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Austenitic Stainless Steel
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Basic Weld Inspection - Part 2
Black Liquor Recovery Boilers - An Introduction
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Boiler/Burner Combustion Air Supply Requirements and Maintenance
Carbon Monoxide Poisoning Preventable With Complete Inspection
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Environmental Heat Exchangers
Factors Affecting Inservice Cracking of Weld Zone in Corrosive Service
Failure Avoidance in Welded Fabrication
Finite Element Analysis of Pressure Vessels
Fuel Ash Corrosion
Fuel Firing Apparatus - Natural Gas
Grain Boundaries
Heat Treatment - What Is It?
How to Destroy a Boiler -- Part 1
How to Destroy a Boiler -- Part 2
How to Destroy a Boiler -- Part 3
Identifying Pressure Vessel Nozzle Problems
Inspection, Repair, and Alteration of Yankee Dryers
Inspection, What Better Place to Begin
Laminations Led to Incident
Lay-up of Heating Boilers
Liquid Penetrant Examination
Low Voltage Short Circuiting-GMAW
Low Water Cut-Off Technology
Low-Water Cutoff: A Maintenance Must
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Maintaining Proper Boiler Inspections Through Proper Relationships
Microstructural Degradation
Miracle Fluid?
Organizing A Vessel, Tank, and Piping Inspection Program
Paper Machine Failure Investigation: Inspection Requirements Should Be Changed For Dryer Can
Pipe Support Performance as It Applies to Power Plant Safety and Reliability
Polymer Use for Boilers and Pressure Vessels
Pressure Vessel Fatigue
Pressure Vessels: Analyzing Change
Preventing Corrosion Under Insulation
Preventing Steam/Condensate System Accidents
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Putting a Stop to Steam Kettle Failure
Quick Actuating Closures
Quick-Actuating Door Failures
Real-Time Radioscopic Examination
Recommendations For A Safe Boiler Room
Recovering Boiler Systems After A Flood
Rendering Plants Require Safety
Repair or Alteration of Pressure Vessels
Residential Water Heater Safety
School Boiler Maintenance Programs: How Safe Are The Children?
Secondary Low-Water Fuel Cutoff Probe: Is It as Safe as You Think?
Short-Term High Temperature Failures
Specification of Rupture Disk Burst Pressure
Steam Traps Affect Boiler Plant Efficiency
Steps to Safety: Guide for Restarting Boilers After Summer Lay-Up
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Suggested Daily Boiler Log Program
Suggested Maintenance Log Program
System Design, Specifications, Operation, and Inspection of Deaerators
Tack Welding
Temperature And Pressure Relief Valves Often Overlooked
Temperature Considerations for Pressure Relief Valve Application
The Authorized Inspector's Responsibility for Dimensional Inspection
The Effects of Erosion-Corrosion on Power Plant Piping
The Forgotten Boiler That Suddenly Isn't
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The Use of Pressure Vessels for Human Occupancy in Clinical Hyberbaric Medicine
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Typical Improper Repairs of Safety Valves
Wasted Superheat Converted to Hot, Sanitary Water
Water Maintenance Essential to Prevent Boiler Scaling
Water Still Flashes to Steam at 212
Welding Consideration for Pressure Relief Valves
Welding Symbols: A Useful System or Undecipherable Hieroglyphics?
What is the Best Welding Process?
What Should You Do Before Starting Boilers After Summer Lay-Up?
Why? A Question for All Inspectors

Temperature And Pressure Relief Valves Often Overlooked

Lee Doran
National Board consultant

Summer 1993  

Category: Operations 


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 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 T&P 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, 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 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.



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 of the ASME Boiler and Pressure Vessel Code for current requirements.


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