Pressure relief valves are designed to release the pressure on a close vessel, at a specfic pressure. As pressure is increased, the boiling point of what is inside (in our industry it is mostly water) is increased. The problem with this is that a increase in pressure puts additional stress on the components in the system. When you are heating the water in a tank the boiling point is increased. If a leak were to occur when the water temperature has exceeded the boiling point, the leaking water would instantly turn to vapor. This is very dangerous, because a cubic foot of water, flashes to 16 cubic feet of vapor. This is a lot of energy. This is an explosion.

  If there was no heat, the leak would just release water in the liquid form (not a vapor) since the boiling point was not reached.

There are two kinds of relief valve becaue of this; a temperature/pressure relief valve and a pressure relief valve. A temperature/pressure relief valve has a stem on it to sense the temperature of the water. A pressure relief valve does not.

Pressure Relief Valve	Pressure/Temperature relief valve

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