National Board consultant
Summary: The following article is a part of National Board
Classic Series and it was published in the National Board BULLETIN. (3 printed pages)
The most neglected safety device on fired pressure vessels is the Temperature
and Pressure (T&P) relief valve on water heaters.
Water heaters are everywhere: in residences, churches, hotels, banks, schools,
garages, etc. These water heaters are usually located out of sight, and
therefore out of mind. Unfortunately, they won't attract any attention unless
there is a lack of hot water.
These water heaters provide many years of trouble-free operation and because of
this, cause complacency on the part of the owner/maintainer.
Because of this complacency, catastrophic failures have resulted in extensive
property damage, injuries and death. Countless other undocumented failures have
resulted in near misses, which could have also resulted in property damage and
In the event of a control failure which may cause a runaway firing condition,
the only safety device which will prevent a catastrophic failure of the hot
water vessel is the T&P relief valve.
In view of this, it is imperative that the T&P relief valve be inspected
and tested regularly.
Since the T&P relief valve is constructed to relieve on either pressure or
temperature, manually testing the valve with the test lever only tests the
mechanical freedom of movement of the valve and ensures the waterways are
Manufacturers recommend that valves that have been in service more than three
years be removed and visually inspected for accumulations of corrosion
deposits, such as those shown in the photographs.
The thermal probe should be inspected for corrosion and scale accumulations
which will insulate the valve and probe from the hot water, and also for any
illegal alterations, repairs or tampering. Leaking T&P relief valves must
The valve should also be inspected for proper installation. The valve probe
must be immersed in tank water and be located in the top six inches of the tank
in order for it to accurately sense tank water temperature. Improper
installation, such as that shown below, could render the valve ineffective
because it cannot sense actual tank water temperature.
Valve piping must also be inspected to ensure the outlet of the valve has not
been reduced, and is pitched down for free draining with no shut-off valves or
other obstructions in the valve drain pipe.
Check the valve nameplate, ensure the pressure relief setting does not exceed
the maximum allowable working pressure of the tank, and be sure that the A.G.A.
rating is in excess of the Btu input
of the heater.
Also, the nameplate should be checked for the ASME "HV" symbol and the National
Board "NB" mark to ensure the valves have been capacity certified by the
National Board and are in compliance with the ASME Boiler and Pressure Vessel
In addition, it is recommended that a log be maintained near the water heater
to provide a record of inspection dates and the results of the inspections.
Don't become a statistic; conduct regular routine inspections and correct all
of the deficiencies discovered.
Editor's note: Some ASME Boiler and Pressure Vessel Code requirements may have changed because of advances in material technology and/or actual experience. The reader is cautioned to refer to the latest edition and addenda of the ASME Boiler and Pressure Vessel Code for current requirements.