High pressure flanges: Class 600 to 2500
Pipelines, compressor stations, refinery high pressure units and power plant feedwater lines need flanges well above Class 150. Here is what each class is rated for, what changes as the class goes up, and what to put on the line list.
High pressure flanges are the ASME B16.5 Class 600, 900, 1500 and 2500 flanges, with Class 400 between 300 and 600 for the few lines that need it. In A105 carbon steel they are rated from 1,480 psig in Class 600 to 6,170 psig in Class 2500 at 100 °F, falling as temperature rises. They are usually weld neck or blind, often with ring type joint facing. We forge and machine them in Class 400 to 2500, NPS 1/2 to 24 (to NPS 12 in Class 2500, the limit of the standard), in A105, A350 LF2, 304L, 316L and duplex, with an EN 10204 3.1 mill test report for every heat.
Ratings by class
| Class | Rating | Typical use |
|---|---|---|
| 150 | 285 psig | Water, HVAC, low pressure process |
| 300 | 740 psig | Process, steam, gas distribution |
| 400 | 990 psig | Rare; some gas and process lines |
| 600 | 1,480 psig | Gas transmission, compressor stations, feedwater |
| 900 | 2,220 psig | High pressure process and pipelines |
| 1500 | 3,705 psig | Hydrotreaters, high pressure steam and gas |
| 2500 | 6,170 psig | The highest B16.5 class; NPS 12 and smaller |
Ratings fall with temperature and depend on the material group: stainless 316L (group 2.3) is rated 230 psig in Class 150 and 600 psig in Class 300 at 100 °F, below A105. Check the B16.5 table for your grade at the design temperature before fixing the class. Our class selection guide works through an example.
What changes as the class goes up
- Thickness and bolting. Higher classes have thicker flanges, larger bolt circles and more or larger bolts for the same pipe size; the flange bolt chart lists the studs for every class.
- Shared sizes. In NPS 1/2 to 2 1/2, Class 900 and Class 1500 flanges have the same dimensions in ASME B16.5.
- Size limits. Class 2500 stops at NPS 12. Above NPS 24, ASME B16.47 covers Series A and Series B flanges up to Class 900.
- Facing. Raised face is still common, but ring type joint facing is usual from Class 600 up in oil and gas service.
- Weld end. Weld neck flanges must be bored to the pipe inside diameter; at high pressure the pipe wall is heavy, so give the schedule or wall on every line.
Testing and documents
Every heat ships with an EN 10204 3.1 mill test report giving chemistry, tensile and hardness results and heat treatment. A350 LF2 reports include Charpy impact results at -50 °F. Stainless and duplex flanges are checked by PMI. Ultrasonic and magnetic particle examination, and EN 10204 3.2 certificates with your inspector, can be arranged when your specification calls for them.
Grades
| Grade | Specification | Typical service |
|---|---|---|
| A105 | ASTM A105 | Most high pressure carbon steel lines down to -20 °F |
| A350 LF2 | ASTM A350, Charpy tested at -50 °F | Cold climate, blowdown and low temperature service |
| F42 to F70 | ASTM A694 | Pipeline flanges matched to API 5L X42 to X70 line pipe |
| F304L, F316L | ASTM A182 | Corrosive and high purity service |
| Duplex S32205, super duplex S32750 | ASTM A182 F60 and F53 | Chloride and seawater service at high pressure |
How to order
A typical line reads: "Weld neck flange RTJ, NPS 6, Class 1500, bored for Sch 160, ASTM A105, 8 pcs". Send the type, size, class, facing, bore or schedule, material and quantity for each line, and we quote every line as specified. Every order ships from our works in Gujarat, India, with an EN 10204 3.1 mill test report for every heat; ordering from India explains the documents, marking, packing and customs.
Frequently asked questions
Is a Class 600 flange rated for 600 psi?
No. The class is a designation, not a pressure. An A105 Class 600 flange is rated 1,480 psig at 100 °F, and less at higher temperatures.
Do you make Class 2500 flanges?
Yes, to ASME B16.5, which covers Class 2500 from NPS 1/2 to NPS 12, in weld neck, blind and the other forged types.
What size is the largest Class 900 flange?
NPS 24 in ASME B16.5; above that, ASME B16.47 Series A and B cover larger Class 900 flanges.
Sources
- ASME B16.5, Pipe Flanges and Flanged Fittings, NPS 1/2 through NPS 24 Metric/Inch Standard
- ASME B16.47, Large Diameter Steel Flanges, NPS 26 through NPS 60
- ASTM A694/A694M, Carbon and Alloy Steel Forgings for Pipe Flanges, Fittings, Valves, and Parts for High-Pressure Transmission Service