riser pipe vs riser

riser pipe and riser both are nouns.

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

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

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

riser pipe (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)