ASME B16.5 Flange Standards: Pressure Ratings, Dimensions & Types

ASME B16.5 Flange Standards: Pressure Ratings, Dimensions & Types

1. Introduction: Global Governing Standard for ANSI Flanges

ASME B16.5 is the most widely adopted piping flange standard across global oil & gas, refinery, chemical, power, LNG and water treatment industries. It unifies flange design, dimension, pressure-temperature rating, marking and testing rules, enabling interchangeability among flanges from different manufacturers worldwide. For piping engineers, procurement teams and EPC contractors, ASME B16.5 is the core reference when selecting forged flange joints for process pipelines. It provides consistent technical rules to reduce leakage risk and simplify project specification.

2. Scope and Nominal Pipe Size Coverage of ASME B16.5

ASME B16.5 applies to forged pipe flanges and flanged fittings, covering nominal pipe size NPS 1/2” to NPS 24” (DN15 – DN600). This standard defines flanges for seven pressure classes: Class 150, 300, 400, 600, 900, 1500, 2500LB. Noted: ASME B16.5 only covers forged flanges. Cast flanges are not within this standard scope. For sizes larger than NPS24, engineers shall switch to ASME B16.47.

3. Pressure-Temperature Ratings (Class 150 to Class 2500)

The Class marking in ASME B16.5 is pressure-temperature rating, not fixed static pressure value. Material strength drops when temperature rises, so the maximum allowable working pressure will derate accordingly. ASME B16.5 publishes dedicated P-T tables grouped by material categories (carbon steel, stainless steel, duplex, alloy steel).

• Class150: Low-pressure general service, common for cooling water, low-pressure gas

• Class300 / 400: Medium-pressure process piping in chemical plants

• Class600 / 900: High-pressure hydrocarbon, steam service

• Class1500 / 2500: Severe high-pressure, high-temperature critical pipelines, usually matched with RTJ ring joint face Before flange selection, engineers must cross-check material group, operating temperature and working pressure against the official P-T chart of ASME B16.5.

4. 8 Flange Types Defined in ASME B16.5 & Typical Applications

1. Weld Neck Flange (WN)：Tapered hub + butt weld, excellent fatigue resistance, for high temperature, high pressure, cyclic load piping.

2. Slip On Flange (SO)：Pipe slips into flange bore, fillet weld, low cost, easy installation for low & medium pressure.

3. Blind Flange (BL)：Solid blank flange, close pipeline ends, for isolation, maintenance and pressure test.

4. Socket Weld Flange (SW)：Socket bore, pipe inserted and welded, preferred for small bore high-pressure piping NPS 1/2”–2”.

5. Threaded Flange (TH)：NPT threaded connection, no welding, for low-pressure non-cyclic service.

6. Lap Joint Flange (LJ)：Matched with stub end, rotating flange for easy bolt hole alignment.

7. Reducing Flange：Transition flange to connect two different pipe sizes.

8. Orifice Flange：Special flange for flow meter / orifice plate installation.

5. Dimension Marking Rules and Tolerance Requirements

All dimension data including flange OD, bolt circle diameter, bolt hole quantity & diameter, flange thickness, hub height, bore size are standardized in ASME B16.5 dimension tables. Mandatory tolerance rules cover overall thickness, bore diameter, bolt hole position, flange face flatness and surface roughness. Sealing face finish is also specified: RF raised face height, FF flat face, RTJ ring groove dimension and finish. Every qualified flange shall carry permanent marking: standard ASME B16.5, Class, NPS, material grade, heat number and manufacturer mark.

6. ASME B16.5 vs ASME B16.47: Boundary and Connection Rules

ASME B16.5: NPS 1/2” ~ 24” ASME B16.47: NPS 26” ~ NPS 60”, split into Series A and Series B

• Series A: Equivalent to MSS SP-44, larger bolt circle, heavier body

• Series B: Compact dimension, smaller outer diameter Two standards cannot be mixed arbitrarily. When a pipeline transitions from NPS24 to NPS26, the designer must use a reducer or transition spool to match B16.5 and B16.47 flange connection. Gasket and bolt sets are not interchangeable between the two standards.

7. ASME B16.5 Compliant Flange Supply | Request Certificate of Compliance

SDFL manufactures and supplies full range ASME B16.5 forged flanges, including WN, SO, BL, SW, LJ, threaded, reducing and orifice flanges. Available materials: A105, A182 F304, F316L, 2205 duplex, F91, Monel, Inconel and other alloy grades. We follow ASME B16.5 dimension, tolerance and marking requirements strictly. All products can be supported with MTC, PMI test, NDT reports, heat treatment documents and Certificate of Compliance (COC). If you need ASME B16.5 flange quotation, technical datasheet or compliance certificate, send us your NPS, Class, material and facing requirement. Our engineering team will provide fast support for your piping project.

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