Check the required rating for
your relief valve by checking the plate on the
water heater, you can not always go with what is
installed on the water heater because the person
that replaced the relief valve before you may
not have installed the correct relief valve.
In the image the water heater relief valve has a copper drain pipe with an elbow pointing it down, I unsoldered the 90 from the pipe so that I can remove the drain pipe from the relief valve. If you can not solder cut the pipe leaving enough room to install a compression coupling to replace the pipe.