riser vs riser pipe

riser and riser pipe both are nouns.

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

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

Other synonyms of riser include riser main, riser pipeline.

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