An expansion joint flange is the end connection that bolts a joint to the mating flanges on your pipe, duct, or equipment. A correct spec defines five details: connection type, drilling standard, pressure class, size and length, and facing and material. Matching each detail to the system leads to a clean bolt-up and a tight seal on day one.
Most rubber expansion joint jobs start with a flanged connection drilled to an ANSI flange pattern, ASME B16.5 Class 150. Threaded and weld-end connections also have a place in many plants. The best choice depends on pipe size, pressure, temperature, and how often you need to replace the joint.
Every set of industrial expansion joints starts with one question: what does it bolt, thread, or weld to? This guide covers the common end connection types and how to specify flange class and size. It also covers the connection mismatches that most often delay an installation.
Common End Connection Types
The end connection determines how the joint attaches, seals, and how easy it is to replace. Bellows design and restraint hardware sit on top of this choice. Getting the expansion joint flange or end fitting right comes first.
Flanged Ends
A flanged connection is the most common end type for rubber, PTFE, and metallic joints. On a rubber expansion joint, the flange is usually integral, with the rubber body extending outward to form the sealing face. Split steel retaining rings sit behind it and spread the bolt load evenly.
Zepco’s EPDM joints come in a flange or belt design. Metallic bellows joints use steel flanges welded to the bellows collars, in a fixed or floating style. Floating flanges, also called lap joint or Van Stone flanges, rotate freely to make bolt hole alignment easy during installation.
Duct and fabric joints attach to angle-frame flanges with a custom bolt pattern or use a belt element clamped between the duct flange and a backup bar. Both single and double flanged designs are common in flue duct service. On these systems, the expansion joint flange pattern comes from the duct drawing itself.
Threaded Ends
Threaded connections use NPT male ends or union fittings on small-bore rubber-and-metal connectors. They often appear at pumps and HVAC equipment, in sizes up to about 3 inches. They install quickly and keep costs low.
Large diameters, high temperatures, and frequent replacement call for a flanged design. Unthreading a connector can disturb the adjacent piping during service. For those lines, flanges or weld ends are the better fit.
Weld Ends
Weld end connections serve metallic bellows joints in high-pressure, high-temperature service such as main steam lines. The pipe ends are beveled and butt welded into the line. The joint becomes part of the pipe, with a fully welded seal.
To specify weld ends, match the pipe schedule and a weld-compatible material to the run pipe, and call out the bevel preparation. Replacement involves cutting the joint out and welding in a new one. That work calls for hot work permits and inspection, so plan the outage window early.
How to Specify Flange Class and Size
A complete flange callout names the standard, class, nominal size, facing, and material. A short entry such as “6 in flanged” leaves the fabricator guessing, and guesses lead to delays. A complete expansion joint flange spec gives the shop every detail it needs on the first pass.
Name Your Drilling Standard
In North America, an ANSI flange usually means ASME B16.5 for NPS ½ through 24. Above NPS 24, ASME B16.47 applies, and its Series A and Series B use different bolt patterns. Water systems may use AWWA C207 drilling, and imported equipment often arrives with EN 1092 (PN) or JIS flanges.
Duct joints follow a custom pattern, whether round or rectangular. For these, list the bolt hole diameter, hole count, spacing, and frame dimensions. A drawing of the existing duct flange works well too.
Separate Flange Class From Joint Rating
This line on the spec sheet causes the most confusion. A rubber or PTFE joint drilled to a Class 150 pattern has Class 150 bolt-hole dimensions, and its pressure rating comes from the joint body and reinforcement. List design pressure, vacuum, test pressure, and temperature as separate entries.
Class also matters for fit. Class 150 and Class 300 flanges of the same size use different bolt circles and bolt counts. Cast iron Class 125 flanges share bolting with Class 150, and Class 250 shares bolting with Class 300, which helps when connecting to older pumps and valves.
Confirm Size, Length and Bore
Nominal size is the starting point. The overall length, measured face to face, must match the gap between the mating flanges, with any precompression or preset noted. For replacements, measure the installed gap and use that figure in the spec.
A clear sizing method also makes sure the length accounts for the movement the joint must absorb. If the line changes size at the joint, specify a concentric or eccentric reducer joint. State which end is the large end and which is the small end.
Specify Facing and Flange Material
Note the mating flange facing: flat face (FF) or raised face (RF). Integral rubber flanges seal on their full face, so the manufacturer needs to know what they will press against. The expansion joint flange facing and the mating flange facing should form a full, even contact.
Specify the retaining ring and flange material, such as carbon steel, galvanized steel, or stainless steel, for the outside environment. High temperature chemical lines may call for Viton joints, which Zepco offers in flat belt or integral flange geometry. Aggressive media may call for Teflon joints, where the liner extends across the flange face.
A complete spec sheet lists the connection type, standard and class, nominal size, and overall length and bore. It also names the facing, the flange, ring, and body materials, and the full service conditions, including pressure, vacuum, temperature, media, and movements. Finally, it calls out any control rods, liners, or covers that attach at the flange.
Avoiding Connection Mismatches
Connection problems tend to show up on installation day, when fixes cost the most. Checking each point below before the order goes in saves time and money. Each check protects the expansion joint flange fit on site.
Mixed Drilling Standards and Classes
ASME and EN flanges of similar size look almost identical, and their bolt circles and hole sizes differ enough to stop bolt-up. Confirm the standard on both mating flanges, especially on imported pumps, chillers, and skid packages. This step matters most on projects with mixed sourcing.
A Class 150 joint fits a Class 150 flange, and a Class 300 flange needs a Class 300 joint. Above NPS 24, confirm B16.47 Series A or Series B before ordering. A series mix-up often shows only when the flanges sit side by side, so check the drawings early.
Facing and Gasket Conflicts
Integral rubber flanges usually seal on their own and need a separate gasket only when the manufacturer calls for one. An added gasket can reduce clamping and let the rubber extrude. Pair flat face cast iron flanges with flat face mating flanges to protect the cast iron from cracking.
Follow the manufacturer’s sealing guidance for PTFE and other lined designs. For this reason, PTFE expansion joints in hazardous service often rely on an integral flange seal. A clean, even sealing face keeps these joints leak-tight.
Valve Clearance and Alignment
Wafer-style valves need a full flange face to seal against. A butterfly valve disc can swing into the joint’s bore, so check the disc clearance against the joint’s inside diameter. A short spool piece solves most clearance issues.
Good piping alignment comes before the joint goes in. Pulling misaligned flanges together with the joint uses up movement capacity before the system ever starts. Sound geometry selection matches the joint to the movement at its location, and it works best on aligned piping.
Bolts and Restraint Points
On arched rubber joints, insert bolts with the heads on the arch side so the threaded ends point away from the body. Correct bolt length keeps the threads clear of the arch and cover during movement. A quick bolt-length check before installation protects the joint.
Joints that need control rods or limit rods require flanges or retaining rings built to accept them. Unrestrained joints in pressurized lines can experience significant pressure thrust. Settle the restraint design before the flange design is final.
Ordering Your Expansion Joint Flange
A few minutes spent on connection details can save days of rework on site. We review connection details, operating conditions, and movements with every customer before fabrication starts. An engineering consultation catches a mismatch on paper, where it costs the least.
When your spec sheet is ready, Zepco’s team can confirm every expansion joint flange detail before the shop cuts steel. We turn around inquiries in under 24 hours. That quick response keeps your project schedule on track.
FAQ
What is an expansion joint flange?
It is the end connection that bolts an expansion joint to the mating flanges of a pipe, duct system, or piece of equipment. Rubber joints usually use an integral rubber face backed by steel retaining rings, and metallic joints use welded steel flanges. The flange must match the system’s drilling standard, class, size, and facing.
Do rubber expansion joints need gaskets?
Most rubber expansion joint designs with integral flanges seal with their own rubber face. An added gasket can reduce clamping force and cause extrusion. Lined and specialty designs follow different rules, so always follow the manufacturer’s sealing instructions.
Can a Class 150 joint bolt to a Class 300 flange?
Each class has its own bolt circle, bolt hole size, and bolt count. A Class 150 joint fits Class 150 flanges, and a Class 300 flange calls for a Class 300 joint. Matching the class on both sides keeps every bolt hole in line.
Is a Class 150 drilled rubber joint rated for Class 150 pressure?
The flange class on a rubber or PTFE joint describes the bolt pattern. The pressure rating comes from the joint body and its reinforcement. List design pressure, vacuum, and temperature as separate items on the spec sheet.
What is the difference between ANSI and DIN flanges on expansion joints?
An ANSI flange under ASME B16.5 or B16.47 uses inch dimensions and pressure classes such as 150 and 300. DIN or EN 1092 flanges use metric dimensions and PN ratings such as PN10 and PN16. Each system has its own bolt pattern, so confirm the standard on both mating flanges, whether the equipment is domestic or imported.
When are weld ends the right choice?
Weld ends suit high-pressure, high-temperature lines such as main steam, where a fully welded seal adds security. They are used on metallic bellows joints with materials and wall thickness matched to the run pipe, a standard approach in steam piping design. Flanges suit joints that need periodic replacement, and the expansion joint flange then becomes the main spec item.
What information is needed to order a replacement expansion joint?
Provide the connection type, flange standard and class, nominal size, measured gap, materials, and full operating conditions. Matching the size, flange rating, and end fittings of the existing system is central to replacing joints smoothly. Photos and measurements of the old joint confirm details that older drawings may miss.
How tight should expansion joint flange bolts be?
Rubber and fabric joints use low bolt torque values, and gentle tightening protects the sealing face. Tighten in a cross-pattern torque sequence to the manufacturer’s values. Recheck the bolts after the first heat cycle.
What are floating flanges on an expansion joint?
Floating flanges rotate freely around the joint ends. This movement makes it easy to line up bolt holes during installation. They are common on molded rubber joints and on metallic joints in small to mid-size pipe.
Which end connection types suit small bore piping?
Threaded NPT ends and union fittings suit small bore lines up to about 3 inches. They are common at pumps and HVAC equipment, where quick installation helps. Flanged ends take over on larger lines and on systems with high temperature or frequent replacement.