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Temperature and pressure
relief valves |
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| What is a T&P relief valve and how
does it work? Most consumers take for granted their
hot water supply and rarely think about their water heaters
once they have been installed. Where water heaters are concerned,
"out of sight" is usually "out of mind."
and to date, many are unaware of the need to have their water
heaters regularly maintained to prevent water heater explosions.
The Temperature and pressure relief valve (T & P) is
the primary "back-up" safety measure which prevents
the water in the pressurized hot water cylinder from overheating.
An explosion of a water heater presumes at least two system
failures. Generally, the safety mechanism in the controls,
coupled with a properly installed and working T & P relief
valve, insures a safe system. |
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Watts T&P relief valve for installation on
unvented water heaters to prevent the temperature of water exceeding
100°C. Pressed brass body. Ethylene-propylene diaphragm.
- Pressure setting: 10 bars max (150psi).
- Temperature setting: 93°/94°C.
- Wrc approved.
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| To insure their safe operation, water heaters
must be installed with temperature and pressure relief valves.
A T&P relief valve is a two-in-one safety device, which,
as its name implies, responds both to excessive temperature
and to excessive pressure in a water heater.
A T&P valve responds to excessive temperature by expansion
of the thermostat which lifts the disc off its seat and discharges
overheated water, allowing cooler water to enter the tank
and replace the discharged hot water in order to moderate
the water temperature. When a valve is relieving water in
volume, it is generally due to excessive temperature. |
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The valve also responds to excessive pressure
by discharging water to prevent further pressure increase. If
a T&P valve is dripping, rather than relieving in volume,
it usually indicates either thermal expansion or debris on the
seat, which prevents the valve from closing tightly.
Thermal expansion is an increase in system pressure over
that regulated by the reducing valve, check valve or backflow
preventor. When water is heated in a closed system, it expands
and causes an increase in pressure. When the system is opened,
pressure will return to normal, and the relief valve should
stop dripping. When a backflow prevention device or check
valve is installed on the water service entering a building,
a closed system is created. When water is heated, it expands.
Because the water has nowhere to go in a closed system, a
potentially dangerous situation is created. |
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| For over a decade it has been known that an inoperative
or improperly installed temperature and pressure (T & P)
relief valve on a water heater can turn an ordinary water heater
into a potentially explosive and life-threatening "bomb."
A safety valve such as a T&P relief valve is a mechanical
device which, like any mechanical device, is subject to failure,
particularly when tampered with or improperly installed. If
the valve cannot do its job, the water in the tank may become
superheated (over 212°Fd, causing a thirty gallon water
heater to explode with the equivalent force of one pound of
dynamite. This is not theory or conjecture. It is a matter
of record that such explosions occur, and have occurred, at
the expense of human life. Yet, to date, only a few are aware
of the need to have their water heaters regularly maintained
to prevent water heater explosions. |
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The T&P relief valve itself is not the cause
of such explosions. Rather, it is the misapplication or malfunction
of the valve, which sets the stage for disaster. Plumbing and
heating engineers are generally the professionals who install
water heaters and safety valves. As professionals who pride
themselves on protecting the health and safety of their customers
and the public, they are in a position to perform reinspections
of T&P relief valves, as well as to persuade building owners,
public officials and code bodies that provisions for reinspection
should be made for the safety of the public. |
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| Hot water heater in school kills seven
What follows is an extract from the fire journal
report on an incident that took place at the Star elementary
school in 1982.
A malfunction of an 85-gallon hot water heater occurred during
a lunch period at the Star Elementary School in Spencer, Oklahoma,
on January 19, 1982. One corner of the cafeteria was destroyed
by the explosion and pieces of concrete block were thrown
across the lunchroom. Six children and one teacher were killed
and 33 others suffered injuries of varying degrees. The malfunction
was caused by overheating of the water in the tank and lack
of emergency temperature-relieving capability. |
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Two gas-fired water heaters were located in a
corner of the kitchen, in a small utility room. The two water
heaters were piped in series; the first heater, Heater A, heated
water to approximately 160°F for the kitchen, a lavatory
in the northwest corner of the kitchen, and a sink on the kitchen
side of the lavatory wall. The second heater, Heater B, was
supplied from Heater A where the temperature of the water would
be boosted to 1800-190°F for use in the rinse cycle of the
dishwashing machine. It was the opinion of the investigative
team that the burner on the high-temperature heater (Heater
B) had fired continuously from the time that the plumber com-
pleted his service on the tank, at approximately 11:00 am, until
the explosion at 12:15 pm -a time span of about one hour and
fifteen minutes. |
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| The probable cause of the continuous firing of
the burner was the failure of the thermostat, because inter
nal corrosion within the thermostat had eroded a portion of
the seating surface, allowing a small amount of bleed gas to
pass through the damaged seat; hence, the leakage of the bleed
gas through the thermostat sent a signal to the burner valve
to remain in the open position. From information available,
this thermostat control was estimated to be 12 to 14 years old.
Markings on screw heads and internal inspection indicated
that the thermostat on Heater B had been opened in the field;
comments by school maintenance personnel indicated that this
was a common practice, the purpose being to recalibrate the
thermostat to allow for higher temperature settings.
Testing and internal inspection revealed that the upper temperature
limit control was inoperable before the explosion. The Fire
officers report concluded that an attempt had been made to
adjust the control; mis-alignment of the control spring indicated
that the control had ceased to function, since the mechanism
would not seal off the flow of bleed gas. This control was
also estimated to be 12 to 14 years old. |
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The pressure and additional temperature-relief
protection for the exploded tank was a combination pressure
and temperature-relief valve (commonly called "T&P
relief valve" ) in a common housing. After close examination,
the investigators determined that the temperature-sensing element
had been removed, leaving the T&P valve with no temperature-relief
capability. They further discovered that the relief valve had
been improperly installed, using a 45° elbow fitting which
would have made it impossible to install the valve with the
temperature-sensing element in place and properly projecting
into the upper six inches of water in the heater. |
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The continuous build-up of heat from an uncontrolled
burner with no emergency temperature-relief capability resulted
in a water temperature above 212°F (i. e. superheated
water). When the tank failed, a portion of the superheated
water instantly expanded to steam, creating the thrust that
propelled the tank pieces and the shock-wave effect identified
as the blast, or explosion. Other factors that had some bearing
on the accident, either directly or indirectly, were reported
by the Fire officer:
- The investigation revealed that there was no preventive
maintenance program in place or in practice.
- The supervision of maintenance engineers to determine
the quality of their work and field practices was inadequate.
- The lack of in-service training for maintenance engineers
permitted practices that were not in the best interests
of safety and performance of the equipment.
- Warehousing of valves and controls at the Maintenance
Center was done in a manner that increased the possibility
of damage to the parts.
The accident demonstrates the need for a means to prevent
temperatures above 212°F in water heaters, because a hazard
does not exist if the liquid in a container is at a temperature
below its normal boiling point. The missing temperature-relief
element, set at 210°F, in the temperature-pressure relief
valve would have been one such means. Without this, the high
temperature thermostat in the control circuit for the automatic
valve was the only such means in place. This was intended
to close the valve when the temperature reached 210°F.
However, it failed -apparently due to corrosion. |
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