I used a left and right nipple and couplings on gas and steam work and even a few times on potable water supply
Reasons
1- Prevents amateurs from taking things apart so they have to call a licensed plumber
2- Less chance of a leak compared to a union
3- Unions are not allowed on gas work prior to the shut off valves
In your case you can call a plumber or rent a Mega press tool and coupling and cut the pipe and then megapress coupling
Or
Cut the pipe open the wall remove the pipe from the female fittings (both sides) get a copper x male adapter and a slip coupling
Or remove the pipe from both ends get two nipples and a union or straight valve with a 1/2 union
The only difference between a hydronic valve and steam is a steam valve is either fully open or fully shut Valve for hot water heating can be throttled and there is a small hole in the cylinder Gate so there is constant circulation to prevent possible freeze up
LEGEND MODEL T-425/S-425 straight circulator valves and straight-way fittings are constructed of heavy duty bronze. They have a male union, provide 100% shut-off and 1/4 turn flow control. For use on forced hot water heating systems up to 60 lbs.
If you cannot get the Legend then the one below will work