Cast Iron Flange Unions

Cast iron flange unions are bolted steam pipe fittings that join two sections of pipe while keeping the joint fully removable without cutting or rethreading. Rated at 125 PSI for steam service, cast iron flange unions are the standard disconnection fitting on low-pressure residential and light commercial steam heating systems, from boiler headers to individual zone supply and return runs. Anvil manufactures the cast iron flange union lineup carried at SupplyHouse, with confirmed sizes running from 3/4" through 2-1/2" under the Anvil brand. A bolted flange joint on a steam line breaks cleanly when service demands it. A threaded union at the same size does not always cooperate.

KEY SUBCATEGORIES / PRODUCT TYPES

Anvil SKU 0308300409 is a 3/4" Black Cast Iron Steam Flange Union with Gasket, rated at 125 PSI. Anvil 0308300409 suits 3/4" NPS steam supply branches at the tail end of a distribution system, including individual zone coil connections and small equipment drops. Steam flange unions function the same way standard bolted flanges do, with two mating flat faces and a gasket compressed between them. Confirm the bolt-hole count and bolt-circle diameter against the mating flange on any existing system before ordering.

Anvil SKU 0308300607 is a 1" Black Cast Iron Steam Flange Union with Gasket, rated at 125 PSI. Anvil 0308300607 is a common spec on 1" zone supply and return piping in two-pipe steam systems serving multiple zones in residential buildings. Steam gaskets are required for all cast iron steam flange union installations. Rubber gaskets aren't suitable for steam service and can fail under operating temperature and pressure.

Anvil SKU 0308300805 is a 1-1/4" Black Cast Iron Steam Flange Union with Gasket, rated at 125 PSI. Anvil 0308300805 bridges the step between smaller branch connections and the larger main distribution sizes, showing up on intermediate distribution runs in multi-zone light commercial steam systems. As with all sizes in the Anvil cast iron flange union lineup, the bolt pattern on Anvil 0308300805 follows ASME B16.1, with likely 4 bolt holes at this pipe size. Non-standard flanges on older systems may not match and should be measured before ordering.

Anvil SKU 0308301407 is a 2-1/2" Black Cast Iron Steam Flange Union with Gasket, rated at 125 PSI. Anvil 0308301407 serves 2-1/2" NPS steam mains and header runs where a full bolted joint is more practical than a threaded union at that diameter. A bolted flange connection at 2-1/2" is easier to break cleanly for service than any threaded alternative. Match the SKU to the pipe size. There's no workaround for a size mismatch on a steam fitting.

BUYING GUIDE / HOW TO CHOOSE

How to choose the right Anvil cast iron flange union starts with pipe size. Anvil SKUs 0308300409, 0308300607, 0308300805, and 0308301407 cover 3/4", 1", 1-1/4", and 2-1/2" NPS respectively. Match the SKU to the nominal pipe size of the steam pipe being connected. Nothing else in the selection process matters if the pipe size is wrong.

Bolt pattern is the second confirmation step, and the one that stops jobs cold when it's skipped. Anvil cast iron flange unions follow ASME B16.1 bolt-hole patterns. Steam systems installed in older buildings may carry non-standard flanges that don't align with those patterns. Before any SKU ships for an existing system, confirm the outside flange diameter, bolt-circle diameter, and the number and size of bolt holes on the mating flange. Standard Anvil patterns should match standard existing flanges. Custom or pre-1980 flanges may not.

Pressure rating is a fixed constraint, not a variable to round up from. All Anvil cast iron flange union SKUs on the SupplyHouse cast iron flange union page carry a 125 PSI steam rating. Residential and light commercial low-pressure steam systems operate well within that threshold. Systems operating above 125 PSI require a different fitting class entirely, and the Anvil cast iron steam flange union lineup isn't the right specification for those applications.

Gasket material is the last decision and one that gets missed more often than it should. Anvil cast iron steam flange unions require a steam-rated gasket. Per product Q&A on the SupplyHouse cast iron flange union listings, rubber gaskets aren't appropriate for steam service. Spiral-wound metallic or compressed non-asbestos sheet gaskets rated for the system's operating temperature and pressure are the correct choices. Confirm the gasket spec before the joint is closed.

FAQs

What PSI rating do Anvil cast iron steam flange unions carry?

Anvil cast iron steam flange unions at SupplyHouse, including SKUs 0308300409, 0308300607, 0308300805, and 0308301407, are rated at 125 PSI for steam service. Low-pressure residential and light commercial steam heating systems operate well within that threshold. Applications pushing above 125 PSI require a different fitting class and the Anvil cast iron steam flange union lineup wouldn't be the right specification for those jobs.

What gasket is required for Anvil cast iron steam flange unions?

Anvil cast iron steam flange unions require a steam-rated gasket, not a rubber gasket. Per product Q&A on SupplyHouse cast iron flange union listings, rubber gaskets aren't suitable for steam service and can deteriorate under steam temperature and pressure. Spiral-wound metallic gaskets or compressed non-asbestos sheet gaskets rated for the system's operating conditions are the correct specification. Verify the gasket's pressure and temperature rating before closing the joint.

How do you confirm an Anvil cast iron flange union will bolt up to an existing fitting?

Confirming compatibility with an Anvil cast iron flange union requires checking the outside flange diameter, bolt-circle diameter, and the number and size of bolt holes on the existing mating flange. Per product Q&A on SupplyHouse listings, standard flanges typically carry 4 or 5 bolt holes and Anvil ASME B16.1 patterns should align with standard existing flanges. Steam systems in older buildings may carry non-standard or custom-drilled flanges that won't match, so measuring before ordering is the right move.

When should Anvil SKU 0308301407 be specified over a smaller cast iron flange union size?

Anvil SKU 0308301407 is the correct specification when the steam pipe being connected is 2-1/2" NPS. Cast iron flange union sizing is determined by the nominal pipe size of the system, not by preference or availability. Specifying a smaller or larger SKU than the pipe size isn't a workaround. Anvil 0308301407 serves 2-1/2" steam mains and header runs where a bolted flange joint is more practical than a threaded connection at that diameter.