Why is the pressure valve repeatedly tripping on my water heater?
If the PT valve opens after hot water is used say from a shower, there are three possibilities. 


There is no expansion tank connected to your hot water tank or the one installed needs serviced or replaced. The cold water that enters the tank expands as it is heated causing the discharge of water due to over-pressure in the tank. 
Your incoming water pressure (the normal house pressure) is too high. Normal incoming supply pressures are usually around 80 PSIG. If that is the case you may need a pressure reducing valve on the incoming house supply line.
The replacement TPRV pressure setting is too low. Most are sold with pressure settings of 125PSI or 150PSI. I prefer the 150PSI as long as that pressure does not exceed the tanks pressure rating.


To correct your problem you may any one or a combination of all three to fix your problem. One last thing, you may want to check with your governing bodies to make sure you do not break any building codes. I never had any problems however I have been retired about 10 years and big brother is always on the prowl and watching.