What a Double-Seal Flange Welded Connection Is
A double-seal flange welded connection is a joint in which two machined sealing flanges are assembled with a gasket between them, clamped together, and then welded to the pipeline. The distinguishing feature is geometric rather than material-related: the sealing faces and the weld are separated, so the weld pool and its heat-affected zone remain at a clear distance from the pipe inner wall. In lined or internally coated pipelines this keeps welding heat away from the internal anti-corrosion layer, which is the main reason the connection is specified instead of a simple butt weld.
Both flanges are normally machined from grade 20 carbon steel to GB/T 699, a low-carbon steel with good weldability that corresponds in practice to the C22 type of EN 10083. The groove is produced by machining, not flame cutting, so the bevel geometry and sealing face tolerances are controlled from the start.
Where the Connection Is Used
The joint is applied in chemical engineering, petroleum engineering, gas transmission and municipal engineering, most often on carbon steel pipelines with a nominal diameter of DN 500 or less. Typical duties include process piping in plants where the line is internally coated, water and gas mains that must be joined in the field, and tie-in sections where a flanged break is needed for future maintenance.
Materials and components are selected against the relevant European or American standards so that the pressure rating of the joint, the pipe and the gasket stay consistent: flanges to EN 1092-1 or ASME B16.5, gaskets to EN 1514-1 or ASME B16.20, welding procedure qualification to ISO 15614-1, welder qualification to ISO 9606-1, and radiographic or ultrasonic examination of the completed weld to ISO 17636-1 or ISO 17640 respectively.
Welding Procedure and Heat Control
The connection is welded with a manual gas tungsten arc root pass followed by manual metal arc filling and capping. The sequence is deliberate: the tungsten arc root gives a clean, fully penetrated first pass with minimal heat input next to the sealing assembly, and the covered-electrode passes then build the joint to full strength at a controlled interpass temperature.
Consumables are matched to the base metal, with a low-hydrogen electrode for the fill and cap passes; electrodes are dried and held in a holding oven before use.
Heat input is limited during the root pass. The objective is a sound weld without letting conducted heat raise the temperature of the internal coating above its service limit.
Interpass temperature is measured and recorded, and no quenching with water is permitted.
Where the design specifies it, a pre-heat is applied for thick-wall sections only; thin-wall low-carbon steel normally needs none.
Because the weld is offset from the inner wall, the residual risk to the lining is thermal rather than mechanical, which is why the pass sequence and the heat input are the two variables that govern joint quality.
Groove Preparation and Clamping
The machined groove, the sealing faces and at least 10 mm of adjacent surface on each side are cleaned of paint, rust, scale, oil and other debris, then ground or brushed to expose clean metal before assembly. Grit, slag or spatter left in the groove is a direct cause of lack-of-fusion defects.
After the two sealing flanges are mated, they must be locked with the gap between them set by the design. Hydraulic clamps, a vice, jacking screws or purpose-made fixtures are all used depending on site conditions. For each pipe diameter, three to five clamping sets are positioned symmetrically and tightened evenly, and the residual gap between the two flange faces is held to 1 mm or less. Uneven clamping distorts the sealing faces, opens the root gap and is one of the most common causes of a failed pressure test.
Construction Sequence, Pressure Testing and Inspection
The works sequence runs from construction preparation and material acceptance through bevel grinding, gasket installation, mating and tightening of the two sealing flanges, base welding, the double-seal pre-test, weld filling and capping, appearance inspection, non-destructive testing, the double-seal hydraulic test, drainage and drying, sealing of the pressure test port, the pipeline system hydraulic test, and a final drain and dry.
| Test stage | Test pressure | Hold time |
|---|---|---|
| Pre-test after base welding | Pipeline design pressure | 30 seconds |
| Strength test after capping | 1.5 times the pipeline test pressure | 30 seconds |
| Tightness test after strength test | Design pressure | 60 seconds |
Each stage is accepted only when the pressure holds with no drop and no visible leakage. Water is drained between stages, and the pressure test port is welded closed and sealed after the final acceptance so that no unfilled opening remains in the joint. Weld appearance inspection checks for cracks, undercut, porosity and incomplete penetration at the visible surfaces; non-destructive testing follows the level and extent specified in the design, typically ultrasonic or radiographic examination of the root and cap regions.
Two practical points are worth recording in the inspection file: the pressure test should be carried out before the pipe is backfilled or insulated, and the gasket must be handled as a single-use item, because a compressed gasket will not reseal after the flanges are opened.
FAQ
Q: Why is a double-seal flange welded connection used instead of a plain butt weld?
Because the weld is physically separated from the pipe inner wall. On internally coated or lined pipelines this keeps welding heat away from the anti-corrosion layer and allows the joint to be tested and reopened at a defined break point.
Q: What welding processes are used for the connection?
A manual gas tungsten arc root pass followed by manual metal arc fill and cap passes. The procedure is qualified to ISO 15614-1 and the welders to ISO 9606-1 before production welding begins.
Q: How is the internal coating protected from welding heat?
By combining the offset weld position with a controlled pass sequence and a limited heat input on the root pass, so that the temperature transmitted to the inner wall stays below the limit of the coating. Interpass temperature is monitored rather than assumed.
Q: What is the maximum permitted gap between the two sealing flanges before welding?
The gap is set to 1 mm or less by the clamping arrangement, with three to five clamps distributed evenly around the circumference and tightened symmetrically to avoid distorting the sealing faces.
Q: What pressure is used for the pre-test and the strength test?
The pre-test after base welding is carried out at the pipeline design pressure for 30 seconds. The subsequent strength test is carried out at 1.5 times the pipeline test pressure for 30 seconds, followed by a tightness test at design pressure for 60 seconds.
Q: Can the joint be dismantled for maintenance?
The flanges can be unbolted and the line opened, but the gasket must be replaced, since a compressed gasket will not reseal reliably. The welded connections to the pipe remain permanent.
