tube in question, and values follow the applicable standard while remaining customizable to what you need. Put a heavier-wall tube and a higher-strength alloy on a high-pressure bulkhead union, and the rating climbs to match. So if you're after high-pressure service, pick your tube wall and grade to land the pressure class you're targeting. There's no elastomer seat in this all-metal design. That means the temperature band comes from the tube alloy itself, not from some soft seal. Crestflo compression fittings are qualified per ASTM F1387, and the tube material conforms to standards such as ASTM A269. Give us the tube OD, wall, and alloy on your RFQ, and we'll confirm a rating for your line.
Finishes & Cleaning: Electropolish, Passivation, Silver Anti-Galling
Standard finish means the material's own finish, as supplied. Beyond that, we offer electropolishing, passivation, and silver anti-galling plating on threads. Oxygen-clean and high-purity preparation are also available, built to a specified cleanliness target. Note your finish or cleanliness requirement on the RFQ. Values and targets follow the applicable standard, and we'll tailor them to what you need.
Standards, Certifications & Traceability: ASTM F1387, NACE MR0175, EN 10204 MTC
Crestflo holds ISO 9001, 14001 and 45001 certification, along with PED compliance. Bulkhead unions are qualified to ASTM F1387, and for sour service they're available in suitable alloys meeting NACE MR0175 / ISO 15156. Each part carries a heat and lot number for full traceability, and ships with an EN 10204 material test certificate.
| Item | Standard / designation | Applies | Reference |
|---|---|---|---|
| Quality & environment systems | ISO 9001 / 14001 / 45001, PED | Yes | Why Crestflo |
| Fitting performance qualification | ASTM F1387 | Yes | ASTM F1387 |
| Sour-service compliance | NACE MR0175 / ISO 15156 | On request | NACE MR0175 |
| Material test certificate | EN 10204 MTC 3.1 (3.2 on request) | Yes | EN 10204 MTC |
| Tube material standard | ASTM A269 | Yes | ASTM A269 |
Testing covers hydraulic proof, burst and impulse, pneumatic, flexure and rotary-flex fatigue, vibration, thermal cycling, pull-out, and vacuum, plus PMI and in-house NDT. All destructive testing happens at ISO/IEC 17025-accredited labs. Third-party inspection is available on request. Crestflo keeps cert numbers in Quality.
Swagelok, Parker, Hy-Lok, Hoke, Fitok & Ham-Let Bulkhead Union Interchange / Drop-In
Crestflo bulkhead unions are built to the same dimensions as major-brand panel-mount bulkhead unions, so they drop in as a direct equivalent. That interchange is backed by ASTM F1387 performance qualification. If your current line runs on a Swagelok, Parker A-LOK, Hy-Lok, Hoke, Fitok, or Ham-Let bulkhead union, we'll build a replacement matched to it in form, fit, and function. Crestflo has no affiliation with any of these brands. Swagelok® is a trademark of Swagelok Company. Parker A-LOK® belongs to Parker Hannifin. Hy-Lok® is a trademark of Hy-Lok Corporation. Hoke® is a trademark of Hoke Inc. FITOK® is a trademark of FITOK Group. Ham-Let® is a trademark of Ham-Let Group.
We don't publish a generic part-number table. Instead, our engineering team builds your interchange to the exact incumbent numbers already on your line. Need a cross-reference? Request your Swagelok cross-reference, Parker A-LOK cross-reference, Hy-Lok cross-reference, Hoke cross-reference, Fitok cross-reference, or Ham-Let cross-reference. Or take a look at the case for a Swagelok alternative.
Ordering Bulkhead Union Tube Fittings
A bulkhead union gets specified by tube OD, body alloy, ferrule type, finish, and whatever reducing or thread option applies. Specs and dimensions sit right on the page in HTML, not buried inside a downloadable catalog PDF, so buyers and engineers can pull them directly rather than digging through a printed catalog. Crestflo, a US supplier of bulkhead union tube fittings, doesn't publish a fixed price list; it runs on an RFQ model instead. Need pricing, current lead time, or a comparison point after searching "bulkhead union near me"? Request a quote and the engineering team will send back firm numbers for your line. For the full part-number schema, request the ordering-code guide. Datasheets and CAD files go out to customers on request. Reach Crestflo at sales@crestflousa.com or +1 346 594 9005, in Houston, TX. As for delivery: standard items ship from ready stock, specials get made to order, and lead time runs 6-8 weeks.
Why Crestflo for Bulkhead Union Tube Fittings
Crestflo is the instrumentation division of an export house that's been in business for four decades, recognized by the Government of India, with a proven delivery record across the Middle East. Solid manufacturing sits behind a brand built specifically for the US market. We Engineer Confidence. For panel fabricators, EPCs, OEMs, and MRO buyers, the pitch is simple:
- ASTM F1387 interchange proof for confident drop-in replacement.
- A 6–8 week lead time against 20+ weeks typical for the majors.
- The full alloy range, SS through duplex, 6Mo, nickel alloys, and titanium.
- Heat-number marking plus EN 10204 MTC on every part.
- Small MOQ and private-label programs.
- Indian origin; US or European melt available on request, with full material traceability. See Crestflo's page on US origin & compliance.
Start with a quote, a free sample, or become a distributor.
Applications for Panel-Mount Bulkhead Unions
Bulkhead unions hold tube runs steady as they pass through instrument and analyzer panels, sampling systems, control enclosures, and skids. They show up across oil and gas service, analytical sampling systems, instrument air lines, and semiconductor work alike.
Bulkhead Union Tube Fitting FAQ
What is a bulkhead union?
A bulkhead union is a gaugeable compression tube fitting. It's built with two tube ends, a threaded body, and a panel locknut, and that combination lets you mount it straight through a panel or bulkhead, joining two tubes of equal outer diameter on opposite sides of the wall.
What is a bulkhead tube fitting?
It's a tube fitting for instrumentation lines that passes through a panel or enclosure wall and locks into place there. The seal happens at the tube itself, using single or double ferrules, not against the panel surface.
How does a bulkhead union work?
The threaded body goes through a prepared panel hole and gets locked in place with a locknut. Each compression nut is then tightened to the gauge position, which drives the ferrule (or ferrules) forward to grip and seal every tube.
What are the different types of bulkhead fittings?
In instrumentation, three fittings do the job: bulkhead unions for tube-to-tube connections, reducing bulkhead unions for unequal ODs, and male-thread bulkhead connectors. Each comes in single- or double-ferrule versions and spans multiple alloys. Note that "bulkhead fittings" used for tank, RV, or aquarium plumbing are a separate product entirely and fall outside this scope.
What sizes do Crestflo bulkhead unions come in?
Imperial and metric, with end connections up to 2" (50 mm) OD; common sizes run 1/8"–1" and 3–25 mm, with reducing configurations also available.
What materials are available?
The range covers 304/304L, 316/316L, 904L, Duplex 2205, Super Duplex 2507, 254 SMO, AL-6XN, Alloy 20, Inconel, Incoloy, Monel, Hastelloy grades, and Nickel 200/201, plus Titanium Gr2. It's stainless and high-nickel/CRA only. No brass appears anywhere in the US range.
Are Crestflo bulkhead unions a drop-in for Swagelok or Parker A-LOK?
They're engineered dimension-for-dimension to swap directly with major-brand bulkhead unions, and each one is qualified per ASTM F1387. Crestflo isn't affiliated with those brands. Swagelok® is a trademark of Swagelok Company; Parker A-LOK® belongs to Parker Hannifin. Want a part-number cross-reference? Reach out to our engineering team.
Are they NACE / sour-service compliant?
Yes, bulkhead unions are available in suitable alloys that meet NACE MR0175 / ISO 15156 for sour service.
What certification and traceability come with each fitting?
Each part carries a heat and lot number and comes with an EN 10204 MTC 3.1 certificate. Need a 3.2 instead? That's available on request. PMI and testing both follow the ASTM F1387 regime.
Where are they made and are they TAA-compliant?
Our standard product comes from India. Need US or European melt instead? We'll supply it on request, with full material traceability, whenever domestic-melt or TAA eligibility applies. Check our US origin and compliance page for the details.
What is the lead time and how do I order?
Standard items ship from ready stock. Specials are made to order, with delivery running 6-8 weeks. MOQs stay small, and private-label options are available. Submit an RFQ or request a sample to place an order.
Related Tube Fittings, Materials & Standards
- Start at the Tube fittings hub for the full lineup; the union tube fitting and the port connector are both worth a look.
- Crestflo works in 316/316L, Super Duplex 2507, and Hastelloy C-276.
- On the standards front, ASTM F1387 and NACE MR0175 set the bar. Not sure which ferrule to pick? The single vs double ferrule page breaks it down.