Pressure Vessel Head Petal Weld Sequence

[bjmc]

Hello,

When welding a vessel head is there a golden rule that you must weld the upper longitudinal welds before welding the circumferential weld immediately below it (assume a bottom head). If so, why? If the upper course has 24 longitudinal welds, for example and the lower course only has 6 longitudinal welds, then it seems that there would be a massive mismatch at the convening circumferential weld because the upper course will shrink much more than the lower course and it may not be possible to keep the circumferential weld fit-up adequately. So, it seems like it might be a good idea to do the circumferential weld first.

Thanks,

 

[fegenbush]

Depending on size, this vessel may benefit from a technique we recently used on a vessel. The technique was to fit up and tack all joints prior to welding the root pass of any joint. This particular vessel was 5/16" thick and all welding was done manual and semi automatic. If you require a different type of process, this may not work as well for you. Otherwise, it will ensure that your fitup remains solid.

An alternate technique to use if the above is not applicable to your size and / or thickness is to weld the (per your description) lower section completely first before fitting the upper section. The upper section will have some shrinkage, but it may be possible to take this into account in your fitup of the upper section (i.e. larger root opening). The downside of this technique is that it is more difficult to control the final diameter of the head.

I hope this helps,

Fegenbush

 

[bjmc]

Thanks for the reply Fegenbush. I was actually wondering about the sequence between longitudinal welds and circumferential welds. Do longitudinal welds on both sides of a circumferential necessarily need to be welded prior to welding the circumferential? If so, can you explain why?

Best regards,

 

[fegenbush]

bjmc,

If the longitudinal joints are fit up and securely tacked along their length, I see no reason that they would need to be completely welded prior to the circumferential joint being welded.

A good fitup goes a long way towards saving you headaches down the road.

Fegenbush