Dual check valve removal
There is a dual check valve off the main line coming into the house in our basement. It is there for a make-shift sprinkler system that is hardly ever used. Because the basement ceilings are finished (Drywall) I had to open up a part of the ceiling. The check valve is in a tight spot, partially above a bulkhead, and so I am unable to cut one end of the pipe in order to try and remove the check valve altogether without having to take the entire bulkhead down. The big nut on this check valve, at the other end, is accessible. If I loosen this large nut which again, is accessible, can I simply remove the spring loaded mechanism inside the check valve, of course rendering the check valve useless, and then tighten the nut back on to the check valve. Therefore there would be no need to cut lines, add new lines etc etc. In other words, no real plumbing issues to be concerned about. I simply made it so that there is no backup of any pressure in the line and that section of pipe now becomes free-flowing so to speak.
This all started when the hot water heater was leaking water from the relief valve.
That type of valve typically has the union nuts on both ends. Your picture is incomplete so we cannot tell the make and model of the valve, but it could have an "open vent" on the backside. In any case, once you remove the valve, it will be just as easy to replace it with a coupling, brass nipple, and brass union.
I have to ask 'why was there a check valve installed in the first place'? Removing or disabling may be illegal in your situation.
FWIW, having a check valve makes your water system 'closed' which nessitates an expansion tank on the water to account for the WH doing its job (i.e., the heated water expands - it needs somewhere engineered to go, not a leaky valve or in your case the safety valve).
The dual check valve is part of the sprinkler system and prevents contaminated water from the sprinklers from being drawn back into the domestic supply-both yours and the city's. They are frequently referred to as "back flow preventers". It is irrelevant weather the system is used regularly or not. My city requires annual certification by a licensed inspector to be certain the seals are still OK and replace them if necessary. This check valve should not be part of the domestic supply line would not make a closed system for the house. If it is in the domestic side, it is serving no purpose and it sounds like it is not in a location where inspection would be easy or even possible. Further, if it several years old, the seals are almost certainly leaking and the valve is useless, but still is causing the expansion problem. These are not install and forget about fixtures, and certainly should be installed in the proper location.