HOT TAPPING – SALDATURE

WELDING IN HOT TAPPING

Welding in hot tapping of a branch fitting is an operation to be considered seriously. The line to be branched is in pressure and may contain hazardous fluid. Two factors reduce the resistance of pipe during welding : the melted area and the heat affected zone. The melted area doesn’t concur to the resistance, the heat affected zone has a lower allowable and shall be derated with the temperature. The task is to verify that the remaining thickness, correctly derated, can resist at the actual internal pressure. If the maximum calculated internal pressure is lower that the actual one we can consider the following actions:

• lower heat input thought a different welding procedure;
• reduce the pressure in the line;
• tie-in relocation where the thickness is higher.

The starting point is the calculation of the maximum allowable internal pressure in the existing pipe at the tie-in location, input data are: the actual thickness and welding procedure (WPS).

The actual thickness is the minimum thickness red with ultrasonic following a procedure defining the point to be measured in the welding area. The ultrasonic test shall detect possible lamination defects, in addition other defects can be relieved with magnetic particles.

The WPS define the sequence of passes, type of electrode, travel speed, current parameters and, as consequence, the heat input; type and passes sequence gives the heat losses toward the ambient. For example a butt welding loose less heat than an angle one so is in an worse condition. Using an low hydrogen electrode we can limitate the heat input.

We said before that heat input determine two zones in pipe thickness: the melted zone considered not resistant and the heat affected zone with a resistance as lower as the temperature is higher (with the diminishing of the material allowable stress with temperature that is more than proportional). A big pipe thickness is fundamental also because it increases the circumferential heat transmission. The material is important as well because a big heat conductance cause a big heat input and a lower resistance under internal pressure. For instance the copper is very difficult to be welded while the stainless steel gives permits a safer weld in hot tapping than the carbon steel. The actual temperature of the pipe is very important because the initial condition is already severe. To be considered welding in steam service that the total heat is the sum of the one given by the temperature and the one delivered with the electrode.