Companion Flanges
The no-weld way to put a flanged joint on threaded pipe. What companion flanges are, where they earn their keep, and the numbers you need to spec one.
What Is a Companion Flange?
A companion flange is the flange that accompanies its mate at a bolted connection. Ask for one at a supply counter and you will get a threaded flange: internal NPT threads per ASME B1.20.1, flat or raised sealing face, and a bolt pattern to ASME B16.5. Screw it onto threaded pipe, add a gasket and bolts, and a threaded line becomes a flanged connection with no welding involved.
That role is why the part exists. Valves, pumps, water meters, and strainers ship with flanged ends. Miles of small-bore pipe in utility, municipal, and commercial service carry threads. The companion flange is the adapter that marries the two, and it comes off just as easily when the equipment needs service.
One naming caution: gearheads use the same term for the flange that couples a driveshaft to a differential. Same words, different world. Everything on this page is piping.
When a Companion Flange Is the Right Call
No Hot Work Allowed
Occupied buildings, wastewater plants, and classified areas where a welding permit is a project of its own. Threads make the connection cold.
Flanged Equipment on Threaded Pipe
Meters, pumps, check valves, and butterfly valves with flanged ends need a mating flange. A companion flange adapts the existing threaded line in minutes.
Planned Disassembly
Seasonal irrigation headers, skid equipment, and temporary process lines that come apart on a schedule. Unbolt, unthread, reuse.
Companion vs. Other Flange Connections
The attachment method drives everything: installation cost, pressure capability, and how the joint behaves over time.
| Flange Type | Attachment | Welding | Typical Service |
|---|---|---|---|
| Threaded Companion | Screws onto NPT pipe threads | None (optional seal weld) | Small-bore, low to moderate pressure, no-weld zones |
| Slip-On | Slides over pipe end | Fillet welds inside and out | General service, easier field alignment |
| Socket Weld | Pipe seats in socket bore | Single fillet weld | Small-bore, higher pressure systems |
| Weld Neck | Butt-welded to pipe bevel | Full penetration weld | High pressure, high temperature, critical lines |
Class 150 Threaded Companion Flange Dimensions
Selected ASME B16.5 Class 150 dimensions, in inches. Threaded companion flanges share the standard B16.5 envelope, so they bolt to any matching flange of the same class and size.
| NPS | Flange OD | Bolt Circle | Bolts | Bolt Size | Min. Thickness |
|---|---|---|---|---|---|
| 1/2" | 3.50 | 2.38 | 4 | 1/2" | 0.44 |
| 1" | 4.25 | 3.12 | 4 | 1/2" | 0.56 |
| 2" | 6.00 | 4.75 | 4 | 5/8" | 0.75 |
| 3" | 7.50 | 6.00 | 4 | 5/8" | 0.94 |
| 4" | 9.00 | 7.50 | 8 | 5/8" | 0.94 |
| 6" | 11.00 | 9.50 | 8 | 3/4" | 1.00 |
Representative values. Always verify against the current edition of ASME B16.5 for the applicable class, size, and material group.
Pressure Ratings and Limits
Class 150 and Class 300 cover nearly all companion flange applications. For A105 carbon steel (Material Group 1.1), ASME B16.5 rates Class 150 at 285 psi and Class 300 at 740 psi at ambient temperature, with ratings stepping down as service temperature climbs.
The threads are the limiting factor, not the flange body. NPT engagement thins the effective pipe wall and can seep under vibration or thermal cycling, which is why codes derate threaded joints in higher classes and why many specs call for a seal weld in critical service. Practical guidance: keep threaded companion flanges on small-bore lines at moderate pressure and temperature, and move to socket weld or weld neck when the service turns severe.
Face types follow the same rules as any B16.5 flange: raised face is standard, flat face mates with flat-faced equipment like cast iron pump housings, and a full-face gasket belongs on every flat-face joint.
Common Materials
| Material | Grade | Typical Service |
|---|---|---|
| Forged Carbon Steel | ASTM A105 | General process, water, air, oil and gas utility lines |
| Stainless Steel 304/304L | ASTM A182 F304/F304L | Corrosive service, food and beverage, water treatment |
| Stainless Steel 316/316L | ASTM A182 F316/F316L | Marine, chloride exposure, chemical service |
| Galvanized Carbon Steel | A105 hot-dip galvanized | Outdoor, municipal, and irrigation systems |
Cast iron and bronze companion flanges exist in the plumbing world; the industrial market runs on forged steel and stainless per the grades above.
Companion Flange FAQ
What is a companion flange?
A companion flange is the flange that mates with another flange to complete a bolted joint. In supply catalogs the term almost always refers to a threaded companion flange: a flange with internal NPT threads that screws onto threaded pipe, letting you adapt a threaded line to flanged equipment without welding.
Is a companion flange the same as a threaded flange?
In industrial piping the two terms overlap heavily. "Threaded flange" describes the attachment method under ASME B16.5. "Companion flange" describes the role: it accompanies a mating flange on a valve, pump, or meter. Most parts sold as companion flanges are threaded flanges by construction.
What about driveshaft companion flanges?
The automotive world uses the same words for the flange that couples a driveshaft to a differential or gearbox. That is a powertrain component, not a piping product. This page covers pipe flanges only.
What sizes and pressure classes are available?
Threaded companion flanges follow ASME B16.5 dimensions from NPS 1/2 through NPS 24, with Class 150 and Class 300 covering the vast majority of applications. Higher classes exist but threads derate quickly, so welded connections usually take over above Class 300.
The Parts You Need, When You Need Them
Companion flanges in carbon steel, stainless, and galvanized, alongside the full range of ASME B16.5 flange types. Send your size, class, and material for pricing.
Request a QuoteRelated reading: all flange types and flange dimension resources.