A compact flange is a weight-optimized flange design that achieves the same (or better) sealing performance as a standard ASME B16.5 or EN 1092-1 flange in a smaller, lighter package. Compact flanges use an engineered metal-to-metal sealing principle and are designed per EN 1591 calculation methods. They are widely specified on offshore platforms and FPSOs where weight and space are critical constraints. NORSOK L-005 is a key reference standard.
How Compact Flanges Differ from Standard Flanges
Feature Compact Flange Standard ASME B16.5 Flange OD 30-50% smaller Standard per B16.5 tables Weight 30-50% lighter Standard Sealing Metal-to-metal (lip seal or IX seal) Gasket-based (RF , RTJ ) Gasket Proprietary metal seal ring Spiral wound or RTJ ringBolt count Fewer, larger bolts More, smaller bolts Face-to-face Shorter Longer Design standard EN 1591; NORSOK L-005 ASME B16.5 Fire safety Fire-tested per API 6FA/ISO 10497 Not inherently fire-tested
Weight Comparison (NPS 6 / DN 150)
Parameter Compact Flange ASME B16.5 Class 600 WN Flange OD ~250 mm 355.6 mm Number of bolts 8 12 Weight (flange only) ~14 kg ~28 kg Weight saving ~50% Baseline
Typical values; exact dimensions depend on manufacturer and design pressure.
Compact flanges are not interchangeable with standard ASME or EN flanges. They require dedicated seal rings (IX type or similar), specific bolting, and trained assembly technicians. Do not attempt to bolt a compact flange to a standard B16.5 flange without an adapter or transition piece.
Applications
Application Benefit Offshore topsides Weight savings reduce structural steel requirements FPSOs Space savings in congested pipe racks Subsea manifolds Compact envelope for subsea equipment HP/HT wells Superior sealing at extreme pressures and temperatures Arctic/cryogenic Metal-to-metal seal performs well at extreme low temperatures
Compact flanges are specified per project or operator standards (e.g., Equinor, Shell, BP internal specs). NORSOK L-005 defines requirements for compact flanged connections in the Norwegian Continental Shelf. Always verify the specific manufacturer and seal ring type accepted by the project specification before procurement.
Seal Ring Types
The sealing element is a precision-machined metal ring (not a standard gasket). Common types include IX seals, DX seals, and proprietary lip seals. These rings are single-use and must be replaced at every joint opening—similar to RTJ rings .
Read the full guide to flanges
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