radiator ?

i want to change a radiator have traced back to the 15mm t join want to disconect it at this join and fit a 15 mm gate valve so it gives me time to change pipe work to new radiator. rather than drain down whole system i was going to use the freeze spray disconnect at 15mm T fit the gate valve on there . but do i fit the gate valve straight on to the T OR DO I HAVE TO FIT IT VIA A SMALL PIECE OF 15MM COPPER FIRST