# InterNACHI » "The Inspectors Journal™"

## TPR Valve Importance

July 15th, 2009

Water is essentially an incompressible solid. It has no latent heat energy within itself to expand when released, unless the water is superheated. Water above 212˚ F is superheated water, and superheated water would really like to turn into steam at atmospheric pressure. It possesses latent heat energy, which can flash into steam and create a force that is not unlike an explosion.

Water would normally boil at 212 ˚F, but inside a tank it can’t expand anywhere so it can’t boil off into steam. Water in a “closed” system and under pressure, such as inside a hot water tank, has a much higher boiling point. For example, where water supplied to a tank is at 50 psi, the boiling point is 297.7˚ F.

Let’s assume a water heater tank has 30 gallons of superheated water inside it. Assume 50 psi and the water temperature is superheated at 300 ˚F. Remember that superheated water really wants to turn into steam. If the tank ruptures, then 30 gallons of superheated water will instantaneously turn into steam in an outwards direction through the rupture.

There is a tremendous amount of energy released as the superheated water is exposed to atmospheric pressure and immediately turns into steam. Every cubic inch of water becomes a foot of steam!

Below is a list of energies developed in a 30-gallon hot water tank.  The following data shows the explosive energy created in a 30 gallon hot water tank at various pressures and temperatures.

At 0 pressure psi, water will boil at 212 F, and the foot-pounds of energy released when 30 gallons of water is exposed to atmospheric pressure is 0.

At 10 pressure psi, water will boil at 239.5 F, and the foot-pounds of energy released when 30 gallons of water is exposed to atmospheric pressure is 479,800.

At 30 pressure psi, water will boil at 274.0 F, and the foot-pounds of energy released when 30 gallons of water is exposed to atmospheric pressure is 1,305,000.

At 50 pressure psi, water will boil at 297.7 F, and the foot-pounds of energy released when 30 gallons of water is exposed to atmospheric pressure is 2,021,900.

At 70 pressure psi, water will boil at 316.0 F, and the foot-pounds of energy released when 30 gallons of water is exposed to atmospheric pressure is 2,642,000.

At 90 pressure psi, water will boil at 331.2 F, and the foot-pounds of energy released when 30 gallons of water is exposed to atmospheric pressure is 3,138.400.

Note that at 50 psi, water flashes into steam at 297.7˚ F, and the energy released equals more than 2 million foot-pounds of energy, similar to the explosive energy released by one pound of Nitroglycerin. A 16-inch gun on a USS Iowa class battleship produced a 7,500,000 foot-pounds of energy.

Combination temperature and pressure relief (T & P Relief or TPR) valves do two things: 1) they open and release water out of the tank if the temperature exceeds 210˚F (just below the boiling point), and 2) they will open if the pressure in the tank exceeds 150 psi (the maximum normal operating pressure for a water heater.)

It is essential to avoid excessively high water temperatures and pressures at a water heater tank.

“How to Properly Inspect Hot Water Tanks” – An online video training course for property inspectors.  http://www.nachi.org/advancedcourses.htm

This blog entry was posted by Ben Gromicko.

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