The only reason you even need an expansion tank is if you have a closed system. This is created either by a pressure regulator valve or a check valve built into the water meter. When your water heater heats water, the water expands. In a closed system it has no place to expand to, so when the resulting pressure reaches the TP valve limit, it trips to prevent the heater from bursting. If the system is open, then expansion is absorbed by the water main, but in a closed system, you need an expansion tank. The expansion tank is charged with air at or near the pressure set by the PRV. If the bladder in the tank goes south, then you need a new expansion tank. The TP valve could be faulty, but my money would be on the tank being the problem. Reset the PRV to 60 psi, charge the expansion tank to that pressure, paying attention to the points Jim gave in his reply, and if the tank appears OK, give it a try.