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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.

Water Heater blow off valve