Resources / Technical Specs
Technical Specifications Overview
A flange specification is a contract between the designer, the supplier, and the fabricator. It carries the geometric data needed to fit the part into a piping system, the metallurgical data needed to qualify the part for service, and the documentation references that let the project owner verify both. This page walks through what each section of a typical spec sheet means and how to read it without missing the points that matter at the bolt-up.
Anatomy of a Flange Spec Sheet
The header carries the part identifier, usually a combination of size, class, type, facing, and material. NPS 6 Class 300 weld neck raised face A105 is a complete identifier for the most common process flange in carbon steel. Below the header the sheet typically opens with the dimensional block: outside diameter, bolt circle, bolt count and size, hub dimensions, face height and diameter, and bore matched to a pipe schedule.
Below the dimensions, the material block calls out the forging specification, usually an ASTM number such as A105 for carbon steel, A350 LF2 for low temperature carbon steel, A182 F304 or F316 for austenitic stainless, A182 F11 or F22 for chrome moly, and A182 F44, F51, or F53 for the duplex and super duplex families. The material block also notes any supplementary requirements such as NACE MR0175 trim for sour service or HIC testing for hydrogen-containing service.
The documentation block lists the deliverable paperwork: mill test report, certificate of conformance, dimensional inspection report, hardness data, and where required NDE results. The applicable standards block references the design code, usually ASME B16.5, B16.47, AWWA C207, or an API spec.
ASME Class to DIN PN Equivalents
The ANSI class system and the DIN pressure nominal (PN) system describe similar things but are not directly interchangeable. The figures below are approximate working pressure equivalents useful for first-pass cross referencing. For a real interchange decision, dimensional compatibility and gasket selection both need separate confirmation.
| ASME Class | Approx PN | CWP (psi) | Typical Service |
|---|---|---|---|
| Class 150 | PN 20 | 285 | General process, utilities |
| Class 300 | PN 50 | 740 | Process, steam |
| Class 600 | PN 100 | 1480 | Refining, gas service |
| Class 900 | PN 150 | 2220 | High pressure process |
| Class 1500 | PN 250 | 3705 | Hydroprocessing, wellhead |
| Class 2500 | PN 420 | 6170 | Very high pressure process |
Pressure-Temperature Derating
A flange class is not a working pressure. It is an index into a pressure-temperature rating table where the actual allowable working pressure decreases as temperature increases. A Class 300 A105 flange holds 740 PSI at 100 degrees F but only about 530 PSI at 600 degrees F. The derating curve is published in ASME B16.5 Table 2 for each material group.
Material group is the key concept. Carbon steels, low alloy steels, stainless steels, and nickel alloys each derate on different curves. A Class 300 F316 stainless flange and a Class 300 A105 carbon steel flange have the same dimensions and the same nominal class, but their working pressures at temperature are different because they sit on different curves in the table.
At low temperatures the picture also matters. Below minus 20 degrees F most carbon steels need impact testing or substitution with A350 LF2 to maintain ductility. The spec sheet should call out the minimum design metal temperature alongside the design pressure to capture both ends of the operating envelope.
Bolt Material Selection
ASTM A193 Grade B7 is the standard stud bolt material for most flanged joints in ambient and elevated temperature service. It is a quenched and tempered chromium molybdenum alloy steel with a minimum tensile of 125 ksi for diameters up to 2-1/2 inches. Paired with ASTM A194 Grade 2H heavy hex nuts, B7 covers the majority of process piping applications.
For low temperature service, ASTM A320 Grade L7 is the parallel material with Charpy impact testing at minus 150 degrees F. L7 is required where the design metal temperature falls below minus 20 degrees F.
B7M is the same chemistry as B7 but with a reduced hardness ceiling of 235 HBW maximum. The lower hardness improves resistance to sulfide stress cracking and is the bolt material specified for NACE MR0175 sour service. The trade-off is lower allowable stress, so the engineering check must confirm that joint preload remains adequate.
Stainless bolting in B8 or B8M is used where the bolt itself must resist corrosive media, typically in chemical service or external marine exposure. Galvanic compatibility with the flange material needs review when mixing stainless bolts with carbon steel flanges.
Gasket Compatibility
Raised face flanges seat soft gaskets, spiral wound gaskets, or kammprofile gaskets, depending on service. Soft sheet gaskets in graphite or PTFE composites suit lower pressure utility lines. Spiral wound gaskets with stainless windings and graphite or PTFE fillers cover most process service through Class 600 and into Class 900 with the right inner ring detail. The gasket outside diameter must clear the bolt circle and the inside diameter must seat against the raised face.
Ring joint flanges use solid metal ring gaskets seated in the machined groove. R-series oval and octagonal rings cover most API 6B and ASME B16.5 RTJ flanges. BX-series rings are pressure energized and cover API 6BX flanges from 5,000 PSI through 20,000 PSI working pressure. The ring number and material must match the groove specification. Soft iron, low carbon steel, stainless, and Inconel are common ring materials, with the choice driven by service media and hardness compatibility with the groove.
For full material data on the flange itself, see our materials section covering carbon steel, stainless steel, alloy steel, and exotic alloys.
Working References
The calculators and articles below cover the day-to-day spec work that follows from the concepts on this page.
Spec Sheet Review?
Send the document to sales@texasflange.com or call (281) 484-8325. Our inside sales group reviews specs every day and will flag any items that need clarification before quote.
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