work execution plan/strategy to retube high pressure boiler

WOW!!!! That is quite the question ...............

I don 't have the time to give a complete answer, HOWEVER .........

Please be sure to hydrotest the boiler upon completion

Alright, I have a little time to provide and hopefully steer you in the right direction-

It seems from your question that you are lacking the most basic/fundamental knowledge regarding boiler repairs. My first piece of advice would be to hire a competent engineer/consultant/contractor to help you with this endeavor.

Apart from that - you must first specify what type of boiler, pressure rating, and size.

From there the issues you must address include:

1. Applicable code to which boiler was manufactured/to be repaired (for example to AMSE BPVC Sec 1)
2. Safety issues including vessel isolation/lock-out and if confined space entries must be done
3. Proper materials for the replacement tubes (see applicable code) and proper certification for these materials
4. Proper qualified workers/contractors (for example certified to National Board 'R')
5. Proper welding procedures (dependent on materials and design)
6. Proper heat treating of weldments (pre/post heat)
7. Inspection/NDT of all repairs (including, but not limited to MPI/LPI, and X-ray requirements)
8. Proper documentation of all repairs (including, but not limited to mill certs for materials, welding procedures, welder qualification card records, NDT results)
9. Finally a hydrotest of the boiler (typically at 1.5 x design pressure) to ensure repairs are sound

Hopefully this provides a little insight into your predicament.

you have to contact boiler manufacturer.

Recommend that you contact the following company - www.limburns.co.uk