Why do we use GTAW/SMAW Welding on Large Pipe?

GTAW is relatively slower than SMAW, so economics would be one reason to include SMAW in welding the larger pipe sizes. I think GTAW remains acceptable, though.

Comparatively GTAW, is much easier to control and can produce a very good weld quality especailly on the root (1st pass of welding).

It is very difficult to produce a good weld on the root pass which form an essential of weld quality and is the 1st pass that intact with the service medium. The deposited weld shall have a good workmanship internally i.e. the oposite side of the weld deposit side and shall be free from any harmful discontinuity (flaw) as required in the applicable codes and standards. The discountinuity occured on root can be the initial opening and consequently propagate to the failure of the joints under the dynamic or static load applied on the welded item during its services. Generally in order to avoid this risk of failure in achieving weld compliance to codes and standards requirements, we will select a process that able to deliver a better quality and lesser weld failure's welding process. And therefore, GTAW is often being used as the 1st and 2nd pass in making the weld joint and filling (other passes) with others welding process that is having a better (higher) depositing rate and higher productity, consequently economically e.g. SMAW, GMAW, SAW etc depending on other consideration factors.

Offcourse, GTAW can be used to weld the whole joints regardless of sizes as well if costs and productivity is not the concerns.

I very much agree with the idea of GTAW for the root pass, but what has been left out is to get a good quality root pass you should purge anywhere from 98% to 100% argon to prevent sugar/grapes whatever you want to call it and give that good quality interior results...