Flanged Joints · Specifier Field Guide

How to Specify Pipe Flanges That Match Your Service, Pressure Class, and Joining System

A pipe flange is not a generic commodity. The wrong combination of face type, material, and pressure class can shorten a piping system's life, force a shutdown, or fail inspection on day one. This guide walks specifiers, buyers, and project engineers through how to ｓｅｌｅｃｔ flanges that actually fit the service — and how to keep the flange, the pipe, and the joining components aligned from RFQ to commissioning.

Why the Flange Choice Echoes Through the Whole Piping System

Every flanged connection is a system decision. The flange has to match the pipe wall schedule, the pipe fittings it sits between, the gasket, the stud bolt grade, and the industrial valves on either end. When one of those goes out of alignment, the bolted joint becomes the weak point in an otherwise well-engineered line.

EZ Steel Industrial has been manufacturing industrial tube, fittings, and flanges from its base in Changsha, Hunan since 1994. That depth matters at the flange stage because the company supplies the entire pressure boundary — carbon steel pipe, stainless, copper-nickel, fittings, flanges, gasket stud bolt nut sets, and valves — from a single quality system. A buyer can hold one supplier responsible for the whole flange-to-flange chain rather than chasing traceability across five different shops.

Step 1: Lock Down the Flange Standard and Pressure Class

Before anyone talks about face type or material, the project specification should already name the flange standard and the pressure class. The two most common families on industrial projects are ASME/ANSI (B16.5 for NPS ½ through 24, B16.47 for larger) and EN 1092-1 for European projects. Within each family, the class is the short-hand for pressure-temperature rating.

• ASME B16.5 Class 150 — low-pressure water, air, and utility service, including building services and process cooling water.
• ASME B16.5 Class 300 / 600 — the workhorses for hydrocarbon, chemical, and refinery process piping, typically paired with raised-face weld neck or slip-on flanges.
• ASME B16.5 Class 900 / 1500 / 2500 — high-pressure hydrocarbon manifolds, steam headers, and wellhead piping; RTJ facings become common above Class 600.
• EN 1092-1 PN 10 / 16 / 25 / 40 — typical European water, HVAC, and process ratings, with PN 63 and above reserved for high-pressure or boiler feed service.

The class chosen here constrains the rest of the design. A Class 150 slip-on flange cannot be reused as a Class 300 flange even if the bore matches, because the hub, bolt circle, and gasket seating dimensions change with each rating step.

Step 2: Pick the Flange Structure to Match the Job

Flange type is a function of the piping duty, not a stylistic choice. The most common industrial structures and where they earn their place:

For buyers sourcing the steel side of this family, the steel flanges line at EZ Steel covers WN, SO, BL, SW, TH, and LJ variants under ASME B16.5, EN 1092-1, GOST, JIS, and BS standards, so a single RFQ can carry the full flange schedule.

Step 3: Match the Flange Material to the Process Fluid

Material selection follows the pipe material on either side of the joint. Galvanic mismatch is the silent killer in mixed-metal systems, especially in seawater, chemical, or offshore service.

Carbon Steel Flanges for General Service

ASTM A105 forged carbon steel is the default for general hydrocarbon, steam, and water service up to about 425 °C. A350 LF2 covers low-temperature carbon service down to –46 °C and is required for impact-tested lines. When the line transitions to a higher alloy — for example, stainless pipe for clean service — a stainless stainless steel pipe section should end at a stainless flange of matching grade, not at a carbon-steel flange with a weldolet, to avoid bimetallic corrosion at the joint.

Stainless and Duplex Flanges for Corrosion Service

A182 F304/F304L and F316/F316L cover most chemical, food, and pharmaceutical service. F51 and F55 duplex are now the standard for seawater and high-chloride service because of their resistance to pitting and stress corrosion cracking. The flange material must match the pipe grade; mixing 316L pipe with a 304 flange is one of the most common MTR failures in receiving inspection.

Copper-Nickel Flanges for Seawater and Marine Service

Where the process medium is seawater or shipside cooling, 90/10 and 70/30 copper-nickel flanges are the industry default. EZ Steel's copper nickel flanges are produced to EEMUA 234 and the related naval specifications, paired with the matching Cu-Ni pipe and butt-weld fittings for full-system corrosion compatibility.

Step 4: Specify the Facing and Gasket Together

The flange face and the gasket are a single mechanical system. A raised-face (RF) flange can use a spiral-wound gasket with inner ring, a flexible graphite sheet, or a compressed non-asbestos sheet depending on temperature and fluid. An RTJ flange requires an RTJ gasket of the correct ring number (R, RX, BX) and a flange groove hardness compatible with the ring material.

Tongue-and-groove (T&G) and male-and-female (M&F) facings add mechanical alignment and are typically used in heat exchangers and small-bore systems. Flat face (FF) facings are reserved for low-pressure cast iron or fiberglass flanges, paired with full-face gaskets. The facing choice should be locked at the same time as the pressure class; changing it in the field requires re-machining the flange and usually invalidates the original pressure-temperature rating.

Step 5: Don't Treat the Stud Bolts as an Afterthought

The stud-and-nut material carries the entire load that holds the joint together. ASME B16.5 and B16.47 call out stud material by flange class and design temperature, and the most common pairings are:

• ASTM A193 B7 / A194 2H — standard alloy steel pairing for elevated-temperature hydrocarbon service.
• ASTM A193 B8 / A194 8 — austenitic stainless, used in low-temperature, corrosive, or cryogenic service, and required for stainless flanges to avoid galling and chloride-related bolt cracking.
• ASTM A193 B16 — chromium-molybdenum-vanadium for high-temperature service above about 400 °C, common in power and refinery fired-heater piping.
• ASTM A320 L7 / A194 4 or 7 — for low-temperature impact-tested service, down to –46 °C and below.

Coating choice matters too. PTFE- or Xylan-coated studs reduce galling during tightening, while zinc-flake and hot-dip galvanizing protect carbon steel studs in offshore or humid environments. Match the coating to the in-service atmosphere and any fire-protection requirement in the project document.

Step 6: Align the Flange with the Valves and Fittings Around It

A flange is only as good as the components it joins. A mismatch in face-to-face length, raised-face height, or bolt-hole alignment between a flange, a industrial valves body, and an adjacent pipe fittings shows up during hydrotest as gasket blowout or flange rotation under torque. The way to avoid it is to source the bundle from a single project-aware manufacturer that can hold a coordinated BOM.

EZ Steel's project model is built around that bundle. A specifier for a refinery, a desalination plant, or a power station can send one RFQ listing the flange schedule, the matching pipe, the pipe fittings (including butt weld fittings, socket weld fittings, and threaded fittings), the stud-bolt sets, and the matching valves — and receive one consolidated offer with one MTC trail.

Common Application Patterns Worth Knowing

Oil and Gas Manifolds and Process Piping

Weld neck flanges in A105 or A350 LF2, Class 300 to 900, are the default. Pair with spiral-wound gaskets, B7/2H stud sets, and matching butt-weld fittings. Sour service (NACE MR0175) needs controlled-hardness bolting and explicit material certification.

Seawater Cooling and Marine Systems

Copper-nickel pipe ends at copper-nickel weld neck flanges per EEMUA 234. Use 90/10 for general seawater and 70/30 for higher-velocity or polluted water. Avoid mixing Cu-Ni with carbon-steel flanges at the joint — that contact point is where galvanic attack starts.

Power Plant Steam and Boiler Feed

Alloy steel flanges (A182 F11, F22, F91) for high-temperature steam. Class 600 and above typically use RTJ facings, B16 studs, and ring-type joint gaskets sized to the standard ring number called out on the drawing.

Pharmaceutical, Food, and Hygienic Process

316L stainless with low-surface-finish flanges, often sanitary ferrule connections rather than ASME B16.5. Gaskets are typically EPDM or PTFE. Documentation should include full traceability, surface finish certification, and an FDA / EU food-contact statement where required.

A Practical RFQ Checklist for Pipe Flanges

• Standard and pressure class (for example, ASME B16.5 Class 300, EN 1092-1 PN 16)
• Flange type (WN, SO, BL, SW, TH, LJ) and quantity per size
• Size and bore (standard bore or pipe schedule-matched bore)
• Material grade and any NACE, impact-test, or sour-service requirement
• Facing (RF, FF, RTJ, T&G, M&F) and surface finish if stainless
• Coating or finish (black, galvanized, anti-rust oil, paint system)
• Documentation required (MTC EN 10204 3.1, PMI report, hydrotest certificate)
• Marking, traceability, and packing expectations for export shipment