Cap Tube Fittings: Stainless & Alloy Instrumentation Tube-End Caps
A cap tube fitting is an instrumentation compression fitting fitted over the outside diameter of a tube end. Its nut and ferrule (or ferrules) draw down to form a leak-tight seal, closing off the tube run for good. Crestflo makes these tube-end caps in 316 stainless and high-nickel/CRA alloys, in both imperial and metric sizes. They're dimensionally drop-in and qualified to ASTM F1387, and every piece carries heat-number traceability. Standard items ship from stock; specials are made to order, with delivery running six to eight weeks.
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An instrumentation compression tube cap seals the outside diameter of a tube end. That's a distinct fitting: not a threaded pipe cap (ASME B16.11), not a cap flange, not a refrigeration capillary tube, and not a protective push-on dust cap.
What Is a Cap Tube Fitting? Tube Cap, Tube-End Cap & Compression Fitting Cap
A cap fits over the tube's outer diameter, and the nut draws it down to advance the ferrule or ferrules onto the tube, gripping and sealing it. What you get is an all-metal, gas-tight termination that shuts off the end of a tube line. The seal happens on the tube's outside diameter, so the cap needs just one connection. No second port, no thread. That single-connection design is what sets a cap apart from an adapter or a plug.
Buyers use several names for the same component. This tube-end fitting also goes by tube cap, tubing cap, tube-end cap, OD tube cap, compression fitting cap, ferrule cap, dead-end cap, tube stub cap, or simply cap fitting; in Swagelok-style catalog logic it carries a "C" body designator. Whatever term is used, it's the same instrumentation cap that terminates a tube run on its OD.
Cap vs Plug: Which Tube Fitting Do You Need?
Most people mix up cap and plug. A cap closes a tube. A plug closes a fitting, that's the real distinction. If you need to dead-end a length of tubing, grab a cap. If you're blanking off an unused female fitting port, use a tube-fitting plug instead.
| Component | Seals | Installs on | Typical use |
|---|---|---|---|
| Cap | Outside of a tube end | Tube OD (nut + ferrule) | Dead-end a tube run, cap a stub-out |
| Plug | Inside of a fitting body | Fitting port | Blank an unused compression port |
How the Seal Works: Single-Ferrule & Double-Ferrule Caps
A cap seals the same way as any Crestflo compression fitting does: the nut drives one or two ferrules onto the tube, creating a leak-tight, metal-to-metal grip with no elastomer in the standard build. As shipped, a cap is a complete cap nut assembly. That means a solid cap body plus the fitting nut and ferrule set (front and back ferrules on a double-ferrule build), so one order gets you a ready-to-install termination. Caps come in single-ferrule and double-ferrule configurations, with hardened or non-hardened ferrules available to requirement. For the trade-off between the two, see single vs double ferrule. For the sealing sub-components, check tube-fitting ferrules, single-ferrule fittings, and the nut-and-ferrule set.
Cap Tube Fitting Sizes: Imperial (1/16″–2″) & Metric (3–50 mm)
Caps come in every size across the compression-fitting range, spanning both imperial/fractional and metric OD series, up to a 2″ (50 mm) tube outside diameter. Sizing follows the standard tube-fitting OD tables. Values track the applicable standard, and they can be adjusted to suit specific requirements.
| Tube OD, imperial | Tube OD, metric | Notes |
|---|---|---|
| 1/16″, 1/8″, 3/16″, 1/4″ | 3 mm, 6 mm | Instrument and sampling lines |
| 3/8″, 1/2″ | 8 mm, 10 mm, 12 mm | General process tubing |
| 5/8″, 3/4″, 1″ | 15 mm, 18 mm, 22 mm, 25 mm | Larger process lines |
| 1-1/4″, 1-1/2″, 2″ | 32 mm, 38 mm, 50 mm | Maximum OD |
Size-specific and system pages: tube-OD basis, metric tube fittings, imperial-fractional tube fittings and the tube-fitting size chart.
Cap Tube Fitting Materials: Stainless Steel Tube Caps in 316/316L, Duplex, Inconel, Monel, Hastelloy & Titanium
Crestflo supplies stainless steel tube caps in 316/316L, along with duplex, super-austenitic, nickel and titanium alloys, the US line carries no brass. Each grade listed below links to its material page. The UNS designations serve as the standard published references for specification.
| Family | Grade (UNS) | Grade page |
|---|---|---|
| Stainless | 304/304L (S30400/S30403) | 304/304L |
| Stainless | 316/316L (S31600/S31603) | 316/316L |
| Super-austenitic | 904L (N08904) | 904L |
| Duplex | 2205 (S32205) | Duplex 2205 |
| Super duplex | 2507 (S32750) | Super Duplex 2507 |
| 6Mo | 254 SMO (S31254) | 254 SMO |
| 6Mo | AL-6XN (N08367) | AL-6XN |
| Nickel | Alloy 20 (N08020) | Alloy 20 |
| Nickel | Inconel 625 (N06625) | Inconel 625 |
| Nickel | Monel 400 (N04400) | Monel 400 |
| Nickel | Hastelloy C-276 (N10276) | Hastelloy C-276 |
| Titanium | Grade 2 (R50400) | Titanium Gr2 |
Incoloy 800/825, Monel K500, Nickel 200/201 and the rest of the material range are available too, the alloy selector will help you match a grade to your media. Finishes on offer include material finish, electropolish, passivation, silver anti-galling plating and oxygen/UHP cleaning. Specify cleanliness targets on the RFQ, or check oxygen & UHP cleaning for details.
Cap Pressure & Temperature Ratings
A cap carries the working-pressure rating of the tube-fitting system it terminates. Take a high-pressure tube cap: it holds the same rating as the tube run it dead-ends. That rating comes from tube OD, wall thickness and alloy, set per the applicable tube-rating standard, such as ASME B31.3. Ratings follow the applicable standard but can be customized to requirement.
A standard cap doesn't use an elastomer; it relies on an all-metal ferrule seal instead. That means its temperature limit tracks the chosen alloy rather than a soft-seat rating. Crestflo verifies every cap under its own test regime: hydraulic proof, burst and impulse, pneumatic, pull-out, vibration, thermal cycling and vacuum. PMI and in-house NDT round out the process, with destructive testing carried out at ISO/IEC 17025-accredited labs and third-party inspection available on request.
Standards, Certifications & Material Traceability
Crestflo compression caps carry performance qualification to ASTM F1387. Bar stock conforms to ASTM A479/A276. Sour-service builds ship compliant with NACE MR0175 / ISO 15156. The company holds ISO 9001 / 14001 / 45001 certification and PED certification too. Each part gets marked with its heat/lot number. An EN 10204 MTC 3.1 comes standard with every shipment, and 3.2 is available on request, details are covered under material traceability.
Drop-In Interchange & Cross-Reference
Crestflo compression fittings are qualified to ASTM F1387. That's why a Crestflo cap is dimensionally an interchange, or equivalent, and it drops into an existing brand's tube-fitting system without any redesign.
Swagelok® belongs to Swagelok Company as a trademark. Parker® (A-Lok® and CPI™), Hy-Lok®, Hoke®, Fitok® and Gyrolok® are trademarks held by their respective owners. Crestflo isn't affiliated with any of them, and none of them endorse Crestflo.
Instead of publishing a part-number table, our engineers build your interchange list against your exact catalog codes. Request your cross-reference for Swagelok, Parker A-Lok or Parker CPI, Hy-Lok, Hoke, Gyrolok or Fitok, or start from the cross-reference hub.
Origin, US Service & Lead Time
Crestflo serves customers across the US, with a sales contact based in Houston, TX. The standard product comes from India, though US or European melt is available on request, and full material traceability comes standard. When a project calls for domestic-melt or TAA-eligible material, our engineering team scopes it as a capability, details are on the origin & compliance page.
Fulfillment follows the same approach as the rest of the line. Standard items ship from ready stock. Specials are made to order. Delivery runs 6-8 weeks. For how that timeline gets held, check the lead-time methodology.
Why Buy Cap Tube Fittings From Crestflo
Crestflo is the instrumentation arm of an export house that's been operating for four decades, carries Government of India recognition, and has a long track record delivering into the Middle East. That history brings real engineering depth to what is now a focused US instrumentation line.
- ASTM F1387-qualified drop-in interchange with the major tube-fitting brands.
- Exotic-alloy availability, duplex, 6Mo, nickel and titanium caps, not only 316.
- Ready stock for standard items · made-to-order for specials · 6–8 week delivery.
- Small MOQ and private-label / OEM programs.
- Heat-number traceability with EN 10204 MTC 3.1/3.2.
- Both imperial and metric on every fitting. We Engineer Confidence.
Order Cap Tube Fittings: Request a Quote
Tell us the tube OD, alloy, ferrule configuration and finish, and our team returns a quote with the correct ordering code, kits and mixed-size sets are built to request. Request the ordering-code guide from our engineers, or configure a part with the part-number configurator.
Request a Quote · Request a Sample · sales@crestflousa.com · +1 346 594 9005
Cap Tube Fitting FAQ
What is a cap tube fitting?
It's a compression tube fitting that fits over the outside diameter of a tube end. The fitting's nut and ferrule (or ferrules) seal it leak-tight, closing off the tube run. Because it seals on the tube OD, it isn't a threaded pipe cap.
What is the difference between a cap and a plug?
A cap seals off the outside of a tube end, closing up a length of tubing. A plug works from the inside, sealing a fitting body and closing off a port that's not in use. Cap tubing when you want to dead-end it. Plug a compression port when you need it blanked off.
How do I install a tube cap?
Slide the cap over the tube OD until it seats, then pull up the nut to drive the ferrule (or ferrules) onto the tube. It's the same pull-up you'd use on any Crestflo compression fitting. The result is an all-metal, leak-tight dead-end, no thread sealant, no elastomer involved.
What sizes do cap tube fittings come in?
Tube outside diameters span 1/16″ to 2″ in the imperial, fractional series. In metric, sizes run 3 mm to 50 mm. Both ranges line up with the standard tube-fitting OD series, so the 2″ imperial size corresponds to the 50 mm metric size.
What materials are cap tube fittings available in?
The lineup includes 304/304L, 316/316L, 904L, Duplex 2205, Super Duplex 2507, 254 SMO, AL-6XN, Alloy 20, Inconel, Incoloy, Monel, Hastelloy, and Nickel 200/201, plus Titanium Grade 2. One exception: Brass isn't available on the US line.
Are Crestflo caps NACE compliant for sour service?
Yes, we can supply caps compliant with NACE MR0175 / ISO 15156 in the appropriate alloys for sour-service duty.
Will a Crestflo cap interchange with my Swagelok or Parker fitting?
Crestflo compression fittings meet ASTM F1387 for dimensional interchange. That means a cap drops right into an existing brand's tube-fitting system without any tweaks. Send us your part-number cross-reference, and our engineers will match it against your catalog codes. Swagelok® is a trademark of Swagelok Company. Crestflo isn't affiliated with or endorsed by Swagelok.
Do cap tube fittings come with material traceability?
Yes. Every part is marked with its heat/lot number, and each order ships with an EN 10204 MTC 3.1 as standard. Need 3.2 instead? Just ask.
Where are Crestflo caps made and are they TAA compliant?
Crestflo serves US customers as a manufacturer. Standard product comes from Indian origin, though US or European melt is available on request, and full material traceability is provided throughout. When TAA-eligible material is needed, that's handled as a capability offered on request, including PED.
What pressure is a cap tube fitting rated to?
A cap's pressure rating matches that of the tube-fitting system it terminates. That figure comes from the tube's OD, wall thickness and alloy, set against the applicable tube-rating standard. Ratings follow the applicable standard, and they can be adjusted to fit specific requirements.
Is a cap tube fitting the same as a refrigeration "cap tube"?
No, they're not the same thing. A refrigeration capillary ("cap") tube belongs to a completely different product category. What Crestflo calls a cap is an instrumentation compression tube cap, and it seals only on the tube's outer diameter.
Related Tube Fittings & Resources
- Tube-fitting plug, blank an unused fitting port
- Nut and ferrule sets, plus standalone ferrules
- Union and connector tube fittings
- NPT tube fittings for a threaded dead-end
- All tube fittings