Both processes utilise the vertical down method of welding. To enable this for SMAW, the electrodes are usually of the cellulosic type. This can lead to problems due to the high level of hydrogen produced by the process.
Mechanical welding uses a low hydrogen process, but the equipment has to be specially made for pipelines. When properly set up, this can produce high quality welds at a high workrate but needs a large numbr of wlds to be economic.
The correct choice of parameters for either of the welding methods needs in-depth knowledge and an understanding of the processes variables.
Welding is normally carried out to either API 1104 or BS 4515 [There are also many Company variations on these specifications in use.]