As the welding takes place on pipes which are still under normal pressure and flow, the weld parameters must be chosen so as not to allow the welder to burn right through the pipe wall as the internal pressure acts strongly on the weld area and cause a blow through. The heat input must not be allowed to be excessive. Conversely, the heat input must not be allowed to be too low as the cooling effect of the product flowing through the pipe will chill the weld area and can cause excessive hardness and can lead to cracking. A careful balance must be achieved.
To minimise the potentially harmful effects of the flow and pressure in the pipe, the weld procedure must be carefully set out. Special attention must be paid to the electrodes, the preheat method, the weld placement, the welder skill, the fit-up and theoperating conditions.
BS6990 requires that adequate testing of the weld procedure is carried out before the procedure is acceptable and the HSE advise that pipes containing certain products such as those containing oxygen or hydrogen under pressure should not be hot tapped.