What are the requirements for pressure vessel flanges?

Pressure vessel flanges must meet strict standards for safety,structural integrity,sealing reliability and material compatibility,governed by codes like GB 150,NB/T 47020–47027,ASME VIII&B16.5.

1.Design&Type Requirements

Preferred type:Weld Neck Flange(WN)or integral forged flange for high pressure/temperature;Slip On(SO)only for low-pressure,non-critical vessels.

Structure:Long tapered neck for stress reduction;full penetration butt weld;no sharp corners(stress concentration control).

Pressure-temperature rating:Must match or exceed vessel design pressure/temperature(PN/Class rating).

Sealing face:RF(Raised Face)for general use;RTJ(Ring Type Joint)for high pressure/hazardous media.

2.Material Requirements

Carbon steel:A105,20#(≤425°C,non-corrosive).

Stainless steel:304/316L(corrosive media,-196°C~+500°C).

Alloy steel:16Mn,CrMo(high pressure/high temp,e.g.,steam,hydrogen).

Low-temperature:35CrMoA(-40°C~-70°C),higher grade below-70°C.

Forging rule:WN flanges must be forged;plate-only flanges are prohibited for pressure vessels.

3.Manufacturing&Heat Treatment

Forging:Forged blanks,no casting for primary load-bearing flanges.

Heat treatment:

Carbon/low alloy steel>76 mm:normalized/fully annealed.

Welded flanges:post-weld stress relief(600–650°C for carbon steel).

Machining:Sealing face flatness≤0.1 mm;no scratches,dents or cracks.

4.Welding&Installation

Weld quality:Full penetration,no porosity/cracks;RT/UT inspection per GB 150.

Bolt specification:8.8/10.9 grade high-strength bolts;nuts≥30HB softer than bolts.

Tightening:Cross-symmetric,2–3 stages to avoid uneven gasket compression.

Gasket:Match media/temp/pressure;no double gaskets or fillers.

5.Inspection&Testing

Visual:No surface defects;seal face damage forbidden.

NDT:Welds per GB 150(RT/UT);defect level≤Ⅱ.

Pressure test:

Hydrostatic:1.25×design pressure,no leakage.

Pneumatic:1.05×design pressure for hazardous media.

Documentation:Material certificates,heat treatment records,NDT reports,test logs.

6.Standard Compliance

China:GB 150,NB/T 47020～47027.

International:ASME VIII Div.1,B16.5/B16.47.