Water Heater Relief Valve
Watts Model 100XL - 4. 3/4 inch - # 150 PSI.

Temperature and Pressure Relief Valve for direct installation into hot water tank. Rated for 150 P.S.I. pressure and 210 degrees F. 3-3/4 inch thermostat probe projection. Protects water heaters to 100,000 BTU per hour.
3/4" M.I.P.S. inlet and 3/4" F.I.P.S. outlet.

Notice: Discharge of the Temperature and Pressure Relief Valve may indicate that an unsafe temperature or pressure condition exists. If a discharge happens shut down the water heater and contact a licensed plumber immediately!

Temperature and Pressure Relief Valves, also called T&P valves, are emergency safety limit devices that will prevent or relieve overheated water and pressure. Without a relief valve during an unsafe condition, the pressure inside the tank would rise to the point the tank may rupture or explode. This would cause potential damage to both people and property.

 Rheem Water Heaters (for instance) are pressure tested to 300 pounds per square inch (PSI) and have a working pressure of 150 PSI. The T&P valve is designed to open when the pressure inside the tank exceeds 150 PSI, allowing pressure to vent safely. The T&P valve will also open on temperature, opening if the water temperature reaches 210 degrees F. The T&P valve will remain open allowing cold water into the tank until the unsafe condition is over.

Here is a lik to the Myth Buster website and a segment on Exploding Water Heaters.   This is very interesting and one of the finest examples of why a relief valve is so important.