Pipe line welding

[QC7]

Hi Team,

Can you just explain me about technical reasons for below clause during pipeline fit up.

1 When a pipe with one longitudinal seam is used, this seam shall be within the top 120 degree of the circumference.

2 The longitudinal seams of consecutive pipes shall be offset by at least 45degree.


Regards,

QC7

 

[abangbikerz]

dont understand the no 1.

but for no 2 it is common to lay long seam that way for to pipe to pipe or shell course to another to avoid too much heat input concentrated at one area.

 

[Fizza453]

QCZ

Following may be reasons:

1- in this way welding seam is not covered by saddle like supports
2- if long seam is in the lower portion, it will see more stresses due to fluid static head

Nasir
Welding Engineer
DESCON ENGINEERING LIMITED
PAKISTAN

 

[weldstan]

Item No. 2 requirement is to mitigate a fracture propogating along multiple weld seams.
Item No. 1 may relate to ease of visual inspection.

 

[gtaw]

I don't know the answer, but perhaps it is so that if the longitudinal seam fails in service, if:
1) the seam is toward the top, the pipe will blow downward and is captured by the earth surrounding it, or
2) if the seam is on the bottom, the pipe would be blown into the air, where it lands, no body knows.

Rotating the longitudinal seam slightly between pipe lengths will hopefully attenuate the propagation of the crack failure along the longitudinal seam. Since this failure mode propagates near supersonic speeds, I wonder if it really makes a difference. I suppose it would to a limited extent if one assumes the weld and HAZ to lack some of the notch toughness of the parent pipe material. Usually the failure is arrested by a valve body placed in the path of the failure.

Best regards - Al

 

[metengr]

Staggering long weld seams in a pressure retaining item is good design practice to avoid continued propagation of cracks within the weld region linking up between shell courses or pipe spools, as weldstan mentioned.

 

[QC7]

Thanks to all....


QC7

 

[steakman911]

all in all...for the same reason bricklayers stagger their bricks.