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."
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.
is it temperature and not pressure that causes a hot water
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
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.
following is directly from the Watts Regulator Co. regarding
installation of T&P valves and Watts 210 gas shutoffs
Instructions: Relief Valves and Automatic Gas Shutoff
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.
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
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