Companion Flanges
The no-weld way to put a bolted, flanged joint on threaded pipe. Here's what a companion flange is, where it earns its keep, and the numbers y'all need to spec one right the first time.
What Is a Companion Flange?
A companion flange is the flange dat pairs up with a mating flange to close a bolted joint. In the flange supply world, dat term almost always means a threaded flange: a flange machined with internal NPT threads per ASME B1.20.1 dat screws directly onto threaded pipe. Just add a gasket and a set of bolts, and your threaded line becomes a flanged connection without a single weld, yessir.
Dat right there is the whole reason the part exists. Valves, pumps, meters, and strainers ship with flanged ends, while a whole heap of small-bore utility, municipal, and commercial pipe runs on threads. The companion flange is the adapter dat brings the two together, and as long as it ain't seal-welded and the threads are still good, it unbolts and unthreads just as clean when the equipment comes out for fixin'.
One caution on the name, now. The driveline world uses “companion flange” for the part dat couples a driveshaft to a differential. Same two words, whole different universe. Everything on this here page is pipe flanges.
How the Joint Goes Together
The companion flange threads right onto the pipe, a gasket seats against the matin' flange, and bolts pull dat joint nice and tight. Ain't no weld needed to assemble it. A seal weld at the thread is optional, is hot work, and gives up dat unthread-later benefit.
When a Companion Flange Is the Right Call
Three situations put a threaded companion flange right at the top of the list:
No hot work permitted. Occupied buildin's, water and wastewater plants, and classified areas where a hot-work permit is its own paperwork project. Threads make the joint cold, so the line goes together with a pipe wrench instead of a weldin' machine.
Flanged equipment on threaded pipe. Meters, pumps, check valves, and butterfly valves arrive with flanged ends and need a matin' flange to land on. A companion flange adapts dat existin' threaded run in minutes, with no bevel prep and no welder on site, I tell you what.
Built to come apart. Seasonal irrigation headers, rental and skid equipment, and temporary process tie-ins that y'all plan to take apart on a schedule. Just unbolt it, unthread it, and use it again next season, friend.
On assembly, make dem threads up with a suitable thread compound or PTFE tape and follow ASME B1.20.1 engagement practice. And y'all gotta know when to reach for somethin' else: severe cyclic, high-temperature, Category M or lethal, and high-vibration service is weld neck or socket weld territory, not threaded.
Companion vs. Other Flange Connections
How a flange attaches to dat pipe drives everything downstream: install cost, pressure ceiling, and how dat joint holds up over years of hard service.
| Flange Type | How It Attaches | Welding Required | Where It Fits |
|---|---|---|---|
| Threaded Companion | Threads onto NPT pipe | None to assemble; optional seal weld (hot work) | Small-bore, low to moderate pressure, for them spots where y'all can't be weldin' |
| Slip-On | Slides right over the pipe end | Two fillet welds, inside and out | General service; easier fit-up than weld neck |
| Socket Weld | Pipe drops right into a bored socket (~1/16" gap) | One external fillet at the hub, per WPS | Small-bore (usually NPS 2 and under) when that pressure's runnin' high |
| Weld Neck | Butt-welds to the pipe bevel | Full-penetration weld | High pressure, high temperature, critical service |
Each type got its own page with all the details: slip-on, socket weld, weld neck, and threaded.
Class 150 Threaded Companion Flange Dimensions
Selected ASME B16.5 Class 150 dimensions in inches. A threaded companion flange sits right in that standard B16.5 envelope, so it'll bolt up to any matchin' flange of the same size and class, no matter how that mate hooks up to its own pipe.
| NPS | Flange OD | Bolt Circle | Bolts | Bolt Size | Min. Flange 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. Y'all better verify against the current edition of ASME B16.5 for the applicable size, class, and material group. Need the full set? Our flange dimensions and weights reference has ever' class and size, our Class 300 dimensions cover the next class up, and our flange size chart guide walks y'all through how to read dem tables.
Pressure Ratings and Limits
Class 150 and Class 300 cover nearly every companion flange application. For A105 carbon steel (Material Group 1.1), ASME B16.5 rates Class 150 at 285 psig and Class 300 at 740 psig at ambient temperature, and both ratings step down as service temperature climbs. For a stainless or alloy companion flange, read the rating from the B16.5 pressure-temperature table for dat material group, not the A105 numbers.
The threads set the ceilin', not the flange body. NPT engagement thins the effective pipe wall and can weep under vibration or thermal cyclin', so a threaded companion flange is rarely the right call for severe cyclic, high-temperature, Category M, lethal, or high-vibration service, and codes derate threaded joints in the higher classes. Where the code or owner requires extra leak integrity, a light seal weld might be added after makeup; it's optional, it's hot work, and it limits future disassembly. A seal weld improves sealin' but it don't give the joint dat weld-neck strength.
The practical rule: keep dem threaded companion flanges on small-bore lines at moderate pressure and temperature, and move on over to socket weld or weld neck when the service turns severe. Larger threaded flanges are dimensionally possible but are rarely stocked above about NPS 3 to 4, so y'all better plan on welded types for big bore. If you're weighin' the trade-offs, our flange buyer's guide lays out the whole decision path for you.
Face types follow the same rules as any B16.5 flange. Raised face is the default, flat face mates up to flat-faced equipment like cast iron pump housings, and a flat-face joint always takes a full-face gasket, yessir.
Common Materials
| Material | Grade | Typical Service |
|---|---|---|
| Forged Carbon Steel | ASTM A105 | General process, water, air, and 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 after machinin' | Outdoor, municipal, and irrigation systems |
Galvanizin' is a finish applied after machinin', not a pressure grade; dat rating comes straight from the base steel. Cast iron and bronze companion flanges live in the plumbing trade, while dem industrial lines run on forged steel and stainless in the grades above.
Texas Flange been keepin' companion flanges right on the shelf in carbon steel, stainless, and galvanized since 1986, right alongside the whole range of ASME B16.5 flange types. Just send us your size, class, and material, and we'll quote it for y'all, ready to ship straight from stock.
