How to Weld Flanged Pipe Correctly?

Verification Area	Applicable Requirement	Example Data	Acceptance / Welding Check
Pipe size and outside diameter	Confirm the nominal pipe size and outside diameter before selecting the flange bore and weld-end preparation.	NPS 2 pipe Outside diameter: 60.3 mm	The flange bore and pipe OD must be compatible. Do not identify the pipe by nominal size alone.
Pipe wall thickness	Verify the schedule or specified wall thickness against the approved piping specification and design calculations.	NPS 2, Schedule 40 Nominal wall: 3.91 mm Approximate ID: 52.5 mm	Measure actual wall thickness where required. The weld procedure, heat input, and joint fit-up must suit the actual thickness.
Flange standard and pressure class	Use ASME B16.5 for pipe flanges and flanged fittings from NPS 1/2 through NPS 24. Confirm the class and material group.	ASME B16.5 NPS 2, Class 150 Welding neck, raised face	“Class 150” is a pressure-temperature designation, not a fixed 150 psi allowable pressure. Check the applicable B16.5 material pressure-temperature table.
Flange dimensions	Verify outside diameter, bolt circle, bolt-hole quantity, bolt-hole diameter, thickness, facing, and bore against the applicable ASME B16.5 table.	Typical NPS 2 Class 150 reference dimensions: Outside diameter: 152.4 mm Bolt circle: 120.7 mm Bolting: 4 × 15.9 mm nominal bolts Raised-face height: approximately 1.6 mm	Use the flange manufacturer’s certified dimensional drawing and the current ASME B16.5 edition for final inspection. Bore dimensions may vary with the connected pipe wall and flange series.
Flange material	Confirm the material specification, grade, heat number, and pressure-temperature rating. ASME B16.5 provides dimensional and rating requirements; it does not replace the material specification.	Carbon-steel example: Forged flange material: ASTM A105N Material certificate: required	Match the flange material to the piping design, service temperature, corrosion requirements, and approved material list. Confirm heat treatment and impact-test requirements when applicable.
Pipe material	Verify the pipe specification, grade, heat number, wall thickness, and condition before welding.	Carbon-steel example: Pipe material: ASTM A106 Grade B NPS 2, Schedule 40	Check that the pipe and flange materials are approved as a compatible weld combination by the qualified welding procedure.
Butt-weld end preparation	Use ASME B16.25 for butt-welding-end dimensions and tolerances, including bevel configuration and alignment requirements.	Common carbon-steel preparation reference: Included bevel angle: approximately 75° Bevel angle per side: approximately 37.5° Root face and root gap: as specified by the qualified WPS	Do not rely on a generic bevel value for every wall thickness. Measure bevel angle, root face, root opening, hi-low, and flange squareness before welding.
Welding process and filler metal	Select the process and filler metal using an approved WPS qualified in accordance with the project’s governing welding code, commonly ASME BPVC Section IX.	Example process: GTAW root / SMAW fill and cap Example filler: AWS E7018 for compatible carbon-steel fill and cap passes	Filler selection must be confirmed by the WPS, base-metal group, service conditions, hydrogen-control requirements, preheat, interpass temperature, and post-weld heat-treatment requirements.
Weld consumable control	Control consumable identification, storage, drying, exposure time, and batch traceability according to the applicable specification and approved welding procedure.	Low-hydrogen electrodes such as E7018 require controlled storage and rebaking or holding conditions when specified by the project procedure.	Record electrode classification, lot number, issue time, storage temperature, and welder identification where required.
Flange alignment and orientation	The flange face must remain square to the pipe axis and the bolt-hole orientation must match the installation drawing.	Inspection points: Face squareness Angular rotation Pipe projection Internal bore transition	Prevent excessive forced fit-up. Verify bolt-hole orientation before tack welding, and protect the raised face from arc strikes, spatter, and distortion.
Pre-weld and post-weld inspection	Inspection scope is governed by the project specification, design code, and approved inspection and test plan.	Typical checks: Visual examination: 100% Dimensional inspection: before and after welding NDE: as specified, such as RT, UT, MT, or PT	Confirm weld profile, undercut, surface defects, flange distortion, bore condition, and final dimensional compliance before pressure testing or service.