union and coupling

Wills ...I agree with you about using unions to deal with leaking threaded pipe! ...BUT since threaded random lengths of pipe come with one threaded coupling (for continuation of a run of pipe), the occurrence of where and when this connection is made would be dictated by construction people. This would be better handled as a rule of thumb by them (1 union every 100’ of pipe). Piping designers only dictate union location when it pertains to "normal" maintenance operation. IMHO

Yes, I agree the location of these particular unions should be by the field contractors. My comment was mainly to point out a possible maintenance issue when using T&C pipe. I however was accountable for providing all materials to the field, so they were required as part of my design (best guess) and were included on isometric drawings. This may not be the case for every project.

Was just something to keep in mind whatever your role (designer, material engg., construction support,etc..).