There are two solutions:
1) Replace TP valve with same temperature/pressure as existing TP valve.
Buy the TP valve at hardware store or home center or plumbing supply.
Turn off water heater so water cools.
Hot water inside a tank is explosive ... so open the TP valve and test water temperature.
If TP is on top of tank, you don't need to drain much water before unscrewing
If TP is on side of tank, use the TP valve to drain water -or- use the drain valve
Unscrew TP to remove.
Put teflon tape on new valve threads so it will seal against leak.
2) If the new TP replacement starts doing the same thing, there may be a thermostat problem -or- thermostat is set too high.
If you keep the water temperature set high, you can add expansion tank to incoming cold water line.
Home Depot sells expansion tanks. Plumbing supply carries them,
The tank for residential water heaters is about same diameter as a football and it goes on cold water line.
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