PWHT on Channel or bonnet of HX with 6 or more tube passes

[Shihabuddin Ramli]

Can anybody explain to me why its stated in latest API660 for this requirement? Its not stated any reason why is shall be PWHT due to lethal sevice or regarding thickness

Shiha

[Hemant Solanki]

in 6 or more passes there will be more welding on surfaces within close proximity. Such welding will make parent metal hard and brittle... in order to avoid such problem, PWHT requirement is added by API, so that parent metal becomes soft and adhere to its properties.

[Shihabuddin Ramli]

Any precaution need to be taken during pwht, especially to avoid any distortion? We have faced a problem before that after pwht, the bonnet was leak at the flange joint after installed and need to be clamped, tq in advance sir

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