1) what is the purpose of this device?
2) how is it different from the relief value that's on the water tank?
3) is it needed?
4) will it, in and by itself, extent the life of the tank?
thanks

I am not an expert but instead a person that has recently had these same questions. I found this article very helpful.

http://www.wvdhhr.org/phs/water/Thermal_Expansion2.pdf

1) what is the purpose of this device?
2) how is it different from the relief value that's on the water tank?
3) is it needed?
4) will it, in and by itself, extent the life of the tank?
thanks
1) this is a bladder tank that controls the expansion of hot water when you have a check valve or a backflow preventer installed on the water service.
2) The relief valve is a safety feature that opens and vents the tank if the pressure or temperature gets too high.
3) It's needed if you have a sprinkler sysstem or any of the items listed in number #1.
4) No! It's function is to absorb and control the extra pressure that occurs as the water heats up.
More questions? I'm as close as a click. Regards, Tom