A T/P and an expansion tank are two separate devices and although both are related to the water heater, they serve different purposes. The T/P, has Redwood has pictured, protects the tank for exploding in the event that the temperature or pressure exceeds safe limits. It is required on every water heater. A thermal expansion tank is only necessary if the water heater is part of a closed system. A closed system is created when there is a check valve in the incoming water supply that prevents the expansion of heated water from being absorbed by the water main. This is commonly caused by a pressure regulator valve. Not all home have or need a pressure regulator valve as their water supply is already at a safe, usable pressure. When a system is closed and there is no expansion tank, when the water heater heats the water, that water expands and the expansion being blocked by the check valve in the PRV causes the pressure in the water heater to rise to the limit of the T/P valve which then trip to relieve that pressure. Even if you are willing to lose the water, you can not rely on the T/P alone to do the job because they sometimes do not close after the pressure is relieved.