for welding of pipelines.

[hemany]

After welding of root run for any process or utility piping copleted, when fill up or final weld pass should be carried out? What time gap should be allowed bbetween two weld runs.

 

[PAN]

It depend on your control interpass temperature. This is indicated in your welding procedure specification.

 

[micjk]

As stated by PAN, control interpass temp is important. Bear in mind also that metallurgy also plays a big part as does the type of system. I've actually seen holes develop in socket weld fittings on a #450 condensate return line due to "hot" welding techniques. The welds were done entirely to fast and to hot, changing the grain structure of the base metal. It was first suspected to have been caused by porosity in the weld but upon investigation was found to be excessive wear around the welded area due to grain structure changes. Another factor was that the line was sized to small for the amount of condensate handled.
There should be welding procedures set up for your facility to be followed. Or you can check ASTM codes for baseline procedures and techniques.