I bought this after a power outage(8.5 hrs) left me with about 5 gallons of water on my living room floor. The way my tank is set up(and size of sump) I needed something to stop back flow during a lengthy power outage because my backup battery can only handle things for a few hours MAX. I installed this (should have bought the tool to install it because it was difficult to do without the specialized tool) and its VERY effective at limiting the back flow. I will say that its 99% effective because VERY MINIMAL water gets through. I performed a simulated power outage and over the course of 2 hours, my sump water level rose about 1/2 inch(approx .75 gallon). Overall, good cheap fix for a pesky problem.

I used this to slow the back siphon of my return lines. I needed to slow it down because the quick siphon would pull air into my pump (causing it to loose it's prime). These check valves absolutely accomplished my goals without much effort or changes to my plumbing system.

They did cut the flow a bit more than I wish they would (hard to say how much it would compare against a flapper valve). One of the two valves I got does NOT stop the back flow, it still flows back within a few minutes. For my needs that is fine.

I used this in a remote deep sand bed and this was the easiest way to prevent backflow. Thankfully it works. The valve does reduce the outflow slightly so be prepared for that. Long term durability is also a question.