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API 650 paragraph

API 650 paragraph

API 650 paragraph

Dear experts,

Many times this issue come from the clients. When the shell material is Group IV, IVA, V, or VI, all opening connections requiring reinforcement in a shell
plate or thickened insert plate more than 13 mm (1/2 in.) thick shall be prefabricated into the shell plate or thickened
insert plate, and the prefabricated assembly shall be thermally stress relieved within a temperature range of 600 °C to
650 °C (1100 °F to 1200 °F) for 1 hour per 25 mm (1 in.) of thickness prior to installation

In my understanding the statement "more than 13 mm thick" is applicable for shell plate and thickened insert plate.

But many people have different interpretation i.e. "more than 13 mm thick" is applicable for thickened insert plate only. So, by this interpretation shell plate under group IV with 10 mm thick and 10" shell nozzles and RF pad need to be prefabricated and stress relieved before installation. In other words thickness of the shell plate is not considered for stress relief. As long as there is a nozzles with RF pad stress relief must be done.

Is there any further explanation in API 650 about the above case ? I mean confirmation that 13 mm is applicable for shell plate also thickened insert plate. I found similar info on API 650 technical query but this is for group I, II, III and IIIA. I understand the explanation shall be the same for group IV, V and VI. But I would like to have your info if there is API 650 statement for group IV, V and VI.


RE: API 650 paragraph

I'm not sure I understand the question, exactly.
Normally, if you have a reinforcing plate on the outside of the shell, it will be the same thickness as the shell. The thickness limitation there is the shell thickness, not the sum of the shell and the reinforcing plate thickness.

If, instead, you use an insert-type reinforcement that is thicker than the shell, then you may run into the 13mm limitation even with 10mm shell. In that case, the simplest solution would be switch to standard reinforcing plates instead of using insert plates.

RE: API 650 paragraph


Thanks for your response.

I will simplify the question. The statement "more than 13 mm thick" is applicable for "thickened insert plate" only or also applicable for shell plate.

If that statement is only applicable for thickened insert plate" than 10" shell nozzle with RF pad on 10 mm shell plate group IV need to be pre-fabricated and stress relieved.

To me that statement is applicable for shell plate and thickened insert plate.


RE: API 650 paragraph

The text is meant to be read literally, and is quite clear:

"...in a shell plate or thickened insert plate more than 13 mm (1/2 in.) thick..."

The thickness limits refers to a single thickness of shell plate or thickened insert plate. It does not apply to a combination of plates such as a shell plate plus a reinforcing plate.

Regarding API groups I, II or III, the previous paragraph is also quite clear:

"...in a shell plate or thickened insert plate more than 25 mm (1 in.) thick..."

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