Like Bill said, we really need a picture. There are a whole lot of limpets and their reef safeness varies. Common names don't help much because they're often used for a group of similar looking species.
That being said, any animal from the Azores isn't likely to do well at reef temps long term. The water there averages quite a bit cooler than reefs. Although some intertidal animals like limpets are adapted to deal with elevated temperatures, their adaptations are for short term increases, not constantly high temps. Also, if these are an intertidal species, they may crawl above the waterline (and even out of the tank) and wait there for the tide to come in, which obviously isn't going to happen.
There are plenty of cheap tropical grazers in the hobby, so I would be inclined to leave the limpets in the wild, even though they are free.