The Power Piping Code, ASME B31.1, (1) applies to tubing outside the boiler setting but does not address tubing as small as is normally used for sample lines. The use of small tubing does not violate the Code; the Code simply does not address the use of small tubing. Therefore the designer is
responsible for the sample line design. Reference
(2) is a source of information for the design of small
tubing systems