riser main vs riser

riser main and riser both are nouns.

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

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

Other synonyms of riser main include riser pipe, riser pipeline.

riser main (noun)	riser (noun)
a vertical pipe in a building	structural member consisting of the vertical part of a stair or step
	a vertical pipe in a building
	a person who rises (especially from bed)