BULLETIN FROM....

Fall 1993

A Word to the Wise

TEMPERATURE & PRESSURE RELIEF VALVES OFTEN OVERLOOKED

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, 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 human suffering.

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 clear.

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 TOP relief valves must be replaced.

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, 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 Code.

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.

Keeping in mind the various water conditions across the province, we suggest the T & P relief valves on water heaters be manually tested monthly. As indicated above, test dates should be kept on a log maintained near the water heater.

Attention ­ precautions should be taken against injury from scalding during the testing.

The relief valves should be equipped with full size discharge pipes installed in accordance with standard approved practice.

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