What you have there is a Duel Check with intermediate atmospheric vent. Watts makes one called a 9D.
If you are getting continuous discharge from it, it is an indication that the internal seating parts and surfaces may need to be cleaned. If its just intermittent discharge it can be caused by water pressure fluctuations. If what you have there is a Watts ¾" 9D here is Watts's repair kit part number 0886010 it includes diaphragm assembly, disc assembly, seat, seat O-ring and two union seals.
I am just wondering what State do you live in. In Illinois there is not supposed to be a shut off valve between the expansion tank and the water heater. To correctly remedy that issue, is to remove the ball valve before the tank and the one after the tee heading back to the heater and install one ball valve before the tee on the recirculation line. If you ever need to change the tank, you just shut off the supply ball valve and the recirculation ball valve to isolate the tank. Also I would of installed the tee the tank is on pointing up to make changing the tank easer.
A properly sized and approved expansion tank shall be located on the outlet side of the check valve in the water heater's cold water supply with no shut-off valve between the heater and expansion tank.