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Kaplan ITA's Monthly Inspection Tips - Free Electronic Newsletter

May, 2003
Water Heater Over-Temperature Protection
Issue #13

Dear Inspector,

This month I would like to discuss the importance of water heater over-temperature protection. This includes a brief discussion of the properties of heating water when under pressure. I highly recommend visiting the website www.wattsregulator.com and perusing their free literature section. Most of the information below is from the Watts information and can be useful when educating clients. I strongly recommend the brochure "52 Questions and their Answers relating to Hot Water Safety."

What causes a water heater or hot water storage tank to explode?
Explosions result primarily from overheating or excessive temperature. Contributing physical causes are corrosion and service weaknesses. The resultant heat rupture exposes its pressurized contents to the atmosphere. This causes the superheated water to immediately flash into steam. This combination of factors creates the explosion.
To illustrate what "heat rupture" means, let us suppose that a heater operating at a normal temperature of 120°F can satisfactorily withstand water pressure of, say, 75 psi. However, the same tank, due to corrosion and other factors, when overheated may only be able to safely handle a pressure of 50 psi.

Why is it temperature and not pressure that causes a hot water explosion?
Water does store energy when heated. The boiling point of water rises when water is under pressure. Explosions resulting from the release of this latent energy can occur at normal operating pressures. The force and energy of the explosion derives from steam
pressure resulting from the superheated water flashing into steam under the atmospheric pressure condition. It is not water pressure that causes an explosion. Tank pressure can be built up to over 500 psi in an unheated tank. When a rupture is caused, nothing happens except a squirt of high pressure water.

If excessive pressure is relieved, why doesn't this give adequate protection against explosion?
A pressure relief valve will only open far enough to reduce the excess pressure. But this amount of discharge is not sufficient to reduce temperature or to prevent the overheating. The reason the volume of pressure discharged is not enough to overcome the BTU heat input is because thermal expansion pressure equals approximately 212% of volume for every 100°F rise. Fluid heat discharge is necessary to relieve the extra BTU heat input for every 1,000 heat units which is about 20 times greater in volume.

The following is directly from the Watts Regulator Co. regarding installation of T&P valves and Watts 210 gas shutoffs

Important Instructions: Relief Valves and Automatic Gas Shutoff Devices
Combination temperature and pressure relief valves with extension thermostats must be installed so that the temperature-sensing element is immersed in the water within the top 6" (152mm) of the water storage tank. They must be installed either in the hot outlet service line or directly in a tank tapping. Combination temperature and pressure relief valves that do not have extension elements must be mounted directly in a tank tapping located within the top 6" (152mm) of the water storage tank. Valves must be located so as to assure isolation from flue gas heat or other ambient conditions that are not indicative of stored water temperature. MC Note: failure to have the temperature sensor in the tank creates thermal lag of the valve compared to tank temperature and a potential hazard before the valve reacts.

WARNING: To avoid water damage or scalding due to valve operation, discharge line must be connected to valve outlet and run to a safe place of disposal. Discharge line must be as short as possible and be the same size as the valve discharge connection throughout its entire length. Discharge line must pitch downward from the valve and terminate at least 6" (152mm) above a drain where any discharge will be clearly visible. The discharge line shall terminate plain, not threaded. Discharge line material must conform to local plumbing codes or ASME requirements. Excessive length over 30' (9.14m), or use of more than four elbows or reducing discharge line size will cause a restriction and reduce the discharge capacity of the valve.

Mike Casey
Kaplan Professional Schools
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