riser main vs riser pipe

riser main and riser pipe both are nouns.

Word	Noun	Adjective	Verb	Adverb
riser main	Yes	No	No	No
riser pipe	Yes	No	No	No

As nouns, riser main and riser pipe are synonyms defined as:

Other synonyms of riser main include riser, riser pipeline.

riser main (noun)	riser pipe (noun)
a vertical pipe in a building	a vertical pipe in a building