ADMADM,

I believe ASME It does not provide any requirement for low temperature or external pressure of the cylindrical shell.

This code applies to boilers where a low temperature is not expected either during normal operation or up-set conditions and start-up or shut-downs.

Additionally, external pressure does not usually apply to boilers. Normally there is no compressor or pump located down stream boilers which can cause a vacuum pressure. Also vacuum due to condensation (say during shut-downs or up-set conditions) is not practically possible.

So, if for any reason you need to consider additional requirements for low temperature applications, this should be included in the specification. And for external pressure design, you can still use section PFT or even ASME VIII Div.1 method, which, by concept, is essentially the same as PFT and uses the same set of external pressure charts! That sound to me a good engineering practice if you really need to consider external pressure on the shell.

Cheers,