[TL;DR] Picking the right pipe flange comes down to four variables: connection method (welded vs. threaded), pressure class (Class 150–2500 per ASME B16.5), facing type (RF, FF, RTJ), and pipe schedule match. Weld neck flanges suit high-pressure, high-temperature lines; slip-on and threaded flanges cut welding cost on medium-pressure runs; blind flanges cap pipeline ends. Tianjin Baode Steel supplies all major flange types from 1/2" to over 24", built to ASME B16.5, ASME B16.47, and BS 3293.
For procurement engineers sourcing pipeline fittings for chemical, petroleum, power, or urban gas projects, the flange line item looks simple on a purchase order - until the wrong facing or pressure class shows up on site. A mismatched flange means a stalled weld crew, a re-order, and a shipping delay measured in weeks, not days. Because flanges sit at every joint, valve connection, and equipment nozzle in a piping system, getting the type, size, and pressure rating right the first time has a direct effect on project schedule and total landed cost.
This guide breaks down the five most commonly specified flange types, compares them side by side, and walks through the exact specification steps a buyer should confirm before placing an order - based on production data from Tianjin Baode Steel Co., Ltd., a Tianjin-based manufacturer of carbon steel, alloy steel, and stainless steel pipe fittings.
Flange Types and Specification Range
Baode Steel's Yanshan County pipe fitting factory produces five core flange categories, each matched to a distinct pressure, connection, and application profile:
Weld Neck (WN) Flange - long tapered hub, one full-penetration butt weld, rated across all ASME pressure classes (150–2500); used on high-pressure, sub-zero, or elevated-temperature lines per ASME B16.5 / B16.47.
Slip-On (SO) Flange - slides over the pipe end, secured with one or two fillet welds; generally rated to Class 600; lower installation cost than WN.
Socket Weld (SW) Flange - single fillet weld, typically 1/2" to 4"; used on smaller, high-pressure piping.
Threaded Flange - screwed connection, no welding required, generally 1/2" to 4" and up to Class 300; practical for thin-wall pipe where welding is restricted.
Blind Flange - solid, no bore, used to seal a pipeline end or isolate a section for testing; available across the full size and pressure range.
All five types are produced in carbon steel (A105N), alloy steel (P91, F91, and other Cr-Mo grades), and austenitic stainless steel, with pressure classes stamped per standard so a mill test report can be traced back to a specific heat.
WN vs SO vs Threaded vs Blind Flange: Side-by-Side Comparison
Each flange type trades off joint strength, installation cost, and pressure ceiling differently. The table below lines up the four decision factors buyers ask about most:
| Flange Type | Max ASME Class | Weld Required | Joint Integrity | Best Fit |
|---|---|---|---|---|
| Weld Neck | 150–2500 | One butt weld | High | High-pressure / high-temp lines |
| Slip-On | ≤ 600 | One or two fillet welds | Medium | Medium-pressure utility lines |
| Socket Weld | ≤ 600 | One fillet weld | Medium | Small-bore, high-pressure piping |
| Threaded | ≤ 300 | None | Low | Thin-wall pipe, no-hot-work areas |
| Blind | All classes | None | N/A (no bore) | Line-end sealing, section isolation |
RF Blind Flange - Blind Flange range
Threaded Flange - Threaded Flange range
How to Specify the Correct Flange: 6-Step Checklist
- Confirm type and facing. State both together, e.g. "weld-neck RF" or "socket-weld RF" - facing (RF, FF, RTJ) determines the gasket seating surface.
- Match Nominal Pipe Size (NPS). The flange bore must equal the connecting pipe or fitting size, e.g. NPS 4 or NPS 6.
- Set the pressure class. ASME B16.5 defines seven classes - 150, 300, 400, 600, 900, 1500, and 2500 - chosen by system design pressure and temperature.
- Pick the governing standard. ASME B16.5, ASME B16.47, BS 3293, DIN, or API 6A, depending on the destination market and project spec.
- Verify material compatibility. Carbon steel A105N for general service, alloy grades (P91, F91) for high-temp service, or stainless for corrosive/hygienic lines - material must match the connecting piping spec.
- Match pipe schedule. For WN and lap-joint flanges, the flange bore must equal the pipe schedule - Sch 40, Sch 80, Sch 120, or Sch 160.
Flange ratings are also temperature-dependent: as operating temperature rises, the pressure-handling capacity of a given class falls, so the class selected at room temperature is not automatically valid at operating temperature - always check the class/temperature curve for the specified material against your maximum service temperature.
Why Buyers Source Flanges From Baode Steel
- Established 2006 in Tianjin, with a dedicated pipe fitting factory in Yanshan County, Cangzhou, producing carbon steel, stainless steel, and alloy steel elbows, tees, reducers, and flanges.
- ISO9001, ISO14001, ISO45001 certified, plus AAA Credit Enterprise rating.
- 420+ sets of production equipment including push machines, press machines, and large heat treatment furnaces; over 20,000 tons of annual pipe fitting output.
- Products exported to 40+ countries across North America, South America, Southeast Asia, the Middle East, and Africa.
- Packaging facility ships 250+ containers per month, with VCI treatment, silica gel packs, and ISPM-15 wooden-case fumigation for overseas shipments.
Frequently Asked Questions
Q1: What are the standard ASME pressure classes for pipe flanges?
A: ASME B16.5 defines seven pressure classes for flanges: Class 150, 300, 400, 600, 900, 1500, and 2500. The class required depends on system design pressure and the flange's rated temperature.
Q2: How does temperature affect a flange's pressure rating?
A: Flange ratings are temperature-dependent - as operating temperature increases, the pressure-handling capacity of the same class decreases, so the class must be checked against the material's rated pressure/temperature curve, not just room-temperature pressure.
Q3: Which flange classes are most common in industrial piping?
A: Class 150, Class 300, and Class 600 are the most frequently specified ratings in general industrial piping, covering the majority of chemical, power, and utility applications.
Q4: What causes a flange connection to leak?
A: Dirty or damaged flange faces are the most common cause - dirt, scale, scratches, weld spatter, or warped sealing surfaces create leak paths or uneven gasket compression at the joint.
Q5: How is a flange connection leak-tested?
A: The piping system is filled with water and pressurized to a specified test pressure above normal operating pressure, then the flange joint is visually checked for any water leakage around the gasket seat.
Matching the wrong flange type to your pressure class is one of the most common - and most expensive - sourcing mistakes on a piping project. Getting the type, class, standard, and facing right up front keeps your weld crew moving and your project on schedule.
For custom flange specifications or a detailed quotation, contact our technical team via email or WhatsApp, or submit an inquiry.
